Cretaceous adakitic Tamba granitoids in northern Kyoto, San’yo belt, Southwest Japan

Abstract

Tamba granitoids, small stocks in northern Kyoto, have adakitic character, i.e., highly Sr/Y and lower Y contents than surrounding granitoids (ilmenite-series) of the San’yo belt and granitoids (ilmenite-series) of the southerly Ryoke belt. The adakitic granitoids including the both ilmenite-series and magnetite-series are further classified into two groups, typical adakite same as Cenozoic type locality and transitional adakite, based on their Ni and Cr contents. Known hornblende and biotite K-Ar ages of the adakitic granitoids are 101–107 Ma, corresponding with those of high magnesian andesite in the Tamba area and older than those (70–80 Ma) of granitoids of other San’yo belt and Ryoke belt. Since the crustal thickness of the San’yo belt in Mid-Cretaceous is not expected to have been thicker than that of present days (37 km), the adakitic Tamba granitoids magma is considered to have been produced by slab melting and subsequent interaction between the magma and host rocks in the wedge mantle. © 2000, Japan Association of Mineralogical Sciences. All rights reserved.