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.

抄録全体を表示

Ferromanganese oxide ores in Sandur, Karnataka, India, consist of the manganese minerals: cryptomelane, romanechite, pyrolusite, lithiophorite and manganite and the iron minerals: magnetite, hematite and goethite. A mixed Mn-Fe-Al phase is invariably present. Quartz, kaolinite, gibbsite and rutile are the minor associates in the ore. The manganese minerals mostly show colloform, mammillary, vein filled, fibrous and other replacement textures. The magnetite mostly occurs as idiomorphic crystals and is the earliest formed phase in the paragenetic sequence.
     The Fe-Mn rich lithologies have high Mn/Fe ratio (av. 2) and are low in trace element and REE content. These are also characterised by : (i) high Al₂O₃ (av. 7.44%), (ii) low phosphorus, (iii) preponderance of K₂O over Na₂O (iv) comparable level of CaO and MgO, (v) average of 0.1% Li₂O and (vi) ubiquitous BaO, markedly concentrated in romanechite.
     Major, trace and REE distribution patterns in the ferromanganese oxide ores and their discriminatory plots provide evidences, though equivocal, do not indicate a single genetic process. A hydrothermal component for the manganiferous lithologies deposited contemporaneously with the detrital material is suggested. The mineralisation is of stratabound replacement type, where the ore acquired all its trace and REE signatures from the surrounding argillite, subsequently subjected to residual concentration.

抄録全体を表示

1対2希釈ガラス円盤による岩石の主成分・微量成分螢光X線分析法を検討した。ガラス円盤は試料1.8 g，融剤3.6 gとし，1200°C，9分間で溶融した。主成分分析には多成分重回帰計算で補正計数を決定し，1対5希釈と同程度の正確度を得た。同じガラス円盤を用い，14微量成分を測定した。検量線は地質調査所標準岩石と，それに標準溶液添加した試料で作成した。両者の検量線は一致している。標準添加した試料を用いて元素間の重なり補正を行った。螢光X線のS/N比が悪いので，測定時間は粉末法の2-10倍必要だった。同一の測定時間，機器条件で粉末法とガラス円盤法の比較を行った。ガラス円盤法は測定再現性にやや劣るが，正確度は優れている。粉末法には粒度効果や鉱物効果があり，測定時間が長くても精度が上がらないためである。さらに標準添加法を用いた微量成分分析を試みた結果，より精度高い分析ができた。

抄録全体を表示