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The Hirao Limestone located in northeast Kyushu is bordered on the north by unmetamorphosed Paleozoic strata (the Kagumeyoshi Formation), and on the south by the Tagawa metamorphic rocks, a member of the Sangun Metamorphic Rocks. They are thermally metamorphosed due to the Cretaceous Hirao granodiorite intrusion. As a result, neither the fossil record nor the record of the regional (Sangun) metamorphism has been preserved. We investigated the geologic relationships among these units. The tectonic collage model associated with accretionary processes proposed by Kanmera and Nishi (1983) seems the most reasonable. The Hirao Limestone and the uppermost part of the Tagawa metamorphic rocks may be a large olistolith formed during the accretion of the Palaeozoic formation. Petrological studies show that the area was thermally metamorphosed as revealed by widespread occurrences of biotite in pelitic rocks both in the Tagawa metamorphic rocks and in the Kagumeyoshi Formation. Garnet occurrences in the pelitic schists close to the Hirao granodiorite suggest about 700°C for the peak metamorphic temperature of contact metamorphism, based on garnet-biotite geothermometers. The high temperature condition is consistent with the occurrence of corundum in quartz-free layers of the pelitic schist, and that of anthophyllite in quartz-free layers of the basic schists consisting of hornblende + plagioclase. These lines of evidence are indicative of the amphibolite facies condition of contact metamorphism.

journal of mineralogical and petrological sciences

Geology and petrology of the Hirao Limestone and the Tagawa metamorphic rocks-with special reference to the contact metamorphism by Cretaceous granodiorite