DETRITAL ZIRCON CONSTRAINTS ON THE TECTONIC EVOLUTION OF THE GRAVINA BELT, SOUTHEASTERN ALASKA

Upper Jurassic - Lower Cretaceous marine elastic strata and mafic to i ntermediate volcanic rocks of the Gravina belt are part of a complex s uture zone separating the Alexander and Wrangellia terranes on the wes t from the Yukon-Tanana and Stikine terranes to the east. U-Pb ages ha ve been determined on 118 single detrital zircon grains from Gravina s trata in an effort to determine the tectonic setting of the Gravina be lt and the paleoposition of outboard terranes prior to their Late Cret aceous juxtaposition against inboard terranes. Samples from five strat igraphic units yield ages of 105-120 (n = 5), 140-165 (n = 56), 310-38 0 (n = 17), 400-450 (n = 19), 520-560 (n = 5), 920-1310 (n = 5), and 1 755-1955 Ma (n = 5). The 105-120 and 140-165 Ma grains were shed prima rily from are-related plutons that lie outboard of the Gravina belt. T he lack of 120-140 Ma ages coincides with a lull in magmatism in the o utboard are and in the western United States, which suggests that Grav ina strata accumulated during major changes in plate motion along the Cordilleran margin. The 400-560 Ma zircons were derived from rocks of the Alexander terrane which also lie to the west. In contrast, the 310 -380 and >900 Ma grains were apparently shed from inboard regions. Lik ely sources include the Yukon-Tanana and Stikine terranes in the north ern Cordillera and assemblages in the northern California region which contain igneous rocks and detrital zircons of the appropriate ages. O ur data accordingly support models in which the Gravina basin formed i n narrow rift or transtensional basins, whereas the outboard Alexander and Wrangellia terranes were located along the California - Oregon - Washington British Columbia - Alaska margin. Our data are less support ive of models in which the Gravina strata and underlying Alexander and Wrangellia terranes were separated from western North America by a la rge ocean basin, or were located along the coast of Mexico.