PETROLOGY AND GEOCHEMISTRY OF ADAK ISLAND PLUTONIC XENOLITHS: IMPLICATIONS FOR CRUSTAL DIFFERENTIATION IN THE ALEUTIAN ISLAND ARC

Plutonic xenoliths from Adagdak Volcano & Mt. Moffett (Adak Island, central Aleutian Island Arc) provide a rare opportunity to study the lower crust of an active island arc. Previous studies have documented in detail the petrologic and chemical characteristics of 8 Adagdak and 15 Mt. Moffett xenoliths (Conrad & Kay, 1984; Debari et. al., 1987) – here, we expand these studies to a much larger xenolith suite from these localities, collected by S. Mahlburg Kay and R. W. Kay. We investigate the petrology and mineral chemistry of a broader xenolith suite (Adagdak, n = 45; Mt. Moffett, n = 21) in order to explore the full lithologic variation of the xenoliths and to lay the groundwork for future geochemical studies.

The expanded sample suite of Adagdak xenoliths comprise dunite, wehrlite, amphibole + olivine clinopyroxenite, amphibolite, & amphibole gabbro cumulates, similar to previously documented lithologies. Olivine Mg#s in dunites, wehrlites, & clinopyroxenites vary from 85-87, 84-86, & 86, respectively, corresponding to equilibrium melts with Mg#s of 63-68. Calculated equilibrium melt Mg#s from clinopyroxene compositions in the amphibolites & amphibole gabbros vary from 59-46. Dunites contain spinel as the sole oxide phase (Cr# 0.33-0.40) whereas, in the amphibole gabbros, titanomagnetite occurs as abundant (5-7 modal %), large (up to 500μm) crystals. Plagioclase first appears as an interstitial phase (8 modal %) in the wehrlites (An = 84-85%) and becomes a dominant phase (35-50 modal %) in the amphibole gabbros (An = 91-96%).

Thermobarometry & oxygenbarometry place dunite crystallization at 950-970^oC (Li et. al., 1995), 0.79-0.95 GPa (Köhler & Brey, 1990), and +2.2-2.4 ΔQFM (Ballhaus, 1991), indicating lower crustal, relatively oxidizing conditions. Plagioclase-amphibole thermometry for the amphibole gabbros suggest formation at 940-1000^oC (Holland & Blundy, 1994). These updated constraints on chemistry & conditions of crystallization are in agreement with the original interpretation of DeBari et al. (1987), amplified by Kay & Kay (1994), that the xenoliths represent crystallization products of near primary magmas at the base of the Aleutian Arc crust. Forthcoming data for the Mt. Moffett xenolith suite will be presented.