Rb-Sr whole rock isochron ages of granites from northern Shikoku and Okayama, Southwest Japan: Implications for the migration of the Late Cretaceous to Paleogene igneous activity in space and time.

Abstract

Rb-Sr whole rock isochron ages have been determined on Late Cretaceous granites occurring in the northern Shikoku, Shodo-shima and Okayama regions. They are 93.1 ± 2.9 Ma (Takanawa Peninsula), 82.1 ± 3.0. Ma (East Sanuki and Shodo-shima), 93 ± 28 Ma (Shirotori granite in East Sanuki) and 84.0 ± 3.7 Ma (Okayama region). Compilation of radiometric age data shows that igneous activity of the age from 120-90 Ma covered almost the whole area of the Chugoku and northern Shikoku Province, and that succeeding activity of the age from 86-80 Ma gradually migrated eastward from the mid Chugoku-northern Shikoku area. On the other hand, igneous activity of the age from 75-60 Ma took place in a narrow zone along the Japan Sea coast, suggesting that an abrupt change in geological situation of igneous activity occurred at around 75 Ma. The field of igneous activities gradually narrowed toward the central area of the Japan Sea coast from Late Cretaceous to Paleogene. This migration of igneous activity may be assumed to have started around 75 Ma, when the field of igneous activities abruptly changed.