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

花岗岩岩浆形成的一个可接受的模型是地壳物质的部分熔融。利用模型和自然系统进行的实验研究揭示了地壳物质在中下地壳条件下的熔融反应。变质岩中的熔融反应以前是在高变质地体中报道的，而火成岩中的熔融反应则鲜为人知。由于花岗岩岩浆的高粘度，任何关于熔融反应的信息都可能被保存在花岗岩套中。在侵入越南Kontum地块麻粒岩相变质岩的花岗岩套中，以及日本Hidaka变质带闪长岩杂岩中的变质包体中，均发现了产生花岗质熔体的反应结构。从结构和化学性质上考虑，菱形石榴石中的黑云母可能是泥质片麻岩脱水熔融时的残留相。因此，在越南孔图姆地块的泥质片麻岩和花岗岩中都保留了产生花岗质熔体的反应结构。在Hidaka变质带，全岩地球化学研究结合变质分析表明闪长岩杂岩中的捕虏体在释放花岗质熔体后被认为是残岩。从东南极洲Napier杂岩西部的Tonagh岛采集的3块超高温片麻岩中测定的涉及逆行石榴石的Sm-Nd内部等时线年龄分别为1870±82 Ma、1897±50 Ma和1557±35 Ma。强调包括Napier杂岩、Rayner杂岩和东高止山脉带在内的东冈瓦纳的组合，如果存在的话，应该在约1600 Ma之前，可以追溯到约2000 Ma的超大陆“哥伦比亚”时代。

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.