Nujiang Suture Research Articles

Hotspot–subduction zone interactions and their resource effects at ∼120 Ma on the central Tibetan Plateau

The central Tibetan Plateau, particularly the Bangong–Nujiang suture zone, hosts abundant and complex Late Mesozoic geological records and world-class mineral resources. However, intense debate persists about the origin and tectonic setting of these geological records and mineral resources, which impedes a comprehensive understanding of regional Late Mesozoic tectonic evolution and mineralization. In this study, we investigated the petrochronology, geochemistry, and Sr-Nd-Hf isotopic characteristics of ∼120 Ma magmatic rocks in this region. We explored their petrogenesis and sources, and in conjunction with their spatiotemporal distribution and scale, we facilitated a first proposal that these magmatic rocks were derived from ∼120 Ma hotspot–subduction zone interaction and that world-class mineral deposits were formed in this geological setting. This study is the first to identify geological records of hotspot–subduction interaction in the central Tibetan Plateau and it has significant implications for gaining a deeper understanding of the closing processes of the Late Cretaceous Epoch in the Meso-Tethys Ocean. This study also highlights the substantial contribution of this hotspot–subduction zone interaction to the formation of world-class mineral deposits in the region, and thus has crucial significance for guiding mineral exploration in the central Tibetan Plateau.

Mesozoic orogenic gold metallogenesis at the Bangong–Nujiang suture belt, central Tibet: Constraints from pyrite textures, composition and sulfur isotope of the Shangxu gold deposit

Pyrite is a good carrier to provide gold within a hydrothermal mineralization system. Under prograde metamorphism, recrystallization of sedimentary and diagenetic pyrite and transformation of pyrite into pyrrhotite in a metamorphic terrane lead to the release of trace elements from the pyrite carrier into an upward migrating liquid phase carrier. In this case study, we report the in situ trace element and δ34S compositions of the pyrite from four different stages, which chronologically comprise sedimentary Py1, diagenetic Py2, metamorphic Py3a, Py3b and hydrothermal Py4a, Py4b, Py5, and the dominant sulfides from the hydrothermal stage at Shangxu, an Early Cretaceous turbidite-hosted orogenic gold deposit in the middle Bangong–Nujiang suture belt (BNSB) in central Tibet, in order to decipher the ore-forming material source and the metallogenesis of the Mesozoic orogenic gold mineralization in the BNSB. Concentrations of trace elements, i.e., Au, Co, Cu, Zn, Mo, Ag, Sb, Te, Hg, Tl, Pb and Bi, that are easy to dissolve during pyrite recrystallization, show a general progressive decline from Py1 to Py4b (excepting in Py3b and Py4a, which have a sudden rise in trace element composition) and a plunge in Py5, indicating that Py1 and Py2 supply the parent source minerals for mineralization, and that the main ore-forming stage are contemporary with the deposition of Py3b and Py4a, ending up with the precipitation of Py5. The δ34S values of pyrite narrow from −46.7 ‰–19.3 ‰ (Py1, Py2), through −7.6 ‰–0.2 ‰ (Py3a, Py3b), to − 3.6 ‰–5.7 ‰ (Py4a, Py4b) and −7.7 ‰–5.6 ‰ (Cp, Gn), indicating that a probable coeval seawater sulfate sulfur source for Py1 and Py2 (by bacterial sulfate reduction), and an inherited sulfur source from Py1 and Py2 for Py3 and Py4. We highlight that contributions from the magmatic materials to the hydrothermal gold mineralization are limited at Shangxu, and the widely distributed Lower–Middle Jurassic pyrite-rich turbidite of the Mugagangri Group is a highly potential primary source rock for the regional gold mineralization.

Early Cretaceous A-Type Acidic Magmatic Belt in Northern Lhasa Block: Implications for the Evolution of the Bangong–Nujiang Ocean Lithosphere

A-type granites have been the subject of considerable interest due to their distinct anorogenic geological background. The A-type and arc-related granites are crucial in deciphering the evolution of the ocean closure and continental collision in the Tibet Plateau. The demise of the Bangong–Nujiang suture zone (BNSZ) and the Yarlung–Tsangpo suture zone was accompanied by the emplacement of volumes of syn-collisional and post-collisional granites. Controversy has persisted regarding the contribution of the collisional granites within the Lhasa Block to the growth of the Tibetan Plateau. This study provides key evidence about the evolution of the Lhasa Block and Bangong–Nujiang Ocean (BNO) by the newly documented 1200 km long, Early Cretaceous A-type acidic magmatic belt. The resolution was achieved through the utilization of petrology, whole-rock geochemistry, zircon U-Pb geochronology, and in situ zircon Hf isotope analysis of the Burshulaling Granites in the eastern segment and previous existing data in the central and western segment of the Lhasa Block. The Burshulaling Granites are characterized as peraluminous, high-K calc-alkaline series, indicating a post-collision setting with high temperature and low pressure. The zircon grains from two granite samples yield 206Pb/238U ages of 115–113 Ma. In situ zircon Hf analyses with 206Pb/238U ages give εHf(t) of −6.2–0.6, showing prominent characteristics of crust-mantle interaction. Granites from east to west exhibit whole-rock geochemical and geochronological similarities that fall within the well-constrained Early Cretaceous time frame (117–103 Ma) and track post-collisional A-type acidic magmatic belt along BNSZ. We argue that this magmatism resulted from slab break-off or orogenic root detachment, leading to melting and mixing of the lower crust. Meanwhile, this study indicates the existence of the Bangong–Nujiang Ocean southward subduction or a collapse following an Andean-type orogen.

Spatial heterogeneity of Cenozoic provenance in the Nima and Lunpola basins of China: Implications for the origin of the low-relief topography in central Tibet Plateau

The central Tibet Plateau is characterised by low-relief topography of high elevation; however, the origin of this surface flatness is uncertain because of a lack of reliable evidence from palaeogeography and palaeodrainage. In this study, we conduct sedimentological and provenance analysis of the Nima Basin, one of several terrestrial basins developed along the Bangong–Nujiang suture zone since the middle Eocene. Our data show that the Dingqinghu Formation in the Nima Basin was deposited in fan delta, braided river and lacustrine environments in the late Eocene after 37 Ma. It received sediment consisting mainly of recycled detritus, including orbitolinid-bearing limestone with volcanic fragments from the Lhasa Terrane in the south. Comparison of the stratigraphic sections in the Nima Basin with those of the adjacent Lunpola Basin suggests that these regions are characterised by local but similar provenance signals. We suggest that a source-to-sink system once existed from the shallower Nima Basin eastward to the deeper and bigger Lunpola Basin. We hypothesise that the Oligocene–early Miocene source-to-sink system from the Nima to Lunpola may have played a role in the surface flattening of the central Tibet Plateau.

Crustal Deformation in Eastern Himalayan Syntaxis Constrained by Ambient Noise Tomography

ABSTRACT As the leading edge of the Indian–Asian collision, the eastern Himalayan syntaxis region has experienced extensive tectonic activities, resulting in complex crustal uplift and deformation in the corner area of the southeastern pathway for the extrusion of Tibetan plateau materials. Despite considerable efforts, the corresponding deformation mechanisms remain uncertain. This study presents a new 3D high-resolution azimuthal anisotropic shear-wave velocity model in the crust and uppermost mantle derived from ambient noise dispersion data. Results show that the upper crustal anisotropy aligns with the geological boundaries and major faults nearby, suggesting shape-preferred orientations. The upper crustal low velocity and weak anisotropy beneath the core of the eastern Himalayan syntaxis (EHS) are closely associated with the high fragmentation of shallow rocks and the upwelling of hot materials during the ongoing subduction of the Indian plate. Our model also reveals relatively complex anisotropic patterns in the midlower crust. The eastern Lhasa terrane, in particular, exhibits low velocity and strong anisotropy with a northwest–southeast-oriented fast axis, supporting the local scale midlower crustal “channel flow” model. In addition, a conspicuous, elongated low-velocity zone along the northwest–southeast direction is observed in the midlower crust and uppermost mantle beneath the Bangong–Nujiang suture. The anisotropy in this region increases with depth, and the fast directions are consistently parallel to the northeast subduction of the Indian plate. We infer that this low-velocity zone may result from partial melting under local compression driven by the Indian–Asian collision. On the basis of newly revealed anisotropic model and previous studies, we construct a new dynamic model, which reveals that the migration of mechanically weak material in the midlower crust and the significant contribution of the northeast subduction of the Indian plate jointly control the crustal deformation of the EHS region.

The Cretaceous suturing process of the eastern Bangong–Nujiang Ocean, Basu area

The Bangong–Nujiang Ocean played an important role in the formation of the Tibetan Plateau prior to the Cenozoic India–Eurasia collision. However, there are still uncertainties about the subduction polarity and timing of the Lhasa–Qiangtang collision. We conducted sandstone petrologic and detrital zircon U–Pb–Hf isotopic analyses on the Cretaceous Wada mélange, trench-fill strata and Duoni Formation in the Basu area in the eastern Bangong–Nujiang Suture Zone. The Wada mélange (∼114 Ma) exhibits block-within-matrix features and a detrital zircon U-Pb age spectrum characterized by multiple peaks at 114–180 Ma, 200–300 Ma, and 1800–2000 Ma. The trench-fill strata consist of coherent chert, sandstone, and mudstone, with a detrital zircon age spectrum dominated by a single peak at ∼120 Ma. The εHf(t) values of the 110–300 Ma detrital zircon grains in the Wada mélange and trench-fill strata range from −20 to +10 and are consistent with those of the South Qiangtang Terrane. The εHf(t) values of the ∼120 Ma detrital zircons are all negative, and these grains were likely derived from a remnant Cretaceous arc in the South Qiangtang Terrane to the north. These data suggest that the accretionary wedge was derived from the South Qiangtang Terrane during the northward subduction of the Bangong–Nujiang Ocean. The Duoni Formation (∼113 Ma) represents peripheral foreland basin deposits with the Lhasa Terrane as the basement. Provenance analysis indicates that these deposits received clastic material mainly from the South Qiangtang Terrane and to a lesser degree from the North Lhasa Terrane. Our results suggest that the Lhasa–Qiangtang collision occurred in the Early Cretaceous (∼113 Ma) in the eastern segment of the Bangong–Nujiang Suture Zone.

Middle–Late Jurassic subduction erosion caused by intra-oceanic arc subduction in central Tibet

Abstract Subduction erosion is crucial in crustal material recycling. However, subduction erosion caused by intra-oceanic arc subduction has not been sufficiently investigated. In this study, we provide new geological, geochronological, geochemical, and isotopic data from Dongco granodiorites in the central Bangong–Nujiang suture zone of central Tibet to explore subduction erosion caused by intra-oceanic arc subduction. Analysis shows that the ca. 158–155 Ma Dongco granodiorites originated from the subducted oceanic plate, and they were contaminated with accretionary wedge when they intruded the Dongco ophiolite. This suggests that the Dongco ophiolite was emplaced in the subducted accretionary wedge before the Late Jurassic. Based on the intra-oceanic arc affinity and lack of volcanic rocks of Middle Jurassic Dongco ophiolite and other regional data, we believe that the main body of the central intra-oceanic arcs and a portion of the western intra-oceanic arcs in the Meso-Tethys Ocean subducted beneath the southern Qiangtang terrane during the Middle–Late Jurassic. In addition, the different degree absence of the Jurassic accretionary wedge, forearc region, and arc magmatic rocks in the southern Qiangtang terrane indicate that the central and western parts of the southern Qiangtang terrane experienced both vigorous and relatively weak subduction erosion during the Middle–Late Jurassic, respectively. Thus, there is a significant spatiotemporal coupling between subduction erosion of the southern Qiangtang terrane and intra-oceanic arc subduction. Based on these studies and the research on subduction erosion, we suggest that subduction of the main body of the central intra-oceanic arcs and partial subduction of the western intra-oceanic arcs in the Meso-Tethys Ocean caused both vigorous and relatively weak subduction erosion of the southern Qiangtang terrane during the Middle–Late Jurassic, respectively. In addition, the increase in subduction rate also promoted Middle–Late Jurassic subduction erosion of the southern Qiangtang terrane.

Provenance Analysis in the Nima Basin during Paleogene and Its Implications for the Decline of the Tibetan Central Valley.

It is unclear what caused the Bangong Nujiang suture zone in the central Tibetan plateau to rise from less than 2 km in early Cenozoic to more than 4 km at present. The zircon U-Pb ages and trace elements of samples from the Niubao Formation in the Paleogene of the Nima basin were analyzed and tested. Combined with the isostasy theory, the surface uplift height of the Nima Basin during the Cenozoic period was calculated. The zircon U-Pb age results of the Niubao formation are consistent with the ages of the Lhasa terrane on the south side of the basin, the Qiangtang terrane on the north side, and the uplift in central. The zircon Eu/Eu* results show that the crust in central part of Tibetan plateau thickened by ∼20 km in Paleogene, resulting in ∼3 km surface uplift. Sediments created a total of about 1 km of surface uplift throughout the Paleogene, and the deposition rate began to slow down significantly at ∼40 Ma. Therefore, it is inferred that in the early Cenozoic, the uplift of the valley was mainly caused by sedimentation. With the continuous downward subduction of the Indian plate, at about 40 Ma, factors such as crustal shortening dominated the uplift of the central valley, and the uplift caused by deposition only accounted for a very small part. In general, the uplift of the Central Valley in the Paleogene was mainly affected by crustal shortening, but a quarter of the surface uplift was caused by the accumulation of sediments.

Petrogenesis of Early Cretaceous andesites and mafic dikes in central Tibet: Implications for the growth of continental crust in collision zones

Post-collisional magmatic rocks in central Tibet provide an opportunity to study the processes of continental crustal growth in a collision zone. Here, we report data for samples of andesites and coeval mafic dikes obtained from the Jiaqiong area of the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating shows that the andesites and mafic dikes were emplaced concurrently during the late Early Cretaceous (ca. 115 Ma). The andesites have high Mg# values (55.2–61.6) and initial 87Sr/86Sr ratios (0.7086–0.7088), negative whole-rock εNd(t) (−6.5 to − 6.1) and zircon εHf(t) (−5.1 to − 1.7) values, and elevated zircon δ18O (6.8‰–7.5‰) values, indicating that the andesites were probably derived by the partial melting of an ancient subduction-modified lithospheric mantle. The coeval mafic dikes have moderately high Mg# values (49.3–50.0) and initial 87Sr/86Sr ratios (0.7076–0.7082), and negative εNd(t) values (−5.4 to − 5.3), implying that they were derived from an enriched lithospheric mantle source. However, the zircon εHf(t) (−5.8 to + 1.6) and δ18O (5.2‰–6.3‰) values of these mafic dikes indicate the influence of depleted mantle components. Furthermore, results of Sr–Nd–Hf–O isotopic mixing modeling suggest that these mafic dikes could have been produced by interaction between asthenospheric and lithospheric mantle. Therefore, we suggest that the parental magmas of the mafic dikes were probably generated by interaction between asthenospheric and overlying metasomatized lithospheric mantle. Combined with data on nearby Cretaceous magmatic rocks and sedimentary strata, we propose that the studied magmatic rocks were formed in a post-collisional slab breakoff setting in central Tibet and have made a substantial contribution to the vertical growth of continental crust in this continental collision zone.

Coexisting High-Al and High-Cr Chromitites in the Dingqing Ophiolite (SE Tibet): Inferences to Compositional Heterogeneity in the Tethyan Upper Mantle

The Dingqing ophiolite represents a significant allochthonous ophiolite nappe in the eastern segment of the Bangong–Nujiang suture zone in southeastern Tibet. The microanalytical data of associated podiform chromitites classify them into two distinct varieties: high-Al and high-Cr. The coexistence of both high-Cr and high-Al chromitites in the Dingqing ophiolite suggests a complex or multistage evolutionary history of the host rocks. New petrological and geochemical analyses are used herein to unravel the interrelationships between the chromitite ores and host rocks and assess the mechanism of formation. The Dingqing ophiolitic nappe is made up mainly of harzburgite, dunite, and less abundant pyroxenite and gabbro. Several small lens-shaped bodies of chromitite ore are mostly confined to the harzburgite rocks, with ore textures varying from massive to sparsely disseminated chromite. In addition to magnesiochromite, the orebodies contain minor amounts of olivine, amphibole, and serpentine. The textural relationships provide compelling evidence of plastic deformation and partial melting of the associated peridotites. Detailed examination of the Cr-spinel grains reveals a wide range of composition, spanning from high-Al (Cr# = 3.18–59.5) to high-Cr (Cr# 60.3–87.32). The abundances of the platinum-group element (PGE) in chromitites are significantly variable (93 to 274 ppb). Formation of the Dingqing peridotites most likely took place in a mid-ocean ridge (MOR) setting, and subsequent modifications by supra-subduction zone (SSZ) melts resulted in heterogenous or mixed geochemical characteristics of these rocks. Chemistry of the spinel–olivine–clinopyroxene assemblage demonstrates multiple stages of partial melting of the source mantle rocks, including an early phase of restricted partial melting (~20%–30%) and a later phase of extensive partial melting (>40%). The formation of the high-Al chromitite type was associated with the early phase (constrained melting), whereas extensive partial melting in the late stages likely led to the accumulation of high-Cr podiform chromitite bodies.

Mesozoic felsic magmatism along the Bangong–Nujiang suture zone, Tibet: Temporospatial variations and tectonic implications

The subduction history and closure of the Meso-Tethys Ocean have not yet been well constrained using data from felsic rocks along the Bangong–Nujiang suture zone. Here we present geochronological, geochemical, and isotopic data from Early Jurassic–Late Cretaceous felsic rocks in the Laguo Co area of central Tibet. We combine our results with published data from coeval felsic rocks to explore the temporospatial variations in magmatism and the implications for subduction in and closure of the Meso-Tethys Ocean. The studied rocks include 182–180 Ma granodiorite, 109 Ma granite porphyry, and 68 Ma rhyolite, and zircon grains from each yield different εHf(t) values (+8.9 to +14.0, −5.5 to +0.7, and −4.9 to +1.1, respectively). The rocks have low-K tholeiitic to medium-K calc-alkaline compositions, low Y and Yb contents, and positive Sr anomalies. These data suggest an oceanic source for the Early Jurassic granodiorite and a crustal source for the Cretaceous granite porphyry and rhyolite, as well as a tectonic transition from an oceanic arc during the Early Jurassic to continental collision during the Cretaceous. The temporospatial variation in felsic magmatism corresponds to intra-oceanic subduction, bidirectional subduction, oceanic ridge or flat subduction, slab rollback, slab breakoff or sinking, and lithospheric delamination. The timing of closure of the Meso-Tethys Ocean is constrained to the Jurassic–Cretaceous, as evidenced by the major differences of the magma sources and formation processes between the Jurassic and Cretaceous felsic rocks. Finally, we present a model showing that oceanic closure occurred earlier in the east and north and later in the west and south.

Early Jurassic lithospheric delamination in the Amdo microcontinent, central Tibet: Inferred from coeval OIB- and MORB-like gabbros

The Bangong–Nujiang suture zone (BNSZ) is the remnants of the Meso-Tethyan main oceanic basin in the Tibetan plateau. It is noted that several microcontinents, e.g., Amdo and Jiayuqiao, occur within the Meso-Tethyan ocean basin. These microcontinents were strongly reworked associated with the subduction and closure of this ancient ocean, thus they play a key role in understanding the evolution of this ocean. In this contribution, we report newly documented Early Jurassic gabbros (ca. 182–175 Ma) from the Amdo microcontinent in the middle segment of the BNSZ. The gabbros can be subdivided into two groups, i.e., OIB (ocean island basalt) and MORB (mid-ocean ridge basalt) groups. The OIB-group gabbros are characterized by highly enriched NdHf isotopic characteristics (zircon εHf(t) = −11.7 to −8.9; whole-rock εNd(t) = −11.0 to −10.1). They are geochemically similar to OIB and are interpreted to be products of the partial melting of a relatively deep mantle source (> 85 km) and subsequent continental crustal inputting. The MORB-group gabbros belong to the tholeiitic series and have similar compositions to those of MORB. They were most likely derived from a depleted mantle source (zircon εHf(t) = +3.7 to +16.5; whole-rock εNd(t) = +4.9 to +8.1) in a relatively shallow depth (< 85 km). Integrating previous studies with the data presented in this contribution, we suggest that the generation of coeval OIB- and MORB-like gabbros are related to lithospheric delamination after the Qiangtang-Amdo collision and the closure of the Bangong-Nujiang ocean basin. This result will give new constraint on the evolution of the Meso-Tethyan Ocean.

Resolving the nature and evolution of the Bangong−Nujiang Tethyan Ocean: New perspectives from the intraplate oceanic-island fragments preserved in Northern Tibet

The nature and tectonic evolution of the Bangong−Nujiang Tethyan Ocean remain unresolved, which seriously restricts our understanding of the role of this ocean in the Tethys tectonic domain and hinders in-depth study of the geodynamics of the Tethyan tectonic evolution. The study of intraplate oceanic-island fragments in the Bangong−Nujiang Suture Zone of Northern Tibet, which are remnants of the oceanic crust, is important for determining the nature and tectonic evolution of the Bangong−Nujiang Tethyan Ocean. Here, we present a combined analysis of petrological, geochronological, geochemical, and Sr-Nd isotope data for the five intraplate oceanic-island fragments preserved in the Bangong−Nujiang Suture Zone. The oldest intraplate oceanic-island fragment in the Bangong−Nujiang Suture Zone is of Late Permian−Triassic age, and the underlying oceanic lithosphere is Late Permian or older, which indicates the development of the Bangong−Nujiang Tethyan Oceanic basin at this time. The opening time of the Bangong−Nujiang Tethyan Ocean must be even earlier than the Late Permian. At least four Triassic intraplate oceanic-island fragments have been recognized in the Bangong−Nujiang Suture Zone, and their assemblages and geochemistry are mostly similar to present-day oceanic-island basalts that erupted onto the thick lithosphere of mature oceanic basins. All of these observations indicate that the Bangong−Nujiang Tethyan Ocean evolved into a mature oceanic basin during the Triassic. The youngest intraplate oceanic-island fragments are of Late Jurassic−Early Cretaceous ages, which effectively argues for the continued presence of the Bangong−Nujiang Tethyan Ocean at this time. The abundant data of the intraplate oceanic-island fragments provide strong evidence that the Bangong−Nujiang Tethyan Ocean was a broad oceanic basin that separated Cimmeria from Gondwana during the Permian−Early Cretaceous and experienced complex evolution over at least 160 m.y.

Tectonic evolution of the Meso-Tethys Ocean in the Jurassic: Birth, growth, and demise of a missing intra-oceanic arc

Reconstructing ophiolite ages and tectonic attributes can elucidate ancient ocean tectonic evolution. We present petrological, chronological, geochemical, and isotopic data for Ritu and Dongco ophiolites (Bangong–Nujiang suture zone, Tibetan Plateau). The ∼171 Ma Ritu and ∼169 Ma Dongco diabase are derived from high-temperature, highly depleted mantle metasomatized by subduction fluids; they are similar to fore-arc basalts. The ∼165 Ma Ritu andesites, from enriched mantle metasomatized by subduction fluids and a small proportion of subduction sediment melts, have affinity with the oceanic plateau. The ∼161 Ma Ritu dacites and Dongco granodiorites arose via interaction between subduction sediment melts and the mantle wedge, and ∼158 Ma Dongco granites were derived from subduction oceanic plate metasomatized by subduction sediment melts; they are all similar to arc magmatic rocks. Their high Na2O and low K2O and Th contents, and depleted zircon εHf(t) values, differ from those of southern Qiangtang terrane continental arc magmatic rocks, indicating intra-oceanic origin. The Middle Jurassic diabase and regional contemporaneous fore-arc basalts and boninites likely formed during intra-oceanic subduction initiation. The Middle–Late Jurassic dacites, granodiorites, granites, and regional contemporaneous high-Mg andesites likely formed in a mature intra-oceanic arc. Therefore, the Meso-Tethys Ocean may contain a ∼1000 km intra-oceanic subduction zone with Middle–Late Jurassic mature intra-oceanic arc. Subduction initiation of the intra-oceanic subduction zone likely arose via subduction transference caused by collage of the oceanic plateau and southern Qiangtang terrane at ∼182–177 Ma. At ∼171–155 Ma, the intra-oceanic arc gradually formed fore-arc basalts, boninites, high-Mg andesites, and tholeiitic/calc-alkaline arc rocks, indicating final maturity. The intra-oceanic arc west section potentially collided with the southern Qiangtang terrane ∼155–145 Ma, while the middle section potentially lasted until Early Cretaceous. Our study constrains Meso-Tethys Ocean evolution, providing an important basis for the study of intra-oceanic arcs in orogenic belts.

Sedimentary Evolution and Geological Characteristics of the Jurassic in the South Qiangtang Basin, China

Based on the analysis of lithology, lithofacies combinations, sedimentary structures, and fossil types in five geological sections measured through fieldwork, this paper comprehensively elucidates the sedimentary evolution characteristics of the Jurassic period in the South Qiangtang area. The South Qiangtang Basin is renowned for preserving the most complete Jurassic marine sedimentary strata in China, and it primarily consists of a mixed platform environment of carbonate and clastic rocks. The Jurassic strata in the South Qiangtang Basin range from the Quse Formation at the base to the Suowa Formation at the summit, with sedimentary facies evolving from the outer shelf to the subtidal zone, and seawater depth gradually becoming shallower. This trend may be associated with the division and expansion of the Bangong–Nujiang suture zone during the Late Triassic, subduction in the Middle Jurassic, and the suture and splicing of the Qiangtang–Lhasa plate during the Late Jurassic tectonic movements. In conclusion, this research presents a comprehensive analysis of the sedimentary evolution of the Jurassic period in the South Qiangtang Basin for the first time. The findings offer significant contributions to the understanding of the region’s geological history and serve as a valuable foundation for future investigations.

Southern Tibetan rifting since late Miocene enabled by basal shear of the underthrusting Indian lithosphere

Syncontractional extension is prominent in present-day Tibet, but its origin remains vigorously debated. Several deep-seated geodynamic processes (e.g., Indian underthrusting, horizontal flow, and mantle upwelling) have been linked to Tibetan rifting. Indian underthrusting is a good candidate because it can well explain why surface rifts are more prominent south of the Bangong–Nujiang suture; however, how Indian underthrusting causes extension is not well understood and lacks observational constraints. Seismic anisotropy, measured by exploiting the birefringence effect of shear waves, can be indicative of the deformation styles within the crust. Here, we unveil the dominant convergence-parallel alignment of anisotropic fabrics in the deep crust of the southern Tibetan rifts using seismic recordings collected from our recently deployed and existing seismic stations. This finding suggests that the strong north-directed shearing exerted by the underthrusting Indian plate is key to enabling present-day extension in southern Tibet.

Petrogenesis of Early Cretaceous volcanic rocks from the Rena-Co area in the southern Qiangtang Terrane, central Tibet: evidence from zircon U-Pb geochronology, petrochemistry and Sr-Nd-Pb–Hf isotope characteristics

AbstractThe subduction of the Bangong–Nujiang Ocean is important in the geological evolution of the Tibetan Plateau. In this paper, we report new zircon U-Pb age and Lu-Hf isotopic data and whole-rock elemental and Sr-Nd-Pb isotopic data for Early Cretaceous dacites from the Rena-Co area (RCA) in the southern Qiangtang Terrane (QT), central Tibet and use these data to better understand the tectonic evolution of the Bangong–Nujiang suture. LA–ICP-MS dating of zircons yields ages of 109.5 ± 0.6 Ma to 109.6 ± 0.8 Ma for the dacites from the RCA. Geochemically, these dacites are medium-K calc-alkaline and show high SiO2 contents of 64.79–70.37 wt.%, high Sr contents of 517–598 ppm and low Y contents of 8.45–10.7 ppm, similar to those of typical adakites. Additionally, all the rocks are strongly enriched in light rare earth elements and some large ion lithophile elements (e.g. Rb, U, K and Cs) but significantly depleted in high-field-strength elements (e.g. Nb, Ta and Ti), consistent with the geochemical characteristics of arc-type magmas formed in the subduction zone. Moreover, these adakite-like dacites show whole-rock initial (87Sr/86Sr)i ratios of 0.705119 to 0.705491, (206Pb/204Pb)i ratios of 18.489 to 18.508, (207Pb/204Pb)i ratios of 15.591 to 15.612, (208Pb/204Pb)i ratios of 38.599 to 38.686, ϵNd(t) values of −0.28 to +1.25 and single-stage Nd model ages of 642 to 818 Ma, as well as significantly positive zircon ϵHf(t) values of 3.9–13.1, with young Hf-depleted mantle ages of 331 to 923 Ma. These geochemical and isotopic data indicate that they are most likely derived from the juvenile thickened mafic lower continental crust, which contains partial melts of metasomatized peridotite and subduction-related fluids in the magma source region. Based on previous studies and our new data, we propose that the RCA adakite-like dacites are most likely a result of the northwards subduction of the Bangong–Nujiang Ocean lithosphere beneath the southern QT during the Early Cretaceous and that a slab rollback model could explain the formation of the RCA adakite-like dacites.

Zircon U–Pb Geochronology, Geochemistry and Geological Significance of the Santaishan–Yingjiang Ultramafic Rocks in Western Yunnan, China

The study of ultramafic rocks in Western Yunnan is of great significance for an understanding of the tectonic evolution of the Neo-Tethys Ocean. The zircon U–Pb data indicated that the Santaishan serpentinized pyroxene peridotite (SSPP) was formed 186–190 Ma, and the Yingjiang hornblende pyroxenite (YHP) was formed 182–183 Ma. The content of MgO in the SSPP is relatively high, but the SiO2, Al2O3, CaO and TiO2 content and ΣREE are low, while the YHP has opposite characteristics. The samples from the SSPP and YHP have similar distribution patterns of trace elements, both being enriched in large ion lithophile elements (LILEs) such as Rb, Ba and Th and depleted in high field strength elements (HFSEs) such as Ti, P and Nb. These characteristics are consistent with the supra-subduction zone (SSZ) type and mid-ocean ridge basalt (MORB) type of ophiolite in the Bangong–Nujiang suture zone. The SSPP rocks have relatively high (87Sr/86Sr)i ratios (0.7091–0.7131) and positive Hf(t) values (11.2–13.8), with εNd(t) values varying from −1.1 to 9.4. The YHP has relatively low εHf(t) values (3.5 to 6.9), with the Nd–Hf isotopic model ages ranging from 610 to 942 Ma. The signatures of Sr–Nd and Lu–Hf isotopes indicate that the SSPP and YHP were derived from the depleted mantle, and the crustal material in the magma source may have originated from the Neoproterozoic Rodinia supercontinent. In the early Middle Jurassic (190 Ma), the Tengchong Block was in the setting of an active continental margin induced by the subduction of the Bangong–Nujiang Ocean, where the SSZ-type SSPP with ophiolite characteristics was formed. With the continuous subduction of the Bangong–Nujiang Ocean, the slab retreated and induced mantle convection, which resulted in the gradual thinning of the continental crust. Meanwhile, the Yingjiang back-arc basin was formed 183 Ma. Under the influence of the upwelling of the asthenosphere and the mixture of crustal materials, the MORB-type YHP was formed.

Constraining the deep dynamic process beneath the Bangong-Nujiang suture zone: A case study from the early cretaceous trachytic rocks

ABSTRACT The geodynamic processes that occurred beneath the Bangong – Nujiang suture zone during the evolution of the Indus – Yarlung Neo-Tethys and Bangong – Nujiang Meso-Tethys are unclear. We conducted a study of Early Cretaceous andesitic rocks from the Nyima area, north – central Tibet, to investigate these processes. The studied rocks have zircon U – Pb ages of 110–104 Ma and negative to positive εHf(t) values (−12.1 to+10.3). These rocks have affinities with trachytes, and have high total alkali contents and Na2O/K2O ratios (5–125). They also have high Sr (281–1409 ppm), Y (21.0–28.5 ppm), and Yb (2.00–2.65 ppm) contents, and exhibit depletions in Nb, Ta, and Ti. Such features indicate these rocks are not adakites, but are similar to typical arc rocks. The geochemical and isotopic data suggest the studied rocks were formed by partial melting of amphibolite-facies oceanic lithosphere, with the addition of enriched sedimentary materials. Based on the regional crustal evolution, we suggest that the Nyima andesitic rocks formed by melting of Bangong – Nujiang oceanic lithosphere. As such, the Early Cretaceous igneous rocks along the central Bangong – Nujiang suture zone were not associated with northward subduction in the Indus – Yarlung Neo-Tethys.

Detrital zircon U–Pb age constraints on the Meso-Tethys Ocean closure in SE Asia

Abstract The location of the Meso-Tethyan suture in SE Asia is ambiguous due to the strong overprint by the later India–Asia collisional tectonics. For the SE Asian extension of the Meso-Tethyan Bangong–Nujiang Suture (BNS) in Tibet, the two main candidates are the Longling–Ruili Suture (between the Tengchong and Baoshan blocks) and the Myitkyina ophiolite belt. Here, we present new detrital zircon U–Pb ages from high-grade metasedimentary rocks of the Gaoligong Group, which is considered to be the basement of the Tengchong Block located between the Longling–Ruili Suture and Myitkyina ophiolite belt. Our data suggest that the Gaoligong Group contains Cambrian, Triassic and Cretaceous zircons and is not a Precambrian basement, with a diagnostic age population peak at c. 1110 Ma. This peak is comparable to the c. 1170 Ma peak reported in the Australia-derived blocks (e.g. Lhasa and West Australia). Such similarities, as well as the similarities in the stratigraphy, palaeobiogeography and magmatic history of the Tengchong Block, suggest that the Tengchong Block was located adjacent to the northern Australia margin in Gondwana, and was most likely to have been linked with the Lhasa Block (southern Tibet) since the Early Paleozoic. Thus, the Longling–Ruili Suture is likely to represent a continuation of the Meso-Tethyan BNS in SE Asia.