Spatial and temporal isotopic domains of contrasting igneous suites in Western and Northern Sulawesi, Indonesia

Palaeocene to Pliocene magmatism in NW Sulawesi shows a progression from an Older Series with calc-alkaline/tholeiitic signatures (51-17 Ma) to a Younger Series of mafic-intermediate high-K magmas (∼ 14-5 Ma) and felsic K-rich calc-alkaline (CAK) magmas (9-2 Ma). The isotopic and geochemical compositions of the Older Series samples indicate that the more westerly samples have been generated in a continental arc setting and the more easterly samples in an oceanic arc; this distinction defines the boundary between the Western and Northern Sulawesi tectonic terranes. The Younger Series high-K magmas have unusual isotopic compositions, with variable but low ¹⁴³Nd/¹⁴⁴Nd, high ⁸⁷Sr/⁸⁶Sr values, and high ²⁰⁸Pb/²⁰⁴Pb for their ²⁰⁶Pb/²⁰⁴Pb ratios compared to subduction-related magmas. The diversity of the isotopic compositions points towards a source with a long and heterogeneous geochemical evolution, most likely located within the Australian subcontinental lithospheric mantle. The Younger Series felsic CAK magmatism has a more homogeneous isotopic and geochemical signature and reflects melting of continental crust of Australian origin. This geochemical progression in time is very similar to that seen in central Sulawesi [Chem. Geol. 156 (1999a) 67], and is best explained by normal subduction of an oceanic plate followed by melting of an underthrust sliver of Australian continent. The size of this microcontinent can be estimated from the areal extent of low-Nd-isotope magmas in Western Sulawesi, ranging from approximately 4°S to 1°N. Underthrusting must have happened prior to 14 Ma, indicating that this event cannot be equated to the collision between Sulawesi and the Sula platform, which occurred at 5 Ma. While subduction beneath western Sulawesi ceased prior to the onset of potassic magmatism in this region, it continued beneath northern Sulawesi producing predominantly calc-alkaline suites.