Geology and metamorphic zonation of the Ryoke Metamorphic Belt on Kasado-jima Island, SW Japan

Abstract

The Cretaceous low-pressure/high-temperature Ryoke Metamorphic Belt contains abundant Ryoke and San-yo Granitoids that intruded to various crustal depths during or after formation of the belt. To clarify the tectono-metamorphic history of the belt, it is necessary to evaluate the thermal effects of the intrusion of the different types of granitoids. The geology of Kasado-jima Island, SW Japan, consists of metasedimentary rocks, massive biotite granite, and migmatitic biotite tonalite. The granite and tonalite occur in the central and southern areas of the island, respectively. Within the central area, granite intrudes metasedimentary rocks and cuts across bedding-parallel schistosity within the metapelites; this indicates that intrusion of the granite postdates Ryoke metamorphism. In contrast, tonalite in the southern area was emplaced parallel to the schistosity. On the basis of the field occurrences and petrographic characteristics of these granitoids, we correlate the biotite tonalite and biotite granite with the Older Ryoke and San-yo Granitoids, respectively. Based on the paragenesis of schistosity-forming minerals within metapelites, we identify two mineral zones in the island: the biotite and cordierite zones. The biotite zone is characterized by a mineral assemblage of muscovite + biotite ± garnet, and the cordierite zone is characterized by the assemblage muscovite + biotite + K-feldspar + cordierite or garnet. On the basis of mineral chemistry, we estimate peak metamorphic conditions for the biotite and cordierite zones to be ∼ 470 °C (reference pressure: 200 MPa) and ∼ 550 °C at 250 MPa, respectively. In the central part of the island, a contact aureole adjacent to the biotite granite is identified by the occurrence of randomly oriented muscovite and very large dendritic cordierite porphyroblasts. These randomly oriented porphyroblasts may have formed under a high degree of supersaturation associated with the thermal perturbation of the shallow intrusion of the biotite granite into cooler crust. In the southern area, most metamorphic minerals are of similar size, and their long axes are aligned to form a schistosity; this indicates a small degree of thermal perturbation and/or a high strain rate. These observations indicate that the biotite tonalite on Kasado-jima Island, as a deep-seated granitoid, was related to regional low-pressure/high-temperature metamorphism.