The versatility of petrological modeling: Thermobarometry of high-pressure metabasites from the renge and sanbagawa belts and phase evolution during warm subduction at nankai

Abstract

Petrological modeling is a powerful technique to address different types of geological problems via phase-equilibria predictions at different pressure–temperature-compo-sition conditions. Here, we show the versatility of this technique by (1) performing thermobarometrical calculations using phase equilibrium diagrams to explore the petrological evolution of high-pressure (HP) metabasites from the Renge and Sanbagawa belts, Japan and (2) forward-modeling the mineral–melt evolution of the subducted fresh and altered oceanic crust along the Nankai subduction zone geotherm at the Kii peninsula, Japan. In the first case, we selected three representative samples from these metamorphic belts: a glaucophane eclogite and a garnet glaucophane schist from the Renge belt (Omi area) and a quartz eclogite from the Sanbagawa belt (Besshi area). We calculated the peak metamorphic conditions at ˜2.0–2.3 GPa and ˜550– 630°C for the HP metabasites from the Renge belt, whereas for the quartz eclogite, the peak equilibrium conditions were calculated at 2.5–2.8 GPa and ˜640–750°C. According to our models, the quartz eclogite experienced partial melting after peak metamorphism. In terms of the petrological evolution of the subducted uppermost portion of the oceanic crust along the warm Nankai geotherm, our models show that fluid release occurs at ˜20–60 km, likely promoting high pore-fluid pressure, and thus, seismicity at these depths; dehydration is controlled by chlorite breakdown. Our petrological models predict partial melting at >60 km, mainly driven by phengite and amphibole breakdown. According to our models, the melt proportion is relatively small, suggesting that slab anatexis is not an efficient mechanism for generating voluminous magmatism at these conditions. Modeled melt compositions correspond to high-SiO2 adakites; these are similar to compositions found in the Daisen and Sambe volcanoes, in southwest Japan, suggesting that the modeled melts may serve as an analog to explain adakite petrogenesis.