(Trace element behaviour in Miocene I-type and S-type granitic rocks in the Ohmine district, central Kii peninsula.)

Abstract

The Ohmine granitic rocks of Miocene age are divided into S-type and I-type. Abundance of trace elements of these rocks is given as well as petrography and major element abundance. The S-type granitic rocks are higher in SiO2, F and Rb/Sr, and lower in Sr than the I-type rocks. The Shirakura rocks (I-type) formed at an early stage are higher in differentiation index, Ce, Rb and Nb and lower in MgO/(MgO + FeO), Co, Cr, Ni and Sr than the Drogara rocks (S-type). On the basis of these data, the authors discuss the genesis of the granitic rocks. The S-type magma was formed by partial melting of biotite- and orthoclase-bearing sedimentary and/or metasedimentary rocks at approx 700oC and 5 kbar. The I-type granitic rocks would be generated by partial melting of Ca-amphibole and plagioclase-bearing intermediate igneous and/or meta-igneous rocks. Fractional crystallization of plagioclase, Ca-amphibole, and biotite occurred in the I-type magma. Judging from the trace element behaviour, it is unlikely that the granitic rocks are the products of large-scale magma-crust interaction and of mixing between basaltic and granitic magma. The authors also propose a revised classification of calc-alkaline granitic rocks; C-type (sub- to meta-aluminous, SrIo >0.708) and W-type (peraluminous, SrIo <0.708) beside I- and S-type; C-type is not yet found. I- and W-type occur in mature continental regions. S-type occurs in the regions consisting of immature crust without basaltic layer and in the region of crust exceeding 50 km in thickness such as continental margin and continent-continent collision zones. -K.O.