Mesoproterozoic To Early Paleozoic Tectonic Evolution Of The North Qaidam Orogenic Belt

The North Qaidam Orogenic Belt(NQOB)is located at the north margin of the Tibet plateau and tectonically in the central region among the North China,Yangtze and Tarim Cratons.It is bounded by the Qilian Block in the north and Qaidam Block in the South,and consists of orthogneisses,felsic and pelitic gneisses with minor lensoid or intercalated eclogites and garnet peridotites.Recent studies on eclogites and their host gneisses show that this belt may record two orogenic cycles,the earlier of which is related to the amalgamation and break-up of the supercontinent Rodinia in Proterozoic whereas the later one involves Early Paleozoic oceanic subduction followed by continental subduction and collision.The special tectonic location and prolonged multi-orogenic records make the Proterozoic to Early Paleozoic evolution of the NQOB of great significance in deciphering the relationship between different cratons and micro-blocks in China.In this thesis,we carried on systemic metamorphism,geochemistry and geochronology studies on different type eclogites and their host gneisses in Yuka-Luofengpo terrane and the granitic gneiss in Shaliuhe area,and obtained the protolith ages and origins,metamorphic ages as well as P-T paths of different type eclogites and their host gneisses;Then the relationship between eclogites and their country gneisses was discusses.Finally,combined our and published data,we discussed the Mesoproterozoic to Early Paleozoic evolution of the NQOB.The main achievements and new understandings of this thesis can sum up as follows:1.Based on detailed field investigation and petrographic observation,we identified four new type eclogites(lawsonite eclogite,talc-kyanite-bearing eclogite,and Luofengpo coarse-and fine-grained eclogite)in Yuka-Luofengpo terrane.In combination of phase equilibrium modelling and geothermobarometry calculation methods,we established their P-T paths and further confirmed that the mineral assemblage of eclogites in Yuka-Luofengpo terrane is mainly controlled by bulk-rock X(MgO)[=MgO/(MgO+FeOT)]and water content.(1)Petrographic observation identified abundant rectangular-or rhombic-shaped mineral aggregates composed of polycrystalline clinozoisite or polyphase clinozoisite+paragonite±phengite±quartz and clinozoisite+amphibole±quartz±Na-rich plagioclase in garnet porphyroblasts of eclogites from Yukahe section for the first time.The peculiar shape,mineral assemblage and reconstructed compositions of these aggregates show that they are pseudomorphs after lawsonite.This discovery make the NQOB be the 20th orogenic belt exposing lawsonite eclogite in the world.Phase equilibrium modelling by Perple_X predicts that the lawsonite eclogite experienced a clockwise P-T evolution.The peak stage mineral assemblage is Grt+Omp+Tlc+Law+Qtz+Rt and the estimated peak P-T condition is 2.7 GPa,570℃,near the quartz-coesite transition boundary.(2)Based on meticulous petrographic observation and mineral chemistry data,we established a clockwise P-T path for the talc-kyanite-bearing eclogite by phase equilibrium modelling using Thermocalc.The obtained peak mineral assemblage and P-T condition are Grt+Omp+Law+Ph+Coe and 3.1-3.2GPa,650-655℃,respectively.And after peak stage,the talc-kyanite-bearing eclogite underwent isothermal decompression.(3)According to petrographic observation,mineral chemistry data and pseudosectioncalculated by Thermocalc,the peak mineral assemblage and P-T condition of the Luofengpo coarse-grained eclogite are Grt+Omp+Rt+Qtz/Coe and P>2.58GPa,T>670℃,respectively.This eclogite underwent strongly amphibolite facies retrogression during decompression because of external fluid infiltration.(4)T-X(MgO)and P-M(H2O)pseudosections calculated using Thermocalc show thatthe mineral assemblage of eclogites in Yuka-Luofengpo terrane is mainly controlled by bulk-rock X(MgO)and water content.Rocks with high X(MgO)favour lawsonite-bearing peak assemblage of Grt+Omp+Law+Rt+Qtz/Coe±Ph and form kyanite-bearing mineral assemblage of Grt+Omp+Ky+Rt+Qtz/Coe±Ph by lawsonite dehydrating reactions during decompression.Whereas eclogites with low X(MgO)tend to form lawsonite-absent peak assemblage of Grt+Omp+Rt+Qtz±Ph and this mineral assemblage will not not evolve without external fluid during exhumation.2.We ascertained that the protoliths of the newly identified lawsonite eclogite,talc-kyanite-bearing eclogite as well as Luofengpo coarse-and fine-grained eclogite are all oceanic crust segments based on bulk-rock geochemistry and zircon U-Pb and Hf-O isotopic data,and confirmed that the protoliths of talc-kyanite-bearing eclogite and Luofengpo eclogites are pieces of Mesoproterozoic oceanic crust produced before amalgamation of the supercontinent Rodina.(1)The talc-kyanite-bearing eclogite has N-MORB-like geochemical characters.LA-ICP-MS and SIMS zircon dating yielded a protolith age of 1273-1280Ma and a peak metamorphic age of ca.437 Ma for it.Zircon Lu-Hf analyses show that the magmatic zircon cores have high εHf(t)of 8.36-15.98 and TDM of 1131-1451 Ma(Mean=1303±55Ma,consistent with its protolith age within error),indicating that its protolith is juvenile crust without continental crust contamination.Therefore,we interpreted the protolith of the talc-kyanite-bearing eclogite as a Mesoproproterozoic oceanic gabbro produced before amalgamation of the supercontinent Rodina;it emplaced in the active continental margin of the Qaidam Block during the assembly of Rodinia and underwent continental deep subduction in Early Paleozoic.(2)Both the Luofengpo coarse-and fine-grained eclogites have MORB-like geochemical signatures.Zircon U-Pb data show that the Luofengpo coarse-grained eclogite has a protolith age of 1070Ma,a Neoproterozoic amphibolite facies metamorphic age of 927Ma and an Early Paleozoic eclogite facies metamorphic age of 439Ma;The fine-grained eclogite also experienced the 916 Ma amphibolite facies metamorphism and partial melted during this Neoproterozoic metamorphic event,as well as the 437Ma eclogite facies metamorphism.Zircon Lu-Hf isotopic data show that the protoliths of both Luofengpo coarse-and fine-grained eclogites are juvenile crust.All these suggest that the protoliths of Luofengpo eclogites are Mesoproterozoic oceanic fragments produced before amalgamation of the supercontinent Rodina;they emplaced in the active continental margin of the Qaidam Block during the closure of this Proterozoic ocean and underwent the first stage metamorphism during the later continental collision;Finally,they underwent continental subduction in Early Paleozoic.(3)Geochemical data show that the lawsonite eclogite has MORB-like character.Zircons from this sample have similar δ18O values(5.38±0.10‰)with the normal mantle(5.3±0.3‰),indicating that the protolith of the lawsonite eclogite is derived directly from mantle rock without continental crust contamination.SIMS and LA-ICP-MS zircon U-Pb dating yielded a 436-438 Ma eclogite-facies metamorphic age for the lawsonite eclogite,similar to the continental subduction age of the NQOB,but did not obtain its protolith age.These data indicate that the protolith of the lawsonite eclogite may be either a piece of Proterozoic ocean that was emplaced in the active continental margin of the Qaidam Block during the amalgamation of the Rodinia supercontinent,or the Early Paleozoic oceanic crust in the transition zone between the South Qilian Ocean(or Tanjianshan Ocean)and the Qaidam Block.3.Based on meticulous petrographic investigation and mineral chemistry data,we established clockwise P-T paths for the Yuka pelitic gneiss and Shaliuhe granitic gneiss in combination of phase equilibrium modelling and geothermobarometry calculation methods,and confirmed that they experienced ultrahigh-pressure(UHP)metamorphism.According to the similar P-T paths and metamorphic ages between eclogites and their host gneisses,we proposed that the eclogites are“in situ" with their country gneisses and they underwent continental deep subduction together in Early Paleozoic.(1)We established a clockwise P-T path for a pelitic gneiss,the direct country rock of Yuka massive eclogite.The predicted peak mineral assemblage is Grt+Jd+Ky+Rt+Coe and the estimated peak P-T conditions is 3.1-3.9GPa,620-700℃,above the quartz-coesite transition boundary,thus the pelitic gneiss experienced UHP metamorphism.We also established a clockwise P-T path for a granitic gneiss,the direct country rock of Shaliuhe continental-type eclogite.The obtained peak P-T condition is 3.0-3.4GPa,655-745℃,indicating that the granitic gneiss underwent UHP metamorphism.In addition,we found relic phengite and high-Al titanite in another granitic gneiss,which is the host rock of Yuka phengite eclogite,indicating that it underwent at least high pressure metamorphism.(2)Zircon U-Pb dating yielded peak metamorphic ages of 432Ma and 447Ma for the Yuka pelitic gneiss and Shaliuhe granitic gneiss,respectively.According to the similar P-T paths and metamorphic ages of eclogites and their host gneisses,we proposed that the eclogites are "in situ" with their country gneisses and they underwent continental deep subduction together in Early Paleozoic.4.We proposed that the eclogite in NQOB can be divided into three types according to their protolith ages and affinities,and they recorded the Mesoproterozoic to Early Paleozoic tectonic evolution history of NQOB.The first type eclogites are represented by the newly identified Yuka talc-kyanite-bearing eclogite and Luofengpo coarse-and fine-grained eclogite.Their protoliths are Mesoproterozoic oceanic fragments produced before the amalgamation of Rodinia and some of them recorded the Neoproterozoic metamorphic event related to the assembly of Rodinia.The second type eclogites are represented by Yuka phengite eclogite and massive eclogite.Their protoliths are within-plate basalts or continental flood basalts formed in a continental rift environment related to the breakup of Rodinia.The third type eclogites are represented by the Shaliuhe kyanite eclogite,phengite eclogite and epidote eclogite.Their protoliths are oceanic fragments in the transition zone between the South Qilian Ocean(or Tanjianshan Ocean)and the Qaidam Block.All the three type eclogites recorded the 420-460 Ma UHP metamorphism.This also confirmed that two oceans developed successively in this area.Therefore,we can NOT use the metamorphic ages of oceanic affinity eclogites without protolith ages to constrain the Early Paleozoic evolution history of the NQOB.5.Combined metamorphism,geochronology and geochemistry study results,we established the Mesoproterozoic to Early Paleozoic tectonic evolution history of the NQOB at a preliminary level.It can be summarized as:the Mesoproterozoic ocean floor spreading stage(1280-1030Ma);the Proterozoic seafloor subduction stage(1030-950Ma);Neoproteorzoic continental collision and Rodinia formation stage(950-890Ma);Rodinia breakup related continental rift formation stage(850-750Ma);Early Paleozoic ocean and Tanjianshan Group formation stage(ca.550-460Ma);Continuous oceanic and continental deep subduction stage(460-420Ma);Breakoff of subducted oceanic slab and exhumation of continental crust stage(<420Ma).