Sources of Plutonic Rocks From the Westernmost Talkeetna Island arc: Fractionation of Mantle-derived Magmas and Intracrustal Melting

The Jurassic Talkeetna island arc extends for ~1300 km across southern Alaska from Tonsina in the east, where a complete crustal section is exposed, to the Alaska Peninsula in the west, where only the uppermost part of the arc is exposed. This arc in its entirety represents a substantial volume of material accreted to the North American margin. In this study, we report new geochemical data for the westernmost Talkeetna arc along the Alaska Peninsula between Naknek Lake and Iliamna Lake. The well-studied eastern portion of arc section is characterized by flat REE patterns and low K at a given SiO2, distinguishing these rocks from continental crust. Previous studies of the eastern arc show that the volcanics, upper intrusives, and lower gabbroic units of the crustal section display very similar trends from LREE to HREE, and that all of these rock units are probably linked by a simple fractionation process. The new suite of plutonic rocks collected from the eastern part of the arc range in composition from 42-73 wt. % SiO2. The suite is comprised of amphibole/pyroxene-rich cumulates, gabbros, diorites, and granodiorites, some of which display a tholeiitic trend, and some of which display a medium-K calc-alkaline trend. The rocks represent mid- to upper-crustal exposures, no lower crust or upper mantle is exposed. Analysis of REE patterns for rocks from the western arc indicates two distinct sources for the magmas. Like rocks from the eastern part of the arc, most of the samples exhibit flat REE patterns (La/YbN = 2) over a wide range of compositions (50-70 wt. % SiO2), where REE abundances generally increase with higher silica content. A second set of compositions show concave-up patterns of LREE enrichment and HREE depletion (La/YbN = 14) where abundances decrease with increasing silica. Of these samples, the most silica-rich exhibit positive Eu anomalies. We believe this second group of samples represents partial melting of amphibole-rich rocks within the arc crust, something not observed in the eastern part of the island arc. These preliminary results from the western arc, and continuing XRF, ICP-MS, and isotopic studies, will be used to quantitatively constrain the crustal composition of the westernmost arc section and help determine how magmatic processes in the arc vary along strike from east to west.