THE ALEUTIAN ARC PUZZLE: PUTTING TOGETHER THE CLUES OF VOLCANISM AND PLUTONISM ON UNALASKA ISLAND

Island arcs are major sites for felsic melt generation, initial development of continental crust, and the recycling of crustal materials back into the mantle. To better understand these magmatic processes and sources, contemporaneous plutonic and volcanic samples need to be studied in a range of arc systems. However, the erosion of extrusive rocks needed to expose co-genetic plutonic rocks means that both types of rocks are typically not found together and their relationships are difficult to determine. The Aleutian island arc is of particular interest due to an extensive record of historic volcanism and well-exposed plutonic rocks and the lavas they intrude, yet only cursory petrologic studies have been completed on plutonic rocks compared to the volcanics. Plutonic and volcanic rocks exposed on Unalaska Island have been analyzed and are good representatives of semi-contemporaneous extrusive and intrusive arc magmatism in the Central-Eastern Aleutians. New, high-quality geochemical and geochronologic data from Unalaska provide important insights into magmatic processes occurring in arc magma chambers and the temporal and petrogenetic relationships between the plutons and associated lavas.

Major element data indicate that the average composition of volcanic and plutonic rocks are similar. The volcanics are andesitic (59% SiO₂) while the composition of plutonics is dioritic (58% SiO₂). Incompatible trace element data also show this similarity, but some of the plutonics are enriched in LREE and depleted in HREE (La_N/Sm_N = 1.93-3.01; Sm_N/Yb_N = 2.1-2.9) compared to the volcanics (La_N/Sm_N = 1.43-2.2; Sm_N/Yb_N = 1.3-1.7), which may be evidence of deeper, higher pressure fractional crystallization of the plutonics. Initial U-Pb zircon analyses reveal a range of ages from ~10 to ~12 Ma in the Captain’s Bay pluton and ~10 Ma in the Unalaska Formation. Results indicate that the plutonics and volcanics are petrogenetically and temporally related but subtle differences in incompatible trace elements and radiogenic isotopes suggest differences in fractional crystallization histories, late assimilation of the country rock by plutons and/or slightly different source components within the mantle wedge. Observations such as these are needed to understand the magmatic processes that create juvenile continental crust.