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Detailed microfacies analysis of carbonate rocks from the Tingri and Nyalam areas of southern Tibet allowed us to reconstruct the evolution of sedimentary environments and relative sea-level changes during the Early to Middle Jurassic. Based on texture, sedimentary structure, grain composition, and fossil content of ~ 500 thin sections, 17 microfacies overall were identified, and three evolutionary stages of sedimentary environments were defined. Stage 1 (Rhaetian?–lower Sinemurian Zhamure Formation) is characterized by siliciclastic and mixed siliciclastic-carbonate sedimentation on a barrier island environment. Stage 2 (upper Sinemurian–Pliensbachian Pupuga Formation) is characterized by high-energy grainstones with rich benthic faunas thriving on a carbonate platform, documenting transgressions in the early and late Pliensbachian. Stage 3 (Toarcian–lower Bajocian Nieniexiongla Formation) is characterized by low-energy mudstones intercalated with frequent storm layers on a carbonate ramp, documenting the climax of the early Toarcian transgression in the beginning of the unit. The transition from Stage 1 to Stage 2 points to a paleogeographic and paleoclimatic change, as the Tibetan Tethys Himalaya moved toward tropical latitudes characterized by highly suitable climatic and ecological conditions for carbonate sedimentation. The abrupt change from Stage 2 to Stage 3 is interpreted as a consequence of the early Toarcian Oceanic Anoxic Event, accompanied by short-term warming and sea-level rise. The failed recovery from the carbonate crisis in the early Bajocian, with continuing deposition on a low-energy carbonate ramp, may reflect tectonic migration toward higher southern latitudes and/or a reduction of the effect of warm tropical currents reaching the southern Tethys. [ABSTRACT FROM AUTHOR]

The Xigaze forearc basin provides information on subduction evolution and magmatic growth of the Gangdese arc as well as on the India–Asia continental collision. Recently obtained sedimentological, biostratigraphic, petrographic, geochemical and geochronological data on Cretaceous to Paleogene strata in the Cuojiangding area (Zhongba county, south Tibet) shed new light on the tectonic evolution of the southern margin of the Lhasa Block during closure of Neotethys and initial collision with India. The uppermost Cretaceous Padana and Qubeiya formations, deposited in deltaic to inner shelf environments, and representing the final filling of the Xigaze forearc basin, were unconformably overlain by the Quxia and Jialazi formations, deposited in fan-delta environments during the Paleocene/earliest Eocene. Petrographic data and U–Pb ages of detrital zircons document the progressive unroofing of the Gangdese arc, which remained the dominant source of detritus throughout the Late Cretaceous to Paleogene. Detrital Cr-spinels in the Quxia and Jialazi formations are geochemically similar to those in Cretaceous Xigaze forearc strata but different from those hosted in Yarlung Zangbo ophiolites, suggesting that the latter were not exposed to erosion in the considered time window. Sandstone petrography, Cr-spinel-geochemistry, U–Pb age spectra and Hf isotopic ratios of detrital zircons in the Quxia and Jialazi formations match those in Paleogene sediments deposited on the distal (Sangdanlin and Zheya formations) and proximal Indian margin (Enba and Zhaguo formations), suggesting that the Quxia and Jialazi formations documents syncollisional fan-deltas deposited on top of the nascent Himalayan orogenic belt. In this scenario, the onset of the India–Asia collision predates deposition of the Quxia and Jialazi formations and is thus constrained as younger than 66 Ma and older than 58 Ma. [ABSTRACT FROM AUTHOR]

Abstract: The geochemistry of detrital chromian spinels is commonly used to discriminate provenance from different tectonic settings of mafic and ultramafic igneous rocks. Detrital spinels in Cenozoic foreland-basin successions fed from the Himalaya Orogen were assertively interpreted as sourced from the ophiolitic rocks of the Indus-Yarlung suture zone. This study compares the geochemistry of detrital Cr-spinels from the Tethys Himalaya passive margin and Cretaceous Xigaze forearc successions with those from the Indus-Yarlung ophiolites. Cr-spinels in the Indus-Yarlung ophiolites have low TiO2 (mostly <0.2%) and high Al2O3 (10–48%). Detrital Cr-spinels from the Tethyan Himalaya have instead high TiO2 (mostly >0.2%) and low Al2O3 (mainly 6–23%), indicating a rift-related basaltic origin. Detrital Cr-spinels from the Xigaze forearc basin have either low TiO2 (mostly <0.2%) and low Al2O3 (4–34%), suggesting provenance from a supra-subduction-zone peridotite, or high TiO2 (>1.0%), indicating intra-plate basaltic origin. Compositional fingerprints of detrital Cr-spinels from Lower Eocene foreland-basin strata in the central-eastern Himalaya indicate provenance from the Lhasa Block without input from the Indus-Yarlung ophiolites. Only Cr-spinels from the Lower Eocene foreland-basin strata in the north-western Himalaya and the Upper Eocene–Lower Miocene remnant-ocean turbidites of the Bengal basin are mostly ophiolite-derived. The Indus-Yarlung ophiolites were thus emplaced and exposed to erosion since the Early Eocene (>50Ma) in the NW Himalaya, but only subsequently (50–38Ma) in the eastern Himalaya. [Copyright &y& Elsevier]

Abstract: Sm–Nd isotopic systematics has been proven to be a powerful tool for investigating the source provenance and tectonic significance of sedimentary rocks. Here, I report the Nd isotopic compositions of Mesozoic to Paleogene siliciclastic sediments from southern Tibet in an attempt to determine the temporal variations in their provenance. The data reveal a shift in ε Nd(0) values of Mesozoic strata in the Tethyan Himalaya, varying from −16.3 to −12.7 in the Jurassic to −8.1 to −3.7 in the Cretaceous. The less negative ε Nd(0) values of the Cretaceous siliciclastic rocks indicate increasing input of a juvenile mantle component from the source region. Such a shift can be attributed to the Early Cretaceous mantle-derived magmatism that was predominant in northern Greater India. This study indicates that there is a significant overlap in the ε Nd(0) values of rocks from the Tethyan Himalaya and Lhasa block, implying that Nd isotopes may not be a good provenance proxy to constrain the initiation of the India–Asia collision. Such an inference is also evidenced by similar Nd isotopic compositions between rocks from the Bhainskati Formation in the Nepalese foreland basin, sourced from the Tethyan Himalaya, and the Enba and Zhaguo formations in southern Tibet, sourced from the Lhasa block. [Copyright &y& Elsevier]

Abstract: Lower Cretaceous volcanic lithic arenites, widely distributed in the Tethyan Himalaya, provide insights into the continental breakup of Eastern Gondwana. In southern Tibet they are represented by the Wölong Volcaniclastics. The volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts, whereas rhyolitic/dacitic volcanism becomes the predominant source of the upper strata. Geochemical analyses of basaltic grains and of detrital Cr-spinels from the Wölong Volcaniclastics demonstrate the alkaline character of the volcanism and suggest “within-plate” tectonic setting for the volcanism. Zircon U–Pb ages confirm that this volcanism continued from ∼140Ma to ∼119Ma. Hf-isotope data on these Early Cretaceous zircons indicate that their parental magmas were mantle-derived, but in the later stage of magmatic activity mantle-derived magmas were mixed with partial melts derived from the continental crust. The Lower Cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of Greater India. The onset of volcaniclastic deposition in the Himalayas appears to become progressively younger toward the west, but it ended synchronously during the Late Albian (∼102Ma). The low volume of volcanic rocks and their intra-plate tectonic setting suggest that they are the result of decompressional melting along extensional deep-seated fractures cross-cutting the continental crust, and reflect changes in regional intra-plate tectonic stresses when Greater India began to separate from the Australia–Antarctica supercontinent. [Copyright &y& Elsevier]

Abstract: Planktonic foraminiferal fossil assemblages identified from the Bolinxiala Formation in Bolin, Zanda, southwestern Tibet of China, determine its age from latest Albian to Maastrichtian. The fossil contents of the Bolinxiala Formation allow its correlation with successions across a platform-to-basin transect of the Late Cretaceous Tethyan Himalaya passive margin. The ocean anoxic event at the Cenomanian–Turonian transition (OAE2) is located at the Whiteinella archaeocretacaea biozone in Zanda, but lithologically it is characterized by grey and bioturbated limestone, implying that during the OAE2 the shallow-water environments of the Tethyan Himalayan carbonate platform remained oxic. Paleogeographic reconstruction indicates that the Upper Cretaceous Oceanic Red Beds (CORBs) in southern Tibet are restricted to the slope and basinal environments but they are entirely missing in the shelf environments. This phenomenon suggests the formation of CORBs by oxidation of Fe(II)-enriched anoxic deep ocean seawater at the chemocline that separated the oxic surface ocean from anoxic deep ocean. For depositional environments above the chemocline, no CORBs would be expected. Because of the chemocline instability across different sedimentary basins, CORBs may be significantly diachronous, consistent with the occurrence of CORBs documented from global sedimentary basins. [Copyright &y& Elsevier]

Abstract: Extensive tectonic activity in southern Tibet (Tibetan Tethys Himalayas) resulted in overthrusting and tectonic deformation of Cretaceous strata, which in the study area was not recognized by earlier researchers. Field studies of three typical cross-sections in southern Tibet has led to the revision of previous stratigraphy. The thickness of the Cretaceous in Gyangze varies between 300 and 700m, but is not over 2000m as previously estimated at Gyabure and Weimei, where the strata are overturned and repeated owing to previously unrecognized thrusts. In upward stratigraphic order four lithological markers were used in the field study: belemnite and black marker, lenticular limestone and white marker, red marker, and olistolith marker. The first two of these are associated with the fine-grained clastic shelf sequence of the Gyabula Formation, which is of Berriasian–early Santonian age. The third is the deep-sea red-beds marker of the Chuangde Formation, dated as middle Santonian–early Campanian. The fourth depicts the slump facies and olistolith of the middle Campanian–Paleocene Zongzhuo Formation. The revised Cretaceous stratigraphy provides a framework for studies of the pelagic red beds of the Chuangde Formation, which is intermittently exposed in southern Tibet. [Copyright &y& Elsevier]

Abstract: Red marine mudstones intercalated with pelagic marlstones, limestones and radiolarian cherts comprise the Chuangde Formation, which overlies mid-Cretaceous dark grey shales in the northern subzone of the Himalayan Tethys of southern Tibet. The red mudstones reflect deposition below the carbonate compensation depth (CCD) in a deep oceanic basin. The intercalated, thin-bedded marlstones represent fine-grained turbidites derived from the upper slope and transported into the adjacent deep basin. Planktonic foraminifera and nannofossils in the marlstones indicate a Santonian–early Campanian age for the formation. The red beds of the Chuangde Formation were deposited in a highly oxygenated deep-sea environment. This is confirmed by the high iron-trioxide content, the negative Cerium anomaly at the bottom of the red sequence, and the very low total organic carbon content (TOC 0.01–ca. 0.14%). Deposition of the red beds coincided with a Santonian–early Campanian diversity peak of planktonic foraminifera. The environmental interpretation is supported by the bulk carbon isotope δ13C value of carbonate turbidites that becomes more positive stratigraphically up from the base of the formation. The highly oxygenated bottom-water conditions were not restricted to a deep ocean basin, but extended up to the continental rise/slope, as indicated by synsedimentary red marls incorporated in slumps and olistoliths near the top of the formation. This indicates that not only the bottom waters but also intermediate waters were highly oxygenated. Deposits of the Chuangde Formation are similar to Upper Cretaceous oceanic red beds (CORBs) in Western Europe, such as the Scaglia Rossa in Italy and the Capas Rojas in southern Spain. They represent the easternmost occurrence of Upper Cretaceous pelagic red beds of the Tethys. Late Cretaceous oxic oceanic conditions extended into the southern Tethys because the formation was deposited in southern palaeolatitudes. Therefore, the change from a dysoxic/anoxic to an oxic bottom ocean environment during the Late Cretaceous was at least hemiglobally if not globally wide. The change to oxic bottom conditions occurred later in the southern Tethys than in the western Tethys, where it began during the late Turonian. Several different earth processes could have caused such change. We suggest that the oxidized character of these deep-sea deposits was a result of a combination of various processes, among which very low sedimentation rates and changes in bottom ocean circulation may have been the most important. [Copyright &y& Elsevier]

During the Paleocene–Eocene Thermal Maximum (PETM, ~56 Ma), a large, negative carbon-isotope excursion (CIE) testifies to a massive perturbation of the global carbon cycle. Shallow-marine settings are crucial to understand the environmental and ecological changes associated with the PETM and the connection between continental and open-marine environments. Detailed sedimentological, paleontological, and geochemical analysis of a quasi-continuous succession of shallow-marine carbonates in the Tethys Himalaya of southern Tibet indicates that a relative rise in sea level coincided with PETM onset, continued through PETM core, and terminated with a regression at PETM recovery. At PETM onset, corresponding to the SBZ4/SBZ5 boundary, no obvious impact on biota and specifically on larger benthic foraminifera (LBF) is observed. The major biotic change occurs later on at PETM recovery, corresponding to the SBZ5/SBZ6 boundary. Our data suggest that relative sea level, rather than temperature, exerted the main control on benthic biota during the PETM. Although the δ13C org values of organic matter are similar in the deep sea and shallow-marine continental margins, the δ13C carb value of bulk carbonates are significantly 13C-depleted, which we attribute to environmental change driven by relative sea-level fluctuations. • A rise in sea level at the PETM onset, continued through the core, and terminated with a regression at the recovery. • No obvious change in marine biota at the PETM onset, rather the major biotic change occurs later in the PETM recovery. • Strongly 13C-depleted values of shallow-marine carbonates might be partly related to sea level fluctuations. [ABSTRACT FROM AUTHOR]

Lower Cretaceous marine strata are widely distributed and well exposed in the Tingri area of Southern Tibet, which includes the Gucuocun and the overlying Gambacunkou Formations. However, the age of the boundary between these two lithologic formations is poorly constrained due to a scarcity of macrofossils. A detailed biostratigraphic study was carried out on the abundant, moderately preserved calcareous nannofossil assemblages obtained from the lower part of the Gambacunkou Formation and the underlying Gucuocun Formation in the Kangsha Section, Tingri. The first occurrence of the marker species Tranolithus orionatus (110.73 Ma) constrains the Gucuocun/Gambacunkou boundary to slightly below the lower boundary of Subzone NC8c (early Albian age), which correlates with Ocean Anoxic Event (OAE) 1b. The absence of Axopodorhabdus albianus and Eiffellithus turriseiffelii throughout the section suggests that the lower part of the Gambacunkou Formation mostly falls within nannofossil Subzone NC8c (110.73–109.94 Ma; early Albian), with a mean sedimentation rate of 12.7 cm/kyr, which is higher than that typical of the open ocean. The consistently high total organic carbon (TOC) and CaCO 3 contents in Interval I (Gucuocun Fm.) indicate that productivity was higher during OAE1b, possibly due to increased terrigenous input (as evidenced by elevated C/N and nutrient/fertility indices) that carried nutrients to promote productivity during this event. Nannofossil assemblages and geochemical proxies, including TOC and elemental ratios (Ti/Al, Zr/Al, K/Al), reveal a parallel upsection increase in surface water fertility and continental weathering intensity from Interval II-1 (0–60 m) to the overlying Interval II-2 (60–102 m) in the Gambacunkou Fm., suggesting that enhanced continental weathering contributed to increase the surface water fertility and dilute the sediment carbonate content. These data also indicate that the depositional environment in the Tingri area was aerobic to hypoxic during the early Albian. Our result confirms the presence of OAE1b at the top of the Gucuocun Formation. • Calcareous nannofossils of the Gucuocun and Gambacunkou Formation, southern Tibet, were studied. • The formation boundary constrained to slightly below the Subzone NC8b/NC8c boundary of early Albian age. • Confirms the presence of OAE1b at the top of the Gucuocun Formation. • High productivity during OAE1b possibly due to increased terrigenous input. [ABSTRACT FROM AUTHOR]

The recycling of continental crust via trench sediment subduction and subduction erosion is well established in modern oceanic subduction zones, and this is fundamental to subduction zone magmatism and mantle heterogeneity. Although a similar process likely took place in ancient oceanic subduction zones, its evaluation is difficult because most geological records were erased by subsequent continental collision and post-collisional reworking processes. To address this issue, a comprehensive geochemical study was conducted for Mesozoic basaltic to andesitic plutonic rocks and surviving contemporaneous paleo-trench sediments from the Himalayan orogen in southern Tibet. These plutonic rocks show variably enriched Sr-Nd-Pb-Hf isotope compositions with ε Nd (t) values varying from −5.6 to +4.7, which are neither positively nor negatively correlated with Mg# values. This indicates that they were not affected by crustal contamination but rather inherited from heterogeneous magma sources. There are consistently continuous variations in the Sr-Nd-Pb-Hf isotope space among the paleo-trench sediments, the arc plutonic rocks, and basaltic rocks from the Yarlung Tsangpo ophiolite suites, indicating that the binary mixing between the paleo-trench sediment-derived melts and the ambient mantle dictated the radiogenic isotope compositions of the arc igneous rocks. These qualitative interpretations are further verified by quantitative geochemical examination of the compositional links in Nd and Hf isotopes between the paleo-trench sediments and arc rocks. As a consequence, the present study has for the first time identified and systematically demonstrated the key role of paleo-trench sediments in constraining the continental crust recycling as well as the origin and compositional heterogeneity of arc igneous rocks above ancient oceanic subduction zones. • Paleo-trench sediments can constrain crustal recycling in ancient subduction zones. • Geochemical features of the Gangdese arc plutonic rocks vary greatly along strike. • The covariations between element and isotope ratios are the feature of primary magmas. • Source composition and melting degree have dictated the composition of the arc rocks. • Mantle sources of the arc magmas would containtrench sediment-derived melts. [ABSTRACT FROM AUTHOR]