Petrogenesis Of Metamorphic Rocks In The Himalayan Orogen:Case Studies Of Gneisses From Yardoi And Migmatites From Nyalam

Collisional orogens record geological processes such as continental collision,subduction,and exhumation,which result in varying degrees of crustal deformation,metamorphism,anatexis and magmatism.They have been a key area for studying continental dynamics and developing plate tectonic theory.As the product of collisional orogen,metamorphic rocks along convergent continental margins can record important information about the formation and evolution of orogenic belts.The Himalayan orogen is the largest and youngest collisional orogen in the world.The collisional event is very young,which not only preserves the HP to UHP metamorphic records related to the collision between India and Asia continents,but also records thermal events at the later stage of the orogenic collision.Therefore,the Himalayan orogen is a natural laboratory for studying anatectic metamorphism of crustal rocks in the collision orogen and testing various models for the formation and evolution of collisional orogens.In this PhD thesis,a combined study of metamorphic petrology and isotope geochronology was carried out for gneisses from the Yardoi dome in eastern Himalaya and migmatites from the Nyalam area in middle Himalaya.The results indicate that the gneisses in the Yardoi dome would not only experience high-pressure Barrovian facies metamorphism in the Eocene,but also experience three periods of decompressional metamorphism in the Eiocene-Miocene.Furthermore,the migmatites in the Nyalam area would have undergone crustal anatexis through mechanisms of muscovite dehydration and hydration melting,and the timing of partial melting can be constrained by zircon U-Pb geochronology and garnet element diffusion modelling.Petrographic observations,mineral major and trace element analyses,phase equilibrium modelling,zircon U-Pb and monazite U-Th-Pb dating were carried out for the gneisses from the Yardoi dome in eastern Himalaya.The results indicate that these metamorphic rocks experienced three metamorphic events,among which the paragneiss records the high-pressure Barrovian facies metamorphism at about 44.8 Ma in the Eocene,and the pressure can reach～11-12 kbar at temperatures of～600-625℃.Such pressures are different from the previous study of low-pressure metamorphism at<9 kbar in the target region.Furthermore,the metamorphic rocks in this area record three episodes of decompressional metamorphism.The first episode occurred from HP Alpine facies series to Barrovian facies series terminated under P-T conditions of～11-12 kbar and～600-625℃ at 44.8 Ma in the Middle Eocene.The second episode of decompression took place at 41-30.3 Ma,and the metamorphic pressure decreases from the high pressures of 11-12 kbar to medium pressures of 6-8 kbar.The third episode of decompression occurred at 37.8-17.8 Ma,and the metamorphic pressure decreases from 7-10 kbar to 3.5-5.5 kbar.The corresponding thermal gradients and metamorphic facies series would have changed from Alpine-type in the early Eocene through Barrovian-type in the middle Eocene to Buchan-type in the Miocene.In view of the contemporaneous occurrence of magmatism along the Himalayan orogen,it is inferred that the formation of the Yardoi gneiss dome may be related to these decompression processes.Immediately following the collision between the Indian and Asian continents,the crustal rocks underwent the Barrovian-type metamorphism at～44.8 Ma in the lower crust after their decompressional exhumation through slice thurst in the middle Eocene.Afterwards the decompressional processes occurred at 37.8-17.8 Ma,and the metamorphic rocks were further exhumated to the shallow upper crust in the Miocene.The decompression in this stage is associated with the domical extrusion of gneiss domes from the lowest crust to the uppermost crust,which leads to the activity of the STDS and the final formation of gneiss domes in the Himalayan orogen.Petrographic observations,mineral major and trace element analyses,phase equilibrium modelling,conventional thermobarometries and zircon U-Pb dating were carried out for the migmatites from the Greater Himalayan sequences in the Nyalam area,middle Himalaya.The results indicate that these migmatites not only record the clockwise P-T trajectory,but also widely develop structures of crustal anatexis.The anatectic metamorphism reached the peak pressure stage at>31 Ma,and the temperature and pressure ranges of 7.3-9.0 kbar and～700℃.Afterwards,the rocks were heated to the peak stage of～725℃ and 6 kbar at 26-21 Ma,leading to partial melting.The melting event occurred at 26.4 Ma and lasted till 18 Ma.In addition,according to the forward modelling of phase equilibrium,it is inferred that these migmatites were produced by two types of melting reaction involving muscovite breakdown,including hydration melting and dehydration melting.Comparing the migmatites with different melting reactions,it is found that melting with water has a significant effect on the composition of plagioclase and the content of K-feldspar,such as enrichment of An in plagioclase and lack of K-feldspar.On the contrary,plagioclase in the dehydration melting reaction is relatively poor in An content,and a large amount of K-feldspar is formed.The above findings provide a new perspective for judging the melting reactions from the viewpoint of migmatites.Combined with the P-T trajectory,it can be seen that the migmatite with hydration melting is in a shallower layer than that with dehydration melting.This coupled layered dehydration-hydration melting relationship is of great significance in the study of anatectic mechanism of continental crust in collisional orogens.