Geochemical variation in tertiary-quaternary lavas of the West Java arc, Indonesia: Steady-state subduction over the past 10 million years

Abstract

Tertiary and Quaternary lavas from the West Java arc, Indonesia, have been analyzed to identify their major and trace elements and Sr-Nd-Pb isotope compositions. The analyzed samples are basaltic to andesitic in composition and are representative of the across-arc setting in both the Tertiary (Miocene) and Quaternary periods. The results indicate that the major and incompatible trace elements are enriched in the rear arc (RA) lavas compared to the volcanic front (VF) lavas. The Nd isotope composition is systematically more radiogenic in the VF lavas, although Pb and Sr isotope compositions in the VF and RA lavas are virtually identical. The incompatible element variation can be simply explained by the differing degree of partial melting in the mantle source, i.e., the melting is greater in the VF and lesser in the RA. The contrast in the Nd isotope composition originates from the different flux rates of the slab fluid relative to the mantle partial melts; these rates are higher in the RA and lower in the VF. The total fluid addition to the mantle mass is greater in the VF; however, a greater degree of melting in the VF dilutes the contribution of radiogenic Nd from the sediment fluid (Sendjaja et al., 2009). The across-arc geochemical variation between the Tertiary and Quaternary lavas does not differ. This implies that the West Java arc has been in a "steady state" over the past 10 million years, with continuous subduction input from the Indian Ocean sediments and continuous upwelling and replenishment of the depleted mantle source from the back arc.