Late Miocene to Pliocene crustal extension and lithospheric delamination revealed from the ~5 Ma Palopo granodioritic intrusion in Western Sulawesi, Indonesia

Abstract

Recent studies have revealed that following collision between the Western Sulawesi and Sula Spur microcontinents during the early to middle Miocene, the Western Sulawesi microcontinent subsequently underwent extension. However, the geodynamic process that resulted in extension is unknown. The petrogenesis of late Miocene to Pliocene granitoids in the region provides insights to the mechanism of extension. To achieve this goal, we studied the Palopo granodioritic intrusion, a well-exposed representative pluton that crops out along the Palu–Koro Fault. Zircon U–Pb dating indicates a 4.8–4.7 ± 0.1 Ma crystallization age for the pluton. Compared with arc igneous rocks, the pluton has high Sr contents and low Y and Yb contents, with relatively high Sr/Y and LaN/YbN ratios. These geochemical signatures are similar to those of typical adakites. The samples exhibit high MgO (1.60–2.54 wt%), Cr (46–65 ppm), and Ni (14.1–19.0 ppm) contents relative to adakitic rocks derived from thickened lower crust. In addition, negative εHf(t) (−9.7 to −5.5) and εNd(t) (−8.5 to −11.1) values, together with relatively low 143Nd/144Nd and high initial 87Sr/86Sr ratios, reflect an origin through recycling of ancient crustal materials, which is clearly different from adakites derived from melting of a subducting slab. The most plausible mechanism for generation of the late Miocene–Pliocene adakitic granodiorites in the Western Sulawesi microcontinent is partial melting of delaminated lower crust that interacted with the mantle. We propose that the late Miocene to Pliocene crustal extension in the Western Sulawesi microcontinent was triggered by delamination of pre-existing thickened lithosphere.