Nature Of The Mantle Peridotite Massifs In Tibet (China) And Associated Geodynamic Processes

The mantle is an important reservoir of material and energy within the Earth, and acts as the medium to circulate them between the core and the crust. Understanding of the structure, composition, origin and evolution of the mantle is a long-standing theme in Solid-Earth science. Kilometer-scale peridotite massifs in collisional or suture zones provide optimal targets for investigating these problems.I have investigated the Shenglikou garnet-facies peridotite body within the North Qaidam ultrahigh-pressure (UHP) orogen of NE Tibet, and the Zedang spinel-facies peridotite massif in the Yarlung-Zangbo suture zone of south Tibet, using petrography, compositional mapping, major- and trace-element analysis and Sr-Nd-Hf-Os-O isotopic analysis of whole-rock samples and minerals, and U-Pb/Hf-isotope studies of zircons. The aims are to:(1) reveal the origins of different rock types in the Shenglikou peridotite massif; (2) determine the petrogenesis of the Shenglikou pyroxenite dykes; (3) understand the role of chemical composition in the behavior of the continental lithosphere during subduction; (4) define the origin(s) of the Zedang peridotite massif; (5) explain the origin and preservation of SiC and related ultra-reduced phases separated from the Zedang peridotite massif; (6) compare the effects of convergent processes on the nature of upper mantle at different stages in the tectonic evolution of the Tibetan plateau.(1) The Shenglikou massif contains two main lithologies:ultramafic rocks rich in olivine (Ol), and others rich in pyroxene (Py). Sulfides have low initial 187Os/188Os (0.1066) and a Re-depletion model age (TRD) of ～3.0 Ga. The Ol-rich group has Mg# up to 0.923, Al2O3 contents down to 0.55 wt%, whole-rock HREE ～0.01-0.1 times chondritic values, and high spinel Cr# (up to 0.71). Modeling suggests that the Ol-rich peridotites are residues after ～40% melt extraction at ～1500-1600℃. Mineral chemistry, Hf-isotope depleted-mantle model ages (TDM= ～1.5-1.4 Ga) and O isotopes (δ18O=5.71±0.10%o) indicate that the Ol-rich group was metasomatized by melts from the convective mantle at ～1.5-1.4 Ga. On the other hand, mineral modes, chemical compositions and Nd-Hf isotopic signatures of the Py-rich rocks show a refertilization of the Archean lithospheric mantle during the period of ～1.0-0.7 Ga, coeval with the assembly and breakup of Rodinia. The Ol-rich and Py-rich groups both show metasomatic enrichment, probably related to early Paleozoic convergence between the Qaidam and Qilian blocks. Zircons from the Py-rich group suggest ultrahigh-pressure recrystallization occurred at-430 Ma.(2) At Shenglikou, pyroxenite dykes (>1m wide) crosscut the peridotite layering, and have weaker shearing, and higher modes of phlogopite (-8 vol%) and garnet (-40 vol%) than the pyroxenite layers, of the Py-rich group. Whole-rock and mineral compositions suggest the dykes are cumulates, formed by fractional crystallization of primitive hydrous arc-related magmas. The trace-element patterns of the whole rocks and minerals, combined with the decoupling of Sr-Nd-Hf-O isotopic signatures (subducted-sediment-like Sr-Nd isotopes vs depleted-mantle-like Hf-O isotopes), suggest that the melt source was sub-arc convective mantle, strongly metasomatized by slab-derived fluids. Lu-Hf isotopes constrain the time of dyke intrusion to the early Cambrian (-500 Ma). During that time, one branch of Proto-Tethyan oceanic lithosphere was subducted northward beneath the Qilian block, and triggered the melting of hydrous convective mantle at ～2.5-4.5 GPa and 1250-1350℃. These arc melts migrated upwards into the lithospheric mantle wedge, to produce the pyroxenite dykes. The pyroxenites were involved in the early Paleozoic (zircon U-Pb ages of -430 Ma) UHP recrystallization, and the peridotite massif was subsequently exhumed. Sm-Nd and Lu-Hf isotopes re-equilibrated at -350 Ma, corresponding to regional orogenic collapse and molasse sedimentation during late Devonian. This study provides a detailed picture of mantle-wedge processes beneath an "Andean-type" continental margin. The dissemination of pyroxenites as dykes within the mantle wedge is not only an effective way to allow the high-density pyroxenites, complementary to continental-arc lavas, to escape delamination back into the convective mantle, but also a significant manner to reduce the filtering effect of lithospheric mantle on the nature of ascending arc magmas.(3) Zircons from UHP eclogites in the Yuka and Xitieshan areas, North Qaidam orogen, record a history from the Neoproterozoic (-785 Ma) generation of igneous protoliths to early Paleozoic (-440 Ma) UHP eclogitization and to later retrogression (-417-409 Ma). The Neoproterozoic mafic magmatism mixed magmas from the depleted mantle with old crustal material. Zircons from the Yuka and Shenglikou gneisses reveal the existence of Neoarchean (～2.8-2.5 Ga) components within the Qaidam crust. The ～1.1-0.7 Ga magmatism added abundant mafic material to the lower crust and lithospheric mantle of the Qaidam and Qilian blocks. This "altered" continental lithosphere, once involved in a convergent system, would form high-density garnet-rich rocks and further subduct to mantle depths (at ～440 Ma). The low-density felsic upper crust was then detached from the high-density subducting slab and exhumed upwards (-417-409 Ma). This study illustrates the important role of the chemical composition of continental lithosphere in its behavior and fate during subduction.(4) The Zedang peridotite massif consists of the NW lherzolite unit and the SE harzburgite unit. The lherzolites have porphyroblastic textures with stronger high-temperature plastic deformation, Cpx porphyroblasts that have exsolved Opx, and higher modes of sulfide than are seen in harzburgites. Whole-rock and mineral chemical data from lherzolites (Opx Al2O3 of 3.61-6.54 wt%; spinel Cr# of 0.17-0.30) show that they are as fertile as abyssal peridotites from slow-spreading ocean ridges. Their equilibration temperatures (up to 1261℃) are ～200-300℃ higher than those of the harzburgites. The harzburgites have refractory signatures (e.g., spinel Cr# of 0.33-0.62) but high concentrations of LREE, Zr, Hf, Nb, Ta, Sr and Th in whole-rocks and Cpx, which suggest the harzburgites were pervasively metasomatized. Whole-rock trace elements and Sr isotopes indicate the lherzolites and harzburgites both experienced seawater metasomatism. Sm-Nd isotopic data suggest that the mantle sources of the lherzolites and harzburgites include both extremely depleted and enriched reservoirs. The harzburgites formed in a high-spreading-rate ridge, probably a forearc setting; the lherzolites were later accreted beneath the harzburgitic lithospheric mantle, and the MORB melts extracted during the lherzolite accretion intruded and metasomatized the overlying refractory mantle at-130-120 Ma; the two mantle units were overthrust onto the Indian continental margin at ～70Ma.(5) Grains of SiC, SiC+K-Al-Si glass+zircon, SiC+Si+SiOx and SiC+Si+ Fe-V-Ti-Mn alloy, and balls of Na-rich and Cu-Ag-Cd-O glass, have been separated from the Zedang peridotites, chromitites and pyroxenites. The assemblages suggest fO2～5 log units below the Fe-FeO buffer. Intergrowths of K-Al-Si-O glasses and silicon oxides with SiC and Si metal suggest these assemblages reflect disequilibrium redox reactions during the rapid upwelling of the enclosing mantle; similar assemblages are found as inclusions within kimberlitic diamonds. The peculiar assemblages may reflect the rapid subduction, breakoff and retreat of the Neo-Tethyan oceanic lithosphere down to the transition zone, producing local high-speed upwelling channels in the mantle wedge; the accelerated dynamics may suppress oxidation reactions, preserving the association of ultra-reduced and relatively oxidized assemblages within the oceanic lithosphere.(6) The Shenglikou peridotite massif records classical "Andean-type" and "Himalayan-type" plate convergence and magmatic dynamics within the sub-continental arc mantle wedge in NE Tibet during the early Paleozoic. These processes can modify the nature of lithospheric mantle beneath ancient continents and trigger their destruction. The Zedang peridotite massif records Mesozoic to Cenozoic ocean-ocean and ocean-continent convergence in south Tibet. The rapid subduction, breakoff and retreat of the oceanic slab could trigger complementary rapid upwelling of deep-mantle material; this process can preserve the un-equilibrated ultra-reduced assemblages within the shallow lithospheric mantle. The two peridotite massifs record the complex accretion and reworking histories of oceanic and continental lithosphere.