Search

We present new geochemistry, geochronology, whole-rock Sr-Nd-Pb, and zircon Hf isotopic data from the Zhongba pluton in the western part of the Gangdese magmatic belt (GMB) and discuss its melt evolution within the collisional tectonic framework of southern Tibet. Our U-Pb zircon dating of the Zhongba plutonic rocks has constrained the timing of their emplacement at 38 Ma. The Zhongba granitoids are enriched in light rare earth elements, large-ion lithophile elements, and Pb and are depleted in high-field-strength elements. They show enriched Sr isotopes, low εNd(t) values, and fairly radiogenic Pb isotopic signatures. They have negative zircon εHf(t) values and ancient crustal model ages of 1483–1180 Ma. These geochemical and isotopic characteristics, combined with the literature data, suggest mixing of different magma types, derived from partial melting of juvenile southern Lhasa crust and ancient Indian continental crust, to produce hybrid magmas of the Zhongba granitoids. We propose that asthenospheric upwelling caused by the breakoff of the northward-subducting Neotethyan oceanic lithosphere provided the heat and material flux to trigger partial melting of the overlying Lhasa arc lithosphere and subducted sediments of the ancient Indian crust. Similar geochemical and isotopic signatures of the coeval Kailas and Zedong granitoids within the GMB indicate that this slab breakoff–induced mid- to late Eocene magmatism was a syncollisional and orogen-parallel magmatic event. The Zhongba pluton and its contemporaneous equivalents thus provide the geochronological and isotopic evidence for the earliest melt flux derived from the subducted continental crust and sediments of India into the melt regime of Cenozoic magmatism in southern Tibet. [ABSTRACT FROM AUTHOR]

Abstract: The Cretaceous Xigaze ophiolite is best exposed at the central Yarlung Zangbo Suture Zone (YZSZ, Tibet) which also includes the Gangdese arc and the Xigaze forearc basin. This study reports new geochronological and geochemical data for this ophiolite to revisit its geodynamic and petrogenetic evolution. The Xigaze peridotites have low CaO and Al2O3 contents and U-shaped Rare Earth Element (REE) patterns, suggesting that they are residues after moderate to high degrees of partial melting and were modified by infiltration of Light Rare Earth Element (LREE)-enriched boninitic melts. The Xigaze crustal rocks belong to two groups: Mid-Ocean Ridge Basalt (MORB)-like rocks and boninitic rocks showing a uniform LREE depletion and flat to LREE enrichment on chondrite-normalized patterns, respectively. Geochemically, both groups show the influence of subducting oceanic slab-derived fluids. LA-ICPMS zircon U–Pb and Lu–Hf analyses from dolerite and quartz diorite dikes, which intruded into the mantle peridotite, and dolerite sheeted sills show that they were generated between 127 and 124Ma. The zircons possess positive εHf(t) values ranging from +7.5 to +17.3. Taking into account the geological and geochronological characteristics of the central-western YZSZ, we propose that ophiolites in this region formed in a forearc spreading setting through rapid slab rollback during subduction initiation between 130 and 120Ma. Following this stage of spreading, the forearc was stabilized and the zone of melting migrated beneath the Gangdese arc producing the voluminous Late Cretaceous granitoids displaying depleted mantle-type Hf isotopic compositions. Our model provides a new explanation for the generation and evolution of forearc-type ophiolites. [Copyright &y& Elsevier]

Abstract: To understand the magmatic and tectonic events in the Eastern Kunlun Range, we conducted systematic zircon U–Pb geochronology, and paired zircon (ZHe) and apatite (AHe) (U–Th)/He themchronology investigations in the central and eastern segments of this range. Zircon U–Pb ages show two-stage magmatic events of Late Silurian and latest Permian–Early Jurassic. ZHe and AHe ages and multi-system thermochronometers reveal multi-stage rapid cooling events. From a combination of these data with previously published geochronological and thermochronological ages, and regional geological setting, we confirm several important Phanerozoic tectono-magmatic events in this region. Whereas the Silurian granites might be related to a small paleo-ocean subduction and subsequent collision, the latest Permian–Early Triassic granitoids were produced by northward subduction of the Songpan–Ganzi Paleo-Tethyan ocean. The Late Triassic–earliest Jurassic granitoids and Late Triassic–Early Jurassic exhumation are interpreted as response to the collision between Kunlun–Qaidam and Qiangtang. Samples close to the Kunlun Fault show rapid Late Oligocene–Early Miocene exhumation (1.3–1.6km/Ma), whereas those from the northern part of central segment display extremely low exhumation rates (0.02–0.05km/Ma). This heterogeneous denudation during 30–20Ma between northern and southern part of the central segment requires that crustal thickening was completed by lower-crustal underthrusting with little exhumation or crustal thickening in pre-Cenozoic times. The activity of the Kunlun Fault and associated normal faulting component were probably responsible for this stage of rapid exhumation in the southern part. The complex tectono-magmatic evolution of the Eastern Kunlun Range is controlled by pre-Cenozoic paleo-ocean subductions and subsequent continent collisions and Paleogene India–Asia collision. [Copyright &y& Elsevier]

Highlights: [•] We analysed the biomarker and hydrocarbon potential of samples in the Lunpola Basin. [•] Quantity, quality and maturity of organic matter were determined. [•] The study indicated that paleoclimate during sample deposition was probably cool and arid. [ABSTRACT FROM AUTHOR]

Abstract: Despite more than 30 years’ studies in the Yarlung Zangbo Suture Zone (YZSZ), southern Tibet is still a region of discovery and the geological evolution of the Neo-Tethys here remains controversial. In this paper we present the new field observation, petrography, zircon U–Pb dating and geochemistry of the Zhongba mafic rocks in the western segment of the YZSZ. Zircon U–Pb analyses from a diabase using SIMS (Secondary Ion Mass Spectrometry) yielded a concordant age of 125.7±0.9Ma. All the Zhongba mafic rocks exhibit LREE enrichment (LaN/YbN =4.7–15.8) without Eu anomalies (δEu=0.97–1.01). Their Primitive-Mantle-normalized trace element patterns are similar to those of the average OIB and Hawaii alkaline basalts, as well as other OIB-type rocks already reported in the central-western YZSZ. These observations lead us to suggest the existence of a series of seamounts within the Neo-Tethys during the Early Cretaceous. These OIB-type rocks could be originated from (1) hotspot mantle; (2) near-ridge seamounts by melting-induced mixing of two distinct mantle sources; (3) late-stage magmatism fed by melts that originated from an asthenospheric window due to slab delamination. Combining literature data from the entire YZSZ, which include studies of the MORB- and IAB-type rocks and mantle peridotites, we prefer the third explanation that the OIB-type rocks represent a late-stage magmatic activity above an intra-oceanic supra-subduction zone within the Neo-Tethys. [Copyright &y& Elsevier]

Abstract: The Gyangze basin, located in southern Tibet, is one of the representative areas for Cretaceous Oceanic Red Beds (CORBs). We investigated and inter-correlated several new CORB outcrops in the western and southeastern part of the Gyangze area in addition to the previously described Chuangde section. The CORBs in the Gyangze basin mainly consist of shales, thin-bedded marls, re-sedimented limestones and clasts supported conglomerates-breccias. Planktonic foraminifers in the carbonate rocks within CORBs belong to the Globotruncana ventricosa and Globotruncanita calcarata zones, indicative of middle Campanian age. The facies assemblages in the different sections indicate that the depositional environments of CORBs range from outer base-of-slope apron to basin zones. Within carbonates, we observed microfacies types characteristic of basin, open deep shelf margin or toe-of-slope environments. Gravity transport resulted from the steepening of the sea floor in the Gyangze basin owing to subduction during the Campanian. Relatively oxic bottom water conditions prevailed below the toe-of-slope environment in the Gyangze basin during CORB deposition. However, the presence of gray clasts within a multicoloured matrix suggests less oxic conditions at shallower water depths within the basin. [Copyright &y& Elsevier]

Quantitative analysis of Early Eocene radiolarian assemblages discovered in the sedimentary mélange (accretionary prism) of the Saga area, southern Tibet provides new information to constrain the timing of Tethys closure and the initial collision of India and Eurasia. The radiolarian species of Saga include Amphisphaera coronata (Ehrenberg), Buryella hannae Bak & Barwicz-Piskorz, Buryella clinata Foreman, Buryella tetradica Foreman, Calocycloma ampulla (Ehrenberg), Lamptonium fabaeforme constrictum Riedel and Sanfilippo, Lamptonium pennatum Foreman, Lithomespilus coronatus Squinabol and Lamptonium (?) colymbus Foreman. The adequate and reliable correlation of these radiolarians specimens indicates that the assemblage is of Early Eocene in age. The age and depositional environment of these radiolarians testify that deep ocean basins existed between India Plate and Asia Plate during the Early Eocene. The complete closure of Tethys must have taken place at least after the Early Eocene. [ABSTRACT FROM AUTHOR]

Presents a study that determined the organic geochemistry of the Cenomanian-Turonian anoxic event in southern Tibet, China. Analysis of the mid-Cretaceous samples; Use of direct combustion; Total organic carbon and its source.

Sedimentary basins adjacent to subduction‐related continental arcs provide important archives for deciphering the intricate history of convergent plate margins. The east‐west trending Gangdese magmatic arc was one of the most predominant topographic features located at the southern margin of Tibet before the arrival of the Indian plate. However, the detailed Cretaceous growth and evolution across the arc system remains ambiguous. Stratigraphy of the adjacent Xigaze forearc basin provides a well‐preserved and well‐exposed record of the tectonic and magmatic evolution of the arc throughout the Cretaceous period. We report new stratigraphic, sedimentological, geochronological, and provenance analyses of the Quarry Ridge sandstone in the Xigaze forearc basin along with compiled zircon U‐Pb ages (n = 9674) and Lu‐Hf isotopic signatures (n = 3389) from the Gangdese arc, the Xigaze forearc basin, and the Linzhou retroarc foreland basin to reconstruct the Early to middle Cretaceous magmatism and uplift of the Gangdese arc and concurrent sedimentary responses within both basins. Exhumation of the arc initiates at around 113 Ma suggested by arc detritus first arriving in both basins. Another episode of inferred uplift occurs at around 108 Ma, which resulted in coarse‐grained sedimentation in adjacent basins, preventing Central Lhasa detritus from reaching the Xigaze forearc basin further south and a facies and provenance change within the Linzhou basin. Finally, a third episode at around 101 Ma is reflected by deposition of the progradational Quarry Ridge clastic succession and marks the initiation of a substantial coarse‐grained depositional stage in the Xigaze forearc basin. Our study emphasizes the connection between coarse‐grained deposition in the forearc basin and arc magmatism and uplift. This study also provides an orogen‐scale assessment of the history of arc magmatism, uplift, and sedimentation across the Gangdese magmatic arc system, which supports interpretations that Tibet was already characterized by complex and substantial topographic relief during the Cretaceous before the collision between the Indian and Eurasian plates. [ABSTRACT FROM AUTHOR]

The questions of how and when the Neotethys Ocean opened are crucial for understanding the evolution of the Tethys Realm and the break-up of Gondwana. However, the evolution of the Neotethys remains controversial. Herein, we report the zircon U–Pb ages, whole-rock major as well as trace elements, and Sr–Nd isotopic data for the basaltic volcanic rocks from the Zhongba Terrane in the western Yalung–Zangbo Suture Zone. The results demonstrate that the Saga basalts were extruded during the Late Permian (257–258 Ma). All samples are characterized based on the low SiO2 content (44.03–50.33 wt%) and belong to the high-K calc-alkaline series. Furthermore, they exhibit light rare earth element enrichment ((La/Yb)N = 12.7–26.7); slightly positive Eu anomaly(Eu/Eu* = 1.01–1.14); prominent Nb and Ta positive anomalies; slightly negative Zr, Hf, and Ti anomalies and exhibit high ratios of Ta/Hf (0.93–1.21), Zr/Y (8.68–10.63), and Nb/Zr (0.39–0.45). The (87Sr/86Sr)i ratio varied from 0.7050 to 0.7063, with relatively constant and depleted εNd(t) values ranging from 0.52 to 1.08. The rocks were probably sourced from a low-degree partial melting of the asthenospheric mantle in a rift environment and had undergone the process of fractional crystallization. These characteristics indicate that the Saga basalts differ from the rift-related magmatic rocks produced by the Panjal Traps. Consequently, combined with the recently reported detailed fieldwork, the Zhongba Terrane disintegrated from the northern margin of Gondwana during the Late Permian, promoting the opening of the Neotethys Ocean, which is most likely caused by the northward subduction of the Paleotethys Ocean. [ABSTRACT FROM AUTHOR]

The >2000 km-long Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of the Mesozoic Neotethyan oceanic lithosphere, and is divided by the Zhada-Zhongba microcontinent into northern and southern branches in its western segment. Zircon U–Pb dating of a doleritic rock from the northern branch has revealed a concordant age of 160.5 ± 1.3 Ma. All of the doleritic samples from the northern branch and the pillow basalt and gabbro samples from the southern branch display consistent REE and trace element patterns similar to those of modern OIB-type rocks. The geochemical and Sr–Nd–Pb isotopic signatures of these OIB-type rocks from the western segment are identical with those of OIB-type and alkaline rocks from other ophiolite massifs along the central and eastern segments of the YZSZ, suggesting a common mantle plume source for their melt evolution. The enriched Sr–Nd–Pb isotopic character of the gabbroic dike rocks from the southern branch points to a mantle plume source, contaminated by subducted oceanic crust or pelagic sediments. We infer that the mafic rock associations exposed along the YZSZ represent the remnants of a Neotethyan oceanic lithosphere, which was developed as part of a plume-proximal seafloor-spreading system, reminiscent of the seamount chains along-across the modern mid-ocean ridges in the Pacific Ocean. [ABSTRACT FROM AUTHOR]

Methane-derived authigenic carbonates with distinctive structures and morphologies have been documented worldwide, but they are rarely found from ancient strata in the Eastern Tethys Ocean. The methane-derived authigenic carbonates found in southern Tibet are developed in calcareous or silty shales of mid-Cretaceous age in the Xigaze forearc basin and in the Tethyan Himalaya tectonic zone. The morphology, mineralogy, elemental geochemistry and composition of carbon and oxygen isotopes of these carbonates are studied in detail. The carbonates have nodular, tubular, and tabular morphologies. They are primarily composed of carbonate cement that binds and partly replaces host sediment grains; host siliciclastic sediments are composed mainly of quartz and plagioclase feldspar; a few foraminifers; and framboidal or subhedral to euhedral pyrite. Carbonate cements dominantly are micritic calcite, with minor contribution of dolomite. Nodular concretions are characterized by depleted δ 13 C values, commonly ranging from −30‰ to −5‰. The δ 13 C values show a gradual decrease from the periphery to the center, and the CaO, SiO 2 , Fe 2 O 3 , Al 2 O 3 , K 2 O, and TiO 2 contents generally show a gradual change. These features indicate that the nodular concretions grew from an early-formed center toward the periphery, and that the carbon source of the nodular concretions was derived from a mixture of methane, methanogenic CO 2 , and seawater-dissolved inorganic carbon. The tubular concretions are characterized by δ 13 C values of −8.85‰ to −3.47‰ in the Shangba Section, and −27.37‰ to −23.85‰ in the upper Gamba Section. Unlike the nodular concretions, the tubular concretions show central conduits, which are possible pathways of methane-rich fluids, suggesting that the cementation of tubular concretions begins at the periphery and proceeds inward. Moreover, the tubular concretions show morphological similarity with the methane-derived carbonate chimneys, pipes and slabs reported in present-day cold seep settings. We suggest that the carbon source of the tubular concretions was derived from a mixture of seawater-dissolved inorganic carbon and oxidized methane formed by released hydrate. The tabular concretions are characterized by δ 13 C values of −21.87‰ to −6.67‰ in the Xiege Sections. These depleted δ 13 C values suggest that the carbon of the tabular concretions was derived at least in part, from AOM. The tabular concretions are characterized by δ 13 C values of −28.81‰ to −12.99‰ in the Gamba Section. According to the δ 13 C values and field observation, we infer that their carbon source was more likely to be a mixture of the oxidized methane formed by released hydrate and seawater-dissolved inorganic carbon. [ABSTRACT FROM AUTHOR]

The Mid-Late Mesozoic magmatic arc in southern Qiangtang block was associated with the long-lasting subduction of the Bangong-Nujiang Tethyan and the subsequent Lhasa-Qiangtang collision. The detailed processes associated with the transition from oceanic lithosphere subduction to lithosphere delamination remain unclear. This paper reports zircon U-Pb age, major and trace element and Sr-Nd isotopic data for the Cretaceous two-phase magmatic rocks in the Shuanghu region of the southern Qiangtang block located in central Tibet. Zircon dating on the Qushenla volcanic rocks yields a 206Pb/238U age of 114 ± 4 Ma. The Abushan volcanic rocks comprise rhyolites. Zircon dating on two rhyolites yields a 206Pb/238U age of 74 ± 1 to 76 ± 1 Ma. In contrast with the Qushenla volcanic rocks, the Abushan rhyolites exhibit features typical of high-fractionated I-type granitoids, high K calc-alkaline and shoshonite, and peraluminous, suggesting that the Abushan rhyolites formed in post-collision setting. The Qushenla volcanic rocks have slightly higher initial 87Sr/86Sr (0.70466–0.70537), and εNd(t) (1.71–2.89) in comparison with the Abushan rhyolites (87Sr/86Sr = 0.70453–0.70491, εNd(t) = 1.63–2.35). The Qushenla volcanic rocks were most likely derived from the mixing of mantle-derived components and crustal material. And the crust assimilation and fractional crystallization played a role during the ascent of magma. The Abushan volcanic rocks derived from magmatic mixing between the crustal materials and mantle-derived components, and undergoing extensive fractional crystallization. Taking into account the regional tectonic and magmatic data, we suggest that the transition from the oceanic lithosphere subduction to lithosphere delamination occurred during the Late Cretaceous in central Tibet. It evolved through three stages: 125–100 Ma, late subduction of Bangong-Nujiang Tethys; 100–87 Ma slab break-off of subduction Bangong-Nujiang Tethys; and 87–73 Ma lithosphere delamination. [ABSTRACT FROM AUTHOR]

Abstract: Major and trace element, Sr–Nd–Pb and zircon U–Pb and Hf isotope data are presented for the newly discovered Abushan volcanic rocks in the southern Qiangtang Terrane, central Tibet. These results offer new insights into the evolution of the Tibetan plateau during Cretaceous. The Abushan volcanic rocks are composed mainly of andesites and trachy-andesites. Zircon U–Pb dating constrains the timing of emplacement as Late Cretaceous (79.9±2.7–75.9±0.49Ma). Major element geochemistry shows that the Abushan volcanic rocks belong to high-K calc-alkaline series. All the andesites and trachy-andesites are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), and depleted in high field strength elements (HFSE) and heavy rare earth elements (HREE). The geochemical features and ages of the Abushan volcanic rocks relate them with post-collisional setting. Furthermore, the volcanic rocks display negative εNd(t) (−3.1 to −2.5), relatively constant 87Sr/86Sr ratios (0.70614–0.70735) and negative εHf(t) values (−5.8 to −2.1). The Sr–Nd–Pb and Hf isotope signature suggests that the andesites and trachy-andesites were derived from the anatexis of mafic lower crust by intrusion or underplating of mantle-derived basaltic magma. The petrogenesis of Abushan andesites provides robust evidence for the crustal thickening, delamination and early uplifting of central Tibet during Late Cretaceous. [Copyright &y& Elsevier]

Abstract: The Yarlung Zangbo Suture Zone (YZSZ), southern Tibet, is a discontinuous belt that is more than 2000km long, composed of the remnants of Neo-Tethyan Mesozoic ocean. One of these relicts is the Xiugugabu ophiolitic massif which is a mantle thrust sheet of more than 260km2 overlying the Cretaceous tectonic mélange south of the YZSZ in SW Tibet. The massif is composed of harzburgites and clinopyroxene–harzburgites with porphyroclastic and porphyromylonitic textures. In the southern part of the massif, peridotites were intruded by amphibole-bearing microgabbro and microgabbronorite sills. A diabase unit which is overlaid by a sedimentary sequence crops out on the NE flank of the massif. Mineral chemistry in harzburgites and clinopyroxene–harzburgites indicates compositions similar to abyssal and forearc peridotites. Peridotites are slightly LREE depleted to enriched with [La/Yb]CN 0.06–2.8 and [La/Sm]CN 0.34–2.64. These ultramafic rocks are inferred to be the residues of 5–25% of partial melting of a depleted mantle that has been enriched by percolating metasomatic melts in a suprasubduction environment. Amphibole–microgabbro and amphibole–microgabbronorite sills are mostly composed of brown to green amphibole, calcic plagioclase, clinopyroxene, ilmenite and orthopyroxene in gabbronorite. Textures and compositions of the brown amphiboles indicate a near-solidus high temperature hydrothermal origin (>800°C). These intrusive rocks are tholeiitic and show N-MORB type REE patterns ([La/Yb]NC 0.35–0.90), a LILE (mainly Th) enrichment and noticeable Nb, Ta and Ti negative anomalies. They have a suprasubduction affinity and were formed in a back-arc basin setting. The diabase unit outcropping to the NE of the massif is not directly related to the ultramafic and mafic ophiolitic rocks. The diabase shows LREE enriched patterns ([La/Yb]NC 8–8.9) and slight Nb, Ta and Ti negative anomalies. The diabase has an intraplate affinity and could have been derived from a mantle source enriched by subduction-related fluids. The absence of continental crustal assimilation indicates that these rocks were probably emplaced in the Jurassic, in an oceanic environment after the Triassic disaggregation of the Indian plate. The data are consistent with the recent geodynamic model proposed for the central part of the suture for the closure of the Neo-Tethys and suggest that the geodynamic evolution of the western part of the basin was comparable to the central part. [Copyright &y& Elsevier]

Abstract: Strongly foliated amphibolite clasts are found embedded within the ophiolitic mélange underlying the Xigaze Ophiolite near Bainang and Angren, Yarlung Zangbo Suture Zone, Southern Tibet. These high-grade amphibolites are interpreted as remnants of a dismembered subophiolitic metamorphic sole that would have formed during the inception of a subduction. They include garnet-clinopyroxene amphibolites, clinopyroxene amphibolites and common amphibolites. Petrographic descriptions, mineral chemistry and thermobarometry of these rocks can be found in a companion paper [Guilmette, C., Hébert, R., Dupuis, C., Wang, C.S., Li, Z.J., 2008. Metamorphic history and geodynamic significance of high-grade metabasites from the ophiolitic mélange beneath the Yarlung Zangbo Ophiolites, Xigaze area, Tibet. Journal of Asian Earth Sciences, 32, 423–437.]. The geochemistry of the amphibolites confirms that their protoliths were igneous mafic rocks of basaltic to pyroxenitic composition that were likely part of an upper oceanic crust. Rare Earth Elements contents are suggestive of an N-MORB origin. However, enriched LILEs and depleted Nb–Ta–Ti when compared to N-MORBs rather suggest a suprasubduction zone influence. A large proportion of the overlying ophiolitic mafic rocks share the same geochemical characteristics, suggesting the protoliths of the amphibolites might have crystallized in the same environment as the Xigaze ophiolitic crust, likely in a back-arc basin. 40Ar/39Ar step-heating dating of hornblende concentrates from three samples yielded ages of 123.6±2.9 Ma, 127.7±2.2 Ma and 127.4±2.3 Ma. These cooling ages are slightly younger or overlapping magmatic and sedimentary ages obtained from the overlying ophiolite. All these new data support a model in which the ophiolitic crust and the protolith of the amphibolites were formed along the same spreading center above a subduction zone. The demise of the early subduction circa 130 Ma forced the inception of a new subduction zone at the SSZ spreading axis, burying SSZ mafic rocks underneath a SSZ ophiolitic mantle. [Copyright &y& Elsevier]

In past decades, the formation of dolomite at low temperature has been widely studied in both natural systems and cultured experiments, yet the mechanism(s) involved in the nucleation and precipitation of dolomite remains unresolved. Late Eocene dolomitic deposits from core in the upper Niubao Formation (Lunpola Basin, central Tibetan Plateau, China) are selected as a case study to understand the dolomitization process(es) in the geological record. Dolomite formation in Lunpola Basin can be ascribed to a different mechanism forming the large quantities of replacive dolostones in the geological record; and provides a potential fossil analogue for primary dolomite precipitation at low temperature. This analogue consists of an alternation of laminated dolomitic beds, organic‐rich and siliciclastic layers; formed in response to intense evaporation interpreted to take place in a continental shallow lake environment. Mineralogical, textural and stable isotopic evaluations suggest that the dolomite from those dense‐clotted laminated beds is a primary precipitate. At the nanoscale, these dolomitic beds are composed of Ca–Mg carbonate globular nanocrystals (diameter 80 to 100 nm) embedded in an organic matrix and attached to clay flakes. Micro‐infrared spectroscopy analyses have revealed the presence of aliphatic compounds in the organic matrix. Microscopic and elemental compositional studies suggest that clay surfaces may facilitate the nucleation of dolomite at low temperature in the same way as the organic matrix does. The dolomite laminae show values for δ18OVPDB from −3.2 to −1.76‰ and for δ13CVPDB from −2.62 to −3.78‰. Inferred δ18OSMOW values of the lake water reveal typical evaporitic hydrological conditions. These findings provide a potential link to primary dolomite formation in ancient and modern sedimentary environments; and shed new light on the palaeoenvironmental conditions in central Tibet during the Eocene. [ABSTRACT FROM AUTHOR]

The early Aptian oceanic anoxic event (OAE 1a) is well known in the western Tethys, the North Atlantic and the Pacific Ocean, but has not been reported in the eastern Tethys to date. In this paper, we present bulk organic carbon isotope data and ammonite biostratigraphy of a lower Aptian succession from the Gucuo area (southern Tibet). These findings document the occurrence of the OAE 1a for the first time from the eastern Tethys. The studied sequence can be attributed to the D. forbesi and D. deshayesi ammonite zones of the lower Aptian. The δ 13 C org data can be correlated with published early Aptian carbon isotope records from the western Tethys and the Pacific. A distinctive negative carbon isotope excursion of 2.4‰ in the upper part of the section corresponds to segment C3 of the OAE 1a, and the following positive excursion correlates to segment C4. The absolute values of the carbon isotope ratio in the Gucuo area are higher than those of known sections in the western Tethys and equatorial Pacific. We suggest that diagenetic alteration is the major cause of the higher absolute values in the Gucuo area. [ABSTRACT FROM AUTHOR]

A radiolarian biostratigraphic study ranging from the latest Barremian to the Aptian of a pelagic sequence (section BB1) near Babazhadong in southern Tibet revealed phyletic relationships of species belonging to the genus Turbocapsula . Three species and two subspecies are included in this genus: Turbocapsula tetras LI & MATSUOKA, n. sp., T. fugitiva O'DOGHERTY, and T. costata ( T. costata (WU) multicostata LI & MATSUOKA, n. ssp. and T. costata (WU) costata ). Turbocapsula fugitiva appears to have arisen from T. tetras by losing the septa between the third and the fourth segments. Turbocapsula fugitiva seems to have given rise to T. costata multicostata following the development of thicker costae between its two to five rows of pores. With the decrease in the number of the costae and the development in the costae, T. costata multicostata evolved into T. costata costata . The phyletic evolution of the genus Turbocapsula has a high stratigraphic value. Based on the phyletic evolution of Aurisaturnalis and Turbocapsula , two radiolarian zones have been defined: the Aurisaturnalis carinatus Zone and the Turbocapsula costata Zone. These zones are defined by evolutionary first appearance bio-horizons (EFABs), which are more reliable than the first and the last occurrence bio-horizons over wide areas. The Turbocapsula costata Zone is subdivided into the Turbocapsula costata multicostata Subzone and the Turbocapsula costata costata Subzone. Based on comparisons with radiolarian biostratigraphic research conducted in other areas, in which age-diagnostic calcareous nannofossils and planktonic foraminifera occur alongside radiolarians, the Aurisaturnalis carinatus Zone and the Turbocapsula costata Zone are assigned to the Lower Hauterivian–Lower Aptian and the Lower Aptian–Lower Albian, respectively. This newly proposed zonation is useful for comparing the stratigraphic relationships of Turbocapsula -bearing horizons in other areas. A zonal scheme like this scheme can be used for identifying certain episodes such as mid-Cretaceous oceanic anoxic events (OAEs) in the pelagic realms. The Early Aptian OAE 1a is located between the first appearance bio-horizon of the genus Turbocapsula and the EFAB of T. costata multicostata . [ABSTRACT FROM AUTHOR]

The lack of magmatic records with high-quality geochronological and geochemical data in the central segment of the southern Qiangtang subterrane in central Tibet inhibits a complete understanding of the subduction polarity of the Bangong–Nujiang Ocean lithosphere during the Mesozoic. In this study, we present the zircon U–Pb age as well as geochemical and Sr–Nd–Pb isotopic data for the Late Jurassic pluton from the Kangqiong area in the central segment of the southern Qiangtang subterrane. The Kangqiong pluton primarily consists of granodiorites (SiO 2 = 62.87–65.17 wt.%) and was emplaced in the Late Jurassic (147.6 ± 2.4–149.9 ± 2.1 Ma). The granodiorites display high Na 2 O numbers (Na 2 O/K 2 O = 1.75–2.24) as well as high MgO (2.21–3.14 wt.%) and Mg-numbers (53–58), are characterized by a low abundance of heavy rare earth elements (e.g., Yb = 1.05–1.92 ppm) and Y (12.63–17.52 ppm), and high Sr/Y (29–61) and La/Yb (14–18) ratios, which are comparable in composition to those of slab-derived adakitic rocks. The Kangqiong adakitic granodiorites have initial ( 87 Sr/ 86 Sr) i ratios of 0.70611 to 0.70669, negative ε Nd (t) values (− 1.06 to − 0.25), ( 206 Pb/ 204 Pb) t ratios of 18.42 to 18.47, ( 207 Pb/ 204 Pb) t ratios of 15.62 to 15.63, and ( 208 Pb/ 204 Pb) t ratios of 38.50 to 38.60. These geochemical signatures indicate that the magmas were most likely derived from the partial melting of the subducted Bangong–Nujiang oceanic crust and minor contaminants from the accretionary complex. Our results, in combination with the coeval magmatism in the western segment of the southern Qiangtang subterrane, indicate that the Bangong–Nujiang oceanic lithosphere was subducted northward beneath the Qiangtang Terrane, forming a west-east magmatic arc over 800 km during the Late Jurassic. [ABSTRACT FROM AUTHOR]

The Avengco Basin is located in the western part of the Tibetan Plateau and is similar to the Nima Basin in the central part of the plateau and the Lunpola Basin in the eastern part in terms of sedimentary characteristics and tectonic settings, which are well known to provide a good source rock potential. However, the organic geochemical characteristics of the Paleocene-Eocene potential source rocks in the Avengco Basin have been under debate. Thirty-four marl and mudstone outcrop samples of the Niubao Formation in the Avengco Basin were collected and subjected to the following analyses: total organic carbon (TOC), Rock–Eval pyrolysis, stable carbon isotopes of kerogen, gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). Here, we present the results indicating the organic matter of the upper Niubao Formation is mainly composed of Type II kerogen with a mixed source, which is dominated by algae. However, the lower Niubao Formation has the less oil-prone Type II–III kerogen, and the sources of the organic matter are mainly terrestrial plants with less plankton. In addition, the samples are thermally immature to marginally mature. The Niubao Formation was deposited in an anoxic–oxic environment which was brackish with an imperceptible stratified water column. The upper Niubao Formation has a medium to good hydrocarbon-generating potential. However, the lower Niubao Formation has a zero to poor hydrocarbon-generating potential. [ABSTRACT FROM AUTHOR]

Growing geologic evidence documents incremental Mesozoic and early Cenozoic shortening and thickening of the Tibetan crust prior to the onset of the main Cenozoic orogenic event. The Tibetan crust shows spatial and temporal variability in thickness, style, and timing of thickening, and in plateau-forming processes. The Songpan–Ganzi area of northeastern Tibet provides evidence for shortening and thickening of the crust in Late Triassic time. An oil exploratory well (HC-1) of 7012.4 m located in the area shows at least six tectonic repetitions, resulting in more than ∼46% thickening of the Triassic sequence. It indicates that the true thickness of the Songpan–Ganzi Triassic flysch is not 10–15 km as previously assumed, but not more than 3–5 km. Based on this evidence, combined with prior tectonostratigraphic studies, we propose that substantial crustal shortening and thickening, leading to initial plateau formation in the northeastern Tibetan Plateau, had already occurred during the Late Triassic. [ABSTRACT FROM AUTHOR]

The mid-Cretaceous greenhouse climate was induced by volcanic outgassing and the release of biogenic or thermogenic methane into the ocean-atmosphere system. During this period, major episodes of oceanic anoxic conditions enabled the large scale deposition of marine black shales rich in organic carbon, serving as a source for methane production. Studies on the anaerobic oxidation of methane, the key biogeochemical process at sites where methane is transported toward the seafloor, and its associated authigenic carbonates from mid-Cretaceous strata can contribute to elucidate such extreme climatic conditions. A total of nine layers of authigenic carbonates were recognized in the sequence of mid-Cretaceous inner shelf sedimentary rocks of the Qiangdong section, Gamba area, southern Tibet. In this study, we decipher the mode and causes of carbonate authigenesis by combining field observation, thin section petrography, mineralogy, and carbon and oxygen stable isotope geochemistry. The lowest obtained δ13C carb value of −30.5‰ is not low enough to exclude oil compounds as a carbon source, but since other evidence of oil seepage is lacking, methane is the most likely carbon source that was admixed to dissolved inorganic carbon to form authigenic carbonate. Typical seep carbonate phases including 13C depleted matrix micrite, clotted micrite, and banded and botryoidal cement as well as three types of authigenic pyrite comprising framboids, zoned aggregates with radial overgrowths surrounding a framboidal core, and euhedral pyrite crystals indicate the occurrence of methane seepage in the mid-Cretaceous depositional environments. In situ brecciation and subvertical tubular carbonates – the former reflecting rupture caused by local fluid overpressure and the latter interpreted as part of a plumping system – agree with widespread and vigorous seepage. Expanded shallow and deep-water anoxia in combination with low seawater sulfate concentration prior to the oceanic anoxic event 2 (OAE 2) event apparently caused an increased flux of organic matter to the methanogenic zone, stimulating methanogenesis and enabling a higher flux of methane toward the seafloor. In the water column, aerobic oxidation of methane would have enhanced oxygen depletion, thereby contributing to black shale deposition and providing a positive feedback to organic matter preservation and burial. Based on correlation with coeval authigenic carbonate deposits from the Tethyan continental margin, we conclude that organic matter accumulation and sea-level change were the main factors controlling carbonate authigenesis in the mid-Cretaceous. • Deciphering the causes of mid-Cretaceous authigenic carbonates in southern Tibet. • Compilation shows common causes for carbonate authigenesis in the mid-Cretaceous. • OAE 2 anoxia in the studied area was accelerated by hydrocarbon seepages. [ABSTRACT FROM AUTHOR]

Abstract: Blocks of highly foliated amphibolite are locally embedded within a serpentinite mélange underlying the Yarlung Zangbo ophiolites in the Xigaze area of southern Tibet. The ophiolites are remnants of an Early Cretaceous back-arc basin within the Permo-Cretaceous Tethys Ocean, which are exposed along in the Yarlung Zangbo Suture Zone (YZSZ). These amphibolites are interpreted as fragments of a dismembered dynamothermal sole. Three types of amphibolite are present: (1) common amphibolite with assemblages of Hbl+Pl±Ep±Ap±Ttn, (2) clinopyroxene-bearing amphibolite with Hbl±Pl±Cpx±Ep±Ttn±Qtz±Ap and (3) garnet–clinopyroxene-bearing amphibolite characterized by the assemblages Hbl+Cpx+Grt+Pl±Rt and Grt+Hbl+Pl (corona assemblage). In all three types, plagioclase is pseudomorphed by late albite–prehnite. Retrograde cataclastic veins containing assemblages of Prh+Ab+Ep±Chl are also present. P–T estimates indicate that the amphibolites reached peak metamorphic conditions of 13–15kbar and 750–875°C. Partial replacement of pyrope-rich (up to 35 mole%) garnet by Al-tschermakite (Al2O3 up to 21 wt%) reflects a high pressure (≈18kbar, 600°C) metamorphic event followed by rapid exhumation. Soon after exhumation, the amphibolites were intruded by very fine-grained diabase dykes that were then hydrothermally altered. The field relationships and metamorphic history of the amphibolites indicate formation during inception of subduction within a back-arc basin prior to obduction of the ophiolites onto the Indian passive margin. [Copyright &y& Elsevier]

The Beimarang massif is one of many ophiolitic remnants which crop out discontinuously along the Yarlung Zangbo suture zone in southern Tibet. The southern contact of these remnants is marked by a highly sheared serpentinite me´lange made up of blocks of serpentinites, diabases, gabbros and composite blocks of serpentinites and mafic injections. The Beimarang me´lange has been investigated in order to bring additional constraints on the origin, metamorphic evolution and obduction processes of the Yarlung Zangbo ophiolite.Petrography and geochemical data suggest that the ultramafic components are similar to moderately depleted upper mantle peridotites. They may represent the already cooled and serpentinized Tethyan upper mantle which was trapped in a mantle wedge at the onset of a north-dipping Early Cretaceous intra-oceanic subduction zone located south of the Gangdese arc. These peridotites were then intruded by back-arc-like mafic magmas whose moderately depleted mantle source was affected by a subduction component. Ultramafic and mafic secondary mineral assemblages suggest that early low-(P/T) metamorphic intra-oceanic conditions reached the amphibolite facies (>550 °C) before being retrograded down to the pumpellyite–prehnite facies (<280 °C). The Beimarang me´lange, interpreted as an obduction me´lange formed near a spreading ridge, was subjected to metamorphic conditions in the pumpellyite–prehnite facies which favored re-serpentinization of the peridotites and partial rodingitization of the mafic rocks. Unlike subduction-related me´langes and their associated lithological units found in the Ladakh area, we found no evidence of high-(P/T) conditions in lithologies from the Beimarang me´lange. [Copyright &y& Elsevier]

• The structures indicate an E-W directed shear associated with sinistral strike-slip. • The deformation is coherent with the eastward extrusion in the eastern Himalaya. • This remagnetization reveals a ~16° clockwise rotation of the Zhongba microterrane. • A pyrrhotite component is acquired post folding as suggested by stepwise fold tests. The India-Asia collision produced the Himalayan orogenic belt. The kinematics of crustal blocks within the orogenic belt can shed insights into the tectonic processes related with the continental collision. We report a combined study of paleomagnetism and microtextures from the Zhongba microterrane, northwestern Tethys Himalaya. The Zhongba microterrane is located in the southwestern part of Tibet. A total of 71 samples collected from Permian formations yield a distinct remanent component carried by pyrrhotite, which failed fold tests. Compared with the apparent polar wander path of India, it is suggested that this overprint component is acquired recently (10–0 Ma), and a ~16° clockwise rotation is also indicated. Microtextures from several localities show E-W directed shear. Together with reported GPS observations and clockwise rotations, a bookshelf-style deformation pattern associated with strike-slip faulting is proposed for the Neohimalayan tectonics in the Tethys Himalaya. Bookshelf-style rotation of small blocks coordinated by strike-slip faults is not only a practical model to absorb the crustal shortening caused by the convergence of the Indian-Eurasian plate, but also an important way for the escape of the Tibetan Plateau to the southeast. [ABSTRACT FROM AUTHOR]