山陰帯,島根県中央部に産する高畑花崗岩および奥山トーナル岩の岩石学的・年代学的研究

Petrological and geochronological data are presented for the Takahata granite and Okuyama tonalite in the central San'in belt，southwest Japan．The Takahata granite consists of coarse to medium grained，equigranular to weakly porphyritic biotite granite，granite porhyry，and aplite．The Takahata granite is intruded by the Okuyama tonalite with sharp contact．The Okuyama tonalite ranges in rock type from medium-grained，biotite-hornblende quartz diorite through tonalite to granodiorite．The Takahata granite is silicic and has high ASI（alumina saturation index）for I-type magnetite series granites，and is characterized by relatively high total Fe_2O_3 content，compared with other fractionated granitic rocks in the central San'in belt．The Okuyama tonalite is metaluminous，and has high magnetic susceptibility．In contrast to the previously reported fission track zircon ages（80 Ma and 82±7 Ma），the Takahata granite gives a Rb-Sr whole rock isochron age of 67.4±4.2 Ma with an initial Sr isotope ratio of 0.7052±0.0004．The Okuyama tonalite yields an age of 65.4±11.3 Ma with an initial Sr isotope ratio of 0.7050±0.0001．Although the Takahata granite has previously been considered to be older than most granitoids in the San'in belt，petrological，geochronological and Sr and Nd isotopic data indicate that the Takahata granite and the Okuyama tonalite belong to the late Cretaceous-Paleocene Inbi Intrusives of the San'in belt．