ALEUTIAN MAGNESIAN ANDESITES - MELTS FROM SUBDUCTED PACIFIC OCEAN CRUST

Several diagnostic chemical characteristics of an uncommon Aleutian magma type support a proposed origin that involves a small amount of partial melting of subducted Pacific ocean crust (basalt) consisting mainly of garnet and clinopyroxene (eclogite or garnet websterite). Among the characteristics are high La/Yb ratios and Sr contents and low ratios of radiogenic to non-radiogenic Sr and Pb. The major element composition of the andesites resembles that of hydrous melts in equilibrium with peridotite: a low ratio of total Fe to Mg is distinctive. These disparate observations can be reconciled if large ion lithophile (LIL)- element-rich hydrous melt from the subducted oceanic crust equilibrates with olivine and orthopyroxene in overlying LIL-element-depleted mantle and then erupts without interacting with the island are crust. The compositional dissimilarity of the magnesian andesites and most other andesites from the Aleutian island arc precludes application of this model to island are magmatism in general. © 1978.