Northern Lhasa Terrane Research Articles

Initial sedimentation after the closure of the Sumdo Paleo-Tethys Ocean: Implications for early Mesozoic tectonics in the East Tethys domain

The closure of the Paleo-Tethys Ocean and subsequent collisional orogenesis in the early Mesozoic played an important role in the formation of the Qinghai–Tibetan Plateau. However, the final closure of the southernmost Paleo-Tethys branching ocean basin in the Qinghai–Tibetan Plateau—known as Sumdo Paleo-Tethys Ocean—and collision between Central and Southern Lhasa terranes remain controversial. In particular, the tectonic significance of early Mesozoic volcanism and sedimentation in the Lhasa terrane is unknown, but crucial for understanding the final stages of ocean closure. Here, we investigate the depositional ages and provenance of detrital zircon in clastic sedimentary rocks, as well as the petrogenesis of the interbedded andesite from the volcano-sedimentary sequence of the Late Triassic to Early Jurassic Xionglai Formation in the Tangjia-Sumdo area. Detrital zircon dates from the Xionglai Formation define five age peaks at 180–200 Ma, 230–340 Ma, 500–620 Ma, 1000–1240 Ma, and 1600–1800 Ma. Detrital zircon trace elements and Hf isotopic characteristics are interpreted to represent derivation from an accretionary wedge and later magmatism related to oceanic slab break-off as well as a minor component from coeval magmatic rocks in the Nyainrong-Nang area in northern Lhasa terrane. Interbedded andesite has a magnesian andesite signature and yields ages of 192–184 Ma with zircon εHf(t) values that range from −3 to +1. We interpret these data as the andesite being sourced from sediment-derived melts that interacted with the overlying mantle wedge in a post-collision extensional setting. Our study suggests that the closure of the Sumdo Paleo-Tethys Ocean and subsequent collision between Central Lhasa and Southern Lhasa terrane may have happened in Late Triassic-Early Jurassic. The final closure of the multi-Paleo-Tethys Ocean and subsequent collision in the Qinghai–Tibetan Plateau may have induced initial subduction of the Meso-/Neo-Tethys Ocean, representing the tectonic transition from collision/post-collision to subduction/accretion in the Qinghai–Tibetan Plateau during the Late Triassic to Early Jurassic.

Partial melting of amphibolite lower crust: A potential magma source for granite-related W mineralization

Magmas for the granite-related tungsten deposits (GTD) are commonly considered to be generated from the upper to middle crustal metasedimentary protoliths, and these magma source materials have previously been regarded as the determinant factor to the granite-related tungsten mineralization. Here we demonstrate, based on high Sr/Y and (La/Yb)N granites related to wolframite mineralization from the Jiaoxi, Jiaodong and Anglonggangri wolframite mining districts in the northern Lhasa terrane, Xizang that this viewpoint is not necessarily correct. Petrological and geochemical evidences suggest that the wolframite mineralization related granites in these mining areas were sourced from partial melting of the K-rich amphibolite lower crust of the Lhasa terrane and underwent significant fractional crystallization. These results indicate that the amphibolite lower crust can also be a candidate magma source for the GTD and the high evolution processes of granite is more important than the nature of the source materials during tungsten mineralization.

Early Cretaceous magmatism archives crustal architecture of northern Lhasa Terrane, central Tibetan Plateau

Identification of continental crustal architecture in orogenic belts is essential for understanding the geodynamic evolution of global continental collision systems. However, ongoing debates surrounding the crustal nature of the Lhasa Terrane in the central Tibetan Plateau impede our comprehension of its tectonic evolution. Mesozoic intermediate-felsic magmatism in the Lhasa Terrane offers key insights into the crustal architecture before the India-Asia collision. Here we report zircon U-Pb-Hf isotope, whole-rock element and SrNd isotope data from Early Cretaceous granitoids in the Baingoin batholith, northern Lhasa Terrane. These granitoids (127–115 Ma) exhibit variable SiO2 (62.99–73.86 wt%), K2O (1.15–4.74 wt%), and MgO (0.49–2.48 wt%) contents. They are classified as low- to high-K calc-alkaline and metaluminous to peraluminous series, enriched in large ion lithophile elements (e.g., Rb) and light rare earth elements ((La/Yb)N = 7.08–18.3), and depleted in high field strength elements (e.g., Nb, Ta, and Ti). These granitoids display geochemical affinities with I-type granitoid rocks and can be further subdivided into three groups. Group 1 granitoids exhibit the highest zircon εHf(t) (−1.57 to 5.85) and whole-rock εNd(t) (−3.37 to −2.45), and lowest (87Sr/86Sr)i (0.706018 to 0.706922) values, whereas Group 2 granitoids have wide ranges of zircon Hf (εHf(t) = −7.78 to 2.26) and enriched whole-rock SrNd isotope compositions ((87Sr/86Sr)i = 0.707214 to 0.707918; εNd(t) = −7.38 to −6.14). These suggest that Group 1 and Group 2 granitoids may be dominantly derived from the juvenile and ancient crustal sources, respectively. Group 3 granitoids are characterized by zircon Hf (εHf(t) = −2.85 to 3.52) and whole-rock SrNd isotope compositions ((87Sr/86Sr)i = 0.706473 to 0.707692; εNd(t) = −4.50 to −3.01) falling between the ranges of the previous two groups, likely represent a mixture of juvenile crustal-derived melt with 10–15% of reworked ancient crustal materials. Integrating previous research, we propose that the persistent ancient crustal basement beneath the northern Lhasa Terrane was concurrently undergoing significant modification through juvenile crustal materials during the Early Cretaceous. These Early Cretaceous Baingoin batholith preserves crucial records of crustal evolution and highlights the influence of upwelling asthenosphere during the transition from Bangong-Nujiang oceanic subduction to Lhasa-Qiangtang collision.

Geodynamics and nature of the basement of the northern Lhasa Terrane: Insights from Early Cretaceous highly fractionated I-type granites in the Beila and Dongga areas, central Tibetan Plateau

Collisional orogenic belts serve as excellent places for investigating geodynamic process and growth mechanism of the continental crust. As one of the largest collisional orogenic belts worldwide, the Tibetan Plateau is formed by the amalgamation of multiple tectonic terranes. However, the nature and composition of the crustal basement beneath the northern Lhasa Terrane, located in central Tibetan Plateau, is still a matter of debate. Here, we investigated a series of Early Cretaceous granites in the Beila and Dongga areas, northern Lhasa Terrane. Zircon U-Pb dating reveals that the Beila and Dongga granites were emplaced at ca. 118 Ma and 110 Ma, respectively. These granites are characterized by high SiO2 and K2O, low TiO2 and P2O5 contents and low Mg#. They exhibit enrichment in light rare earth elements, significant negative Eu anomalies, and depletion in Ba and Sr. These geochemical features suggest that they belong to highly fractionated I-type granites. Both the Beila and Dongga granites display similar and enriched whole-rock Sr-Nd and zircon Hf isotope compositions, implying a likely common ancient crustal origin. Given the temporal sequences and redox conditions during their formation, it is highly probable that the Beila and Dongga granites formed during the syn-collision stage between the Lhasa and Qiangtang Terranes and the breakoff of the Bangong-Nujiang oceanic slab, respectively. Integrated with coeval magmatism in the central Tibetan Plateau, our new data reveal the presence of a regionally preserved ancient basement beneath the northern Lhasa Terrane, analogous to the equivalent basement beneath the central Lhasa Terrane.

Late Cretaceous-Eocene exhumation of the northern Lhasa terrane and topographic implications for the Central Tibet

The central Tibetan Plateau has an average altitude of ∼5000 m; its exhumation and chemical weathering greatly influence the global climate and ocean chemistry. The modern central Tibet is characterized by low-relief, high elevation topography with endorheic drainage. When and how these geomorphic characteristics of central Tibet were initiated remains controversial. Here, we have applied zircon UPb dating and low-temperature thermochronology on the Cretaceous plutons from Coqin Basin of central Tibet in order to assess timings of exhumation. The thermal history modeling indicates a period of relatively rapid cooling (2.5–4 °C/Ma) occurred in Late Cretaceous to Middle Eocene times (from ∼80 Ma to ∼40 Ma), with the exhumation rates of 0.2–0.4 mm/yr assuming a geothermal gradient of 25 °C/km. Cooling rates then slowed to ∼0.5 °C/Ma during the Middle Eocene to the present, with a relatively lower exhumation rates of ∼0.02–0.03 mm/yr. Synchronous rapid cooling and exhumation has also been identified in central Tibet; this signal of widespread Late Cretaceous exhumation across the region may be viewed as evidence for the initial surface uplift and erosion of the central Tibetan plateau. Lower exhumation rate since ∼40 Ma, combined with sedimentological data suggests that the low-relief, internally drained topography of central Tibet was initiated around this time.

Cretaceous magmatism in the northern Lhasa Terrane: Implications for the tectonic evolution and crustal growth tempos of central Tibet

The subduction and demise of an ocean plate are generally recognized as essential processes that result in the reworking and maturation of the continental crust. The northern Lhasa Terrane in central Tibet represents the forefront of the Lhasa-Qiangtang collision belt following the closure of the Bangong-Nujiang Ocean. Thus, it is a pivotal location to study the transition processes from oceanic lithosphere subduction to continental collision as well as pertinent crustal growth mechanisms. Here, we present zircon U-Pb dating, whole-rock major and trace element and Sr-Nd isotope, and zircon Hf isotope and trace element data of the Mendang igneous complex, Baingoin County, northern Lhasa Terrane. Geochronological results show that the granodiorites, trachydacites, and rhyolites in the Mendang igneous complex formed at ca. 122−116 Ma, 97 Ma, and 73 Ma, respectively. The Early Cretaceous granodiorite samples are peraluminous with high SiO2, Al2O3, and K2O contents, and moderate A/CNK (molar ratio of Al2O3/[CaO + Na2O + K2O]) values, which are similar to those of typical felsic peraluminous I-type granites. The granodiorites are characterized by enrichment in light rare earth elements and large ion lithophile elements (e.g., Rb) and depletion in high field strength elements (e.g., Nb, Ta). They also show the most enriched whole-rock Sr-Nd [(87Sr/86Sr)i = 0.7072−0.7078; εNd(t) = −7.60 to −5.08] and zircon Hf [εHf(t) = −4.46 to +1.02] isotope compositions, indicating that the Early Cretaceous granodiorites were likely derived from an ancient basement under a subduction setting. The trachydacites have uniform SiO2, high Al2O3, Sr contents, and Sr/Y values, and low Y and Yb contents, belonging to adakitic rocks. They show more depleted whole-rock Sr-Nd [(87Sr/86Sr)i = 0.7065−0.7066; εNd(t) = −0.56 to −0.22] and zircon Hf [εHf(t) = 4.36−7.84] isotopes than the granodiorites, suggesting that the trachydacites may have generated from partial melting of the juvenile thickened lower continental crust. The rhyolites have the highest SiO2 and K2O contents in the Mendang igneous complex, and significant depletion of Ba, Sr, Eu, P, and Ti. They have slighter more enriched whole-rock Nd [εNd(t) = −3.71 to −1.16] and zircon Hf [εHf(t) = 1.03−4.31] isotope compositions than the trachydacites. These features suggest that the rhyolites were highly fractionated products of the crustal melts. Whole-rock Sr-Nd and zircon Hf isotopes of the Mendang igneous complex show a kink trend from enrichment to depletion and then transfer to enrichment again, signifying an increased contribution of juvenile materials in the northern Lhasa Terrane toward progressively replacing the ancient lower crust and accumulating to newly formed crust. The estimated crustal thickness beneath the northern Lhasa Terrane shows a sharp increase from the Early to Late Cretaceous and peak at ca. 97 Ma, whereas it largely decreases in the Late Cretaceous. Integrated with previous studies, we propose that the formation of the Mendang igneous complex (122−73 Ma) elaborately documents the regional tectonic transition from oceanic lithosphere subduction to demise as well as continental crustal differentiation and maturation. The rollback and breakoff of the southward subducted Bangong-Nujiang oceanic slab in the Early Cretaceous initiated diverse magmatism in the northern Lhasa Terrane. During the early Late Cretaceous, the widespread adakitic and Mg-rich magmatism was attributed to the delamination of the thickened lithosphere following the final amalgamation of the Lhasa and Qiangtang Terranes. In the late Late Cretaceous, post-collisional extension induced the formation of the rhyolites in the northern Lhasa Terrane. The growth and destruction of the continental crust had certain tempos from the oceanic lithosphere subduction to continental post-collision stage.

Strongly Peraluminous Highly Fractionated I-Type Granite from Bangong–Nujiang Metallogenic Belt, Tibet: Implications for Continental Evolution and Evaluation of Economic Potentiality

The research on highly fractionated granite has significant implications for both the evolution and compositional maturation of the continental crust and metallogenic exploration. As a means of further understanding crustal evolution and promoting ore exploration in the Bangong–Nujiang metallogenic belt (BNMB), we present the petrography, zircon LA–ICP–MS U–Pb age, and Hf isotopic data, along with the whole-rock geochemical and Sr–Nd isotopic composition on Kese highly fractionated granite in the Baingoin area within the BNMB, central Tibet. The results show that Kese granite possesses a zircon U–Pb age of 127.8 ± 1.7 Ma and a relative enrichment in zircon Hf isotopic composition (−12.8~+0.3) with a two-stage Hf model age of 1.2~2.0 Ga. This granite belongs to the high-K calc-alkaline series, characterized by a strongly peraluminous feature, and is enriched in large-ion lithophile elements (LILEs) and Nd isotopes (−7.86~−7.74). The granite was likely to have been derived from the mixed melts derived from 40%~45% juvenile basaltic lower crust, 15%~20% ancient lower, and 40% middle–upper, following intense fractional crystallization processes involving amphibole, biotite, plagioclase, and some accessory minerals during the magma’s evolution. We infer that Kese highly fractionated granite can be formed from the continental collision of the Lhasa–Qiangtang terranes initiated before 128 Ma. The reworking of pre-existing juvenile and ancient crustal materials drove the composition of the northern Lhasa terrane to that of a mature continental crust. Moreover, the distinctive geochemical features have shown that the high degree of differentiation led to intense magmatic–hydrothermal interaction during the formation of Kese granite. A comparison of the geochemical characteristics of mineralized and barren granites suggests that the highly fractionated granites in Baingoin from the BNMB have a high economic potential and are suitable for preliminary exploration of Sn–W-(U) deposits.

Divergent double subduction of Bangong-Nujiang Ocean revealed by high-resolution magnetotelluric data at 86°E in the northern Tibetan Plateau

The Tibetan Plateau was mainly produced by the assembly of several terranes resulting from ocean closure and continental collision. The formation and evolution of the Bangong-Nujiang Ocean (BNO) played a prominent role in the Tibetan Plateau accretion before the Cenozoic collision between the Indian and Eurasian continents. The closure mode of the BNO between the Lhasa and Qiangtang terranes has been controversial, involving subduction polarity, i.e., regarding whether there was only northward subduction beneath the Qiangtang terrane or both southward and northward subduction beneath the Lhasa and Qiangtang terranes. To resolve this issue, a dense broadband magnetotelluric (MT) profile transecting the Bangong-Nujiang suture zone (BNS) at 86°E was completed. After standardized data processing and analysis, geological interpretation can be performed on the two-dimensional (2-D) inversion results of the MT data. Through a large number of inversion experiments, we obtained a reliable lithospheric electrical structure beneath the BNS and its adjacent regions. Our results show that the upper crust in our study area is featured by high resistivity overall, but it is cut by several fault zones, indicating that it may be influenced by the late strike-slip fault caused by the India-Asia collision. The middle-lower crust of the northern Lhasa terrane is highly conductive and may represent juvenile crust. The existence of a northward and southward inclined electrical gradient zone in the BNS may indicate that the BNO was closed by north-south bidirectional subduction.

3-D electrical structure and tectonic dynamics in the Yangbajing area based on the array magnetotelluric data

The well-known N-S-trending fault in the Yangbajing area plays a crucial role in the tectonic evolution of the Tibetan Plateau. Previous researches on a few E-W geophysical profiles suggested that the eastern shear at the base of the upper crust and/or lithosphere deformation brought on by asthenosphere upwelling are the major causes of the Yadong-Gulu rift’s creation. Here we propose a 3-D electrical resistivity model derived from the magnetotelluric (MT) array data spanning the Yadong-Gulu rift (YGR), and the distribution of temperature and melt fraction is estimated by the experimental calibrated relationships bridging electrical conductivity and temperature/melt fraction. The result reveals that the Indian slab subducted steeply in the east of the Yadong-Gulu rift, while Indian slab may have delaminated with a flat subduction angle in the west. The temperature distribution shows that the upper mantle of the northern Lhasa terrane is hotter than that of the southern Lhasa terrane. This is likely the result of mantle upwelling caused by either the subduction of the Indian slab or thickened Tibetan lithosphere delamination. Moreover, the strength of the mid-lower crust is so low that it may meet the conditions of the local crust flow in the west-east direction. The local crustal flow and the pulling force from the upwelling asthenosphere jointly contributed to the formation of the Yadong-Gulu rift. These main factors exist in different stages of the evolution of the Yadong-Gulu rift.

An Oligocene-Miocene intermontane narrow lowland in the central Tibetan Plateau: Insights from provenance analysis and palynological record of a Cenozoic sedimentary succession

• The Niubao Formation in the Avengco Basin, the western part of the BNSZ, was deposited during Oligocene-Miocene with bidirectional sources. • The Oligocene-Miocene paleoelevation of the Avengco Basin was 3090-3290 m. • A 3 km high intermontane lowland on the BNSZ were formed during the Oligo-Miocene. • This intermontane lowland was probably formed by the reactivation of thrust belts and cloud extend westward to the Avengco Basin. A series of Cenozoic basins are distributed along the Bangong-Nujiang suture zone and amalgamate into an east-west trending topographic lowland. The reconstruction of the formation process for the lowland is critical to understanding the evolution of the Tibetan Plateau and paleoclimate. Here we present results from sedimentology, zircon U-Pb ages, and palynological records of the Niubao Formation in the Avengco Basin, the western part of central Tibet. The Niubao Formation consists of conglomerate, sandstone, siltstone, marl, and limestone, which were deposited in alluvial fan and shallow lake. The palynological assemblages suggest that the Niubao Formation in the Avengco Basin was deposited during the Oligocene-Miocene. Provenance analyses suggest that the Niubao Formation can have bidirectional sources, including the Southern Qiangtang terrane and/or Bangong-Nujiang suture zone in the north and the Northern Lhasa terrane in the south. By using the co-existence approach, palynological records from the Niubao Formation yielded a paleoelevation of 3090-3290 m after temperature and lapse rate correction, which was 1-2 km lower than those of the Lhasa and Qiangtang terranes during the Oligocene-Miocene. Based on the above observations and previous studies from the coeval sediments of the Lunpola and Nima Basins in central Tibet, we propose that the intermontane lowland along the Bangong-Nujiang suture zone was formed as a whole during the Oligocene at the latest and extended to the Avengco Basin in the west.

Detrital zircon U-Pb geochronology of the Lunpola basin strata constrains the Cenozoic tectonic evolution of central Tibet

The Cenozoic tectonic evolution of central Tibet plays an important role in the growth of the Tibetan plateau. Paleoelevation studies show that the central Tibetan plateau hosted an east-west trending belt of basins in the Paleogene, but how they relate to the regions uplift history remains unclear. This study addresses the uplift history of the central Tibetan plateau by focusing on Lunpola basin strata deposited in the Cenozoic through detrital zircon basin provenance and zircon geochemical indicators for crustal-thickening. U-Pb dating of detrital zircon demonstrates that the Lunpola basin was mainly derived from mixed sources composed of Paleozoic–Mesozoic sedimentary rocks of the Lhasa terrane, the Cretaceous igneous rocks of the northern Lhasa terrane, and the Paleozoic–Mesozoic sedimentary rocks of southern Qiangtang terrane. The modal framework-grain composition of sandstone samples from the basin strata indicates a recycled orogen as the source. Sandstone composition analysis reveals an increase in the volcanic component with time associated with a pulse of quartz grains in the middle Eocene. We interpret such observations to indicate input from early Cretaceous clastic sediments belonging to the Duba Formation and the Baingoin Plutonic Complex, which imply passive exhumation along the hanging wall of the Gerze-Siling Co thrust. Trace element geochemistry of zircon grains demonstrates an increase in europium anomaly (Eu/Eu*), which we interpret to reflect ∼20 km crustal thickening in central Tibet from the Eocene to Oligocene, as well as an increase of >3 km surface elevation which partly was induced by the Cenozoic sedimentary accumulation. This significant uplift event may have been associated with convective removal of a thickened Tibetan lithosphere that in turn led to magmatic inflation of the Tibetan crust.

Sm-Nd isotopic compositions of deep-marine mudstones, Xigaze forearc basin, southern Tibet: Implications for drainage evolution and expansion

Provenance analysis of fine-grained sedimentary rocks has great importance to understanding the tectonic and paleogeographic evolutions of convergent margin systems. This study conducted Sm-Nd isotopic analysis of mudstone samples from Cretaceous deep-marine strata in the Xigaze forearc basin to evaluate the provenance, paleogeography, and regional drainage patterns in the arc-forearc region. Ten mudstone samples yield εNd(t) ranging from −7.44 to + 0.58 and Nd model age from 0.862 to 1.50 Ga. The mudstones were sourced from a mixture of the Gangdese arc and the Central to Northern Lhasa terrane. The Gangdese arc was the dominant source area, and the relative contribution of the Central to Northern Lhasa terrane sources increased up-section. Our interpretation is consistent with studies on coarser components. We interpret these results as a long-term signal of drainage capture of drainage networks that originated in the southern end of the Gangdese arc and brought Gangdese sources into the forearc basin during its early stages. Fostered by the uplift of the Lhasa terrane, drainage capture eventually reached the Gangdese mountains and regions to the north, resulting in the erosion and supply of more materials with negative εNd(t) from the Central and/ or Northern Lhasa to the basin. Similar patterns of Sm-Nd provenance evolution and drainage expansion have been observed in other well-studied forearc basins (e.g., the Great Valley forearc basin of California) and may reflect an important exhumation trend in convergent margin systems.

Albian–Cenomanian granitoid magmatism in Eastern and Central Tibet as a result of diachronous, continental collision induced slab tear propagation

AbstractA discrete belt of Albian–Cenomanian granitoid plutons occurs in the Lhasa and Qiangtang terranes in the Central (CTP) and Eastern Tibetan Plateau (ETP) and represents a major magmatic pulse in the plateau’s crustal evolution during the Cretaceous. The geochemistry, petrogenesis, and magmatic development of these granitoids are different from those of magmatic arc granitoids along the southern edges of the Lhasa and Qiangtang terranes, indicating different heat and melt sources and tectonic setting of their formation. We present here new mineral, whole-rock and isotope geochemistry, and zircon U-Pb age data from the Xiasai pluton in the ETP and discuss its geochemical-petrological characteristics and magmatic development in comparison to the other Cretaceous plutons in the ETP and CTP, and within the tectonic framework of the Mesotethyan geology of Tibet. Zircons from the Xiasai and other plutons in the ETP have yielded U-Pb ages ranging from 106 Ma to 93 Ma in comparison to 115 Ma and 100.3 Ma zircons from the Southern Qiangtang Terrane (SQT) and 113.4 Ma and 109 Ma zircons from the Northern Lhasa Terrane (NLT) farther west. The Cretaceous granitoids in the ETP and CTP range in composition from granite, K-feldspar granite to monzogranite and biotite monzogranite, representing highly fractionated I-type granites with relatively high SiO2 and K2O contents, variable (Na2O + K2O)/CaO and FeOT/MgO ratios, and (Zr + Nb + Ce + Y) abundances. They display significant negative Eu anomalies (Eu/*Eu) = 0.04–0.12) and strong depletions in Sr and Ba, and are strongly enriched in large ion lithophile elements but depleted in high field strength elements. Their εHf(t) values correspond to wide ranging Hf isotope crustal model ages (TDMC) of 0.3–1.9 Ga, and their Sr-Nd isotopic signatures show elevated (87Sr/86Sr)i ratios (0.7034–0.7105) and negative εNd(t) values of –8.8 to –4.9. These high whole-rock (87Sr/86Sr)i ratios and relatively high Th/Nb and Th/Yb ratios indicate incorporation of melts derived from partial melting of subducted sediments into the melt evolution of these granitoids that involved partial melting of the subduction-metasomatized lithospheric mantle and the mafic- to intermediate-composition continental crust. The extant zircon crystallization ages of the granitoid intrusions in the CTP and ETP show eastward younging of their emplacement from 115 Ma to 93 Ma, suggesting an apparent eastward migration of the heat source through time. A diachronous collision of the NLT with the SQT during 145–120 Ma and the subsequent slab breakoff induced, eastward propagated slab tear and asthenospheric upwelling produced the hybrid melts of the Albian–Cenomanian granitoids and their emplacement in a discrete, narrow magmatic belt in the CTP and ETP.

Evolution of the Sumdo Paleo-Tethyan Ocean: Constraints from Permian Luobadui Formation in Lhasa terrane, South Tibet

Reconstructing the evolution of the Sumdo Paleo-Tethyan Ocean (SPTO) is important for understanding the transition from the Paleo-Tethyan system to the Neo-Tethyan system. The existence of the SPTO is inferred from the presence of an (ultra-)high pressure metamorphic belt and dismembered ophiolite bodies that separate the southern Lhasa and northern Lhasa terrane. This paper presents new geochronological and geochemical data for the Luobadui Formation in the Lhasa terrane. The geochemical difference between the Lower Luobadui volcanics and the Upper Luobadui volcanics suggests a source transition from a depleted mantle source to an enriched mantle source, which indicates continental-arc subduction. The integrated geological evidence suggests that the SPTO initially opened during the late Carboniferous (ca. 304 Ma), as verified by the earliest record of oceanic crust. Our new data for the Luobadui volcanic rocks, together with the metamorphic and sedimentary records, support northward subduction of the SPTO lithosphere beneath North Lhasa, most likely starting during the Early Permian (ca. 275 Ma) and lasting until the Middle Permian (ca. 260 Ma). The main closure of the SPTO occurred during Middle–Late Triassic (ca. 240–220 Ma), as recorded by medium-pressure metamorphism and syn-collisional magmatism. The Wilson cycle of the SPTO is hereby reconstructed, providing significant insights into the amalgamation of the Tibetan Plateau.

Post-subdution evolution of the Northern Lhasa Terrane, Tibet: Constraints from geochemical anomalies, chronology and petrogeochemistry

Bangong-Nujiang collisional zone (BNCZ) is an older one in Qinghai-Tibet Plateau and resulted in the famous Bangong-Nujiang metallogenic belt, which plays an important role in evaluating the formation and uplift mechanism of plateau. The northern and central Lhasa Terrane composed the southern part of the BNCZ. Since ore deposits can be used as markers of geodynamic evolution, the authors carried 1∶50000 stream sedimental geochemical exploration in the Xiongmei area in the Northern Lhasa Terrane to manifest the mineralization, and based on this mineralization with geochemical and chronological characteristics of related magmatic rocks to constrain their geodynamics and connection with the evolution of the Lhasa Terrane. The authors find Early Cretaceous magma mainly resulted in Cu, Mo mineralization, Late Cretaceous magma mainly resulted in Cu, Mo, and W mineralization in the studying area. The results suggest a southward subduction, slab rolling back and break-off, and thickened lithosphere delamination successively occurred within the Northern Lhasa Terrane.© 2022 China Geology Editorial Office.

Provenance of Late Cretaceous accretionary complex within the Yarlung–Zangpo suture zone, Bainang, southern Tibet: Implications for the subduction–accretion of the Neo-Tethyan ocean

Accretionary complexes offer important source of information for studying the subduction history and source-to-sink system and neighboring terranes. In this study, we conducted field mapping, detrital zircon U–Pb age and trace element analyses, and electronic microprobe composition analysis of detrital Cr-spinel in sandstones from the Cretaceous Bainang accretionary complex in the central part of the Yarlung–Zangpo suture zone (YZSZ). The Bainang accretionary complex is located in a structural window that is mainly composed of Xigaze ophiolite, Late Triassic–Early Cretaceous radiolarite chert with a Late Cretaceous hemipelagic siliceous shale matrix, and Late Cretaceous sandstone. The obtained detrital zircon geochronological dataset confirms two spectra. Detrital zircons from the Canggasaba and Lianxiang sections are dominated by Precambrian grains with subordinate numbers of Mesozoic grains clustering at 243–178, 164–117, and 109–91 Ma, consistent with the mélange and trench-fill Rongmawa Formation, and all the trace elements of the Mesozoic grains match continental arc affinity. Provenance analysis suggests that detritus in these sections was mainly derived from the eastern part of the Gangdese arc and the central–northern Lhasa terrane. These results confirm the existence of the ancestral Lhasa River that transports sediments from the eastern Lhasa terrane to the subduction zone and further westward along the east–west axial system parallel to the trench. Conversely, the Qiajiu section is mainly dominated by Mesozoic detrital zircon grains, representing submarine fan deposits that were sourced from the adjacent Gangdese arc and transported through a local north–south-flowing river system. Detrital Cr-spinel compositions and detrital zircon trace elements from the Qiajiu section also revealed the contribution of ophiolites. Considering the abundance of Eurasia-affinity detrital zircons, this study supports the single subduction model that was developed along the southern margin of the Lhasa terrane.

Late Eocene palaeogeomorphology and uplift process of the Central Tibetan Plateau: Constraints from intermediate-felsic magmatic rocks

The Eocene palaeogeomorphology and uplift process of the Central Tibetan Plateau including the Lhasa and Qiangtang terranes have been controversial. In this paper, we study Eocene rhyolites in Wuma area of the Northern Lhasa terrane on the Central Tibetan Plateau by zircon U-Pb dating, geochemistry, zircon Lu-Hf isotopes, and Sr-Nd isotopes. The results show that the ages of Wuma rhyolites are 40-39 Ma, and the petrogenesis of them is related to partial melting of lower crust. The La/Yb, Dy/Yb, Sm/Yb and zircon Eu/Eu* of lower-crust-derived intermediate-felsic magmatic rocks can well reflect crustal thickness and Moho depth. Based on statistics of intermediate-felsic magmatic rocks in the Central Tibetan Plateau at 47–37 Ma, we find that lower-crust-derived intermediate-felsic magmatic rocks of the Northern Lhasa terrane have lower La/Yb, Dy/Yb, Sm/Yb and zircon Eu/Eu* than those of the Southern-Central Lhasa and Qiangtang terranes, indicating that Northern Lhasa terrane had thinner crust than Southern-Central Lhasa and Qiangtang terranes during late Eocene. Further, according to Airy isostatic compensation, in which thick crusts correspond to high altitudes and thin crusts correspond to low altitudes, we speculate that the Northern Lhasa terrane had lower altitude, while the Southern-Central Lhasa and Qiangtang terranes had higher altitude during late Eocene; that is, the Central Tibetan Plateau had the late Eocene palaeogeomorphology of two mountains and one basin. In addition, through statistics of intermediate-felsic magmatic rocks in the Northern Lhasa terrane and near its suture zones on both sides since 100 Ma, we find that La/Yb, Dy/Yb and Sm/Yb of these intermediate-felsic magmatic rocks increase at 40-34 Ma, indicating that the crust of the Northern Lhasa terrane have thickened and the surface may have been uplifted during late Eocene. Based on that there are abundant late Eocene lower-crust-derived intermediate-felsic magmatic rocks in the Central Tibetan Plateau and the late Eocene palaeogeomorphology of Central Tibetan Plateau is two mountains and one basin, and other evidences, we speculate that the late Eocene uplift of Northern Lhasa terrane may have been related to lower crustal channel flow.

Mid-Cretaceous thick carbonate accumulation in Northern Lhasa (Tibet): eustatic vs. tectonic control?

Abstract Widespread accumulation of thick carbonates is not typical of orogenic settings. During the mid-Cretaceous, near the Bangong suture in the northern Lhasa terrane, the shallow-marine carbonates of the Langshan Formation, reaching a thickness up to ~1 km, accumulated in an epicontinental seaway over a modern area of 132 × 103 km2, about half of the Arabian/Persian Gulf. The origin of basin-wide carbonate deposits located close to a newly formed orogenic belt is not well understood, partly because of the scarcity of paleogeographic studies on the evolution of the northern Lhasa. Based on a detailed sedimentological and stratigraphic investigation, three stages in the mid-Cretaceous paleogeographic evolution of northern Lhasa were defined: (1) remnant clastic sea with deposition of Duoni/Duba formations (Early to early Late Aptian, ca. 125–116 Ma); (2) expanding carbonate seaway of Langshan Formation (latest Aptian–earliest Cenomanian, ca. 116–99 Ma); and (3) closure of the carbonate seaway represented by the Daxiong/Jingzhushan formations (Early Cenomanian to Turonian, ca. 99–92 Ma). Combined with data on tectonic subsidence and eustatic curves, we emphasized the largely eustatic control on the paleogeographic evolution of the northern Lhasa during the latest Aptian–earliest Cenomanian when the Langshan carbonates accumulated, modulated by long-term slow tectonic subsidence and high carbonate productivity.

Post-subdution evolution of the Northern Lhasa Terrane, Tibet: Constraints from geochemical anomalies, chronology and petrogeochemistry

Bangong-Nujiang collisional zone (BNCZ) is an older one in Qinghai-Tibet Plateau and resulted in the famous Bangong-Nujiang metallogenic belt, which plays an important role in evaluating the formation and uplift mechanism of plateau. The northern and central Lhasa Terrane composed the southern part of the BNCZ. Since ore deposits can be used as markers of geodynamic evolution, the authors carried 1∶50000 stream sedimental geochemical exploration in the Xiongmei area in the Northern Lhasa Terrane to manifest the mineralization, and based on this mineralization with geochemical and chronological characteristics of related magmatic rocks to constrain their geodynamics and connection with the evolution of the Lhasa Terrane. The authors find Early Cretaceous magma mainly resulted in Cu, Mo mineralization, Late Cretaceous magma mainly resulted in Cu, Mo, and W mineralization in the studying area. The results suggest a southward subduction, slab rolling back and break-off, and thickened lithosphere delamination successively occurred within the Northern Lhasa Terrane.© 2022 China Geology Editorial Office.

A photomicrograph dataset of mid-Cretaceous rocks from Langshan Formation in northern Lhasa Terrane, Tibet

A photomicrograph dataset of mid-Cretaceous rocks from Langshan Formation in northern Lhasa Terrane, Tibet