Hybridization of a shallow 'I-type' granitoid pluton and its host migmatite by magma-chamber wall collapse: The Tokuwa pluton, central Japan

The Miocene Tokuwa pluton of 'I-type' granitoid affinity was emplaced discordantly into a Cretaceous to Paleogene accretionary complex and induced a contact aureole in which various thermally metamorphosed rocks were developed, including hornfels, metatexite, diatexite and cordierite-bearing tonalite (Crd-tonalite) of 'S-type' granite affinity. The thermally metamorphosed rocks show low-pressure reaction textures culminating in partial melting. Peak P-T conditions of similar to 3 kbar at similar to 780 degrees C are estimated on the basis of the TWQ thermobarometer for the garnet-bearing rocks. The rocks in the contact aureole exhibit a gradual transition from hornfels, through metatexite and diatexite to Crd-tonalite. The Sr-isotopic composition at the time of Tokuwa pluton emplacement at 12 Ma decreases systematically from metatexite (0.7100-0.7112) through diatexite (0.7078-0.7094) to Crd-tonalite (0.7067-0.7068); this trend is interpreted in terms of mixing between the Tokuwa magma and the aureole migmatites. The field relationships, geochemical data, and isotopic data collectively suggest that the emplacement of the Tokuwa pluton triggered partial melting of the surrounding metasedimentary rocks. Subsequent hybridization of the Tokuwa magma with the metatexite in variable proportions produced the Crd-tonalite and diatexite. The hybridization was caused by invasion of the Tokuwa magma into the migmatite zone, accompanied by gravitational collapse of the previously crystallized wall of the magma chamber. The data presented demonstrate that even a relatively low-temperature, shallow, 'I-type' granitoid pluton can induce contact anatexis and hybrid 'S-type' granitoid formation at the intrusive contact.