Isotopic equilibration ages for the Miyanohara tonalite from the Higo metamorphic belt in central Kyushu, Southwest Japan: Implications for the tectonic setting during the Triassic

Miyanohara tonalite occurs in the middle part of the Higo metamorphic belt in the central Kyushu, Southwest Japan. This tonalite intrudes into early P ermian Ryuhozan metamorphic rocks in the south and is intruded by Cretaceou s Shiraishino granodiorite in the north. The Miyanohara tonalite yielded th ree mineral ages: (i) 110-100 Ma for Sm-Nd and Rb-Sr internal isochrons and for K-Ar hornblende; (ii) 183 Ma for Sm-Nd internal isochron; and (iii) 21 1 Ma for Sm-Nd internal isochron. The ages of 110-100 Ma may indicate cooli ng age due to the ther mal effect of the Shiraishino granodiorite intrusion . The ages of 183 Ma and 211 Ma. are consistent with timing of intrusion of the Miyanohara tonalite based on geologic constraints. The hornblende in t he sample which gave 183 Ma shows discontinuous zoning under microscope, wh ereas the one which gave 211 Ma does not show zonal structure. These minera logical features suggest that the 183 Ma sample has suffered severely fi om later tectonothermal effect compared with the 211 Ma sample. Therefore, th e age of 211 Ma is regarded as near crystallization age for the Miyanohara tonalite. The magmatic process, geochronology and initial Sr and Nd isotope ratios for the Miyanohara tonalite are similar to those of early Jurassic granites from the Outer Plutonic Zone of the Hida belt that constitutes a m arginal part of east Asia before the opening of the Japan Sea. Intrusion of the Miyanohara tonalite is considered to have taken place in the active co ntinental margin during the late Triassic.