Cretaceous-Paleogene magmatism in the central San-in district, Southwest Japan

Cretaceous to Paleogene intrusive rocks associated with small amounts of contemporaneous volcanic rocks cluster in the central San-in district, SW Japan. Geological and Rb-Sr isochron age data are presented for these intrusive and volcanic rocks in order to clarify the time and correlation of the Cretaceous-Paleogene magmatism in the San-in district. The intrusive rocks comprise a composite batholith, occupying an area of about 2,400 square kilometers. Greater part of the batholith was formed by intrusions of intermediate to felsic magmas during about 25 million years from 75 Ma to 50 Ma, and the batholith was completed by intrusions of late Paleogene small stocks. Based on presently and previously reported geochronological data, the Cretaceous-Paleogene magmatism in the San-in district is divided into three stages, i.e., Cretaceous volcanic rocks and the Mochigase-stage intrusive rocks (~78 Ma), the Inbi-stage intrusive rocks and contemporaneous volcanic rocks (75 to 50 Ma), and the late Paleogene Tamagawa-stage volcanic and intrusive rocks (44 to 30 Ma). A hiatus for six million years is recognized between the Inbi and Tamagawa stage magmatism.