DETRITAL GEOCHRONOLOGIC, GEOCHEMICAL, AND STRATIGRAPHIC EVIDENCE FOR PALEOGENE SEDIMENT ACCUMULATION AND VOLCANISM ALONG THE ARCWARD MARGIN OF A REMNANT FOREARC BASIN, SOUTH-CENTRAL ALASKA

Upper Paleocene-Eocene strata (Arkose Ridge Fm.) exposed in the southern Talkeetna Mountains of south-central Alaska record nonmarine deposition and volcanism along the arcward margin of a remnant forearc basin modified by Paleogene spreading ridge subduction. Measured sections and lithofacies analyses document chiefly pebble-boulder conglomerate, feldspathic sandstone, and lava flows in westernmost ARF outcrops. At Willow Creek, two sandstone samples reveal three main U-Pb detrital zircon age populations: Latest Cretaceous to Early Paleocene (85-60 Ma; 63% of 189 grains); early Late Cretaceous (100-85 Ma; 30%) and Early Cretaceous to Jurassic (200-100 Ma; 5%). ARF strata unconformably overlie a granitoid pluton that yields Late Cretaceous U-Pb zircon ages (79-69 Ma; 74 grains from three samples). Sparse Late Paleocene detrital zircon ages constrain the maximum depositional age to <59-58 Ma. In the Government Peak area, U-Pb ages of 160 detrital zircon grains from two sandstones reveal a unimodal Late Cretaceous population (97-69 Ma) that overlaps the age of the underlying pluton (86-79 Ma; 24 spot analyses from one sample). Geochemical analyses demonstrate that western ARF lavas are tholeiitic, range from basaltic andesite to andesite in composition, and share geochemical characteristics with continental arcs, including evidence for crustal assimilation. Younger volcanic and intrusive rocks exposed in the nearby Matanuska Valley and Caribou Creek volcanic field yield more primitive/depleted compositions that reflect little crustal contamination. Collectively, our new datasets support tectonic models that invoke spreading ridge subduction beneath the arc/forearc basin during late Paleocene-Eocene time. Latest Cretaceous-Paleocene uplift of arc plutons followed by late Paleocene-early Eocene alluvial-fluvial sediment accumulation and effusive eruptions is consistent with the exhumation/subsidence history expected from ridge subduction. Moreover, geochemical compositions provide evidence for initial crustal extension and magma derivation from an arc-like mantle prior to more widespread extension and eruption of more depleted basalts erupted from a mid-ocean-ridge-like mantle attributable to emplacement of a slab window beneath the forearc basin and remnant arc.