Analytical study of mineral parageneses of the Abukuma metamorphic rocks

点击翻译按钮获取中文摘要

Geological and petrological studies has been carried out on the Abukuma metamorphic rocks (the Gosaisyo series and the Takanuki series), northeastern Japan, to reveal their P(pressure)-T(temperature) evolution.Jurassic radiolarian fossils were found for the first time in a Fe-Mn-rich metachert of the Gosaisyo series and, thus, the age of original rocks of the Gosaisyo series was confirmed to be Jurassic. Consequently, the Abukuma regional metamorphism took place after Jurassic and before the emplacement of the Cretaceous intrusive complexes about 120 Ma ago.Rocks of the Gosaisyo series have not shown any evidence of high- to mediumpressure metamorphism. On the contrary, textures and compositions of garnet and plagioclase suggest low-pressure conditions during metamorphism.The Takanuki pelitic rocks are characterized by the occurrence of garnet porphyroblasts showing complex chemical zoning and a sector structure. Their P-T-t paths based on the compositions of garnet, plagioclase, and biotite suggest nearly isothermal loading at high-temperatures at an early stage of regional metamorphism. The high-temperature loading is also evidenced by the occurrence of sillimanite and hercynite along with staurolite within almandine and corundum in the Takanuki lareritic rocks. It may be due to the overthrusting of the Gosaisyo series on the Takanuki series.