Development of Microcracks in Granites Clarified by Fluid Inclusion Study :Examples from the Quaternary Takidani Pluton, Japan

Abstract

The Takidani Granodiorite is the youngest exposed plutonic rock on Earth. Hydrothermal processes, and crack formation in the Takidani Granodiorite, characterized by analyzing fluid inclusions in primary quartz. Most of the fluid inclusions are secondary, and have been classified as liquid-rich inclusions, vapor-rich inclusions, and polyphase inclusions.

Homogenization temperatures of the fluid inclusions ranged from about 70?C to over 600?C, which of salinity varies from 1.2 to 73 NaCl eq wt.%. High salinity fluid inclusions occur in samples from the ìcoreî of the zoned pluton. The origin of the high salinity fluid is interpreted to derive from a magmatic fluid which had not been diluted by low temperature, low salinity meteoric water, which prevailed during recent time.

The distribution of healed microcracks in a sample collected from Shiradashi-zawa (S1) show clear preferred orientations; comprising two N-S oriented microcracks dipping to west and east respectively, and E-W striking vertical cracks. These features correspond to the orientation of joints evident at an outcrop scale. Open cracks show one preferred orientation, in places corresponding to the orientation of healed cracks and joints. The homogenization temperature of secondary inclusions trapped along healed cracks from outcrop S1 are between 256?Cand 340?C with high salinity (2.6-57 NaCl eq wt.%). The homogenization temperatures were pressure corrected to obtain actual trapping temperatures, assuming a lithostatic pressure obtained by PSHA techniques. Assuming that the generation of the cracks and trapping of fluid inclusions occurred simultaneously, then we infer that microcracks in the Takidani Granodiorite formed at a temperature range of between 280?C and 390? C, close to the transition from subcritical to supercritical hydrothermal conditions.