Chemical and Mineralogical Characteristics of the Kanamaru Pegmatite Deposit, Niigata Prefecture, Japan
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概要
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Late Cretaceous (81.5 Ma) granitic rocks of the Kanamaru mine area are composed of coarse-grained biotite granite, fine-grained biotite granite and two-mica granite-aplite. They were found to be all ilmenite-series, and are high in K<SUB>2</SUB>O, and low in total Fe<SUB>2</SUB>O<SUB>3</SUB>, MgO and CaO, as compared with the average Japanese granitoids. The Rb/Sr ratio is 0.3-1.6 in the coarse-grained biotite granite, 0.9-1.1 in the fine-grained biotite granite, 15-52 in the two-mica granite, 68-108 in the garnet-biotite rocks and higher than 76-104 in the K-feldspar pegmatite. The most-fractionated, two-mica granite appears to be genetically related to the pegmatite deposit. Biotite in the two-mica granite and garnet-biotite rocks are iron rich, as a result of fractionation of the original ilmenite-series magmas. Garnets are almandine > spessartine in composition.<BR>The pegmatite deposit, which has a rugby-ball shape with an E-W axis, is hosted in the fine-grained biotite granite and the two-mica granite. It is leucocratic, containing a large amount of K-feldspar, some quartz and only a little mafic silicates, reflecting the K<SUB>2</SUB>O-rich and Fe-poor characters of the original granitic magma. No mineralogical and textural zonings are observed in the pegmatite. Fluid inclusion study on quartz indicates low homogenization temperature and presence of CO<SUB>2</SUB>-bearing fluids. K-feldspars of the two-mica granite and pegmatite have high degrees of triclinicity. These rocks may have been solidified under volatile-rich environment, which made Al-Si ordering of the K-feldspars during the sub-solidus stage.
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