Subducted upper and lower continental crust contributes to magmatism in the collision sector of the Sunda-Banda arc, Indonesia

Abstract

Pb isotopes in igneous rocks from the Banda-Sunda arc show extreme along-arc variations, which correspond to major lithologic changes in crustal components entering the subduction system. An increase in 206Pb/204Pb ratios toward the zone of collision with the Australian continent reflects input of subducted upper-crustal material; maximum values coincide with anomalously radiogenic 3He/4He ratios that have been earlier attributed to the involvement of the continental margin. The collision zone is further characterized by 208Pb/204Pb ratios that are higher for a given 207Pb/204Pb value than observed in the noncollisional sectors, and in the central part of the collision zone, the 206Pb/204Pb ratios are lower than the most radiogenic values in the adjacent areas. We propose that these Pb isotope signatures reflect input of subducted lower crust, mobilized as a result of slab-window formation during arc-continent collision. Variations in Pb isotopes in the collision zone are solely determined by variations in the nature and proportions of different subducted components. The Pb isotope arrays in the noncollision area may be dominated by slab components as well and could reflect mixing between subducted oceanic crust and entrained sediments, rather than between subarc mantle and subducted sediments. Our new interpretation of the Indonesian Pb isotope data does not call for involvement of ocean-island basalt (OIB)–type mantle or Australian subcontinental lithospheric mantle, as has been suggested previously.