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Date Permissions Signed

5-25-2018

Date of Award

Spring 2001

Document Type

Masters Thesis

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

DeBari, Susan M., 1962-

Second Advisor

Babcock, R. Scott (Randall Scott)

Third Advisor

Sisson, Thomas W.

Abstract

Major element, trace element, and mineral compositions have been determined for four Quaternary mafic monogenetic cinder cones and flows south of Glacier Peak, a dacitic stratovolcano in the northern Cascade arc. The flows are the Whitechuck basalt, and the basaltic andesites of Indian Pass, Lightning Creek, and Dishpan Gap. Whitechuck has high concentrations of AI2O3 (≥ 18 wt.%) and low concentrations of K2O (≤ 0.45 wt.%) and shares similar trace element characteristics with high alumina olivine tholeiites reported in the central and southern Cascades. The three basaltic andesites are calc-alkaline. Indian Pass and Lighting Creek have primitive compositions, with Mg# [100*Mg/(Mg+Fe*)] > 65, Cr > 200 ppm, and Ni > 100 ppm. Most samples have olivine in equilibrium with bulk rock compositions. Rb and Sr are relatively low in most flows, especially at Whitechuck and Dishpan Gap. Sr and La decrease with increasing SiO2 in the Lightning Creek and Dishpan Gap cinder cones, indicating possible mixing with a Sr and La-poor felsic component represented by the most evolved Glacier Peak dacites. Petrographic observations of disequilibrium textures such as xenocrysts, xenoliths, quenched glass inclusions, and strongly-zoned phenocrysts in these two flows also indicate possible melting. All flows are enriched in large ion lithophile elements and light rare earth elements relative to high field strength elements, showing that the mantle beneath Glacier Peak has been fluxed by a hydrous subduction component. Ba/Nb ratios (~40-110) are highest for the three basaltic andesites, indicating that they had the greatest amount of subduction component enrichment. Nb and Ta abundances are highest at Indian Pass (Nb =~7 ppm) and lowest at Whitechuck (Nb = ~2-3), indicating that Indian Pass and the other two basaltic andesites were produced by relatively low degree hydrous melts of depleted mantle. In comparison, the Whitechuck basalts were produced by relatively dry (2O) slightly higher degree melting of a more depleted mantle (similar to NMORB) source. Of the three mantle-domains inferred beneath the Cascades, only a MORB-like mantle and a subduction-fluxed depleted mantle are represented beneath the Glacier Peak region. No OIB-like mantle domain is thus far represented in the Cascades north of the Mt. Rainier region.

Type

Text

Keywords

Mafic cinder cones

Publisher

Western Washington University

OCLC Number

1080644289

Subject – LCSH

Lava flows--Washington (State)--Glacier Peak Region; Geochemistry--Washington (State)--Glacier Peak Region; Petrology--Washington (State)--Glacier Peak Region

Geographic Coverage

Glacier Peak Region (Wash.)

Format

application/pdf

Genre/Form

masters theses

Language

English

Rights

Copying of this document in whole or in part is allowable only for scholarly purposes. It is understood, however, that any copying or publication of this thesis for commercial purposes, or for financial gain, shall not be allowed without the author's written permission.

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