GEOCHEMICAL VARIABILITY AND U-PB DATING OF COEVAL PLUTONIC AND VOLCANIC ROCKS FROM UNALASKA ISLAND, ALASKA

Island arcs are the products of subduction and they preserve geologic evidence of chemical recycling of crustal materials. The petrology and geochemistry of coeval intrusive and extrusive rocks provides critical information on magmatic systems beneath arc volcanoes. At oceanic arcs, it is commonly difficult to compare plutonic and volcanic rocks because either the plutons are not exposed, or the volcanic rocks have been weathered away.

On Unalaska Island, in the Central-Eastern Aleutians coeval plutons and spatially associated volcanic rocks are exposed. Samples from the Miocene Unalaska Formation, Captains Bay Pluton and Shaler Pluton on Unalaska Island were analyzed for major elements, trace elements and radiogenic isotopes. U-Pb dating of zircons in 7 samples indicates that the plutonic and volcanic rocks both formed ca. 10 Ma. Results show that the Captains Bay pluton spans the compositional range from gabbroic (48 SiO₂ wt.%) to granitic (72 SiO₂ wt.%), while Unalaska Formation volcanics display the equivalent range of volcanic compositions (47 SiO₂ wt.% to76 SiO₂ wt.%). Shaler pluton has an intermediate to felsic range of compositions (54 SiO₂ wt.% to 74 SiO₂ wt.%). Trace element abundances in the plutonic and volcanic rocks are quite similar. Relative to primitive mantle, Eu shows subtle positive and negative anomalies, Nb and Ta exhibit negative anomalies and Sr and Pb show positive anomalies that are characteristic of magmatic rocks formed in island arc settings. Nd isotopes show that the of Unalaska Island samples (e_Nd 7.2 to 9.1) are more radiogenic than the central and eastern Aleutians (e_Nd 5.4 to 8.3), while Sr and Hf isotopes of Unalaska Island (⁸⁷Sr/⁸⁶Sr = 0.70297 to 0.70400; e_Hf = +10.36 to +15.91) are of similar composition (⁸⁷Sr/⁸⁶Sr = 0.70291 to 0.70366; e_Hf = +10.68 to +16.80). Pb isotopes (²⁰⁷Pb/²⁰⁴Pb = 15.515 to 15.597) display a wider range of compositions than the central and eastern Aleutians (²⁰⁷Pb/²⁰⁴Pb = 15.546 to 15.631). The data were modeled using MELTS and compared to other Central-Eastern Aleutian samples. Model outputs indicate that the mafic plutonic and volcanic rocks originated from a basaltic parental source while the more evolved samples resulted from mixing of this basaltic melt with a felsic melt.