Geochronology and geochemistry of Cretaceous-Eocene granites,Tengchong Block(SW China):Petrogenesis and implications for Mesozoic-Cenozoic tectonic evolution of Eastern Tethys

The Early Cretaceous-Early Eocene granitoids in the Tengchong Block record the evolutionary history of the Mesozoic-Cenozoic tectono-magmatic evolution of Eastern Tethys.(a)The Early Cretaceous granitoids with relatively low(^(87)Sr/^(86)Sr)iratios of 0.7090-0.7169 andε_(Nd)(t)values of-9.8 to-7.8 display metaluminous,calc-alkaline dominated by I-type granite affinity and hybrid mantle-crust geochemical signatures.They may have been derived from melting of the subducted Meso-Tethyan BangongNujiang oceanic crust with terrigenous sediments in an arc-continent collisional setting.(b)The Late Cretaceous-Paleocene granitoids with relatively high(^(87)Sr/^(86)Sr)iratios of 0.7109-0.7627,andε_(Nd)(t)values of-12.1 to-7.9 exhibit metaluminous to peraluminous,calc-alkaline dominated by S-type granite affinity and hybrid Lower-Upper crust geochemical signatures,which may be originated from partial melting of the Meso-Proterozoic continental crust in the collision setting between the Tengchong Block and Baoshan Block.(c)The Early Eocene granitoids have metaluminous,calc-alkaline I-type and S-type granites dual affinity,with relatively high(^(87)Sr/^(86)Sr)iratios of 0.711-0.736,ε_(Nd)(t)values of-9.4 to-4.7,showing crust-mantle mixing geochemical signatures.They may have been originated from partial melting of the late Meso-Proterozoic upper crustal components mixed with some upper mantle material during the ascent process of mantle magma caused by the subduction of the Neo-Tethyan Putao-Myitkyian oceanic crust,and collision between the Western Burma Block and the Tengchong Block.It is these multi-stage subductions and collisions that caused the spatial and temporal distribution of the granitic rocks in the Tengchong Block.

Granitic Magmatism in Eastern Tethys Domain(Western China) and their Geodynamic Implications

Western China locates in the eastern section of the Tethys domain, granitic rocks in this region with variable formation ages and geochemistry record key information about the crust-mantle structure and thermal evolution during the convergent process of Tethys. In this study, we focus on some crucial granitic magmatism in the western Yangtze, Qinling orogen, and western Sanjiang tectonic belt, where magma sequence in the convergent orogenic belt can provide important information about the crust-mantle structure, thermal condition and melting regime that related to the evolution processes from Pre-to Neo-Tethys. At first, we show some features of Pre-Tethyan magmatism, such as Neoproterozoic magmatism(ca. 870–740 Ma) in the western margin of the Yangtze Block were induced by the assembly and breakup of the Rodinia supercontinent. The complication of voluminous Neoproterozoic igneous rocks indicated that the western Yangtze Block underwent the thermodynamic evolution from hot mantle-cold crust stage(ca. 870–850 Ma) to hot mantle and crust stage(ca. 850–740 Ma). The Neoproterozoic mantle sources beneath the western Yangtze Block were progressively metasomatized by subduction-related compositions from slab fluids(initial at ca. 870 Ma), sediment melts(initial at ca. 850 Ma), to oceanic slab melts(initial at ca. 825–820 Ma) during the persistent subduction process. Secondly, the early Paleozoic magmatism can be well related to three distinctive stages(variable interaction of mantle-crust to crustal melting to variable sources) from an Andeans-type continental margin to collision to extension in response to the evolution of ProtoTethys and final assembly of Gondwana continent. Thirdly, the Paleo-Tethys magmatism, Triassic granites in the Qinling orogenic display identical formation ages and Lu-Hf isotopic compositions with the related mafic enclaves, indicate a coeval melting event of lower continental crust and mantle lithosphere in the Triassic convergent process and a continued hot mantle and crust thermal condition through the interaction of subducted continental crust and upwelling asthenosphere. Finally, the Meso-and Neo-Tethyan magmatism: Early Cretaceous magmatism in the Tengchong Block are well responding to the subduction and closure of Bangong-Nujiang Meso-Tethys, recycled sediments metasomatized mantle by subduction since 130 Ma and subsequently upwelling asthenosphere since ca. 122 Ma that causes melting of heterogeneous continental crust until the final convergence, this process well recorded the changing thermal condition from hot mantlecold crust to hot mantle and crust;The Late Cretaceous to Early Cenozoic magmatism well recorded the processes from Neo-Tethyan ocean slab flat subduction, steep subduction, to initial collision of India-Asia, it resulted in a series of continental arc magmatism with enriched mantle to crustal materials at Late Cretaceous, increasing depleted and/or juvenile materials at the beginning of early Cenozoic, and increasing evolved crustal materials in the final stage, implying a continued hot mantle and crust condition during that time. Then we can better understand the magmatic processes and variable melting from the mantle to crust during the evolution of Tethys, from Pre-, Paleo-, Meso-, to Neo-, both they show notably intensive interaction of crust-mantle and extensive melting of the heterogeneous continent during the final closure of Tethys and convergence of blocks, and thermal perturbation by a dynamic process in the depth could be the first mechanism to control the thermal condition of mantle and crust and associated composition of magmatism.

Decoupled δ^(13)Ccarb and δ^(13)Corg records at Triassic–Jurassic boundary interval in eastern Tethys: Environmental implications for spatially different global response

Althoughδ^(13)C data(eitherδ^(13)Ccarb orδ^(13)Corg)ofmany Triassic–Jurassic(T-J)sections have been acquired,pairedδ^(13)Ccarb andδ^(13)Corg from continuous T-J carbonate sections,especially in eastern Tethys,have been scarcely reported.This study presents paired and decoupledδ^(13)Ccarb andδ^(13)Corg data from a continuous T-J carbonate section in Wadi Naqab.The T-J Wadi Naqab carbonate section,located in United Arab Emirates,Middle East,represents tropical and shallow marine sedimentation in eastern Tethys.At the T-J boundary interval,an initial carbon isotope excursion(CIE)is observed with different magnitude of isotope excursion and timing inδ^(13)Ccarb andδ^(13)Corg,while subsequently a positive CIE is only distinct inδ^(13)Ccarb.Based on petrological,carbon isotope,Rock-Eval and elemental analyses,theδ^(13)Ccarb is thought to record marine inorganic carbon,and theδ^(13)Corg to record terrigenous organic carbon.Therefore,the pairedδ^(13)Ccarb andδ^(13)Corg herein potentially document simultaneous changes in T-J atmospheric and marine settings of eastern Tethys.Their decoupled behavior may likely be caused by different changes or evolution of carbon pool between marine and atmospheric settings.The initial CIE present in bothδ^(13)Ccarb andδ^(13)Corg may indicate influence of isotopically light carbon release related to CAMP activity in both atmospheric and marine settings.The following positive CIE only inδ^(13)Ccarb suggests relatively steady carbon isotope composition in atmosphere,but enhanced burial of isotopically light carbon in marine settings.Furthermore,the T-J carbonates in the studied section were possibly deposited in normal and oxic shallow marine conditions.Global correlation based on the Wadi Naqab section and other T-J sections suggests spatially different T-J environmental parameters:in eastern Tethys and western Panthalassa,oxic condition,lacking organic-rich sediment,weaker ocean acidification and less influence of isotopically light carbon are more prevalent;in western Tethys and eastern Panthalassa,oxygen-depleted condition,black shales,stronger acidification and heavier influence of isotopically light carbon are more prevalent.These differences may be related to spatial distance from the CAMP or to different paleogeography.

Permo-Triassic paleogeographic,paleoclimatic and paleoceanographic evolutions in eastern Tethys and their coupling

Based on reconstructions of paleogeography, paleoclimate and paleoceanography of the Chihsian, Wujiapingian, Anisian and Norian intervals in the eastern Tethys, the multiple intrinsic relationship and interaction among the lithosphere, the hydrosphere and the atmosphere as well as their evolutions are discussed. It was demonstrated that paleogeographic change of the eastern Tethys and the northward shift of the Pangea during the Permo-Triassic periods governed the coeval paleocurrent pattern and its evolution, which in turn seems to be a key contributor forcing the Permo-Triassic megamonsoon to the climax.

A Toarcian Ocean Anoxic Event record from an open-ocean setting in the eastern Tethys: Implications for global climatic change and regional environmental perturbation

The Early Toarcian“Oceanic Anoxic Event”(T-OAE)is recorded by marked disruption to both the climate system and marine ecosystems.Here,we present intergraded high-resolution carbon-isotope data(δ^(13)C),bulk geochemistry,mineral characterization from an open-ocean setting in the eastern Tethys.With these data,we(1)construct the high-resolution record of the T-OAE from an open-ocean setting in the eastern Tethys;(2)show that the T-OAE in the Sewa succession was marked by coarser-grained deposits associated with high-energy conditions within the otherwise low-energy claystone deposits that likely linked to a globally increased supply of clastic sediments into marginal and deeper marine basin;(3)propose that the low C_(org):P_(total) ratios,in combination with bioturbated structure and depletion or slight enrichment in redox-sensitive trace elements of V,Mo,and U suggest a long-term oxygenation event throughout the T-OAE interval at the Sewa succession,and hence,anoxia may not play a fundamental role during the Toarcian negative CIE in this setting;(4)exhibit that a warming and more humid climate began at the start of the T-OAE,and many episodic changes in sediment provenance throughout the T-OAE interval occurred at this location;and(5)suggest that accumulation of organic-matter sediments during the T-OAE is generally controlled by global climatic changes,but a regional environmental perturbation also might influence the preservation of organic matter.

Late Triassic bivalves associated with a hydrothermal vent system in the Yidun Island Arc (SW China) of the eastern Tethys

The Yidun Island Arc in the Three Rivers (Jinsha River, Lancang River, Nujiang River) region of southwestern China is one of the most important Kuroko-type volcanogenic massive sulfide deposits (VMS) in China. Intra-arc rifting of Yidun Island occurred during the Late Carnian-Norian when VMS deposits such as the Gacun Pb-Zn-Cu deposit were formed. A bivalve fauna was found in fine-grained tuffaceous slate and in mineralized tuffaceous siltstone containing very high contents of Pb (45.01-103.37 ppm) and Zn (135.78-300.03 ppm) of the upper Tumugou Formation in the Changtai-Gacun volcanic-sedimentary rift basin. Stratigraphically, the bivalve-bearing beds are equivalents of the Gacun Pb-Zn-Cu deposits. The diversity of this bivalve fauna is very low. It consists mainly of the thin-shelled, epibyssate suspension-feeding bivalves Pergamidia eumenea and Parapergamidia changtaiensis, the burrowing large, elongated, suspension-feeding Trigonodus keuperinus and Unionites? sp., and occasional specimens of the endobyssate suspension-feeding Trigonodus? sp. and the deep burrowing suspension-feeding Pleuromya markiamensis. Individuals of the first four taxa are so abundant that the specimens are sometimes concentrated in shell beds, probably indicating a gregarious habit. This bivalve fauna is associated with internal moulds of cylindrical, slightly conical tubes most likely produced by a worm-shaped organism. Composition, morphology, diversity, and high abundance of this fauna, chemical features of the surrounding sediment, and the tectonic setting all suggest that this bivalve fauna lived in a deep-water environment in or around a hydrothermal vent system.

Organic geochemistry and source rock potential assessment of Late Paleocene Patala Formation in Margalla Hill Range,North Pakistan

Black shales of the Paleocene Patala Formation are proven source rocks for conventional hydrocarbons in southern Potwar Basin of Pakistan and are assumed to be effective source rocks towards north in the Margalla Hill Range of Pakistan.In this regard,the current study focuses on source rock geochemistry of the Paleocene Patala Formation to assess its source rock potential,organic matter types and thermal maturity levels in the Margalla Hill Range of North Pakistan.Source rock generative potential,kerogen types and thermal maturation of the analysed rock samples were unraveled by using Rock-Eval pyrolysis T_(max),TOC(total organic carbon)and vitrinite reflectance(R_(0))analyses.TOC analysis coupled with S_(2) yield revealed poor to fair source rock quality encountered within the formation.The pyrolysis T_(max) vs Hydrogen Index(HI),showed mostly Type Ⅲ kerogen dominated by thermally immature to mature organic matter.The HI and genetic potential is low and revealed poor hydrocarbon generation potential of the formation.The S_(1) vs TOC plots confirm the indigenous nature of the hydrocarbons hosted by Patala Formation.The vitrinite reflectance outcomes indicated immature to mature source rock beds lying in dry gas zone.The HI and OI signatures and abundance of Type Ⅲ kerogen are indicating dominance of terrestrial organic matter within the formation.Overall,the investigated Patala Formation exposed at the studied section of Margalla Hill Range,Pakistan acts as a poor source rock unit for liquid hydrocarbon generation but holds prospects for dry gas generation in the study area.

Constraints of C-O-Sr Isotope and Elemental Geochemistry on Carbonate Sedimentary System Paleoenvironments of the Middle Jurassic Hepingxiang Formation,Western Simao Basin

Western Yunnan is an important area for Mesozoic marine strata,development of which helps to reconstruct paleoenvironments in the eastern Tethys.To bring knowledge of eastern Tethys up to that of western Tethys,this study focuses on its sedimentary characteristics and paleoenvironmental significance concentrating on the Xiushan section of the Simao Basin with analyses of the petrology,element geochemistry,and C-O-Sr isotopes.Samples are micrite,bioclastic limestone,marl,and mixed calcareous clastic rocks with the 87Sr/86Sr values of limestone ranging from 0.708255 to 0.708933;theδ^(13)C values of the limestone range from−5‰to+1.5‰and theδ^(18)O values range from−13.7‰to−9.1‰.Based on the results,it is concluded that the Middle Jurassic limestones in the Simao Basin were deposited in a tidal flat environment with a strong influence of terrigenous input.At least one transgression event occurred during the depositional period.The paleoclimate as a whole changed from dry hot to humid hot and back to dry hot again.These enriched and improved results provide further data support for a comparison of the paleoenvironments between the eastern and western Tethys.