Abstract
K-Ar analyses of hornblendes and biotites separated from the Kibe cauldron that is a type locality of the Shunan Group and the associated intrusives (Iwagoyama granite) , and from the Yamaguchi cauldron and Shikuma plutonic complex that are correlated with the Kibe cauldron, are carried out to reveal temporal and spatial variations of Cretaceous volcano-plutonism in the western Chugoku district, SW Japan. The age results including the previous radiometric ages on the Shunan Group and the closely associated intermediate intrusive rocks have revealed that a series of volcano-plutonism of the Shunan Group occurred between 100 and 90 Ma in the restricted southern part of the Chugoku Mountain ranges (Setouchi side) . Volcanic rocks of the Kanmon Group predating the Shunan stage are inferred to be 110–100 Ma in age, being spreaded over the whole western Chugoku district from Japan Sea side to Setouchi side. The volcano-plutonism of the Abu Group and the related intrusive rocks following the Shunan stage is dated to be 90–80 Ma, and expands on the northern part of the Chugoku Mountain ranges (Japan Sea side).
