兵庫県北部矢田川花崗岩体のRb-Sr全岩アイソクロン年代

The Yadagawa granite in the northern part of Hyogo Prefecture is a stock composed mainly of medium- to coarse grained biotite granite，granite polphyry and aplitic granite．Fine-grained hornblende biotite quartz diorite and medium grained hornblende biotite granodiorite are minor rock types．The stock intrudes the late Cretaceous Yadagawa Group，which consist of andesite to rhyolite lavas and pyroclastics．Seven granites and a granite porphyry give a well-defined Rb-Sr whole rock isochron of 69.3 ± 3.6 Ma with an initial Sr isotope ratio of 0.70603 ± 0.00016．This indicates that the Yadagawa granite can be assigned to the latest Cretaceous to Paleocene Inbi Intrusives，which are widely distributed in the San'in district．The Yadagawa granite has a slightly higher initial Sr isotope ratio than most of the Inbi Intrusives．This is consistent with the concept that sources ofCretaceous felsic magmas in the eastern San'in and northern Kinki districts had higher ^<87>Sr/^<86>Sr ratios than those in the central and western San'in districts．