Integrated P-T Paths of the High-Pressure Rocks and Their Tectonic Implications for the Mountain-Building of the North Qilian, China

摘要

After the integration of petrographic study, geothermobarometry and Gibbs method, the synthetic P- T pathsfor the rocks from different geological profiles in the North Qilian, China, have been derived. The composite P-T pathsfrom different methods indicate that all the high-pressure rocks in the Qilian area recorded P-T paths with clockwise loopsstarting at the blueschist facies, later reaching peak metamorphism at the blueschist facies, eclogite facies or epidote-amphibolite facies and ending up with the greenschist facies. The incremental Ar-Ar dating shows that the plateau ages forthe high-pressure rocks range from 410 to 443 Ma. The plateau ages could be used as a minimum age constraint for thesubduction that resulted in the formation of these high-pressure rocks in the Qilian area. It is proposed that the late-stagedecompressional and cooling P-T paths with ends at the greenschist facies for these high-pressure rocks probably reflectthe uplift process which could occur after shifting the arc-trench tectonic system to the system of continental orogenic belts.The retrograde paths for the high-pressure rocks in the North Qilian tectonic belt are characterized by dramatic decom-pression with slight cooling, which suggests very rapid exhumation. Petrography supports that the mountain-building forthe Qilian mountain range could undergo a very fast process which caused rapid uplift and denudation.