Abstract

New field and laboratory data from western Sulawesi, Indonesia, integrated with available data establish its Late Cenozoic igneous framework and a new model for its tectonic evolution. Western Sulawesi contains three major Neogene N-S-trending tectonic domains (from W to E): (1) an active foldbelt, in which Pliocene and Miocene volcanogenic rocks are involved in W-vergent thrusting which extends into the Makassar Strait; (2) a central belt comprised of a deformed submarine Miocene volcanoplutonic arc built on an Oligocene-Eocene clastic and carbonate platform with Latimojong Mesozoic basement metamorphic and sedimentary rocks thrust over its eastern margin on W-vergent faults; and (3) an accreted Cretaceous-Palaeogene(?) ophiolite (Lamasi Complex) between the Latimojong basement block and Bone Bay. The Lamasi Complex ophiolite includes dioritic plutons, basaltic sheeted dykes, pillow lavas, greenstones, tuffs and volcanic agglomerates with depleted (MORB-like) Sr & Nd isotope and REE characteristics of probable normal oceanic crust with possible subduction-related or back-arc affinity. New K-Ar, ⁴⁰Ar-³⁹Ar, Rb-Sr, and Nd-Sm isotope data suggest Cretaceous to Eocene crystallization and Oligocene to Miocene obduction. Late Miocene to Poliocene extrusive and intrusive rocks form a cogenetic volcanoplutonic complex of calc-alkalic to mildly alkalic, potassic, and shoshonitic felsic and mafic magmatic rocks of bimodal composition which were erupted and intruded during a short episode of Middle Miocene to Pliocene (3–18 Ma) lithospheric melting. Based on new Rb-Sr, Nd-Sm, and U-Pb isotope, and major and trace element geochemical data, parental source rocks of the Miocene melts were Late Proterozoic to Early Palaeozoic crustal and mantle lithospheric assemblages which became heated and melted owing to a continent-continent collision in which west-vergent continental lithosphere derived from the Australian-New Guinea plate was subducted beneath eastern-most Sundaland. The timing of this magmatism and subsequent cooling and denudation history are constrained by 113 new K-Ar, ⁴⁰Ar-³⁹Ar, and fission track ages. The new tectonic model differs significantly from previous models: the Makassar Strait is now interpreted as a foreland basin bound on both sides by converging Neogene thrust belts, in contrast to previous models suggesting Late Tertiary oceanic spreading or continental rifting. West-vergent obduction of a pre-Eocene oceanic, primitive arc, or back-arc crust onto western Sulawesi occurred during late Oligocene to Miocene times. The Late Miocene western Sulawesi magmatic arc is envisioned as a continent-continent collision product, in contrast to previous models involving a normal ocean-continent or ocean-ocean subduction-related magmatic arc (west or east vergent) or post-subduction rifting. The east Sulawesi ophiolite extends into western Sulawesi, suggesting that Bone Bay resulted from collapse of the over-thickened Miocene orogen. The new tectonic model illustrates the central role western Sulawesi plays in unlocking the complex evolution of Indonesia as well as the temporal and magmatic details of a continent-continent collision zone.