PALEOZOIC MAGMATISM IN THE WRANGELLIA TERRANE CONSTRAINED BY U-PB GEOCHRONOLOGY OF MODERN RIVER DETRITAL ZIRCONS, WRANGELL MOUNTAINS, ALASKAN CORDILLERA

Accretion of the Alexander-Wrangellia-Peninsular composite terrane represents one of the largest additions of crust to western North America during Phanerozoic time. Assembly of the composite terrane prior to accretion remains enigmatic, including the timing and nature of Paleozoic magmatism. U/Pb geochronology of detrital zircons and ⁴⁰Ar/³⁹ Ar geochronology of detrital igneous grains from modern rivers encircling the type locality of Wrangellia in the Wrangell Mountains provides new age constraints on Paleozoic magmatism. Sampled rivers drain >15,000 km² of partly glaciated bedrock assigned to Wrangellia, including Carboniferous-Permian volcanic and sedimentary rocks (Skolai Group). Only a handful of Paleozoic bedrock ages have been reported from the river catchments. Cenozoic detrital ages are most abundant in the sampled rivers, reflecting erosion of the <30 Ma Wrangell volcanic belt. However, Late Devonian to Permian (380-250 Ma) ages are a persistent secondary population, representing 427 of >2700 zircon ages in 15 of 17 rivers and 61 of 1312 igneous grain ages in 7 of 15 rivers. Nearly all (99%) Late Paleozoic detrital zircons yield U/Th values <10, indicating an igneous provenance, although some may have been recycled from Mesozoic sedimentary strata. Abundant ~320 to ~275 Ma ages in fifteen rivers suggest nearly continuous Middle Pennsylvanian to Early Permian magmatism. Our results from northern Wrangellia support recent geochronologic studies that report Late Pennsylvanian to Early Permian magmatism within southern Wrangellia and the Peninsular and Alexander terranes (Bacon et al., 2012; Beranek et al., 2014), indicating the three terranes may have shared a similar tectonic history since late Paleozoic time. Moreover, our results also demonstrate the utility of detrital geochronology from modern river sediments to elucidate the geochronology of glaciated bedrock.