The Easton metamorphic suite is a Mesozoic era subduction complex in northwest and central Washington, USA, which contains amphibolite-facies units structurally overlying separate high- and low-grade blueschist units. New structural, petrographic, and geochronologic data record a complex history related to Early Jurassic subduction initiation. Two types of amphibolite occur as (1) meter-scale coarse garnet amphibolite blocks with an upper amphibolite- to granulite-facies assemblage and (2) continuous layers of ≤10-m-thick foliated garnet amphibolite. The amphibolite blocks are encased in the foliated amphibolite, quartzose schist, and serpentinite. Garnet Lu-Hf geochronology records prograde garnet growth at 203 Ma in the amphibolite blocks and 183 Ma in the foliated amphibolite unit. In situ titanite U-Pb ages on amphibolite blocks, quartzose schist, and foliated amphibolite cluster at 168−163 Ma, with minor inherited components of up to 200 Ma. Amphibole 40Ar/39Ar cooling ages from the amphibolite blocks are 160−158 Ma, which is slightly younger than previously published 40Ar/39Ar cooling ages of 167−165 Ma from the foliated amphibolite. The deformation-temperature-time history of foliated amphibolite records subduction initiation in the Easton metamorphic suite at ca. 183 Ma, followed by cooling to high-grade blueschist facies, ∼500−600 °C, at ca. 165 Ma, and <400 °C by ca. 160 Ma. The 203 Ma coarse amphibolite blocks may have formed in an earlier metamorphic belt before being incorporated into the newly initiated subduction zone at 183 Ma, though an older age of subduction initiation is possible. Combined with existing data from the lower-grade regional blueschists that lie structurally beneath the high-grade rocks, the Easton metamorphic suite preserves >70 m.y. of subduction metamorphism and deformation. Early Jurassic subduction initiation in both the Easton metamorphic suite and the Franciscan Complex of California, USA, reflects broadly synchronous initiation of east-dipping subduction along portions of the Cordilleran margin by 183−176 Ma.
Skip Nav Destination
Article navigation
Article Contents
Research Article|
May 07, 2025
Early Publication
Early Jurassic subduction initiation recorded in the Easton metamorphic suite, Northwest Cascades (Washington, USA) Available to Purchase
Nicholas S. Cunetta;
Nicholas S. Cunetta
1
Geology Department, Western Washington University, Bellingham, Washington 98225, USA
Search for other works by this author on:
Sean R. Mulcahy;
Sean R. Mulcahy
1
Geology Department, Western Washington University, Bellingham, Washington 98225, USA
Search for other works by this author on:
Elizabeth R. Schermer;
Elizabeth R. Schermer
1
Geology Department, Western Washington University, Bellingham, Washington 98225, USA
Search for other works by this author on:
Matthijs A. Smit
Matthijs A. Smit
2
Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada3
Department of Geosciences, Swedish Museum of Natural History, Frescativägen 40, SE-114 18 Stockholm, Sweden
Search for other works by this author on:
Nicholas S. Cunetta
1
Geology Department, Western Washington University, Bellingham, Washington 98225, USA
Sean R. Mulcahy
1
Geology Department, Western Washington University, Bellingham, Washington 98225, USA
Elizabeth R. Schermer
1
Geology Department, Western Washington University, Bellingham, Washington 98225, USA
Matthijs A. Smit
2
Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada3
Department of Geosciences, Swedish Museum of Natural History, Frescativägen 40, SE-114 18 Stockholm, Sweden
Publisher: Geological Society of America
Received:
22 Nov 2024
Revision Received:
05 Mar 2025
Accepted:
03 Apr 2025
First Online:
07 May 2025
Online ISSN: 1943-2674
Print ISSN: 0016-7606
© 2025 Geological Society of America
GSA Bulletin (2025)
Article history
Received:
22 Nov 2024
Revision Received:
05 Mar 2025
Accepted:
03 Apr 2025
First Online:
07 May 2025
Citation
Nicholas S. Cunetta, Sean R. Mulcahy, Elizabeth R. Schermer, Matthijs A. Smit; Early Jurassic subduction initiation recorded in the Easton metamorphic suite, Northwest Cascades (Washington, USA). GSA Bulletin 2025; doi: https://doi.org/10.1130/B38167.1
Download citation file:
You could not be signed in. Please check your email address / username and password and try again.