Abstract

The mid-Cretaceous, cale-alkaline, LREE-enriched plutons on southern Revillagigedo Island, southeastern Alaska, form two distinct groups: (1) biotite-epidote-hornblende tonalite of the Moth Bay pluton and (2) biotite-bearing, hornblende-absent leucotonalite. Higher total REE and lower Na2O, Al2O3, and Sr distinguish the Moth Bay tonalite from the leucotonalite. The plutons are surrounded by narrow contact metamorphic aureoles containing kyanite and staurolite. Pressure estimates for emplacement based on metamorphic conditions of contact rocks and from igneous amphibole compositions are ∼8-9 kbar. Detailed analyses of pluton/host rock relationships indicate that pluton emplacement was synchronous with orogen deformation. Pre-emplacement thrusting placed the metavolcanic and metasedimentary rocks of the Taku terrane structurally above the younger metasedimentary rocks of the Gravina belt. The contact between these units was subsequently cut by the Moth Bay pluton. The southern margin of the Moth Bay pluton has been transformed to blastomylonite. Contact metamorphic textures and undeformed dikes which cut the blastomylonite indicate that shearing did not outlast crystallization of the pluton. A still later phase of deformation transposed the pluton-related structures and generated a northeast-dipping, ductile thrust fault (Southern Revillagigedo shear zone) along the southern shore of Revillagigedo Island. The shear zone and earlier structures all trend approximately northwest.

Fission-track age determinations and track-length measurements in apatite were used to determine the cooling histories of the plutonic units. Fission track ages from the western part of the study area are 51.3 ± 4.0 Ma (apatite), 57.8 ± 4.4 Ma (zircon), and 69.8 ± 4.2 Ma (sphene); eastward, toward the Coast batholith, fission track ages are ∼10 m.y. younger. These younger ages may result from a tectonic/thermal event associated with batholith emplacement. This same event may have generated north-trending structures that overprint older northwest-trending structures on the east side of the study area.

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