Bacan is located in the zone of convergence between the Eurasian, Philippine Sea and Australian plates. The oldest rocks in Bacan belong to the Sibela Continental Suite and are probably of Precambrian age. The complex includes continental phyllites, schists and gneisses of upper amphibolite facies. Isotopic dating yielded extremely young ages due to interaction with hydrothermal fluids. Juxtaposed against the continental rocks is the mostly unmetamorphosed, arc-related Sibela ophiolite, probably derived from the Philippine Sea plate. Isotopic dating yielded a Cretaceous age with an Oligocene-Miocene overprint. In north Bacan, the oldest formation is the Upper Eocene Bacan Formation which comprises interbedded arc volcanic and turbiditic volcaniclastic rocks, metamorphosed under conditions between the prehnite-pumpellyite and greenschist facies. A similar Lower Miocene sequence, assigned to the same formation, is exposed in south Bacan. The Oligocene Tawali Formation on Kasiruta, NW of Bacan, consists of arc basalts and volcaniclastic turbidites, metamorphosed to zeolite facies. The Bacan and Tawali Formations represent different parts of an arc, active from Late Eocene until Early Miocene, resulting from northward subduction of the Australian plate under the Philippine Sea plate. There is a major Lower Miocene unconformity, representing collision of the Australian continent with the Philippine Sea plate, above which shallow marine limestones of the Lower-Middle Miocene Ruta Formation were deposited. This deposition was interrupted by sudden influxes of volcaniclastic sands, forming the Amasing Formation. The Upper Miocene-Pleistocene Kaputusan Formation, rests locally unconformably on older rocks, and includes three members. The Goro-goro Member consists of arc andesites, originating from four eruption centres, which erupted from Late Miocene to Pleistocene, with the oldest in south and the youngest in north Bacan. The Pacitak Member consists of shallow marine pyroclastic rocks. The Mandioli Member formed fringing coastal reef limestones. The volcanic rocks of this formation were produced by eastward subduction of the Molucca Sea plate. Quaternary basalts are related to movement along the Sorong fault. It is concluded that most of the Bacan region has been part of the Philippine Sea plate since the Cretaceous; volcanic rocks of different ages all have an arc character and chemistry, and lithological variations reflect different positions within the volcanic arc; and there is evidence for continental crust of Australian origin in the Bacan area by the Early Miocene.
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