紀伊半島中部，大峰地域の中新世ＩタイプおよびＳタイプ花こう岩質岩の微量化学組成

Abstract

The Ohmine granitic rocks of Miocene age occur in the central Kii peninsula, Outer Zone of Southwest Japan. These rocks fall into two distinct types; S-type and I-type (Dorogawa and Shirakura rocks). Trace elements (Ba, Ce, Co, Cr, Cs, Nb, Ni, Rb, Sc, Sr, Y, Zr, and F) of the rocks were determined by photon activation, colorimetry, and ion-electrode analyses. The S-type granitic rocks are high in SiO2, F, and Rb/Sr and low in Sr compared with the I-type ones. The Shirakura rocks formed early stage in the intrusive sequence are higher in D. I., Ce, Rb, and Nb and lower in MgO/(MgO+FeOt), Co, Cr, Ni, and Sr than the Dorogawa ones. The S-type magma, from which biotite, plagioclase, and orthoclase must have fractionated, would be formed by partial melting of biotite and orthoclase containing sedimentary and/or metasedimentary rocks at about 700°C and 5 kbar. The I-type granitic rocks were probably generated by partial melting of Caamphibole and plagioclase containing intermediate igneous and/or metaigneous rocks. Fractional crystallization mainly of plagioclase, Ca-amphibole, and biotite must have occurred in the I-type magma. The Shirakura magma would be produced by lower degree of partial melting than the Dorogawa one. The crystallization course of the I-type magma was different from that of the S-type one ; orthoclase precipitated next to quartz in the I-type magma, on the contrary quartz crystallized after orthoclase in the S-type magma. On the basis of trace elements behavior, it is unlikely that the granitic rocks are products of large scale magma-crust interaction and of mixing between basaltic and granitic magmas. On the criteria of alumina saturation and isotope ratio, revised classification of calc-alkali granitic rocks are proposed; C-type (sub to metaluminous, SrIo〉 0.708) and W-type (per. aluminous, SrIo〈0.708) in addition to I-type and S-type. C-type rocks are, however, not found yet. The parental materials and fractionating phases of the I-type, W-type, and the S-type in the world are estimated from publicated data. The estimated parental materials and fractionating phases of the I- and W-type and the S-type are the same as those of the Ohmine I-type and the Ohmine S-type, respectively. The I- and W-type rocks are, therefore, in lower range of Rb/Sr than the S-type ones. Many I- and W-type granitic rocks occur in mature continental regions composed of basaltic and granitic layers. The S-type granitic rocks would, on the other hand, occur in the regions consisting of immature crust without basaltic layer (e. g. southern part of the Outer Zone of Southwest Japan), and in the regions of crust exceeding 50km in thickness such as continental margins (e. g. western Bolivia) and continent-continent collision zone (e. g. High Himalaya).