Qiangtang Terrane Research Articles

New Late Cretaceous Paleomagnetic Results From the Eastern Qiangtang Terrane: Implications for the Postcollisional Extrusion and Convergence of the Southeastern Tibetan Plateau

AbstractTo refine the postcollisional extrusion and convergence history of the southeastern Tibetan Plateau, we report a quality paleomagnetic data set from Upper Cretaceous redbeds of the eastern Qiangtang Terrane (QT), where is situated at the front edge of eastern Himalayan Syntaxis. The 32 site‐mean directions provide a mean pole at 75.4°N, 176.8°E (A95 = 2.4°) and a paleolatitude of 31.8 ± 2.4°N for the Mangkang area. Comparison with reliable Late Cretaceous paleomagnetic data from the eastern QT and East Asia indicates that the eastern QT experienced a southward extrusion of 940 ± 310 km after the Late Cretaceous. Comparing Late Cretaceous paleolatitudes estimated from the eastern QT and Lhasa terrane (LT) reveals a paleolatitude difference of 2,390 ± 620 km. Such a great latitudinal convergence can be attributed to the fact that some continental terranes originally located between the eastern LT and QT were laterally extruded after the Late Cretaceous.

New Paleomagnetic results from Late Triassic limestone of the Eastern Qiangtang Terrane: Implications for the closure of the Paleo-Jinshajiang Ocean

New Paleomagnetic results from Late Triassic limestone of the Eastern Qiangtang Terrane: Implications for the closure of the Paleo-Jinshajiang Ocean

Petrogenesis of Early Permian within-plate-type gabbros in the Western Qiangtang Terrane, Tibet: partial melting of lithospheric mantle metasomatized by subducted oceanic sediments on the northern margin of the Gondwana

ABSTRACT The extensively distributed Early Permian mafic rocks in the Qiangtang Terrane of the central Tibetan Plateau are the magmatic records of separation of this terrane from the Gondwana. Nevertheless, the origins and mantle source characteristics of these rocks continue to be subjects of debate. This study reports zircon U-Pb and Lu-Hf isotopic and whole-rock geochemical and Sr-Nd-Hf isotopic data on gabbros exposed in the Zougou Region, western Qiangtang Terrane. Our zircon U-Pb isotope age suggests that the Zougou gabbros (ZGs) were formed at ca. 290 Ma, representing newly discovered Early Permian mafic rocks within the Qiangtang Terrane. The ZGs display geochemical affinities with within-plate-basalts given that they are tholeiitic and have high Zr/Y and Ti/V ratios. Compared to previously examined Early Permian mafic rocks in the western Qiangtang Terrane, the ZGs are characterized by their distinct, nonradiogenic Nd (εNd(t) = -0.54 to −2.18) isotopic signatures, which cannot be attributed to crustal contamination or assimilation. This assertion is supported by their uniform whole-rock εNd(t), εHf(t), and zircon εHf(t) values, as well as by magma mixing modelling. In conjunction with regional geology, we propose that the ZGs were derived from an isotopically enriched lithospheric mantle metasomatized by previous subduction events. Furthermore, the positive decoupling of Hf-Nd isotopes (ΔεHf(t) = +2.21 to + 2.72), negative Nb-Ta anomalies, and variable Ba/Th ratios within the ZGs indicate that the metasomatic agents were fluids released from oceanic sediments. Our study provides insights into the compositions of the lithospheric mantle beneath the Qiangtang Terrane during the Permian, which is pivotal to discriminating the role of asthenospheric and lithospheric components in the formation of the contemporaneous mafic rocks in response to Gondwana break-up.

The Crust-Mantle Interaction of the Qiangtang Terrane: New evidence from the Effective Elastic Thickness of the Lithosphere

The crust-mantle interaction in the Qiangtang terrane is significant to study continental rheology and evolution. Its mechanism remains a subject of considerable debate for the reason of lack of sufficient geophysical evidence. The effective elastic thickness (Te) of the lithosphere can provide important constraints on this issue because it is sensitive to the state of mechanical coupling between the crust and the lithospheric mantle. We present new high-resolution Te of the Qiangtang terrane by using the multitaper admittance method with fusion of different window, based on satellite gravity and topographical data. Thus, a detailed study of the lithosphere is conducted, for the first time to utilize the spatial variations of Te and associated parameters, including thermal structures of the lithosphere, uppermost mantle seismic Pn-velocity, and crustal deformation. The results indicate that crust-mantle interaction in the Qiangtang terrane primarily occurs in the middle Qiangtang terrane (87°E ∼ 95°E, 32°N ∼ 34°N), where Te values are lower. In the eastern and western Qiangtang terrane, Te values are higher, implying weaker late-stage modification. Due to Rayleigh-Taylor instability, lithospheric delamination occurred beneath the south Qiangtang terrane. Based on the extent of these low Te values (Te < 50 km), we conclude that the delaminated lithospheric slab sinking into the mantle is ∼400 km in length and elongated in shape. The delamination induces the upwelling of mantle material, upward stress, volcanic activity, extensional faults, and hot springs in the Qiangtang terrane.

A 4000‐km‐Long Late Devonian Continental Arc in Eastern Asia and the Opening of the Palaeo‐Tethys

ABSTRACTThe Palaeozoic–early Mesozoic Palaeo‐Tethyan evolution was crucial for the formation of present‐day eastern Asia. However, the exact timing and mechanism of the opening of the main Palaeo‐Tethys Ocean, represented by the Longmuco‐Shuanghu‐Changning‐Menglian‐Bentong‐Raub Suture, remain debated. Here, we report U–Pb–Hf isotopes and trace elements for Late Devonian detrital zircons from the Batang area, which exhibit arc‐like geochemical signatures, indicating strong continental arc magmatism in the North Qiangtang terrane. These characteristics match well with those of coeval zircons from the Indochina and Hainan terranes, thus defining a 4000‐km‐long Late Devonian continental arc. Supported by available geological and geophysical evidence, our results suggest that the southward subduction of the Jinshajiang‐Ailaoshan‐Song Ma Ocean led to the northward migration of the North Qiangtang‐Indochina‐Hainan continental ribbon and the subsequent opening of a back‐arc basin, which evolved into the main Palaeo‐Tethys Ocean.

Two-stage Mesozoic oceanic subduction and related mantle metasomatism beneath the South Qiangtang terrane with implications for post-collisional magmatism

Subduction-related metasomatism at convergent margins is a crucial process for the lithosphere evolution of the overriding plate. In the central Tibetan Plateau, tectonic transitions and related mantle metasomatism history of the oceanic subduction beneath the South Qiangtang terrane (SQT) remain ambiguous, which impeded understanding the geological evolution of the SQT during the Mesozoic and interpreting the geodynamic process responsible for the Cenozoic post-collisional magmatism. This paper integrated the new and published geochronological and geochemical data from the Jurassic-Cretaceous magmatic rocks in the western part of the SQT to explore the variations of magmatism and processes of mantle modifications. The Jurassic arc magmatism is characterized by the southward younging migration with increasing zircon εHf (t) values and the east–west linear distribution of ca. 155–150 Ma slab-derived adakites. After a magmatic gap at ca. 145–130 Ma, the Early Cretaceous magmatism (130–100 Ma) with distinct juvenile isotopic compositions reinitiated firstly in the south of the Jurassic magmatic belt. In comparison, a younger phase of felsic rocks (120–100 Ma) spatially and geochemically overlaps the northern Jurassic granitoids with ancient crustal sources. These spatial–temporal-geochemical covariations of magmatism could be best explained by the southward transference of oceanic subduction at the earliest Cretaceous that was induced by the Jurassic accretion of an oceanic plateau onto the SQT. Under such subduction regimes, several high-Nb mafic rocks in the northern and southern magmatic belts which were formed by partial melting of the slab-melts-metasomatized lithospheric mantle respectively recorded the Jurassic and Cretaceous mantle metasomatism beneath the SQT, based on their spatial and geochemical relationships with two phases of the adakites. Combined with other geological evidence, these results contribute to elucidating the Mesozoic geodynamic evolution of the oceanic subduction beneath the SQT, which further has implications on the deep dynamic processes responsible for post-collisional magmatism.

Cretaceous-Cenozoic cooling history of central-northern Tibet: Insights from the fission track thermochronology of detrital apatite from sediments of the Tuotuohe Basin

Deciphering the interactions between tectonic and exhumation processes in the Tanggula Mountains (central-northern Tibetan Plateau) can provide insights into the processes of the Tibetan plateau uplift and its geomorphic evolution. In this study, we present new detrital apatite fission track (AFT) data from Cenozoic sediments of the Tuotuohe Basin (northeastern part of the Qiangtang terrane) and its periphery (including the Tanggula Mountains), with the aim to reconstruct the cooling history of the Tanggula Mountains during the Cretaceous and the Cenozoic era. Our results show that the provenance of detrital material evolved in the Tuotuohe Basin and highlight that previously deposited sediments were recycled into the Tuotuohe Basin at ∼ 27.5 Ma. The data further outline that the Tanggula Mountains and the Tuotuohe Basin experienced three major phases of tectonic uplift and exhumation: 122–106, 65–54, and 44–35 Ma. These exhumation-induced cooling phases might be related with three phases of primary tectonic activity, i.e., the collision between the Qiangtang and Lhasa terranes (central part of the Tibetan Plateau) that started during the Early Cretaceous, the collision of the Indian and Eurasian plates in the Early Cenozoic and finally, the “hard collision (the Indian and Eurasian continents)” that occurred during the Early Eocene–Oligocene.

3D Shear‐Wave Velocity Structure of the Crust and Upper Mantle Beneath India, Himalaya and Tibet

AbstractWe perform Rayleigh‐wave group‐velocity dispersion measurements from 14,706 regional‐waveforms at periods of 10–120 s, followed by ray‐based tomography and inversion to obtain 3D‐Vs structure of the crust and upper mantle. The group‐velocity maps have 3–5° lateral resolution, and Vs models have ∼3%–7% average‐Vs uncertainty. The Moho depth is assigned to the bottom of the steepest‐gradient layer with Vs between 4.1 and 4.5 km s−1, and the sedimentary‐layers have Vs ≤ 2.9 km s−1. Indian cratons have high average‐crustal‐Vs of 3.6–3.9 km s−1 and thickness of 40–50 km. The intervening rift‐basins are filled with low‐Vs sedimentary‐rocks. The Himalayan Foreland Basin has along‐arc variation in sedimentary thickness with the thickest layer (8–10 km) beneath the Eastern Ganga Basin. The Indian lithospheric mantle has high‐Vs (&gt;4.4 km s−1), and along with high‐Vs crust attest to a cold, rigid lithosphere. This lithosphere underthrust entire Western Tibet and up to the Qiangtang Terrane in Central‐Eastern Tibet. The top of the underthrusting Indian‐crust is marked by lower‐Vs and thrust‐fault earthquakes. The shallow crust beneath Tibet (0–10 km) has high‐Vs and is mechanically strong; whereas, the mid‐crust (20–40 km) has ∼5%–10% low‐Vs anomalies due to radiogenic/shear heating within the thickened crust. This layer is weak and decouples the deformation of the shallow and deep layers. Low‐Vs upper‐mantle with deeper high‐Vs layer is present beneath the Deccan and Raj‐Mahal Traps, suggesting plume‐volcanism related thermal anomaly and refertilization of the upper mantle. The intra‐cratonic basins with circular geometry, high‐Vs lithosphere and no basement earthquakes, possibly formed by thermal subsidence of isostatically‐balanced cratonic lithosphere.

Final closure of the Paleo-Tethys Ocean: Insights from Triassic granitoids in the central Qiangtang area, northern Tibetan Plateau

The geodynamic evolution during the closure of the Paleo-Tethys Ocean in the Tibetan Plateau remains to be fully understood. The Longmu Co−Shuanghu suture zone in the northern Tibetan Plateau has usually been considered to represent the main ocean basin of the Paleo-Tethys Ocean, so it plays a key role in understanding the evolution of the Paleo-Tethys Ocean. In this study, we focused on the Gacuo and Bensong batholiths on the north and south sides of the Longmu Co−Shuanghu suture zone, respectively. We conducted detailed zircon geochronology and whole-rock geochemical and Sr-Nd isotopic analyses, as well as zircon Hf isotope studies. Zircon U-Pb dating indicates that the Gacuo batholith was formed ca. 223−209 Ma, and the age of the Bensong batholith is ca. 213−203 Ma. The Gacuo batholith is mainly composed of I-type granitoids, which are most likely attributed to partial melting of ancient sedimentary materials of the North Qiangtang terrane with a mixture of ∼0%−30% amounts of mantle-derived components. In contrast, the Bensong batholith has granitoids of A-type affinity, and it was probably generated by partial melting of Mesoproterozoic crust of the South Qiangtang terrane with limited mantle contribution (&amp;lt;5%). Finally, we suggest that the Gacuo batholith was probably generated by the break-off of the oceanic slab beneath the North Qiangtang terrane, while the Bensong batholith was related to a possible lithospheric delamination process of the South Qiangtang terrane after continental collision. Therefore, the Gacuo and Bensong batholiths both developed in a postcollisional tectonic setting, and they recorded the evolutionary process of the subduction and closure of the Paleo-Tethys Ocean during the Late Triassic.

Late Triassic paleolatitude of the Southern Qiangtang terrane, central Tibet: Implications for the closure of the Longmu Co-Shuanghu Paleo-Tethyan Ocean

Studies on the paleogeographic evolution of the Southern Qiangtang terrane, as an integral component of the Cimmerian continent, are essential for understanding the geodynamic evolution of the Tethys domain. Here, we present a paleomagnetic study of Late Triassic volcanics (∼222–206 Ma) from the Southern Qiangtang terrane, with the aim of enhancing our understanding of the evolutionary processes of the Longmu Co-Shuanghu Paleo-Tethyan Ocean. The characteristic remanent magnetizations (ChRMs) carried by magnetite and hematite, which formed during the high-temperature oxidation in the initial cooling phase of the volcanic rocks, are effectively isolated through successive thermal demanetizations. The ChRMs pass the fold test, indicating a primary magnetization. It appears to average out paleosecular variation well enough to yield a reliable paleolatitude estimate. Following tilt correction, the ChRMs yield an overall mean direction of Ds = 185.4°, Is = 53.8°, ks = 94.3, α95 = 3.8°, N = 16, indicating a paleolatitude of 34.3°N and a paleopole located at –22.5°N, 82.9°E with A95 = 4.4°. Comparing the Triassic latitudinal history of the Southern Qiangtang with that of the Northern Qiangtang suggests that the western part of Longmu Co-Shuanghu Paleo-Tethyan Ocean likely closed during the early Late Triassic. Moreover, integration with other geological evidence from the Southern Qiangtang terrane suggests that the Longmu Co-Shuanghu Paleo-Tethyan Ocean closed diachronously from east to west.

Early–Middle Devonian paleomagnetic results from the Zhongba microterrane, Tibetan Plateau: Evidence for its origin from the northern margin of Greater India

Abstract The initial disintegration of Gondwana during the Paleozoic laid the foundation for the formation of the Tibetan Plateau in the Cenozoic. Determining the relative positions of the microterranes in Gondwana during the Paleozoic not only informs the subsequent drift and accretion processes of these microterranes but is also crucial to the paleogeographic reconstruction of Gondwana. However, the lack of paleomagnetic constraints on the Devonian paleogeography of the microterranes in the northern part of Gondwana makes this effort challenging. Here, we report paleomagnetic results for the first time from the Early–Middle Devonian sediments of the Zhongba microterrane. The site-mean direction is declination (Ds) = 310.7°, inclination (Is) = −67.2°, ks = 31.2, α95 = 8.3°, and n = 11, in stratigraphic coordinates. Positive fold and reversal tests, together with rock magnetism results and microscopic observations, strongly suggest that the remanence carriers are of depositional origin. The paleomagnetic results meet the paleomagnetic reliability criteria and therefore can be used for tectonic reconstructions. Our results constrain the paleolatitude of the Zhongba microterrane to be 50.0°S ± 11.7°S in the Early–Middle Devonian. Combined with published detrital zircon ages as well as paleomagnetic results and geological data, our data indicate that the Zhongba microterrane, which had no tectonic affinity with the Lhasa terrane, was part of the northern margin of Greater India during 408–388 Ma and coupled with the South Qiangtang terrane, Tethyan Himalayas, and other terranes to form the continuous northern continental margin of East Gondwana.

First identification of Mid-Miocene north–south trending dikes in the eastern Qiangtang terrane, eastern Tibet: Mantle melting and implications for plateau uplift

The uplift of the Tibetan Plateau during the Miocene is crucial to understanding continental deformation processes and global climatic events. However, the eastern Tibetan Plateau remains poorly investigated. Mantle-derived magmatism provides crucial insights into the deep dynamic processes and surface uplift of the plateau. In this paper we report the first discovery of north–south trending lamprophyre dikes from the Aduo Basin in the eastern Qiangtang terrane, eastern Tibet. Our new age data show that these lamprophyre dikes were generated in Mid-Miocene (15–13 Ma). This new discovery has bridged the Mid-Miocene mantle-derived magmatic gap in the Qiangtang terrane. Trace element and Nd–Sr isotopic data indicate that they were derived by partial melting of enriched subcontinental lithospheric mantle. Our study implies that the eastern Tibetan Plateau had its attained near-maximum elevation by the Mid-Miocene. Combined with previous research results, we propose that the entire plateau almost simultaneously reached its near-maximum elevation by the Mid-Miocene.

Moderate magnitude clockwise rotation of the Yunlong Basin: Implications for synchronous Eocene rotation of the southeastern Tibetan Plateau

The clockwise rotation and southeastward extrusion of the southeastern Tibetan Plateau have played important roles in accommodating the uplift and deformation of the plateau. Numerous paleomagnetic studies have suggested post−late Eocene clockwise rotation of the southeastern Tibetan Plateau along the eastern Himalaya syntaxis, whereas few researchers have addressed the specific Eocene deformation, leading to ambiguous interpretations of the tectonic evolution in the region. Herein, we conducted a paleomagnetic study of the Yunlong Formation in the Yunlong Basin, which is Late Cretaceous to early Paleocene in age. In total, 386 oriented samples were collected. Rock magnetic, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS) analyses revealed detrital carriers, such as hematite and some magnetite. In thermal demagnetization processes, 332 characteristic remanent magnetizations were isolated, which yielded positive reversals and tilt tests, providing a site-mean direction of declination (Ds) = 56.0° ± 2.6°, inclination (Is) = 34.3° ± 3.8°, α95 = 2.7°, k = 91.0, and N = 31 after tilt correction. Magnetostratigraphic analysis was performed, and a depositional age of 79−61 Ma for the section was obtained, which is consistent with the previous paleontological and detrital zircon ages. Compared with the Eurasia reference pole of the period, the data revealed a 45.2° ± 5.1° clockwise rotation of the Yunlong area since 79−61 Ma. The integrated regional paleomagnetic results suggest the occurrence of ∼20° of clockwise rotation of the Lanping-Simao terrane during the Eocene, which is similar (in terms of magnitude and time of occurrence) to that of the Gonjo Basin in the eastern Qiangtang terrane. Integrated with other lines of geologic evidence, we propose a new deformation model in which the entire southeastern Tibetan Plateau experienced ∼20° of rigid clockwise rotation during the Eocene, followed by subsequent oroclinal bending.

Tenfold lower late quaternary throw rates in central Tibet compared to southern Tibet reflect different extensional deformation mechanisms

Present-day tectonic deformation in central Tibet is characterized by a series of ∼NS-trending grabens which accommodate EW extension. Quantifying the geometry and kinematics of these grabens is essential to understand Cenozoic tectonic deformation and Tibetan Plateau evolution. Here, we focus on the Norma Co graben (NCG), i.e., the southern segment of the Shuanghu-Norma Co graben (SH-NCG) system, which is the most prominent graben system within the Qiangtang terrane in central Tibet. We study its tectonic and geomorphologic characteristics to determine the late Quaternary throw rates of the normal faults bounding the graben, based on high-resolution satellite images interpretation, field investigation, and cosmogenic 10Be dating (n = 23). Using terrestrial LiDAR, UAV, and kinematic GPS, we precisely measure vertical offsets (up to 15 m) of ∼90–120 ka-old alluvial surfaces, yielding a throw rate of 0.10(+0.04/-0.03) mm/yr. This rate is ten times lower than those published along other NS-trending grabens in southern Tibet, reflecting different deformation mechanisms, as previously suggested: grabens in eastern Qiangtang formed by rapid eastward block extrusion, those in western Qiangtang formed by distributed extension on numerous scattered normal faults, and those in southern Tibet result from divergent orthogonal thrusting along the curved Himalayan arc.

Petrogenesis of late triassic high-silica granites in the North Qiangtang terrane, central Tibetan Plateau

Although high-silica granites can provide unique insights into the maturity of the continental crust and rare metal enrichment, the origin of high-silica granitic magmatism remains uncertain. In this paper, we present an integrated study of zircon U-Pb geochronology and trace elements, whole-rock geochemistry, and Sr-Nd isotopes for two typical high-silica granites (namely, the Longbao granitic porphyry and the Yushu granite) found in the North Qiangtang terrane, central Tibetan Plateau. Zircon geochronological data indicate that these high-silica granites crystallized at 217–214 Ma. All the samples from the Longbao granitic porphyry and the Yushu granite exhibited high SiO2, low MgO, depletion of Ba, Nb, Sr, P, and Ti, and enrichment of Th and U. They exhibited relatively high (87Sr/86Sr)i ratios of 0.7120–0.7136 and low εNd(t) values of −8.58 ∼ −7.58; together with their old ages according to the two-stage Nd model (1.6–1.7 Ga), these features indicate the involvement of crustal materials. Geochemical and isotopic variation indicated that the high-silica granites studied were mainly produced by the dehydration melting of a muscovite (Ms)-bearing source, and that the Triassic turbidites might be a good candidate for the magma source. Combining this evidence with new regional studies, it can be concluded that partial melting of Triassic turbidites induced by slab roll-back might be the key factor controlling the origin of Late Triassic magmatism in the North Qiangtang terrane.

Remagnetization of magnetite-bearing rocks in the Eastern Qiangtang Terrane, Tibetan Plateau (China): Mechanism and diagnosis

Remagnetization is a common yet notorious phenomenon that interferes with paleogeographic reconstruction. Classical paleomagnetic field tests are helpful in detecting remagnetization but their diagnostic power is limited: remagnetization may occur before folding, the tilting age may be ambiguous, or protracted remagnetization may yield dual polarities. Rock magnetic information can provide other constraints on our understanding of the origin of natural remanent magnetization (NRM). Here we focus on the rock magnetic properties of acknowledged remagnetized limestones and unremagnetized rocks of the Zaduo area in the Eastern Qiangtang Terrane, Tibetan Plateau (China). Chemical remanent magnetization is suggested as a more frequent mechanism than the thermoviscous resetting of the NRM. The secondary NRM resides in authigenic magnetite of stable single domain and superparamagnetic (SP) size which grew during post-depositional burial processes. Both high-field and low-field thermomagnetic runs reveal the alteration of existing iron sulfides to magnetite in the remagnetized limestones. NRM decay curves show that the maximum unblocking temperature of the remagnetized samples is significantly lower than that of the unremagnetized samples. Component analysis of acquisition curves of the isothermal remanent magnetization (IRM) reveals a hard component that represents SP magnetite in remagnetized limestones. This component is absent in unremagnetized rocks. End-member modelling reveals a convex curve in the coefficient of determination versus the number of end-members plot for the unremagnetized limestones, whereas the remagnetized rocks exhibit both near-linear and convex shapes. In addition, quantitative analysis of the hysteresis loop shape for different lithologies indicates its validity in detecting remagnetization. Furthermore, we show the differences in the hysteresis data distributions of the two rock types on the Day plot, the Néel diagram, the Borradaile diagram, and the Fabian diagram. Our research emphasizes that rock magnetic properties can serve as tools to diagnose remagnetization in magnetite-dominated rocks. We recommend a comprehensive rock magnetic study to discriminate remagnetization, involving the Day plot, Fabian diagram, thermal demagnetization curves, IRM component analysis and end member modelling.

Petrogenesis and tectonic implications of late Permian andesites from Rongzharinian, northern Qiangtang, Tibet

At convergent margins, high-magnesian andesitic rocks (HMAs) can offer vital information about the formation of the continental crust and the interactions between the deep crust and mantle. This study focuses on new identified HMAs from the Rongzharinian area, northern Qiangtang Terrane (NQT). We report the major, trace elemental, Sr-Nd-Hf isotopic data, and zircon U-Pb age of volcanic rocks with the aim of understanding their petrogenesis and tectonic setting. A concordant age of 257.7 ± 1.5 Ma for the Rongzharinian high-Mg andesites is determined by zircon U-Pb dating using LA-ICP-MS techniques. The Rongzharinian HMAs have high MgO contents (5.88–7.39 wt%), Mg# values (63–68), and Cr (143–185 ppm) and Ni (48–66 ppm) concentrations, close to primitive HMAs. They underwent variable degrees of fractionation of amphibole and minor clinopyroxene and negligible crustal contamination during magma evolution. REE modeling results show that the Rongzharinian HMAs can be generated by 4 %–7% melting of hydrous spinel lherzolite at relatively shallow depths. Geochemical data indicate that the Rongzharinian HMAs were dominantly derived from the mixed mantle wedge source composed of N-MORB-like and E-MORB-like components. Based on the regional tectonic evolution, we propose that the Late Permian rollback of the Paleo-Tethyan slab may induce upwelling of the asthenosphere mantle, resulting in melting of the mantle source and the generation of the Rongzharinian high-Mg andesites.

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.

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.

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.