Sm-Nd, Rb-Sr and K-Ar geochronology of the Higo metamorphic terrane, west-central Kyushu, Japan

The Higo metamorphic terrane situated in west-central Kyushu island, southw est Japan, is composed of greenschist- to granulite-facies metamorphic rock s. The southern part of the metamorphic terrane consists mainly of gar net- biotite gneiss and garnet-cordierite-biotite gneiss, and orthopyroxene or c ordierite-bearing S-type tonalite with subordinate amounts of hornblende ga bbro. Rb-Sr, Sm-Nd and K-Ar isotopic ages for these rocks have been determi ned here. The garnet-biotite gneiss gives an Sm-Nd age of 227.1 +/- 4.9 Ma and a Rb-Sr age of 101.0 +/- 1.0 Ma. The hornblende gabbro has an Sm-Nd age of 257.9+/-2.5 Ma and a K-Ar age of 103.4+/-1.1 Ma. These age differences of the same samples are due to the difference in the closure temperature fo r each system and minerals. The garnet-cordierite-biotite gneiss contains c oarse-grained garnet with a zonal structure conspicuously distinguished in color difference (core: dark red; rim: pink). Sm-Nd internal isochrons of t he garnet core and the rim give ages of 278.8+/-4.9 Ma (initial Nd-143/Nd-1 44 ratio = 0.512311+/-0.000005) and 226.1+/-28.4 Ma (0.512277+/-0.000038), respectively. These ages are close to formation of the garnet core and the rim. It has been previously suggested that the Higo metamorphic terrane bel ongs to the Ryoke metamorphic belt. But Sr and Nd isotopic features of the rocks from the former are different from those of the Ryoke metamorphic roc ks, and are similar to those of the granulite xenoliths contained in the Ry oke younger granite.