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Master of Science (MS)
Babcock, R. Scott (Randall Scott)
Brown, Edwin H.
Pevear, David R.
The Lake Ann stock is a two pyroxene quartz monzodiorite to granodiorite epizonal intrusive emplaced into the Shuksan thrust fault in the latest Pliocene. It is similar to other Late Tertiary Cascade intrusives in mineralogy, texture, chemistry, and setting. These similarities are attributable to near surface emplacement, quick cooling and magmatic arc setting. Epizonal emplacement is indicated by fine, equigranular textures, granophyre, late stage alteration, late miarolitic dikes, plagioclase resorbed by alkali feldspar, and low pressure contact metamorphic assemblages. Temperature of emplacement was between 600°C and 925°C; pressure was less than 2 kb. Chemistry of individual samples and variation diagrams closely resemble those of Sierra and Cascade arc suites.
Petrologic data from the stock are compared to two recently developed theories of magmatic evolution, magma mixing and the ''restite'' model. Evidence for formation of the stock by mixing of basaltic and rhyolitic magmas is equivocal. However, complexily zoned plagioclase crystals, the chemical composition, and the stock’s magmatic arc setting suggest that the stock contains a small restite component from an igneous source.
The Late Cenozoic history of the Lake Ann area is:
1) eruption of the Hannagan volcanic rocks about 3.4 Ma and emplacement and cooling of the Lake Ann stock about 2.5 Ma
2) a lull in magmatism follows and erosion unroofs the stock
3) Swift Creek volcanic rocks erupt and mantle the topography
4) valleys develop in the Swift Creek volcanics and are filled with flows of Mt. Baker andesite.
The timing and westward migration of magmatism fits the regional pattern which may be related to episodic accelerated spreading of the Juan de Fuca Ridge.
Geology, Petrology, Lake Ann, North Cascades
Western Washington University
Lake Ann Region (Wash.)
Copying of this thesis 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.
James, Eric William, "Geology and Petrology of the Lake Ann Stock and Associated Rocks" (1980). WWU Graduate School Collection. 733.