Title

Geology and Geochemistry of Volcanic Rocks of the Menagerie Wilderness, Western Cascades, Oregon

Publication Date

5-2002

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Major Advisor

Craig White

Abstract

The early magmatic history of the Cascade Arc is recorded in the Oligocene and early Miocene volcanics of the Western Cascade province. These rocks consist chiefly of silicic tuffs, tholeiitic basalts and basaltic andesites. This study focuses on a suite of volcanics exposed within the Menagerie Wilderness, east of Sweet Home, Oregon and immediately north of the Middle Santiam River. There, three main rhyolite centers have been identified that are overlain in part by High Cascade mafic volcanics. The study area is on the northern edge of a series of rhyolite exposures that define a rough circle approximately 15 km in diameter, and which may represent an eroded caldera complex. Silicic lavas in the Menagerie area range in composition from medium K rhyolite to trachydacite (high K) and dacite (low K). The three silicic centers were distinguished mainly through petrographic characteristics. Phenocryst assemblages include plagioclase-hornblende-magnetite (Rooster Rock rhyolite), plagioclase-quartz-magnetite (Soda Fork rhyolite) and quartz-plagioclase-biotite-hornblende-magnetite (Moose Mountain rhyolite).

The silicic rocks related to the three domes in the study area are similar in terms of mineral content and overall chemical composition. Despite this, it is very unlikely that the rhyolites are closely related to one another in a petrologic sense. Rather, the field units appear to be separate magmatic pulses that are unrelated by any simple petrologic process. It is likely, however, that the rocks underwent similar petrogenetic processes separately from one another.

Based on trace element and REE chemistry it appears that no single process of magmatic differentiation can account for the rhyolites. Chondrite normalized REE plots have moderate Eu anomalies and flat MREE-HREE patterns, consistent with fractionation of plagioclase and hornblende. Comparisons to other silicic centers show that the Menagerie rocks share affinities with High Cascade rocks that have been derived through fractionation of a mafic parent. However, variations in excluded trace element ratios suggest that some component of partial melting may have occurred. The most plausible scenario based on the available data suggests that some combination of FC and partial melting may have played a role in the differentiation of unidentified mafic magmas to create the Menagerie rhyolites.

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