R8: Metamorphic rocks and tectonics

[R8-02] Pressure-Temperature-Time Conditions of The Basement Rocks in The Banggai-Sula Microcontinent, Indonesia

「発表賞エントリー」

*Fransiska Ayuni Catur Wahyuandari¹, Nobuhiko Nakano¹, Tatsuro Adachi¹, Nugroho Imam Setiawan² (1. Kyushu Univ., 2. Gadjah Mada Univ.)

Keywords:Banggai-Sula microcontinent, Paleozoic basement, monazite EPMA ages

The Banggai-Sula microcontinent in the eastern Indonesia region is known to be derived from the Northwest Australia continental margin (Sula Spur). The basement rocks of Banggai-Sula microcontinents are often associated with the high-temperature/low-pressure metamorphic rocks of Kemum Basement High of the Bird’s Head region in Papua which intruded by granitoids of Devonian–Carboniferous and Permian–Triassic age. This study reports the petrological and geochemical characteristics, and monazite EPMA ages of these basement rocks to understand its origin and the relation with tectonic events along the northeastern margin of Gondwana in the Paleozoic. Metamorphic rocks collected from Banggai and Peleng Islands imply differences in metamorphic conditions between Banggai Island in the east (garnet-andalusite-muscovite schist) and the western part of Peleng Island (garnet-staurolite-biotite-muscovite schist). The preservation of garnet growth zoning during prograde metamorphism is indicated by decreasing Mn from the core towards the rim of the garnet grains obtained from metamorphic rocks in this region. The monazite EPMA ages were determined from biotite granite in Banggai Island and staurolite-bearing pelitic schist in Peleng Island. The biotite granite from Banggai Island gives a weighted mean age of 271 ± 8 and 263 ± 14 Ma from the core and rim of monazite. The staurolite-bearing pelitic schist from Peleng Island yields a weighted mean age of 434 ± 3 and 449 ± 12 Ma from the core and mantle, and 403 ± 8 Ma from the rim of monazite. This Silurian-Devonian metamorphic age has not been reported before from the Banggai-Sula microcontinent and Eastern Indonesia region. The differences in metamorphic grade and timing with the Kemum Basement High suggest that an earlier tectonic event influenced the metamorphism in the Banggai-Sula microcontinent. Such early Paleozoic igneous activity and metamorphism happened along the northern margin of East Gondwana related to the subduction of the Proto-Tethys Oceanic crust. Silurian–Devonian metamorphism in the Banggai-Sula microcontinent might represent the eastern continuation of this tectonic event. In the presentation, we will also discuss the pressure-temperature conditions of these basement rocks.