Qiangtang Block Research Articles

The Mesozoic Closure of the Paleo-Tethys Ocean: Constraints from Zircon U–Pb Ages, Hf and Nd Isotopes of The Granitic Plutons in the Hoh-Xil-Songpan-Ganzi

Dong, J.-H.; Zou, H.; Liu, H.; Li, Y., and Jiang, X.-W., 2019. The Mesozoic closure of the Paleo-Tethys Ocean: Constraints from zircon u–pb ages, hf and Nd isotopes of the granitic plutons in the Hoh-Xil-Songpan-Ganzi. In: Gong, D.; Zhu, H., and Liu, R. (eds.), Selected Topics in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 94, pp. 101–106. Coconut Creek (Florida), ISSN 0749-0208.The Hoh-Xil-Songpan-Ganzi (HXSG) complex is a large triangular fold located in the eastern part of the Tibet Plateau, which recorded of the Closure of Paleo-Tethys Ocean and recorded the tectonic activities of convergence between the three blocks of the Yangtze, North China and Qiangtang during early Mesozoic. This contribution combines geological and Hf-Nd isotopic data for igneous rocks from the HXSG complex, together with the salient features to gain insights into the crustal architecture and tectonic evolution. Two period tectonic-magmatism activities of sys-collision magmatism (ca. 243-227 Ma) and post-collision magmatism (ca. 219-185 Ma) have been recognized in the HXSG complex during Early Mesozoic. The large-scale intermediate-felsic magmatism include high Ba-Sr granite, adakite granitoids, I-type and A-type granitoids, strong aluminous leucogranites. The depth of the source region is different from the mantle, the crust and mantle transition zone, the lower crust and the upper middle crust. As for the closure of the Paleo-Tethys Ocean, the subduction of the Paleo-Tethys oceanic lithosphere leads to the convergence of the North China Block, the Yangtze Block and the Qiangtang Block, the HXSG complex shortened and thickened, the thickened lithosphere was delaminated, then the asthenosphere further upwelled to melting the thicken lower crust, the HXSG complex entered the intraplate extensional tectonic setting, the magmatism involved the partial melting of the lithospheric mantle and even the asthenosphere mantle.

Challenges in isolating primary remanent magnetization from Tethyan carbonate rocks on the Tibetan Plateau: Insight from remagnetized Upper Triassic limestones in the eastern Qiangtang block

Carbonate rocks, widely used for paleomagnetically quantifying the drift history of the Gondwana-derived continental blocks of the Tibetan Plateau and evolution of the Paleo/Meso/Neo-Tethys Oceans, are prone to pervasive remagnetization. Identifying remagnetization is difficult because it is commonly undetectable through the classic paleomagnetic field tests. Here we apply comprehensive paleomagnetic, rock magnetic, and petrographic studies to upper Triassic limestones in the eastern Qiangtang block. Our results reveal that detrital/biogenic magnetite, which may carry the primary natural remanent magnetization (NRM), is rarely preserved in these rocks. In contrast, authigenic magnetite and hematite pseudomorphs after pyrite, and monoclinic pyrrhotite record three episodes of remagnetization. The earliest remagnetization was induced by oxidation of early diagenetic pyrite to magnetite, probably related to the collision between the northeastern Tibetan Plateau and the Qiangtang block after closure of the Paleo-Tethys Ocean in the Late Triassic. The second remagnetization, residing in hematite and minor goethite, which is the further subsurface oxidation product of pyrite/magnetite, is possibly related to the development of the localized Cenozoic basins soon after India-Asia collision in the Paleocene. The youngest remagnetization is a combination of thermoviscous and chemical remanent magnetization carried by authigenic magnetite and pyrrhotite, respectively. Our analyses suggest that a high supply of organic carbon during carbonate deposition, prevailing sulfate reducing conditions during early diagenesis, and widespread orogenic fluid migration related to crustal shortening during later diagenesis, have altered the primary remanence of the shallow-water Tethyan carbonate rocks of the Tibetan Plateau. We emphasize that all paleomagnetic results from these rocks must be carefully examined for remagnetization before being used for paleogeographic reconstructions. Future paleomagnetic investigations of the carbonate rocks in orogenic belts should be accompanied by thorough rock magnetic and petrographic studies to determine the origin of the NRM.

Geochemistry and Nd isotopes of Lower Permian siliciclastic rocks from central Tibet, western China: Evidence for Early Permian initiation of the rifting between the Lhasa and Qiangtang blocks?

Fifteen Lower Permian siliciclastic rocks from central Tibet were analysed for the major and trace elements, and ten samples among them were chosen for determination of Nd isotopic compositions. They are characterized by low concentrations of SiO2 and low ratios of SiO2/Al2O3, moderate abundances of Fe2O3 + MgO (mudstones, 7.28–9.72 wt.%; sandstones, 4.62–10.02 wt.%) and TiO2 (mudstones, 0.55–0.95 wt.%; sandstones, 0.53–0.99 wt.%), significant enrichments of light rare‐earth elements (LREEs) relative to the heavy REEs (HREEs; average (La/Yb)N = 9.8 [mudstones] and 9.48 [sandstones]), flat HREE distribution, obvious negative Eu anomalies (average Eu/Eu* = 0.68), higher Th/Sc and La/Sc ratios, and negative initial εNd values (on average −7.2 and −8.5 for sandstones and mudstones, respectively). The mixing modal analysis based on some elemental pairs and Nd isotopic composition, respectively, Nd depleted mantle model age (TDM) and initial εNd values, uniformly indicates that about 30–40% mafic materials could have been added into these sedimentary rocks. These siliciclastic rocks could have formed in a setting of rifting between the Lhasa and Qiangtang blocks, when a 280 Ma mantle plume upwelling event was occurring in northern Gondwana.

Provenance and palaeogeographic implications of detrital zircons from the lower Carboniferous Riwanchaka Formation of the central Tibetan Plateau

AbstractThe Longmu Co–Shuanghu suture zone, which divides the Qiangtang terrane into the northern and southern Qiangtang blocks, is regarded as a key locality in reconstructing the evolutionary history of the Palaeo-Tethys Ocean and the break-up of Gondwana. However, although low-temperature – high-pressure metamorphic rocks and ophiolites have been documented within the Longmu Co–Shuanghu suture zone, it remains unclear whether it is an in situ suture zone and represents the relic of the main Palaeo-Tethys Ocean. The uncertainty stems mainly from the limited systematic studies of the provenance, palaeontological evidence and depositional settings of strata on either side of the Longmu Co–Shuanghu suture zone (i.e. northern and southern Qiangtang blocks). Here we report new detrital zircon U–Pb ages and palaeontological data from Lower Carboniferous strata (Riwanchaka Formation) of the northern Qiangtang block, central Tibet. The Riwanchaka Formation contains warm-climate biota with Cathaysian affinities. Provenance analysis reveals that the formation has detrital zircon spectra similar to those from strata of the Yangtze Plate, and it contains a large proportion of zircons with ages (~360 Ma) similar to the timing of synsedimentary magmatic arc activity, implying an active continental margin setting associated with northward subduction of the Palaeo-Tethyan oceanic lithosphere. Conversely, the Carboniferous–Permian strata from the southern Qiangtang block contain cool-water faunas of Gondwanan affinity and exhibit minimum zircon crystallization ages that are markedly older than their depositional ages, suggesting a passive continental margin setting. The differences in provenance, palaeontological assemblages and depositional settings of the Carboniferous to Permian strata either side of the Longmu Co–Shuanghu suture zone indicate the existence of an ancient ocean between the northern and southern Qiangtang blocks. Combining the new findings with previous studies on high-pressure metamorphic rocks, arc magmatism and ophiolites, we support the interpretation that the Longmu Co–Shuanghu suture zone is an in situ suture zone that represents the main suture of the Palaeo-Tethys Ocean.

Zircon U–Pb age and geochemical constraints on the origin and tectonic implication of the Tuotuohe Cenozoic alkaline magmatism in Qinghai–Tibet Plateau

A large number of Eocene–Oligocene alkaline/alkali-rich igneous rocks were developed in the Tuotuohe region of the Qinghai-Tibetan Plateau. In this study, we present zircon U–Pb ages, Hf isotope data, and major and trace element compositions of the Cenozoic alkaline rocks from the Tuotuohe region in order to constraint the petrogenesis and tectonic evolution history of Qiangtang Block. Zircon U–Pb ages were measured via LA–ICP–MS to be 39.6, 37.6 and 32.0 Ma. The 39.6 Ma trachyte was characterized by low SiO2 and high K2O and MgO contents. The 37.6 and 32.0 Ma orthophyres show enrichment in SiO2 and K2O, but deficient in MgO. All of the samples belong to the alkaline rocks. These rocks display enrichment in REE, LREE, and LILE, depletion in HFSE, and no obvious Eu anomalies. Geological and geochemical features suggest that there were two possible mechanisms for the origin of the alkaline rocks in the Tuotuohe region: (1) the removed mafic lower crust dropped into the asthenosphere, forming the mix magma (Nariniya trachyte); (2) the upwelling asthenosphere triggered the crustal melting (Nariniya and Zamaqu orthophyre). The Eocene–Oligocene alkaline rocks in the study and adjacent areas are likely to be the result of the same tectonic–magmatic event of deep lithospheric evolution that is the crustal material melting triggered by lithospheric delamination. This conclusion extends the influence scope of lithospheric delamination eastward to the Tuotuohe region (~ 92°E) from Banda Co (~ 82°E).

The northern Qiangtang Block rapid drift during the Triassic Period: Paleomagnetic evidence

As one of the pivotal Gondwana–derived blocks, the kinematic history of the northern Qiangtang Block (in the Tibetan Plateau) remains unclear, mainly because quantitative paleomagnetic data to determine the paleoposition are sparse. Thus, for this study, we collected 226 samples (17 sites) from Triassic sedimentary rocks in the Raggyorcaka and Tuotuohe areas of the northern Qiangtang Block (NQB). Stepwise demagnetization isolated high temperature/field components from the samples. Both Early and Late Triassic datasets passed field tests at a 99% confidence level and were proved to be primary origins. Paleopoles were calculated to be at 24.9°N and 216.5°E with A95 = 8.2°(N = 8) for the Early Triassic dataset, and at 68.1°N, 179.9°E with A95 = 5.6° (N = 37) for the Late Triassic, the latter being combined with a coeval volcanic dataset published previously. These paleopoles correspond to paleolatitudes of 14.3°S±8.2° and 29.9°N±5.6°, respectively. Combining previously published results, we reconstructed a three-stage northward drift process for the NQB. (1) The northern Qiangtang Block was located in the subtropical part of the southern hemisphere until the Early Triassic; (2) thereafter, the block rapidly drifted northward from southern to northern hemispheres during the Triassic; and (3) the block converged with the Eurasian continent in the Late Triassic. The ∼4800 km northward movement from the Early to Late Triassic corresponded to an average motion rate of ∼11.85 cm/yr. The rapid drift of the NQB after the Early Triassic led to a rapid transformation of the Tethys Ocean.

Heterogeneous lithospheric mantle beneath the southeastern Tibetan Plateau: Evidence from Cenozoic high-Mg potassic volcanic rocks in the Jinshajiang–Ailaoshan Cenozoic magmatic belt

Cenozoic (∼40–30 Ma) high-Mg potassic volcanic rocks are abundant in the Jinshajiang–Ailaoshan magmatic belt that stretches from eastern Tibet to western Yunnan. The petrogenesis and compositional variation of the mantle source of these rocks are poorly known. In this paper, we obtained U-Pb zircon ages and whole-rock major and trace element and Sr-Nd-Pb-Hf isotopic data for these volcanic rocks from the Nangqian basin in the Eastern Qiangtang block, eastern Tibet. High-Mg volcanic rocks in Nangqian are potassic trachybasalts, shoshonites and latites and contain 6.49–10.54 wt% MgO, 48.04–56.89 wt% SiO2 and 3.36–4.14 wt% K2O, with K2O/Na2O ratios of 0.94–1.24. Zircon U-Pb dating shows that they are slightly older (40–38 Ma) than their counterparts (37–32 Ma) from the Dali district, western Yunnan. They are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and have significantly negative Nb-Ta-Ti anomalies. Isotopically, they have primitive mantle-like 87Sr/86Sr ratios (0.7050–0.7055) and εNd(t) values of −1.06 to +0.10 and restricted ranges of 206Pb/204Pb (18.63–18.71), 207Pb/204Pb (15.52–15.57) and 208Pb/204Pb ratios (38.42–38.59). Zircon εHf(t) values vary from +4.1 to +9.4 and are thus decoupled from their whole-rock Nd isotope ratios. Previous studies have shown that Cenozoic high-Mg potassic volcanic rocks in the Dali district have more enriched Sr-Nd-Pb-Hf isotopic compositions, higher LILE/HFSE (high field strength elements) and LILE/LREE ratios but less enrichment in LREE than the counterparts from the Nangqian basin. Geochemical differences between high-Mg volcanic rocks from these two locations can be explained by different mantle sources caused by different metasomatic processes. Underneath the Nangqian basin, a juvenile, isotopically “depleted”, and garnet-bearing lithospheric mantle was metasomatized by Paleozoic and Cenozoic subduction, whereas an ancient (probably as old as Neoproterozoic), fertilized, and spinel facies-dominated lithospheric mantle was metasomatized by Neoproterozoic subduction underneath the Dali district.

Early Cretaceous sedimentary evolution of the northern Lhasa terrane and the timing of initial Lhasa-Qiangtang collision

Lower Cretaceous strata in the Baingoin basin of the northern Lhasa terrane record initial collision between the Lhasa and Qiangtang blocks, followed by the early uplift of central Tibet. North-south traverses across the Baingoin basin highlight major differences between the Duba Formation in the north and the quasi-coeval Duoni Formation in the south. The Duba Formation documents upward transition from shallow shelf and deltaic environments to coarse-grained siliciclastic fluvial sedimentation. Abundance of detrital zircons yielding Jurassic-Cretaceous ages with εHf(t) values mainly between −2 and +10, occurrence of chert, Cr-spinel, and pyroxene grains, together with southward paleocurrent directions indicate that the Duba Formation was sourced from the southern Qiangtang terrane and Bangong-Nujiang suture zone to the north. The Duoni Formation in the south was deposited in shelfal to fan-delta and fluvial environments. Abundant volcanic clasts, detrital zircons yielding Cretaceous ages with mainly negative εHf(t) values, and northward paleocurrents indicate an active volcanic source located in the central Lhasa terrane to the south, with minor input from the northern Lhasa terrane. Only the northern part of the Baingoin basin was directly controlled by the Lhasa-Qiangtang collision and may thus be considered a peripheral foreland basin, whereas the southern part was mainly influenced by tectonic processes related to the northward subduction of Neotethyan lithosphere, and may thus be comparable to a retroarc foreland basin. But these sedimentary features and the 139–79 Ma Baingoin plutonic intrusion do not fit well with classical foreland-basin models. Zircon chronostratigraphy constrains the final consumption of Bangong-Nujiang oceanic lithosphere and initial collision between the Lhasa and Qiangtang microcontinents to have taken place by 122 Ma, which has major implications for paleotectonic reconstructions of the Tibetan Plateau.

Inverting Rayleigh surface wave velocities for crustal thickness in eastern Tibet and the western Yangtze craton based on deep learning neural networks

Abstract. Crustal thickness is an important factor affecting lithospheric structure and deep geodynamics. In this paper, a deep learning neural network based on a stacked sparse auto-encoder is proposed for the inversion of crustal thickness in eastern Tibet and the western Yangtze craton. First, with the phase velocity of the Rayleigh surface wave as input and the theoretical crustal thickness as output, 12 deep-sSAE neural networks are constructed, which are trained by 380 000 and tested by 120 000 theoretical models. We then invert the observed phase velocities through these 12 neural networks. According to the test error and misfit of other crustal thickness models, the optimal crustal thickness model is selected as the crustal thickness of the study area. Compared with other ways to detect crustal thickness such as seismic wave reflection and receiver function, we adopt a new way for inversion of earth model parameters, and realize that a deep learning neural network based on data driven with the highly non-linear mapping ability can be widely used by geophysicists, and our result has good agreement with high-resolution crustal thickness models. Compared with other methods, our experimental results based on a deep learning neural network and a new Rayleigh wave phase velocity model reveal some details: there is a northward-dipping Moho gradient zone in the Qiangtang block and a relatively shallow north-west–south-east oriented crust at the Songpan–Ganzi block. Crustal thickness around Xi'an and the Ordos basin is shallow, about 35 km. The change in crustal thickness in the Sichuan–Yunnan block is sharp, where crustal thickness is 60 km north-west and 35 km south-east. We conclude that the deep learning neural network is a promising, efficient, and believable geophysical inversion tool.

Sources and petrogenesis of Late Triassic Zhiduo volcanics in the northeast Tibet: Implications for tectonic evolution of the western Jinsha Paleo-Tethys Ocean

The western Jinsha suture, representing a branch of the Paleo-Tethys, makes the junction between the Songpan-Ganzi Complex and the North Qiangtang Block (NQB). However, the Triassic tectonic evolution of the western Jinsha Paleo-Tethys Ocean is poorly constrained. This paper shows new LA–ICP–MS zircon U-Pb geochronology, major and trace elements, and Sr–Nd–Hf isotopic compositions of the Zhiduo dacitic volcanic rocks in the northern margin of the NQB. LA–ICP–MS zircon U-Pb dating displays that they erupted between 227 ± 2 and 228 ± 1 Ma. These rocks exhibit high silica, medium-K calc-alkalic signature, and low MgO, TiO2, P2O5, Cr, and Ni compositions. They are enriched in light rare earth elements and large-ion lithophile elements with moderate to strong negative Eu anomalies (Eu/Eu* = 0.24–0.56), and depleted in heavy rare earth elements and high field strength elements. They possess relatively low initial 87Sr/86Sr ratios (0.7055–0.7084), positive whole rock εNd(t) (+0.2 to +1.4), and magmatic zircon εHf(t) values (focus on 9.2 ± 2.7) with young Nd and Hf model ages (T2DM (Nd) = 885–983 Ma; T2DM (Hf) = 467–1035 Ma). These features are comparable to those of typical arc volcanic rocks and indicate that they were probably derived from partial melting of a juvenile crust source. The eruption of these volcanic rocks could be attributed to southward subduction of the western Jinsha Paleo-Tethys Ocean. In combination with previous studies, we propose that the subduction of the western Jinsha Paleo-Tethys Ocean initiated at the beginning of the Late Triassic (ca. 228 Ma) and closed at the end of the Late Triassic (ca. 208 Ma). In addition, there is a universal northeastward-younging trend of arc magmas in the northern margin of the NQB, which may indicate a model involving the initial shallow subduction and subsequent slab roll-back.

Petrogenesis of late Early Oligocene trachytes in central Qiangtang Block, Tibetan Plateau: crustal melting during lithospheric delamination?

ABSTRACTLate Early Oligocene mantle-derived potassium (K)-rich lavas occur widely in the Qiangtang Block, central Tibet. However, it remains ambiguous whether partial melting of crust beneath central Tibet occurred in the Oligocene. Here, we report the newly identified trachytes in the Gemuchaka area of central Qiangtang Block. Zircon U-Pb and biotite 40Ar-39Ar analyses suggest that they were erupted at ca. 30 Ma, coeval with other K-rich lavas in the central Qiangtang Block. They have relatively constant and high SiO2 (62.4–62.9 wt.%), Al2O3 (17.2–17.4 wt.%), K2O (6.0–6.1 wt. %), and Na2O (4.6–4.7 wt.%) contents. They are characterized by enrichments of light rare earth elements (LREEs) and large-ion lithophile elements (LILEs), and by depletions of heavy REEs (HREEs) and high field strength elements (HFSEs), with negligible Eu but clear negative Ta-Nb-Ti anomalies. They have low εNd(t) (−5.5 to −4.6) but high (87Sr/86Sr)i (0.7076 to 0.7077) values, with variable zircon εHf(t) (−5.78 to 0.92) and δ18O (6.51 to 8.54) values. Except for obviously higher SiO2 contents, the trachytes are geochemically similar to the K-rich rocks in Gemuchaka area. We suggest that the trachytes were probably generated by remelting of the K-rich lower crust, which was possibly formed by the underplating of enriched mantle-derived K-rich lavas during the delamination of the lower part of thickened lithospheric mantle. After the delamination, the upwelling asthenosphere provided heat for partial melting of the lithospheric mantle and newly-formed lower crust. Our study provides the first petrological evidence for the existence of late Early Oligocene crust-derived rocks in the Qiangtang Block.

Petrogenesis and tectonic implications of Late Cretaceous highly fractionated I-type granites from the Qiangtang block, central Tibet

The tectonic evolution of the Lhasa and Qiangtang collision zone is poorly understood due to a lack of pivotal magmatic records and complex geology in central Tibet. In this study, we present zircon U–Pb ages, whole-rock major and trace elements, and Sr–Nd–Pb isotopic data for the newly discovered Chuburi pluton in the southern Qiangtang block. Zircon U–Pb data reveal that the Chuburi pluton was emplaced during the Late Cretaceous (ca. 73 Ma). Bulk-rock analyses show that this intrusion is characterized by high SiO2, Na2O and K2O, but low MgO, CaO and P2O5. The granites are enriched with large-ion lithophile elements and light rare earth elements, with marked Eu anomalies, and there are few heavy rare earth elements and high field strength elements. These geochemical features indicate that the Chuburi pluton is a highly fractionated I-type granite. The nine granitic samples show high (87Sr/86Sr)i ratios (0.7064–0.7089), negative εNd(t) values (−1.94 to −5.59) and have Nd isotopic model ages ranging from 0.76 to 0.97 Ga. The initial Pb isotopic ratios of the Chuburi pluton is uniform: (206Pb/204Pb)t = 18.774–18.884; (207Pb/204Pb)t = 15.695–15.713; (208Pb/204Pb)t = 39.362–39.613. Based on geochemical characteristics of the Chuburi pluton, and consideration of the region geological setting, we suggest that the parent magma was generated by magmatic mixing of mantle and lower crust, and then undergoing extensive fractional crystallization with removal of K-feldspar, plagioclase, mafic mineral and minor accessory mineral. The crust beneath the central Tibet may have been significantly thickened following the final Lhasa-Qiangtang amalgamation. We argue the generation of the Chuburi pluton can be interpreted as a result of regional lithospheric delamination.

Provenances of Cenozoic sediments in the Jianghan Basin and implications for the formation of the Three Gorges

ABSTRACTThe formation of the Three Gorges (TG), the most important capture point along the Yangtze River, has been debated since the last century. Most of hypotheses on its formation are based on the timing of unconformities detected in the sedimentary basins along the riverbed. These unconformities may mark periods of tectonic activity and enhanced hydrodynamic condition. Herein, we detail the Cenozoic sedimentary provenance of the Jianghan Basin through LA-ICP-MS zircon U-Pb geochronology and heavy mineral analyses. The obtained data suggest that during Palaeocene-Oligocene time, the Jianghan Basin was supplied by sediments from the nearby mountain belts. However, the occurrence of zircons with ages <32 Ma above the unconformity at ~24.6 Ma indicates a provenance from Cenozoic igneous rocks which are exposed only in the Qiangtang Block and the Songpan-Ganzi Orogen. We propose therefore that the TG formed between the late Oligocene and the early Miocene and that the ~24.6 Ma unconformity in the Jianghan Basin represents the best dating of the TG formation.

Geochronological, geochemical, and Sr-Nd-Hf isotopic characteristics of granitoids in eastern Tibet and implications for tectonic correlation with southeastern Asia

Understanding the configuration of continental blocks within the Tibetan orogen prior to Indo-Asian collision is vital to determining their accretionary history. However, complex tectonic overprinting, including the development of the eastern and western Himalayan syntaxes, has hampered the investigation of correlations between tectonic blocks in the Tibetan Plateau with those in southeastern Asia. The North Lancangjiang tectonic belt is known to connect with the Longmu Co–Shuanghu suture to the west and the Changning-Menglian suture to the south, but limited work has been undertaken in eastern Tibet, and no ophiolite suites or high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic belts have been identified within this belt. This paper presents new in situ zircon U-Pb, whole-rock major- and trace-element, and whole-rock Sr-Nd-Hf isotopic data for three granitic plutons (Chaya, Jinba, and Batang) within the North Qiangtang block. These plutons yield ages from ca. 265 Ma (Chaya) to ca. 230 Ma (Batang). The Chaya and Batang granites are chemically similar to I-type granites, whereas the Jinba granite is an S-type granite. Sr-Nd-Hf isotopic data suggest that the Chaya granite formed from magmas generated by the partial melting of subduction-modified mantle wedge material and that the Jinba granite formed from magmas generated by the partial melting of Precambrian metasedimentary rocks mixed with a small amount of juvenile mantle–derived crustal material. The Batang granite formed from magmas generated by the partial melting of mafic lower-crustal material. The convergence of the North Qiangtang block with neighboring blocks was a consequence of the closure of the Paleo-Tethys Ocean, and the plutons in this area record subduction-related or syncollisional magmatism. New data from granites within the North Lancangjiang magmatic belt and the Jinshajiang suture indicate that the North Lancangjiang Paleo-Tethyan slab subducted eastward, whereas the Jinshajiang Paleo-Tethyan slab subducted westward under the North Qiangtang block at ca. 267 Ma. The timing and style of block convergence between North Qiangtang–Simao–Indochina and neighboring blocks are similar to those recorded within central Tibet and southeastern Asia. This, combined with the presence of similar Middle Mesoproterozoic–Lower Paleozoic basement materials and Paleozoic sediments hosting Cathaysian warm-climate fossils, indicates that North Qiangtang, Simao, and Indochina represent a single block.

Protoliths and tectonic implications of the newly discovered Triassic Baqing eclogites, central Tibet: Evidence from geochemistry, Sr[sbnd]Nd isotopes and geochronology

Geochemical, SrNd isotopic and geochronological methods were used to reveal the protoliths and geodynamic implications of the newly discovered Triassic Baqing eclogites, eastern Qiangtang terrane, central Tibet. We assessed the mobility of trace elements, and it turned out that high field strength elements (HFSEs) and rare earth elements (REEs) were immobile in the high-pressure (HP) metamorphic process and reliably employed to discriminate the protoliths of the Baqing eclogites. Whole-rock geochemical data (especially immobile elements) suggested both arc-related (negative NbTa anomalies, high light (L) REEs/heavy (H) REEs ratios) and mid-oceanic ridge basalt (MORB)-related (without negative NbTa anomalies, relatively low REEs contents) characteristics, indicating the contribution of different proportions of subduction material with a backarc basin origin. According to the SrNd isotopic ratios (initial 87Sr/86Sr ratios: 0.7078–0.7086; εNd(t) values: −0.58 to +1.96), the protoliths of the Baqing eclogites originated from mantle which experienced continental crustal contamination in the subduction zone. Compared with nearby subduction-related magmatic rocks from eastern Qiangtang terrane, backarc basin was the most likely tectonic environment for the Baqing eclogite protoliths. The Jinsha Paleo-Tethyan Ocean southward subduction underneath East Qiangtang block (EQB) and rollback led to the formation of this backarc basin between the Late Permian and Early Triassic, and subsequent northward subduction of the backarc basin formed the Baqing eclogites in early Late Triassic (227–221 Ma).

Collisional processes between the Qiangtang Block and the Lhasa Block: Insights from structural analysis of the Bangong–Nujiang Suture Zone, central Tibet

The Bangong–Nujiang Suture Zone (BNSZ) is one of the main suture zones in the Tibetan Plateau and indicates the existence of the Bangong–Nujiang Neo‐Tethys Ocean (BNO). It was formed by the collision between the Qiangtang and Lhasa blocks after the closure of the BNO. However, its evolutionary processes and subduction polarity remain controversial. Research on the BNSZ is of great significance for exploring plate tectonic evolution and ocean–continent connection. The scarcity of the BNSZ structural data is one of the most important reasons for the debate of the BNO tectonic evolution. Based on structural analysis in the field and combined with previous petrology and palaeomagnetic data, this paper has determined that the closure time of the BNO is progressive and scissor‐like, from Middle Jurassic in the east to Early Cretaceous in the middle and late Early Cretaceous to early Late Cretaceous in the west. There was a three‐stage deformation in the BNSZ: (a) N–S‐directed compression induced by the initial collision; (b) WNW–directed transpression related to the stress adjustment; and (c) NE–SW‐directed compression caused by the low‐angle NE‐directed subduction of the Indus–Yarlung Zangbo Ocean. The different structural pattern along the eastern segment of the BNSZ may have resulted from orogenic bending due to the later Himalayan Orocline caused by the India–Eurasia collision in the Cenozoic.

Detrital zircon age spectra of the Yungou Formation and its constrain to the related block affinity, western Yunnan

滇西允沟岩组为一套低绿片岩相的泥质浅变质岩，岩性以云母石英片岩、云母石英千枚岩、云母片岩、细晶灰岩为主，局部夹硅质岩，一直以来被认为是前寒武纪变质基底的重要组成部分。其原岩以碎屑岩为主，含有部分灰岩、白云岩，是一套形成于特提斯洋被动大陆边缘的半深海-深海沉积物，因此，是研究特提斯洋构造演化的重要窗口。但是，对允沟岩组的形成时代至今仍存在较大争议，为此本文选取允沟岩组中的片岩进行了碎屑锆石U-Pb精确定年。分析结果表明，允沟岩组原岩沉积时代为新元古代晚期-寒武纪期间（551~491Ma），主要由新太古代、新元古代及少量古元古代碎屑物质组成。其中，大量太古宙晚期碎屑锆石表明其源区有太古宙基底的存在，而1749Ma这组碎屑锆石可能与哥伦比亚超大陆聚散有关，956Ma和848Ma这两组锆石记录的事件可能是对罗迪尼亚超大陆三阶段裂离事件的前两期的响应。此外，还含有少量泛非运动信息，但泛非运动对其源区的影响极其有限。结合前人研究成果和本文碎屑锆石年龄谱，显示允沟岩组形成于原特提斯洋的被动陆缘，与印度板块、南羌塘地块具有显著的亲缘性。;The Yungou Formation in western Yunnan Province, a suit of metapelites of lower greenschist facies, consists dominantly of mica quartz schist, mica quartz phyllite, mica schist and fine crystalline limestone, with some siliceous rocks. It has always been considered as an important part of the Precambrian metamorphic basement. The protoliths are mainly clastic rocks with some limestone and dolomite, belonging to a set of semi-deep sea sediments formed in the passive continental margin of the Tethys Ocean. So the Yungou Formation witnessed the origin, development and extinction of the Tethys Ocean, and played a great role in understanding the tectonic evolution of the Tethys Ocean. However, its forming age is still under controversial. In this paper, the zircon U-Pb dating of schists from the Yungou Formation, southwestern Yunnan Province, has been carried out. The analysis results show that the depositional age of the protolith of the Yungou Formation is in Late Neoproterozoic-Cambrian (551~491Ma), and mainly composed of Neoarchean, Neoproterozoic and some Paleoproterozoic detrital rocks. The discovery of massive Neoarchean detrital zircons indicates the existence of the Archean basement in the source area. The 1749Ma zircons may be related to the Columbia Supercontinent from assembly to breakup. The zircons of 956Ma and 848Ma may be a response to the former two stages of the three-stage-rifting evolution of the Rodinia Supercontinent. In addition, the dating result also revealed the existence of pan-African tectono-thermal event, but its impact on the source region of the Yungou Formation is extremely limited. Combined with previous study, the U-Pb age spectra of detrital zircons shows that the Yungou Formation developed on the passive continental margin of the Proto-Tethys Ocean and when it deposited this block has the good affinity of the Indian and South Qiangtang blocks.

Complex deformation within the crust and upper mantle beneath SE Tibet revealed by anisotropic Rayleigh wave tomography

Knowledge about Rayleigh-wave phase-velocity azimuthal anisotropy can provide important insight into the dynamic processes and complex deformation mechanisms beneath Southeastern (SE) Tibet. In this study, we imaged the azimuthal anisotropy of the Rayleigh-wave phase-velocity in the period range of 10–60 s beneath SE Tibet. The model provides seismological evidence for the existence of asthenospheric upwelling beneath the Indo-China block (ICB) and east-central Tibet. Stratified anisotropy architectures around the Eastern Himalaya Syntaxis (EHS) are observed, and the upper boundary of the subducting Indian slab at least extended to the center Lhasa block (LSB). Additionally, our models also reveal the complex deformation pattern of different blocks in the crust and upper mantle beneath SE Tibet, and the crustal deformation is much more complicated than that of the upper mantle. The EHS and LSB are deformed in the whole crust, which is affected by the subducting Indian lithosphere. The Qiangtang block, ICB and Songpan-Ganzi block deformation was mainly controlled by the strikes of the faults and the west-east tectonic movement of the plateau materials. The deformation of the Sichuan-Yunnan diamond block is related to mid-lower crustal flow. In the upper mantle, the fast-propagation directions of most of the blocks match the fast-polarization directions of SKS and the absolute plate motion, excluding the ICB and east-central Tibet. Based on the combined results of the isotropic and anisotropic models and previous studies, two dynamic models are proposed to shed new light on the complex deformation and dynamics processes in SE Tibet.

Postcollisional delamination and partial melting of enriched lithospheric mantle: Evidence from Oligocene (ca. 30 Ma) potassium-rich lavas in the Gemuchaka area of the central Qiangtang Block, Tibet

Postcollisional potassium (K)-rich magmatic rocks have important implications for unveiling the deep geodynamic processes active during the evolution of orogenic belts and the surface uplift of plateaus. However, their petrogenesis remains highly controversial. Here we report on zircon U-Pb and mineral Ar-40-Ar-39 age, zircon Hf-O isotope, mineral composition, whole-rock major and trace element composition, and Nd-Sr isotope data for the postcollisional K-rich lavas in the Gemuchaka area of the central Qiangtang Block (central Tibet). Age dating suggests that these lavas were generated in the late Early Oligocene (ca. 30 Ma). Two series of rocks were recognized: silica-undersaturated and -saturated lavas. Both series are geochemically characterized by continuous variation trends. All rocks are enriched in light rare earth elements (LREEs) relative to heavy REEs (HREEs) and depleted in Ta-Nb-Ti, with negligible Sr and Eu and obviously positive Pb anomalies. The rocks have higher Th/ La (0.2-0.8) and Nb/Ta (18.6-19.8) values than asthenospheric mantle-derived rocks but slightly variable values of epsilon(Nd) (t) (-6.0 to -4.6), (Sr-87/Sr-86)(i) (0.7076-0.7083), and zircon epsilon(HF)(t) (-7.2 to +0.8) and delta O-18 (7.1-8.5 parts per thousand). We suggest that the parental magmas of these K-rich lavas were likely derived by variable partial melting of lithospheric mantle metaso-matized by subducting continental sediment-derived melts with the addition of minor asthenospheric components. Element modeling results suggest that the basic end members were likely generated by fractionation processes dominated by olivine and minor clinopyroxene from parental magmas, and some other evolved rocks were generated by crystal (olivine, clinopyroxene, Fe-Ti oxides) fractionation from basic magmas. Combining geological and Cenozoic magmatic rock data in the Qiangtang Block, we propose that the Gemuchaka magmatism was most likely triggered by the delamination of lithospheric mantle thickened by early (Paleocene- Eocene) intracontinental subduction and subsequent asthenospheric upwelling.

Changhsingian (Late Permian) foraminifers from the topmost part of the Xiala Formation in the Tsochen area, central Lhasa Block, Tibet and their geological implications

A foraminiferal fauna consisting of 15 genera and 19 species is documented for the first time from the topmost part of the Xiala Formation in the Aduogabu area in Tsochen County, central Lhasa Block. This foraminiferal fauna is dominated by Colaniella, indicating a broadly Changhsingian age. The presence of Colaniella, Reichelina and absence of the typical tropical fusuline genus Palaeofusulina in the fauna indicate that the Lhasa Block has not merged into equatorial regions during the Changhsingian. More importantly, the widespread Lopingian (Late Permian) marine carbonates with warm-water faunas and the stable platform carbonate sequence through the Guadalupian and Lopingian in the Lhasa Block present an obvious contrast against the contemporaneous sequences in the South Qiangtang Block and Gondwana margin. It suggests that both the Bangong-Nujiang Ocean and the Neotethys Ocean would have opened by the Guadalupian and Lopingian.