Geological and petrological studies on the late Cretaceous and early Paleogene igneous rocks in central San-in District, Southwest Japan (I)

Abstract

Igneous and metamorphic rocks of the central San-in district can be classified into three groups, according to their stages of activities. They are: (1) The late Cretaceous volcanic rocks resulted by volcanic activities in three different stages and accompany dykes in their intrusive facies, (2) The late Cretaceous and early Paleogene intrusive rocks resulted from the plutonic activities in two different stages, (3) The late Cretaceous and early Paleogene metamorphic intrusive rocks which were re-metamor-phosed under plutonic activities of latter stage. The late Cretaceous volcanic rocks, composed largely of rhyolite, rhyodacite and andesite, have been known to distribute mainly on the westside of the “Sanbe-Hiroshima Structural Zone” whereas the late Cretaceous and early Paleogene intrusive rocks, composed largely of diorite, quartz diorite, aplitic granite and biotite granite, occupy chiefly on the east side of this structural zone. The geological structure of basement rocks, Paleozoic in age, seems to govern the late igneous activities of this district.