MAGMATIC EVOLUTION OF SULAWESI (INDONESIA) - CONSTRAINTS ON THE CENOZOIC GEODYNAMIC HISTORY OF THE SUNDALAND ACTIVE MARGIN

Tertiary and Quaternary magmatic rocks from West Sulawesi record the c omplex history of part of the Sundaland margin where subduction and co llision have been and are still active. The present study, based on pe trographic data, major- and trace-element chemistry and K-40-Ar-40 dat ing aims to document the age and chemical characteristics of the magma tic formations from West Sulawesi and to determine the corresponding c onstraints on the geodynamic evolution of the Sundaland border. The We st Sulawesi magmatic province includes the South Arm of Sulawesi (Ujun g Pandang area), the western part of Central Sulawesi with the Toraja and Palu areas, and finally, the North Arm, extending from Palu to Man ado, which includes the Tolitoli and Manado areas. Paleocene magmatic activity seems to be restricted to an episode of calc-alkaline magmati sm in the Ujung Pandang area (61-59 Ma). The major Eocene (50-40 Ma) m agmatic event is tholeiitic and is documented in all areas except in U jung Pandang. It led to the emplacement of tholeiitic pillow-lavas and basaltic dykes of back-are basin (BAB) affinity. These rocks are pote ntial equivalents to the Celebes Sea basaltic basement. From Oligocene to Miocene, magmatic eruptions produced successively island-are thole iitic (SAT) and calc-alkaline (CA) rock series. The youngest IAT activ ity occurred around 18 Ma in the central part (Palu area) and around 1 4 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 a reas) during the Late Miocene (9 Ma) and is still active in the Manado region. In other areas (Palu, Toraja and Ujung Pandang areas) an impo rtant and widespread magmatic event occurred between 13 and 10 Ma and emplaced K-rich magmas, either silica-undersaturated alkali-potassic b asalts (AK), ultrapotassic basanites (UK) or shoshonites (SH), K-rich activity continued in the south until the Pleistocene (0.77 Ma) with a lkali-potassic, ultrapotassic and shoshonitic magmas. In Central Sulaw esi (Toraja and Palu areas) the most recent magmatic event occurred be tween 6.5 and 0.6 Ma. The corresponding products are granitic rocks an d widely distributed rhyolitic pyroclastic flow deposits, All these ro cks are acidic in character (SiO2 > 60%), with trace-element and isoto pic 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 are a s previously assumed. With the exception of the Manado area beneath wh ich subduction is still active, calc-alkaline and island-are tholeiiti c lavas and plutonics are volumetrically minor with respect to K-rich magmas. Their occurrence through time is also fairly restricted, mostl y to the period between 30 and 15 Ma. Another important feature is the occurrence of island-are tholeiitic and calc-alkaline magmas crosscut ting an older terrane of BAB affinity, the Tinombo Formation (Manado, Tolitoli and Palu areas). As this formation is being regarded as an eq uivalent to the Celebes Sea floor, the most likely explanation for thi s feature is the hypothesis of tectonic erosion linked to the NW-dippi ng subduction beneath the North Arm. The Late Miocene high-g magmatic activity in Central and South Sulawesi reflects the prevalence of a po st-collisional tectonic regime following the docking of microcontinent s of Australian origin to Central Sulawesi during Neogene times. The i ncompatible element-enriched character of these high-K rocks might ref lect their derivation from a mantle source enriched through metasomati sm related to a previous subduction event. Such a model cannot account for the Plio-Pleistocene CAK magmatism of Central Sulawesi, the acidi c composition of which does not support a derivation from an ultrabasi c source. The trace-element patterns of the CAK rocks are very similar to those of the high-grade metamorphics of Central Sulawesi, suggesti ng that the latter might represent their possible source. Such an anat ectic model implies a collisional to post-collisional tectonic regime limited to Central Sulawesi, while a post-subduction regime prevailed in the south.