RbSr dating of acidic rocks from the middle part of the Inner Zone of southwest Japan: tectonic implications for the migration of the Cretaceous to Paleogene igneous activity

Abstract

RbSr age determinations by mineral and whole-rock isochron methods have been performed on Cretaceous to Paleogene granitic and rhyolitic rocks from the middle part of the Inner Zone of southwest Japan in order to examine some chronological problems and tectonic models for the migration of igneous activity. Our chronological results include: (1) isotopic homogenization of granitic and rhyolitic rocks in the Miyazu-Izushi area was disturbed by mixing of acidic and basic magmas; (2) the Rokko granite was emplaced at 77.8 Ma and reheated at 72 Ma; (3) mineral isochron ages obtained for the acidic volcanic rocks range from 62.6 to 85.8 Ma and are considered to date the time of eruption. Igneous activity migrated eastward from ∼ 95 Ma to ∼ 70 Ma, which may be related to a similar eastward migration of the development of sedimentary basins in fore-arc regions. The observed eastward migration has been discussed with relevance to a ridge subduction model in which the heated position by subducted ridge moved from west to east. Although the cause for the migration of the igneous activity is poorly understood, several difficulties with the simple ridge subduction model are pointed out.