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Master of Science (MS)
Brown, Edwin H.
Burmester, Russell F.
Babcock, R. Scott (Randall Scott)
The focus of this study is on the metamorphism, plutonism, and structures of the Mt. Buckindy-Snowking region of the North Cascades crystalline core.
In a 15 by 30 km region in the northwest portion of the crystalline core, Cretaceous plutons in a northwest-southeast belt intrude rocks of the Napeequa unit. Metamorphic grade in the country rock ranges from lower greenschist facies in the northwest to amphibolite facies in the southeast. Thermobarometry applied to the equilibrium assemblages garnet-biotite-muscovite-plagioclase and garnet-homblende- plagioclase defines orogen-normal isobars varying from less than 4 Kb in the northwest to over 9 Kb in the south.
Plutons are granodioritic to tonalitic in composition and include, from southeast to northwest, the Downey Creek, Bench Lake, Cyclone Lake, and Jordan Lake plutons. Migmatite, composed of dikes and sills intruding country rock, formed during the emplacement of the Downey Creek and Bench Lake plutons. All of these plutons are presently undated except for a 73 Ma U/Pb zircon date for the Jordan Lake tonalite. However, the Downey Creek and Bench Lake plutons are inferred to be 95 to 96 Ma based on dates obtained from dikes in migmatite associated with the intrusion of these plutons.
The Downey Creek and Bench Lake plutons are interpreted to be syn-tectonic based on the following textural evidence: 1) dikes in the migmatite crosscut metamorphic foliations; 2) dikes in the migmatite display solid-state deformation textures; and 3) plutons display solid-state foliations parallel to the regional metamorphic fabric. There is also mineralogic evidence of emplacement at depths equal to those reached during peak metamorphism. Geobarometry of the Bench Lake pluton, using the A1 content of hornblende, yields pressures of 9.0 Kb (calibration of Schmidt, 1992) and 9.5 Kb (calibration of Hollister et al., 1987). Also, the occurrence of magmatic epidote supports emplacement at depths greater than 6 Kb. Country rock pressure is indicated to be 7-8 Kb from thermobarometry in this region.
A penetrative metamorphic foliation is recorded in all metamorphic and plutonic rocks except for the mostly undeformed Jordan Lake pluton. This fabric dominantly strikes northwest-southeast and dips steeply to moderately to the northeast. Mineral lineations within the foliation plane of the country rock generally trend northwest with a shallow to moderate plunge. Sparse kinematic indicators indicate non-coaxial, dextral shear, as found elsewhere in the crystalline core. Post-metamorphic sinistral shear is recorded in rocks along the west and southwest margin of the Triassic Marblemount meta-quartz diorite. This shear fabric is crosscut by the largely undeformed Jordan Lake pluton. But a sinistral shear zone is also found in the Jordan Lake pluton.
The findings of this study are consistent with an orogenic model involving dextral transpression within a magmatic arc; high-pressure, diachronous metamorphism is best explained by a magmatic loading model.
North Cascades crystalline core, Plutonism, Metamorphism
Western Washington University
Subject – LCSH
Geology, Structural--Washington (State)--Cascade Range; Metamorphic rocks--Washington (State)--Cascade Range; Intrusions (Geology)--Washington (State)--Cascade Range
Cascade Range; Washington (State)
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.
Fluke, Steve M. (Steven Milton), "Metamorphism and Plutonism of the Mt. Buckindy – Snowking Region, North Cascades, Washington" (1992). WWU Graduate School Collection. 801.