Ailaoshan The Southern Section Of Ailao Mountain Rock Group, Yao Rock Group Metamorphic Characteristics

The researching area, where border on Vietnam and across two first level tectonic units, Yangtze Platform and Cathaysia Block, locates from Ma'andi to Yaoshan area, where is dominated by Jinping County and Hekou County respectively, southwestern Yunnan Province. This article mainly study metamorphic and deformed characteristics of Ailaoshan Group and Yaoshan Group by means of systemic field geological investigation and actually measured section plane, as well as petrochemical and geochemical data, and comebacks their protolith and depositional environment, tectonic setting by means of studying petrologic, petrochemical and geochemical information, and discuss relationship between Ailaoshan Group and Yaoshan Group by means of studying rock association, metamorphic mineral, metamorphic facies, metamorphic temperature and pressure and history of deformation and metamorphism. Based on the above, this article offers some information to solve the important dispute in basic geology, that Yaoshan Group, which is a part of Ailao Group, belongs to Yangtze Platform, or Cathaysia Block.This study indicates that the protolith of Palaeoproterozoic Ailaoshan Group is a suit of volcanic-sedimentary construction, including basic, intermediate and intermediate-acid volcanic construction and continental debris construction carbonatite-bearing, which reflects the sedimentary environment in active continental margin. Correspondingly, the protolith of Paleoproterozoic Yaoshan Group is a suit of sedimentary construction, including continental debris construction carbonatite -bearing, which reflects the shallow sea sedimentary environment in passive continental margin. Two Groups all underwent regional dynamic heat current metamorphism in Lvliang stage and regional dynamic heat current to regional low temperature dynamic metamorphism in Indo-China stage, linear dynamic metamorphism in Yanshan stage and Himalayas stage.By the common methods (relationship between minerals, mineral thermometer, mineral manometer, temperature and pressure determined by mineral experiment and so on), author can get the P-T-t trendlines of Ailaoshan Group and Yaoshan Group in Lvliang stage metamorphic cycle and Indo-China-Yanshan metamorphic cycle.By this research, we can get such important cognitions as follows:1. P-T-t trendlines open out that metamorphisms, which Ailaoshan Group and Yaoshan Group underwent in Lvliang stage and Indo-China-Yanshan stage, correlated with extrusion-collision tectonic regime, but there are differences in the aspects of the intumescing cause, magma action and heat source.(1). Tectonic uplift happened in the two metamorphic cycles, metamorphism in Lvliang stage maybe resulted from gravity equilibrium, but in Indo-China-Yanshan stage maybe resulted from thrust-faulting.(2). The heat current value increasing in Lvliang cycle maybe derived from the heat relaxation and the supply of deep heat flow after subductionâ†'collisionâ†'crust thickened. Therefore the metamorphic climax (highest metamorphic temperature) is concomitance with the pressure fallen, that implied the course of tectonic uplift, and granitic magma come from the crust remelting in metamorphic climax, their restrictive relationship is tectonismâ†' metamorphismâ†'magmatism. The heat current value increasing in Indo-China-Yanshan cycle maybe derived from the deep heat flow upwelling along shear zone when land-land collided, metamorphic climax is concomitance with the course of invariable pressure condition, their restrictive relationship is tectonismâ†'metamorphism or tectonismâ†'magmatism.2. There is notable difference in the course of main metamorphic process between Ailaoshan Group and Yaoshan Group. Most differences are as follows:Difference in source of heat current: Ailaoshan Group underwent a shortdated temperature increasing and pressurizing course in prograde metamorphism, afterward it went through a long period of almost isopiestic and temperature increasing course, it implied that Ailaoshan Group has gotten the deep heat current supply, however Yaoshan Group went down constantly, the deep heat current supply is shortdated. Perhaps, it reflected the differences in metamorphism environment of them.Difference in buried depth: Ailaoshan Group went through the highest pressure at 0.8 Gpa, corresponding buried depth is about 30 kilometers. Yaoshan Group went through the highest pressure at 1 Gpa, and corresponding buried depth is about 36 kilometers.Difference in returning phase: after Ailaoshan Group underwent temperature reduction and pressure down, metamorphic process ended in andalusite face, and it went through fleetly uplifting in the last phase. Yaoshan Group metamorphic process ended in sillimanite face. According to the end point of metamorphism, the final buried depth, Yaoshan Group returned after metamorphism, is deeper than Ailaoshan Group.According to P-T-t trendlines and estimating P/T values by mineral thermometer, mineral manometer, the highest temperature Yaoshan Group reached in metamorphism is higher than Ailaoshan Group's. The P-T-t umbriferous points of Ailaoshan Group are dispersive, but Yaoshan Group's concentrate on the sillimanite face area. It maybe reflect that Yaoshan Group went through an action with a longer time and higher temperature. So, only a little metamorphic mark in the embedding phase is residual.3. Yaoshan Group maybe belong to the crystalline basement of Cathaysia Block.