T4［トピック］変成岩とテクトニクス

10:15 〜 10:30学生優秀発表賞エントリーECS: Early Career Scientist

[T4-O-5] P–T constraints on high-pressure metamorphism of the Pompangeo Schist Complex, Central Sulawesi: Pseudosection modeling of lawsonite-bearing blueschis

*MAULANA Baso Rezki^1,2、NISHIKAWA Osamu¹、OKUDAIRA Takamoto³ (1. Graduate School of International Resource Sciences, Akita University, Japan、2. Geological Engineering, Hasanuddin University, Gowa, South Sulawesi, Indonesia、3. Department of Geosciences, Osaka Metropolitan University, Japan)

キーワード：

Central Sulawesi Metamorphic Belt、Pompangeo Schist Complex、lawsonite-bearing blueschist、phase-equilibria modeling、Sundaland margin

The Pompangeo Schist Complex (PSC) is the most extensively exposed metamorphic terrane in the Central Sulawesi Metamorphic Belt (CSMB). It covers roughly 5,000 km², making it one of the largest high-pressure (HP) exposures in Southeast Asia. Yet its metamorphic evolution remains poorly understood, and no quantitative pressure–temperature (P–T) estimates have been reported for the complex. Parkinson (1998) mapped metamorphic zones defined by lawsonite- and glaucophane-bearing assemblages but could not quantify P–T conditions because mineral compositional data and suitable thermobarometers were lacking. Here we integrate optical petrography, SEM-EDS imaging, and WDS-EPMA mineral analyses with thermodynamic modeling in Perple_X to constrain the P–T evolution of lawsonite-bearing blueschist from the Poso Regency. From 28 oriented field stations, we selected three lawsonite-bearing metabasite samples (BS15MS, BS51MS, BS58BS) for pseudosection modeling. They preserve HP assemblages in which glaucophane coexists with lawsonite, almandine-rich garnet (grossular > pyrope > spessartine), and epidote. We modeled two coexisting metapelitic samples (BS47MS, BS32MS) in parallel. Both preserve phengite-bearing assemblages (phengite–chlorite ± albite in BS47MS; quartz–muscovite–phengite in BS32MS) that lack sodic amphibole and lawsonite, and they provide independent compositional constraints within the same P–T interval recorded by the blueschists. Cross-cutting relationships and textural evidence, most fully documented in BS15MS and BS51MS and corroborated by assemblages in BS58BS, define a four-stage history for the metabasites. Stage I is HP greenschist (phengite + chlorite ± albite). Stage II marks the blueschist peak (glaucophane + lawsonite ± garnet ± epidote), recorded in the cores of garnet porphyroblasts. Stage III is an early exhumation stage (glaucophane ± epidote ± chlorite ± albite), together with an epidote- and chlorite-bearing overgrowth domain that mantles Stage II garnet cores in porphyroblasts. Stage IV represents a retrograde greenschist-facies overprint (chlorite + albite ± calcite, with calcite locally developed in marble-adjacent lithologies). We modeled the metabasites in the MnNCKFMASHTO system and the metapelites (BS47MS, BS32MS) in TiNCKFMASHO using Perple_X v7.2.2 (hp62ver.dat); this reproduces the natural assemblages and constrains Stage II to the blueschist stability field. Phengite Si contents recorded in Stage I of the metabasites and independently in BS47MS corroborate these HP conditions and are consistent with the pseudosection-derived estimates for the blueschist. Collectively, these data provide quantitative P–T constraints for lawsonite-bearing blueschist of the PSC. They are consistent with Cretaceous subduction along the Sundaland margin (Parkinson, 1998), part of the same regional HP metamorphic system that also produced the higher-pressure eclogite-facies rocks of the Bantimala Complex in southwestern Sulawesi (Parkinson et al., 1998; Hsia et al., 2024).

References:
Hsia, J., Zhang, J., Qian, J., Liu, J., Tian, Y., Xian, W. W., Yin, C., Gao, P., Forster, M., & Guo, M. (2024). Metamorphic P-T-t constraint on the Bantimala garnet-epidote-glaucophane schist and its implication on Mesozoic evolution of Western Sulawesi, Indonesia. Tectonophysics, 873, Article 230228. https://doi.org/10.1016/j.tecto.2024.230228
Parkinson, C. (1998). An outline of the petrology, structure and age of the Pompangeo Schist Complex of central Sulawesi, Indonesia. The Island Arc, 7(1–2), 231–245. https://doi.org/10.1046/j.1440-1738.1998.00171.x
Parkinson, C. D., Miyazaki, K., Wakita, K., Barber, A. J., & Carswell, D. A. (1998). An overview and tectonic synthesis of the pre-Tertiary very-high-pressure metamorphic and associated rocks of Java, Sulawesi and Kalimantan, Indonesia. The Island Arc, 7(1–2), 184–200. https://doi.org/10.1046/j.1440-1738.1998.00184.x