NEW INSIGHTS INTO GEOCHEMISTRY OF PALEOGENE VOLCANISM FROM THE TALKEETNA MOUNTAINS, ALASKA

We present preliminary results of new stratigraphic, geochronological, and geochemical analysis of Paleogene volcanic rocks from the Talkeetna Mountains C–4 and parts of the C–3 and B–4 quadrangles, Alaska. This area was mapped at a scale of 1:50,000 during the summer of 2014 by the Alaska Division of Geological & Geophysical Surveys (DGGS) Minerals Section as part of a multi-year effort to understand and improve the geologic framework of the western portion of the Wrangellia terrane. The Paleogene section mapped during the project comprises at least 600 meters of volcanic flows, ash falls, and volcaniclastic breccias ranging in composition from basalts to rhyolites. The section is generally horizontal and unconformably overlies a typical Permian to Triassic Wrangellia stratigraphy as well as Jurassic to Cretaceous metasediments of the Kahiltna assemblage. In the southern portion of the mapping area, the volcanics are cut by a major NE–SW trending fault, which sets a boundary between flow-dominated rocks to the north and highly devitrified ash fall dominated rocks to the south. The age of volcanism in this region has previously been determined to be between 46 and 39 Ma. Although this time period coincides with the Kula Ridge subduction and onset of Pacific plate subduction and widespread arc magmatism in the Alaska Range, the origin of these rocks is still controversial.

In order to better understand the processes involved in the Paleogene magma formation, we have undertaken petrologic and geochemical investigations of whole rock samples from the region and combined them with ⁴⁰Ar/³⁹Ar geochronology of whole rock and mineral separates. Our analysis of the trace element geochemical patterns suggests a volcanic arc origin. Elevated Ba and Pb could be evidence for partial melting of a fluid-somatized mantle wedge, but the possibility of crustal material input cannot be ruled out. Although the Kula Ridge was subducting, influence of a potential slab window is not clearly evident in the geochemical data. To test whether fractional crystallization or an assimilation of partial melts from continental crust and fractional crystallization processes is responsible for formation of the wide range of observed compositions, we present preliminary results from trace element modeling.