Sources for magmatism in Central Sulawesi: Geochemical and Sr-Nd-Pb isotopic constrainsts.

Abstract

Middle Miocene to Quaternary magmatism in Central Sulawesi (Indonesia) is characterised by radiogenic Sr and Pb isotopic signatures, and unradiogenic Nd isotopic ratios. Although the geochemistry of the samples is heterogeneous, all samples show a distinct subduction signature (negative anomalies for Nb and Ti, positive for K and Pb) in their trace element patterns. Mafic magmatism is represented by lamprophyric magmas, and a suite of ‘gabbros' (ranging from gabbro to clinopyroxenite). The isotopic signature of the lamprophyres could be explained by simple mixing between a mantle source, similar to that for mid-ocean ridge basalts (MORB), and sediments, as is typical for subduction-related volcanics, but the Pb–Sr isotopic systematics of the gabbros preclude this interpretation. They are interpreted as containing an important contribution from an old sub-continental lithospheric source, located within a sliver of Australian continent that has been thrust underneath Central Sulawesi. Felsic magmatism is likely to reflect high degrees of crustal contamination or intracrustal melting. These interpretations suggests that contemporaneous subduction did not play a major role in determining the isotopic signature of Miocene–Pleistocene magmatism in the area, despite the ‘subduction signature' seen in their trace element patterns.