Usual isotopic, unusual elemental compositions for western Aleutian lavas

New isotopic and compositional data for Quaternary lavas from Gareloi volcano (178.8°W) and Tanaga Island (178°W), in the western Aleutian island arc, shed light on along-arc geochemical trends and processes of differentiation beneath western Aleutian volcanoes. Tanaga lavas are dominantly basalt and basaltic andesite; Gareloi lavas are mostly basaltic andesite and andesite. Most Gareloi samples and half the Tanaga samples - those from Takawangha volcano on the island's eastern half - are richer in LILE, U, Th, and Pb than any previously known lavas from the Aleutian arc front. These are shoshonites through trachytes based on their total alkali content (Na2O to 5.4 wt%, K2O to 3.8 wt% at 61 wt% SiO2). In contrast, HFSE and REE are at normal Aleutian arc-like concentrations. Despite their unusual elemental compositions, Gareloi and Tanaga samples are isotopically similar to other western Aleutian lavas. They have limited ranges in Pb isotopes (206Pb/204Pb = 18.71 - 18.78) that plot in the middle of values from other western arc volcanic samples (Adak and west), but are less radiogenic than almost all lavas east of Adak. The values fit into a regional trend of slightly decreasing values of Pb isotopes from east to west, thought to reflect decreasing amounts of sediment input. The ranges of 87Sr/86Sr and 143Nd/144Nd for Tanaga and Gareloi lavas are also small (0.7031 - 0.7033 and 0.5130 - 0.5131, respectively), and plot in the middle of fields defined by Aleutian lavas. These isotopic results suggest that Tanaga and Gareloi lavas derive from similar mantle sources as other Aleutian magmas. Compared to high Mg# andesites from other locations in the western Aleutians that may be melts of slab or sub-crustal eclogite with variable peridotite interaction, Gareloi and Tanaga have relatively low Mg#s (Gareloi = 0.31 - 0.54; Tanaga = 0.35 - 0.61), Sr/Y (Gareloi = 14 - 46; Tanaga = 16 - 38), and (Dy/Yb)n (Gareloi = 1.1 - 1.6; Tanaga = 1.4 - 1.7). Trace element modeling suggests that the unusual elemental character of Tanaga and Gareloi lavas may be explained by ~10% partial melting of a mantle peridotite source fluxed by up to 1.5% slab-derived fluid, followed by fractionation-differentiation involving common arc mineral assemblages (plagioclase + pyroxene + oxides +apatite +/- amphibole).