South Qiangtang Research Articles

Tectonic Evolution of the Meso-Tethys Ocean: Insights from Geochemistry and Geochronology of the Jurassic ophiolitic complex in the Asa area, central Tibet

ABSTRACT Although voluminous Jurassic ophiolites have been reported along the Bangong–Nujiang Suture Zone (BNSZ), the tectonic setting of ophiolites and the Jurassic evolution history of the Meso-Tethys Ocean remain mysterious. This study presents petrological, geochronological and geochemical data for newly discovered Jurassic ophiolitic complex in the Asa area, the Shiquanhe–Namco Ophiolite sub-belt (SNO sub-belt) of BNSZ. Detailed geological investigations and zircon U-Pb dating reveal that the Jurassic ophiolitic complex are composed of a suite of Mid-Late Jurassic (165–158 Ma) ophiolite and Early Jurassic gabbros (ca. 190 Ma). The Early Jurassic gabbros show affinities with island arc tholeiitic (IAT) basalts, which were derived from depleted mantle sources modified by oceanic slab-derived fluids and formed in a forearc settings. The Mid-Late Jurassic ophiolite mainly consists of peridotites, cumulate gabbros, isotropic gabbros, plagiogranites and greenschists. Whole-rock geochemical and zircon Hf isotopic data suggest that the Mid-Late Jurassic ophiolite formed in an intra-oceanicisland arc setting. Together, the Asa ophiolitic complex document magmatic processes from forearc extension to the development of intra-oceanicisland arc, confirming the occurrence of intra-oceanicsubduction system within the Meso-Tethys during the Jurassic. We tend to regard that both northward intra-oceanicsubduction and ocean–continent subduction were initiated during the Early Jurassic in the Meso-Tethys. During Mid-late Jurassic, a northward intra-oceanicarc–back-arc basin system developed in the Meso-Tethys, along with a continental forearc spreading basin resulting from northward ocean-continental subduction beneath the South Qiangtang terrane, producing the complex Jurassic ocean structure of the Meso-Tethys.

India in the Nuna to Gondwana supercontinent cycles: Clues from the north Indian and Marwar Blocks

Evolution of the Indian Block can be traced through Earth9s Phanerozoic and Precambrian supercontinent cycles. The Paleoproterozoic tectonostratigraphic record of the North Indian Block and the Aravalli Delhi Fold Belt in the Nuna supercontinent assembly shows a close link with the events in the Cathaysia Block of South China. Accretion of the two terranes is documented by 1.97 to 1.92 Ga continental arc igneous rocks and 1.91 to 1.81 Ga syn- and post-collisional magmatism in the Lesser Himalaya, along with 1.88 to 1.86 Ga granulite metamorphism in both continental blocks. The connection between the North Indian Block and the Cathaysia Block continued through Nuna dispersal and was followed by the accretion of a series of terranes/microcontinents along the western margin of this united North India-Cathaysia Block during Rodinia assembly (ca. 1.0 Ga). This is recorded by accretion of the Marwar Block to the North Indian Block and Yangtze Block to the Cathaysia Block. Long-lived active continental margins continued along Marwar (NW India), Yangtze, Madagascar and the Seychelles until ca. 720 Ma that jointly occupied a peripheral or even independent paleoposition in the Rodinia reconstructions. The eastern margin of India sutured with the Western Australia-Mawson blocks along the Kunnga Orogen during the final assembly of Gondwana in the early Paleozoic, whereas microcontinental blocks including south Qiangtang and north Lhasa, were accreted to the northern margin of Gondwana in the vicinity of India. The collision of this ensemble of blocks with Africa (western Gondwana) is marked by the East African Orogen/Mozambique Belt, extending through central east Africa, Madagascar, South India and Antarctica. However, further north, India was separated from the Arabian-Nubian Shield by an embayment of the proto-Tethys that remained integral until the breakup of Gondwana. The accretion of Laurussia to Gondwana in the mid-Paleozoic during the assembly of Pangea corresponds with lithospheric extension along the northern margin of India (Gondwana) and separation of several continental blocks including South China, south Qiangtang, and north Lhasa, which then drifted northward across the Paleo-Tethys to collide with the Asian segment of Pangea in the Permo-Triassic.

Lower Cretaceous turbidites in the Shiquanhe–Namco Ophiolite Mélange Zone, Asa area, Tibet: Constraints on the evolution of the Meso-Tethys Ocean

Turbidites from the Shiquanhe–Namco Ophiolite Mélange Zone (SNMZ) record critical information about the tectonic affinity of the SNMZ and the evolutionary history of the Meso-Tethys Ocean in Tibet. This paper reports sedimentologic, sandstone petrographic, zircon U-Pb geochronologic, and clastic rocks geochemical data of newly identified turbidites (Asa Formation) in the Asa Ophiolite Mélange. The youngest ages of detrital zircon from the turbiditic sandstone samples, together with ~115 Ma U-Pb concordant age from the tuff intercalation within the Asa Formation indicate an Early Cretaceous age. The sandstone mineral modal composition data show that the main component is quartz grains and the minor components are sedimentary and volcanic fragments, suggesting that the turbidites were mainly derived from a recycled orogen provenance with a minor addition of volcanic arc materials. The detrital U-Pb zircon ages of turbiditic sandstones yield main age populations of 170–120 Ma, 300–220 Ma, 600–500 Ma, 1000–700 Ma, 1900–1500 Ma, and ~2500 Ma, similar to the ages of the Qiangtang Terrane(age peak of 600–500 Ma, 1000–900 Ma, ~1850 Ma and ~2500 Ma) and the accretionary complex in the Bangong–Nujiang Ophiolite Zone (BNMZ) rather than the age of the Central Lhasa Terrane (age peak of ~300 Ma, ~550 Ma and ~1150 Ma). The mineral modal compositions, detrital U-Pb zircon ages, and geochemical data of clastic rocks suggest that the Asa Formation is composed of sediments primarily recycled from the Jurassic accretionary complex within the BNMZ with the secondary addition of intermediate-felsic island arc materials from the South Qiangtang Terrane. Based on our new results and previous studies, we infer that the SNMZ represents a part of the Meso-Tethys Suture Zone, rather than a southward tectonic klippe of the BNMZ or an isolated ophiolitic mélange zone within the Lhasa Terrane. The Meso-Tethys Suture Zone records the continuous evolutionary history of the northward subduction, accretion, arc-Lhasa collision, and Lhasa-Qiangtang collision of the Meso-Tethys Ocean from the Early Jurassic to the Early Cretaceous.

Crustal material recycling induced by subduction erosion and subduction-channel exhumation: A case study of central Tibet (western China) based onP-T-tpaths of the eclogite-bearing Baqing metamorphic complex

AbstractSubduction and exhumation processes, interacting with each other, play a key role in crustal recycling. Downgoing oceanic lithosphere constitutes the dominant input at subduction margins, but subduction erosion, the removal of crustal material from the overriding plate, may add additional ingredients and complexity to the subduction factory. Different exhumation models have been proposed to explain how subducted materials are exhumed and therefore contribute to crustal recycling, e.g., exhumation up the subduction channel versus diapiric rise through the mantle wedge that overlies the subducted plate. The recently discovered Baqing eclogite-bearing high-pressure metamorphic complex, central Tibet, China, provides an excellent opportunity to decode the exhumation process, the origin of subduction-related magmatism, and the crustal structure of the North Qiangtang block, in addition to elucidating processes of crustal recycling. Pressure-temperature-time (P-T-t) paths and zircon U-Pb ages and trace-element compositions for Baqing high-pressure rocks were used to evaluate exhumation processes and to determine the geochemical and tectonic affinity of the Baqing metamorphic complex.The Baqing metamorphic complex is mainly composed of eclogite, gneiss, and schist. It is located between two geologically distinct terranes—the South Qiangtang block, which has early Paleozoic basement, and the North Qiangtang block, which has Proterozoic basement. In the schist, zircon cores with steep heavy rare earth element (HREE) slopes and oscillatory zoning yielded inherited ages that are similar to detrital zircon ages for the South Qiangtang block schist; in contrast, zircon rims with flat HREE slopes yielded metamorphic ages of 224 Ma that are similar to the metamorphic ages obtained for the Baqing eclogite. In contrast, zircons from the gneiss yielded an upper-intercept age of 1033 ± 32 Ma (interpreted as the crystallization age) and a lower-intercept metamorphic age of 198 ± 4 Ma. Field relations indicate that gneiss and eclogite/amphibolite were exhumed together, so the ∼20 m.y. gap between the gneiss and the metabasite metamorphism may indicate a long exhumation duration. In the region, Proterozoic ages of ca. 1000 Ma are known only from the North Qiangtang block; we thus propose that the Baqing gneiss originated from North Qiangtang block Proterozoic basement, which, along with North Qiangtang block Triassic arc magmatic rocks and the discrepancies between ancient and current arc-trench distances, results in estimates of ∼20–170 km of Triassic subduction erosion. Results of P-T analyses show that most eclogite, amphibolite, and schist shared a similar clockwise P-T path, different from that of the gneiss, which records a higher geothermal gradient. The clockwise P-T trajectory, long exhumation duration, lack of significant heating during exhumation, and the South Qiangtang block affinity of the schist (host rock of the Baqing eclogite) are consistent with subduction-channel exhumation rather than diapiric rise through the mantle wedge. Geochemical similarities between the North Qiangtang block Triassic subduction-related rocks and the Baqing gneiss may signal the involvement of unexhumed Baqing metamorphic complex in the recycling of the Qiangtang crust.

Early Paleozoic granitic rocks of the South Qiangtang Terrane, northern Tibetan Plateau: Implications for subduction of the Proto- (Paleo-) Tethys Ocean

Knowledge of the early Paleozoic tectonic framework of the northern margin of Gondwana continent is critical for reconstructions of the Gondwana supercontinent. Early Paleozoic leucogranite and gneissic granite units within the South Qiangtang Terrane, northern Tibetan Plateau, can constrain the timing and geological processes that occurred along the northern margin of Gondwana continent at this time. Here, we present new field, petrological, zircon geochronological, geochemical, and Nd–Hf isotopic data for this suite of granitic rocks, and use these data to provide insights into the evolution of this Gondwanan margin. Zircon U–Pb dating yielded two groups of ages at 469–453 and 486–473 Ma. All of the granitic rocks in the Gemuri area are high-K calc-alkaline and peraluminous, and record the fractionation of variable amounts of plagioclase, K-feldspar, and biotite. These leucogranites formed from magmas generated by the dehydration melting of muscovite within Paleoproterozoic–Mesoproterozoic metapelites under relatively low-temperature conditions (TZr = 616 °C–695 °C). In contrast, the protoliths of the Duguer gneissic granite formed from magmas generated by the partial melting of Paleoproterozoic–Mesoproterozoic metapelites contaminated by mantle-derived material under relatively high-temperature conditions (TZr = 730 °C–884 °C). The coeval nature and linear distribution of the igneous rocks within the northern Gondwanan margin, and the fact that the granites in the Gemuri area are enriched in large-ion lithophile elements (LILEs; e.g., Rb and K) and depleted in high-field-strength elements (HFSEs; e.g., Zr, Nb, Ta, and Ti), suggest that this magmatism occurred in an active continental margin setting involving the formation of arc-type rocks. The presence of an early Paleozoic suprasubduction zone-type ophiolite within the South Qiangtang Terrane provides evidence of the southward subduction in the Paleo-Tethyan Ocean at this time, suggesting that the two stages (i.e., the Gemuri leucogranites and Duguer gneissic granites) of arc-type magmatism in the Gemuri area may both be related to this subduction event.

Magmatic gap associated with stepwise arc‐continent collision during the subduction of Banggong‐Nujiang Tethys Ocean: Evidence from Late Jurassic bimodal magmas

The magmatic suites in the southern margin of South Qiangtang Terrane (SQT) provide important clues to the process of northward subduction of the Banggong‐Nujiang Tethys Ocean (BNTO) and its geodynamic evolution. Previous studies identified a magmatic gap from Late Jurassic to Early Cretaceous (ca. 150–130 Ma) in the southern margin of SQT. Although the gap is generally correlated with the process related to BNTO subduction, the previous models on this remain equivocal. In this study, we investigated a suite of bimodal magmatic rocks newly recognized in the Rigenco area of SQT, including gabbros and granodiorites which were dated at 151.7 ± 3.8 Ma and 150.0 ± 2.0 Ma respectively. Based on geochemical and isotopic analyses, it is inferred that the gabbroic magmas were derived from a sub‐arc mantle source enriched by metasomatism of voluminous slab‐derived and crustal compositions, and that the granodiorites were derived from sub‐arc juvenile lower crust. Combining the petrogenesis of gabbros and granodiorites, it is proposed that the bimodal magmas were produced under a tectonic setting of general compression with local extension. A detailed evaluation of the tectonic setting of the magmatic bimodal suite suggests crustal thickening at ca. 150 Ma and that the BNTO involved multiple arcs and sub‐basins. A new model of arc‐SQT collision is proposed to account for the magmatic gap which commenced at ca. 150 Ma. The model of arc–continent collision in this case can well explain the present distribution and occurrence of magmatic rocks of late Jurassic age in the southern margin of the SQT, especially the magmatic gap. Our discovery of Late Jurassic bimodal rocks provides a critical evidence for step‐wise arc–continent collision during BNTO subduction, and contributes to the understanding of the magmatic gap in SQT and the evolution of BNTO.

Late Cretaceous High‐Sr/Y Granitic Rocks in the Eastern Tibetan Plateau: Crustal Thickening During Continental Collision

AbstractHigh‐Sr/Y granitic rocks, characterized by intermediate–high SiO2 contents (> 56 wt.%), high La/Yb ratios (> 20), and low Y (< 18 ppm) and Yb (< 1.9 ppm) contents, have drawn much attention from geologists in the past decades. They were originally named adakites and defined as granitic magma in convergent plate margins formed by partial melting of subducted young (< 25 Ma), hot, and hydrated oceanic slab. Recent studies suggest that these rocks can also be formed in a variety of non‐subduction‐related tectonic settings through different petrogenetic processes including partial melting of thickened lower continental crust, providing important constraints on crustal growth and evolution throughout the Earth's history.The Tibetan plateau, generally considered as an archetype of collisional orogen, was formed by the successive accretion of different tectonic terranes to the southern margin of the Eurasia plates since the Early Paleozoic. Geologically, the Tibetan plateau comprises several W–E trending terranes (the Kunlun, North Qiangtang, South Qiangtang, and Lhasa terranes) and northernmost India (the Himalaya region). These terranes have undergone significant crustal thickening during the continental collision events. The crustal thickness of the Tibetan plateau is asymmetric, being thickest under the Lhasa terrane and thinning more gradually towards the north than to the south. From south to north, the crustal thickness increases from ∼50 km in the Himalaya region, to ∼70–80 km under the Lhasa terrane, and decreases to ∼65 km under the North and South Qiangtang terranes and ∼55 km beneath the Kunlun terrane. The present crustal thickness of the Tibetan plateau has generally been attributed to the Cenozoic collision between the Indian and Eurasian plates and the subsequent intracontinental convergences since ∼55 Ma. However, doubts were raised about this interpretion in the past few years. For example, recent studies indicated the crust of the Lhasa terrane had been significantly thickened during ca. 140–130 Ma in response to the subduction of the Neo‐Tethyan Ocean, although the Cretaceous marine strata across the Lhasa terrane demonstrates that it was thin enough to be within the reach of sea level during at least the early Late Cretacous. Whether the South Qiangtang terrane underwent crustal thickening before Cenozoic or not remains an unanswered question.In this study, new zircon U–Pb geochronologic and Hfisotopic, and whole‐rock geochemical data of the Late Cretaceous high‐Sr/Y granitoids from the eastern South Qiangtang terrane are presented. LA–ICP–MS zircon U–Pb dating produced ages (92.0 ± 1.1 Ma, 88.8 ± 0.7 Ma, and 91.5 ± 0.6 Ma) of formation for the Leiwuqi granitoids of the South Qiangtang terrane, Tibetan Plateau, indicating that they were formed during the Late Cretaceous. The granitoids have low A/CNK ratios (0.94–1.06; <1.10) and P2O5 contents (0.02–0.12 wt.%), geochemically similar to I‐type granitoids. They are characterized by high Sr (350–938 ppm, most > 400 ppm), low Y (5.94–9.67 ppm), with high Sr/Y (36–119, most > 40) and (La/Yb)N ratios (67–394). They exhibit negative zircon ∊Hf (t) values (−9.8 to −1.1) and low MgO (0.32–0.69 wt.%), Cr (2.07–12.17 ppm), and Ni (1.10–5.94 ppm) contents. These features suggest that the granitoids probably were derived from partial melting of thickened lower continental crust (> 50 km). The crust of the South Qiangtang terrane (at least its eastern part) may have been significantly thickened before the Late Cretaceous.

The Ren Co MOR‐type Ophiolite in the North‐central Tibetan Plateau: A Fast‐spreading Ridge Segment of the Meso‐Tethys Ocean?

AbstractThe Meso‐Tethys Ocean is generally considered to have opened in the late Early Permian as a result of the Cimmerian continental block drifting away from the Gondwana supercontinent. This ocean is also termed the north branch of the Neo‐Tethys Ocean, and is now represented by the Bangong–Nujiang suture zone in the north‐central Tibetan plateau. However, it is still unknown for the evolutionary history for as such a huge ancient ocean basin.Ophiolites are remnants of oceanic lithosphere and preserve key information in rebuilding the evolutionary history of ancient oceans. In this study, we undertook detailed geological mapping for the Ren Co ophiolite in the middle part of the Bangong–Nujiang suture zone, and a typical Penrose‐type ophiolite sequences was newly documented in the Ren Co area. The Ren Co ophiolite comprises serpentinized peridotite, cumulate rock, gabbro/diabase, sheeted dike, massive and pillow lavas, and minor red chert. These rocks exhibit well‐preserved mantle and crust rock suites, and show close similarities to oceanic lithospheres at modern fast‐spreading ridges. Zircon U–Pb dating for gabbro and plagiogranite yielded ages of 169–147 Ma, which suggest that the Ren Co ophiolites were formed during the Middle to Late Jurassic. Harzburgite in the Ren Co area shows similar features to those of abyssal peridotite indicating the residues of the oceanic mantle. Mafic rocks (basalt, diabase and gabbro) of the Ren Co ophiolite show geochemical features similar to those of typical N‐MORB. Moreover, all samples have positive whole‐rock ∊Nd(t), and zircon ∊Hf(t) and mantle‐like δ18O values. These features suggest that these rocks were derived from a depleted mantle source, thus the Ren Co ophiolite was typical MOR‐type ophiolite. We suggest that the Ren Co ophiolite was formed in a fast‐spreading mid‐ocean‐ridge (MOR) setting, and they most likely represented the ridge segment of the Bangong‐Nujiang Meso‐Tethys Ocean. The Bangong–Nujiang Meso‐Tethys Ocean was a wide paleo‐ocean, and experienced continuous oceanic spreading, subduction, accretion before final Lhasa and South Qiangtang continental assembly.

The Sedimentary Evolution During Late Triassic‐Jurassic Period and the Hydrocarbon Exploration Direction in the Eastern Part of the Qiangtang Basin, Tibet

AbstractBased on the field outcrops surveyed, combined with recent published the regional tectonic evolution and geochronology data, we analyzed the lithologies and rock associations of strata, identified the sedimentary facies types, and discussed the distribution sedimentary facies and the hydrocarbon accumulation in the eastern Qiangtang basin during the Late Triassic – Jurassic. Marked by regional unconformities, there are two tectono‐stratigraphic units (from the Carnian to the Norian and from the Rhaetian to the Kimmeridgian, respectively) in the eastern part of Qiangtang basin. We systematically described the distribution range, thickness variation and lithological characteristics of different formations in the tectono‐stratigraphic units. The Late Triassic‐Jurassic is dominated by marine facies and marine ‐ continental transitional facies. The marine ‐ continental transitional facies include deltaic and tidal‐lagoon facies. Marine facies including gentle carbonate slope, evaporative platform, restricted platform, littoral, neritic, bathyal and abysmal facies. The Carnian stage is dominated by littoral–neritic–bathyal–abysmal facies in the north Qiangtang depression otherwise the littoral–neritic facies in the south Qiangtang depression. The early Norian stage is dominated by carbonate gentle slope‐mixed continental shelf facies. The late Norian, Bajocian, Callovian and Kimmeridgian stage are dominated by tidal flat‐delta facies in the north Qiangtang depression and littoral‐neritic facies in the south Qiangtang depression. The Bathonian and Oxfordian stage are dominated by evaporative platform‐ restricted platform‐ mixed continental shelf facies. The sedimentary facies formed zones from north to south and extended in an E–W direction. The Eastern Lower Uplift (ELU) played an important role in the division zones of sedimentary facies from north to south. During the Bathonian and Oxfordian, the ELU developed below the sea level and controlled the distribution of restricted platform, evaporative platform and platform margin. We analyzed 20 source rock samples from the upper Triassic‐Jurassic. The total organic carbon (TOC) value from Qoimaco, Buqu and Adula Formations. of late Triassic‐Jurassic in the eastern Qiangtang basin are ranges from 0.17∼0.33% (average 0.28%), 0.05∼0.25% (average 0.15%) and 10.32∼28.78% (average 19.33%), respectively. Obviously, the Adula Fm. developed good source rocks. The values of Tmax and S1+S2 in the Adula formation are 459–461°C (average 460°C) and 6.75‐28.55 mg/g (average 18.18mg/g), indicating that the Adula source rock has reached high‐over‐maturity stage. The bathyal, gentle slop and platform facie belts of the Upper Triassic can configurate the good hydrocarbon prospects in the northeastern area of the Qiangtang basin.

Late Cambrian to Early Silurian Granitic Rocks of the Gemuri Area, Central Qiangtang, North Tibet: New Constraints on the Tectonic Evolution of the Northern Margin of Gondwana

AbstractThe Paleozoic tectonic framework and paleo–plate configuration of the northern margin of Gondwana remain controversial. The South Qiangtang terrane is located along the northern margin of Gondwana and records key processes in the formation and evolution of this supercontinent. Here, we present new field, petrological, zircon U‐Pb geochronological, and Lu‐Hf isotopic data for granitic rocks of the Gemuri pluton, all of which provide new insights into the evolution of the northern margin of Gondwana. Zircon U‐Pb dating of the Gemuri pluton yielded three concordant ages of 488.5 ± 2.1, 479.9 ± 8.9, and 438.5 ± 3.5 Ma. Combining these ages with the results of previous research indicates that the South Qiangtang terrane records two magmatic episodes at 502–471 and 453–439 Ma. These two episodes are associated with enriched zircon Hf isotopic compositions (∊Hf(t) = –10.1 to –3.9 and –16.6 to –6.5, respectively), suggesting the granites were formed by the partial melting of Paleoproterozoic–Mesoproterozoic metasedimentary rocks (Two–stage Hf model ages (TDMC) = 2094–1704 and 2466–1827 Ma, respectively). Combining these data with the presence of linearly distributed, contemporaneous Paleozoic igneous rocks along the northern margin of Gondwana, we suggest that all of these rocks were formed in an active continental margin setting. This manifests that the two magmatic episodes within the Gemuri area were associated with southward subduction in the Proto‐ (Paleo‐) Tethys Ocean.

Oldest Jurassic wood with Gondwanan affinities from the Middle Jurassic of Tibetan Plateau and its paleoclimatological and paleoecological significance

Jurassic flora of the Tibetan Plateau is poorly known, with limited fossil records. The paleobiogeography of terrestrial biota is, however, one of the major evidences to reconstruct the intricate paleogeography of an area. Well-preserved fossil wood is described from the Middle Jurassic Xiali Formation (Callovian) in the South Qiangtang terrane of the Tibetan Plateau, SW China. The wood specimen is a well-preserved secondary xylem without growth rings. Bordered pits on the radial tracheid walls are mostly uniseriate distant, locally contiguous. Cross-fields have 1–2 large oopores. A systematic study indicates that the current fossil wood represents a new species, Circoporoxylon tibetense sp. nov. This is the earliest Laurasian data for the genus Circoporoxylon, which has a Gondwanan origin and subsequently became common in Laurasia during the Cretaceous. The present new finding indicates that Circoporoxylon might have reached the Laurasia via a Tibetan terrane. The presence of fungal remains and wood rotting structures resembling the extant white-rot testifies for the plant–fungal interaction in the Callovian. The absence of growth rings in Circoporoxylon wood and the absence of gypsum layers in the South Qiangtang terrane, together with the widely occurrence of gypsum layers in the North Qiangtang terrane, suggests the South Qiangtang terrane might be wetter than the latter during the Callovian age. The paleogeomorphological highlands of Central Uplift were most likely the boundary between the two. This paleoclimate zonation implies that the Permian to Mesozoic megamonsoon of the supercontinent Pangaea might have terminated in the northern of Paleo-Tethys before the Callovian.

Provenance of early Paleozoic sedimentary rocks in the Altyn Tagh orogen: Insights into the paleoposition of the Tarim craton in northern Gondwana associated with final closure of the Proto–Tethys Ocean

Abstract The evolution of the northern margin of Gondwana, especially to the north of India and Australia, remains enigmatic. Much controversy concerns when and where the Tarim craton was amalgamated with northern Gondwana due to final closure of the North and South Altyn Oceans (two branches of the Proto–Tethys Ocean between southeastern Tarim and northern Gondwana). This study addressed these issues through systematic field-based zircon U-Pb dating and Hf-isotope analyses of early Paleozoic sedimentary rocks in the Altyn Tagh orogen. New dating results reveal depositional ages from ca. 494 to 426 Ma. Provenance tracing indicates the ca. 494–477 Ma samples were dominantly sourced from local Altyn Tagh areas to the south of the North Altyn Ocean, whereas the ca. 465–449 Ma samples are characterized by a significant increase in ca. 2.7–2.4 Ga, 2.0–1.7 Ga, and 840–780 Ma detrital zircons, indicating an augmented supply of detritus from the Tarim craton to the north of the North Altyn Ocean. This change indicates a major provenance shift from a single to multiple source regions between ca. 477 and 465 Ma, marking the timing of the final closure of the North Altyn Ocean. Zircon U-Pb and Hf-isotopic data from the ca. 444–426 Ma samples resemble those from the ca. 465–449 Ma samples, suggesting local sediment recycling related to a postcollisional regime. Considering the South Altyn Ocean and other branches of the Proto–Tethys Ocean, we infer that the entire Proto–Tethys Ocean might have been progressively closed at ca. 500–420 Ma, leading to the amalgamation of most East Asian blocks with northern Gondwana. Detrital zircon U-Pb and Hf-isotope comparisons indicate that Tarim shared a North Indian affinity with many East Asian terranes (such as North Qilian, North Qinling, South China, Indochina, South Qiangtang, etc.), rather than with Arabia-Iran or other terranes (e.g., Lhasa and Sibumasu) that were adjacent to western Australia along the northern margin of Gondwana.

Organic geochemical characteristics and hydrocarbon generation potential of Jurassic shales and mudstones in Qixiang Co area southern Qiangtang Basin, Tibet

The Qixiang Co area is located in the South Qiangtang Depression Zone. Jurassic shales and mudstones with large thicknesses are widely developed in this area. The Jurassic was a potential exploration horizon in the Qiangtang Basin; however, few detailed analyses have been conducted on the geochemical characteristics of mudstones and shales in this area. In this paper, the hydrocarbon generation potential and sedimentary environment of shales and mudstones are analyzed using organic geochemical data. The results show that the abundance of organic matter is low, as indicated by the low TOC value (averaging 0.70%) and hydrocarbon potential (S1+S2) (0.13 mg/g). The main macerals of kerogen are exinites with an average value of 82%, and they are mainly type II2 kerogen. A summary of the results is presented: the thermal maturity of organic matter is high; the average Ro value is more than 1.2%; n-alkanes are dominated by a high carbon number and have no obvious odd-even advantage; the average value of Pr/Ph is 0.74; the distribution of C23 triterpenoids satisfies C20< C21< C23; the content of the gammacerane index is moderate (0.21) the variation range is small; and the C27 content in regular steranes is dominant. It can be concluded that the organic matter precursors in this area were a mix of higher plants and marine phytoplankton. Organic matter was deposited under a reductive environment with stable water and slightly high salinity. The hydrocarbon generation potential of the Jurassic shale and mudstone in this area is poor, which is unfavorable to oil and gas exploration.

Geology and geochronology of the Tiegelongnan porphyry-epithermal Cu (Au) deposit, Tibet, China: Formation, exhumation and preservation history

The Tiegelongnan (Rongna) deposit is a giant porphyry-epithermal Cu (Au) deposit within the Duolong porphyry district, in the south Qiangtang terrane, central-western Tibet, China. The deposit is centered on multiphase granodiorite porphyry (GP) intrusions intruded in Jurassic sandstone. Phase 1 and 2 GP are associated with biotite and sericite alteration, whereas GP3 and 4 occur in middle and shallow parts of the deposit affected by sericite and advanced argillic alteration (alunite-kaolinite-dickite-pyrophyllite). Sericite alteration is pervasive in the deep part of the deposit replacing biotite alteration and hosting porphyry stage chalcopyrite-bornite-pyrite ± molybdenite mineralization. Advanced argillic alterationis associated with epithermal stage high sulfidation (tennantite, enargite, bornite, and digenite) mineralization, overprints the sericite and biotite alteration from the shallow part of the deposit. The top of the Tiegelongnan hydrothermal system was weathered with a layer of supergene oxidation and enrichment zone, exhumated, and covered by andesite and gravel.Muscovite sampled from deep sericite alteration yields a 40Ar-39Ar age of 120.9 ± 0.8 Ma, which is consistent with previously reported hydrothermal biotite 40Ar-39Ar age of 121.1 ± 0.6 Ma, molybdenite Re-Os ages of 121.2 ± 0.6 and 119.0 ± 1.4 Ma, and GP 1 and 2 zircon U-Pb ages of 121.5 ± 1.5, 120.2 ± 1.0 Ma (LA-ICP-MS results), and 118.7 ± 0.9 Ma (SIMS result). Phase 3 GP 3 yields a CA-ID-TIMS zircon U-Pb age of 119.9 ± 0.2 Ma, which constrains the age of porphyry magmatic-hydrothermal events. The porphyry system was then unroofed, weathered, and superimposed by discrete stages of advanced argillic alteration and high sulfidation mineralization. Two pulses of alunite events yield 40Ar-39Ar ages of 116.3 ± 0.8 Ma and 111.7 ± 1.0 Ma, which represent the principal epithermal alteration and mineralization ages at Tiegelongnan. The first alunite pulse age is consistent with younger porphyry (GP4) age of 116.2 ± 0.4 Ma (zircon LA-ICP-MS age). Weathering and exhumation of the porphyry-epithermal deposit were continuing until the cover of andesite at ~110 Ma and by post-andesite gravel. The andesite was affected by a younger weak hydrothermal alteration at 108.7 ± 0.7 Ma, constrained by a muscovite 40Ar-39Ar age. Another barren alunite yields a 40Ar-39Ar age of 100.6 ± 2.0 Ma, which may represent the youngest hydrothermal event. The long life-span discrete hydrothermal activities from ~120 to ~100 Ma in the Tiegelongnan deposit is consistent with the long-lasting tectonic-magmatic events of the Bangong-Nujiang suture zone.The discrete epithermal mineralization events at Tiegelongan are younger than the porphyry mineralization events, which is analogous to several other epithermal deposits worldwide. The ~10 m.y exhumation history at Tiegelongan is slower than the typical and rapid erosion history of ~1–2 m.y porphyry deposits in low-altitude tropical climates regions, and during a specific abrupt uplift pulse in the Andean region. The slow exhumation at Tiegelongnan is interpreted to be an integrated result of an arid climate environment, relatively flat topography, and a slow uplift condition during Cretaceous in central-western Tibet.

Study of molasse within the middle segment of the Bangong‐Nujiang suture zone, central Tibet: Constraints of ocean–continent transform

The Bangong‐Nujiang suture zone, separating the Lhasa and South Qiangtang terranes of the Tibetan Plateau, is marked by remnants of the Bangong‐Nujiang oceanic basin. In the Zanzong Co area of central Tibet, non‐marine molasse deposits of the newly identified Jingzhushan Formation record the eventual collision between these two terranes and subsequent ocean–continent transform along the middle segment of the Bangong‐Nujiang suture zone. In this contribution, it is observed that the Jingzhushan Formation, characterized by lacustrine facies and braided delta facies, rests unconformably on the underlying marine strata and ophiolite. Zircon U–Pb dating has been conducted with samples of four interlayer volcanic rocks and two sandstones from the Jingzhushan Formation. The interlayer volcanic rocks constrain the depositing age from ca. 115 to 72 Ma. Based on the youngest detrital zircon ages, the maximum depositional ages of the Jingzhushan Formation are suggested to be ca. 78 Ma. Petrographic and detrital zircon isotopic results indicate that sediments in the Jingzhushan Formation have mixed sources from the South Qiangtang terrane and the intervening Bangong‐Nujiang suture zone. Provenance analysis, together with regional data, suggests that the newly identified Jingzhushan Formation reflects continental molasse deposition in an intermontane basin during the post‐collisional stage. The development of the intermontane basin evidenced by molasse deposition of the Jingzhushan Formation reveals that the ocean–continent transform of the middle segment of the Bangong‐Nujiang Ocean had been completed in the late Early Cretaceous (ca. 115 Ma).

Southward subduction of the Bangong-Nujiang Tethys Ocean: insights from ca. 161–129 Ma arc volcanic rocks in the north of Lhasa terrane, Tibet

The Meso-Tethys Ocean was mainly represented by the Bangong-Nujiang suture zone between the South Qiangtang and Lhasa terranes in the Tibetan plateau. The subduction polarity of this Tethys Ocean during the Mesozoic is still being debated and it has been suggested to be northward or bidirectional subduction. A series of volcanic rocks, including andesite, dacite, rhyolite and volcaniclastic rocks, are documented in the Xinji area, north of Lhasa terrane, Tibet. These samples yielded zircon U–Pb ages between 161 and 129 Ma, which represent the emplacement age of the volcanic rocks. The volcanic rocks show typical arc calc-alkaline signatures, with strong depletion in Nb, Ta and Ti, enrichment in Rb, Th, U and Pb. These features are indicative of subduction-related arc magmatism. The Jurassic andesite shows high Mg# values, and high Th but low Sr contents, and it is interpreted as a result of interaction of subduction sediment-derived melting with mantle wedge peridotite. The Early Cretaceous andesite, dacite and rhyolite show similar geochemical features and are suggested to be formed by melting of ancient crustal materials with contribution of mantle-derived magma, following by varying degrees of fractional crystallization and assimilation. Finally, we suggest that the Xinji volcanic rocks represent an Andean-type magmatic arc along the north of Lhasa terrane. They were produced by southward subduction of the Bangong-Nujiang Tethys Ocean. Consequently, a bidirectional subduction model was suggested for the evolution of the Bangong-Nujiang Tethys Ocean during the Middle Jurassic to Early Cretaceous.

Middle Permian fusulines from the Thitsipin Formation of Shan State, Myanmar and their palaeobiogeographical and palaeogeographical implications

AbstractFusuline faunas including 29 species belonging to 19 genera/subgenera are described from the Thitsipin Formation in eight sections/localities on the Shan Plateau in eastern Myanmar. These fusulines broadly indicate a Midian (middle Permian, Guadalupian) age. The lower diversity and the presence of some genera, such as Monodiexodina and Eopolydiexodina, suggest that the Sibumasu Block belonged to the palaeobiogeographic Cimmerian Province during the Midian. Furthermore, quantitative cluster analysis of middle Permian fusulines from the Shan Plateau and adjacent Cimmerian blocks suggests close faunal affinities between the Sibumasu Block and the Baoshan Block in western Yunnan. More importantly, the widespread occurrence of fusulines Eopolydiexodina afghanensis and Jinzhangia shengi on the Shan Plateau is similar to that of contemporaneous fusuline faunas from the Baoshan Block and the South Qiangtang Block but different to that of the Tengchong and Lhasa Blocks, which are dominated by the characteristic Nankinella–Chusenella assemblage. This faunal discrepancy provides strong evidence that the Bangong–Nujiang suture passes through the Gaoligong Orogen in western Yunnan rather than through the Myitkyina ophiolites in northern Myanmar. Additionally, palaeobiogeographical analysis of these fusuline faunas from the Lhasa, Tengchong, South Qiangtang, Baoshan and Sibumasu Blocks implies that the Bangong–Nujiang Ocean might have been present before the Midian.

Petrogenesis and tectonic setting of the Early Cretaceous granitoids in the eastern Tengchong terrane, SW China: Constraint on the evolution of Meso-Tethys

Abstract As a result of the evolution of Meso-Tethys, Early Cretaceous granitoids are widespread in the eastern Tengchong terrane, SW China, which is considered as the southern extension of the Tibetan Plateau. These igneous rocks are therefore very important for understanding the tectonic setting of Meso-Tethys and the formation of the Tibetan Plateau. In this paper, we present new zircon U-Pb dating, whole-rock elemental, and Nd isotopic data of granitoids obtained from the eastern Tengchong terrane. Our results show that these granitoids are composed of monzogranites and granodiorites and formed at ca. 124 Ma in the Early Cretaceous. Mineralogically and geochemically, these granitoids display metaluminous nature and affinity to I-type granites, which are derived from preexisting intracrustal igneous source rocks. The predominantly negative whole-rock εNd(t) values (−10.86 to −8.64) for all samples indicate that they are derived mainly from the partial melting of the Mesoproterozoic metabasic rocks in the lower crust. Integrating previous studies with the data presented in this contribution, we propose that the Early Cretaceous granitic rocks (135–110 Ma) also belong to I-type granites with minor high fractionation. Furthermore, in discriminant diagrams for source, granitoids are mainly derived from the partial melting of metaigneous rocks with minor sediments in the lower crust. The new identification of the Myitkyina Meso-Tethys ophiolitic suite in eastern Myanmar and mafic enclaves indicate that these Cretaceous igneous rocks were the products of the tectonic evolution of the Myitkyina Tethys Ocean, which was related to post-collisional slab rollback. Moreover, the Tengchong terrane is probably the southern extension of the South Qiangtang terrane.

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 &gt;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.

The Cretaceous crustal shortening and thickening of the South Qiangtang Terrane and implications for proto-Tibetan Plateau formation

The timing and magnitude of deformation across the central Tibetan Plateau, including the South Qiangtang Terrane (SQT), are poorly constrained but feature prominently in geodynamic models of the Tibetan Plateau formation. The Ejiu fold and thrust belt (EFTB), which is located in the SQT, provides valuable records of the Mesozoic-Cenozoic deformation history of the central Tibetan Plateau. Here we integrate geochronology of volcanic rocks, low-temperature thermochronology, geologic mapping and a balanced cross section to resolve the deformation history of the SQT. Geochronologic data suggest that major deformation that initiated in the early Cretaceous continued until at least 80 Ma and ceased by ∼40 Ma. The balanced cross section resolves ∼66 km upper crustal shortening (34%) mainly during the Cretaceous Qiangtang-Lhasa collision. However, the Cenozoic crustal shortening is not well constrained because of a lack of successive Cenozoic strata. We also discussed whether the observed crustal shortening can account for the modern crustal thickness and elevation in the SQT. Our observations indicate that crustal shortening and thickening within the central Tibetan Plateau was mostly accomplished during the Cretaceous Lhasa-Qiangtang collision. A thick crust could be maintained since the Cretaceous due to slow erosion rates since ∼40 Ma. Minor Late Cenozoic shortening also contributed to a small amount of crustal thickening in the central Tibetan Plateau. However, close to modern >4700 m elevation was finally attained by lithospheric mantle foundering in the Qiangtang Terrane at ∼25 Ma.