Abstract

The island arcs of Indonesia exhibit a range of subduction-related tectonic environments and consequently are of particular interest in studying the relationship between the nature of the subducted plate and volcanism. The Sunda arc in Java manifests the underthrusting of oceanic lithosphere beneath the newly accreted margin of Asia. In the Banda arc, continental material appears to be subducting beneath a volcanic arc that is underlain by oceanic crust.

87Sr/86Sr ratios (0.7040 to 0.7059) in Holocene andesitic lavas from Java are higher than those from other volcanic arcs, except those arcs where sialic contamination has been inferred. A group of anomalous calc-alkaline lavas, characterized by high 87Sr/86Sr ratios (0.7054 to 0.7059) and relatively high abundances of K and related elements, appear to be the result of sialic contamination of a mantle-derived melt. High 87Sr/86Sr ratios, as large as 0.7095, together with values of δ18O as large as +9.2‰ (SMOW) in lavas from the Banda arc, imply mixing of mantle-derived and sialic components. Because of the tectonic peculiarities of the region, the most likely source of contamination is from subduction of terrigenous material derived from the northwest continental shelf of Australia. Lead isotopic ratios in lavas from Java are generally transitional to the trends defined by other arc lavas and are characterized by higher 207Pb/206Pb ratios than oceanic basalts. The anomalous calc-alkaline lavas, in particular, are characterized by relatively high 207Pb/206Pb ratios, interpreted to result from subduction and melting of sediments derived from the Precambrian shields of Australia and India during the opening of the Indian Ocean. Neodymium-isotope ratios lie within the range defined from other volcanic arcs. In terms of 143Nd/144Nd and 87Sr/86Sr, the Banda arc samples lie on a mixing line between compositions typical of the mantle and the upper continental crust.

That subducted sediments appear as a component in at least some of the Holocene Indonesian lavas implies melting at the top of the subducted lithospheric slab and provides constraints on the thermal structure of the descending plate.

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