The Banda arc of eastern Indonesia manifests the collision of a continent and an intra-oceanic island arc. The presently active arc is located on what appears to be oceanic crust whereas the associated subduction trench is underlain by continental crust.
Recent lavas from the Banda arc are predominantly andesitic and range from tholeiitic in the north through calc-alkaline to high-K calc-alkaline varieties in the southern islands. Defining this regular geochemical variation are significant increases in the abundances of K (2,600-21,000 ppm), Rb (10-90 ppm), Cs (0.5 7.0 ppm), and Ba (100-1,000 ppm) from tholeiitic to high-K calc-alkaline lavas. STSr/S6Sr ratios in the tholeiites are relatively low, from 0.7045 to 0.7047. In the calc-alkaline lavas, STSr/S6Sr ratios range from 0.7052 to 0.7095, and in the high-K calcalkaline lavas from 0.7065 to 0.7080. There is no correlation between STSr/S6Sr and major and trace element abundances, even among lavas from the same volcano. Late Cenozoic cordierite -bearing lavas from Ambon, north of the presently active arc, are highly enriched in K, Rb and Cs, which together with STSr/S6Sr ratios of approximately 0.715 is consistent with their derivation from partial melting of pelitic material in the locally -thick crust.
The high STSr/S6Sr ratios in the Recent calc-alkaline lavas are interpreted to result from mixing of a sialic component with a mantle derived component. The most likely cause is subduction and subsequent melting of either sea-floor sediments or continental crust. However, it is probably unrealistic to model this type of deep contamination by simple two-component mixing. Such contamination implies that the volcanic rocks from the Banda arc are at least partly a manifestation of melting at or near the Benioff seismic zone. Temperatures of at least 750 800~ * This paper is the result of a cooperative project with the