POST-EOCENE IGNEOUS AND TECTONIC EFFECTS OF SPREADING CENTER SUBDUCTION UNDER SOUTHEAST ALASKA

Eocene to Quaternary subalkaline igneous rocks intrude the Chugach and Alexander terranes. The Alexander terrane (AXT) of southeastern Alaska occupies the forearc region of a Late Cretaceous-Eocene Andean-type arc that underlies the Coast Mountains along the northwest margin of North America. Collision of the AXT with North America in Jurassic to mid-Cretaceous time was followed by growth of the Chugach accretionary complex on the outboard margin of the AXT. Eocene (50 Ma) anatectic near-trench plutons and associated gold-quartz veins extend for the length of the Chugach terrane in southeastern Alaska, suggesting near-parallel introduction of an oceanic spreading center into the Chugach trench. The timing, location, and chemistry of these plutons are compatible with emplacement above a slab window. After 50 Ma, dextral translational structures, including the Chatham-Denali and Queen Charlotte-Fairweather fault (QCFF) systems, superceded convergent structures and pervasively dissected the Chugach and Alexander terranes. Arc magmatism never resumed, suggesting the trailing oceanic plate may not have been subducted. The Chugach accretionary complex was later intruded by 40-29 Ma ultramafic bodies and by trondhjemitic-tonalitic plutons yielding U-Pb zircon ages as young as 25 Ma. These 50-25 Ma plutons intrude regionally metamorphosed Chugach terrane sediments that yield biotite 40Ar/39Ar cooling ages ranging from 50-35 Ma. The range of igneous and metamorphic ages in the accretionary prism suggests protracted elevated thermal conditions related to a slab window. In the AXT east of the Chatham Strait fault, pull-apart basins containing Paleocene to Miocene fossils formed between northwest-trending dextral faults. Basaltic to rhyolitic subalkaline volcanic rocks that lack the Ta and Nb depletion typical of subduction related rocks intrude these features. Subalkaline granites broadly decrease in age eastward from 40 Ma in western AXT to 12 Ma in the western Coast Mountains. Ubiquitous Recent basalts have similar chemistry. Subalkaline “within plate” chemical compositions for AXT Neogene igneous rocks are compatible with melts that correspond to protracted residence of a slab window beneath the Chugach and Alexander terranes following Eocene truncation of convergent activity on the west by the QCFF.