Magma emplacement in a convergent tectonic orogen, southern Revillagigedo Island, southeastern Alaska

Abstract

The western metamorphic belt of the British Columbia – Alaska coastal orogen consists of lithostratigraphic units assigned to the Taku terrane and the Gravina sequence. On Revillagigedo Island and Cleveland Peninsula in the Ketchikan quadrangle of southeastern Alaska, greenschist- and amphibolite-grade schists of the western metamorphic belt are intruded by mid-Cretaceous tonalite and leucotonalite plutons and sills. A number of these igneous bodies are spatially associated with northwest-trending shear zones developed along thrust faults that formed during the main phase of metamorphism of the schists. Observations of field relations and structural fabric in the igneous rocks and the country-rock schist show the following: (i) Along the southern shore of Revillagigedo Island, the Moth Bay pluton was emplaced along a shear zone. Magmatic and submagmatic flow foliation in the southern border zone of the pluton parallels foliations in the sheared country rock; rocks in the interior of the pluton are generally unfoliated. (ii) At Carroll Point a group of sills intrude along the same shear zone. In some of the sills, tabular plagioclase phenocrysts are oriented parallel to the shear fabric. (iii) Foliation-parallel sills lie within a shear zone that separates Gravina sequence and Taku terrane rocks at Point Francis on Cleveland Peninsula. Older sills within the shear zone are deformed, and younger intrusions are unfoliated and cut across the fabric. Based on these and similar observations elsewhere in the area, we suggest the mid-Cretaceous tonalite and leucotonalite magmas in the Ketchikan quadrangle intruded during the regional compressional deformation that formed the shear zones.