Search

The widespread mid-Cretaceous igneous rocks in the northern margin of the Lhasa Block play an important role in understanding deep geologic processes, matter exchange at depth, and tectonic evolution of the Tibetan Plateau. In this paper, we report new zircon U-Pb ages, whole-rock major and trace element data, and Sr-Nd-Pb-Hf isotope data from the Talabuco andesites and basalts, which were dated at ~111 Ma. These rocks belong to the high-K calc-alkaline and shoshonite series, and show enrichment in terms of large-ion-lithophile elements (LILEs, e.g. Rb, U, and Th) and light rare earth elements (LREEs), but depletion of high-field-strength elements (HFSEs, e.g. Nb and Ti). The (87Sr/86Sr)iratios of the Talabuco andesites range from 0.7043 to 0.7048, and theεNd(t) contents range from 0.68 to 4.33. The ratios of206Pb/204Pb,207Pb/204Pb, and208Pb/204Pb are 18.6064–18.8993, 15.6233–15.6707, and 38.8634–39.1720, respectively. The176Lu/177Hf and176Hf/177Hf ratios of one sample range from 0.00081 to 0.00206 and 0.28280 to 0.28296, respectively. TheεHf(t) values for this sample range from 3.4 to 9.1, and the two-stage model Hf age (TDM2) is 0.59–0.95 Ga. Combined with previous studies, the geochemical and isotopic data reveal that the parental magma of the Talabuco andesites was probably derived by partial melting of EM II-type sub-continental lithospheric mantle (SCLM). The Talabuco andesites are most likely generated by fractionation of mafic magma contaminated by subducted oceanic sediment and represent product of arc magmatism due to northward subduction of the Yarlung Zangbo Neo-Tethyan slab or southward subduction of the Bangong Meso-Tethyan slab. [ABSTRACT FROM AUTHOR]

The role of the Meso-Tethyan oceanic plateau in the growth of the Tibetan Plateau is debated. In this paper, we report coexistence of large-scale mid-ocean ridge basalt (MORB)-like and enormous ocean island basalt (OIB)-type mafic rocks from the Dongco ophiolitic rocks of the Bangong–Nujiang Suture Zone. The MORB-like rocks host tholeiitic geochemical characterizes, and have supra-subduction zone affinity such as enrichments in large ion lithophile elements (LILEs), depletions in high field strength elements, high ε Nd (t) values (+6.5 to +9.5) and low initial 87Sr/86Sr ratios (0.7033–0.7050). The OIB-type rocks have an alkaline affinity and are characterized by enrichments in LILEs and light rare earth elements, strongly negative Sr anomalies, variable ε Nd (t) values of −3.7 to +7.3, and low initial 87Sr/86Sr ratios ranging from 0.7034 to 0.7050. Metamorphic zircons from amphibolites in the sole of the ophiolitic rocks yielded U Pb ages of 104–100 Ma. Detrital zircons from the upper Cretaceous sandstone of the Abushan Formation (K 2 ab) yielded young U Pb ages of 100–86 Ma. Considering the regional geology, we suggest that the Dongco MORB-like rocks were derived from a depleted mantle source and formed from an intra-oceanic subduction-related setting, representing remnants of normal oceanic crust, whereas the OIB-type rocks were derived from an enriched mantle source representing remnants of a Meso-Tethyan oceanic plateau. Our preferred hypothesis is that the Meso-Tethyan oceanic plateau experienced a phase of accretion at 104–100 Ma, which probably increased local subduction erosion of the upper plate and providing provenance for the deposition of the K 2 ab sandstone. The maximum depositional ages of K 2 ab sandstone constraint the minimum ages for closure of the Bangong–Nujiang Tethys Ocean most likely at 100–86 Ma. • Identified a new 104 Ma metamorphic event in the western Bangong–Nujiang Suture • Closure of the Bangong–Nujiang Tethys Ocean most likely occurred during 100–86 Ma • Dongco ophiolitic rocks contain remnants of normal oceanic crust and oceanic plateau [ABSTRACT FROM AUTHOR]

Abstract: The elevation of the Tibetan plateau is a revolutionary event in the Earth history, which resulted in change not only of Cenozoic regional and global climate but also of monsoon intensity. A critical aspect for the development of the Tibetan plateau that remains open to intense debate is whether the collision among the Tibetan continental blocks or the subduction of the Tethys prior to the Indo-Asian collision in the Cenozoic contributed to the plateau growth. We propose here that an Andean-type orogen could have been present in southern Tibet from the Jurassic to Early Cretaceous, but collapsed during the Middle Cretaceous, based on a synthesis of existing structural, sedimentary, geochronological, and geochemical data. The orogeny is marked by north-verging Jurassic–Early Cretaceous thrusting in the Lhasa block, a result of flat subduction of the Yarlung-Zangpo Neo-tethys that is recorded by a narrow belt of 205–174Ma adakitic rocks along the southernmost margin of the Lhasa block. Denudation of the orogen produced a thick (up to 15km) sequence of Jurassic turbidites and molasse-type sediments on the northern Lhasa block. Widespread Middle Cretaceous (135–100Ma) postorogenic, strongly peraluminous to calc-alkaline, magmatism and extensional deformation in the Lhasa block and an extensive marine transgression in Tibet indicate that the orogen broke down due to extension resulting from lithosphere delamination and asthenosphere upwelling. Meanwhile, the north-dipping subduction of the Bangong Meso-tethys produced a wide Jurassic–Middle Cretaceous magmatic arc along the southern margin of the Qiangtang block. Findings of 132–108Ma-aged ophiolites rich with Middle Cretaceous radiolarians in central Tibet indicate that the Bangong Meso-tethys did not close until the Late Cretaceous. Repeated flat slab subduction of the Yarlung-Zangpo Neo-tethys occurred during the Late Cretaceous (90–78Ma) as indicated by the adakites in the southernmost Lhasa block, which, together with the ongoing Qiangtang–Lhasa collision, could have contributed to the growth of the Tibetan plateau. [Copyright &y& Elsevier]

The article talks about the tectonic compression pamir syntax in the Tibetan Plateau, the distribution of Taxkorgan alkalic complex, being the largest intrusive rock in the region and its importance in lithosphere shortening and thickening. It talks about zankangou rock mass located in Taxkorganarae, diopside syenite which has gray and equigranular texture, the mineral composed of granet, titanite and apatite and diopside syenites being rich in alkali.