Isothermal compression record in the high-pressure Bantimala garnet amphibolite: Implications on the Mesozoic subduction system between the Sundaland and Western Sulawesi (Indonesia)

Abstract

The isothermal compression is a special prograde metamorphic P-T path in high-pressure rocks. However, the exact geodynamic processes and associated tectonic significance of this path remain unclear. The Mesozoic subduction and collision between the Sundaland Block and east Java-west Sulawesi (EJWS) generated various metamorphic rock assemblages, thus leading to the crucial need to understand the evolution of the Meso-Tethys. The Bantimala Complex is a key metamorphic basement in Western Sulawesi that preserves records of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism during Cretaceous subduction. This manuscript presents a detailed petrological, mineralogical, geochronological, phase equilibrium modelling, and Zr-in-rutile thermometry study of a garnet amphibolite from the Bantimala Complex. Petrographic observations indicate that the garnet amphibolite underwent three metamorphic stages that include prograde (M₁), peak (M₂), and retrograde (M_3a and M_3b) stages. Pseudosection modelling constrains the P-T conditions of these stages at 13–15.5 kbar/565–585 °C (M₁), ∼23.5 kbar/588 °C (M₂), 8–9.5 kbar/410–480 °C (M_3a), and 8.3–8.8 kbar/415–440 °C (M_3b), respectively. UPb dating utilizing magmatic zircon reveals a protolith formation age of 212 ± 3.1 Ma. ⁴⁰Ar/³⁹Ar dating results from sodic-calcic amphibole and phengite indicate the timing of HP metamorphism at 136–128 Ma (Early Cretaceous). In contrast to other HP metamorphic rocks in the Bantimala Complex (eclogite and blueschist), garnet amphibolite exhibits a unique counterclockwise P-T path that includes an isothermal compression stage. This implies that the rock likely detached from the upper plate and was transported to the lower plate within the subduction zone. Following peak metamorphism at a depth of approximately 75 km, it was exhumed along with other HP metamorphic rocks to a shallower crustal depth of approximately 25 km. Combined with the available data, this study also reveals the coexistence of diverse P-T trajectories within the Bantimala Complex, thus providing robust evidence for a Mesozoic subduction system between the Sundaland Block and Western Sulawesi (Indonesia).