Along-strike variation in the Aleutian Island Arc: Genesis of high Mg# andesite and implications for continental crust

Abstract

Based on a compilation of whole rock geochemistry for approximately 1100 lava samples and 200 plutonic rock samples from the Aleutian island arc, we characterize along-strike variation, including data for the western part of the arc which has recently become available. We concentrate on the observation that western Aleutian, high Mg# andesite compositions bracket the composition of the continental crust. Isotope data show that this is not due to recycling of terrigenous sediments. Thus, the western Aleutians can provide insight into genesis of juvenile continental crust. The composition of primitive magmas (molar Mg# > 0.6) varies systematically along the strike of the arc. Concentrations of SiO 2 , Na 2 O and perhaps K 2 O increase from east to west, while MgO, FeO, CaO decrease. Thus, primitive magmas in the central and eastern Aleutians (east of 174°W) are mainly basalts, while those in the western Aleutians are mainly andesites. Along-strike variation in Aleutian magma compositions may be related to a westward decrease in sediment input, and/or to the westward decrease in down-dip subduction velocity. 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, 208 Pb /204 Pb and 87 Sr/ 86 Sr all decrease from east to west, whereas 143 Nd/ 144 Nd increases from east to west. These data, together with analyses of sediment from DSDP Site 183, indicate that the proportion of recycled sediment in Aleutian magmas decreases from east to west. Some proposed trace element signatures of sediment recycling in arc magmas do not vary systematically along the strike of the Aleutians, and do not correlate with radiogenic isotope variations. Thus, for example, Th/Nb and fractionation-corrected K concentration in Aleutian lavas are not related to the flux of subducting sediment. Th/La is strongly correlated with Ba/La, rendering it doubtful that Ba/La is a proxy for an aqueous fluid component derived from subducted basalt. Ce/Pb > 4 is common in Aleutian lavas west of 174°W, in lavas with MORB-like Pb, Sr and Nd isotope ratios, and is also found behind the main arc trend in the central Aleutians. Thus, Ce/Pb in Aleutian lavas with MORB-like isotope ratios is not always low, and may be affected by a component derived from partial melting of subducted basalt in eclogite facies. Enriched, primitive andesites, with high Sr/Y, steep REE patterns, and low Yb and Y, are an important lava type in the Aleutians west of 174°W. High Sr/Y and Dy/Yb, indicative of abundant garnet in the source of melting, are correlated with major element systematics. Lavas with a garnet signature have high SiO 2 , Na 2 O and K 2 O.