GEOCHEMICAL AND TECTONIC RELATIONSHIPS IN THE EAST INDONESIAN ARC-CONTINENT COLLISION REGION - IMPLICATIONS FOR THE SUBDUCTION OF THE AUSTRALIAN PASSIVE MARGIN

Variations in the isotopic signatures of volcanics along the East Sunda Banda Arc reflect changes in the nature and amount of sedimentary material supplied by the northeast Indian Ocean floor and the adjacent Australian passive continental margin, which form the two major domains of the Indian Ocean plate that approach the arc system. A compilation of isotopic data for 200-500-km-long arc sectors shows that the trend in magmatic signatures follows distinct subduction/collision stages reached by the corresponding oceanic and continental-margin sections entering the trench system. Maximum amounts of magma source contamination are inferred for volcanics near an extinct sector north of Timor, where the Australian continent started to collide with the arc first. Pb-Nd isotopic source mixing models point to contamination by sediments with variations in composition, similar to observed along-arc changes in sediments entering the trench. The results indicate an increasing contribution of subducted continental material in the direction of the collision region. Mass-balance calculations, considering the magmatic output and minimum input of subducted continental material required to generate the composition of the volcanic arc in the collision region, are difficult to reconcile with subduction of ocean-floor sediments alone. Thicknesses of sediments presently covering oceanic crust near the margin are close to calculated thicknesses of the sediments fluxed into the trench and magmatically returned to the arc crust, but cannot account for the additional volumes of material accreted on the overriding plate in the same period of time. It is inferred that leading portions of the Australian continental margin have reached magma generation zones in the easternmost Sunda arc and western Banda arc, which implies subduction to depths greater than 100 km.