Tectonic History And Crustal Evolution Of The Northern And Central Lhasa Terrane

Continental crust is characteristic as its spatial-temporal heterogeneity in structure and composition and as its secular and complex history.Detrital zircons are a minor component of shallow clastic sedimentary rocks but provide robust information about their sources,geological setting of deposition and tectonic affinity.On the other hand,deep-seated xenoliths captured in igneous systems can provide critical understanding about the physicochemical nature and age framework of deep continental crust,as well as crustal accretion and reworking and crust-mantle interaction.The combination of the shallow sediments and deep xenoliths can provide a comprehensive perspective of continental-crust evolution.As the southernmost Gondwana-derived block in the Tibetan Plateau,the Lhasa Terrane is bounded by the Yarlung-Zangbo Suture Zone(YZSZ)in the south and the Bangong-Nujiang Suture Zone(BNSZ)in the north.The Lhasa Terrane experienced secular and multistage tectonism,magmatism and metamorphism related to the Tethyan subduction and continental collision,since it was separated from East Gondwana.In this thesis,a combination of petrological,geochemical and geochronological studies was carried out on the Precambrian metamorphic strata and crustal xenoliths in northern and central Lhasa Terrane,aiming to provide important constraints on the crustal evolution of the central Tibetan Plateau.(1)Neoproterozoic clastic rocks in the upper Nyainqentanglha Group,which represents the earliest sedimentary strata in the Lhasa terrane,were studied to reveal the origin of this block and its response to the evolution of Rodinia supercontinent.U-Pb dating,REE geochemistry and Lu-Hf isotopic data of detrital zircons from the Neoproterozoic clastic rocks are used to constrain the provenance of sediments and the tectonic models for the Precambrian crustal evolution in the Lhasa block.Detrital zircons in a meta-sandstone and two conglomerates yield three Neoproterozoic age populations peaks at 0.82 Ga,0.90 Ga,and 0.99 Ga,respectively.The age patterns and the youngest age population at 805±4 Ma represent the sedimentary deposition age(Tonian time).Most 793 Ma to 850 Ma zircons have ?Hf(t)values between-0.2 and +4.3,while grains with ages between 870 Ma and 940 Ma have ?Hf(t)values between-18.6 and +4.2,implying a transition in the source area from a reworked-crust domain to a more juvenile origin.Mesoproterozic zircons show increasing abundance from 1.8 Ga to 1.0 Ga with weak peaks at 1.32 Ga and 1.68 Ga.The Lu-Hf isotopic fingerprints reveal that the Mesoproterozoic sources changed from a source of 1.68 Ga juvenile crust to a mixed origin of reworked material after Grenville time.Sporadic grains with ages >1.8 Ga have average-crustal Hf model ages(TDM2)ages varying from 3.7 Ga to 2.8 Ga with a peak at 2.5 Ga,implying an ancient continental origin.Felsic veins in the Nyainqentanglha Group consist of quartz,feldspar and zoisite-clinozoisite,and were crystalized at 789±13 Ma.Phase equilibria modeling implies that the felsic vein was produced by partial melting of mafic lower Nyainqentanglha unit under granulite-facies conditions(>740?,<1.1 GPa)with water saturation components.These data help to define the first-order stratigraphic architecture of the Lhasa basement and indicate a close affinity of the Lhasa Terrane with the circum-Rodinia subduction system during the Tonian time.(2)U-Pb-Hf isotopes of zircons,bulk-rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host Early Mesozoic granites(205 ±6 Ma)in the central Lhasa terrane were shown to probe the deep crustal evolution.The xenoliths contain plagioclase,amphibole,titanite,allanite,quartz,biotite and muscovite,with accessory Fe–Ti oxides,apatite and zircon.Bulk-rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite-facies metamorphism(equilibration at pressures of 0.46-0.83 GPa and temperatures of ~650?),before they were captured by the host granite at ~205 Ma.Zircons in these xenoliths show complex microstructures,including inherited cores,magmatic or metamorphic bands,and high U-Th hydrothermal rims.Inherited zircon cores record U-Pb ages from 2277 Ma to 517 Ma.Igneous zircons show a range of concordant U-Pb ages,suggesting a protracted magmatism from 236 Ma to 203 Ma.Metamorphic zircon zones record the timing of amphibolite-facies metamorphism from 224 Ma to 192 Ma,while the high U-Th hydrothermal rims show a subsequent fluid activity until ~150 Ma.Unradiogenic Hf isotopic compositions of both xenoliths and host granites [xenolith ?Hf(t)=-11.2 to 0;host granite ?Hf(t)=-17.3 to-3.3] indicate that the Early Mesozoic deep crust in the central Lhasa terrane was originated mainly from ancient(i.e.,Proterozoic)crust,with little or no interaction with juvenile magmas.This study suggests a possible continental differentiation mechanism during crustal reworking;progressive melting may initiate from the lower mafic crust(at ca.236 Ma)and gradually migrate into the sediment-rich upper crust(until ca.203 Ma).The reworking results in the transition from small fluxes of intermediate magmas to voluminous peraluminous S-type granite.(3)A combined study of petrology,phase equilibria modeling and zircon U-Pb geochronology was carried out on the Baingoin complex from the North Lhasa terrane.The Baingoin complex consists of quartz diorite plutons and metasedimentary xenoliths.The quartz diorite pluton was generated at ~138 Ma,with zircon ?Hf(t)between 0.8 and +4.0.The metasedimentary xenolith contains biotite + sillimanite + cordierite + plagioclase + quartz,with enclosed leucosomes veins less than 2 mm wide(mainly feldspar + quartz + biotite).Phase equilibria modeling shows that the xenolith underwent amphibolite-facies metamorphism and partial melting under peak P-T conditions of ca.680-770 ? and <0.55 GPa.LA-ICP-MS U-Pb zircon dating on the xenolith yield concordant ages from 3167 Ma to 138 Ma,with two groups of age cluster at 763±58 Ma and 426±25 Ma.This study reveals that the Cretaceous crustal growth and sediment underplating in the continental arc system in north Lhasa.Through the combined study of the Precambrian metamorphic strata and crustal xenoliths within Mesozoic plutons,three tectonic-thermal events(late Neoproterozoic,early Mesozoic and late Mesozoic)were identified to control the crustal evolution of the central-north Lhasa terrane.During late Neoproterozoic,the Lhasa Terrane was located in the circum-Rodinia subduction system,receiving continental clastic materials eroded from Mesoproterozoic Himalaya crust and Neoproterozoic arc terranes and depositing on the mafic oceanic crust.After the oceanic basin was closed,Lhasa was possibly attached to north India in Gondwana assembly and experienced ~660 Ma orogenesis.After the closure of the Sumdo Paleotethyan Ocean,the central Lhasa crust experienced a middle-pressure metamorphism and crustal reworking event from bottom to up during the early Mesozoic.The southward subduction of the Bangonghu-Nujiang Tethyan slab induced the Early Cretaceous arc magmatism in north Lhasa,leading to the crustal growth and the underplating and relamination of accretionary sediments into the arc melting regions.