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Keywords:

  • arc magma;
  • Indonesia;
  • slab fluid;
  • Sr–Nd isotopes;
  • Sunda Arc;
  • trace element

Abstract

The Sunda Arc of Indonesia developed along the convergent margin between the Eurasian and the Australian Plates. More than 100 Quaternary volcanic centers occur along the arc. The West Java Arc is a segment of the Sunda Arc in which more than 10 volcanic centers are located, corresponding to the 120 to 200 km depth contours of the Wadati–Benioff zone. The geochemistry of 207 Quaternary lavas from six centers across the arc was investigated. The lavas range from basalt to dacite. Incompatible element abundances increase from the volcanic front to the rear-arc in response to a change from low-K to high-K suites. Nd–Sr isotope compositions of the basalts scatter between mid-ocean ridge basalt (MORB) source mantle and Indian Ocean sediment (SED) compositions, with volcanic front low-K basalts having more radiogenic Nd than the rear-arc basalts. It is suggested that mixing between slab-derived fluids mainly from the SED and melt from MORB source mantle played a significant role in determining the geochemistry of the West Java basalts. Incompatible element patterns in primitive mantle normalized multi-element plots are almost identical across the arc, except for greater inclination and weaker positive Sr spikes in the rear-arc basalts. This suggests a lower degree of partial melting in the rear-arc mantle, accompanied by change in SED fluid composition between the volcanic front and the rear-arc. The latter is confirmed by fluid-fluxed melting model calculations using multiple trace elements and Nd and Sr isotopes. All the West Java Arc lavas require deficit of Sr from the slab SED. This may occur due to selective breakdown of Sr-rich hydrous silicate minerals, such as zoisite, at shallower depths before the SED component reaches the depth of dehydration effective for magma genesis. The rear-arc basalts need further Sr deficits along with lesser fluid. These features are commonly observed in many arc basalts, and are likely attributable to the same mechanism.