東北日本，月山火山新期噴出物の岩石学的研究

Abstract

The Gassan volcano is part of the Chokai volcanic zone, and consists of a number of volcanic edifices (Gassan, Ubagatake, Yudonosan, Amamoriyama, Sen-nindake, Yakushidake, Waratahageyama, Shinakurayama, and Ohirayama) which were formed during the Quaternary in northeastern Japan. This study has been focussed on Gassan, Ubagatake, Waratahageyama, Amamoriyama, and Ohamaike, which are defined here as the younger stage lavas of the Gassan volcano. Younger stage lavas of the Gassan volcano are andesites, except in the case of Amamoriyama (dacites). These rocks have some or all of the following features; (1) presence of plagioclase phenocrysts with sieve-textured zones (spongy cores, or dusty cores or zones) and penetrative resorption features, (2) overgrowth of clinopyroxene around orthopyroxene, (3) orthopyroxene reaction rims around olivine, (4) disequilibrium phenocryst assemblages such as Mg-rich olivine and quartz, and (5) existence of mafic inclusions. Mineralogical compositions of host rocks and mafic inclusions have been determined and identified as follows. Cores of plagioclase phenocrysts have a wide range in composition (An 40-90) and show a bimodal distribution (peaks: An 40-60, An 70-90). The Mg/(Mg+Fe) ratios of the cores of clinpyroxene and orthopyroxene phenocrysts have a unimodal distribution (peaks Cpx, 0.72-0.76; Opx, 0.64-0.68) with minor amounts of Mg-rich ones (Cpx, 0.76-0.84; Opx, 0.72-0.76). Mg-rich phenocrysts show Mg-Fe partitioning in equilibrium with Mg-poor olivine phenocrysts. The crystallization temperatures of both clinopyroxene and orthopyroxene phenocrysts estimated by using pyroxene thermometry are about 800°C (Mg-poor pairs) and 1000°C (Mg-rich pairs). These characteristics of phenocrysts strongly suggest that the Gassan younger stage lavas are the products of mixing between silicic (ca. 800°C) and minor amounts of mafic (ca.1000°C) magmas.