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Exhumed high pressure/low temperature metamorphic belts provide evidence of subduction processes at depth, but the mechanisms by which subduction zones initiate and evolve to steady thermal state remain contested due to a paucity of recovered subduction infancy rocks. The Easton metamorphic suite in northwest Washington is a Jurassic-Cretaceous subduction complex that preserves a high-grade metamorphic sole inferred to have formed during subduction initiation. Near Gee Point, WA, coherent garnet amphibolite, hornblende-bearing white mica quartzose schist, and underlying garnet blueschist are characterized by their proximity to overlying serpentinized peridotite and the presence of blocks and layers of foliated and unfoliated metasomatic rock. These high-grade rocks are exposed in a series of northeast vergent, steeply inclined, tight-isoclinal folds that post-date regional high pressure/low temperature metamorphism. Two shear zones dip steeply to the southwest, concordant to adjacent blueschist-greenschist, quartzose schist, and amphibolite fabrics. Pervasive retrogression from amphibolite to blueschist-greenschist facies from high to low structural levels is documented at the map, outcrop, and micro scale. Amphibolites contain a foliation defined by aligned hornblende, or in places are weakly foliated. Retrogression of amphibolite is associated with a well-developed glaucophane-white mica-chlorite fabric that wraps older garnet and hornblende, with pargasite cores rimmed by glaucophane and chlorite. Both garnet blueschist and quartzose schist preserve at least one younger foliation. Prior 40Ar/39Ar thermochronology and thermobarometry are consistent with a cooling subduction zone, from amphibolite facies conditions of 760C at >167 Ma to <400C at ~163 Ma. Paired garnet 176Lu-176Hf geochronology and 40Ar/39Ar thermochronology samples will elucidate the timing of (1) proposed subduction initiation, (2) early subduction zone cooling rates, and (3) two shear zones that deformed the Easton metamorphic suite.
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Early Jurassic subduction initiation recorded in the Easton metamorphic suite, Northwest Cascades (Washington, USA)
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.
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- Award ID(s):
- 1948584
- PAR ID:
- 10588971
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Bulletin
- ISSN:
- 0016-7606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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