Magmatic evolution of Sulawesi (Indonesia): constraints on the Cenozoic geodynamic history of the Sundaland active margin

Abstract

Tertiary and Quaternary magmatic rocks from West Sulawesi record the complex history of part of the Sundaland margin where subduction and collision have been and are still active. The present study, based on petrographic data, major- and trace-element chemistry and 40K 40Ar dating aims to document the age and chemical characteristics of the magmatic formations from West Sulawesi and to determine the corresponding constraints on the geodynamic evolution of the Sundaland border. The West Sulawesi magmatic province includes the South Arm of Sulawesi (Ujung Pandang area), the western part of Central Sulawesi with the Toraja and Palu areas, and finally, the North Arm, extending from Palu to Manado, which includes the Tolitoli and Manado areas. Paleocene magmatic activity seems to be restricted to an episode of calc-alkaline magmatism in the Ujung Pandang area (61-59 Ma). The major Eocene (50-40 Ma) magmatic event is tholeiitic and is documented in all areas except in Ujung Pandang. It led to the emplacement of tholeiitic pillow-lavas and basaltic dykes of back-arc basin (BAB) affinity. These rocks are potential equivalents to the Celebes Sea basaltic basement. From Oligocene to Miocene, magmatic eruptions produced successively island-arc tholeiitic (IAT) and calc-alkaline (CA) rock series. The youngest IAT activity occurred around 18 Ma in the central part (Palu area) and around 14 Ma in the North Arm (Tolitoli area) while CA magmas were emplaced in the North Arm at ca. 18 Ma (Tolitoli and Manado areas). Typical calc-alkaline activity resumed only in the North Arm (Tolitoli and Manado areas) during the Late Miocene (9 Ma) and is still active in the Manado region. In other areas (Palu, Toraja and Ujung Pandang areas) an important and widespread magmatic event occurred between 13 and 10 Ma and emplaced K-rich magmas, either silica-undersaturated alkali-potassic basalts (AK), ultrapotassic basanites (UK) or shoshonites (SH). K-rich activity continued in the south until the Pleistocene (0.77 Ma) with alkali-potassic, ultrapotassic and shoshonitic magmas. In Central Sulawesi (Toraja and Palu areas) the most recent magmatic event occurred between 6.5 and 0.6 Ma. The corresponding products are granitic rocks and widely distributed rhyolitic pyroclastic flow deposits. All these rocks are acidic in character (SiO 2 > 60%), with trace-element and isotopic signatures (SrNdPb) typical of a strong continental imprint. The most striking tectonic implication of this magmatic evolution is that West Sulawesi can no longer be considered as a typical magmatic arc as previously assumed. With the exception of the Manado area beneath which subduction is still active, calc-alkaline and island-arc tholeiitic lavas and plutonics are volumetrically minor with respect to K-rich magmas. Their occurrence through time is also fairly restricted, mostly to the period between 30 and 15 Ma. Another important feature is the occurrence of island-arc tholeiitic and calc-alkaline magmas crosscutting an older terrane of BAB affinity, the Tinombo Formation (Manado, Tolitoli and Palu areas). As this formation is being regarded as an equivalent to the Celebes Sea floor, the most likely explanation for this feature is the hypothesis of tectonic erosion linked to the NW-dipping subduction beneath the North Arm. The Late Miocene high-K magmatic activity in Central and South Sulawesi reflects the prevalence of a post-collisional tectonic regime following the docking of microcontinents of Australian origin to Central Sulawesi during Neogene times. The incompatible element-enriched character of these high-K rocks might reflect their derivation from a mantle source enriched through metasomatism related to a previous subduction event. Such a model cannot account for the Plio-Pleistocene CAK magmatism of Central Sulawesi, the acidic composition of which does not support a derivation from an ultrabasic source. The trace-element patterns of the CAK rocks are very similar to those of the high-grade metamorphics of Central Sulawesi, suggesting that the latter might represent their possible source. Such an anatectic model implies a collisional to post-collisional tectonic regime limited to Central Sulawesi, while a post-subduction regime prevailed in the south.