EOCENE MAGMAS AS PROBES OF WRANGELLIA COMPOSITE TERRANE CRUST IN THE TALKEETNA MOUNTAINS, ALASKA

Eocene volcanic rocks in the southern Talkeetna Mountains are part of a 90 km-long northwest trending volcanic graben that is oriented perpendicular to the trend of the Aleutian arc. The magmatic rocks intrude and overlie the Wrangellia-Peninsular terrane boundary and range in composition from basalt through rhyolite, with most being mafic to intermediate in composition. Basalts from across the volcanic field are characterized by flat REE patterns and low Ba/Nb, whereas intermediate to felsic samples show elevated Ba/Nb and positive correlations between La/Sm and SiO2. Two distinct REE vs. SiO2 trends are present, with samples from northern part of the volcanic field having higher La/Yb and La/Sm relative to SiO2 than samples from the southern part of the field. Mixing between a mafic magma and two different crustal sources is also evident in the isotopic composition of the felsic end members: rhyolite from the southern part has a mantle-like isotopic signature (87Sr/86Sr(i)= 0.70384; 143Nd/144Nd(i) = 0.513001), whereas rhyolite from the north has a more evolved isotopic signature (87Sr/86r(i) = 0.70639; 143Nd/144d(i) = 0.512798). Intermediate rocks from both the northern and southern end of the volcanic field have isotopic compositions intermediate between the basalt and the rhyolite samples from those areas. We interpret the difference in isotopic composition as resulting from anatexis and AFC processes involving crust of varying age and composition across the Wrangellia Composite terrane. Mantle-derived magmas interacted with juvenile crustal rocks associated with the Jurassic arc of the Peninsular terrane to the south, and a more evolved crustal source associated with Wrangellia to the north. The orientation of the volcanic field is consistent with right-lateral simple shear models. Its formation may have been related to an increase in the inboard transfer of strain from the Border Ranges fault zone to inboard faults such as the Denali and Tintina faults. This transfer of strain may have increased during the Eocene due to more orthogonal convergence along the western limb of the Border Ranges fault caused by the 65-50 Ma counterclockwise rotation of western Alaska.