Timing of subduction initiation and deformation history of amphibolite and blueschist in the Easton metamorphic suite, Northwest Cascades, Washington

Author

Nick S. Cunetta, Western Washington UniversityFollow

Date of Award

Winter 2024

Document Type

Masters Thesis

Department or Program Affiliation

Geology

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Schermer, Elizabeth, 1959-

Second Advisor

Mulcahy, Sean

Third Advisor

Walowski, Kristina

Abstract

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

Type

Text

Keywords

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

Publisher

Western Washington University

OCLC Number

1426862160

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.)

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 document for commercial purposes, or for financial gain, shall not be allowed without the author’s written permission.

Rights Statement

http://rightsstatements.org/vocab/NKC/1.0/

Recommended Citation

Cunetta, Nick S., "Timing of subduction initiation and deformation history of amphibolite and blueschist in the Easton metamorphic suite, Northwest Cascades, Washington" (2024). WWU Graduate School Collection. 1270.
https://cedar.wwu.edu/wwuet/1270