Petrogenesis of granitic magma and its thermal history : disclosure of melting reaction from granitic suites

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An acceptable model for the generation of granitic magma is partial melting of crustal materials. Experimental studies using both model and natural systems have revealed melting reactions for crustal materials under conditions of lower to middle crust. The melting reactions in metamorphic rocks were previously reported from high-grade metamorphic terranes, whereas those in igneous rocks are less understood. It is possible that any information with respect to melting reactions is preserved in granitic suites because of their high-viscosity of magma.We found reaction texture to produce granitic melt in the granite suites that intruded granulite facies metamorphic rocks in the Kontum Massif, Vietnam and also metamorphic xenoliths included in the diorite complex in the Hidaka metamorphic belt, Japan. Taking texture and chemistry into account, the biotite included in the anhedral garnet would be of a residual phase at melting of the pelitic gneiss by dehydration melting. The reaction texture producing granitic melt would therefore be preserved in the pelitic gneiss as well as the granite in Kontum Massif, Vietnam. In the Hidaka metamorphic belt, whole-rock geochemical studies combined with metamorphic analyses indicate that the xenoliths in the diorite complex are regarded as restites after releasing granitic melt.Sm-Nd internal isochron ages involving retrograde garnet determined from three ultrahigh-temperature (UHT) gneisses taken from Tonagh Island, the western part of the Napier Complex, East Antarctica gave 1870 ± 82 Ma, 1897 ± 50 Ma and 1557 ± 35 Ma. It is stressed that the assembly of East Gondwana including the Napier Complex, the Rayner Complex, and the Eastern Ghats Belt, if existed, should be before ca 1600 Ma, and may be traced back to ca 2000 Ma of the supercontinent "Columbia" era.