ROCK AND AGE RELATIONSHIPS OF THE TALKEETNA FOREARC SUBDUCTION COMPLEX IN THE NELCHINA AREA, SOUTHERN ALASKA

Subduction-zone processes are still poorly understood due to the paucity of exposures and the complexity of rock relationships within the accretionary prism. A remarkably well-preserved exposure of subduction-related rocks is located at the foot of Nelchina glacier in south-central Alaska. Here, the crystalline basement of the Talkeetna volcanic arc contacts the mélange of its related accretionary complex along the Border Ranges fault. I mapped and sampled the mélange exposures of the complex in July 2016. The mélange consists of relatively undeformed argillite, metavolcanic rocks, and chert, as well as a ~100-m-diameter block of pillow basalt. Preliminary x-ray fluorescence data shows that this pillow basalt formed in a within-plate setting, suggesting it is unrelated to Talkeetna arc volcanism. Detailed compositional data from a transect across this basalt will show effects of progressive alteration from core to rim of the block. A zircon U-Pb age of a gabbro from the Talkeetna arc basement is 188.9 ± 2.2 Ma, which is consistent with the age of Talkeetna arc volcanism. A zircon U-Pb age of a felsic dike that crosscuts the mélange is 53.0 ± 0.9 Ma, which is both consistent with the ages of near-trench plutonic rocks across southern Alaska, and sets a minimum age of mélange deformation.