Rb-Sr whole rock age of granitic rocks from the Sanyo zone in the central Hiroshima Prefecture

Abstract

Rb-Sr age deteminations were carried out on some granitic rocks of the Sanyo zone in Hiroshima Prefecture, Southwest Japan. The “Hiroshima” and the “Gu” types (after Hiroshima Pref., 1963), which are two major rock types of the granitic rocks of the Sanyo zone, give Rb-Sr whole rock isochron ages of 90.5±5.1Ma and 88.2±15.3Ma with initial 87Sr/86Sr ratios of 0.70631±0.00015 and 0.70678±0.00155, respectively. The age difference between the two types is insignificant because of a large error in the age of the “Gu” type, and all the samples of both types give a more reliable isochron age of 92.8±4.0Ma and a Sr initial ratio of 0.70627±0.00020 for the major part of the granitic rocks of the Sanyo zone. These data indicate that the “Hiroshima” and the “Gu” type granites were emplaced contemporaneously with a single igneous activity during middle Cretaceous age.