Search for: All records

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. 

The Easton metamorphic suite of the Northwest Cascades thrust system is an exhumed Jurassic-Cretaceous subduction complex that can be used to test models for subduction zone deformation. Regional blueschist and greenschist within the Easton were accreted in a thermally evolving subduction zone between 150 – 136 Ma. Two units of the Easton metamorphic suite, the Darrington phyllite and the Shuksan greenschist, were originally interpreted as a coherent unit that subducted together in a zone of distributed deformation. The phyllite and greenschist are exposed in a gently SE plunging, steeply inclined, NE vergent, regional synclinorium in the Finney Creek area. New field mapping, microstructural analysis, and thermometry suggest the Darrington phyllite includes a new unit, the Silver phyllite. The Darrington phyllite contains two foliations with the dominant S2 foliation defined by aligned Gr + Ms. The Silver phyllite also preserves two foliations with S1 defined by aligned Ep + Gr in Ab porphyroclasts and relict Ms in fold hinges. The S1 foliation is variably overprinted by the dominant S2 axial planar cleavage of aligned Ms + Chl (148 –140 Ma) that intensifies with proximity to the greenschist contact. In contrast, the Shuksan greenschist dominantly preserves an S1 foliation (~140 Ma) of aligned Ep + Act/Gln + Ms + Chl that is variably overprinted by a weak S2 (140 – 136 Ma) that is axial planar to tight folds in the greenschist and the regional synclinorium. The contact between Silver phyllite and Shuksan greenschist is marked by a Ms + Chl + Ab mylonite that is parallel to the S2 fabric in both units. Raman spectroscopy of carbonaceous material (RSCM) suggests the three units have different thermal histories. Graphite in the Darrington phyllite S2 foliation yields temperatures of 374 – 400 C. The Silver phyllite S1 assemblage records RSCM temperatures of 430 – 450 C while previous Chl ­– Ms thermometry from S2 yielded 310 – 340 C. In contrast, the Shuksan greenschist S1 formed at peak metamorphic conditions of ~360 C. The deformation and thermal history combined with existing Ar/Ar ages suggest that cooling of the Silver phyllite from S1 to S2 was synchronous with prograde metamorphism and formation of S1 in the greenschist during underplating, and that these unit were subducted as discrete tectonic slices rather than as a coherent unit. 

Numerical models of subduction initiation and observations of exhumed subduction complexes indicate that the early stages of subduction are characterized by rapid cooling followed by a prolonged steady thermal state that can last tens of millions of years. Several mechanisms are proposed to drive cooling in subduction zones and include thermal relaxation, exhumation, and underplating, but determining the relative contribution of each mechanism in the history of an exhumed subduction complex can be difficult. The Easton metamorphic suite in the Northwest Cascades of Washington is a Jurassic-Cretaceous subduction complex that records the subduction and accretion of distinct units within a thermally maturing nascent subduction zone. The region preserves an inverted metamorphic sequence with metamorphic temperatures and ages that decrease structurally downward from an early accreted metamorphic sole to younger regionally extensive blueschist facies units. In the metamorphic sole Grt±Cpx amphibolite was metamorphosed at 750-800 C at 1.0 GPa prior to 167 Ma. The amphibolite is underlain by a high temperature Grt-Ab-Gln blueschist that was metamorphosed at ~530 C and 1.0 GPa at 165 Ma. The contact between the units is gradational and the general lack of deformation suggests initial cooling to lower temperatures may have been caused by cooling of the overall subduction zone. Retrograde Lws-Ep-Gln-Ms assemblages suggest that cooling of both the amphibolite and high-grade blueschist units to below 400-500 C was caused by exhumation to 0.7 GPa by 157 Ma. In the regionally extensive blueschist units, Ep-Ab-Chl-Ms±Grt±Lws phyllite was metamorphosed at 430-450 C and 0.7 GPa by 149 Ma. Retrograde fabrics in the phyllite record similar temperatures to peak metamorphic conditions in an underlying Ep-Ab-Gln/Act greenschist/blueschist unit that was accreted and metamorphosed at ≤350 C by ≤140 Ma. The contact between the phyllite and greenschist is marked by a high strain mylonite zone and the combined observations suggest that cooling of the phyllite was driven by underplating of the younger greenschist unit. The observed assemblages and fabrics within the Easton metamorphic suite record cooling as the result of thermal relaxation, underplating, and exhumation at distinctly different times in the subduction history. 

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.