Study On Melting Sequence Of Minerals In Granitoids Under High Temperature And High Pressure

Since Wyllie and his team proposed the theory of Dehydration melting in 1990s, the melting characteristics of hydrous mineral have been getting more and more attention in the Academia. High-pressure and high temperature experiments on the dehydration melting of granite were carried out in a closed water-unsaturated system in this study to analyze and discuss this phenomenon, using powder sample of biotite granodiorite, with the mineral assemblage of Plagioclase(45%) + K-feldspar(15%) + Quartz(20%) + Biotite(13%) + Hornblende(7%). The products were sliced to be observed under the microscope and analyzed by Electron microprobe analyzer. Experiment and analysis results are described as follows:A) Most of the melts is located in Quartz-Biotite or Quartz-Hornblende contact areas, and interconnect each other, few in Plagioclase-Quartz contact area.B) Hydrous minerals remain their form at 850°C. Only few of them break down at their rims. Few melts present at this temperature with low K, high Ca and Na in average. Al₂O₃ content of the melts keep in the range of plagioclase. Melts close to the hydrous minerals contain a small amount of Mg, Fe. It is concluded that the dehydration causes the melting of plagioclase at the very early time.C) As the temperature rises, the degree of melting increases, with an increasing content of Fe, Mg in the melts. The hydrous minerals further decompose, and almost lose their form at 1050°C. Meanwhile, some garnets are observed, with an average composition of SiO₂-31.24%, Al2O3-18.49%, FeO-27.90%, and MgO-21.45%, which can be considered as the productions of the decomposition of Biotites or Hornblendes.D) K contents of melts rise as the T increases, which mean that the K-feldspar participated into the melts, especially in run CG1-3.E) Quartzes appear anhedral crystal with apparent grain boundaries at low temperature, while smooth grain boundaries when the temperature is high. SiO₂ contents of melts close to quartzes and feldspar are significantly increased with increasing temperature.The melting characteristics of hydrous minerals are mainly constrained by water in the system. In a closed water-saturated system eutectic melting of felsic minerals will be first to be observed to generate acidic melt at a lower melting degree, compared to the mafic melt by incongruent melting of hydrous minerals at a higher melting degree. In an open system dehydration melting will go first until the hydrous minerals disappear, at which moment the eutectic melting begins, and the melts evolution from mafic to acid.In this study experiments under closed but water-unsaturated system were carried out, which generated melts that evolved from mafic to acid, too, with an increase contents of Fe, Mg. It shows that the melting sequence in this study is Biotite→Plagioclase→K-feldspar→Quartz, and biotites have been involved all the time, to provide liquid to the system. This is different from both the open system and closed water-saturated system. So we can conclude that in a closed but water-unsaturated system, dehydration melting will go first, resulting in an abnormal melting sequence. Thus rocks in the deep continent crust under this situation will generate a wide range of magmas, and that will bring in the earth's continent crust with a very complex structure, horizontally and vertically.