Kaersutite-bearing metagabbro of the Chichibu terrane in the Central Kii Peninsula, Southwest Japan

Abstract

Greenstones commonly occur within the Jurassic accretionary complex of the Chichibu terrane (central Kii Peninsula, Southwest Japan). In Gobanseki pass and Dorogawa areas, sharp contacts between the metagabbro with adjacent chert are exposed. Long axes of the metagabbro intrusive bodies are less than several ten meters in diameter, and makes stock or sheet. Field relationships indicate that metagabbro exhibited chilled margin and wavy, sometimes irregular boundary surface against chert. In the contact zone, the metagabbro changed to become dark green chilled margin, sometime a white glassy rock about 1cm from the boundary. Field occurrences and relationships indicate that the metagabbro intruded into unconsolidated pelagic sediments. The K-Ar age on kaersutite yielded age around 220±5.9 Ma. The metagabbro shows holocrystalline textures and is mainly composed of euhedral titanaugite, kaersutite, plagioclase, apatite and Fe-Ti oxide minerals. Some kaersutite grains overgrowth with titanaugite. Some metamorphic minerals including chlorite, albite, epidote, sphene and actinolite are present in the metagabbro, indicating that they have been affected by greenschist facies conditions. Their occurrence and petrography suggest they are of magmatic origin rather than representing tectonic blocks metamorphosed under amphibolite facies conditions. The chemical composition of the metagabbro is characterized by high contents of alkali (Na_2O+K_2O=1.9-6.8wt%), TiO_2 (2.5-7.8wt%), P_2O_5 (0.6-1.0wt%), Nb (66-113ppm) and Zr (262-414ppm). The geochemical characteristic suggests that they are similar to OIB-type alkali basalt and basalt originating from superplume.