GEOLOGOYF -MT. MCLOUGHLIN by LEROYC ARSONM AYNARD A THESIS Presented to the Department of Geology and the Graduate School of the University of Oregon in partial fulfillment of the re~uirernents for the degree of Master of Science June 1974 ii iii VITA NAMEO F AUTHOR1L eRoy Carson Maynard PLACEO F BIRTH1 Hayward, Oklahoma DATEO F BIRTH1 25 June 1945 UNDERGRADUAANTDEG RADUATSEC HOOLAS TTENDEDs University of Oregon DEGREEAS WARDEDs Bachelor of Science, 1967, University of Oregon AREASO F SPECIALI NTEREST Igneous Petrology Geomorphology PROFESSIONALE XPERIENCE1 Assistant Field Geologist, under Professor Ewart M. Baldwin, University of Oregon, Eugene, 1967 Teaching Assistant, Department of Geology, University of Oregon, Eugene, 1971 Reseach Assistant, Center for Volcanology, Department of Geology, University of Oregon, Eugene, 1972 iv ACKNOWLEDGEMENTS I would like to thank Drs, A, R, McBirney and G, G, Goles for their time and effort in directing this study, Discussions with Drs, E, M, Baldwin and B, H, Baker Froved most fruitful dur.lng the preparation of this thesis, This research was supported in part by NSF research grant GA-19382 and a student research grant from the Department of Geology, Univers.ity of Oregon, V TABLEO F CONTENTS Introduction 1 Description of Area 2 Regional Geology 12 Western Cascade Stra"l;.igraphy 17 High Cascade Series 28 Petrography 48 Phenocryst Petrography 63 Nomenclature 80 Volume Relationships 82 Analysing Techniques 86 Peacock Index 89 Rock Compositions 92 Petrology and Petrogenesis 118 Appendix 124 Bibliography 137 vi LIST OF FUGURES Figure No. Subject Page 1. Regional tectonic map 3 2. Thesis location map 5 3. Spheroid.al weathering of Plio-Cascades 32 4. Early stage agglomerate of Mt, McLoughlin 5. Flows exposed in cirque headwall 38 6, Blocky flow erupted on flank of Mt, McLaughlin ?. Orthopyroxena jacketed with clinopyroxene 50 8. "Ghosts" of olivine phenocrysts 9, Pilotaxitic floK texture in a Heppsie Andesite 52 10. Apatite needles in Plio-Cascades lava 11. Texture of glassy Fish Lake lava 55 12. Glomerophenocryst in glassy Fish Lake lava 13, Zoning of plagioclase in Fish Lake lava 59 14. Texture of fine-grained flank lava 15, Clusters of subparallel augite needles 61 16. Texture of Mt. McLaughlin main cone lava 1 ?. Single grain of olivine-orthopyroxene-clinopyroxene 64 18. Concentric ring of olivine granules in orthopyroxene 19. Cumulate olivine in ophitic.pyroxene 66 20. Euhedral, twinned, and zoned clinopyroxene 21. Pyroxene quadrilateral 70 22. Albite-anorthite binary system, wet 74 23• . Diopside-anorthite binary system, wet 24. Cumulate olivine as inclusions in clinopyroxene 76 25. Texture of Brown Mountain lava 26. Diagram of KzO+ Na20/ca9 vs. S102 90 27. AMFt ernary diagram 98 28. NCKt ernary diagram 99 vii LIST OF FIGURES,c ontinued Figure No. Subject 29. Diagram of Ti02 vs. S102 100 :30. Diagram of MgOv s, Si02 .101 31. Diagram of Fe203 vs. ~102 102 32. Diagram of CaO vs. SiOz 103 33. Diagram of K20 vs, S102 104 34. Diagram-of P205 vs, S102 105 35. Melting characteristics of a natural high-alumina 115 basalt system under wet conditions 36. Fo-Di-Qz diagram at 20 kb under wet conditions 120 ° viii LIST OF TABLES . Table No. Sub,ject Page 1. Variables in data on XRF determinations 88 2. Analyses of Heppsie Andesite, Plio-Cascad.es, and 106 olivine basalts I J. Analyses of Fish Lake series andesites 107 4. Analyses of Fish Lake series andesi tes 108 5. Analyses of Fish Lake series andesites 109 6. Analyses of Mt. McLaughlin main cone andesites 110 ?. Analyses of Mt. McLaughlin flank lavas and Brown 111 Mountain andesite 8. Analyses of lavas suspected of phenocryst enrichment 112 9. Analyses of pyroxenes from Mt. McLaughlin lavas 113 10. Analyses of pyroxenes from Crater Lake rocks 11. Correlation of glacial successions 133 12. Trace element abundances from Mt. McLaughlin area 134 • rocks 13. Trace element abundances from Mt. McLaughlin area 13.5 rocks 14. Trace element abundances from Mt. McLaughlin area 1:36 rock and three standard rocks ( ix Mt. McLoughlin viewed from the ·southeast 1 INTRODUCTION The purpose of this investigation is to contribute information I about High Ca1:1cadev olcanoes by studying one of them, Mt. McLoughlin, in detail.. 'nle study provides data on l) the distribution and abundance• of rock types, 2) the geologic history of the area, 3) the structural and contact relation of the High Cascades to the Western ea,scades, and 4) the variation and evolution of rock types. 2 Description of Area The mapped area is entirely within the Cascade Range geomorphic provi:-nce. 'nlis province is a north-south linear belt of Eocene to Recent volcanic rocks, 40 to 70 miles wide and nearly 700 miles. long. The range extends from· Mt. Lassen in California north to Mt. Garibaldi in British Columbia, cutting Oregon a_nd Washington into a western third' and an eastern two thirds, (gigure 1), The ·mapped area (115 square ~iles in Jackson and Klamath Counties) includes a major portion of the Kt. McLoughlin quadrangle and parts of the Lake O' WQods quadrangle to the east and the Rustler Peak quadra~gle to the north. 'l'he quadrangle maps are part of the 15' series and were first published in 1955 by the U.S.G.S. at a scale of 1:62,500. The" area is bounded on. the east by a north-south trending normal fault, ; which may be related to the faults that form the Klamath graben to the east. The west and south boundaries are near .the west fork of -Willow Creek and the·south fork of Little Butte Creek respectively. Both of these cree~s expose underlying Oligocene pyroclastic deposits. The northern bou~1ary is the south fork of Fourbit Creek. It marks the northern limit of Mt. McLoughlin lavas. .Between the confluence of Pourbit Creek and Willow Creek is a flat area known as Parker Meadows. It is formed _from stream deposits which were later covered by younger flows of olivine andesite. Figure~ shows these major geographic features. - --~--- 3 ..D'- \ ,, ~B' \ ~ ," \ ~ " ' / ~j l~ ~ - ~ \ -- --:s:: tH __-_(_) _.A- _~ ' -·- ----:-. -♦•- • • - .-<. ~ - / ~ ; t ~~. ~~'\f ,a,, _- ...... I u ✓ J-S_ :-:-_,/ &~ / .\ 4 Figure 1. Tectonic map showing the spatial relationship of the High Cascades in Washington, Oregon and northern California and the West- ern Cascades in Oregon to pre-Tertiary structures. Labeled peaks and centers are B-Mt. Baker; G-Glacier Peak; R-Mt. Rainier; SH-Mt. St, Helens; A-Mt. Adams; H-Mt. Hood; J-Mt. Jefferson; TS-Thrree Sisters; CL- Crater Lake; S-Mt. Shasta; L-Mt. Lassen; N-Newberry Crater; and ML-Medicine Lake. ~lack areas are pre-Tertiary intrusions and lines. are fold axes. Map is modified from Wise (1969), N 1 THESIS AREA llll!lf••S TIOT 1ur11 SCAlf 0 1 2 IN MILES Figure 2, Location map• .. 6 Access The area can be reached by traveling 40 miles east from Medford or 30 miles west from Klamath Falls on U.S. Highway 140. A network of cinder 'surfaced Forest Service roads provides access to nearly all portions of the area in the summer months. In the winter months, Highway 140 is kept open by repeated plowing of the pass which 1s 5,200 feet in elevation. All other roads are left closed after the first heavy snow in the fall until the following May. Vegetation, soil and colluvium cover most of the area, except the youngest lava flows, and restrict good outcrops mostly to roadcuts. Traveling on foot through the area, even aiong streams, is hamp~red by a thick growth of alder, madrone, and hazel which has grown up over logged areas. The northwest flank of Mt. Mcloughlin is an unpenetrable dense growth of live oak, snow brush and manzanita brush which has grown over an old burn. Two Forest Service trails at one time passed through this brush toward Mt. McLoughlin, but they have not been maintained in recent years. Th.e trails are now difficult to recognize and barely passable. Physiographic Features . The principal features, of the area are Mt. McLaughlin, a composite volcano; and Robinson Butte and Brown Mountain, two shield volcanoes immediately to the south. Elevations range from 3000 feet on basalt- covered Parker Meadows in the northwest corner, to the' top of Mt. McLaughlin at 9495 feet. Mt. McLoughlin, formerly known as ~t. Pitt, 7 is the highest and the most prominent peak between Mt. Shasta and Crater Lake. Fresh lava flows cover most of Brown Mountain and portions of the flank of ~t. McLoughlin. Some of these flows have a few trees and bushes on them but o~hers are barren. '11tere are a few cinder cones in the area, but they are old enough to have a forest cover. Land forms ate constructional and only slightly eroded except where glaciers cut into the upper slopes of Mt. McLoughlin, the head of Willow Creek and Little Butte Creek in the west and south, respectj.vely. The fault . scarp near the east boundary of the mapped area is a tectonic feature. Culture The area is almost entirely within the Rogue River and Winema .. National forests. The principal land u~e is logging. Most new roads are made by logging companies and then selected ones are maintained by the Forest Service, Some areas are now being logged for the second or third time because the pine forests'gro' w fast " in this climate of cold wet winters and hot dry summers. Experimental plots and other hand planted groves o_f Ponderosa pine have been planted over many clear-cut areas in the past 20 years. There is no farming in the area, Hay is grown for winter cattle feed a few miles to the west. in the larger meadows and clearings. Cattle are allowed to graze on federai lands during the sUlllll'ler. The area is extensively used for recreation. It has six large ,campgrounds; and at Fish Lake and Lake 0 1 Woods there are resort 8 facilities and surmner homes. There is fishing and hunting in season, camping and picnicing, boating, swimming, and water skiing on the resort lakes, hiking on the Oregon Skyline Trail and on the trail up Mt. McLoughlin, snow-mobiling in the winter, and sightseeing all year round. Most of the thesis area is an important water shed for the city of Medford. The outflows of Willow, Fourmile, and Fish Lake are regulated. Willow Lake is completely man-made, while Fourmile and Fish Lake are natural but have had their capacity increased by cement retaining walls around their outlets. Water flowing out of Fourmile Lake is diverted by canal into Fish Lake, and the overflow of Fish Lake is discharged into the North Fork of Little Butte Creek. The headwaters of Beaver Creek are siphoned across the upper canyon of the South Fork of Little Butte Creek and into Howard Prairie Reservoir. Methods of Investigation Field mapping and sampling were done in the summers of 1968 and 1971. Flows, vents, and different lithologies were plotted on 15' quadrangle maps enlarged to a scale of 1:32,000. Forest Service fireman's maps and aerial photographs taken by the U.S.G.S. in 1955 at a scale of 1:63,000, were used in the field, but because of their small scale and poor reproduct~on quality, they were of limited use. A newer set of photographs covering the area surrounding Mt. McLoughlin was purchased in February 1972 to map detailed features on the mountain and around its base. These photographs were flown in August 1971 and are at a scale of 1:16,000. 9 Nearly 140 rock samples were collected. Of these, 90 were selected for thin sectioning and 35 were chosen for chemical analysis. Trace element abundances were determined in ten samples by instrumental neutron activation techniques (INAA). Major element~, except Na20, were determined by the X-ray fluorescence. Sodium abundances were obtained using INAA. Of the thin sections studied, modes were determined on 10 samples by counting 300 grains per slide. The universal stage was used to measure optic angles of pyroxenes, and imnersion oils were used to determine the index of refraction of pyroxenes and olivines. These measurements were used to estimate the composition of olivine and pyroxene phenocrysts of selected samples. The anorthite contents of phenocryst and ground mass plagioclases were determined by using combined albite and Carlsbad twins after the method of Tobi (1963). Previous Studies Callaghan (1933) was the first to divide the Cascade Range into two linear belts, the older Western Cascades and the younger High Cascades. Thayer (1936-1937) further developed concepts of the structure and stratigraphy of the range and was the first to recognize the pre• dominance of basaltic rocks in the High Cascades which underlie the more spectacular but less voluminous strato•volcanoes. The earliest paper describing Mt. McLoughlin was entitled "Notes on Mount Pitt" by Arthur B. Emmonsw hich was read before the California Academy of Sciences in 1885. Emmonsv isited the mount&in in the su111Der 10 of 1875 to measure its height barometrically and to study its geology. He sampled the coarse- and fine-grained rocks of Mt. Mcloughlin and described the huge "crater" on the north side. There was no further significant description until a paper on Kt. 'l'hielsen by Bowel Williams appeared in 1935. Williams listed F.mmons' analysis of a coarse-grained rock and noted that the rock was notably lower in sodium and higher in potassium than were Crater Lake rocks. Seven years later, Williams (1942) published his well-known m~nograph on Crater Lake which included a reconnaissance of the Cascades between Crater Lake and Mt. Shasta. The brief history and description given in this reconnaissance and in Emmons' "notes" are the only pub- lished descriptions of Mt. McLoughlin. No previous geologic map of the volcano has been published, although a small scale cross-section and block diagram was given by Williams (1942). The Medford 30' quadrangle map published by the U.S.G,S. was authored by Francis Wells and his co-workers (1956). The map includes most of the section of Western Cascades east of Medford and terminates about four miles from the thesis area. In an accompanying text, Wells divided the rocks of the Western Cascades into the Colestin, Roxy and Wasson formations, and the Heppsie Andesite. He grouped the Roxy and Wasson formations into the Little Butte Series. The Oregon State Department of Geology and Mineral Industry published the Butte Falls 30' quadrangle map, produced by Wilkinson and others (1941). This map was printed well before the Medford map even though 11 Wells completed his field work in 1938. Wilkfnson I s map roughly outlines the major lithologic units. A bulletin was to accompany the map but the State Department of Geology never received the text. Williams (1957) produced a geologic·map of the Bend quadrangle which was also published by the Oregon Department of Geology. 'Ibis map and accompanying text included a reconnaissance map of a portion of the High Ca111cadesf rom the Three Sisters south tQ Crater Lake. In the text·, Williams lists criteria to be used in distinguishing Western and High Cascades rocks. Dallas Peck and others (1964) in U.S.G.S. Profess~onal Paper 449 reviewed and described a large area •o f the Wes.tern Cascades between the South Umpqua River and the Columbi~ River. 'nlelr purpose was to determine "the gross aspects of volcanic stratigraphy, the geolog-ic structure, and distribution of major geologic units," .. A.general summary of the -geology of the Cascade Range with an exhaustive list of references is included in the Oregon State DOGAMI bulletin #64 (Griggs, 1969), 12 REGIONALG EOLOGY General Features The Cascade Range in Oregon is a long, thick, pile of Cenozoic volcanic ro~k. lavas and pyroclastic rocks ranging in age from Eocene to late Miocene make up most of the western two thirds of the range. During. the Miocene epoch, the rocks- were uplifted, gently folded, faulted, intruded by shallow stocks, generally altered to a greenish hue, and then deeply eroded. Where attitudes can be determined, the rocks dip gently to the east until they are partially overlain by fresher undeformed flows from shield volcanoes of Pliocene to :Recent age. These shields coalesced to form a high, narrow plateau between the older lavas to the west and th~ high lava plains of Central Oregon. Situated on this undulating plateau is the long chain.of Pleistocene s~rato-volcanoes, ~umerous cinder cones, and some fresh pre-historic lava flows. Callaghan (1933) divided the Cascades into two north-south belts. He called the older deformed rocks to the west the Western Cascades, and the yo~nger undeformed rocks along the eastern edge of the range the High Cascades. Both terms are loosely used as physio- graphic areas as well as rock series units. 13 Western_Cascades The volcanic rocks of the Western Cascades presumably overlie the Unpqua Formation or its equivalent. 'lbe lower portion of this formation is a sequence of submarine lavas and breccias with associated dark shales of deer marine origin, It is exposed in the Roseburg area. The upper part of the sequence consi~ts of marine sandstones and con- glomerates which grade laterally ihto tuffs toward the east. South of Roseburg, the lhpqua Formation overlies upper Cretaceous marine sediments which in turn rest upon a Pretertiary "basement complex." The basement rocks consist of Jurassic sediments, Triassic schists, "older" . ; undated schists, and large fault~bounded bodies of serpentine and peridotite. All of these r9ck types have been intruded by Jurassic to Cretaceous granitoid dikes and stocks. 'lbese plutonic rocks are exposed in the Klamath Mou~tains forty miles to the southwest of Mt. McLaughlin, and in the Blue Mountains in north-central Oregon. After implacement of the plutons but before the creation of the ·Cascade lineament in late Cretaceous time, small bodies of marine sandstone and shale were enfolded on top of the basement rocks, Trends in the Pretertiary rocks and ,the ·pronounced crosscutting of the Cascade Range are shown in figure l• Volcanic rocks of early Tertiary age are found not only in the Western Cascades, bµt also over a wide area in eastern Oregon. Pre- suinably .these two sequences were deposited in the same tectonic basin, are roughly equivalent in age, and were at one time continuous beneath 14 the High Cascades. Rocks which make up the Western Cascades were deposited on the western edge of a large basin. Because of the great thicknesses exposed today, it appears that the western portion of the basin deepened into a north-south trough. The subsidence may have been self-generating. That is to say, as the outpourings of lava and pyroclastics continued, they sank under their own weight. The terrain maintained itself above sea level, but never attained great height. The rocks in this belt are varied lava flows, ignimbrites, tuffs and breccias, and minor lacustrine and fluviatile interbeds. Many of the rocks are of local character so that upon comparison, different sections of the Western Cascades differ greatly in lithology &nd thickness. The only persistent horizon is a light colored silicic ash unit near the center of the section. Its thickness ranges between O and over 5,000 feet. Peck and his co-workers (1964) thought that the source vents for the Western Cascades were aligned in north-south belts which moved progressively eastward with time. During late Miocene time, after more than 10,000 feet of rock had accumulated, the belt was uplifted, folded, faulted and intruded by shallow stocks. The folding is best observed in a series of broad NNE trending anticlines and synclines in the North Santiam River section east of Salem (Thayer, 1937). The faulting is best seen in the range south of the Middle Fork of the Willamette River. Here, the interbedded sediments show a consistent dip of 5 to 10 degrees to the east or northeast; northwest-trending faults are the major geologic structure (Peck et nl., 1964). 15 The shallow intrusions are thought to be contemporaneous with uplift and faulting. Stocks have yielded radiometric ages as old as Oligocene, but there is evidence that deformation continued to late Miocene time. Mineralization accompanied the intrusions and large areas of the surrounding rock have been propylitized, so that the rocks have a characteristic greenish hue. The uplifting of the belt created a p~ateau thousands of feet higher than the previous elevation. This plateau was vigorously attacked by erosive for.ces and by early Plio.- cene time it had been maturely dissected. Griggs (1969) estimates that nearly half of the section is still depressed below sea level. High Cascades To the east and partially overlying the Western cascades is the younger volcanic belt which Callaghan named the High Cascades. Nearly all the rocks in this series were extruded as lava flows, some of which are several miles in length. West of the crest lavas formed ~nter~ canyon flows down stream and river canyons. The more viscous flows formed a high narrow plateau of coalescing shield volcanoes with a chain of high peaks spaced along the crest. The predominant rock types are basaltic andesite, 53-57'1. silica. Andesite, 57% silica, is only a common rock type near high peaks arid the more silicic differentiates are confined exclusively to these peaks. R9cks of the High Cascades are generally assumed to range in age from early Pliocene to Holocene (Griggs, 1969). Some of the early 16 lavas are known to 1nterfinger with Pliocene nonmarine sediment~ to the east but, in most areas, direct dati~ ts not possible and the ages of rocks can only be inferred by such criteria as stratigraphic. position and amount of weathering and erosion. nie high peaks are considered very young since many have post glacial flows and no rocks of reversed magnetic polarity have been observed on them. Pleistocene to Recent normal faults have broken the lavas, espec- ially south of Crater Lake, but no tilting or warping has been de- te.cted. Faulting in the High Cascades is only prominent south of Crater Lake. nte faults trend north-south and some have measured displacements on the order of 800 feet. 17 WESTERNC ASCADEST RATIGRA:f}IY The sequence of lower Tertiary rocks that overlies the rocks of the Klamath Mountains has a regional northeast dip away from the Medford-Ashland valley and, therefore, presumably extends beneath the . High Cascades and Mt. McLoughlin, The rocks exposed t,n Bear CreJk ~- .... valley and .the canyons of Little Butte Creek have been describ~g; ·by t! «?..; , Francis G. Wells in "The Geology of the Medford Quadrangle, Oregon• California" (Wells and others, 1956). Since my interest in this sequence in the field was limited, the following descriptions, except where _noted, are summarized extensively from Wells' text. Hornbrook Formation In the Medford-Ashland valley, the first marine sediments which lap on the Klamath Mountains are Upper Cretaceous sands of the Horn- brook Formation, The sands are nearshore in character; they are "fairly uniforat in texture and compesition and consist of well-bedded, 1:'lard, fine-grained .greenish-gray arkosic sandstone with local. lenses of coarse conglomera~es and sandy shale," The sands have a maximum thick- ness of .600 feet 'in Oregon but further south in California they attain a thickness of 2,000 feet, Fossils found by Peck and Imlay (1956) were considered Cenomanian and Turonian in age. 18 Umpgua Formation Resting disconformably on the Hornbrook is the Umpqua Formation of lower Eocene age. Farther north near Roseburg, the U:npqua is predominantly submarine volcanic rocks and dark shales, but in the Medford valley it consists mostly of tuffaceous sand with lenses of shale and gravel. Some of the shale is carbonaceous and east of Ashland small seams of coal have been mined. The sediments contain fossil leaf prints and poorly preserved molluscs. Near Ashland, the formation dips generally 20 to 25 degrees to the northeast and is estimated to be nearly 8,000 feet thick. Intrusive Bodies The Umpqua and all later Western c.ascade formations up to Plio~ene in age are intruded by dikes and sills of various compositions .. Al though Wells collectively gri;mps them as Pliocene (?), there are probably many episodes of volcanism represented, ranging in age from late Eocene to Pliocene. Thus, the intrusions may be vent feeders, or their offshoots, which have supplied the volcanic material for later formations but have only recently been exposed by erosion. Colestin Formation· North of Medford near the town of Eagle Point, the marine Umpqua sands grade u;iward lnto a variety of stratified cunti.nental volcanic fragmehtal material which Wells named the Colestin Formation. The 19 stratified rocks interfinger with flows, lahars, and agglomerates of predominantly andesite and rhyolite composition. The Colestin Formation has a maximum thickness of 2,000 feet, Leaf prints ,ound at the top of the formation near the California border suggests a late Eocene age. In this southern exposure area there is an angular discordance between the Umpqua and Colestin rocks. Roxy Formation Between Medford and the Siskiyou Pass the Colestin Formation is not exposed. In its place, lying unconformably upon the Umpqua, is the Roxy Formation, named by Wells as the lower unit of the Little Butte Series. Wells described the Roxy as "a great variety of volcanic rocks, including dense andesite, platey porphyritic andesite, vesicular and scoriaceous rock, glassy rock or vitrophyre, blocky flow breccias, and direct products of explosive eruptions ranging from bomby agglom- erates to fine-grained tuff." The contact between the Colestin and Roxy is defined by Wells as ''the top of the uppermost layer ·of water- laid pyroclastics that is succeeded by at least 100 feet of flows with or without interlayered nonwaterlaid pyroclastics. 11 Leaf prints found in the Roxy have been correlated with early Oligocene flora of central Oregon. Wells estilll8tes the Roxy to be nearly 2,000 feet thick. Wasson Formation The Wasson Formation, the upper unit of Wells' Little Butte Series, rests with apparent conformity on the Roxy formation. The Wasson 20 Formation consists mostly of siliceous tuffs. Wells divided the form- ation into four members. All members are present and well exposed in Wasson Canyon but south of the south fork of Little Butte Creek only one member persists. No members are mapped south of Dead Indian High- way. Wells has the Wasson terminated in the north, near the top of the Medford sheet, by a fault. Leaf prints found in Wssson Canyon were said to resemble middle Oligocene fossils of eastern Oregon. The description of the Wasson formation is confusing and, because of vague references to the different members, the text seems to con- flict with the map explanation of the rock types. The bottom member in the legend is described a~ a "buff fine-grained tuff with fragments of flow rock." The probable text description is "a bed about 300 feet thick of massive, dirty-yellow tuff consisting of some angular frag- ments in a basaltic tuff matrix." The second member in the legend is described as "pyroclastic rocks of dacitic or rhyolitic composition." In the text, this member ia mistakingly described as "about 300 feet of platey andesite flows and agglomerates." One of the possible causes of this error could be the fifteen year time period between field mapping and writing of the text, even though Wells revisited the area in 1951. Since the thick- ness of these silicic pyroclastic units is not given, it has been estimated from the map at near 300 feet. The third member of the Wasson Formation is the ''most conspicuous and extensive layer ... a chalky white tuff. •· Wells described it further as "about 250 feet thick and in most places does not show / 21 distinctive bedding but breaks off in flakes or thin slabs. IL is uniformly fine grained and of siliceous composition," The fourth and uppermost member of the Wasson Formation consists of 300 feet of lavas and agglomerates. It varies "from red flow breccia to scoriaceous and vesicular flows.,'" Wells considered it andesitic in composition, Wasson in Thesis Area In the Medford quadrangle, the Wasson ash dips gently to the northeast and disappears under the overlying lava flows. On the west side of the thesis area, faulting has brought the ash back to the surface, so that the third and fourth members of the Wasson have been repeated. As is shown on plate 1, a northwest trending normal fault crQ&ses route 140 at mile post 25. 6 ,in section. l. This structural relationship is affirmed· by a water-lain ash bed found in a roadcut at mile post 26.3. The bed dips approximately 8 degrees to the northeast and is cut by a small three foot fault which shows that the west block is down. With the aid of a cross•section drawn perpendicular to the strike, the stratigraphic displacement of this fault, using a 5 degree dip, is estimated to be at least 2000 feet, cross-section A of plate 1. In the appendix is a road log along route 140 between mile posts 19 and 35. It describes the geology seen from the highway from the base of the Wasson ash in the Medford quadrangle through the High Cascade lavas near the summit of the pass. Although nearly 300 22 stratigraphic feet of ash is exposed along the highway in,the thesis area, no distinct mappable horizons, such as Wells found, were noted. Outcrops are poor over a wide section, both laterally and vertically. The Wasson ash generally has subdued topography and a thick soil covering. What is thought to be stratigraphically the lowest Wasson outcrop- in the thesis area is exposed along State Highway 140 at M.P. 25.8. It is a massive red, brown and grey mud flow unit which consists mostly of light and dark pumice fragments with a small amount of ash and small lithic fragments as the coarse matrix. Because of its seemingly low· stratigraphic position and its lithology, it is considered a possible outcropping of the second Wasson member. The rest of the ash is mapped as the third Wasson member and most of it is regarded as non-welded ignimbrite deposits. It consists of varying proportions of lithic fragments and flattened pumice surrounded by a light-colored silicic dust, The following descriptions of selected exposures will attempt to show the variations on this generalized lithology. At M.P. 26.0 route 140, there is a high road-cut of a massive white to grey ignimbrite with abundant pumice and lithic fragments. At M.P. 26,3, the road-cut is in a buff colored ash without lithic fragments. It is massive and has an air or water-lain white ash bed cutting through it. In sections 25 and 26 of upper Willow Creek, the ash again has pumice and lithic fragments but part of the matrix contains abundant plagioclase crystals and rare quartz, The color of the ash can change 23 noticeably over a few tens of feet to many different pastel shades. Cream, grey, buff, brown and pink are the dominant colors. Four and one-half miles west of Butte Falls there is a road-cut in Wasson ash similar to the Willow Creek lithology which has a bright green ash matrix. Not all of the Wasson tuffs are loose ignimbrite deposits. Th~ top unit in the floor of the south fork canyon is a medium grey welded tuff. Because of poor outcroppings, its stratigraphic relation to loose ash higher on the north canyon wall is unknown. Wells' map shows that the fourth member of the Wasson is more restricted in area than are the underlying tuffs. Along route 140 from mile post 21.8 to 25,6, roadcuts expose over a thousand feet of red flows and agglomerates. The exposure along its strike, which is apprdximately NW, is only seven miles. North of Salt Creek and south of the south fork of Little Butte Creek this member was not mapped by Wells. There are a variety of lithologic and textural types in the fourth member. now agglomerates are more common low in the section while vesicular flows and tuffaceous agglomerates are more prevalent high in the section. Along the road that follows the south fork, the lower part of this member is represented by several thick beds of black agglomerate. Massive outcrops tens of feet high consisting of blocks of black vesicular rock several inches in diameter are held together with little or no ash. Lava flows are inconspicuous in this section. Toward the top, there is a persistent dirty brown tuffaceous agglomerate, 24 and here the inch-wide fragments of scoria are completely separated by the ash which makes up about 50 percent of the rock. In many places, especially along ridge tops where weathered surfaces may be preserved, the flows and agglomerates have been altered to a light purple and grey mass. It is possible that there was a substantial time lag between the last agglomerates and the first Heppsie flows. This lag could account for the deep weathering of the rocks. There are several possible source vents for the agglomerates along the west edge of the thesis area. A string of rounded knobs trends approximately N60Wj ust north of route 140 between mile posts 25 and 26, in sections 31 and 36. The speckled-grey to black color of these proposed vents differs from the dark reds and greens of early flows of this member, but later fourth-member deposits tend to contain more ash, are lighter in color, and commonly contain blocks of lava similar to rocks surrounding the vents, Directly west of Robinson Butte in sections 7, 12, and 18, there are several more eroded knobs which may be fourth- member vents. The rocks, which are a very light-grey pyroxene andesite, are distinct from all others in the area. Because of the great disparity in lithologic features, it is doubtful that this last member should have been included with the tuffs as part of the Wasson formation, If it is not important enough to be mapped separately, it would be more logical to include it as a basal member of the overlying Heppsie Andesite. 25 Heppsie Andesite Above the fourth Wasson member a thick series of andesite flows dip eastward out of the Medford quadrangle and under the base of the high Cascade lavas. Wells named these flows the Heppsie Andesite after Heppsie Mountain 1~cated between the forks of Little Butte Creek. He· described them as "typical 'andesites of the Western Cascades' of other writers." "In general, they are thickly bedded, massive though locally platy, gray to blue-gray porphyritic rocks with a microcrystaline groundmass in which are varying amounts of phenc:,crysts of either horn- blende or pyroxene or both." Since the flows rest on Oligocene tuffs and are capped by High Cascade lavas of a presumed Pliocene age, Wells considered them Miocene. Thickness varies greatly because of active erosion but at Bieberstedt Butte just off the map to the east, the Heppsie andesite may exceed 1,500 feet. Wells did not find an unconformity between the Wasson formation and the Heppsie andesite in the Medford Quadrangle, but notes that there is one indicated on Wilkerson's Butte Falls map, the quadrangle directly to the north of the Medford map. Peck (1964) considers the Heppsie Andesite to be equivalent to the upper part of the Sardine formation, Heppsie Andesite in the Mt, McLaughlin Area The areas mapped as Heppsie Andesite on plate l may actually contain many variations of the basic lithology described by Wells, except that olivine should be substituted for hornblende. No rocks 26 with hornblende phenocrysts were found, whereas olivine is common as phenocrysts. At the head of the canyon of the South Fork of Little Butte C~eek, there are many tens of flows of a dense blue-grey lava that directly overlie a welded ignimbrite. The flows are massive, 10-20 feet thick, fine grained, and contain dull, light green olivine phenocrysts. A few flows have pyroxene as the dark phenocryst. The flows continue up to the base of the High Cascade lavas. In the west branch of Willow Creek canyon, a light grey lava overlies Wasson tuffs. The rock is fine-grained, platy, and contains dark olivine phenocrysts, Most of Juniper Ridge consists of this same rock type. Two eroded Heppsie vents are exposed near the head of the east branch of Willow Creek. The one located near the center of section 19 is a dense medium grey andesite with black olivine phenocrysts. The associated lavas next to the vent closely resemble the flows in South Fork canyon. They are dense, medium-grey andesite with opaque yellow-green olivine phenocrysts. A similar rock outcrops on the east edge of Juniper Ridge and was sampled in a roadcut near the southern edge of section 7 on the Butte Falls road. A second Heppsie vent is located near the center of section 20. The rock contains black ghosts of olivine along with relatively fresh olivines which have rust-colored rims. Clustered phenocrysts of plagioclase and the fresh olivine are present. Chemical analyses of the two vents show that they appear to be the same. 27 Another source for the Heppsie Andesites is an eroded knob at the North Fork Little Butte Creek Forest Camp located at the northeast base of Robinson Butte. It is a multiple vent which includes two Heppsie lithologies. The first type is a medium-grey-massive rock containing yellow-green olivine phenocrysts. The second type is a light-grey-platy rock which has black olivines. 28 HIGH CASCADSEE RIES The High Cascade lavas were first designated as a separate series by Callaghan (1933). He reconnoitred the crest of the Oregon Cascades and came to the conclusion that it was "convenient to divide the Cascade Range south of Mount Hood in two parts, the Western Cascades and the High Cascades, on the basis of a pronounced unconformity in the strati- graphic sequence of lavas. 11 Thayer (1937) studied the North Santiam River section and also agreed that there was a marked unconformity between the Western and High Cascades along the entire length of the range in Oregon. He then divided the High Cascade lavas in the North Santiam area into four sub-series. nie oldest lavas he named the Outerson basalts. They were assumed to have erupted during Pliocene time along the east scarp boundary of the uplifted Western Cascades. The Outerson lavas were greatly eroded before the next ep;sode of volcanism which came later in Pliocene and early Pleistocene time. These later lavas were named the Minto basalts and they make up the greater part of the High Cascades in this region. They form the broad bases of volcanoes found near the present crest of the range. The Minto lavas were also deeply dissected before the next series, the Olallie lavas, were erupted, Olallie flows consist mainly of andesite and basalt and form the high peaks and the small, steep-sided shield volcanoes along the crest. 29 Contemporaneous with the Olallte lavas were the Santiam basalts which are the fluid intercanyon flows in many of the stream valleys. Williams (1949) in descri~ing the rocks of the Macdoel quadrangle of northern California divided the high Cascade lavas in another manner. The first lavas were quiet extrusions of basalts which formed broad, coalescing shield volcanoes, and were thought to be of Pliocene age. They rest di_rectly upon middle Tertiary tuffs. Volcanism continued sporadically into the Pleistocene. Different relative ages of the volcanoes resulted in their being in various stages of erosion when a noticeable change in rock type occured. ·A few shields were covered by late Pleistocene pyroxene andesites similar to the lavas of the high peaks; others were covered by Recent flows of basalt or andesite, while some were left unchanged except for erosion. Williams' account of Cascade History contrasts with Thayer's in that: 1) there are no lavas comparable to the Sardine in his Western Cascade stratigraphy, 2) there is no distinct double cycle of erosion as after the Outerson and .Minto basalts, and 3) there are no lavas comparable to the Santiam basalts although similar ''mesa basalts'' outcrop to the east and southeast. , Some of these differences could be due to individual interpretation but it is more likely that the histories of the two areas are substantially different in their details even though the general sequence of events and rock descriptions may seem the same. Only after many more parts of the Cascade range have been studied in detail will it be possible to distinguish events that are unique to certain areas from those that 30 are part of the overall sequence of High Cascades history. In mapping the rocks of the High Cascades, I found that rock types of different ages may be very similar when weathered, _and even different lithologies of comparable age tend to grade into one another. This makes the mapping of lithologies very difficult and for this reason there are many places where only approximate boundaries could be delineated. e. Because lavas are extruded intermittantly, the High Cascades must have many erosional surfaces, but only two unconformities shall be recognized besides Pleistocene glaciation. I shall treat the history of the High Cascades as divided into Plio-Cascades, olivine basalts, Fish Lake series, Mt. McLaughlin and associated lavas, and post-glacial lavas. The first unconfonnity is a weathered surface on the Plio- Cascades and the second is the faulting which occured after the Fish Lake series. Plio-Cascades The Plio-Cascades consist of basaltic lavas which formed broad, coalescing shield volcanoes. The first flows erupted presumably during late Pliocene time upon a faulted surface of eroded Heppsie andesite . . The present outcrop areas cover only about ten square miles in the thesis area, mainly southwest and southeast of Mt. McLaughlin. Other outcrops south of the mapped area suggest that, as in the Macdoel quadrangle, old High Cascade volcanoes underlie much of the younger lava. 31 Two Plio-Cascade volcanoes have been left uncovered on the south- west side of Mt. McLoughlin. One is located at Rye Plat in section 27 and the other is east of Dogwood Spring in section 16. At Rye Flat, magma cooled in the upper conduit to form a light brown-grey, fine- grained gabbro. Elsewhere, the rock is massive, blue-grey basaltic andesite. Although the vent areas appear to be slightly eroded, the slopes are smooth and close to their original form even though the slopes are also deeply weathered. Fresh rock is found in some of the deepest highway cuts where one can see the massive interior of flows. Elsewhere there is only intensive spheroidal weathering, figure 1• Olivine Basalts After the Plio-Cascades shield volcanoes became extinct, the Mt. McLoughlin area was quiet for some time. Erosion was not pronounced but weathering ~as deep. When this break ended, there was an out- pouring of basalts which were chemically distinct from Plio-Cascade lavas. Olivine basalts are found covering a large area north of Mt. McLaughlin, but are mostly covered by younger lavas in the mapped area. They are assumed to range in age from Pliocene to Pleistocene. Although several intercanyon flows of olivine basalt overlie a glassy flow of early Fish Lake series in the valley of Big Butte Creek west of Butte Falls, most of the basalts appear to be older than Fish Lake lavas. No olivine basalts were erupted while Mt. McLoughlin was being formed and there are no Holocene basalts. 32 Figure 3. Outcrop in roadcut at M.P. 29.9 showing intense spheroidal weathering of Plio-Cascades. Figure 4. Tuffaceous agglomerate from the early pyroclastic cone of Mt, McLoughlin, 33 Fish Lake Series While olivine basalts were still being erupted, another rock type of distinct character began pouring out. The centers were aligned north-south and are located on the east edge of the map. The oldest flow outcrops at the west end of Fish Lake and for this reason lavas of this episode are referred to as the Fish lake series. The first flows of the series were very fluid, glassy andesites. They formed flat surfaces. such as shown by the topography immediately east of Robinson Butte. and they traveled many miles down stream channels to form intercanyon flows. One flow traveled more than ten miles down the valley of Big Butte Creek from its source near the High Cascades. It can be seen outcropping along the highway 4.0 miles west of Butte Falls. The rock is distinctive in hand specimen. It is a glassy black lava and contains large clear blades of plagioclase phenocrysts whose cleaved surfaces flash reflected light. The plagioclase is commonly found clustered with phenocrysts of augite, hypersthene and olivine, The rock is massive and has a hackly fracture. Succeeding flows were less glassy and have a dark to medium grey color. They were also more viscous and formed short, thick flows, but the rocks still have the same distinctive plagioclase, massive texture, and fracture. These less giassy rocks are found southwest of Brown Mountain in sections 23 and 24, and they still have the topographic form of flows. Two small window~ of this rock type at the northwest and south base of 34 Mount McLoughlin (plate 1) are the youngest lavas of this series known to underlie that area. A possible source vent for these early lavas was found in the northwest corner of section 21 south of Robinson Butte. Most of the vents are assumed to lie further east beneath younger flows. 'nle early flows of the Fis.h, , T,('.ke series have a narrow range of silica content between 56-67%. Higher in the section south of Brown Mountain, the silica content of the lavas increases to 57 to 61%. These lavas will be referred to as the late differentiates of the Brown Mountain Series; they are light grey with white patches of plagioclase phenocrysts. Clusters of phenocrysts, which include clino-pyroxene and olivine, are still abundant. The most silicic lavas, with 59 to 611o Si02 tend to be platy, and femic phenocrysts are seldom seen in hand specimen. Present physiography indicates that most of the late differentiate lavas in the Fish Lake series came from vents aligned along the north-south trending Applegate Ridge southeast of Brown Mountain. Perhaps other sources have been hidden by the Recent lavas of Brown Mountain. After volcanic activity ceased, movement along north-south fractures cut several volcanoes in half, including Rye Spur east of Mount Mcloughlin and those aligned on Applegate Ridge. Some investigators in the High Cascades believe that the shield volcanoes were actively being built at the same time that the high strato-volcanoes were forming, Williams (1942, 1949, and 1957), Wise (1969), and Griggs (1969). This is also true in much of the region 35 surrounding the thesis area. But the volcanoes of the Fish Lake series became inactive during the first half of the Pleistocene epoch) and theiT lavas were faulted before the early phases of Mount Mcloughlin. In this respect, the Fish Lake series of andesites and the olivine basalts found north of Mt, McLaughlin hold a position in the histnry' cf the High Cascades similar to Thayer's Minto basalts. Since many of the old volcanoes in the Macdoel quadrangle were later covered by younger Pleistocene lavas) it is possible that this area also had a corresponding break in its nistory. The lack of chemical analysis in both the North Santiam and Macdoel areas makes further comparisons along this line speculative. The nearest volcanic activity outside of the thesis area that may be contemporaneous with Mount McLoughlin is Pelican Butte and the Mountain lakes volcanic center located to the northeast and the south- east, respectively. Mount Mcloughlin and Associated Lavas The first eruptions of Mount McLoughlin were no doubt violent, for glaciation has exposed pyroclastic deposits in the core of the mountain which affirm an explosive beginning. This debris formed a tall cone on what had been an undulating terrain of broad shield volcanoes of the Fish Lake series, The pyroclastic rocks consist of well consolidated tuffaceous agglomerates with some minor lapil li beds, No outcrops were found containing beds of pure ash, nor were any Strombolian bombs or lava flows found interbedded in the agglomerates. 36 The cinders are dark grey to black and commonly have phenocrysts of plagioclase and olivine. The fresh light grey ash makes up as much as half of the rock (figure~). In places the ash is light buff or rust colo~ed from hydrothermal alteration. This sulfataric activity 0 was less intense than Williams (1942) inferred from his reconnaissance studies. The relatively high elevation at which some of the pyroclastic rocks are found suggest that the cone probably attained a height of 3,000 feet above the surrounding terrain. However, cones which consist only of tephra are not known to exceed 1500 feet. They then reach a point of instability and begin slumping. To reach this great height, Mt. Mctoughlin's early cone may have been strengthened by an occasional lava flow (although none are exposed) or the cone grew on an older platform volcano .0 This :rre-;foLou~hlin volcano, if present, would have been between one and two thousand feet high so that the present depth of erosion has not exposed it. Perhaps it was similar to the form which Brown Mountain has today. When explosive activity ceased, flows from the summit area com- pletely enveloped the cone, forming what McDonald (1972) has called an armored volcano. These flows are very uniform in composition and texture. Most seem to have been erupted from a central conduit with perhaps a few coming from nearby offshoots. All of the flows are highly vesiculated but, because phenocrysts are so numerous, round vesicules are lacking. Instead, there are many small, interconnecting pore spaces. The lavas are porphyritic andesites which, like many of the large Cascade peaks, 37 contain large phenocrysts of olivine, plagioclase, clinopyroxene 1 and sparse hypersthene. The long dimensions of the largest crystals reach% centimeter and olivine attains twice that size. Plagioclase crystals are white, clinopyroxene dark green, hypersthene dark red- brown, and olivine a vitreous light yellow-green. Twenty-one flows were examined closely in the headwall of the northeast cirque,(figure .5). The lavas range from one to three meters thick. Individual flows are fairly uniform in thickness and show no pronounced-changes of dip. The centers of the flows are solid but the tops and bottoms consist of scoria, which in places accumulated to thicknesses greater than the solid centers. Because there is no inter- bedded ash, the rubble is considered to come entirely from the moving flow surfaces. The possibility remains that a portion of the rubble consists of bombs that were thrown out between flows, The predominant color of the lavas is dark grey but there is a range from black to red. The red color is from the original oxidation state of the lava. No chemical weathering of the lavas is evident but the loose rubble between the flows make them susceptible to erosion, During this period of flow accumulation, only small volumes of lava were erupted at one time. Therefore, none of the flows originating near the summit was-·able to flow far from the mountain even if they. .. reached the base. The high viscosity of the lavas is shown by the·--.fact o;--- that they were able to flow only on the steep slopes of the cone, and the change in gradient at the base of the cone was enough to halt them. The last magma cooled in the top of the conduit to form a massive ,. 38 Figure 5. Flows exposed in the headwall of the north~ast cirque. Figure 6. Stage three;, blocky lava at northwest" base of Mt. McLaughlin. 39 green rock. Erosion has stripped the surrounding rock so that it now stands as a monolith 100 feet high and SO feet wide. Aside from the less glassy groundmass, the rock is not noticeably different from the lavas in thin section_- Lavas of Associated Vents 'flte growth of Mt. Mcloughlin was contemporaneous with the fonnation of Robinson Butte, the eruption of flows covering the ridge southeast fl of the mountain, and the growth of a few cinder cones. The flows coverin~ the southeast ridge are merely a thin covering on older topography. Several source vents are shown on the map. Three are in section 18 just north of the Mcloughlin trail. They were either implaced as viscous plugs or their lavas were eroded away by glaciers. A fourth vent is located in section 17 just east of Frey Lake and still another is in section 33, three miles to the southeast. These flows are highly porphyritic and similar in texture to the main cone lavas. The Frey Lake flow has very large (% cm) euhedral phenocrysts of dark green clinopyroxene. Robinson Butte In plan view (Plate 1) Robinson Butte is elongated in a north- south direction. This was caused by a shift in the central vent\ mile to the south during a late stage of its growth. Williams (1949) noted that many volcanoes in the Macdoel quadrangle are elongated either north-south or approximately N30°w. He suggests that this is caused 40 by lavas coming up along two sets of fault fractures. In Butte Valley, to the east of the volcanoes, faults have been mapped which have similar north and northwest orientations. Because Robinson Butte has not been deeply eroded, one can only speculate on the rock type which makes up its bulk. The last flows that erupted from the north vent are medium red-grey andesites similar in texture and phenocrysts content to main cone lavas. The last eruptions from the younger vent produced a basaltic cinder cone on the crest of the butte, but erosion and vegetation have now effectively obscured it. Two other cinder cones were built on the south flank of the butte during this late phase activity. As on Mount McLaughlin, most Robinson Butte lavas did not travel far past the base. There is no evidence of intercanyon flows down the nearby canyons of the North and South Forks.of Little Butte Creek, but one flow traveled two miles down the head of Grizzly Creek. A few flows fanned out on Robinson Prairie to the southwest and one formed a mile long lobate feature at the southeast base of the butte. Cinder Cones In addition to the three cinder cones on Robinson Butte, there is one on the north side of Juniper Ridge in section 1 and another, named Pearce Point, located one mile north of Lake O' Woods. All of the cones are composed of a mixture of red basaltic lapilli and small scoria bombs. Very little ash is present. The Pearce Point cone has been excavated for road ballast exposing a vertical north-south dike 41 approximately one meter thick. The rock is dark red like the cinders, and has dark green clinopyroxene and highly oxidized olivine phenocrysts. Pearce Point is situated along a fault that bisects Rye Spur volcano. The cinder cones on Robinson Butte probably have a slmila-r fault relationship as discussed above. The cone on t~e north side of Juniper Ridge'has no apparent connection with north-south faulting but an alder northwest trending fault ,may be on the north side of Juniper Ridge, Any evidence which may bav~ connected this cinder cone with fault~ng has been buried under the younger flank lavas. More evidence of cinder cones being associated with faulting is found outside the thesis area six miles east of Pearce Point. bn · the north flank of the Wilderness Lakes volcanic center, a north-south stri~g of seven cinder cones is aligned with the· west fault of the Klamath Graben. The lavas of Mt. McLoughlin would belong to Thayer's Olallie group since, by definition, high pea,k lavas form a part of thi~ group. Flows comparable.to the contemporaneous Santiam basalts are not represented in the thesis area and they are not found in the Macdoel quadrangle. However, light grey intercanyon fiows similar to the ~ntiam basalts fill the South Fork canyori of Big Butt~ Creek northwest of Mt. McLoughlin. These flows have the diktytaxitic texture that is so common to this rock type. There is no evidence that they origin~ted in the thesis area and I believe that thefr sources were somewhere to the nor.th ot Mt. McLoughlin. 42 Flank Eruptions On the northwest side of Mt. McLoughlin I have been able to map three different ages of flank lavas which I have labeled stages one, two, and three. The first two stages are interpreted as being con- temporaneous with the growth. of the main cone of Mt. McLaughlin, wit-h stage three lavas being post-glacial. Stage One Lavas The oldest flank eruptions are olivine andesites which are located on the northwest flank of Mt. McLoughlin. The lavas are black, fine- grained and v~sicular. The olivines are approximately l mm in size. The flows extend for 8 miles down the valley of Fourbit Creek,. The flow limits are outlined by Willow and Fourblt Creeks. Big Butte • Springs are located near the end of these flows which act as a large reservoir for the springs. The city of Medford has used thEise large springs as a source of water since the late l920's. The flat surface of the flows suggests that the lavas were very fluid when they were erupted. The.soil and colluvium on top of the flows is now more than a foot deep. In the valley of Fourbit Creek, stage one lavas in places form a thin coveri!lg on a relatively thick deposit of alluvium which appears to be glacial outwash. The sediments, instead, am interpreted as stream deposits which were washed dow-n during the early explosive phase ' of Mt. McLoughlin. The large quantity of material choked the stream valley and caused ancestral Fourbit Creek to deposit a thick layer of cinders and coarse sand in its channel. A gravel pit in these deposits is located on the west edge of Parker meadows (SW\, sec. 26) just off the map. The size of the sediments ranges from coarse sand to pebbles several inches in diameter. Most of the rocks are dark, scoriaceous main- cone andesite pebbles. Stage Two Lavas After a break of indeterminant length, lavas again erupted on Mt. McLoughlin's flanks. These flows differ from those of the pre- ceeding stage in having a distinct rubbly character, but they are otherwise similar in composition. The rubble may make up one half or more of the flow. In the center of the flow is a solid mass of medium-grey, fine-grained lava containing olivine phenocrysts. In some flows the core may be reddish like the rubble and it may tend to be diktytaxitic in texture. One of these flows crosses route 140 at mile post 31.2 in the west half of section 35. It is thin (c. 20 meters) in the roadcut but fans out on flatter terrain. It forms the bulge in the north shoreline of Fish Lake known as Dne Point. Similar flows can be seen in roadcuts between the Doe Point flow and mile post 32.7. What appears to be the longest flow in the area is a second stage lava on the west flank of Mt. McLaughlin. Although its source is hidden under a more recent flow, it emerges in section 21 and extends for at least seven miles down the east branch of Willow Creek. I shall refer to it later as the Bear Pan flow since it is near Bear 44 Pan Spring, section 19, where the flow rubble was quarried for road surface material. A large area of second stage flows is located near the northwest edge of the map. The flows emerge from under glacial outwash in section 27 and extend nearly four miles west to where Whisky Springs come forth from beneath them. The south fork of Fourbit Creek marks their northern boundary. Most of the stage two flows are covered with vegetation which includes full grown trees, suggesting that soil formed quickly on the rubbly surfaces. Some vegetated areas grade into small patches of open lava and, since some flows are blocky like stage three lavas, a distinct age difference between the lavas could not everywhere be recognized. This is particularly true in section 36 along route 140 east of Fish Lake. The sources for flank lavas seem to have been fissures but few vents have been mapped. Some have been hidden by the lavas mounding over their vents while others have no doubt been covered by younger lavas and glacial deposits. Some vents may have been active during more than one stage of volcanism. Post-glacial (Stage Three) Lavas There are three different textural types of olivine andesite and one silicic andesite that have erupted during the last episode of volcanism. They are very fresh but none is historic. Judging from their appearance, they may be only a few thousand years old, as are the Recent flows in the McKenzie Pass area which have been dated radio- metrically. The first type of stage three lavas are coarse-grained olivine- pyroxene aodesites. They were erupted from North Squaw.Tip and South Squaw Tip, located above the 7000 foot level on the west side of the mountain, and near Rye Spring on the southwest flank. Chemically and texturally these andesites are main-cone lavas but they are block lavas ins~ead of the rubbly flows which came from the central vent. The second type of stage three lavas is located at the bottom of the northeast cirque in section 12. The rock is a massive, medium- grained olivine-pyroxene andesite. It is light-grey or red-grey in color, and has plagioclase, dark green clinopyroxene, and vitreous honey-colored olivine as phenocrysts. The mafic phenocrysts are very large and are commonly over one half centimeter across. Chemically, these flows are also like the main-cone lavas but their flow texture is quite different. They consist of large angular blocks which form thick, narrow flows, or what is sometimes called coulees. The third type of lava is a fine-grained olivine andesite which was erupted on the flanks of Mt. McLoughlin after the Squaw Tip lavas. The flows look much like the stage one and two flank eruptions. They are fine-grained, dark, and contain small olivine phenocrysts. Unlike the fluid stage one and rubbly stage two lavas, the stage three lavas form blocky flows as do all post-glacial lavas. On the west side of the mountain stage three flank lavas are vesicular and tend to form long, blocky, steep-sided flows, (figure ~. On the south flank the lavas are not vesicular and form short mounded flows. Snme of the mounds cover less than an acre in area and are not individually mapped. Blocks making up stage three flows may be several feet across, producing a coarse textured flow surface, Still, the tops of some flows show moving-lava features such as gutters, pressure ridges, and.collapsed tubes. Brown Mountain Lava The fourth type of stage three lavas is a silicic andesite which covers Brown Mountain. These lavas look very much like the stage three blocky lavas on the south side of Mt. McLaughlin. Since these two types are found side by side in the field, it is fortunate that there is a way to distinguish between them. The Brown Mountain andesites very rarely contain olivine as phenocrysts, ~nd are among the few lavas in .the area which have no phenocrysts. The crest of Brown Mountain is crnwned by a cinder cone that still retains its crater. The crater is oval shaped and approximately 100 meters long. The cone consists of fine light•grey andesitic cinders and ash, some of which has washed back into the crater so that it is now less than 10 meters deep. The youngest flows on the mountain postdate the cone and were erupted from a vent on the northwest side of it. Three Stage Growth The sequence of the types of eruptions suggest that there were three fairly distinct periods of. growth for Mt. McLoughlin. The textures seen in the lavas are in agreement with what one might expect from a volatile-ri~h magma that is progressively degassing with time. The first eruptions (stage one), being rich in volatiles, are very ·explosive and produce large amounts of ash. If there are lava flows, ~hey would be expected to be fluid and vesicular. As eruptions continue (stage two), the magma slowly loses the volatile components and less violent eruptions of less fluid lavas follow. In time, the magma loses most of its volatiles and the lavas are viscous, forming blocky flows (stage three). Cinders and ash are still evident during the last two stages but they are produced in rather minor amounts, 48 PETROGRAPHY Wasson Fourth Member Two samples of the fourth member of the Wasson were sectioned for study. Sample LS-41 is an olivine andesite of an eroded vent just north of route 140 near mile post 26, southwest corner of section 31, This is considered a possible ~ource for the fourth member flow breccias. Phenocrysts which were once olivine have been completely altered to ore minerals, red-brown iddingsite, and green-brown serpentine. The phenocrysts were not large, less than 1 millimeter, and there is no olivine in the groundmass. Ortho- and clinopyroxene occur in the rock, both as phenocrysts and in the groundmass. The orthopyroxene phenocrysts are subhedral and more numerous than clinopyroxene. Orthopyroxene, however, is subordinate in the groundmass. Plagioclase is the most common phenocryst phase. Both phenocryst and groundmass plagioclase is strongly·, zoned. The composition of the phenocrysts ranged from calcic andesine to sodic labradorite. The groundmass, which makes up about one half of the rock, contains mostly plagioclase atd pyr0xene with small patches of dark grey glass. A few apatite needles are present. Sample LS-8 is another plug immediately west of Robinson Butte. The rock is a leucocratic pyroxene diorite in hand specimen. In thin section, one can see tha,t some of the interstitial material, which may have contained glass, has been altered and partially replaced by optically continuous calcite. Large black phenocrysts of pleochroic bronzite, En 81, attain lengths of over 5 millimeters. Some of the smaller orthopyroxene laths, .\ mm in size, are jacketed with clinopyroxene (figure L). The large bron~ite phenocrysts are slightly rounded but show no reaction on their rims. Olivine is not present but may have been converted to bronzite, l;>ecause these phenocrysts have many ore granules as inclusions, ~linopyroxene phenocrysts are present but most of them are in clusters with large plagioclase phenocrysts. Heppsie Andesite Two types of Heppsie Andesite can be recognized in hand specimen. They have been described in an earlier section as a light grey rock with dark phenocrysts and a medium grey rock with light-colored phenocrysts. Sam_ples LS-2 and LS-58, respectively, are representative of these two types. Sample LS-2 is an olivine andesite that has olivine phenocrysts (4 percent) set in a felted groundmass of plagioclase laths (72 percent), granular clinopyroxene (17 percent), and ore (4 percent). A small amount of brown glass .makes up about l percent of the volume. The remaining 2 percent of the rock consists of ''ghosts" of former phenocrysts. Most of these ghosts are now composed of ore 50 Figure ?. ;. Groundma.ss orthop,roxene jacketed with cliriopyroxene. Scale .on all microphotographs .is •. . app:r:~ximately 4 cm = 1 mm, ~less otherwise noted. Ffgure 8. "Ghosts" of olivine phenocrysts consisti:gg of ore gra~mles and rimmed with clinopyroxene granules, 51 granules whose! aggregate shape shows good crystal outline, The outline is rimmed with' clinopyroxene granules while the core of the ghosts is filled with clinopyroxene, orthopyroxene, and plagioclase (figure ~.). The former phe·nocrysts were probably olivines which were unstable and reacted wi°th part of the magma which contained fresher olivine phenocrysts. The rims and fractures of fresh olivine phenocrysts are stained a dark red ..b rown. This gives the olivine its dark appearance in hand specimen., The central portions are fairly well preserved. They commonly have good double pyramid or rectangular subhedral shapes and attain lengths up to 3 millimeters. Laths of calcic andesine are% millimeter in length. The amount of glass is so small that the rock has an intergranular texture. In other :areas sampled, where flow movement produced platy jointing, the rocks have good pilotaxitic texture. This texture is well developed in section 25, west of Willow Creek campground, figure ,2_. Sample LS. .5 8 is of the second type of Heppsie Andesite. Its finer grained size accounts .for its darker color. The rocks ·are from a vent in which the olivine phenocrysts were extensively oxidized to ore. The rectangular crys.tal outline is not rimmed with pyroxene and the cores still show recognizable olivine. This alteration effect is apparently-a phenomenon confined to the vent; lavas from the same vent have the fresher, light green olivine which is characteristic of this second type of Heppsie Andesite. 52 F1gure99. P1lotax1t1c texture found in some of the Heppsie 5ndes1te flows west of Willow Prairie Forest Camp. Figure 10. Apatite needles radiating from glassy areas in a Plio-Cascades lava~ Approximate scale is 1 cm= 0.1 I1lllla 53 Plio-Cascades Samples LS-4, a plug rock, and LS-24, its associated lava, are examples of Plio-Cascades olivine andesites. The lava rack contains brown, stained olivine phenoc!"Jsts (4 percent), plagioclase (58 percent), clinopyroxene granules (14 percent), ore (.5 percent), dusty brown glass (17 percent), and aoundant small needles of apatite (2 percent) radiating out ·rrom the gl~ssy areas (figure 10). The texture is intersertal although in pJ.aces there is enough glass to completely surround the plagioclase laths in hyalo-ophitic texture. Olivine ranges in size from 0.1 to 1,0 mma nd clinopyroxene less than 0.1. Plagioclase laths are 0,5 mm long and have a composition near An 60. The plug sample is mostly intergranular with rare small patches of glass. The grains are equigranular and measure about 0,5 mma crosso Some of the larger grains of pyroxene are subophitic around olivine. and plagioclase, The olivine is very fresh and has clinopyroxene over-growths but a small amount. of orthopyroxene is also presente One sample located near LS-4 was sectioned which was intermediate in texture between the two samples descr.1.bed, It has abundant orthopyroxene laths, some of which are coated with clinopyroxene. The plug rock has fewer apatite needles than the lava but they are twice as large (0,1 mml ong). Olivine Basalts Sample LS-56 was taken from glacial material at the north base of Mt. McLaughlin but derived from the area to the north. The sample contains olivine phenocrysts (8 percent) which form subhedral pr.isms 1 to 5 mml ong. Some of the olivine is granuJar, less than 1 mm, but there ls none in the finer, granular grcu·n·-dmass. Plagi.oclase phenocrysts -· comprise 30 percent of the rock and have an estimated composition of An 60-70. The groundmass contains plagioclase (25 percent), granular clinopyroxene (JO percent), ore (5 percent), glass (2 percent), and a few needles of apatite (less than 1 percent). Fish Lake Series The Fish Lake Series on the south side of Brown Mountain is a group of andesite flows in which an olivine andesite grades into a silicic andesite. The striking characteristic of the early flows of this series is the large amount of black glass. In sample I, the glassy groundmass is 70 percent of the bulk in which large (:> 1/2 mm) phenocrysts of plagioclase, olivine and pyroxene are imbedded (figure ll). The glass has a patchy appearance that appears to result from mixing of different portions of the lava which had different stages of microlite development. The microlites, when large enough to recognize, are plagioclase, pyroxene and ore grains. The phenocrysts consist of sodic labradorite (20 percent), olivine (4 percent), clinopyroxene (4 percent), and orthopyroxene (2 percent). Most phenocrysts are 1 to 3 millimeters in length except for olivine which is less than 1/4 millimeter. 'The olivine is stained red-brown or riuaned with small grains of ore, The plagioclase and pyroxene phenocrysts are rounded in their outline, zoned, and have .55 Figure 11. Typical texture of early Fish Lake lavas showing glassy ground.mass and large phenocryst phases. Figure 12. Plagioclase-orthoclase glomerophenocryst commonly found in Fish Lake lavas south of Brown Mountain. cavities filled with glass. They also contain inclusions of olivine grains. Another noteworthy feature in this rock, and for the series as a whole, is the common occurance of cumulate phenocryst clots. There are two types o~ clots. The first is simply a cluster of similar large phenocrysts scattered throughout the rock. The second consists only of plagioclase (70 percent) and orthopyroxene (15 percent). The interstices are filled with glass similar to that of the host rock (figure ]1). The plagioclase laths are 1 millimeter in length, or half that of the regular phenocrysts. The composition of the plagio- clases is approximately the same. The orthopyroxene grains are much smaller than those in the host rock, They are rounded crystals which may approach 0,25 millimeter in size but are commonly only 0.1 mm. Younger rocks in this series generally become lighter in color as microlites in the glass grow larger, but this may vary even in a single flow. Sample III is similar to I in having four types of phenocrysts but in a less glassy matrix. The plagioclase again shows strong zoning and resorption. Many of the larger phenocrysts are so spotted with resorption scars and pyroxene inclusions that the twin lines can scarcely be seen. A crystal of this type was found to have a composition in the bytownite range. Other plagioclase grains have less pronounced resorption or none at all. Most of these crystals are smaller and fall in the labradorite composition range. The olivine content of sample III is approximately the same as in sample I. The grains are commonly stained and show a reaction 57 relationship with their surroundings. Those found in the glass are rimmed with augite and ore granules. Those found in phenocryst clusters have overgrowths of orthopyroxene. Orthopyroxene was not present as intermediate sized phenocrysts in sample I. In sample III, it appears in small but erratic amounts. It is still the least common mafic phase. Ore grains up to 0.25 mm are now present as small phenocrysts also. Phenocryst clusters are still abundant and vary in their mineral proportions. All contain plagioclase which usually makes up about half of the volume, The predominant or only mafic phenocryst may be olivine or one of the pyroxenes. A rare third type of cluster was found in sample III. It consists almost entirely of olivine grains (0.1 to .25 mm in size) with a small amount of poikilitic plagioclase. Sample Vis from a later flow which is completely crystaline. There are still four phenocryst phases: plagioclase (45 percent), ortho- pyroxene (7 percent), clinopyroxene (3 percent), and olivine (a~ly converted cumu).ate 0livine·,material. i. . ,. . r • Figure 20 •. 'Euhe(iral•, t.'wµmeci,a nd. zoned c~i~~pyr9xt,he . .w ith olivine i:ricl'Us1ons~ •'! • • I• I The birefringence ranges from a dark grey to yellow-orange and only laths which show the lowest birefringent colors are seen to be coated. This indicates that the clinopyroxene prefers t0 grow on the (100) faces of the orthopyroxene crystals. As in. the olivine crystals. optic angles of the orthopyroxene megacrysts are very large and their optic signs are negative. Clinopyroxene Megacrysts of clinopyroxene occur in all of the main cone lavas studied. Modal concentrations in many sectioned samples appear to be less than 1 percent. In sample LS-6, one of the last flows to come from the summit area, the mode is near 8 percent. The crystals are characterized by numerous inclusions of olivine and plagioclase. Some inclusions are very large compared to the size of the pyroxene grains, (figure 16). There are also resorption fillings of glass and plagioclase. Orthopyroxene is sometimes seen as small grains on the rims, hut most of the crystal outlines show no reaction with the liquid. As noted with orthopyroxene, some crystals of clinopyroxene in sample LS-7 occur as ophitic overgrowths in remnant olivine cumulate material. Faint zoning can be seen in some crystals which have been cut close to basal sections. It is not uncommon for the phenocrysts to be euhedral and t~inned, (figure 20). Phenocryst Composition Many of the rocks studied contain large mafic phenocrysts and it was fairly easy to pick the grains from the crushed rock. Once the grains were separated and finely crushed, a small quantity was sprinkled on a gla~s slide with a drop of calibrated index oil. When a favorably oriented grain is found, the refractive index of a principle crystal ray can be determined by matching it with that of the index oil that surrounds it. The index of the beta ray was used in olivine and clinopyroxene and the gamma ray in orthopyroxene. The beta ray in olivine and the gamma -r;ay in orthopyroxene vary directly with the Mg to Fe ratio in the mineral lattices so that once the ray indices are determined, the mineral compositions can be read fr.om the graphs drawn by Poldervaart (1950). To estimate the compositions in the clinopyroxenes, both an index and the 2V of the mineral is needed, because the composition varies with the three major components of calcium, iron, and magnesium. Clinopyroxene data was plotted on the graph drawn by Hess (1949). Accuracy of'index determinations for olivine and orthopyroxene was approximately j: .002 which corresponds to error in the molecular content of-± 1 percent. Because of the high index 9f clinopyroxene .beta rays, the index oils had to be mixed to give approximate ·intermediate values. Thus, the accuracy for this mineral was on ± ..0 05 which gives a large molecular error of!. 5 percent. The 2V measurement varied several degrees from grain to grain in one thin section, ranging from 52 to 56 degrees. The combined errors and variation produced a very approximate chemical composition for the clinopyroxenes using petrographic techniQUes. Assuming there is a continuous range of 2Vs from 52 to 56 degrees, the clinopyroxene compositions fall in the calcic augite field of Polder- vaart and Hess (1954). In tables 9 and 10, page,113, are listed chemical analyses of phenocrysts from rocks of Mt. McLaughlin and Crater Lake. The phenocrysts were picked from crushed rock, mounted in epoxy, and anal- ysed with the electron probe. All analyses were provided to me by William P. Leeman of the Center for Volcanology, University of Oregon. Data for the pyroxenes are plotted on a quadrilateral diagram in figure 21. The clinopyroxenes from both centers form a tight grouping in the calcic augite field. The orthopyroxenes have a ra.nge in compositjons from bronzlte (En 80) to hypersthene (En 65). Tie lines connect analysed pyroxene pairs from the same sample. t CaSi03 Oiopside : : Solite •:. . • • • • • • ·2116d' ~ .. _:~L •L ~~.: ·.:. .: • ·- . :-;- .. ~ . . • • : Ls,1--J} t : : • • • ' · .SS0 .•• :, . ,,r,•, .. .I, . . I • .: . . • • • •r. ·...... . . / ff ,I II: . . . • . .:. . · 1· •• • .• • ... . . : . I-··.. ··...... . . • I. . ·soo I i: • ·:· ••.... • • . • • : I I o- . • • -450 • • ca r r~ o /"2/: ~ /I. 1 I ' Augite I I I 11 I I /J r I ,, : I o Mt. Mcloughlin pyro11enes f- ,,_ 1-- ....I. II I I fl I I 7 Subcalcic *Crater Lake pyroxenes I II : l I Augite I I I I I I : : I I 1 L I I I I ~, ,_ _J_ _J I • I I I I I I I I I l Pigeonite / ' I rI f I I ~ I T ; ,- -, 7 HP-2 * 0 1 AC·I Bronzite LS·6 Hy~enthene 50 Figure 21. Pyrox~n,? quadrala+.eral 1-1lth c-:imposi tions of pycoxcnes from Mt, McLaughlin and C.rater Lake andesttes, All po:i.nts a:ce microp-robe ci,nalyses by William P, Le-?.man, (197.3. unpublished data), Analyse::, are listed in ta.bl.es 9.t and :)b. Nomenclature by Poldervaart and Hess (1951), Refractive ind.ices and ?.V opti.cal propcrti~s fl·om !foss (19L1,9) and t1uir (19.51). --.J 0 71 Plagioclase. The composition of plagioclase feldspars was approximated by using the combined albite-Carlsbad twin technique. In this method, angles between the twins are measured on a favorably oriented grain by rotating the _st~ge of the microscope, and then plotting these angles on a chart to estimate the anorthite content. The chart used was a ~igh temperature one drawn by Tobi (1963). When possible, three separate measurements were taken on a favorable grain. These conform to three. natural divisions in the crystal of core, outer zone, and rim. In main cone lavas, the cores are assumed to have been crystalized at a great depth and range in composition between An 50-60. The largest crystals generally show resorption features and their cores are sometimes more calcic than An 70. The outer.zone is assumed to have crystalized at a shallower depth than the core and ranges in composition from An 40-50. The outer rim is believed to have crystalized during eruption and is connnonly An 30-40.in composition. The large plagioclase phenocrysts in nearly all of the lavas of the area are characterized by extreme normal, oscillatory, and patchy zoning. These types of zoning are common in orogenic andesites as a whole. The core has normal and patchy zoning, the outer zone normal and oscillatory zoning, and the rim normal zoning. Normal zoning is commonly believed to be the result of simple crystal fractionation as proposed by Bowen (1934). In this process 72 both the liquid and the crystals become progressively more sodic as crystalization proceeds. The commonly held interpretation for oscillatory zoning is that it results from periodic slight increases in volatile pressure. After a pressure increases, crystalization of plagioclase is more calcic and this reversed zone is then followed by normal zoning as the effect of the pressure increase is reduced. Several episodes such as this can produce a series of reversals or oscillatory zoning. Patchy zoning is a term proposed by Vance (1966) to describe the mottled cores of feldspar crystals. The patchy appearance is caused by resorbtion blebs being filled with more sodic plagioclase in crystalographic continuity. Some rejsorbed areas contain clear glass and some have very small poikiliti~ inclusions. The inclusions are interpreted as the crystals formed from the other components of the trapped liquid in the cavity. This is evidence that the resorbtion features were caused by a cavity forming process and not merely by ion exchange within a stable crystal lattice framework. For the cause of patchy zoning, Vance proposed that resorbtion of the calcic centers of crystals is caused by a decrease in pressure resulting from upward movement of magma. While some resorbtion is taking place in the cores as pressure diminishes, the magma is also cooling. The temperature drops until it intersects the liquidus at a point where a more sodic phase is deposited in the central cavities and at the rim. In his argument, Vance used a set of hypothetical liquidus-solidus 73 curves for the albite-anorthite system which were extrapolated from the limited knowledge of the system at that time. I have visually compared Vance's curves with those that have since been determined by Yoder (1969). If Yoder's curves are a closer approximation of how plagioclase crystalizes in a magma chamber, then those drawn by Vance are much too flat in the solidus and should be suspect. Although Vance's explanation for patchy zoning is still possible, I prefer to invoke a volatile pressure buildup-and-release mechanism as a more plausible means of producing these features. Because of the explosive character of orogenic andesites, many workers have assumed that these lavas maintain a high but variable volatile pressure and that most of this is due to water in the system. Volatile pressure-buildup before an eruption and the amount of pressure release during an eruption should have a marked effect on the crystal- ization history of plagioclases in the magma chamber. Using P(pressure) = 1 bar as a reference pressure, Yoder (1969) has shown that a moderate increase in the water pressure, Pff20: 150 bars, causes a larger shift in the plagioclase system than a large increase in dry pressure, P = 10kb, and in the opposite direction (figure 22). My argument is as follows: A small but sudden isothermal increase in volatile pressure on a magma would lower the liquidus-solidus curves. The equilibrium liquid could then be more calcic than the crystals and resorption could occur. If this pressure is later rapidly released, the curves would then be raised, presumably high enough for a more sodic phase to crystalize in the resorption holes and on the rim. 1800 74 1700 L1qu1d ( + Ga~} 1600 'I I 1~00 .t.'). w 1400 a: :::i ~a: w n. ::l,: w I- Plag,atlase ( + Gas) 90~1:-i,,1:-,--::10:----:zl:o---}:30=----4.Lo ___ :;oL__ _ 6.l_0_ _ 71._o_ _JOOL--9 10--Anct1_J "''• WEIGHTP ER CENT Figure 22 • The al bl te-anorthi te syster,1 at 1 bar and 10 kb and the hydrous system at 150 bars. Adapted from Yoder (1969). 1soo,-~--.---r---.-----,---.--------,------.---..--- 1700 1600 1~00 u 0 ----1----- ------?- 1000 -----------------~~----~ S00'---,!;10---:-~20:;--,3';;:o----:4::l.:o:----;l~o::----.:6:l:::0--,!-.70,-·----,eLo----,90.1.._ _ __J Oiop~ide - Anot1hite Figure 2J. The diopside-anorthi te system at 1 bar and 20 kb dry and at 5 and 10 kb wet. Adapted from Yoder (1969). ?5 Associated Vents Several volcanic vents are located on the ridge that trends south- east, away from Mt. McLoughlin. The lavas are coarse-textured, porph- yritic andesites and basic CW!lulate andesites which are believed to be contemporaneous with the second stage growth of the main cone. Sam:pleLS-28, a Frey Lake lavat is a pyroxene andesite which contains megaphenocrysts of clinopyroxene over 0.5 cm across. In thin section these are seen to be tight clusters of clinopyroxene with individual crystals ranging from 0,5 to 3 mm in size. They constitute nearly 4 percent of the bulk of the rock, Twinning of the grains and inclusions of olivine and plagioclase are common features. Some olivine inclusions are embayed and several may go extinct at once under crossed nichols. This indicates that original cumulate olivine has now been changed to pyroxene, (figure 24). Many crystals not in clusters are euhedral. Olivine is not conspicuous in hand specimen since the granules are generally less than 0.5 mm.. This mineral makes up nearly three percent of the mode. Small crystal clusters consisting of granular ollvine and plagioclarein nearly equal proportions are fairly common. Orthopyroxene is rare. Labradorite phenocrysts make up 40 percent of the rock. They are zoned from approximately An 60 in the cores to near An 40 on the rims. Over 53 ·percent of the rock is groundmass. It is composed of plagioclase (29 percent1 pyroxene (18 percent), and ore (5 percent) 76 Figure 24. Cumulate olivine as inclusions in clino- pyroxene. Figure 25. Texture of BrownH ountain lava showing rare olivine pljenooryst. 77 set in a microcrystaline matrix {1 percent). Both pyroxenes are present in nearly equal amounts. Sample LS-82 was taken in section 33, to the southeast of the Frey Lake lava previously described. Although similar in hand specimen, it is more basic than LS-28 and is an olivine-pyroxene bearing basaltic andes1te. The most abundant mafic phenocryst is olivine {7 percent). The subhedral grains are commonly 0.25 to·o.5 mm in sise with a thick, red- brown alteration zone at their rime. The mineral appears much fresher when found in granular form, eithor as separate grains, as inclusions, or in clusters with plagioclase and pyroxene. Olivine crystals contain many small ore grains, some appear to be chrome spinal. Clinopyroxene (2 :Percent) and plagi:oclase {.34 pe;rnent) also occur as phenocrysts, The plagioclase laths are co11m0nly1 11111i n length and are zoned from calc1c labradorite (An 67) to andesine. Ortho- pyroxene 1s present in minor amounts and is found as intermediate sized grains, some of which are coated with clinopyroxene. The groundmass constitutes 57 percent of the rock. Granular pyroxene accounts for 42 percent of the groundmass material or 24 percent of the total bulk. uround.mass plagioclase represents 27 percent and ore 5 percent, with m1crocrystaline material making up less than one percent of the bulk. Sample BL-9, from the top of Robinson Butte, is associated with the late stage cinder/cone on top. The rock has a large proportion of mafic phenocrysts (11 percent), which may be due to cumulate 78 enrichment and give the rock a basaltic composition. This lava may not. therefore, be representative of Robinson Butte as a whole. Only one other Robinson Butte sample was sectioned and although it was taken f'rom the north flank of the butte and is from the older north vent, it also seemed to be enriched in mafic phenocrysts. Olivine makes up 5 percent of sample BL-9 and clinopyroxene 6 percent. Olivine 1s strongly stained along fractures and rims but is otherwise little altered. Grain sime ranges from less than 0.25 mmt o more than 2 mm. Both large and small grains are found in phenocryst clusters and in the ground.mass. The largest grains contain chrome spinel inclusions, commonly grouped near the centers of crystals. When in contact with the groundmass, olivine is rimmed with granular clinopyroxene. Clinopyroxene occurs as euhedral to subhedral phenocrysts over J mmi n length and as granular. material in the groundmass. Most large crystals are found in clusters with plagioclase and granular olivine. The latter two minerals commonly occur as inclusions in the clinopyroxene. Often, pyroxene crystals are twinned and some show faint, broad oscillatory zoning in basal sections, similar to that shown in figure 20. Several large phenocrysts contain olivine inclusions which go to extinction together, again suggesting that a former olivine crystal has largely been converted to pyroxene. No orthopyroxene was found. 79 Brown Mountain Lava The blocky flows ,which cover most of Brown Mountain are am"bung the youngest flows in the Cascades. Sample BL-4 is one of the latest of these lavas. It erupted from the northwest base of the summit cinder cone. The rock is dark grey, fine-grained, and looks very much like a basalt but is in fact a silicic artdesite at 59 percent silica. The dark color is a result of a dark, glassy base which makes up 67 percent of the rock. (figure 25). The base, in thin section, is a light brown glass which contains small crystalites of pyroxene and plagioclase. Crystal~ of orthopyroxene (6 percent) and labradorite (2? percentr An 55) are considered groundmass since they form laths less than 0.5 mmi n length. The lack of mafic phenocrysts attests to the s111c1c nature of the rock. An occasional phenocryst of plagioclase (1 mmi n size) which show resorption and. a few rare olivines are found but the rock is essentially non-porphyritic. All of the Brown Mountain lavas studied were of this same char- acter. glassy and non-porphyritic. In places, especially on the east side, schl1eren are very noticeable on weathered surfaces of blocks. Fresh surfaces do not show these lighter bands to such a degree. As seen under the microscope, this feature could be explained by the mixing of two separate portions of the magmaw hich had different degrees of crystalization in the groundmass or by segregation of a filter-press liquid. 80 NOMENCLATURE The chemical compositions of the lavas sampled range from basalt to silicic andesite. Silica values range from 52 to 61 percent. Rocks were named on the basis of a combination of factors which would prove both reasonable and workable. These factors ares 1) natural discontinuities in the compositional sequence of lavas, 2) petrographic changes, 3) cation nonn values, and 4) a desire to conform as closely as possible to rock terms now in use in the literature, Because of the limited number of chemical analyses, many workers in the past have distinguished andesites from basalts by the disap- pearance of olivine in the more silicic rocks. On this basis, some basalts may have silica values above 58 percent. More commonly, however, the tenn andesite refers to orogenic lavas which have silica values ranging between 53 and 63 percent, Basalts Rocks in this study are considered basalts if the normative value of the anorthite component of total plagioclase exceeds 50 percent. This break is near the silica value of 53 percent and fortuitously falls on a natural break in the compositional sequence of lavas studied. Other means used to characterize basalts are a normative quartz value of less than 2.5 and a color index greater than JO. 81 Andesites For convenience, and to conform to recent usage, andesites are commonly divided into two groups called basaltic andesites and silicic andesites. Basaltic andesites are those lavas which have a silica content between .53 and'58 percent and which may or not contain olivine phenocrysts. The normative anorthite content of these lavas is less than 50 percent, normative quartz is between 2 • .5 and 10 percent, and the color index is·between 20 and JO. Silicic andesites are lavas which normally contain no olivine. They are characterized by a silica value above 58 percent, normative quartz above 10 percent, and a color index of less than 20. There is no natural discontinuity between basaltic andesites and silicic andesites, nor between silicic andesites and dacites. 'These divisions are fundamentally the same as those used by Greene (1968) for the Mt. Jefferson area and by Wise (1969) for the Mt. Hood area, Other commonly used terms are "low-silica andesite" and "olivine andesite" which are essentially basaltic andesite. The terms "high-silica andesite" and "pyroxene andesite" are comparable to sillcic andesite. 82 VOLUMREE LATIONSHIPS It is lmportant to know the abundance of the different lava types, but since limits on the thicknesses and the area covered by older flows could vary greatly, only crude estimates are possible. Consequently, volume estimates are presented only for the major rock types which post-date the Heppsie Andesite. As a starting assumption, the upper surface of the Heppsie Andesite is believed to lie at an average elevation of JOOO feet when extended beneath the High Cascades. Since on the western edge of the thesis area these older lavas dip between 3 and 10 degrees to the northeast, some mechanism is assumed, such as repeated faulting or lessning_of dip, to keep the formation near this elevation. The area used in the calculations is a 12 by 7.5 mile north-south strip which lies direptly west of the Lake 0' Woods fault and runs the length of the thesis area. Plio-Cascades and Olivine Basalts Pliocene and Pleistocene basalts and basaltic andesites which erupted before the Fish Lake series have an estimated average upper surface elevation of 5000 feet. The calculated volume of these lavas (with average dimensions of 12 miles long, ?.5 miles wide, and 2000 feet thick) is 35 cubic miles. The high-pha:;p,orus basaltic andesites comprise one-third to one-half of this volume with the younger olivine basalts making up the remainder. Fish Lake Series The volume of the Fish Lake series in the strip area is estimated to be nearly 6 cubic miles. This is the sum of a calculated northern 8J and southern portion. That portion of the series north of route 140 is about 2 cubic miles, The average dimensions are 7 miles long, J wide, and 500 feet thick, These numbers take into account the high basement (6000 feet) of olivine basalt at the north end and the thinning of the Fish Lake lavas near route 140. The volume of the southern portion of this series is estimated to be 4 cubic miles, The top of the older olivine basalts is believed to stay near 5000 feet elevation from route 140 to the south edge of the· map, Near the south edge, the Fish Lake series appears to thin once again. The average dimensions used for calculating this volume are 4 miles long, 5 miles wide, and 1000 feet thick, The silicic andesite differentiates were found to occur only southeast of Brown Mountain and probably add lP.r;s than 0.5 cubic mile to the volume of the series, Late Pleistocene and Recent Lavas The volume of Mt, McLaughlin, before glaciation, was computed from the equation for the volume of a simple cone, V=~R2H/J, where R is the radius and H the height of the cone. With an estimated elev- ation of 3500 feet above the surrounding terrain and a radius of 1.5 miles, the volume is close to 1.8 cubic miles. Even when the flows around the base are added, the volume is still less than 2 cubic miles, The size of Mt. McLaughlin may seem insignificant when compared with volumes of Mt. Hood (about 10 miJ), Mt. Rainier (about JO miJ), and Mt, Shasta (more than 50 miJ). Mt. McLaughlin is comparable to one of the Three Sisters in volume, 84 My interpretation of Mt. McLoughlin's beginning, shown in cross section BB', plate 1, is that the older units beneath the mountain form a high base at 6500 feet elevation. Included with these older units could be a pre-stage-one buildup of McLaughlin lavas to form a shield "platform" before stage one pyroclastic rocks were laid down. Although this is known to have occurred at other Cascade volcanoes, there is no evidence to support this interpretation at Mt. McLaughlin. If the original base were at 6500 feet elevation, the stage one pyroclasic cone would not be near enough to the surface to be exposed by erosion. However, if large sections of the cone were raised several hundred feet by stage two injections of lava, they could form outcrops now seen in the cirque headwalls. An alternate explan- ation of why stage one pyroclastic rocks are found at such a high elevation would assume that explosive activity continued well into stage two but the cone was reinforced by hidden lava ..f lows to enable it to grow to great height. The volume of Robinson Butte is estimated to be 0.5 cubic mile. The Recent silicic andesite:.flows covering Brown Mountain have an estimated average thickness of 250 feet and cover more than 13 s4uare miles. This amounts to another half cubic mile. Other late Quaternary flows on the flanks of Mt. McLaughlin are less than 0.1 cubic mile in volume. Therefore, all late Pleistocene and Holocene volcanism in the mapped area totals three to four cubic miles. The estimated br.eak-down of the volume by rock type for- the High Cascades in the thesis area is as followss 85 Plio-Cascades basaltic andes.lte •1 5 miJ Oliv:i.ne basalt (very speculative) 20 mi3 Basaltic andesite 9 mi3 Silicic andesite 1 mi3 86 ANALYSINGTE CHNIQUES Major oxide cont~nts were determined on 35 samples using the X-ray fluorescence (XRF) method described by Nor+ish and Hutton (1968). A general description of the method 1s as follows1 A rock sample is crushed and powdered using equipment which minimizes contamination. Two glass discs are made from each sample using a fusion-stamping process. The discs c~ntain a mixture of precisely weighed portions of rock powder, lanthanum tetraborate flux, and sodium nitrate, Discs of known mixtures, standards, and unknown samples are subjected to x-rays from a specific source tube which cause the disc surface to fluorescee An analysing crystal disperses the resulting fluorescence and a flow timer counts the number of photons of a specific (small band width) energy received during a known time interval. The number of counts . , recorded for an unknown is more-or-less directly proportional to the amount of the oxide in the sample as compared to a known sample. Oxide percentages are computed from nominal values which are fed into a computer "matrix" program. This program is designed to sort out the effects that one oxide has on the determination of the other oxides. As the final step. the percentages of the oxides from the two discs are averaged, Details of the proceedure are contained in the reference given abovee Table 1 lists the ten major and minor oxides determined for each sample. Also listed is the x-ray source tube, analysing crystal, counting time in seconds, the one-sigma counting error, estimated actual 87 error, and the actual error in percentage of the oxide weight. Sodium abundances were determined using the instrumental neutron activation analysis (INAA) technique described by Gordon, et al,, (1968). In this method, the sample is first irradiated and the resultant x-ray emissions are counted and compared to a known stand- ard much as in the X.RFm ethod. The errors listed take in the actual counting error, which is a relationship between the counting peak height and the level of back- ground, and also electronic errors which are inherent in the system, The final estimated limits of error, columns f and g, are found by comparing both sample discs for all 35 samples and listing the max-imumd ifference found for each oxide, Most of the additional error between counting error and total estimated error is considered to result from the weighing of the components, differences in batches of flux, and unresolved discrepancies in the computer "matrix" corrections. 88 Table 1 a b C d e f g Oxide Type x-ray Analysing counting one sigma limit of limit of source tube crystal time in counting error in error as seconds error total % total oxide wt. oxide wt. SiOz Cr PET 10 .20 .40 1 TiOz Cr PET 20 001 ,OJ 4 Alz03 Cr PET 10 .10 ,20 1 MgO Cr ADP 2 counts@ 100 .20 .20 4 FezOJ* w LiF (200) JO .05 ,20 J, MnO** w LiF (200) 30 .01 .01 10 CaO Cr PET 20 ,03 ,20 3 NazO Gemanium INAA method 40 .07 detector KzO Cr PET 20 ,02 .05 5 Pz05 Cr PET 10 .01 .02 5 * Total iron calculated as Fez03 ** Mno abundances are so low (.09 to ,17 % of total rock wt,) that the final error was essentially limited by the method of analysis and the total error can be represented by the statistical counting error. LiF (200) - fluorescence was detected using the (200) face of LiF crystal 89 PEACOCKIN DEX The alkali-lime index proposed by Peacock (1931) is the silica value for which CaO equals t.ot~ alkalies when values are plotted for a suite of rocks. This index is not readily obtained from the plots -of McLaughlin area lavas since none of, the rocks are rich enough in silica to make the trends on plots of K20 + Na20 / CaO against Si02 go above unity, (figure 24). The plots do show a vague trend which would put the Peacock Index for the Fish Lake series between 61 and 62, If th.is projected value is valid for the rocks of the Mt, McLaughlin area, the suite is calc-alkal.lne to calclc in nature and is similar in chemical and mineralogical composition to the rest of the High Cascades and to other continental margin orogenic andesites. The lavas of Mt, McLaughlin vary less than do the lavas of the. Fish Lake series so that the index for Mt, McLaughlin alone could not be determined, Mt, McLaughlin lavas a-re slightly richer in CaO and poorer in alkalies than are the Fish Lake series, These values group in a position on the variation diagram which indicates that the index might be 62 or higher, This is the same value as found at Crater Lake, higher than that for the Modoc lavas (60,5) 1 but less than that for Mt. Shasta (63.7). The Peacock Index is not widely used today. It was initiated as a means of arbitrarily comparing and classifying rock suites by using this slight difference in their compositions, More recently, Dickinson and Hatherton (1967) have raised the possibility that differences in the alkali co~tent (specifically K2o) are related to depth of magma .9 ■ ■ •• .8 • ■ • ■ WT.% • ■ IS1 • K 0+N~O •7 2 • ■ CAO • .6 oO •• • 0 ISi • ••• • • .5 . 4 t:,. 51.0 53.0 55.0 57.0 59.0 61.0 WT. % S102 '\!) 0 Figure 26~ Closed circles, main-cone lavas of Mt. McLaughlin; closed triangles, flank lavas; closed squares, Fish Lake series. Other symbols, figure 29, page 100. 91 generation. This difference is maintained as the magma works its way to the surface and can can be seen in variations of surface lavas. If this hypothesis is valid, the Peacock Index, or a modification of it, could take on genetic significance, Relative depths of origin of the primary magma could possibly be determined between rock suites and between individual series within a rock suite. 92 ROCKC OMPOSITIONS Western Cascades No lavas from the fourth member of the Wasson Formation were chemically analysed but because they are dark and contain olivine they are probably basalts or basaltic andesites. Some of the agglomerates may be andesitic but they are so strongly weathered that it is difficult to assess their former rock type. If the chemical character of these rocks is consistent with that of other rocks of the Western Cascades sampled by Peck and others (1964), they are slightly more alkaline than High Cascade rocks. Peck reported the Peacock .Index to be near 60 for the Western Cascade series of lavas. Two samples of Heppsie Andesite were ~nalysed. They are listed in table 2, page 106. Analysis 1 is of the medium grey type of rock and analysis 2 is of the light gtey type. The two chemical compo- sitions are essentially the same in silica, but there are differences in MgO, total Fe, Cao, and AlzOJ• Little Butte Forest Camp near the west end of Fish Lake is located on an eroded Heppsie Andesite vent in which the two lithologies are intermingled within a 10 meter radius, suggesting that they share a common time and space relationship. Plio-Cascades • Plio-Cascade lavas are mapped separately from Heppsie Andesites Because of the angular unconformity between them, and their differences in composition. Again, only two samples from this unit were analysed 93 but petrographic features of other samples indicate that the two are representative of the group. ; Plio-Cascade lavas are tran~itional between basalts and amledtes. Their low average silica value (52 percent) and the presence of normative olivine put them in the basalt field, but their color index is slightly less than JO and the normative An content is less than 50 percent which is in the andesite field, The transitional nature of the lavas could not be a result of a ·slight concentration of mafic phenocrysts ln a magma which would other- wise be a Hlgh Cascade basaltic andesite. Besides having a substantial age differenc~, the high relative values for K20 and P2o5 indicate that phenocrysts enrichment did not occur and that the lavas have a compo- sition rUstln,ct from and not genetically related to the High Cascade lavas. The Plio-Cascade lavas are more alkaline in nature than are the· High Cascade lavas. The K20 level of the two samples, figure JJ, is almost t.wice a.s high when ~arr.paring rocks of equal silica values. It is uncertaln whether sodium really shows this same contrast, SampJ.e LS-4 shows a tiigh sodium level but LS-24 does not. Because it is so di.ff'icult to obta5n fresh samples of Plio-Cascade lavas, it is probable that a. substantial amount of sodium was leached out of LS-24 during the intense weathering of the unit. The most important difference between Pllo-Cascade lavas and High Cascade lavas is the high phosphorus content of the Plio-Cascades, (figure J4, page 105). The P205 content is nearly 0.5 percent, twice the level of other lavas analys~do 94 When studying the North Santiam district, Thayer (1937) listed three chemical analyses in which the P205 content is near 0.5 percenti all three are labeled Outerson basalts. Outerson lavas were given a Pliocene age_since they unconformably lie between the Miocene Sardine lavas and the Plio(?)-Pleistocene lavas of the High Cascades. Perhaps there 1s an extensive high phosphorus rock unit which lies between the Western Cascades and the High Cascades. It may not be continuous or have uniformly high phosphorus levels, but if these characteristics are indeed distinctive throughout the Cascades, this unit will be a valuable stratigraphic marker. Most of the high- . phosphorus lava.s should be easily found by looking at rock thin sections and noting the relative abundance Qf apatite needles, Plio-Peistocene Olivine Basalts The basalts in the Mt. McLaughlin area are typical of the high- aiumim lavas of orogenic regions, and similar to those which have been reported from other parts of the Cascades, for example the Santiam basalts of Thayer. They are compositionally similar and at times texturally similar to the diktytaxitic olivine basalts found east of the Cascades. Kuno (1960) was the first to separate and name high-alwnlna basalts as a distinct basalt type. It has been noted that they are 9ommonly associated with orogenic andesites, Some workers have argued that high-alumina basalts are the parent magma from which andesites are derived through the process of crystal fractionation. 95 In the Mt, McLoughl1n area, few olivine basalts are exposed at the surface. Most are assumed to pre-date the Fish Lake lavas. Two olivine basalts were analysed, LS-45 and LS-56. They are listed 1n table 2 as analyses 5 and 6. Fish Lake Series The Fish Lake series is best exposed on the south side of Brown Mountain. The nowe stairstep up to the east toward the former vents which are located immediately west of Lake O' Woods, The ten analysed samples, labeled I through X, show a gradual change in major element composition.,The earliest flows have a silica value of 56 percent, a basaltic andesite1 the latest flows reach a maxlmwnv alue of 61 percent, a silicic andesite. Harker diagrams for Fe, Mn, Ti, and Mg show linear depletion trends with increasing silica. This fact along with petrographic and field evidence indicates that differentiation of the initial magma has occurred. Sinc9 the lavas progressively become more crystal1ne and contain numerous phenocryst clusters, it is believed that crystal fractionation is the main process working to cause differentiation., Although not as linear as those previously mentioned, Ca also shows a depletion trend while the alkalies show enrichment. There is a strange character to the trends of the alkalies. Although both K20 and Na2o increase, the most siliceous lavas start to show a decrease in the~e components. The KzOd iagram, figure JJ, displays this "hook" most prominently. Publil!hed data from other Cascade volcanoes were plotted on a similar diagram. The K2o values for Mt, Shasta, Crater Lake, the Three Sisters, Mt. Hood, and Mt. Rainier produce a scatter in the silic1c andeaite field. The K20 values of rocks from the Kt. Jeff- erson area show the same hooked pattern as the Fish Lake series and at the same KzOa nd Si0 2 levels. Not all of the lavas mapped as Fish Lake series belong to a sequence which has differentiated. Rye Spur volcano due east of Mt. McLaughlin is composed of lavas which did not. The lavas from this volcano are basaltic andesites which are rich in glomero- phenocrysts but the groundmass is fine-grained and dark grey in all the lavas, Two other volcanoes to the north of Rye Spur show the same constructional form, the same stage of erosion, and are also bisected by north-south faults with the east side down. These cones were not visited and the composition of their lavas are unknown. Mt. McLaughlin Lavas After the faulting of the now extinct Fish Lake volcanoes, or perhaps commencing with it. Mt. McLaughlin and its associated vents began to erupt. The general chemical nature of the lavas is the same as for other High Cascade centers. On both the NCKa nd AMFd iagrams, figures 27 and 28, Mt. McLaughlin lavas plot near the basic ends of the other suites. Lavas of the Fish Lake series which have differ- entiated overlap the basaltic andesite field of the Mt. McLaughlin lavas and the silicic andesite field of the Mt, Rainier lavas. 97 Mt. McLaughlin has erupted both coarse- and fine-grained lavas almost simultaneously at different stages of its growth, Both types vary only slightly in composition. All are basaltic andesitos which are almost identical in major element chemistry to the initial lavas of the Fish Lake series. When Mt. McLaughlin lavas are plotted on silica variation diagrams, the coarse- and fine-grained types form separate groupings. The groupings are close together but seem to be mutually exclusive, The silica values for the two groups are the same, varying between 55 and 57 percent, but all other major oxides are lower in the coarse- grained lavas except CaO, Na2o, and Al2o3, which are higher. (see figures 29 through 34). Most of the differences in major oxide components between the two lava types can be explained be their obvious contrasting crystal~ne texture. Since the coarse-grained lavas are ver:y porphyritic.with plagioclase, an. enrichment in this phase can raise the values for CaO, Na2o, and Alz03• The other components are then lowered slightly by the dilution effect of the added plagioclase. This effect can be confirmed by computing a 15 percent by weight enrichment in labradorite crystals of composition An 65. This method does not explain all of the differences in the oxide values. The concentrations of K20 and P2o5 are lower than can be expected from the dilution effect described above. Since these two oxides are not removed by any phenocryst phase present in the lava, they are being lost through some other differentiation process or were deficient in the original magma. Na + K,__.......,___.,__~-~--JL..-~~-_j,[__J.L___li_~'---~___.:y__~-~~-.J,/___J/_~~~Mg 50 Figure 27. AMFd iagram of rock sunes from the Cascade Range. R-Mount Rainier, StH-Ho,mt St. Heler,s, CL-Crater Lake, McL-Mount McLaughlin, F'L-Fish Lake series, Sh-Mount S!1asta, L-~wunt Lassen, I \ I \ Na Ca 50 Figure 28, NCK d."iagr.a,:i of r0':!k suites from the C3,scade Range. Symbols sam9 as in fir,ure 27, 1.3 ISi I. I WT. % ISi II Tl0 Ill 2 II 6. • 0.9 D,. AA A /J,,. II l'I a ■■ ■ ~ • • 0 II 0 e 0.7 0 • • I! a 0;5 __.._...._ _._.,.__ _ ___. __ _._ ___ ..._ __ ■ ■ 51.0 53.0 55.0 57.0 59.0 61.0 ·WT. % S102 j>..A Figure 29. Closed circles, main-cone lavas1 open circles, lavas suspected. o£ uhenacryst 0 0 enrichment, closed triangles, fine-grained flank lavas; open triangles, olivine basalts an~CU11ulate lavas1 closed squares, Fish ·Lake series and Brown Mountain lavas slashed squares, Plio-Cascadesr slashed circles, Heppsie Andesites. 10.0 0 0 8.0 wr. ISl 6. % MGO 6.0 ISl (9 0 .... A. A. • • -• ■ ■ ■ 4.0 Ila ■ al ■ 0 ■ II ■ ■ ■ ■• ■ 2O6- ______ _._ __ _._ ___ &.- __ ......_ _____ _,, ___ .1- __ ....,L.._ _ --L, __ _ 51.0 53.0 55.0 57.0 59.0 61.0 WT.% S102 Figure JO. S)'llbols are the same as in figure 29. 10.0 ,ISi 9.0 ■ 8.0 C ISi ■ B li1I Ill Iii 7.0, 0 0 0 "· • ■ II Ill ■ 0 6.0 II ■ II 11 ·5.0L..---..L..-----L-__,;;.-..:1- __ ....r._.. . _,..;,_..J. ___ ..1..,_ __ -'- __ --11....-- __ ..L- __ __._ __ 51.0 53.0 55.0 57.0 59.0 WT. 0/4 S102 .... Figure J1, Symbols are the same as in figure 29. 0 N· 10.5. 9.5 8.5 6. WT.% .6. • • CAO ISl • • •J, ,. ISl 00 J,,. 0 J,,. ri1 Cl 7.5 m A • • ■ • IZ II ■ 6.5 • ■ II ■• 51.0 53.0 55.0 57.0 59.0 61.0 WT. % S102 Figure J2. Symbols are the same as in figu.re 29. 1.6 m II II 1.3 WT. II % ISi ■ • II K20 l!I A. ■ 151 1:1 A. l'J 1.0 A. CX) • •• ' 0.7 • • 0.4 51.0 53,0 55.0 57.0 59.0 61.0 WT. % S102 Figure '.33. Symbols are the same as in figure _29. .55 !SI ISi .46 .37 II .28 a A mA A A ■ SIii • ■ ■ .1_9 D II • • ■ A 0 • 00,M 0 • • .10 5!,.0 ~3.0 ~5.0 !57.0 59.0 61.0 WT. % S102 Fi-gure 34-, Symbols are the same as::in figure 29, 106 Table 2. Chemlcal analyses and norms of Heppsie andesites, Pllo- Cascades, and olivine basalts. 1 2 J 4 5, 6 Sa·nplc No. LS-2 LS-58 LS-4 LS-24 LS-45 LS-56 :::aoz 54.42 _54.39 52.55 52.03 53~03 52.43 1'102 0.76 0,87 1.00 1.21 .1~09 0,88 Al20J 17.97 18.57 17.72 18.14 17~29 18.lJ F~ t.ob.l 6.74 s.13 7.93 9.97 8.48 8,64 a::; Fe203 t•:nO o. 10 0.14 0.15 0.17 0.14 0.15 :•lgO 7.42 5.10 7.58 5.26 7 .•5 8 ✓ 6.34 CaO 6.52 7,41 7.69 8,10 8.23 9.45 Na?.O 3.90 4.06 4.20 J,47 3.31 J,J8 lute norms was 0.15. 107 Table 3. Chemical analyses and norms of Fish Lake series andesites. 7 8 9 10 11 Sa:nple No. I IX LS-81 LS-75 II SlOz 56.18 56,J8 56,59 56.70 56,91 'I'i02 0.98 1.00 0,8J 0.95 ,0.97 Al20j 18.06 18,04 18.72 17.50 17,14 Fr. t.0t.1.l 8.4J 7,98 03 7.JJ 7.61 7,81 as F\.,2 MrO 0.14 0.14 0,12 0.12 0.14 l·'.gO J.95 J,01 J.85 3,92 4.10 C:aO 7.22 7,65 7.56 6,75 6,97 Na~O J.85 J.96 4.13 4,27 3,97 K20 1,09 1.16 o.84 1.61 1.53 I'205 o.?5 0,29 0,17 0,27 0.26 Tot.al 100.15 99.61 100,14 99,70 99,80 Caln.norms 01 Qz 5.3 6.o 5.0 3,6 5,0 Or 6.5 6.9 5,0 9.6 9, 1 Plag 63.7 64,4 67.2 62.4 6o.4 Di 4.4 6.5 5.1 6.J 6.8 Hy 17.1 13.0 15.J 15,1 15.7 Ar, 0,5 o.6 o.4 o.6 0:5 Tl 1,4 1 .4 1.2 1 .J 1 ,4 Mt 1.2 1.1 1.0 1.1 1,1 '.~ rlu.g An 44,8 J8,J Color Index 22,6 23,8 7, Glassy olivine andesite, Lava at junction of Forest Service roads 3705 and 3706, sec. 16, east of Robinson Butte, 8, Olivine andesite. Lava in road cut along F.S. road J640, west edge of sec. 1.0, between Brown Mountain and Lake O' Woods, 9, Olivine andesite from Rye Spur volcano. Lava on east side of Billie Creek on F.S. road 3633, NW¼,s ec. 21, 10. Olivine-pyroxene andesite, Lava along unmaintained logging road, NW¼, sec. J1, NWb ase of Mt. McLoughlin, 11. Glassy olivine andesite, Lava along F.S. road 3705, south side of Beaver Dam Creek, SW¼, sec. 14, SW of Brown Mountain, 108 Table 4. Chemical analyses of Fish Lake series andesites, continued. 12 13 14 15 16 Sa·nple No. VI III BL-2J BL-8 X S10z 56.93 57.68 57.90 58.07 .58,29 r10 2 o.8J o.s1 0.80 0.77 0.79 Al>OJ 18.48 18.05 17,91 17,46 i7.50 F'< I~ l> 7000 1::->-u 3 l ::r er ~ I a a n .o 6000 I .< (D ::, (D 3000 Amphibolite 2000 X X X X 1000 High-Alumina Basolx - -- ----• 500 600 700 800 900 1100 1200 1300 Temperature, °C Figure 35. Pressure-temperature diagram of a natural high- alumina basalt-water system, (Yoder and Tilley, 1962). The coarse-grained lavas of the main cone include LS-7, LS-14, LS-68, LS-71, LS-80, 21F, and two special cases, LS-3 and LS-6. The latter is one of the last flows to erupt from the central vent and LS-3 1s a Holocene flow_located at the southwest base of the mountain. These two samples plot very close together on all of the Harker diagrams. They are 4 percentage points higher in MgOa nd 2 points lower in silica than in the other coarse-grained lavas, (open circles, figures 29 through 34). The norm values given in table 8 indicate that there has been a 15 percent enrichment in olivine. The petrography of LS-6 confirms the existence of mafic phenocryst accumulation, but it is of clinopyroxene, (8 percent mode). Details of the petrography, however, reveal that many large grains of pyroxene have grown around olivine cumulate material and has almost entirely replaced it. Only slight accumulations above the other lavas can be detected in LS-J. In sample LS-71 there is a noticable modal concentration of large olivines. This enrichment 1s also apparent in the lower silica and higher MgOl evel of the sample. The level of olivine, Fo 80, accumulation is computed to be about 2 to J percent. Brown Mountain Lava Brown Mountain lava is the only,silicic andesite in the mapped a,ea which has erupted since the Fish Lake volcanoes ceased their activity. This lava is not like the former Fish Lake differentiates 1n texture. Brown Mountain flows are post-glacial lavas which have the da~k color and the blocky flow surface texture of the fine-grained 117 fla~ lavas of Mt. MciLoughlin. These lavas are .also the only non- porphyrlttc lavas in the area. These two facts point to the pro~ble origin of the lavas, which 1s that Brown Mountain lavas are a resuit of the filter- pressing of porphyritic olivine andesite similar to the flank lavas on Mt. McLaughlin. Petrographic ev~dence supporting this premise 1s the occassional appearance of patchy-zoned plagioclase phenocrysts and the rare ocurrance ~fa fresh olivine grain in a lava which is a siltcic andesite. The sample plots on the variation diagrams with the Fish Lake series differentiates. This, is to be expected since the initial lavas for both groups were the same. 118 - PETROLOGYA ND PETROGENESIS Parental Marona A basaltic andesite 1s the most likely parental magman Qt only in the Mt. McLaughlin area but for the whole High Cascade subprovince. The evidence for this is of several typesa 1) Even though the parental magmat ype may not be the dominant lava.at the surface, one would expect that occasionally some of this magma will find its way to ths surface in an undifferentiated form. Features of a parenta;l. magmaw ould include recurring eruptions and an aphyric texture in lava flows. Especially important is the lack of piagloclase phenocrysts. Their presence would indicate that the magma had started to change under low pressure fractionation conditions, There are lavas of three different ages around the base of Mt. McLaughlin that fulfiil these requirements. They are all fine-graineds they h?-ve the same chemical composition: and they are a recurring rock type, 2) If basaltic andesite was derived by near-surface fractionation of an olivine basalt, one would expect to flnd basalts as a recuurring rock type, There are no known, fine-grained, high-alumina basalts that are definately the same age as Mt. McLaughlin. J) The initial lavas of the Fish Lake series also started out with the same bulk chemistry as Mt, McLoughlin lavas and then differ- entiated to s111c1c andesites, 119 4) The large composite cones of the High Cascades are dominated by silicic andesite but most of them have at least small exposures of olivine andesite. Mt. Rainier and Mt. Baker are commonly said to be comprised solely of silicic andesites. This is not true, Both centers are reported in the literature to have olivine andesites, (Fiske and others, 1953s Coombs 1939). Unfortunately only silicic andesites were chosen by the workers for analysis, Many of the olivine andesites of the various centers are dark,.fine-grained, and occur more than once in the eruptive sequence. 5) A parental magma would be expected to be the dominant rock type at all centers which did not differentiate. Even though olivine basalts and silicic andesites are locally voluminous, the most abundant lava in the High Cascades since the Fish Lake series began has been basaltic andesite. 6) When Mt, McLoughlin lavas are plotted on a high water pressure forsterite-diopside-quartz diagram, they lie on the invariant point for liquids co-existing with olivine and pyroxene, (figure 36). Kushiro (1969) deduced this diagram from the results of experimental runs at PH2o= 20 kilobars. Yoder (1969) later showed the implications of this system as it relates to the Cale-alkaline lavas. The first melt of an• olivine normative basalt, or even of an olivine plus two pyroxene per1dotite (lierzolite) under these conditions would be at the high-temperature invariant point. This implies that basaltic andesites are primary magmas at high pressures when water is present. 120 DI 'i I I FO I I I I I EN I I I 'FO EN QZ Figura :,6. Fo-Di-Qz diagram at 20 kilobars wet, adapted from ·xushiro ' (1969). Circles, coarse-grained Mt. McLoughlin lavas, solid triangles, fine-grained flank lavas, open triangles, basalts and basic cumulate lavas, squares, Fish Lake lavas and Brown Mountain lava (bml).. 121 By making adjustments of the partial pressure of water, this diagram can explain the common association of andesites and high- alumina basalts, If the_ partial pressura of water is low, the invariant point shifts toward basaltic compositions and to higher temperatures. According to the flow lines, silicic andesites would plot along the join XY as olivine and pyroxene are subtracted from the liquid, This is also the case indicated by the plots of the Fish La.ke lava series. Their trend is subparallel to the XY flow line which proceeds toward silica enrichment. Differentiation One of the factors that must contribute to the differentiation of a magma 1s the development of a large magma chamber. In such a chamber, crystal fractionation could be the major process in the differentiation of calc-alkaline lavas. Therefore, an efficient magma chamber would be one that is able to separate phenocrysts from the liquid effectively, eithor by crystal settling, depostion from a down-going convection current, filter-pressing, or some other unknown method. Several aspects of a differentiating calc-alkaline volcano can be justifiably proposed from the chemical, petrographic, and field evidence. These include the following, 1) A magma chamber should be large and shaped in such a way that early formed phenocrysts can effectively separate and be left behind when the magma erupts. If convection currents are needed to 122 separate the crystals, then a large~roidal or cylindrical shape may be needed as opposed .to an.irregular system of anastomosing dikes and sills. 2) The magma chamber should be located at an optimum depth such that it is not so shallow that it cools rapidly forming an intrusive plug·, but not so deep that convection currents can not operate. J) Volatile pressure 1s fairly high but it is variable. This. can produce the high anorthite content and oscillatory and patchy rz:oning seen in the plagioclases. High volatile pressure,, especially in an andesite melt which is more viscous than basalt, can produce explosive eruptions.such as those which have occurred on Mt. M~oughlin. 4) The appearance of olivine, pyroxene, and calcic plagioclase in glomerophyrlc clusters indicates that these phases can be concentrated and separated. This can cause the magma to change toward silicic andesite, a rock type that predominates in the large, composite volcanoes of the Cascades. At Mt. McLoughlin, the mineral. clusters were not effectively separated from the liquid. Conclusion In the past. most work~rs in the High Cascades have a.oncentrated their attention on the large volcanic centers of the range. These centers usually have large volumes of silicic andesite with subordinate dacites, rhyolites. and basaltic andesites. Even as early as 1937 Thayer states, "Overemphasis has ap~ntly been placed on andesites in the High Cascades •. The evidence that great quantities of basaltic lavas were erupted while the large andesite peaks were being built is 12) indisputable • • " Of course, in Thayer's frame of reference andesites are sil1c1c andesites and basaltic lavas a.re basaltic andeaites, At Mt. Hood. the dominant lava 1s a silicic andesite which Wise (1969) concluded must have been the primary magma. Although olivine andesites are found in the deepest exposure on Mt. Hood which are chemically and mineralogically similar to a Holocene flank lavat he states that they are not genetically related to the silicic andesites. In his study of Mt. Jefferson, Greene (1968) found that basaltic andesites dominate all other lavas in volume. He deduced that a magmaw ith 53 to 56 percent silica was the parental magma of that volcano. The presence of Recent•int:ra.canyon flows which have the same chemical composition as the most commont ype of older lavas convinced Greene that this must be a reappearance of undifferentiated pa.i;ental magma. Mt. Jefferson is a large center which apparently did not differentiate its lavas efficiently or possibly had just started to do so when the system stopped. The .same is true of Mt. McLoughlin. The lavas of the main cone have abundant crystal clusters. but they were not effectively separated before eruption. Even though the coarse-grained lavas have the general appearance of a silicic andesite, the lavas retain the bu~k composition of the initial fine-grained magma, namely a basaltic , andesite. 124 APPENDIX 125 ROADL OGA LONGRO UTE1 40 From mile post 19 to 35.2 M.P. Description 19.0 Road cut showing Wasson' tuff -overlying a red now breccia unit of the Roxy Formation. The ash dips to the northeast. A dike cuts the ash, trending approximately NE, on the east end of the cut. The ash is rich in lithic fragments. The breccia 1s well cemented~ perhaps silicified. 19,5 Note the cliff~form1ng units high on the north side of the high- , way. The top units are Heppsie Andesite flows.~ 19.7 Outcrop of r~d agglomerate and flow breccia. Where aenae rock is found, it is a dark, green-black holocrystaline basalt •. A series of these now. breccias comprise the cliff outcrops:much higher above the road. This particular outcrop may ·be a. :siwnp• . block since the subdued topography indicates that Wasson ash should be here. Colluvium has been covering the ash since the last stop. ..·, 21.8 First of ' a succession of roadcuts which expose. a:.long a o·7. mile section the floM breccias of the fourth me'mber··o:ft he Wasson. It is described by Wells as "flow agglomerate ranging from red flow breccia to scoriaceous and vesicular flows." 21.9 The flows are cut by & vertical dike which is oriented almost pa.rallel·to the highway. Although the scoriace'l)us tops and bottoms of the flows undulate, the attitude of the flows can clearly be seen to dip eastward. 22.9 More scor1aceous and vesicular lava. Some of the vesicles have been filled, forming amygdw.es. 23.1 Roadcut in dark, olivine basalt which may be the dense phase of the altered, vesicular rock at the last stop. It looks· very much like the 1 dike rock at MP 21.9. 24.,5 Grizzly road branches off to the right (south). The roadcut shows more dark rock grading into a red, scoriaceous now breccia. Near The east end of the aut, there are two blocks severai meters wide which were caught u:p in the flow, They are similar to Wasson tuff. One block has cobbles which contain crystals of hornblende up to 1 cm in length. 126 M.P. Description 25.0 Two more flows which have breccia on the top and the bottom. The top of the lower flow developed a soil horizon which smoothed over the irregular surface. The top flow baked the soil to a bright red-orange. 25.2 East end of exposure is cut by a dike two meters wide. 25.6 This 1s the last roadcut of flows and agglomerates until MP 26.7 where it may ue repeated. A fault crosses the road in the subdued area east of the cut. The west block is downdropped approximately 2000 feet. The red agglomerate-flow breccia member of the Wasson seems to be very thick in the highway section. This is not due to repetition.by faulting. Several vents on both sides of the high- way have been mapped as possible sources for this member. If these vents are the primary sources, the member would be expected: to thicken toward them. In mapping this member in the type section, Wells has it thickest around Little Butte Creek, but a. few miles • ,. •, - to the north and the south it thins and is not mapped further.· 25.8 Red, brown and grey pyroclastic rocks of the Was~on tuff, 26.0 A 20 foot high roadcut of a light grey pyroclastic flow containing pumice and lithic fragments, 26,3 A thin, white ash bed is off-set about 2 meters, west side down. Last of the ash and into a red agglomerate flow unit. This unit may be a repeated fourth member flow or possibly an irregular contact with dark Heppsie Andesit flows. 27.0 He.ppsie Andesite with several large blocks of dark, green-black basalt inclusions. Overlying this flow is a flow of the younger Plio-Cascades unit which has very similar lithology when fresh. Red agglomerate. This may be the fourth member or a scoriaceous flpw of Heppsie andesite. ti;, .; 'Heppsie Andesite flow showing lots of basal scoria overlying an essentially flat lying lens of watef-lain sediments. . ~· 4"' .( Basaltic andesites of Flio-Ca.sca~es unit. Butte Falls road to the left (north). 127 M.P. Description 28.J Road to :right to Big Elk Guard Station, Robinson Butte. and Little B~tte Forest Camp. 29.1 Road north to Rye Spring and Rye Flat. 29.9. Road south to Doe Point Forest Camp. Roadcut shows spheroidally weathered Plio~Cascade rocks. 30.4 Road to right goes in to Fish Lake Resort, Next roadcut 1s -through a rubbly flow of dark, fine-grained, olivine andesite which erupted on the south flank of Mt. McLaughlin. A stage two flow. For approximately the next mile, fresh• blocky lava nows can be seen on both sides of the road. These are post-glacial (stage three) olivi~e andesites which were erupted on the south flank of Mt. McLaughlin. 32·.5 Summit 32.7 First roadcut through Brown Mountain lava. The flows l,ook like stage three flank lavas fro~ Mt, McLoughlin but they contain no oli_vine an_d are chemically silicic andesi tes. 3J,8 The easternmost roadcut through Brown Mountain lava.. Rock type changes to coarse-grained andesites which were erupted on the ridge to the southeast of Mt. McLoughlin. 35.2 De~p roadcut in spheroidally weathered rock. The rock may be Plio ..C ascades ar early Qua.ternary olivine basalt. 128 Age of. Mt, McLoughlin The ages· of the high peaks of the Cascade Range have .never been well resolved. Most High Cascade lavas are too young to be dated using the K/A method and too old to be dated using the c14 method, even if charcoal inclusions could be found. The pea.ks must be older than the close of the last major glaciation 15,000 years ago pecauae the former glacier~ have left large cirques and till deposits. The peaks are younger than the earth's last major magnetic reversal approximately 700,000 years ago, all of the rocks of tba high peaks tested for natural remnant magnetism, including Mt. McLaughlin. have normal magnetism. That 1s, rocks, cooled in the earth's magnetic field which had the same polarity that it has today. The last period of reversed polarity, the Matuyama.r eversed epoch, lasted from 2. 34 million years ago (MYA)t o 0 •.6 9 MYA. Except for a poss- 1b le reversal of short duration 30.000 to 20,000 YA, the Brunhes epoch, 0.69 MYAto present, has been normal, (Cox, 1969). This short reversal has not, been recognized in the Cascades. According to E.M• . Taylor ( orai communication, 1973) even the ' • deeply burled rocks of the High Cascades near the Three Sisters ha~e normal remnant.magnetism. These rocks are exposed in deeply glaciated valleys and form the base on w]';iicht he Three Sisters grew. The high, composite cones are considerably younger than these basal rocks, but a.t the prasent•time, there is no Jfay of knowing how much younger they are. 129 PLEiSTO<;::EGNLEA CIAT~ON During the maximumi ce advances during Pleistocene time, there was an ice cap on the high, narrow plateau area of th~ High Cascades. This fact was first recognized in the early 1900's but not widely appreciated, perhaps because of the lack of published glacial studies in the Oregon Cascades. Evidence for this ice cap can easily be seen on topographic maps of the area between Mt. McLaughlin and Crater Lake. The ice cap formed along the crest of the range and upon melting left a large humucky area of moderate relief and, poor drainage. Leading out of the area to the east and west are U-shaped glacially scoured valleys. There are no individual catchment basins or cirques that could have accumulated snow to feed these valley glaciers~ They. must have been fed by the overflow from the ice cap. Mt. McLaughlin was the southern limit of the ice cap in Oregon. The average elevation of the Cascade crest between Mt. McLoughlin and the Klamath River in California was below 5600 feet and too low to support an ice cap. Two cirque-fed .glaciers flowed north and northeast from Mt. McLaughlin and c,oalesced with ice moving south from the ice cap_into the Fourmile Lake basin. Most of this ice was then diverted west, down the south fork of Fourbit Creek. Although a small portion of the ice probably flowed down Billie Creek on the east ·side, no moraines could be discerned on aerial photographs. 130 Some of the ice in the northeast cirque overflowed its edge and left three lobe-shaped end moraines south of Frey Lake. It is in the middle lobe that a post-glacial block lava flow erupted and contributed to the for111$t1ono f an unusual geomorphic feature. The dark blocks from this flow are creeping downhill and feeding a blockstream in a small draina:ge depression. From the other side of the drainage, weathered buff-colored Frey Lake lava blocks are being fed into.the blockstream. This results in a sharp color line which runs down the center of the blockstream.and can be clearly seen on the gro4_I1da nd on aerial photographs. Blockstreams also occur on the west slope of the mountain where they are fed by the block lava which was erupted from North and South Squaw Tip. During maximumg lacia.tion, a small glacier sta~ed to develope a cirque on the northwest side.of Mt. McLaughlin. Two well-defined troughs between North Squaw Tip and the north cirque are a result of this action. Ice flowed down the mountain for more than a mile and deposited a great volume of huge blocks be:low the 5600 foot. level. Movement probably continued for some time after the main glacier melted. The huge field of blocks may have moved as a rock glacier. The surface of the field' has no matrix but it shows strongly ridged and convex toes which are characteristic of rock glaciers. Below the surface, there may be an accumulation of fine material which may have provided a matrix so that true solifluction movement could occur. 131 The glacial features on the north side of Mt. McLoughlin shown on plate 2 were transferred from aeria.l photograghs (scale, 1113,000; dated 1969) onto an enla_rged fifteen minute topographic map ( 1132 ,oo~) using a stereo-plotter. Gary Carver (1973, unpublished thesis) studied the glacial strat- igraphy of the Mountain Lakes Wilderness area located to the southeast of Mt. McLaughlin. Although no tills in the area were dated radio- 'metricly, Carver has compared the drifts in the area with those from the Washington Cascades and the Sierra Nevada to show approximate age correla t:1:ons, (table 11 ) • Obs.cure glacial moraine features are located in sections 3 and J4 on the northwest flank'of Mt. McLaughlin. They may be Varney Creek age or older, even though 1n his table, Carver tentatively has the age of Mt. McLaughlin younger than the Varney Creek till. The prominent glacial features on Mt. McLoughlin are assUJlled to be no older than the last major glaciation and would therefore correlate with the Waban dr1:ft. Carver reports that• a drift of Zephyr Lake age (which may be a late Waban drift) is found four kilometers down the north slope of the mountain, _or three kilometers beyond the cirque ; thresholds, and ended below the 1900 meter level. Neoglacial-I till 1s said to show up, w~ll on Mt. McLoughlin. It is found on the north slope to within two kilometers of Fourmile Lake and ends at an elevation slightly less than 2000 meters. After the Mazama aah was deposited ( c. 7000 YA), the glaciers were confined to 132 their pre-existing cirques, Such features as fossil rook glaciers and block streams are assumed to have been active during the Neoglacial- I and -II episodes, Protalus ramparts and rock glaciers in and around the present cirque basin are still active today. 133 Table 11. Comparison of Quaternary glacial successions in the Sierra N~va4aa nd Cascade Range• after Carver (1973). Cascade Range Sierra Nevada Appoximate Southern Oregon California Age Neoglacial II Matthes Till unnamedt ill .500 Neoglacial I (McL till) Recess Peak Till 2009 MazamaA sh MazamaA sh 6700 Zephyr Lake drift Hilga.rd Till 9400 Waband rift Tioga Till 13,000 0 Tena.ya Till Varney Creek drift Tahoe '1'111 - basalt - Moss Creek till Mono Basin Till ' Winemat ill Casa Diablo Till 240,000 Bishop Tu:f'f 700.000 older tills Sherwin Till 134 T.able 12. Trace element de.ta (INAA) for Mt. McLoughlin area.rocks. LS-4 Cs Ba 260 !43 235 :!:35 386 ;!:61 La 6.22 ;t.16 9.02 ;t.20 7o12 ;t.12 18.08 !o35 8.74 :!:o18 Ce 14.5 ,!;.3 19.0 ;t.6 14.7 :!:•3 39.7 :!:•.5 19.6 :!:.•.5 Sm 2.28 ;t.OJ J.18 :!,.03 2.35 :!,.02 .5.23 ;t.06 J.OB ;t •.0 4 Eu 0.91 ;t.02 1.11 !,,OJ o.89 ;t.02 1.50 ;t.03 1.10 ;t.02 Tb o.40 ;t.02 0.55 ;!:.OJ 0.27 ;t.02 0.63 :!;.02 o.48 ;t.03 Lu o.16 :!;,02 00 29 ;t.02 0.18 :!,.1 0,27 ;t.02 0.31 :!:,.02 Th 0.96 ;t.04 1.23 :t,.08 1.02 ;!:.,04 1.21 :!:,.07. 0,94 !,.OB Zr Hf 1 • .55+.06 1,.B.5:!:,11 1.?? :!;.05 2.82 ;!.:.OB 1.91 ;t.09 Co • 23.2 +.3 32.7 ;t.4 23,0 ;t.2 21.6 ;t.2 32.0.+-..3 Sc 19.4 ;t.2 21.6 ;t.2 32.6 ;t.3 Cr 35 :!: 1 180 :!: 2 2.5 :!: 1 32 :!: 1 178 :!: 2 All abundances in ppm except as indicated. 13.5 Table 13, Trace element data (INAA) for Mt. McLaughlin area rocks, BL-4 ~ LS-75 LS-26 !&:fil. Na% 3.07 .:!;.04 2,89 :!:.,03 3.17 !,04 2,73 :!:_.03 3,07 :!:.,04 Cs 0, 78 :!;0 04 0.53 :!:.,05 0.67 :!:,05 o. 34 :!:,08 Ba, 376 :!:,38 429 :!:71 40 :!:.10 La 11.42 :!_,2.5 11 •.2 9 :!:.,21 18,.57 :!:.•3 4 14.24 :!:,2.5 9.10 !,21 Ce 23e2 :!;o3 22, 9 :!:,.5 40,7 :!:,} 28,J :!;.4 18,4 !,,4 Sm 2,83 :!;.04 3,36 !,04 5.21 !• 06 3,79 .:!;.04 3,10 +,04 Eu 0.90 :!;.02 1.16 !,03 1,42 !,,02 1.20 :!:.,02 1.03 +,02 Tb 0.36 !,02 0,48 !,02 0,74 :!:.,02 0 • .52 :!:,.02 0,38 .:!;.02 Yb 1.1 :!:,1 1.1 -+,1. 2.7 !,1 1 • .5 .:!;,1 1.1 .:!;.1 ' ~-f!t Lu 0.20 :!:.,02 o. 18 :!:•0 2 '?j· o.48 !,02 0,26 .:!;,02 0,18 :!:,02 Th 2,77 :!;o0.5 1,48 :!:,07 3.69 !,08 1.,54 !,.06 1. 22 :!:, 06 Zr 230 ;!J8 Hf 2.21 :!:.,0.5 2,34 :!:,,08 3o26 :!;o07 2.9.5 :!:,08 1,7.5 :!:,06 Co 17.0 :!:,2 21,7 :!:.,3 24.o .:!;.3 230 7 :!:o3 22.1 !,2 Sc 13.6 :!:,1 17.1 . -+.2 17,6:!:.,2 19,9 :!:,,2 19.4 :!:.,2 Cr 32 :!: 1 73 :!:.1 76 :!:. 1 9.5 ! 1 33 :!: 1 136 Table 14, Trace element data (INAA) for Kt, McLoughlin area rocks, LS-71 BCR-1* ' - ~* I Na % ' 2,88 ;!,03 1,65 :!:,02 2,45 :!:,03 3,22 !,04 Cs 1.00 !,13 1,15:!:,,10 Ba 170 :!:20 1213 :!:.33 La 6.10 :!:,16 10.33 :!:,24 24.27 ;!,44 37,50 :!:,53 Ce 54,1 :!:•7 74,1 !• 7 Sm 2,13 +,03 3.55 -+.. 04 7,10 :!:,08 6,07 :!:,,07 Eu 1.09 :!:.,03 1.95 ;t.04 1.71 :!:,,04 Tb o.66 :t,o4 1,07 ;t.o4 0.70 +.03 Yb 2.2 :!:_.1 3.6 !•2 2,0 :!:,,1 Lu 0,17 ;!:,02 0.32 !,02 0.55 :!:,,03 0,35 +.02 Th 2,41 :!:,07 6.63 +.u Zr 536 !53 Hf 2. 39 :!:,10 4,96 :!:,,12 4,76 :!:,,09 Co 43.2 +.5 35.4 +.4 14,7 :!:,,2 Sc 12.8 :t,1 Cr 121 :!:.2 8 :!:.1 10 :!:.1 * S:tandard rocks which were• tested with the unknowns, 137 BIBLICGRAPHY Bowen, N.L., 1934, The Evolution of Igneous Rocks, Princeton Univ. 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Div, of Mines Bull, 151, p, 7-60, 139 Williams, Howel, 1957, A geologic map of the Bend quadrangle, Oregon, and a reconaissance geologic map of the central portion of the High Casqade Mountains, Oreg. Dept. 9f Geol. Publ. in cooperation with USGS. Wise, w.s., 1969, Geology and petrology of the Mt. Hood areas a study of High Cascade volcanism1 GSA Bull. v. 80. P• 969~1006. Yoder, H.S., 1969, Calcalkalic andesites1 experimental data bearing on the origin of their assumed characteristics1 in Proceedings of the andesite conferences Oreg. Dept. of Geol.~ubl., Bull #659 p. 77-89. - • Yoder, H.S. 1 and Tilley, C.E., 1962, Origin of basalt magma.s1 an exper• imental study of natural and synthetic'rock systems, Journ. of Petrol., v. 3, p. J42-.5J2. _,,_. ___-_