Pliocene obducted, rotated and migrated ultramafic rocks and obduction-induced anatectic granite, SW Seram and Ambon, Eastern Indonesia
Geochemistry, petrography and thermobarometry of the ultramafic and associated rocks of SW Seram and Ambon indicate that they were obducted as weakly depleted oceanic lithosphere which was formed about 10 Ma prior to obduction. Occurrences of cordierite-bearing granite are closely associated with the ultramafic rocks. Petrography and geochemical analysis based on major, RE and other trace elements, indicate that the granite is of the S-type, containing a significant restite component; a VAG signal is interpreted as inherited from the partly melted and included Palaeozoic metasediments. The granite magma was formed as a consequence of the obduction of very hot mantle peridotite inflicting high-grade metamorphism and partial anatexis on the Palaeozoic continental substratum. The oldest age in the literature for cordierite-bearing dacite, connected to this partial anatexis, sets the minimum age of the obduction event at 4.4 Ma. The obduction took place at the eastern end of the Banda Arc, near the NE margin of the Weber Deep, where SW Seram must have been at that time. The ophiolite as part of the Buru-Seram Microplate, rotated and migrated about 400 km westward to its present position. Since the Early Pliocene, the rate of upheaval in Kaibobo (SW Seram) has averaged about 260 cm Ka -1, but may well have been over 1000 cm Ka -1 in the early period immediately after the formation of the granite melt. As the average uplift rate over the same period for Central Seram was previously estimated at 110 cm Ka -1, strongly differential upheaval prevailed in Seram in the Early Pliocene. It is postulated that young oceanic lithosphere has been created by transtensional pull-apart in the easternmost part of the Banda Sea Plate due to oblique convergence of the northward moving Australian Plate, of which fragments gradually became incorporated in the regime of the westbound SW Pacific Plates. Obduction was effected during the strong Pliocene rotation of the Buru-Seram Microplate, at the maximum of change between regimes of motion. It is speculated that the histories of ophiolite complexes of E Sulawesi and Buton are similar to those of SW Seram and Ambon. In this view, oceanic lithosphere, formed by transtensional pull-apart near the NE margin of the Weber Deep in the Early Oligocene, obducted on Australian continental fragments in the Early Miocene and migrated westward until collision in the Middle Miocene.