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Date of Award

Winter 2024

Document Type

Masters Thesis

Department or Program Affiliation


Degree Name

Master of Science (MS)



First Advisor

Schermer, Elizabeth, 1959-

Second Advisor

Mulcahy, Sean

Third Advisor

Walowski, Kristina


High-temperature metamorphic soles form in nascent subduction zones when a thin sliver of newly subducting oceanic plate is accreted and metamorphosed by the overlying mantle. The Easton metamorphic suite is a Jurassic to Cretaceous subduction complex in northwest Washington state that preserves a relatively coherent metamorphic sole overlain by serpentinized peridotite. Within the metamorphic sole, amphibolite-facies units structurally overlie a high-grade blueschist unit, with shear zones marking major contacts. Two types of amphibolite were identified, including meter-scale, coarse-grained diopside-bearing garnet amphibolite blocks with an upper amphibolite to granulite facies assemblage and continuous layers of fine to medium-grained foliated amphibolite. The coarse amphibolite blocks are surrounded by garnet-hornblende-white mica quartzose schist, foliated amphibolite, and serpentinite. Garnet major and trace-element geochemistry show zoning patterns consistent with prograde garnet growth. Garnet cores have elevated Lu, suggesting that garnet-whole rock 176Lu-176Hf ages reflect prograde to peak metamorphism. Two garnet-whole rock 176Lu-176Hf isochrons from the coarse amphibolite blocks are ~203 Ma. A garnet-whole rock 176Lu-176Hf isochron from the foliated amphibolite is ~183 Ma, ~16 m.y. older than a previously published 40Ar/39Ar hornblende cooling age from the same sample. Combined with prior cooling ages and thermometry data, the ~183 Ma age is interpreted to reflect the initiation of subduction in the Easton metamorphic suite. The foliated amphibolite accreted at ~183 Ma at ~760°C, followed by cooling to high-grade blueschist-facies at ~500-600°C at ~165 Ma and




metamorphic sole, subduction initiation, amphibolite, blueschist, Shuksan, Easton, Northwest Cascades, garnet Lu-Hf, shear zones, geology


Western Washington University

OCLC Number


Subject – LCSH

Metamorphism (Geology)--North Cascades (B.C. and Wash.); Shear zones (Geology)--North Cascades (B.C. and Wash.); Subduction zones--North Cascades (B.C. and Wash.); Rock deformation--North Cascades (B.C. and Wash.); Amphibolite--North Cascades (B.C. and Wash.); Schists--North Cascades (B.C. and Wash.); Metamorphic rocks--North Cascades (B.C. and Wash.); North Cascades (B.C. and Wash.)

Geographic Coverage

North Cascades (B.C. and Wash.)




masters theses




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Rights Statement

DR Fig2_GrtLAICPMS.xlsx (152 kB)
Data Repository Figure 2

Available for download on Saturday, March 15, 2025

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