POST-ACCRETION MAGMATISM WITHIN THE KUIU-ETOLIN IGNEOUS BELT, SOUTHEASTERN ALASKA

Abstract

Petrological and geochronological studies of Tertiary granite and gabbro plutons in southeastern Alaska provide information about the Mid-Miocene post-accretionary magmatic history of the region. The Burnett Inlet Igneous Complex is a 20 Ma bimodal granite–gabbro complex located on Etolin Island and adjacent areas in central southeastern Alaska. It consists of three main members: granite, alkali granite, and a gabbro–diorite unit consisting of intermingled mafic, hybrid and granitic rocks. Mafic magmatic enclaves are ubiquitous in the complex. They occur as isolated inclusions in the silicic plutons and as packed masses in the intermingled zones of the gabbro–diorite unit. The mafic enclaves typically display round and pillow-like shapes, igneous textures, and chilled margins, and thus denote the mingling of mafic and felsic magmas. Petrographic and geochemical features indicate that the complex developed via a combination of fractional crystallization, crystal accumulation and magma mixing. Systematic changes in mineralogy and coherent, curvilinear trends on geochemical diagrams indicate that the mafic and felsic magmas each evolved separately, mainly by crystal accumulation and fractionation. Magma-mixing textures, such as micro-inclusions and quartz xenocrysts in some mafic rocks, suggest mafic–felsic magma mixing and hybridization. However, the compositional and viscosity differences precluded bulk mixing between end-member granite and basalt magmas. Mixing probably involved intermediate magmas, whose smaller differences in composition and viscosity would have permitted hybridization. Both the mafic and felsic units have within-plate geochemical characteristics, indicating that post-accretionary magmatism occurred within an overall extensional tectonic setting. Distributions of apatite fission-track lengths and mineral-cooling curves imply that the magmatic rocks cooled to less than 100°C within 5 million years of emplacement. The complex is interpreted to have initially evolved as a shallow-level silicic magma chamber heated by underplated basaltic magma derived from partial melting of enriched upper mantle. Subsequent invasion of the silicic magma chamber by basaltic intrusions induced an explosive eruption phase, followed by rapid crystallization and cooling of the entire complex.