Analyses of igneous rocks from the Eocene calc-alkaline and Miocene potassic volcanic arc in southwest Sulawesi indicate that magmas became more heterogeneous in their trace element and Pb–Sr–Nd isotopic signature following the collision of the Buton microcontinent with the arc at 15 Ma. Isotopic ratios become more ‘continental’ 4 my after the collisional event (87Sr/86Sr 0·7085, 143Nd/144Nd 0·5125, 206Pb/204Pb 19·2, 207Pb/204Pb 15·73, 208Pb/204Pb 39·4). As the overriding plate consists of young Sundaland crust, whereas the subducted sediment is likely to have been shed from a compositionally distinct microcontinent of Australian derivation, we can be certain that the continental isotopic signature reflects subduction of continental material rather than crustal contamination. The isotopic compositions of the magmas can be explained by the melting of a mixed mantle wedge, consisting of fluid-fluxed and sediment-modified MORB mantle. In this model, the maximum amount of sediment added to the mantle source is 10%. ; Marlina A. Elburg, Theo Van Leeuwen, John Foden and Muhardjo ; Copyright © 2002 Oxford University Press