The metamorphism of the Tagawa metamorphic rocks, eastern North Kyushu

Abstract

The Tagawa metamorphic rocks in eastern North Kyushu consist of the Main and Honjo bodies, which are mainly composed of pelitic-psammitic schist and amphibolite. The pelitic-psammitic schist exhibits preferred orientation of biotite, graphite and fibrous sillimanite, and cordierite and garnet include those minerals. This suggests that the Tagawa metamorphic rocks underwent low-P/high-T regional metamorphism at amphibolite facies condition. The metamorphic conditions of the Main and Honjo bodies are estimated to be 530-600°C, 0.3-0.4 GPa, and 600-650°C, 0.2-0.3 GPa, respectively. Andalusite porphyroblasts that include biotite and graphite might have been produced from contact metamorphism by the Ochiai Granodiorite and Masaki Granite. Detrital zircons in pelitic-psammitic schist display two major U-Pb age peaks at 1850 Ma and 250 Ma. These age peaks are similar to those of detrital zircons from the Suo metamorphic complex from the Kurume and Omuta districts, suggesting that the Tagawa metamorphic rocks also belong to the Suo metamorphic complex. This metamorphic complex sustained low-P/high-T regional metamorphism in the interval between the high-P/low-T Suo metamorphism and contact metamorphism by the Cretaceous granitic rocks.