K-Ar ages of a late Cretaceous granitic ring complex around southern Lake Biwa, southwest Japan - cooling history of a huge cauldron

Abstract

Late Cretaceous granitoids of the San'yo Belt around southern Lake Biwa form a huge ring complex measuring 60 km by 40 km, and are associated with rhyolitic pyroclastic rocks called the Koto Rhyolites. Felsic magmas formed a huge magma chamber, with a part of the magma erupted as pyroclastic flows, followed by a large-scale cauldron subsidence with concentric ring fractures. Part of the magma formed a granitic ring complex, which reached the solidus temperature at 80-78 Ma. The cooling rate was 40-80°C/m.y., reaching the closure temperature for the biotite K-Ar system at 75-70 Ma. The residual magma remained up to about 66 Ma. It was squeezed and intruded into the granitic ring complex and basement as small-scale granitic and pyroclastic dikes. The fission track zircon ages were reset by hydrothermal or extreme geothermal activity which continued until at least 56 Ma. -from English summary