Amphibolitization within the lower crust in the termination area of the Godzilla Megamullion,an oceanic core complex in the Parece Vela Basin

Gabbroic rocks and amphibolites were collected from the KR03‐01‐D10 dredge site located on the West Arm Rise of the Godzilla Megamullion, close to the Parece Vela Rift which appears to correspond to the termination area of a detachment fault, the Philippine Sea. The gabbroic rocks and amphibolites reveal the occurrence of a high hydrothermal activity in the lower crust close to a paleo‐ridge. In the gabbroic rocks, plagioclase compositions of both porphyroclasts and matrix were transformed into sodium‐rich compositions close to albite. Amphiboles are of secondary rather than igneous origin based on their microstructural occurrences. In the amphibolites, anorthite contents of porphyroclasts and matrix plagioclase are relatively lower than those of the gabbroic rocks, whereas the chemical compositions of amphibole within the amphibolites are similar to those of amphibole within the gabbroic rocks. Amphibolites represent the product of retrograde metamorphism associated with hydrothermal alteration of the gabbroic body by the reaction: clinopyroxene + calcic plagioclase + fluid → amphibole + sodic plagioclase. The estimated temperatures of the amphibolites derived from the amphibole thermobarometer and the gabbroic rocks derived from the hornblende–plagioclase geothermometer show ～700–950°C and 650–840°C, respectively. The hydrothermal alteration recorded in the gabbroic rocks possibly occurred under high‐T conditions; the rocks were then metamorphosed to the amphibolites during a retrogressive stage. Our study indicates that amphibolitization took place with various degrees of deformation. It may imply that the hydrothermal activity increased as the Godzilla Megamullion developed as an oceanic core complex in the paleo‐ridge.