Formative periods and source materials of Cretaceous-Paleogene granitoids from Honshu Arc

Abstract

Rb-Sr whole rock isochronRS-WIis one of the major dating methods for granitoids. The ages defined by this isochron generally have large age-errors, and sometimes cannot be understood clearly. Analysis of the scattering of data in the isochron diagram is a useful tool to overcome these negative aspects. We found out that the Sr isotopic homogenization is not generally attained in whole-area of granitic mass but only within a narrow extent of several square kilometers and occurs in harmony with the shape of the mass. By careful sampling with full consideration of this fact, we will be able to obtain reliable ages indicating emplacement of granitic magma. It is needless to say that the samples must be co-genetic suites and fresh. Based on the RS-WI ages reported previously, we clarified the migration of magmatism from Cretaceous to Oligocene in the Honshu Arc HA. Especially, the magmatism from Late Cretaceousca. Mato Oligocene gradually migrated or concentrated in a narrow area along the Japan Sea coast. The analysis of such a migration of magmatism can be linked with other data to clarify the cause of the spreading of the Japan Sea basin at Middle Miocene. The oxygen and SrI and NdI characteristics of the Cretaceous-Pale-ogene granitoids of the HA can be divided into the North, South, Kitakami and Sado zones. The granitoids belonging to different zones are formed from isotopically different lower-crust and upper-mantle materials. Transitional zones sometimes exist between two zones. Nd model ages were calculated from the Paleozoic to Mesozoic sedi-mentary rocks in the western part of the HA. The main aim of this study is to ascertain whether the above differences in the O-Sr-Nd isotopic ratios from upper-mantle to lower-crust are also observed in the upper crustal materials. Abstract CThe Geological Society of Japan 2005 441