Deformation And Metamorphism Characteristics Of The Garnet-bearing Phengite Schist In Lanling Area, North Tibet

The lanlingmetamorphic belt is located in the suture zone of Gondwana and Eurasian Continent, and it is also a typical orogenic zone accretionary complex, consisting of eclogite, glaucophane schist and garnet-muscovite schist. As the main component of the complexe, the garnet-muscovite schist recorded the information of its forming and deformation process, which have great significance for deciphering the tectonic evolution of the plate suture zone. In this paper, the microtectonic analysis of the garnet-muscovite schist have been combined with outcrop structure and metamorphic characteristics study, in order to rebuild the deformation and metamorphism history, deformation mechanism and forming environment of the rock, and further, the tectonic evolution of the region.According to the study, four stages of deformation have been distinguished. The first stage of deformation is related to the oceanic subduction, forming the N-S construction. The new foliation transversal structural transported the original foliation, and syntectonic quartz vein formed at the same time. And this stage of deformation happened in a high pressure and low temperature environment, which was recorded by the garnet inclusion quartz’s fabric(basal<a> and rhomb<a>) and the garnet inclusion phengite’s Si composition. The second stage of deformation was developed under consistent construction background. Longitudinal structural transposition occurred, and a penetrating foliation(S2), which was the axial plane cleavage of recumbent fold, replaced the S2. In the micro-scale, the quartz in matrix grain boundary migration dynamic recrystallized and its prism<a> and prism<c> slip system activated and wiped out the former basal<a> and rhomb<a>, meaning the deformation was related to a high temperature environment. The next deformation is associated with E-W compression, making relieved upright fold and a series of crenulation cleavage(S3), and in micro scale, the BLG recrystallization quartz overlying the GBM recrystallized quartz. The BLG+GBM recrystallized quartz shows a different fabric pattern, charactering by the prism<c> moved to the basal<a> slip system. The last stage of deformation developed in surface or near surface, and was respond to the conjugate shear joint and normal fault. The rock has been experienced three stages of metamorphism. The first was blueschist facies, and the second was epidote-amphibolite facies, which was followed the greenschist facies respectively. And the peak metamorphic temperature and pressure is above 550℃ and 1.24 GPa, which are calculated by the quartz fabric and phengite’s Si(p.f.u).