Zircon melt inclusion study on the Miocene Miuchi granitoid pluton, SW Japan: A new approach to estimate crystallization pressures of granitic rocks

Abstract

Granitic rocks (sensu lato) are a major rock type in orogenic belts. The crystallization pressures of granitic rocks can help to unravel the magmatic processes and tectonic evolution of these regions, particularly in areas where metamorphic rocks are not exposed. However, geobarometric techniques that are suitable for granitoids are quite limited. In this study, we propose a new approach to estimate crystallization pressures of granitoid magmas using melt inclusions in zircon, a ubiquitous accessory mineral in these lithologies. Homogenization experiments of polymineralic inclusions hosted in zircon have been conducted from a biotite granite sample in the Miocene Miuchi pluton, southwest Japan arc, using a piston-cylinder high-pressure–high-temperature apparatus. The homogenized compositions of the inclusions have high SiO2 contents (76–80 wt%) implying that they represent fractionated interstitial melts trapped in growing zircon crystals. The rhyolite-MELTS geobarometer for the homogenized melt inclusions in oscillatory-zoned zircon rim yields pressures ranging from 80 to 114 MPa (with error of ±25 MPa), interpreted as the crystallization pressures of the Miuchi pluton. These results are consistent with field and petrographic observations suggestive of a shallow emplacement level. The approach presented here would be applicable to most granitoids, which could provide fundamental data to better understand granite petrogenesis and the tectonic evolution of orogenic belts.