Central Qiangtang Metamorphic Belt Research Articles

Metabasic rocks as important nitrogen carriers to forearc depths: Implications for deep nitrogen cycling

Understanding deep nitrogen (N) cycling better requires investigating the delivery of N to subduction systems via various lithologies. Input to subduction zones through mafic rocks is more voluminous and massive as compared to sedimentary rocks which calls for a thorough investigation of the behavior of N in metabasic rocks. Here we estimate the delivery of N to subduction zones at forearc depths by investigating the geochemistry of amphibolites and epidote-blueschists from the Central Qiangtang Metamorphic Belt in Tibet where the metabasic rocks likely represent the transition from oceanic to continental subduction. The rocks contain 21–147 ppm N with δ15N values from +1.8 ‰ to +10.0 ‰, and 147 ppm N is the highest that has been reported in a metabasic rock thus far. Given the N abundances for most of the rocks are much higher than those of altered oceanic crust (i.e. basalts, sheeted dikes and gabbros; 6.0 ± 4.7 ppm), the N is likely neither magmatic nor was introduced in the rocks during hydrothermal alteration prior to subduction. This is confirmed by the K2O/Th versus Ba/Th, Th/U versus Th and Ba/Rb versus K2O plots where these rocks align with the trend of metamorphic fluid alteration rather than seafloor hydrothermal alteration. A two-step process led to N acquisition in the metabasic rocks. In the first step, the metabasic rocks acquired their N from metasediment-derived fluids during metamorphism in the subduction channel. In the second step, some of the metabasic rocks underwent N loss and concomitant enrichment in 15N due to devolatilization within the subduction channel. We modeled the N fluxes at forearc depths in 55 modern-day subduction zones via metasedimentary and metabasic rocks assuming their minimum, median and maximum N concentrations to assess their relative importance in delivery of N to subduction zones. We find that metabasic rocks supply comparable fluxes of N to metasedimentary rocks at forearc depths, even though metasedimentary rocks have at least an order of magnitude higher N abundance than metabasic rocks. The importance of metabasic rocks in N delivery reinforces the need to investigate the behavior of N in them from more locations globally to improve our understanding of deep N cycling.

The central Qiangtang Metamorphic Belt in northern Tibet is an in-situ Paleo-Tethys Ocean: Evidence from newly discovered Late Devonian radiolarians

The paleogeographic position of the North Qiangtang Block, as well as the origin of the Central Qiangtang Metamorphic Belt (CQMB) have subjected to considerable debate that hampers the understanding of the early evolution of the Paleo-Tethys Ocean. This study reports a new radiolarian fauna of a Famennian age (Late Devonian) from the ophiolitic mélange south of Gangtang Co, northern Tibet, including Callela parvispinosa Won, Entactinia foveolata Nazarov, and Plenoentactinia pinguis Won. The discovery of Devonian radiolarians in the CQMB strongly supports the model that the Longmu Co–Shuanghu suture zone represents the main branch of the Paleo-Tethys Ocean. A correlation of the Late Devonian radiolarian in Tethys realm reveals that the Longmu Co–Shuanghu suture zone was connected to the Changning–Menglian suture zone in western Yunnan, the Chiang Mai–Inthanon and Chanthaburi suture zones in Thailand, and the Bentong–Raub suture zone in Malay Peninsula. The synchronous advent of Late Devonian radiolarians suggests that the Paleo-Tethys Ocean may have opened during that time.

Triassic record of Paleo-Tethyan subduction: Evidence from prograde metamorphic P–T paths of the Baqing eclogite, eastern Qiangtang, central Tibet

ABSTRACT The prograde metamorphic P–T conditions of eclogites correspond to the subduction processes of oceanic plates. The conditions may be recorded in mineral inclusions and the compositional zoning of garnets, and they play significant roles in revealing the dehydration depth, geothermal structure of the subduction plate and in understanding arc magmatism. The P–T conditions of the Baqing eclogite have been determined previously using traditional geothermobarometers and Ti-in-zircon geothermometers. To obtain more precise prograde metamorphic P–T conditions and details of the Paleo-Tethyan subduction process, we carried out phase equilibrium modeling for a newly collected Baqing eclogite sample. The sample is characterized by well-developed prograde zonings in metamorphic garnets with increasing Mg and decreasing Mn contents from cores to rims, a peak metamorphic mineral assemblage of garnet + omphacite + phengite + rutile + quartz, and a retrograde metamorphic mineral assemblage of amphibole + plagioclase that replaced omphacite. We calculated the peak metamorphic conditions of the Baqing eclogite using the Garnet–Omphacite–Phengite–Quartz thermobarometers to be 26.5 ± 1 kbar and 730 ± 60°C, consistent with the results (23.5–27.5 kbar, 730–750°C) calculated using a P–T pseudosection. The prograde metamorphic P–T conditions were constrained to be ~21.4 kbar and ~580°C using pyrope and grossular isopleths. These results are consistent with previous calculations for the Baqing eclogite. Eclogites of the Central Qiangtang metamorphic belt, the Baqing metamorphic complex, and Changning–Menglian metamorphic belt show parallel P–T paths with different geothermal gradients, that relate to their different protoliths but similar heating and burial processes during Triassic subduction of the Shuanghu Paleo-Tethyan oceanic plate.

Lithospheric extension of the accretionary wedge: An example from the Lanling high-pressure metamorphic terrane in Central Qiangtang, Tibet

Deciphering the exhumation mechanism of high-pressure−low-temperature (HP-LT) metamorphic rocks can provide important insights into the tectonic evolution of oceanic subduction zones at active continental margins. Here, we present a multidisciplinary study examining the exhumation tectonics of the Permo−Triassic eclogite-bearing HP-LT terranes of the Central Qiangtang metamorphic belt (CQMB) in the central Tibetan Plateau. Field geological relations and microscopic observations show that the HP-LT rocks in the Lanling area are separated from the epimetamorphic late Paleozoic ophiolite mélange of the hanging wall by low-angle detachment faults and primarily exhibit five stages of deformation. The HP-LT terranes were exhumed as a metamorphic core complex and show pervasive synexhumation top-to-the-SW and -S shear structures. The results of the petrological and mineralogical analysis and pseudosection modeling of retrograde eclogites indicate that these rocks are characterized by synexhumation mineral growth pulses with hairpin-type decompressional pressure-temperature trajectories. A compilation of previous geochronological data and our 40Ar/39Ar dating results of synexhumation minerals and shear bands in HP-LT rocks indicate continuous exhumation at ca. 244−215 Ma prior to the continental collision between the North Qiangtang Block (NQB) and South Qiangtang Block. Moreover, the CQMB is likely an autochthonous accretionary wedge resulting from northward subduction of the Paleo-Tethys Ocean beneath the NQB. Combined with ca. 243−230 Ma mantle upwelling to the north of Lanling, we infer that the CQMB, namely, the accretionary wedge of the Paleo-Tethys Ocean, experienced Middle−Late Triassic lithospheric extension in the NW-SE direction.

Permian foraminifers from the exotic limestone blocks within the central Qiangtang Metamorphic Belt, Tibet and their geological implications

• The age of the exotic limestone blocks within the Central Qiangtang Metamorphic Belt is late Kungurian to Roadian. • The exotic limestone blocks came from the Shuanghu type of Permian sequences in the South Qiangtang Block. • A strong subduction and accretion process happened during the closure of the Longmu Co-Shuanghu Paleotethys Ocean. The Central Qiangtang Metamorphic Belt (CQMB) is a subject of long-standing controversy for decades with respect to its origin and tectonic evolution. In this study, two foraminiferal faunas are reported from the exotic limestone blocks in the Jiaomuchaka and Jiangaizangbu sections within the CQMB. In total, 9 species of fusulines and 32 species of smaller foraminifers have been discovered in the Jiaomuchaka section, and 10 species of fusulines and 15 species of smaller foraminifers have been documented in the Jiangaizangbu section. In terms of the presence of the fusulines Pseudodoliolina ozawai , Presumatrina neoschwagerinoides , Neoschwagerina simplex and Afghanella sp., an age of late Kungurian to Roadian can be assigned to the faunas. A comparative study on the stratigraphic sequence and foraminiferal faunas between the exotic limestone blocks and contemporaneous deposits in the adjacent areas reveals that the exotic limestone blocks resemble strata of the Lugu Formation in the Shuanghu and Minzhuochaka areas in the South Qiangtang Block. The presence of these exotic limestone blocks within the CQMB suggests that the in-situ deposits of the Lugu Formation have been partly subducted beneath the North Qiangtang Block during the closing of the Paleotethys Ocean along the Longmu Co-Shuanghu Suture Zone.

In-situ central Qiangtang metamorphic belt in western Tibet as a typical suture zone: Evidence of crust-mantle structural footprints from P-wave receiver function analyses

Whether the central metamorphic belt of the Qiangtang terrane in the Qinghai-Tibet Plateau should be attributed to a suture zone, is still heavily debated, due to inadequate deep structure constraints. Using the data collected from 16 stations in the northern part of the Hi-Climb profile and 16 broadband seismic stations recently arranged in the northern part of Gaize County, this study developed a broadband seismic profile running through the in-situ Central Qiangtang Metamorphic Belt (CQMB) roughly along 84.5°E. Significantly characteristics of the crust and mantle structure beneath the profile were obtained through P-wave receiver function analyses: (i) The south Qiangtang terrane has a complex Moho structure at depth 56–82 km, while the north Qiangtang terrane has a nearly horizontal Moho at depth approximately 62 km. Moreover, the northward-inclining Moho is noticeably dislocated with a maximum offset of approximately 18 km under the CQMB. (ii) The crustal low-velocity zones on both sides of the CQMB are transversely discontinuous and are also distinct from each other. (iii) A northward-subducting interface occurs beneath the in-situ CQMB (34.0°N–34.5° N). Based on the geological outcrops and their petrological analysis, the structural characteristics beneath the CQMB are interpreted as the traces of the Longmucuo-Shuanghu suture zone, which was formed by the northward subduction-collision of the south Qiangtang terrane left after the closure of the Paleo-Tethys Ocean at the end of the Triassic.

Provenance evolution during passive- to active-margin transition unraveled from an accretionary complex from the Bangong-Nujiang suture zone: Insights into Early Mesozoic Meso-Tethys subduction and source-area tectonics

The South Qiangtang terrane (SQT), the Tibetan representative of the Cimmerian continent, is bounded in the south by the Bangong-Nujiang suture zone (BNSZ). The BNSZ is geologically manifested by the Mugagangri Group (MG) accretionary complexes developed during northward Meso-Tethyan subduction, whose timing and processes are, however, contentious. Herein, we conducted provenance analysis on the MG trench deposits, via petrography, whole-rock geochemistry and detrital-zircon geochronology. The results reveal a complete picture of provenance evolution during passive- to active-margin transition (from subduction initiation through fringing oceanic-arc, to continental-arc developments) at the SQT margin. In particular, the earliest trench deposits (subunit Trma; ~222 Ma), which contain olistostromes with blocks of oolitic limestone shed from the destructing SQT passive margin, mark the subduction initiation. A second subunit (Trmb; ~210 Ma) has periodically elevated volcanogenic components. Conglomerates of subunit Trmb have abundant basaltic clasts of intra-oceanic arc geochemical affinities, which were likely derived from an intra-oceanic arc fringing the SQT. The youngest subunit (Jmd; ~190 Ma) exhibits enriched ferromagnesian elements and a major youngest detrital-zircon age peak, indicating a continental-arc provenance. Moreover, detrital-zircon age spectra suggest that, at ~210 Ma, the central Qiangtang metamorphic belt became an important source area, and the North Qiangtang terrane (NQT) replaced SQT as the main provenance. This is best explained by that the SQT underthrusted beneath the NQT and was downwarped to form a foreland basin. Our findings confirm that the initial subduction of Meso-Tethys beneath SQT was coeval to other Cimmerian terranes, likely a uniform response to the Paleo-Tethys closure and collision between Eurasia (NQT) and Cimmeria (SQT).

A late Permian–Triassic trench‐slope basin in the Central Qiangtang metamorphic belt, Northern Tibet: Stratigraphy, sedimentology, syndepositional deformation and tectonic implications

AbstractRecords of sedimentation and deformation in trench‐slope basins contain valuable tectonic information about the associated oceanic subduction zone. Here, we present a multidisciplinary study on newly discovered late Permian–Triassic sedimentary successions in the >500‐km‐long Central Qiangtang metamorphic belt (CQMB) to better understand the type of basin and the concomitant tectonism. The Mayer Kangri succession contains lithofacies associations of submarine fan siliciclastic rocks, slope‐environment limestone, deep marine chert and minor olistostromes from the forearc basin. The conodont assemblages and sandstone and andesite interlayers yield continuous stratigraphic ages from the Lopingian to middle Norian. The clastic sediments had two provenances, including the epicontinental arc in the North Qiangtang block (NQB) and synchronous volcanism in the accretionary wedge. Moreover, a large suite of the Anisian–early Carnian radiolarian cherts (>30 m thick) was discovered in the Lanling area. Regionally, the CQMB shows evident spatiotemporal variations in late Permian–Triassic sedimentation, with a general depositional trend of southward deepening and getting younger. The three identified subzones include a bathyal setting, a carbonate platform setting and a deep marine setting from north to south. These observations indicate that the late Permian–Triassic sedimentary successions in the CQMB were deposited in a trench‐slope basin environment during the northward subduction of the Longmu Co–Shuanghu Tethys Ocean beneath the NQB. Generally, the CQMB and the concomitant trench‐slope basin is among the well‐preserved ancient analogs characterized by extensional tectonism. The syndepositional horst‐graben‐like structure, forearc basin‐derived olistostromes, abyssal radiolarian cherts and synchronous volcanism provide new implications for the geological evolution of the trench‐slope basin.

Petrology and P–T path of blueschists from central Qiangtang, Tibet: Implications for the East Paleo-Tethyan evolution

The Paleo-Tethys Ocean records the initial geodynamic processes related to the dispersion of Gondwana and the subsequent accretion of Laurasia. However, whether the blueschist-bearing central Qiangtang metamorphic belt (CQMB) within the Qiangtang Block represents an in situ Paleo-Tethys suture remains controversial. Since the starting point of this debate is the subduction type (i.e., warm subduction vs. cold subduction) recorded by the associated central Qiangtang blueschists, here, we approach this debate by investigating their petrogenesis, which records the subduction and exhumation of an oceanic plate. We reconstruct the clockwise pressure–temperature (P–T) paths using phase equilibrium modeling for two types of blueschists from the Rongma and Gangma Co areas, and their peak P–T conditions reveal typical cold subduction with a uniform low geothermal gradient of 5–10 °C/km. Combining this information with published radiometric dating and bulk-rock geochemical data, we find that the central Qiangtang blueschists record almost all of the early Permian-Late Triassic subduction of the LSPTO and subsequent continental subduction, as well as the Late Triassic exhumation of oceanic and continental material. This study confirms that the central Qiangtang blueschist-bearing mélange marks an in situ suture, which formed during the northward subduction and subsequent exhumation of the LSPTO.

Crystal preferred orientations, deformation mechanisms and seismic properties of high pressure metamorphic rocks from the Central Qiangtang metamorphic belt, Tibetan Plateau

Crystal preferred orientation (CPO) of high pressure metamorphic rocks is an important contributor to seismic anisotropy in the subduction interface and continental crust. However, the mechanisms of deformation and CPO development, as well as the effect of CPO patterns on the seismic anisotropies remain not fully solved. By analyzing the microstructures, CPOs and seismic properties of the high pressure rocks from the Central Qiangtang metamorphic belt in the northern Tibetan Plateau, we found that the mineral CPOs (e.g., omphacite and amphibole) are developed dominantly by non-dislocation based deformation mechanisms in the eclogites and amphibolites, while quartz and phengite CPOs are primarily formed by dislocation creep in the garnet-quartz-mica schists. The variations of CPO patterns are mainly controlled by the strain geometry and/or strain magnitude, which also regulate the first-order symmetry patterns of seismic anisotropies among samples. Besides, amphibole enrichment can remarkably increase the anisotropies and decrease the velocities of seismic waves in the eclogite‒amphibolite suite. A combination of seismic parameters such as Vp, Vs, Vp/Vs ratio, intensities and symmetries of P-wave anisotropy (AVp) and shear wave splitting (AVs), and P- or S-wave reflection coefficients can be used to discriminate eclogites, amphibolites and their garnet-quartz-mica schists country rocks at depth when high resolution seismic methods can be applied. Notably, amphibolite and garnet-quartz-mica schists showing moderate-to-large AVp and max. AVs can make considerable contributions to field-observed crustal anisotropy near the strike-slip continental shear zones or mantle wedge anisotropy in the subduction zones.

Electrical Conductivity of Eclogite, Amphibolite and Garnet-Quartz-Mica Schist with Implications for the Conductivity in the Qiangtang Terrane of Northern Tibetan Plateau

Understanding the electrical conductivity of high pressure metamorphic rocks is essential to constrain the compositions in the subduction zone and continental crust. In this study, we calculated the electrical conductivity for such rocks sampled from the central Qiangtang metamorphic belt in the northern Tibetan Plateau. The results reveal that, when aqueous fluids are absent, the conductivity of meta-mafic rocks (e.g., eclogite and amphibolite) is strikingly higher than that of meta-felsic rocks (e.g., garnet-quartz-mica schist). The conductivity of eclogite decreases due to the enrichment of amphibole, but this effect is diminished when a critical degree of amphibolization is reached. Our calculated conductivity of eclogite and amphibolite differs greatly from the experimentally derived results for the eclogites from other localities, partly owing to the strong effects of different mineral assemblages and chemical compositions on the conduction mechanisms and efficiencies. However, the disparity of conductivity between our calculated and the previously measured results for a similar amphibole-rich eclogite sampled from the same locality suggests that trails of highly conductive rutile-ilmenite aggregates may contribute to the higher bulk-rock conductivity in the laboratory measurements. Moreover, since the calculated conductivity of eclogite and amphibolite is not high enough at the temperatures relevant to their metamorphic thermal condition, partial melts or aqueous fluids originated from the upwelling asthenosphere are more likely to explain the anomalously high electrical conductivity zones in magnetotelluric images in the Qiangtang terrane in the northern Tibetan Plateau.

Sedimentary Evolution and Provenance of the late Permian‐middle Triassic Raggyorcaka Deposits in North Qiangtang (Tibet, Western China): Evidence for a Forearc Basin of the Longmu Co‐Shuanghu Tethys Ocean

AbstractThe tectonic origin of the >500‐km‐long E‐W trending Central Qiangtang metamorphic belt (CQMB), which separates the North Qiangtang block (NQB) and South Qiangtang block (SQB), remains controversial. Moreover, the coeval geological evolution of the southern NQB has been poorly investigated, particularly its tectonic relationship with the CQMB. Here we present stratigraphic, sedimentary, and provenance analyses of the late Permian‐middle Triassic depositional succession at Raggyorcaka in the southern NQB and test two radically different hypotheses for origin of the CQMB. A complete marine transgression‐regression sequence with two‐sided provenance characterizes the late Permian‐Triassic sedimentary rocks in the southern NQB. Sandstone petrological analyses reveal a prominent provenance transition to an active volcanic source beginning in the late Changhsingian. Detrital zircon U‐Pb geochronological results of the transgression subsequence show a concentrated youngest zircon group of 236–288 Ma (peak at ~248.1 Ma), with negative εHf(t) values (−25.3 to −0.2) and large Hf crustal model ages (T ; 1,311–2,887 Ma). These new findings show that the Raggyorcaka sequence was most likely deposited in an active continental margin. Combined with other evidence, we further infer that the Carboniferous‐Triassic successions of the southern NQB were most likely deposited in a forearc basin under the in situ suture model, that is, the northward subduction of the Longmu Co‐Shuanghu Tethys Ocean beneath the NQB. Moreover, the detrital zircon age distribution of the southern NQB suggests that the NQB probably drifted from the Gondwana supercontinent in the early Paleozoic and became adjacent to peri‐Cathaysian blocks no later than the Carboniferous.

Survived Seamount Reveals an in situ Origin for the Central Qiangtang Metamorphic Belt in the Tibetan Plateau

The origin of the central Qiangtang metamorphic belt (CQMB) has long been in debate, which is not clear whether this belt is the exhumed Jinsha oceanic plate that had been subducted and underthrusted beneath the Qiangtang Block, or the in situ Longmu Co-Shuanghu suture that separated the south and north Qiangtang blocks. Here we report field observations, zircon U-Pb ages and Lu-Hf isotopes, as well as whole rock geochemistry and Sr-Nd isotopes of the Late Triassic volcanic rocks near the Chabo Co within the southern margin of the CQMB. The ca. 229 Ma Chabo Co volcanic rocks and limestones possess characteristic lithologies of a seamount. Their geochemical and isotopic compositions are similar to OIB-type lavas. Unlike other metabasalts (eclogites and blueschists) in the CQMB, the Chabo Co volcanic rocks are OIB-type lavas that did not experience high-grade metamorphism; this is likely because that the Chabo Co seamount was detached from the subducting Longmu Co-Shuanghu oceanic slab. This work provides new solid evidences for an in situ origin of the CQMB.

Newly discovered Late Triassic Baqing eclogite in central Tibet indicates an anticlockwise West\u2013East Qiangtang collision

The Triassic eclogite-bearing central Qiangtang metamorphic belt (CQMB) in the northern Tibetan Plateau has been debated whether it is a metamorphic core complex underthrust from the Jinsha Paleo-Tethys or an in-situ Shuanghu suture. The CQMB is thus a key issue to elucidate the crustal architecture of the northern Tibetan Plateau, the tectonics of the eastern Tethys, and the petrogenesis of Cenozoic high-K magmatism. We here report the newly discovered Baqing eclogite along the eastern extension of the CQMB near the Baqing town, central Tibet. These eclogites are characterized by the garnet + omphacite + rutile + phengite + quartz assemblages. Primary eclogite-facies metamorphic pressure–temperature estimates yield consistent minimum pressure of 25 ± 1 kbar at 730 ± 60 °C. U–Pb dating on zircons that contain inclusions (garnet + omphacite + rutile + phengite) gave eclogite-facies metamorphic ages of 223 Ma. The geochemical continental crustal signature and the presence of Paleozoic cores in the zircons indicate that the Baqing eclogite formed by continental subduction and marks an eastward-younging anticlockwise West–East Qiangtang collision along the Shuanghu suture from the Middle to Late Triassic.

Mayer Kangri metamorphic complexes in Central Qiangtang (Tibet, western China): implications for the Triassic–early Jurassic tectonics associated with the Paleo-Tethys Ocean

The Mesozoic orogeny in Central Qiangtang Metamorphic Belt, northern Tibet, provides important insights into the geological evolution of the Paleo-Tethys Ocean. However, the Triassic–early Jurassic tectonics, particularly those associated with the continental collisionstage, remains poorly constrained. Here we present results from geological mapping, structural analysis, P–T data, and Ar–Ar geochronology of the Mayer Kangri metamorphic complex. Our data reveal an E–W-trending, ~2 km wide dome-like structure associated with four successive tectonic events during the Middle Triassic and Early Jurassic. Field observations indicate that amphibolite and phengite schist complexes in this complex are separated from the overlying lower greenschist melange by normal faulting with an evident dextral shearing component. Open antiform-like S2 foliation of the footwall phengite schist truncates the approximately north-dipping structures of the overlying melange. Microtextures and mineral chemistry of amphibole reveal three stages of growth: Geothermobarometric estimates yield temperatures and pressures of 524 °C and 0.88 GPa for pargasite cores, 386 °C and 0.34 GPa for actinolite mantles, and ~404 °C and 0.76 GPa for winchite rims. Peak blueschist metamorphism in the phengite schist occurred at ~0.7–1.1 GPa and ~400 °C. Our Ar–Ar dating of amphibole reveals rim-ward decreasing in age bands, including ~242.4–241.2 Ma, ≥202.6–196.8, and 192.9–189.8 Ma. The results provide evidence for four distinct phases of Mesozoic tectonic evolution in Central Qiangtang: (1) northward oceanic subduction beneath North Qiangtang (~244–220 Ma); (2) syn-collisional slab-break off (223–202 Ma); (3) early collisional extension driven by buoyant extrusion flow from depth (~202.6–197 Ma); and (4) post-collision contraction and reburial (195.6–188.7 Ma).

An early bird from Gondwana: Paleomagnetism of Lower Permian lavas from northern Qiangtang (Tibet) and the geography of the Paleo-Tethys

The origin of the northern Qiangtang block and its Late Paleozoic–Early Mesozoic drift history remain controversial, largely because paleomagnetic constraints from pre-Mesozoic units are sparse and of poor quality. In this paper, we provide a robust and well-dated paleomagnetic pole from the Lower Permian Kaixinling Group lavas on the northern Qiangtang block. This pole suggests that the northern Qiangtang block had a paleolatitude of 21.9±4.7°S at ca. 296.9±1.9 Ma. These are the first volcanic-based paleomagnetic results from pre-Mesozoic rocks of the Qiangtang block that appear to average secular variation accurately enough to yield a well-determined paleolatitude estimate. This new pole corroborates the hypothesis, first noted on the basis of less rigorous paleomagnetic data, the presence of diamictites, detrital zircon provenance records, and faunal assemblages, that the northern Qiangtang block rifted away from Gondwana prior to the Permian. Previous studies have documented that the northern Qiangtang block accreted to the Tarim-North China continent by Norian time. We calculate a total northward drift of ca. 7000 km over ca. 100 myr, which corresponds to an average south-north plate velocities of ∼7.0 cm/yr. Our results do not support the conclusion that northern Qiangtang has a Laurasian affinity, nor that the central Qiangtang metamorphic belt is an in situ Paleo-Tethys suture. Our analysis, however, does not preclude paleogeographies that interpret the central Qiangtang metamorphic belt as an intra-Qiangtang suture that developed at southernly latitudes outboard of the Gondwanan margin. We emphasize that rigorous paleomagnetic data from Carboniferous units of northern Qiangtang and especially upper Paleozoic units from southern Qiangtang can test and further refine these paleogeographic interpretations.

Stepwise exhumation of the Triassic Lanling high‐pressure metamorphic belt in Central Qiangtang, Tibet: Insights from a coupled study of metamorphism, deformation, and geochronology

AbstractThe E‐W trending Central Qiangtang metamorphic belt (CQMB) is correlated to the Triassic orogeny of the Paleo‐Tethys Ocean prior to Cenozoic growth of the Tibetan Plateau. The well‐exposed Lanling high‐pressure, low‐temperature (HP‐LT) metamorphic complex was chosen to decipher the process by which it was exhumed, which thereby provides insights into the origin of the CQMB and Qiangtang terrane. After a detailed petrological and structural mapping, three distinct N‐S‐trending metamorphic domains were distinguished. Microscopic observations show that core domain garnet (Grt)‐bearing blueschist was exhumed in a heating plus depressurization trajectory after peak eclogitic conditions, which is more evident in syntectonic vein form porphyroblastic garnets with zoning typical of a prograde path. Grt‐free blueschist of the mantle domain probably underwent an exhumation path of temperature increasing and dehydration, as evidenced by pervasive epidote veins. The compilation of radiometric results of high‐pressure mineral separates in Lanling and Central Qiantang, and reassessments on the published phengite data sets of Lanling using Arrhenius plots allow a two‐step exhumation model to be formulated. It is suggested that core domain eclogitic rocks were brought onto mantle domain blueschist facies level starting at 244–230 Ma, with exhumation continuing to 227–223.4 Ma, and subsequently were exhumed together starting at 223–220 Ma, reaching lower greenschist facies conditions generally after 222–217 Ma. These new observations indicate that the CQMB formed as a Triassic autochthonous accretionary complex resulting from the northward subdcution of the Paleo‐Tethys Ocean and that HP‐LT rocks therein were very probably exhumed in an extensional regime.

The subduction-accretion history of the Bangong-Nujiang Ocean: Constraints from provenance and geochronology of the Mesozoic strata near Gaize, central Tibet

The Mesozoic strata, within the Bangong-Nujiang suture zone in central Tibet, recorded critical information about the subduction-accretion processes of the Bangong-Nujiang Ocean prior to the Lhasa-Qiangtang collision. This paper reports detailed field observations, petrographic descriptions, sandstone detrital zircon U-Pb ages and Hf isotopic analyses from an accretionary complex (preserved as Mugagangri Group) and the unconformably overlying Shamuluo Formation near Gaize. The youngest detrital zircon ages, together with other age constraints from literature, suggest that the Mugagangri Group was deposited during late Triassic-early Jurassic, while the Shamuluo Formation was deposited during late Jurassic-early Cretaceous. Based on the differences in lithology, age and provenance, the Mugagangri Group is subdivided into the upper, middle and lower subunits. These units are younging structurally downward/southward, consistent with models of progressive off-scrapping and accretion in a southward-facing subduction complex. The upper subunit, comprising mainly quartz-sandstone and siliceous mud/shale, was deposited in abyssal plain environment close to the Qiangtang passive margin during late Triassic, with sediments derived from the southern Qiangtang block. The middle and lower subunits comprise mainly lithic-quartz-sandstone and mud/shale, containing abundant ultramafic/ophiolitic fragments. The middle subunit, of late Triassic-early Jurassic age, records a transition in tectono-depositional setting from abyssal plain to trench-wedge basin, with sudden influx of sediments sourced from the central Qiangtang metamorphic belt and northern Qiangtang magmatic belt. The appearance of ultramafic/ophiolitic fragments in the middle subunit reflects the subduction initiation. The lower subunit was deposited in a trench-wedge basin during early Jurassic, with influx of Jurassic-aged zircons originating from the newly active southern Qiangtang magmatic arc. The lower subunit records the onset of arc magmatism related to the northward subduction of the Bangong-Nujiang Ocean. The Shamuluo Formation, comprising mainly lithic-feldspar-sandstone with limestone interlayers, was deposited in a post-collisional residual-sea or pre-collisional trench-slope basin, with sediments derived entirely from the Qiangtang block.

Lawsonite- and glaucophane-bearing blueschists from NW Qiangtang, northern Tibet, China: mineralogy, geochemistry, geochronology, and tectonic implications

The occurrence of high-pressure (HP) blueschists within the central Qiangtang terrane of northern Tibet has a significant bearing on plate-suturing processes. In order to contribute to the ongoing debate on whether the central Qiangtang metamorphic belt represents an in situ suture within the Qiangtang terrane, we examined lawsonite- and glaucophane-bearing blueschists from the northwest Qiangtang area (84° 10′–85° 30′ E, 34°10′–34° 45′ N). All studied rocks are metapelites, metasandstones, or metabasalts, characterized by lawsonite + glaucophane + phengite, lawsonite + glaucophane + epidote + albite + quartz, or glaucophane + phengite + quartz assemblages. The meta-mafic rocks contain very high TiO2 and low Al2O3 contents. They are typified by abundant ferromagnesian trace elements, and an absence of Eu anomalies and Nb–Ta deletions; all the above features indicate that these mafic rocks represent oceanic island basalt (OIB) protoliths. Most of the metasediments contain high SiO2, moderate Al2O3 + K2O, and low TiO2 + Na2O. They display high CIA (chemical index of alteration) values (74% ± 5%) and distinctly negative Eu anomalies (Eu/Eu* = 0.64 ± 0.05). This, along with their high field strength elemental characteristics, indicates that they were deposited in a passive continental margin environment, intercalated with OIB-type basalts. We estimate the peak metamorphic conditions for these blueschists as T = 330–415°C and P = 9–11.5 kbar. This HP event occurred at ca. 242 Ma, indicated by a well-defined 40Ar/39Ar plateau age for glaucophane. Retrograde metamorphism occurred at T = 280–370°C, P = 6.5–9.5 kbar, t = ca. 207 Ma (40Ar/39Ar dating of phengite). Therefore, a cold subduction (geotherm ∼8°C/km) attended the passive continental margin during the Triassic when the eastern Qiangtang collided with the western Qiangtang. The northwest Qiangtang HP metamorphic belt is an extension of the central Qiangtang metamorphic belt that defines the suture between eastern and western Qiangtang, and indicates an anticlockwise, diachronous closure of the Shuanghu Palaeo-Tethys.

40Ar‐39Ar Geochronology and Tectonic Implications of the Blueschist from Northwestern Qiangtang, Northern Tibet, Western China

Abstract:Blueschist exposed in the northwestern Qiangtang terrane, northern Tibet, western China (∼84°30′ E, 34°24′ N), provides new constraints on the tectonic evolution of Qiangtang as well as northern Tibet. The blueschist represented by lawsonite‐ and glaucophane‐bearing assemblages equilibrated at 375–400 °C and ∼11 kbar. 40Ar‐39Ar analysis on mineral separate from one blueschist sample yielded a well‐defined plateau age of ∼242 Ma. Geochemical studies show the blueschist is metamorphosed within‐plate basalts. The high pressure‐low temperature blueschist indicates a Triassic event of lithosphere subduction, and clearly represents an extension of the central Qiangtang metamorphic belt, and defines an in situ suture between eastern and western Qiangtang.