Bangong-Nujiang Suture Research Articles

Paleomagnetic study on the Triassic rocks from the Lhasa Terrane, Tibet, and its paleogeographic implications

We present paleomagnetic results from the newly discovered Early-Middle and Late Triassic marine sediments of the Lhasa Terrane. Orientated samples were collected from 32 sites (330 samples) on the north side of the Dibu Co Lake (84.7°E, 30.9°N), Coqên County, in the western region of the Lhasa terrane. Rock magnetic data revealed that most of the samples were dominated by magnetite and/or pyrrhotite. The stepwise demagnetization curves illustrated three-components: a low temperature component (Component A) near the present-day field (PDF), a secondary remanent magnetization (Component B) that may be from the Cretaceous Period, and a high-temperature component (Component C). The Component C were isolated from the Early-Middle Triassic rocks in 8 sites (47 specimens) and from the Late Triassic rocks in 6 sites (37 specimens). The Component C of the Early-Middle Triassic rocks passed a reversal test (B class, 95% confidence level) and a fold test (99% confidence level), that of the Late Triassic rocks passed a fold test (95% confidence level). The corresponding paleopoles for the Early-Middle and Late Triassic periods of the Lhasa Terrane were at 18.9°N, 208.4°E with A95=3.9° and 19.6°N, 211.8°E with A95=10.7°, respectively. We suggest that the Lhasa Terrane maintained a relative stable latitude (16.5±3.9°S and 18.4±10.7°S) in the southern hemisphere during the Triassic Period before moving northwards and amalgamating with the main body of Eurasia. The Qiangtang and Lhasa terranes, which were located at the mid-low latitudes of the southern hemisphere, might have been isolated between Eurasia and Gondwanaland since the Early Triassic Period. The Meso-Tethys, potentially represented by the Bangong–Nujiang suture zone (BNS) between the Lhasa and Qiangtang terranes, opened up in the Early-Middle Triassic Period and expanded during the entire course of the Triassic Period.

Tectono-magmatic evolution of Late Jurassic to Early Cretaceous granitoids in the west central Lhasa subterrane, Tibet

There is ongoing debate as to the subduction direction of the Bangong–Nujiang Ocean during the Mesozoic (northward, southward or bidirectional subduction). Arc-related intermediate to felsic intrusions could mark the location of the subduction zone and, more importantly, elucidate the dominant geodynamic processes. We report whole rock geochemical and zircon U–Pb and Hf isotopic data for granitoids from the west central Lhasa subterrane (E80° to E86°). All rocks show metaluminous to peraluminous, calc-alkaline signatures, with strong depletion of Nb, Ta and Ti, enrichment of large ion lithophile elements (e.g., Cs, Rb, K), a negative correlation between SiO2 and P2O5, and a positive correlation between Rb and Th. All these features are indicative of I-type arc magmatism. New zircon U–Pb results, together with data from the literature, indicate continuous magmatism from the Late Jurassic to the Early Cretaceous (160 to 130Ma). Zircon U–Pb ages for samples from the northern part of the west central Lhasa subterrane (E80° to E82°30′) yielded formation ages of 165 to 150Ma, whereas ages of 142 to 130Ma were obtained on samples from the south. This suggests flat or low-angle subduction of the Bangong–Nujiang Ocean, consistent with a slight southward decrease in zircon εHf(t) values for Late Jurassic rocks. Considering the crustal shortening, the distance from the Bangong–Nujiang suture zone, and a typical subduction zone melting depth of ~100km, the subduction angle was less than 14° for Late Jurassic magmatism in the central Lhasa interior, consistent with flat or low-angle subduction. Compared with Late Jurassic rocks (main εHf(t) values of −16 to −7), Early Cretaceous rocks (145 to 130Ma) show markedly higher εHf(t) values (mainly −8 to 0), possibly indicating slab roll-back, likely caused by slab foundering or break-off. Combined with previously published works on arc magmatism in the central Lhasa and west part of the southern Qiangtang subterranes, our results support the bidirectional subduction of the Bangong–Nujiang Ocean along the Bangong–Nujiang Suture Zone, and indicates flat or low-angle southward subduction (165 to 145Ma) followed by slab roll-back (145 to 130Ma).

The fate of the Indian lithosphere beneath western Tibet: Upper mantle elastic wave speed structure from a joint teleseismic and regional body wave tomographic study

We investigate the fate of the Indian lithosphere following its descent beneath western Tibet by means of tomographic imaging based on arrival times of body waves from regional and teleseismic sources recorded by a portable network deployed in the region from 2007 to 2011. We use a non-linear iterative algorithm that simultaneously models absolute, regional, and relative teleseismic arrival times to obtain a 3-D velocity structure in a spherical segment that extends from 26°N to 37°N, from 76°E to 89°E, and from the surface to 430km depth. We find that variations in P and S wave speeds in the upper mantle are similar, and identify a number of prominent fast anomalies beneath western Tibet and the adjacent Himalayas. We associate these fast anomalies with the mantle lithosphere of India that is likely colder and hence faster than the ambient mantle. Resolution tests confirm the ability of our dataset to resolve their shapes in the upper 300km, and the lack of significant downward smearing of these features. We interpret the presence of faster material below 300km as being consistent with former Indian lithosphere having reached these depths. There are two main fast anomalies in our model. One resembles a ∼100km wide sub-vertical column located directly beneath the India-Asia plate boundary. The other anomaly is thinner, and has the shape of a dipping slab that spans the north-south width of the Lhasa block. It dips towards the NE, starting near the Indus-Yarlung suture and ending north of the Bangong-Nujiang Suture at depths in excess of 300km. Another finding of our study is the absence of major fast anomalies west of ∼80°E, which our resolution tests show to be significant. Our results do not support the notion of a continuous body of formerly Indian lithosphere being presently underthrust northward, and extending all the way to the northern boundary of the plateau. Rather, shapes of fast anomalies in western Tibet suggest colder material beneath the northernmost Himalaya and the southernmost part of the plateau. The presence of multiple relatively small-scale anomalies, rather than a single large fast body aligned with the India-Asia boundary, rules out subduction of the intact Indian lithosphere. Our preferred scenario for the process that currently removes the mantle lithosphere of India from the collision zone envisages viscous behavior, with gravity-driven instabilities leading to the formation of multiple drips of different size. To reconcile our findings with evidence of past underthrusting of Indian lithosphere beneath Tibet we postulate a geologically recent change in the mode of lithosphere removal. Our scenario includes initial underthrusting, subsequent gravitational destabilization, and removal of the mantle lithosphere of Indian plate. At present the mantle lithosphere of India that continues to underthrust southern Tibet develops a drip-like instability that is seen as a fast columnar feature in our model.

Complex Indian subduction style with slab fragmentation beneath the Eastern Himalayan Syntaxis revealed by teleseismic P-wave tomography

On the eastern margin of the Himalayan orogenic belt, the rapid uplift of the Namche Barwa metamorphic terrane and significant bending of the Yarlung Zangbo suture zone occur. The formation mechanism and dynamics of the Eastern Himalaya Syntaxis (EHS) is still debated. In order to better understand the deep structures beneath the EHS, we deployed 35 broadband seismic stations around the Namche Barwa Mountain. The data were integrated with existing datasets for a 3-D teleseismic P-wave tomography. The results demonstrate complex deep structures and significantly contrasting Indian subduction styles in the eastern Himalaya. In the western region of the EHS, the Indian slab flatly subducts under southern Tibet and might extend to the Bangong-Nujiang Suture. In contrast, a (north)eastward steep subduction occurred in the eastern region of EHS. The contrasting subduction styles result in tearing and fragmentation of the Indian lithosphere between the flat and steep subducting slabs beneath the EHS. Consequently, the hot asthenospheric mantle may rise through the slab window, which might further lead to the rapid uplift of Namche Barwa and the formation of EHS. The lateral variation in subduction/collision mode and slab tearing induced asthenospheric mantle upwelling is similar to that observed in the Hellenide and Anatolide domains of the Tethyan orogen.

Tectonic significance of the Dongqiao ophiolite in the north-central Tibetan plateau: Evidence from zircon dating, petrological, geochemical and Sr–Nd–Hf isotopic characterization

The Dongqiao ophiolite occurs in the central segment of the Bangong–Nujiang suture zone, in north-central Tibet, China. It is still debated on the tectonic setting of the Dongqiao ophiolite despite after more than 30years’ studies. The Dongqiao ophiolite has a complete section of a typical ophiolite, composed of harzburgite, dunite, layered and isotropic gabbros, pillow and massive basalts, as well as radiolarian chert. Whole-rock geochemical analyses show that harzburgite displays a broad U-shaped REE pattern and has a fore-arc affinity, whereas basalts show affinities of E-MORB, OIB and IAB. The basalts were probably formed in different tectonic settings, that is, mid-ocean ridge, oceanic island and island arc. The gabbros and basalts are characterized by positive εNd(t) (+1.6 to +6.7) and εHf(t) (+8.1 to +13.9) values. Zircon U–Pb dating yielded ages of 188±1Ma for the layered gabbro and 181±1Ma for the amphibole gabbro. The new ages and the published age data of the Dingqing and Dong Co ophiolites led us to conclude that the Bangong–Nujiang Ocean existed from the Late Triassic to Early Cretaceous. The new geochemical data also suggested that the Dongqiao ophiolite was a typical SSZ-type ophiolite formed in an initial fore-arc oceanic basin. Fore-arc ophiolites are probably widely distributed along the Bangong–Nujiang suture zone. If so, the Tethys Ocean of the Bangong–Nujiang area probably existed as a fore-arc oceanic basin during the Late Triassic to Early Jurassic.

Significant seismic anisotropy beneath southern Tibet inferred from splitting of direct S-waves

This study presents a total of 12008 shear wave splitting measurements obtained using the reference-station technique applied to direct S-waves from 106 earthquakes recorded at 143 seismic stations of the Hi-CLIMB seismic network. The results reveal significant anisotropy in regions of southern Tibet where null or negligible anisotropy has been hitherto reported from SK(K)S measurements. While the individual fast polarization direction (FPD) at each station are found to be consistent, the splitting time delays (TDs) exhibit deviations particularly at stations located south of the Indus–Tsangpo Suture Zone. The fast polarization directions (FPDs) are oriented (a) NE–SW to E–W to the south of the Indus–Tsangpo Suture Zone (b) NE–SW to ENE–SSW between Bangong–Nujiang Suture Zone and the Indus–Tsangpo Suture Zone (ITSZ) and (c) E–W to the extreme north of the profile. The splitting time delays (δt) vary between 0.45 and 1.3s south of the ITSZ (<30°N latitude), while they range from 0.9 to 1.4s north of it. The overall trends are similar to SKS/SKKS results. However, the differences may be due to the not so near vertical paths of direct S waves which may sample the anisotropy in a different way in comparison to SKS waves, or insufficient number of SKS observations. The significant anisotropy (∼0.8s) observed beneath Himalaya reveals a complex deformation pattern in the region and can be best explained by the combined effects of deformation related to shear at the base of the lithosphere and subduction related flows with possible contributions from the crust. Additional measurements obtained using direct S-waves provide new constraints in regions with complex anisotropy.

Age and structure of the Shyok suture in the Ladakh region of northwestern India: Implications for slip on the Karakoram fault system

A precise age for the collision of the Kohistan-Ladakh block with Eurasia along the Shyok suture zone (SSZ) is one key to understanding the accretionary history of Tibet and the tectonics of Eurasia during the India-Eurasia collision. Knowing the age of the SSZ also allows the suture to be used as a piercing line for calculating total offset along the Karakoram Fault, which effectively represents the SE border of the Tibetan Plateau and has played a major role in plateau evolution. We present a combined structural, geochemical, and geochronologic study of the SSZ as it is exposed in the Nubra region of India to test two competing hypotheses: that the SSZ is of Late Cretaceous or, alternatively, of Eocene age. Coarse-continental strata of the Saltoro Molasse, mapped in this area, contain detrital zircon populations suggestive of derivation from Eurasia despite the fact that the molasse itself is deposited unconformably onto Kohistan-Ladakh rocks, indicating that the molasse is postcollisional. The youngest population of detrital zircons in these rocks (approximately 92 Ma) and a U/Pb zircon date for a dike that cuts basal molasse outcrops (approximately 85 Ma) imply that deposition of the succession began in the Late Cretaceous. This establishes a minimum age for the SSZ and rules out the possibility of an Eocene collision between Kohistan-Ladakh and Eurasia. Our results support correlation of the SSZ with the Bangong suture zone in Tibet, which implies a total offset across the Karakoram Fault of approximately 130–190 km.

Underplating of basaltic magmas and crustal growth in a continental arc: Evidence from Late Mesozoic intermediate–felsic intrusive rocks in southern Qiangtang, central Tibet

Phanerozoic growth of continental crust has widely been considered as an important geological phenomenon and mainly occurs in an arc setting. However, the crustal growth models (mantle-derived basalt underplating or accretion of island or intra-oceanic arc complexes or oceanic plateau) have been disputed. Here we present new zircon LA–ICPMS U–Pb age, whole-rock major and trace element, Sr–Nd and zircon Hf isotopic data for Late Mesozoic intermediate–felsic intrusive rocks in the Rena Co area in southern Qiangtang, central Tibet. LA–ICP–MS zircon U–Pb dating for two granodiorite and three diorite samples and one granodiorite porphyry sample gives ages of ca. 150Ma, ca. 112Ma, respectively, indicating they were generated in the Late Jurassic–Early Cretaceous. All rocks are sub-alkaline in composition and belong to the high-Kcal-alkaline series. The ~150Ma diorites (SiO2=57.9–61.2wt.%) exhibit relatively high MgO (3.13–3.88wt.%) and Cr (52.4–282ppm) contents and Mg# (47–51) values, similar to magnesian diorites. They are geochemically characterized by uniformly low εNd(t) (−5.5 to −5.2), high (87Sr/86Sr)i (0.7071 to 0.7078) and Th/La (0.22–0.32), and variable zircon εHf(t) (−8.7 to +4.8) values. They were probably generated by melting of oceanic sediment diapirs, followed by interaction with the surrounding mantle during the northward subduction of Bangong–Nujiang Oceanic lithosphere. The ~150Ma granodiorites and ~112Ma granodiorite porphyries are characterized by low MgO (<3wt.%) contents and Mg# (<45) values, high Al2O3 (>15%wt.%) and Sr (>400ppm) and low Y (<18ppm) and Yb (<1.9ppm) contents, and high Sr/Y and La/Yb ratios, which are similar to those of typical adakites. The granodiorites have low εNd(t) (−7.6 to −3.7) and zircon εHf(t) (−9.8 to +0.2) and high (87Sr/86Sr)i (0.7069 to 0.7086) values, and were likely produced by partial melting of a thickened and heterogeneous ancient lower continental crust. The relatively depleted isotope compositions [(87Sr/86Sr)i=0.7054–0.7065; εNd(t)=−0.61 to +0.25; zircon εHf(t)=+4.7 to +9.7] of the granodiorite porphyries indicate that they were most probably generated by partial melting of newly underplated and thickened basaltic lower crust. Taking into account ophiolites in the Bangong–Nujiang Suture and Late Mesozoic magmatic rocks in the southern Qiangtang sub-block, we suggest that this area was located in a continental arc setting. Moreover, from the Late Jurassic to Early Cretaceous, the ancient lower crust in the southern Qiangtang sub-block was gradually replaced by mantle-derived juvenile materials. The crustal evolution indicates that, in a continental arc, basaltic magma underplating plays a key role in vertical crustal growth.

Ages and geochemistry of the Renacuo granitoids in the Gaize area, central Tibet: implications for the northward subduction of the Bangong Suture Ocean

Subduction of Bangong Suture Ocean and collision between the Lhasa and Qiangtang blocks were keys to better understand the development of central Tibet. However, the initiation and closure timing, and nature and structure of the Bangong suture are still poorly constrained. Mesozoic intrusions are widely distributed in central Tibet. Petrology, geochronology and geochemistry of the Renacuo pluton and their microgranular enclaves in the Gaize area to the north of the Bangong–Nujiang suture zone, south of the Longmu Co–Shuanghu suture zone, are used to constrain the tectonic history of the Bangong suture. Results of our SHRIMP U–Pb dating of the zircons are as follows: quartz diorite is 139 ± 4 Ma, MSWD = 1.6; the microgranular enclave is 147.3 ± 3.3 Ma, MSWD = 0.45. The evolutionary trend of the major elements, trace elements and rare earth elements of the microgranular enclaves and the host rocks are almost similar, suggesting that the enclaves and the host rocks may have had the same source. They most likely originated from magma mixing between mantle-derived and crust-derived melts. On the basis of our mapping and geochronologic studies, we suggest that the formation of the Renacuo pluton is consistent with previously suggested northward subduction of the Bangong Suture Ocean, which occurred in the Late Jurassic–Early Cretaceous. Copyright © 2015 John Wiley & Sons, Ltd.

Evolution of the Bangong–Nujiang Tethyan ocean: Insights from the geochronology and geochemistry of mafic rocks within ophiolites

The Bangong–Nujiang suture zone (BNSZ) is located in the central Tibetan Plateau, and represents the relict of the Bangong–Nujiang Tethyan Ocean (BNTO). This paper presents zircon U–Pb ages and whole-rock geochemical and Sr–Nd isotope data for the ophiolitic rocks from the Rutog, Dongco, Dongqiao, Amdo, and Dengqen areas (from west to east) in the BNSZ. Zircon U–Pb ages obtained from five gabbros and one leucogabbro from the five areas are 169±2, 167±2, 187±2, 184±2, 177±3, and 164±2Ma, respectively. Mafic rocks (gabbros and basalts) in the BNSZ can be divided into MORB-like and OIB-type, based on geochemical data. The MORB-like rocks are tholeiitic and can be further divided into N- and E-MORB subtypes, marked by depleted and flat REE patterns, respectively. All MORB-like rocks show supra-subduction zone (SSZ) geochemical affinities such as mild enrichment in the large ion lithophile elements and depletion in the high field strength elements, coupled with high and positive whole-rock εNd(t) values (+8.1 to +5.4). The OIB-type rocks, which formed in the Early Cretaceous (~132–108Ma), are exposed in the Dongco, Dongqiao, and Tarenben areas, and they consist mostly of alkali basalts with within-plate geochemical characteristics and positive εNd(t) values (+5.9 to +4.5). On the basis of these new data, combined with information from the literature, we further support that the BNTO was an intra-oceanic arc–backarc basin that existed between the North Lhasa subterrane (NLT) and the South Qiangtang subterrane (SQT) during the Early–Middle Jurassic. This basin may have been active until the Early Cretaceous, when the OIB-type basalts erupted.

Petrology, geochemistry, and geochronology of boninitic dikes from the Kangqiong ophiolite: implications for the Early Cretaceous evolution of Bangong–Nujiang Neo-Tethys Ocean in Tibet

The Kangqiong ophiolite is exposed in the central–western part of the Bangong–Nujiang suture zone (BNSZ) of central Tibet. This study reports new data for boninitic dikes with the aim of reconstructing the geodynamic and petrogenetic evolution of the Kangqiong ophiolite. Ten samples of boninitic dikes that cross-cut the mafic cumulates have very low TiO2 (0.34–0.42%) contents and high MgO (6.65–8.25%) contents. LA-ICP-MS U–Pb analyses of zircon from the boninitic dikes yield an age of 115 Ma. They are characterized by positive εHf(t) values varying from +13.1 to +15.0. Taking into account the geochemical characteristics of the mantle section, the Kangqiong ophiolite should be generated in a fore-arc spreading setting resulting from intra-oceanic subduction. Based on our data and previous studies, we propose that the BNSZ represents the major suture and records the Early Cretaceous intra-oceanic subduction of the Bangong–Nujiang Neo-Tethys Ocean, and the Shiquan River–Yongzhu–Jiali ophiolitic mélange belt represents a back-arc basin. These two belts, together with the northern Lhasa subterrane should, represent an Early Cretaceous intra-oceanic subduction system and back-arc basin in central Tibet that is similar to present-day active intra-oceanic subduction systems in the western Pacific Ocean. The final closure of the Bangong–Nujiang Neo-Tethys Ocean might have taken place later than the Early Cretaceous.

Assembly of the Lhasa and Qiangtang terranes in central Tibet by divergent double subduction

Integration of lithostratigraphic, magmatic, and metamorphic data from the Lhasa–Qiangtang collision zone in central Tibet (including the Bangong suture zone and adjacent regions of the Lhasa and Qiangtang terranes) indicates assembly through divergent double sided subduction. This collision zone is characterized by the absence of Early Cretaceous high-grade metamorphic rocks and the presence of extensive magmatism with enhanced mantle contributions at ca. 120–110Ma. Two Jurassic–Cretaceous magmatic arcs are identified from the Caima–Duobuza–Rongma–Kangqiong–Amdo magmatic belt in the western Qiangtang Terrane and from the Along Tso–Yanhu–Daguo–Baingoin–Daru Tso magmatic belt in the northern Lhasa Terrane. These two magmatic arcs reflect northward and southward subduction of the Bangong Ocean lithosphere, respectively. Available multidisciplinary data reconcile that the Bangong Ocean may have closed during the Late Jurassic–Early Cretaceous (most likely ca. 140–130Ma) through arc–arc “soft” collision rather than continent–continent “hard” collision. Subduction zone retreat associated with convergence beneath the Lhasa Terrane may have driven its rifting and separation from the northern margin of Gondwana leading to its accretion within Asia.

Petrogenesis of diabase from accretionary prism in the southern Qiangtang terrane, central Tibet: Evidence from U–Pb geochronology, petrochemistry and Sr–Nd–Hf–O isotope characteristics

AbstractThe Bangong–Nujiang suture (BNS) between the Lhasa and Qiangtang terranes is an important boundary and its petrogenesis is controversial. Diabase from the accretionary prism in the southern Qiangtang terrane yields a zircon U–Pb age of 181.3 ± 1.4 Ma. All the diabases show tholeiitic basalt compositions, gentle enrichment patters of light rare earth elements (REE), variable enrichment in incompatible element concentrations (e.g. Th and Rb), and no anomaly in high field strength elements (e.g. Nb and Ta), similar to that of enriched mid‐ocean ridge basalt (E‐MORB). They have relatively homogeneous whole rock Nd (εNd(t) = 7.3–9.1) and zircon Hf–O isotopic compositions (εHf(t) = 14.8–16.1, and δ18O = 4.57–6.12‰), possibly indicating melting of the depleted mantle and no significant crustal contamination during the petrogenesis. The element variations suggest that the diabases were formed by plume–ridge interaction at a mid‐ocean ridge within the Bangong–Nujiang ocean.

Late Triassic island-arc–back-arc basin development along the Bangong–Nujiang suture zone (central Tibet): Geological, geochemical and chronological evidence from volcanic rocks

A major debate related to the evolution of the Tibetan Plateau is centered on whether or not an island arc–back-arc basin system occurred along the Bangong–Nujiang suture zone, central Tibet. Here we present new zircon U–Pb geochronology, rare earth elements (REEs) and bulk-rock geochemistry of these magmatic rocks in the Amdo area, the middle Bangong–Nujiang suture zone, central Tibet, to identify significant and new records of Mesozoic tectonomagmatic processes. Zircon U–Pb dating using LA-ICP-MS techniques yields a concordant age with a weighted mean 206Pb/238U age of 228.6±1.6Ma (n=7, MSWD=1.19) for the Quehala basalts, and a mean age of 220.0±2.1 (n=8, MSWD=1.5) for the Amdo pillow lavas. On the normalized REE patterns of zircon, significant Ce enrichment indicates that the magma sources of these magmatic rocks have been subjected to modification of slab-derived fluid. Geochemical features suggest that the Quehala basalts (ca. 228Ma), displaying an island arc tholeiites (IATs) affinity, resulted from partial melting of an relatively enriched mantle wedge in the subduction zone, whereas the Amdo pillow lavas (ca. 220Ma) characterized by both arc-like and N-MORB-like geochemical characteristics occurred as associated back-arc basin basalts (BABBs) at the spreading center of back-arc basin after the formation of island arc tholeiites. In conclusion, the volcanic rocks in the Amdo area have documented the magmatic processes from early-stage subduction to development of associated back-arc basin, confirming the occurrence of intra-oceanic subduction within the Bangong–Nujiang Tethys during the late Triassic. Furthermore, the spatial relationships among the Quehala formation, Tumengela formation and Amdo pillow lavas indicate likely northward subduction of the Bangong–Nujiang Tethyan Ocean during the Late Triassic to middle Jurassic.

The evolution of the Bangong–Nujiang Neo-Tethys ocean: Evidence from zircon U–Pb and Lu–Hf isotopic analyses of Early Cretaceous oceanic islands and ophiolites

We conducted in situ U–Pb analyses of zircons from three basalts and one gabbro from the Zhonggang oceanic island, one basalt from the Zhaga oceanic island, and one gabbro from the Kangqiong ophiolite (all located in the middle segments of the Bangong–Nujiang suture zone of Tibetan Plateau), as well as in situ Hf isotope analyses of zircons from one gabbro from the Zhonggang oceanic island to constrain the tectonic evolution of the Bangong–Nujiang Neo-Tethys ocean. All samples contain numerous inherited zircons, and all the zircons contain magmatic oscillatory zoning and have Th/U ratios exceeding 0.4. Moreover, the average ΣREE content of these zircons is less than 2000ppm, and they display clear negative Eu and variable positive Ce anomalies, indicating a magmatic origin. LA–ICP–MS U–Pb dating of the zircons revealed three clear peaks in the age distribution, at 248–255, 162–168, and117–120Ma; Lu–Hf isotopic analyses of zircons from the gabbro of the Zhonggang oceanic island yielded a 269Ma crust–mantle separation age. Taking into account the regional geology, previous data, and our new analyses, we infer that the middle and western segments of the Bangong–Nujiang Neo-Tethys ocean had initially opened in the late Permian (254–269Ma) and that the ocean opened substantially between the late Permian and the Early Triassic (248–255Ma). In addition, we infer that the initiation of subduction of the Bangong–Nujiang Neo-Tethys ocean took place at ~162–168Ma, which is Middle Jurassic. The117–120 Ma age is the time when the oceanic islands and ophiolites were formed, indicating that the Bangong–Nujiang Neo-Tethys ocean was, to some extent, still open at that time.

Petrology and U–Pb zircon geochronology of bimodal volcanic rocks from the Maierze Group, northern Tibet: Constraints on the timing of closure of the Banggong–Nujiang Ocean

We present new zircon U–Pb dates, major and trace element chemistry, and Hf isotopic compositions for bimodal volcanic rocks of the Maierze Group (MG) in the Maierze area of the Southern Qiangtang–Baoshan block, northern Tibet. We discuss the implications of these data for the evolution of this region. The MG bimodal volcanic rocks consist of basalts and dacites that yield LA-ICP-MS zircon U–Pb ages of 122 and 120Ma, respectively. The MG basalts have light rare earth element (LREE)-enriched chondrite-normalized REE patterns (LaN/YbN=13–14), high Ti/V ratios (45–64), high Zr (190–270ppm) and Nb (22–41ppm) concentrations, and Zr/Y ratios (7–9) that are similar to those of within-plate basalts. The MG basalts also have low MgO and total Fe2O3 (TFe2O3) concentrations, significant enrichments in the LREE and the light ion lithophile elements (LILEs; Rb, Ba, Th, U and Pb), and weak depletions in the high field strength elements (HFSE; Nb, Ta, and Ti), all of which are clearly evident in primitive-mantle-normalized multi-element variation diagrams. The MG dacites are more enriched in the LILE (e.g., Rb, Ba, Th, U, K, and Pb) and more depleted in the HFSE (e.g., Nb, Ta, and Ti) than the MG basalts. Moreover, the dacites have variable zircon εHf(t) values (−6.3 to +6.3). These features indicate that the parental magma for the MG basalts was likely derived from an enriched lithospheric mantle source that was contaminated by subduction-related fluids or melts. In contrast, the MG dacites were derived from mixing of the MG basaltic magma with a second magma derived from partial melting of the continental crust. The geochemical and Hf isotopic characteristics of the MG bimodal volcanic rocks suggest that they formed during the initial stages of development of a back-arc basin. From south to north, the Bangong–Nujiang suture zone, the Duolong gold-rich porphyry copper deposit, and the Maierze bimodal rocks are interpreted to represent a remnant of a complete volcanic arc system that comprises the Bangong–Nujiang Ocean, an island arc, and a back-arc basin, respectively. The fact that this volcanic arc system was still active during the Early Cretaceous indicates that closure of the Bangong–Nujiang Ocean occurred later than the previously suggested period of between the Late Jurassic and Early Cretaceous.

Re-Os isotopic constraints on the evolution of the Bangong-Nujiang Tethyan oceanic mantle, Central Tibet

Geochemical (including Re-Os isotopic) studies of the mantle rocks of ophiolites in the Bangong-Nujiang suture zone in central Tibet have provided a coherent picture of the evolution of the Bangong-Nujiang Tethyan oceanic mantle from mid-ocean ridge (MOR) to subduction-zone (SSZ) settings.Clinopyroxene (cpx)-harzburgites and lherzolites in the Bangong Lake ophiolite were formed in a MOR setting, as demonstrated by the Cr# of spinels (<0.60) and whole-rock LREE-depleted patterns. Suprachondritic 187Re/188Os ratios (up to 1.833) of cpx-harzburgites and their spinels can be explained by interaction with melts derived from high Re/Os sources. Re-depletion (TRD) model ages (0.48-0.55Ga) suggest these rocks may represent a Pan-African domain beneath the Gondwana continent. High TiO2 contents of spinels and whole-rock samples imply that the lherzolites were formed through a refertilization process. Similarly, Re-Os isotopic systematics of sulfides in the lherzolites (187Re/188Os: 0.173-1.717, 187Os/188Os: 0.12646-0.17340) demonstrate that they are mixtures of primary and secondary sulfides. 187Os/188Os ratios (0.1211-0.1226) of whole-rock lherzolites give TRD ages of 0.73-0.97Ga, indicating the presence of Neoproterozoic lithospheric mantle under the spreading ridges.Mantle rocks in the SSZ-type ophiolites from Bangong Lake, Dongqiao and Nagqu reflect the complex evolution of the Bangong-Nujiang oceanic mantle during the SSZ stage. Most harzburgites from the Bangong Lake ophiolite give TRD ages of 1.0-1.5Ga, possibly representing relics of a Mesoproterozoic lithospheric mantle. However, three samples have both high Os contents (1.32-4.45ppb) and near-chondritic 187Os/188Os (0.1260-0.1297), and may represent Mesozoic oceanic lithospheric mantle. 187Os/188Os ratios of dunites and harzburgites from the Dongqiao and Nagqu ophiolites vary from 0.1174 to 0.1316 and give TRD ages up to 1.43Ga, also suggesting the existence of a Mesoproterozoic lithospheric mantle which has experienced mantle refertilization and later metasomatism caused by percolation of several generations of melts and/or fluids.We interpret the old mantle domains in the Bangong-Nujiang ophiolite belt as relics of ancient sub-continental lithospheric mantle (SCLM), which may have survived during the opening of the Bangong-Nujiang Tethyan ocean basin, and subsequently were incorporated into Mesozoic oceanic lithospheric mantle.

Age and nature of the late Early Cretaceous Zhaga Formation, northern Tibet: constraints on when the Bangong–Nujiang Neo-Tethys Ocean closed

This article reports the depositional environment, provenance, and U–Pb zircon age constraints for the newly identified Zhaga Formation in northern Tibet and uses these to better understand the tectonic evolution of the Bangong–Nujiang suture. One transect across the Zhaga Formation was investigated. The Zhaga Formation is ~2 km thick, dominated by greywacke and conglomerate at the base, basalt and limestone in the middle, and greywacke and shale at the top. Greywacke in the Zhaga Formation typically contains 70–75% quartz, 5% feldspar, 3–5% rock debris, and >15% matrix, with normal grading and convolute bedding, basal flow structures, and distinct Bouma sequences interpreted as bathyal to abyssal turbidites. One rhyolite sample and one greywacke sample from the studied transect were collected for zircon U–Pb dating. The rhyolite yields a concordia age of 118 Ma, and the greywacke yields nine age peaks of 247, 330, 459, 541, 611, 941, 1590, 1871, and 2482 Ma, indicating that the Zhaga Formation formed during the late Early Cretaceous and the provenance of its detritus was the Qiangtang area. These data, combined with the Early Cretaceous ocean islands, indicates that the Bangong–Nujiang Neo-Tethys Ocean must have been open during the late Early Cretaceous. We conclude that the Bangong–Nujiang Neo-Tethys Ocean closed after the late Early Cretaceous and not during the Late Jurassic or the early Early Cretaceous as proposed by previous workers.

Variation of Moho Depth across Bangong-Nujiang Suture in Central Tibet—Results from Deep Seismic Reflection Data

There is a long-term dispute at Moho depth across the Bangong-Nujiang suture (BNS). Due to the complicated and changeable seismic geological condition, it is not easy to acquire images of the reflective Moho in central Tibet. In the support of the SinoProbe project, a series of deep seismic reflection profiles were conducted to image Moho structure across the BNS and the Qiangtang terrane. These profiles extend from the northern Lhasa terrane to the Qiangtang terrane crossing the BNS. Both shot gathers and migration data show clear Moho images beneath the BNS. The Moho depth varies from 75.1 km (~24 s TWT) beneath the northmost Lhasa terrane to 68.9 km (~22 s TWT) beneath southmost Qiangtang terrane, and rises smoothly to 62.6 km (~20 s TWT ) at ~28 km north of the BNS beneath the Qiangtang terrane. We speculate that the Moho appears a 6.2 km sharp offset across the BNS and becomes ~12.5 km shallower from the northmost Lhasa terrane to the south Qiangtang terrane at ~28 km north of the BNS. The viewpoint of Moho depth across the BNS based on deep seismic reflection data is inconsistent with the previous 20 km offset.

Late Cretaceous tectonic framework of the Tibetan Plateau

New research, coupled with previous data, reveals the Late Cretaceous paleo-geography, and related paleo-tectonic movement of the Tibetan Plateau. A vast ocean, the Neo-Tethys Ocean, perhaps as wide as ∼7000km, existed between the Indian and Eurasian Continental Plates in the early Late Cretaceous. In addition, a Himalaya Marginal Sea lay along the border of the Indian Plate and other marginal seas were present to the north in both the southern Lhasa and southwestern Tarim Blocks. Northward subduction of the Neo-Tethys Oceanic Plate along the Yalung-Zangbu Suture closed most of the ocean and led to intensive thrusting, tight folding, magmatic plutonism and volcanic eruptions in the central plateau to the north. A magmatic arc up to 500km wide formed across the southern margin of the continental plate in central Tibet and its varying granitic composition appears to reflect the depth to the subducted plate and define its geometry. A series of large, chiefly north-dipping thrust systems also developed across central Tibet. These include thrusts along the Yalung-Zangbu and Bangong-Nujiang Sutures, the North Gangdese and North Lhasa Thrusts in the Lhasa Block, the Qiangtang and North Tangula Thrusts in the Qiangtang block, the Hoh-Xil and Bayan Har Thrusts in the Hoh-Xil Block, as well as the sinistral-slip South Kunlun and Altyn Tagh Faults in northern Tibet. Uplifts formed above the hanging walls of the major thrusts and their eroded debris formed thick red-beds in basins below them. The central Tibetan Plateau maintained a low elevation and coastal vegetation was dominant during the Late Cretaceous.