ALEUTIAN ISLAND ARC MAGMAGENESIS REVISTED: GEOCHEMICAL EVIDENCE FROM UNALASKA VOLCANIC AND PLUTONIC ROCKS

Studying island arcs provides answers to important questions about the petrogenesis of crustal rocks because they are major sites of felsic melt generation and crustal recycling into the mantle. Intraoceanic arcs are particularly useful tectonic settings to investigate these processes because they generate chemically distinct arc magmas. The Aleutian island arc is of particular interest because of its extensive record of historic volcanism, well-exposed plutonic rocks, and its crustal thickness comparable to that of continental crust. The present Aleutian arc was created due to past subduction of the Kula plate and subsequent subduction of the Pacific plate beneath the North American plate, with the western arc forming a mature oceanic island arc built on old oceanic crust and the Alaska Peninsula, forming a continental arc built on allochthonous terranes accreted to the North American continent in the Phanerozoic. Although the Aleutians have these intriguing features, only cursory petrologic studies have been completed on the plutonic rocks compared to extensive studies on volcanic rocks.

As a result of the dearth of modern geochemical data and radiometric dating of the Aleutian arc, we have initiated a geochemical and petrologic study of the igneous rocks of Unalaska Island in the Western Aleutians with samples of volcanics from the Unalaska Formation and the plutons that intrude them. Preliminary major element results indicate that the volcanic rocks range from basaltic to andesitic (47%-62% SiO₂) while plutonic rocks have a greater compositional range from gabbroic to granitic (51%-74% SiO₂). Trace element analysis shows that Sc/MgO ranges from 0.0009-0.0028, Cr/MgO from 0.0008-0.0086 and Ni/MgO from 0.0007-0.0040 for plutonic rocks, while volcanic Sc/MgO ranges from 0.0015-0.0031, Cr/MgO from 0.0028-0.0042 and Ni/MgO from 0.0007-0.0040. REE data of La/Yb (4.6-8.3) for plutonic rocks indicate that they are depleted compared to volcanic rocks (4.7-14.2). These results indicate that the plutonics and volcanics originated from similar magma sources with the plutonics being more evolved than volcanics and could also provide a clearer understanding of the processes that occur during the generation of continental crust.