Origin And Early Evolution Of The Lhasa Terrane, South Tibet: Constraints From The Bomi Gneissic Complex

Tibet plateau,termed as“Third Pole”in the Earth,is the youngest and most spectacular of all the continent-continent collisional belts on Earth.It is widely assumed that the high elevation and thick crust of Tibet plateau is largely a consequence of the Paleocene collision and continued convergence between the Indian and Eurasian plates.The Lhasa terrane,the southern margin of the South Tibet,has received much attention as it records the information on the India-Eurasia collision and associated geodynamic processes.However,the Cenozoic deformation may have resulted from a combined effect of Pre-Cenozoic and Cenozoic tectonic activities.In order to solve the problem about the Cenozoic continental dynamic of the Lhasa terrane,we must understand the early evolution of the Lhasa terrane.However,the origin and early evolution of the Lhasa terrane during the Precambrian are still ambiguous.Generally,there are three hypotheses for its derivation:?1?the Lhasa terrane in the Precambrian originated from the northeastern India;?2?the Lhasa terrane might be derived from the northern Australia during the Precambrian;?3?the Lhasa terrane in the Precambrian originated from the northern EAO.Previous studies suggested that the Bomi Complex in the eastern Lhasa terrane records the most complete Precambrian magmatism and metamorphism.In this paper,we carried out an integrated study of zircon U-Pb age,major and trace element geochemistry,and Lu-Hf isotope composition for the granitoid gneisses from the Bomi Complex.We establish the Precambrian magmatic and metamorphic frameworks of the Lhasa terrane,and then to compare with Indian craton.It provides important insights into understanding the origin and early evolution history of the Lhasa terrane.The main research results are as follows:?1?Our new geochronological data reveal three episodes of magmatism at ca.1866 Ma,ca.1288-1264 Ma,ca.824 Ma,and one metamorphism at ca.618-600 Ma in the Bomi Complex.It is note that the Neoproterozoic magmatism was first reported in the Bomi complex.Together with data from literatures,the Bomi Complex had experienced three episodes of magma activities at ca.1866-1782 Ma,ca.1343-1250Ma,and ca.824 Ma,and two episodes of metamorphism at ca.1117 Ma and at ca.625-600 Ma.Thus the Bomi complex is the only one of the four complexes in the Lhasa terrane that witnessed the whole Precambrian magmatism and metamorphism.?2?Paleoproterozoic and Neoproterozoic magmatic rocks are medium to high-K,metaluminous to weakly peraluminous granites,which had experienced later amphibolite-facies metamorphism.These granite gneiss samples have variable REE contents but fractionated chondrite-normalized REE patterns with elevated LREE,depleted HREE with negative Eu anomalies.They are characterized by a relative enrichment in LILEs and LREEs,but a significant depletion in HFSEs,which are similar to those of arc granites.The zircon magmatic cores of the Paleoproterozoic and Neoproterozoic granite gneiss samples have positive?_Hf?t?values of+0.8 to+6.4 and+1.1 to+7.0,respectively,indicating that the two episodes of magmatism with a dominantly juvenile mantle contribution.These rocks show geochemical affinities to volcanic arc granites,and could be associated to the Paleoproterozoic and Neoproterozoic subduction processes,respectively.By comparing the geochemical data,the Bomi Complex and the Himalayan sequence are generally correlative,which formed in a coherent Paleoproterozoic magmatic arc setting along the margin of the supercontinent Columbia,and an Andean-type orogen along the northwestern margin of supercontinent Rodinia,respectively.?3?The Mesoproterozoic magmatic rocks in the Bomi Complex could subdivide into two types,the earlier granite gneisses at ca.1343-1276 Ma and later biotite gneisses at ca.1250 Ma.The Mesoproterozoic granite gneisses have high contents of silicon?SiO₂=71.71-73.08%?and alkalis?K₂O+Na₂O=8.18-9.12%?.They show enrichment of light REE relative to heavy REE and moderate negative Eu anomalies?Eu/Eu*=0.44-0.57?on the chondrite-normalized REE patterns,and negative anomalies of Ba,Nb,Ta,Sr,P and Ti on the primitive mantle-normalized spider diagram.Geochemical features suggest that the granite gneisses have an aluminous A-type granite affinity,and formed in the‘Within-Plate'environment.In contrast,the biotite gneisses have low SiO₂ contents?63.87-67.69%?.They also show enrichment of light REE with?La/Yb?_N ratios of 10.1-28.3 and weakly negative Eu anomalies?Eu*/Eu=0.78-0.83?on the chondrite-normalized REE patterns,and negative anomalies of Ba,Nb,Ta,Sr,P and Ti on the primitive mantle-normalized spider diagram.The protoliths of the biotite gneisses were probably tonalities.They have higher zircon?_Hf?t?values of+10.5 to+3.6 than those?+4.0 to+0.2?of the early granite gneisses,indicating magmatism with a dominantly juvenile mantle contribution.The biotite gneisses have similar geochemical characteristics to those of calc-alkaline granitoids,and probably formed in a subduction-related environment.The geological record in the eastern Lhasa terrane is well comparable to that along the cratons-EGB contact in the southeastern India.Thus,the eastern Lhasa terrane,as the northern extension of the cratons-EGB contact,witnessed an early rifting correlated to the final breakup of the supercontinent Columbia,and later subduction and collision correlated to the amalgamation of supercontinent Rodinia.?4?The metamorphic ages,obtained from metamorphic zircons from the gneisses in the Lhasa,are consistent with the metamorphic timing from the cratons-EGB contact.Combine Mesoproterozoic magmatism,we suggested that Lhasa terrane was probably the northern extension of the cratons-EGB contact in late Neoproterozoic,and witnessed the initial assemblage within the East Gondwana.In summary,the Lhasa terrane and the Indian plate were tectonically linked before the Cambrian,and both witnessed the assembly or breakup of the supercontinents Columbia,Rodinia,and Gondwana.We thus considered that the Lhasa terrane was located in the northern margin of the Indian craton during its early evolution.