U-PB AND 40AR/39AR DETRITAL GEOCHRONOLOGY OF MODERN RIVER SEDIMENTS IN THE WRANGELL MOUNTAINS, ALASKA: IMPROVED CONSTRAINTS ON THE AGE AND TECTONIC EVOLUTION OF JURASSIC-QUATERNARY MAGMATISM

Jurassic-Quaternary magmatic belts crop out for > 200 km across the Wrangell Mountains of eastern Alaska. The temporal-spatial evolution of magmatism is only loosely constrained by bedrock ages from this remote, glaciated landscape. We present U-Pb detrital zircon and ⁴⁰Ar/³⁹Ar detrital volcanic lithic grain analyses on modern sand deposits of glacial-fluvial catchments. The detrital sampling approach allows for large spatial coverage > 12,000 km² and access to material eroded beneath ice. The combination of methods allows characterization of both felsic, zircon-bearing and mafic, zircon-poor igneous rocks.

New single-grain ages, together with published bedrock ages and geochemical data, provide the following results: (1) Low U-Th ratios in all zircons together with ⁴⁰Ar/³⁹Ar ages from volcanic lithic grains indicate that the single-grain ages record magmatism, not metamorphism. (2) Magmatism was nearly continuous from ~155 Ma to ~100 Ma and ~26 Ma to < 1 Ma; 135-125 Ma magmatism was more pronounced than previously recognized. (3) 155-140 Ma magmatic products are limited to the south flank of the Range whereas 140-100 Ma products crop out chiefly along the north flank. ~26 to 17 Ma products crop out only along the north flank, whereas ~17 Ma to <1 Ma bodies crop out throughout the Range. (4) The magmatic history is consistent with a tectonic model involving a late Jurassic-Cretaceous arc (~155-140 Ma Chitina arc, ~140-100 Ma Chisana arc) along the northern Cordillera margin during orthogonal terrane accretion, low-flux magmatism during latest Cretaceous-Paleogene dextral transpressional motion along the Denali fault; and Neogene slab-edge magmatism (Wrangell arc) during flat-slab subduction of the Yakutat microplate. The Wrangell arc experienced a complex migration history due to upper-plate translation and Yakutat flat-slab progression. Between ~17 Ma and ~12 Ma the Wrangell arc migrated trenchward relative to its original position as the upper plate was translated north along the Denali fault. From ~6 Ma to Present the Wrangell arc migrated away from the trench relative to its ~12 Ma to ~6 Ma position, as the leading edge of the Yakutat slab progressed northwestward. These results offer new insight into the evolution of an accretionary margin that experienced discrete pulses of arc magmatism for >155 my.