Sponsor
Portland State University. Department of Geology
First Advisor
Michael L. Cummings
Term of Graduation
Fall 2000
Date of Publication
11-9-2000
Document Type
Thesis
Degree Name
Master of Science (M.S.) in Geology
Department
Geology
Language
English
Subjects
Magmatism -- Oregon -- Klamath County, Volcanic ash tuff etc -- Oregon -- Klamath County, Stratigraphic geology -- Pliocene, Stratigraphic geology -- Pleistocene, Stratigraphic geology
Physical Description
1 online resource (2, ix, 162 pages)
Abstract
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.
Rights
In Copyright. URI: http://rightsstatements.org/vocab/InC/1.0/ This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).
Recommended Citation
Lee, Christy L., "Magmatic Processes Operating During the Middle to Late Pliocene and Pleistocene Along the Transition Zone Near 43º North" (2000). Dissertations and Theses. Paper 6748.