The rock record of east-dipping Jurassic subduction along the western Cordilleran margin
Sean Mulcahy, Western Washington University
Elizabeth Schermer, Western Washington University
Nicholas Cunetta, Western Washington University
Abstract
The age and polarity of subduction along the western margin of the North American Cordillera in the Jurassic and Cretaceous are widely debated. The rock record of this subduction is preserved in subduction-accretion complexes that span the length of the Cordilleran margin where the metamorphic age of the earliest formed rocks dates subduction initiation and the structural patterns of accreted units constrain the polarity of subduction. The Easton metamorphic suite and associated terranes in the San Juan Islands of Washington contain Jurassic to Cretaceous oceanic rocks that have been interpreted to record subduction initiation followed by prolonged cooling during continued subduction-accretion. The structurally highest levels of the Easton preserves an inverted metamorphic gradient with granulite and amphibolite structurally above Na-amphibole schist. The granulite to amphibolite facies assemblages occur as coarse grained Grt-Cpx-Amp blocks within continuous layers of fine to medium-grained foliated Grt-amphibolite. Both amphibolite types are variably retrograded to blueschist facies. The structurally lower and younger Na-amphibole schist contains Ep-Ms-Gln/Brs±Grt±Pl schist and Grt-Gln/Brs-Omp eclogite. Garnet major and trace element geochemistry combined with Lu-Hf geochronology record prograde to peak metamorphism at ~203 Ma in the coarse-grained Grt-Cpx-Amp blocks and ~183 Ma in the foliated Grt-amphibolite. The 203 Ma age of Grt-Cpx-Amp blocks records a previously unrecognized phase of Triassic metamorphism in the Easton that may be associated with an earlier subduction or arc related event. The garnet age of the foliated amphibolite is interpreted to record subduction initiation in the Easton at 183 Ma. In-situ U-Pb titanite ages of 168-162 Ma from the amphibolite blocks and foliated amphibolite suggest both units were juxtaposed prior to 168 Ma. The titanite ages overlap with previously published 40Ar/39Ar ages and are interpreted to reflect cooling during exhumation and underplating of structurally lower and colder Na-amphibole schist.
Structurally lower regional blueschist assemblages dominated by metagraywacke and phyllite display a general decrease in metamorphic age and grade structurally downward and to the west. A continuous temperature-time history from the ~183 Ma amphibolite to the lowest metagraywackes at ~124 Ma suggests the Easton and related units record >50 million years of subduction metamorphism and accretion. The prolonged HP/LT metamorphism and the progressive decrease in metamorphic grade and age from east to west is consistent with east-dipping (west facing) subduction. When combined with data from the Franciscan complex in California, the rock record of subduction reflects broadly synchronous initiation of east-dipping subduction along portions of the Cordilleran margin.
Session
Subduction zone geology