The Exhumation Mechanism And Process Of Blueschist In Lanlin Area, Central Qiangtang Basin, Tibet

Qinghai-Tibet Plateau which located in southwest China, with the total area of-250km2and the average altitude of4000-5000m, is the largest plateau in China, and also the latest and higest in the world. A large number of studies reveal that the Tibetan Plateau is due to the convergence of India and Eurasia Plate. It is composed of the the collision the "multi-terrain, multi-island-arc and multi-subduction complex rocks" in the Proto-Tethys, Paleo-Tethys and Neo-Tethys. And it is the composite orogenic belts and terranes that formatted in the evolution of the Tethys. In a lot of blocks in Qinghai-Tibet Plateau, Qiangtang block is the one that preserve the tectonic system of Paleo-Tethys best, and is the key areas that reveal the transition from Paleo-Tethys to Neo-Tethys.Recent geological survey and comprehensive study shows that central Qiangtang metamorphic rocks is actually a tectonic melange belt, including marble, quartzite, gabbro, epidote blue schist, garnet blue schist and eclogite el.al.The tectonic melange belt with NW-SE strick extended more than500km, and the widest area has the width of nearly100km. Lanling Blue schist is located at the central of the Qiangtang tectonic melange belt(QTMB), and is the largest outcrop of blue schist in QTMB.Based on the combination of the previous work, this paper report the geochemistry, mineral composition and geochronology characteristics of Lanling Blue schist. This protolith of this blue schist is alkaline basalt with45-50wt%SiO2,7.8-15.1wt%Al2O3and2.5-4.9wt%total alkalis (Na2O+K2O). Most samples are characterized by high TiO2content (3.4-5.7wt%), relatively high∑REE and ELREE contents, are derived from a typical Ocean Island Basalt(OIB) source. Immobile elements covariance diagrams also show that they are formed in an OIB setting with little or no continerntal crust contamination. This result proves the existence of Paleo-Tethys ocean between the Northern and southern Qiangtang terrane.With the subduction of oceanic crust, this basalt subducted under the Northern Qiangtang block in northeast direction. And the exhumation process can be divided into four phases:First, the rock experience lawsonite eclogite-facies metamorphism in about66.2km deep, with the metamorphic condition of2.0GPa in470℃. Then the rocks have a short stay in this depth, may be accompanied by a small amount of uplift, increased from66.2km to55.7km, metamorphic condition is changed to1.7-1.8GPa in530-540℃.Second, the eclogite underwent the ductile shearing in the stretching environment and returned back to the boundary between crust and mantle (at the depth of about33.3km). And experience blueschist facies retrogressive metamorphism with the condition of1.0GPa in500℃.Third, maybe due to underplating of the Gangtangcuo Granite, the rock rose to the depth of14.7km, and transfer to epidote-amphibolite facies metamorphism through isothermal decompression, and the metamorphic condition was0.3-0.67GPa in460-480℃.Fourth, with the effection of surface erosion and the extrusion in NE direction, this rock uplifted to11km deep, and experience greenschist facies metamorphism at the condition of about0.4Gpa in318℃. Then rose to shallow surface environment, and underwent the later brittle deformation in this condition.