Petrology and geochemistry of a calcic and ferrous granitoid pluton: the Mitsuhashi Granite in the Ryoke Belt, southwest Japan

The Mitsuhashi Granite pluton, 8×8 km in extent, was emplaced within the Ryoke metamorphic rocks, yielding a radiometric age of c.80 Ma. The main rock types are coarse-grained hornblende-biotite tonalite and low-K2O granodiorite, with subordinate quartz diorite which bears cummingtonite or garnet. A...

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Published inGankō : Ganseki Kōbutsu Kōshō Gakkai shi Vol. 92; no. 6; pp. 231 - 244
Main Author KUTSUKAKE, Toshio
Format Journal Article
LanguageEnglish
Published Japan Association of Mineralogical Sciences 1997
Subjects
Calcic granitoid
Geochemistry
Mitsuhashi Granite
Petrography
Ryoke Belt
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Summary:The Mitsuhashi Granite pluton, 8×8 km in extent, was emplaced within the Ryoke metamorphic rocks, yielding a radiometric age of c.80 Ma. The main rock types are coarse-grained hornblende-biotite tonalite and low-K2O granodiorite, with subordinate quartz diorite which bears cummingtonite or garnet. At the margin of the pluton, wallrock assimilation produced garnet-biotite granite and granodiorite. Geochemically the rocks are unusually calcic (alkali-lime index=65), peraluminous and metaluminous, and low-potassic, and also have characteristically high Fe/Mg ratios. They have high (av. 280 ppm) Zr and low (<50 ppm) Rb concentrations.Geobarometric calibrations and mineral parageneses indicate that crystallization took place deeper than 15 km (∼0.42 GPa), under a reduced condition. Trace element variations and REE patterns suggest mainly biotite and some plagioclase fractionation. Modal and geochemical variations are modelled principally by varying the ratios of cumulus plagioclase and intercumulus liquid. The potential protoliths of this pluton are mafic rocks of continental lower crust beneath the Ryoke Belt.
ISSN:0914-9783
1881-3275
DOI:10.2465/ganko.92.231