The Ogawa granodiorite mass in the western part of Agematsu Town, central Japan. Part 2, petrology of the marginal facies and the origin of various rock facies.

Abstract

The marginal facies of the Ogawa granodiorite mass is observed not only at the margin of the mass but as dykes in the surrounding rocks. The marginal facies in question show particular porphyritic structure which is distinguished from the petrographic fabrics ubiquitously observed in the granodiorite mass. Characteristic features of the marginal facies are as follows; 1) The phenocrysts comprise plagioclase, quartz, hornblende and biotite. It is noteworthy that hornblende phenocryst is also generally observed. 2) Quartz and other phenocrysts are corroded and enclose the groundmass minerals at their margins. 3) K-feldspar is almost absent as phenocryst. The chemistry of the marginal facies corresponds to an average of the chemical compositions of the three rock facies (medium-grained, porphyritic and fine-grained facies) which form the main part of the granodiorite mass. From these petrographic and chemical features, the author was led to the conclusion that the magma which formed the Ogawa granodiorite mass had the chemical composition corresponding to the marginal facies in question with about 71% silica and that after the consolidation of chilled marginal facies, the medium-grained facies formed first, followed by the formation of porphyritic facies and finally fine-grained facies as the results of the crystallization differentiation in situ.