EARLY CENOZOIC TALKEETNA MOUNTAINS MAGMATISM IN SOUTH-CENTRAL ALASKA: RECORD OF CRUST-MANTLE VARIATIONS, TERRANE ACCRETION, AND PLUTON EXHUMATION

Paleocene-Eocene plutonic and volcanic rocks of the Talkeetna Mountains (TM) form ~200 km north-south belt across the Wrangellia composite terrane (WCT) in the orocline hinge area of southern Alaska. These rocks are largely bimodal suites, have intra-plate characteristics, contain adakites, and show north-south geochemical and age trends. Rocks in the northern TM, in the suture zone at the leading edge of the WCT, are more geochemically enriched (basalts with ⁸⁷Sr/⁸⁶Sr_(i) 0.7033 to 0.7049 and εNd_(t) 2.0 to 7.4) whereas the southern TM rocks, adjacent to the forearc region at the trailing edge of the WCT are geochemically depleted (basalts with ⁸⁷Sr/⁸⁶Sr_(i) 0.7028 to 0.7032 and εNd_(t) of 8.4 to 10.9). These data reveal an OIB-type mantle in the north, consistent with lithospheric mantle of the accreted WCT (Triassic oceanic plateau) and a MORB-type mantle in the south, consistent with underplating sub-oceanic depleted mantle beneath southern Alaska through a slab window. Granitic rocks and rhyolites in the northern TM show geochemical evidence that they formed by anatexis and assimilation of Kahiltna assemblage flysch which forms the mid to shallow crust in the WCT suture zone. Acidic rocks in the southern TM evolved more directly by fractional crystallization with minor assimilation of Jurassic-Triassic Talkeetna island arc crust which forms the southern portion of the WCT. ⁴⁰Ar/³⁹Ar ages show that the northern TM volcanic rocks are older, ranging from 56 to 49.5 Ma with southern TM volcanic rocks ranging from 49 to 36 Ma, consistent with progressive southward tapping of the mantle reservoirs in the orocline hinge area during rotation of western Alaska and/or transtension on strike-slip faults. Granitic plutons in the WCT suture zone are 64 to 56 Ma. Conglomerate units at the base of the volcanic rocks contain clasts of these granitic plutons indicating uplift and exhumation of the plutons between ~64 and 56 Ma (before onset of volcanism). The TM igneous rocks were not part of a continental margin arc, but were formed during crustal thickening in the WCT suture zone (collisional magmatism) and slab window processes. These results provide a case example for the role that accreted terranes and slab windows can have in continental margin mantle growth as well as crustal growth throughout the northern Cordillera.