U-PB GEOCHRONOLOGY AND HF ISOTOPE ANALYSIS OF DETRITAL ZIRCONS FROM THE GRAVINA BELT, SOUTHEAST ALASKA

The Gravina belt consists of Upper Jurassic through mid-Cretaceous marine clastic strata and mafic to intermediate volcanic rocks that occur along the western flank of the Coast Mountains in southeast Alaska. These strata rest unconformably on Triassic and older rocks of the Alexander and Wrangellia terranes to the west, and are overthrust by Triassic and older rocks of the Taku and Yukon-Tanana terranes to the east. Detrital zircons from eight samples have been analyzed for U-Pb geochronology and Hf isotope geochemistry to evaluate provenance linkages with these inboard and outboard terranes, as well as coeval plutons of the Coast Mountains batholith. Our data, together with previous U-Pb analyses by Kapp et al. (1998) and Gehrels (2000), record dominant age groups of 105-115, 120-130, and 140-165, subordinate age groups of 200-250, 320-390, 400-480, and 520-560 Ma, as well as a minor proportion of 920-1310, 1755-1980, and 2400-2800 grains. Peak ages for these groups are 113, 122, 154, 228, 340, 432 and 550 Ma. The timing and proportions of 105-130 Ma and 140-165 Ma age groups are very similar to the record of plutonism in the adjacent Coast Mountains Batholith (CMB). The Hf isotope compositions for 105-165 Ma grains are also similar to the juvenile to intermediate values reported by Cecil et al. (2011) from the CMB. Hf isotopic compositions for the 400-480 Ma grains are highly juvenile and resemble Hf isotopic compositions of igneous and detrital grains of the same age from the Alexander terrane. Collectively these data support previous interpretations that much of the detritus in the Gravina Belt was shed from the outboard Alexander terrane and Jura-Cretaceous magmatic arc. The significance of 320-390 Ma and >1.0 Ga grains in the Gravina belt remains uncertain, as grains of the same age and intermediate Hf isotopic signatures occur in both the Alexander and Yukon-Tanana terranes. The present data are therefore insufficient to test the hypothesis of Sigloch and Mihalynuk (2013) that the Gravina Belt was separated from the Yukon-Tanana terrane by a Jura-Cretaceous subduction zone.