Petrography and Petrology of Some Intrusive Bodies in the Southern Willamette Valley, Oregon.
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Date
1964
Authors
Holmes, John Shaw
Journal Title
Journal ISSN
Volume Title
Publisher
University of Oregon
Abstract
Many post-middle Oligocene bodies of intrusive rocks
are located in the southern Willamette Valley near Eugene,
Oregon. Six of these which fora topographic highs,
Creswell Butte, Spencer Butte, East Butte, Skinner Butte,
Gillespie Butte, and North Butte, are examined in detail.
From outcrop patterns, aerial photos, available
literature, and field observations, the author believes
the buttes to be sills or sill-like structures, concordant
or slightly discordant with the surrounding Eugene Formation.
Specifically, Creswell Butte,. Spencer Butte, and
East Butte are eastward dipping sills; Skinner Butte is
a sill-like structure dipping northwest; Gillespie Butte
is a phacolith occupying a local synclinal fold plunging
to the northwest, North Butte 1s a small laccolith with
a feeder dike below the outcrop.
All six buttes have similar lithological characteristics.
They are medium- to fine-grained basalts with an
ophitic and in places glomeroporphyritic texture. Plagioclase
(labradorite) 1s the predominant mineral and coexists
with lesser amounts o! the pyroxenes, hypersthene
and ferroaugite. Some samples have trachytic texture.
Hypersthene and ferroaugite are present at Creswell, Spencer, Gillespie, and North Buttes, but only the monoclinic
pyroxene is present at East and Skinner Buttes.
The ferromagnesium grains are moderately altered to
chlorophaeite. This chloritic material appears to have
altered from hypersthene mainly, but in some samples it
has replaced ferroaugite. Other minerals, in minor quantities,
are magnetite, ilmenite, apatite, and zircon.
There is no evidence supporting differentiation
within the buttes. The constant Fe2/Mg2 ratio suggests
the same magma source tor all six buttes and thus rules
out the possibility of extensive differentiation. There
is, however, variation in the pyroxene content, which
suggest a temperature variation during intrusion. The
two-pyroxene basalts in the thesis area represent higher
temperature conditions where the hypersthene was 'frozen'
in the rocks and not allowed to undergo a normal inversion
to pigeonite. The basalts which contain only ferroaugite
indicate a temperature of formation lower than that normal
for a two-pyroxene basalt.
Description
88 pages
Keywords
Topography, Eugene, OR, Creswell Butte, Spencer Butte, Skinner Butte, Gillespie Butte, Geology, Willamette Valley