STUDY OF THE ORIGIN OF MICRO-INTERGROWTH OCCURRED IN THE GRANITOID ROCKS
花岗质岩石中微粒交生体的成因研究

A mineral aggregate or micro-intergrowth made of fine grains often appears in the interstice or periphery of the feldspar grains within the rocks which underwent ductile shear or plastic deformation. At present, it is generally believed that the micro-intergrowth would be products of recrystallization and the occurrence of the micrc-intergrowth shows that the rocks not only suffered from the plastic deformation under stress, but the anatexis could also be caused by increasing temperature and pressure. Therefore, the micro-intergrowth is the prcducts of crystallization of intergranular melt. According to the texture the micro-intergrowth can be divided into the following types:granular, sutured, myrmekitic, graphic and the quartz intergrowth with deformational riband and undulatory extinction. Judging from micro-electron prbe analyses, the micro-intergrowth is mainly made of albite, potasium feldspar and quartz,and the average composition of the normal mineral corresponds to that of "Iower-limit melting point"of Q-An-Ab-Or-H_2O system.The occurrence of the microintergrowth demonstrates that the "lower-limit melt" could be migrated and concentrated into the fissure, fold bend and other places with released stress forming the veins during the plastic deiormation. The mineralogy and chemistry of the serokinematic potash granites in the areas of Archean high-grade metamorphism (e.g. Qin Huandao granites of East Hebei Province) are similar to those of the micro-intergrowth. This means that the "lower-limit melt" produced and accumulated during the plastic deformation could finally form a large scale potash granitic rock.