17:15 〜 19:15
[SMP26-P08] Paleozoic and Mesoproterozoic metamorphic history of the Banggai-Sula microcontinent, eastern Indonesia
キーワード：Banggai-Sula, Eastern Indonesia, U-Th-Pb Monazite age, Pressure-temperature conditions, Paleozoic basement, Gondwana
Continental fragments with Australian or Gondwanan affinities are widely distributed in eastern Indonesia. The Banggai-Sula microcontinent (BSM) is one of the most notable examples. Previous studies have correlated BSM with Papua and New Guinea based on similarities in pre-Cretaceous stratigraphy overlying Paleozoic basement rocks (e.g. Pigram and Panggabean, 1984; Garrard et al., 1988). More recent studies suggest that BSM was part of the extended Australian continental crust (Sula Spur) (Hall, 2009). While the Mesozoic and Cenozoic histories of BSM have been extensively studied, its Paleozoic geological history remains underexplored. This study investigates metamorphic rocks of western BSM, using petrography, mineral chemistry, and U-Th-Pb monazite geochronology to reconstruct metamorphic evolution and identify tectonic events related to Paleozoic and earlier history of BSM.
Field observation and petrographic analysis reveal distinct basement rock distributions across the region. In Banggai Island, Grt-And-Bt-Ms schist with relict staurolite is found in the south, while granites dominate the north. In Peleng Island, the St-bearing Grt-Bt-Ms schist occurs in the west and the Grt-Sil-Bt augen gneiss in the central region. In Banggai Island, the timing of peak metamorphism of 395±9.6 Ma is defined by U-Th-Pb monazite geochronology from low-Y monazite rims in the Grt-And-Bt-Ms schist. On the west of Peleng Island, the peak metamorphism, based on low-Y monazite rims of St-bearing Grt-Bt-Ms schist, has a mean age of 407±7.8 Ma. Monazites from granitoids yield a mean age of 244±2 Ma, suggesting that metamorphism was regional and unrelated to granitoid intrusions. Additionally, in the central Peleng Island, preliminary U-Pb zircon dating of zircon rims in the garnet-sillimanite-biotite augen gneiss provides a metamorphic age of 1508±18 Ma.
Based on petrographic observations and mineral chemistry, metamorphic P-T condition is estimated using conventional thermobarometers (Holdaway, 2000; 2001; Wu and Zhao, 2006; 2015; Wu, 2018). The peak Silurian-Devonian metamorphism, characterized by equilibrium assemblage St + Grt + Bt + Ms + Qz ± Pl, occurred at 503–620°C and 0.65–0.76 GPa. This was followed by decompression into the andalusite stability field (477–598°C, 0.11–0.23 GPa), with Grt + And + Bt + Ms + Qz ± Pl as equilibrium assemblages. The presence of staurolite inclusion in andalusite from the Grt-And-Bt-Ms schist on Banggai Island supports this transition. Sector-zoned garnet grains in both staurolite-bearing pelitic rocks also indicate multiple stages of garnet growth during prograde metamorphism. In contrast, garnets in Grt-Sil augen gneiss are homogenous except near the rims. The peak condition of Mesoproterozoic metamorphism, characterized by Grt + Sil + Bt + Kfs + Pl + Qz as equilibrium assemblages, is estimated at 578–648°C, 0.40–0.61 GPa.
The basement rocks of BSM are often linked with the Kemum Basement High in the Bird’s Head region of Papua, which records metamorphism that can be tentatively correlated with two phases of magmatic activity in the Devonian–Carboniferous and the Permian–Triassic periods (Jost et al., 2018; 2021). However, this study shows that the basement rocks of BSM underwent Mesoproterozoic and Silurian-Devonian metamorphism, followed by Triassic granite intrusions. Early Paleozoic igneous activity and metamorphism have also been reported from the Semitau Block in Borneo (Zhu et al., 2022) and other parts of Southeast Asia, reflecting the configuration of continental fragments along northern Gondwana due to Proto-Tethys Ocean subduction (e.g., Wang et al., 2021; Cawood et al., 2021). The Silurian–Devonian metamorphism in the Banggai-Sula microcontinent may represent the eastern continuation of this tectonic event. Further discussion on the protolith and origin of the BSM basement rocks will be included in the presentation, along with additional U–Pb zircon data.
Field observation and petrographic analysis reveal distinct basement rock distributions across the region. In Banggai Island, Grt-And-Bt-Ms schist with relict staurolite is found in the south, while granites dominate the north. In Peleng Island, the St-bearing Grt-Bt-Ms schist occurs in the west and the Grt-Sil-Bt augen gneiss in the central region. In Banggai Island, the timing of peak metamorphism of 395±9.6 Ma is defined by U-Th-Pb monazite geochronology from low-Y monazite rims in the Grt-And-Bt-Ms schist. On the west of Peleng Island, the peak metamorphism, based on low-Y monazite rims of St-bearing Grt-Bt-Ms schist, has a mean age of 407±7.8 Ma. Monazites from granitoids yield a mean age of 244±2 Ma, suggesting that metamorphism was regional and unrelated to granitoid intrusions. Additionally, in the central Peleng Island, preliminary U-Pb zircon dating of zircon rims in the garnet-sillimanite-biotite augen gneiss provides a metamorphic age of 1508±18 Ma.
Based on petrographic observations and mineral chemistry, metamorphic P-T condition is estimated using conventional thermobarometers (Holdaway, 2000; 2001; Wu and Zhao, 2006; 2015; Wu, 2018). The peak Silurian-Devonian metamorphism, characterized by equilibrium assemblage St + Grt + Bt + Ms + Qz ± Pl, occurred at 503–620°C and 0.65–0.76 GPa. This was followed by decompression into the andalusite stability field (477–598°C, 0.11–0.23 GPa), with Grt + And + Bt + Ms + Qz ± Pl as equilibrium assemblages. The presence of staurolite inclusion in andalusite from the Grt-And-Bt-Ms schist on Banggai Island supports this transition. Sector-zoned garnet grains in both staurolite-bearing pelitic rocks also indicate multiple stages of garnet growth during prograde metamorphism. In contrast, garnets in Grt-Sil augen gneiss are homogenous except near the rims. The peak condition of Mesoproterozoic metamorphism, characterized by Grt + Sil + Bt + Kfs + Pl + Qz as equilibrium assemblages, is estimated at 578–648°C, 0.40–0.61 GPa.
The basement rocks of BSM are often linked with the Kemum Basement High in the Bird’s Head region of Papua, which records metamorphism that can be tentatively correlated with two phases of magmatic activity in the Devonian–Carboniferous and the Permian–Triassic periods (Jost et al., 2018; 2021). However, this study shows that the basement rocks of BSM underwent Mesoproterozoic and Silurian-Devonian metamorphism, followed by Triassic granite intrusions. Early Paleozoic igneous activity and metamorphism have also been reported from the Semitau Block in Borneo (Zhu et al., 2022) and other parts of Southeast Asia, reflecting the configuration of continental fragments along northern Gondwana due to Proto-Tethys Ocean subduction (e.g., Wang et al., 2021; Cawood et al., 2021). The Silurian–Devonian metamorphism in the Banggai-Sula microcontinent may represent the eastern continuation of this tectonic event. Further discussion on the protolith and origin of the BSM basement rocks will be included in the presentation, along with additional U–Pb zircon data.