Research On Several Key Questions Of Yarlung Zangbo Ophiolite Zone, Tibet And Its Adjacent Area

Yarlung Zangbo Ophiolite Zone (YZOZ) and its adjacent area preserve excellent geologicalrecords on the evolution of Tethyan ocean and the uplift history of Tibetan Plateau, thus it is thenatural laboratory for plate tectonics and geodynamics. However, timing and mechanism of theYZOZ and the uplift history of its adjacent area remain elusive. Here we report results fromfield geology, petrology, geochemistry, sedimentology, geochronology and (U-Th)/Helow-temperature thermochronology in the YZOZ and its adjacent area, in order to reveal the ageand tectonic setting of the YZOZ, the early uplift history of the YZOZ and its adjacent area, andthe exhumation history of Yarlung Zangbo.Zircon U-Pb analyses from one representative gabbro sample in the western YZSZ yieldeda crystallization age of 363.7±1.7 Ma (1σ). All gabbro samples show typical Ocean IslandBasalt (OIB) affinity with no to little continental crust contamination. These observations, incombination with the Early Carboniferous layered gabbros reported at Luobusa, indicate thatthey could represent remnants of the Paleo-Tethys. Thus, a branch ocean separating the WesternQiangtang terrane and the Lhasa terrane from the Gondwana continent should have occurredduring the Late Devonian and the Early Carboniferous.Peridotites from Zhongba and Xigaze ophiolite massifs have highly depleted modal,mineral and bulk rock compositions, indicating that they are residues after moderate to highdegrees of partial melting (13-24%) mainly in the spinel-stability field. These rocks displaytypical U-shaped chondrite-normalized Rare Earth Element (REE) patterns and fractionatedchondrite-normalized Platinum Group Element (PGE) patterns. These characteristics, incombination with their hybrid mineral and whole-rock compositions intermediate betweenthose of abyssal and forearc peridotites, indicate melt-rock interaction resulting in the selectiveenrichment of LREE and Pd. A two-stage model was used to explain the generation of theZhongba harzburgites: 1) original generation from a MORB-source upper mantle, and 2)subsequent trapping as part of a mantle wedge above a subduction zone. Mafic rocks from theseophiolites show OIB, MORB-like(Mid-Ocean Ridge Basalt) and IAB (Island Arc Basalt)affinities. The MORB-type rocks show enrichment in LILE(Large Ion Lithophile Element, e.g. K, Cs, Ba, Sr) and some HFSE (High Filed Strength Element, e.g. U, Pb), but depletion inNb-Ta element, indicating that they were metasoamtized by subducted slab-releasedfluids/melts. Formations of the western-central Yarlung Zangbo Ophiolites involveddecompression melting in fore-arc setting during 130-120 Ma. After 120 Ma, a new flat-slabsubduction zone developed along the southern margin and zone of melting moved underGangdese, resulting in the formation of 116-80 Ma granites and the development of Xigazefore-arc basin.Sedimentary lithofacies and facies associations analyzed in the western Hoh Xil basinindicate they were deposited in a braided fluvial system and alluvial fan, similar with theFenghuoshan Group, eastern Hoh Xil basin. Provenance analyses from conglomerate clastcompositions, paleocurrent orientations, and detrital zircon U-Pb and Lu-Hf isotopiccompositions document sediments in both western and eastern basins were derived from theQiangtang and Lhasa blocks. These observations, in combination with comparativepaleomagnetic results, imply that Hoh Xil basin was a single, wide basin during Paleogene.Apatite and zircon (U-Th)/He thermochronology data from central Songpan-Ganzi and centralTibet confirm that the most plateau underwent rapid to moderate exhumation during Cretaceousand early Cenozoic. The thermal history modelling results show that both Songpan-Ganzi andcentral Tibet have undergone continuous slow exhumation at rates of 0.06-0.02 km/Ma for thepast 50 Ma at least, suggesting tectonic and morphologic stability of the most plateau since atleast early Eocene. Apatite (U-Th)/He ages from southern and eastern Tibetan Plateau revealsignificant regional river incision occurred since Miocene, following a period of relativestability of the landscape from ca. 40-20 Ma. The possible drive forces for the regional riverincision are base-level fall and intensification of Asian monsoon.