贺兰山北段孔兹岩系脱水熔融实验研究——临界熔体比例的确定及意义

 在充分研究贺兰山北段区域地质的基础上,选定该区孔兹岩系中最有代表性的两类岩石:变粒岩类及富铝片麻岩为实验对象,进行天然块状样品的脱水熔融实验研究。根据成分变异的特点,确定岩浆的临界熔体体积比为30%,其不仅影响变质作用p-T-t轨迹的演化路径,而且与原地、半原地及异地型花岗岩的形成有着密不可分的关系。

DEHYDRATION MELTING EXPERIMENT OF THE KHONDALITE SERIES OF THE NORTHERN SEGMENT IN THE HELAN MOUTAINS:DETERMINATION AND THE SIGNIFICANCE OF CRITICAL MELT FRACTION

Based on the regional geology, two types of rocks had been selected as the experimental samples:Al-rich geniss(H029) and Biotite granular(H013). The results of solid samples dehydration melting experiments show that the composition of the melt is unhomogenouse except at 4kb,850℃ and its melt fraction is nearly 30%. The study on rheology shows that the mechanical nature of the rocks can change with melt fraction. When the melt fraction is high,the system follows the rule of Binnhan fluid.It means that the fluid could transfer stress, but could not resist it.When the melt fraction is low, the agent between solid grains is enhanced, and the system becomes nearly solid.We think that changes of the melt fraction can result in changes of the melt composition. Because the melt fraction is high at 0.4 GPa 850℃(30%±),the behavior of the system coincide nearly with that of Binnhan fluid,so changes of the melt composition depend on convection.when the melt fraction is low at other conditions(<30%),changes of the melt composition depend on diffusion.The rate of diffusion is by far lower than convection so that the melt composition could be changed into homogenouse at 0.4 GPa 850℃. Based on the change of the characteristics of the melt composition,this article concludes that the Critical Melt Fraction (CMF) is 30% ± and the change of rhelogical properties of the melt results in the change of composition of the melt.Combined with the metamorphic p-T-t path of the Khondalite Series and the characteristics of granite in Helan Moutains, the Critical Melt Fraction is shown to be closely related to the formation of granite body.