Download Abstract An abstract of the thesis of Christy L. Lee for the

Abstract
An abstract of the thesis of Christy L. Lee for the Master of Science in Geology
presented November 9, 2000.
Title: Magmatic processes operating during the middle to late Pliocene and Pleistocene
along the Cascades - Basin and Range transition zone near 43° North.
A geochemical and structural transition zone exists between the Cascades and
Basin and Range provinces in Oregon. Geologic mapping in this area has allowed the
definition of six volcanic rock sequences: 1) interfingered alkali-rich basaltic
trachyandesite to trachydacite lava flows and low-alkali basaltic andesite lava flows, 2)
HAOT lava flows, 3) basaltic andesite lava flows, 4) andesite lava flows and vent
deposits with minor basaltic andesite lava flows, 5) HAB lava flows, and 6)
interfingered basaltic andesite, andesite, and trachyandesite lava flows.
Geochemical models of sequence 1 alkali-rich rocks suggest the fractionation of
the parent assemblage plag (An55)+magn+ol±aug at relatively dry (H2O < 0.4 weight
percent) and shallow (P = 1.5 to 3 kbar (< 9 km)) initial conditions. As water content
increases, augite and olivine disappear to form the daughter assemblage plag
(An29)+opx+magn+amph+ap. Approximately 50 percent fractionation combined with
contamination by a LIL and LREE enriched and HFSE and HREE depleted contaminant
account for the observed geochemical trends in these rocks.
Sequence 2 HAOT have similar geochemistry to N-MORB and are LIL, HFSE,
LREE and incompatible element enriched when compared to the composition of
primitive mantle. In addition, sequence 2 HAOT geochemical trends are more similar to
HAOT erupted along the Cascades than HAOT from the Basin and Range.
Andesitic lavas of sequences 1 (low-alkali), 3, 4, and 6 show evidence of batch
melting and mixing in their scatter on geochemical diagrams and in their textures (e.g.,
complex plagioclase zoning patterns, abundant inclusions in plagioclase phenocrysts,
and multiple plagioclase phenocryst populations).
The variety of lava types produced reflect multiple magmatic processes that may
be distinctive of the Cascades - Basin and Range transition zone. HAOT and HAB
lavas were able to rise to the surface with little to no ponding in the crust. Magmas were
able to rise to shallow levels where they underwent fractionation and contamination to
produce alkali-rich trachytic suites. Multiple andesitic lavas are most likely the product
of batch melting and combined fractionation, contamination and magma mixing. These
sequences are consistent with extension in the transition zone during the middle to late
Pliocene and Pleistocene.
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