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Journal of Petrology | Volume 43 | Number 4 | Pages 581-606 | 2002
© Oxford University Press 2002
Origin of Geochemical Variability by ArcContinent Collision in the Biru Area, Southern Sulawesi (Indonesia)
1DEPARTMENT OF GEOLOGY AND GEOPHYSICS, ADELAIDE UNIVERSITY, ADELAIDE, S.A. 5005, AUSTRALIA
2RIO TINTO EXPLORATION, JAKARTA, INDONESIA
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 PbSrNd 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%.
KEY WORDS: arc volcanism; continental collision; Indonesia; radiogenic isotopes; subduction
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