Jurassic granitoids intruding into the Hida and Sangun metamorphic rocks in the central Sanin District, Japan

概要

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So-called "Older Granites" are distributed in Paleogene granitic terrain of the eastern Shimane and westernmost Tottori Prefecture region, and some petrographical studies including U-Pb dating on the contained zircon were performed and reported here. These granitoids are weakly foliated quartz diorite to granodiorite in composition, and fine-grained secondary biotite was formed close to the contact with Paleogene biotite granites, thus indicating older than Cretaceous in age. The contained zircon showed 198.6±2.7 Ma, thus the earliest Jurassic. The Jurassic granitoids could well be correlated to those of the northern Hida Belt, but here they occur with both the Hida high-T/low-P type and Sangun low-T/high-P type metamorphic rocks. The Jurassic granitoids are considered as an independent plutonic product formed much later than the regional metamorphic rocks.