Jian-Gang Wang

Provenance of the Upper Cretaceous–Eocene Deep-Water Sandstones in Sangdanlin, Southern Tibet: Constraints on the Timing of Initial India-Asia Collision

The first arrival of sedimentary material from Asia onto the Indian continental margin provides a minimum constraint on the timing of initial India-Asia collision. A combination of petrology, detrital Cr-spinel geochemistry, and zircon U-Pb dating of the Upper Cretaceous to Eocene deep-water succession at the Sangdanlin section, southern Tibet, provides evidence for major provenance change within the strata. The Upper Cretaceous–Paleocene Denggang Formation quartzarenites contain zircons with dominant Proterozoic-Ordovician U-Pb ages, with an additional age peak of Early Cretaceous, which we interpret to be derived from the northern Indian margin. By contrast, the lithic sandstones of the Early to Middle Eocene Sangdanlin and Zheya formations are dominated by zircons younger than 200 Ma, showing one major peak at ∼80–125 Ma and two subdominant peaks at ∼54–70 and ∼180–196 Ma, comparable to those from the Gangdese magmatic arc. Cr-spinels in the Sangdanlin and Zheya formations are abundant and characterized by extremely low TiO2 wt%, indicating an ophiolitic source. We consider the Sangdanlin and Zheya formations syncollisional, deposited in a foredeep basin, with the provenance being the Gangdese arc and the Yarlung Zangbo suture zone. The abrupt sedimentary provenance change between the Denggang and Sangdanlin formations denotes the onset of India-Asia continental collision occurring before the late Ypresian (∼50 Ma). Comparison of our data with those from coeval strata along the Himalaya suggests limited diachroneity in the India-Asia continental collision process.

Zircon U–Pb and Hf isotopic constraints on the onset time of India-Asia collision

The time of initial collision between India and Asia has been extremely controversial despite the fact that it is vital to constraining the orogenesis and subsequent evolution of the Himalayas and Tibetan Plateau. Here we report U–Pb and Hf isotope analysis of detrital zircons from two principal foreland basins, that is, the Sangdanlin and Gyangze basins respectively in the western and central parts of southern Tibet. Our data suggest that Asian-derived clastic sediments started contributing to sedimentation on the Indian continental margin earlier than generally thought, at ∼60 Ma in both basins near the Yarlung-Zangbo Suture Zone. In southern Tibet, no evidence for the existence of an intra-oceanic arc within the Neotethys is observed. We conclude that ∼60 Ma can be used to constrain the onset time of the India-Asia collision, at least in central Tibet. After this initial collision, closure of the Neotethys propagated both westward and eastward, with the final closure occurring at ∼50 and ∼45 Ma in northwestern and eastern Himalayas, respectively. Our conclusion differs significantly from the previous view that the India-Asia collision may have started in northwestern Himalaya and propagated eastward with diachronous suturing.

Xigaze forearc basin revisited (South Tibet): Provenance changes and origin of the Xigaze Ophiolite

Our new stratigraphic, sedimentological, and micropaleontological analysis, integrated with basalt geochemistry, sandstone petrography, and detrital-zircon U-Pb and Hf isotope data, suggests the revision of current models for the geological evolution of the Asian active margin during the Cretaceous. The Xigaze forearc basin began to form in the late Early Cretaceous, south of the Gangdese arc, during the initial subduction of the Neotethyan oceanic lithosphere under the Lhasa terrane. Well-preserved stratigraphic successions document the classical upwardshallowing pattern of the forearc-basin strata and elucidate the origin of the associated oceanic magmatic rocks. The normal midocean-ridge basalt (N-MORB) geochemical signature and stratigraphic contact with the overlying abyssal cherts (Chongdui Formation) indicate that the Xigaze Ophiolite formed by forearc spreading and represents the basement of the forearc sedimentary sequence. Volcaniclastic sedimentation began with thick turbiditic sandstones and interbedded shales in the late Albian–Santonian (Ngamring Formation) followed by shelfal, deltaic, and fl uvial strata (Padana Formation), with fi nal fi lling of the basin by the Campanian age. Forearc sandstones do not show the classical trend from feldspatholithic volcaniclastic to quartzo-feldspathic plutoniclastic compositions, indicating limited unroofi ng of the Gangdese arc prior to collision. U-Pb age spectra of detrital zircons are unimodal with a 107 Ma peak in the lower Ngamring Formation (104–99 Ma), bimodal with a subordinate additional peak at 157 Ma in the middle Ngamring Formation (99– 88 Ma), and multimodal with more abundant pre-Mesozoic ages in the upper Ngamring and Padana Formations (88–76 Ma). These three petrofacies with distinct provenances document the progressive erosional evolution of the Gangdese arc, with uplift of the central Lhasa terrane and expanding river catchments to include the central Lhasa terrane during the Late Cretaceous.

Tethyan suturing in Southeast Asia: Zircon U-Pb and Hf-O isotopic constraints from Myanmar ophiolites

Ophiolites that crop out in Southeast Asia represent the relics of the Tethys Ocean, which existed between the continents of Gondwana and Laurasia during much of the Mesozoic. Two ophiolite belts in Myanmar, i.e., the Eastern Belt and the Western Belt, have been conventionally regarded as parts of a single suture connecting with the Yarlung-Tsangpo suture in the Tibetan Plateau and displaced by the dextral Sagaing fault. Here we present for the first time a combined analysis of zircon secondary ion mass spectrometry U-Pb ages and Hf-O isotopes of two Myanmar ophiolites, the Kalaymyo ophiolite from the Western Belt and the Myitkyina ophiolite from the Eastern Belt. Our results show that the Kalaymyo ophiolite has an Early Cretaceous age (ca. 127 Ma), coeval with Neo-Tethyan ophiolites along the Yarlung-Tsangpo suture. In contrast, the Myitkyina ophiolite was formed during the Middle Jurassic (ca. 173 Ma) and thus the Eastern Belt is the southern continuation of the Meso-Tethyan Bangong-Nujiang suture in the Tibetan Plateau. Consequently, we argue that the two Myanmar ophiolite belts belong to two different sutures of the Meso-Tethys and Neo-Tethys, and that the boundary between the Sibumasu and west Burma blocks is a Jurassic suture rather than a transcurrent shear zone.

Eocene Neo-Tethyan slab breakoff constrained by 45 Ma oceanic island basalt–type magmatism in southern Tibet

Slab breakoff is one of the primary processes in the evolution of many collisional orogens. In the Tibet-Himalaya orogen, the timing of breakoff of the Neo-Tethyan slab remains controversial because of a scarcity of solid evidence. This study reports the discovery of Eocene gabbros, dated at 45.0 ± 1.4 Ma (in-situ U-Pb age of titanite) using secondary ion mass spectrometry, from the eastern segment of Tethyan Himalaya in southern Tibet. These rocks show geochemical characteristics similar to those of HIMU (high μ)–type oceanic island basalt and have depleted Sr-Nd isotopes [87Sr/86Sr(t) = 0.70312-0.70317; eNd(t) = +4.9 to +5.0]. It is suggested that the gabbros stand as the first direct evidence for partial melting of the asthenosphere followed by rapid magma ascent with negligible crustal contamination. This event, combined with results from relevant studies along the Indus-Yarlung suture zone, is best explained by a sudden and full-scale detachment of subducted Neo-Tethyan slab at great depth. The breakoff model may account for coeval tectonomagmatic activities (development of small-scale, short-lived magmatism and subsequent termination of the Gangdese arc magmatism) in southern Tibet and for the abrupt slowdown (ca. 45 Ma) of Indo-Asia convergence.

Upper Oligocene–Lower Miocene Gangrinboche Conglomerate in the Xigaze Area, Southern Tibet: Implications for Himalayan Uplift and Paleo-Yarlung-Zangbo Initiation

The Gangrinboche Conglomerate, exposed along the Yarlung-Zangbo suture zone, records a crucial stage of the Himalayan orogeny. The type section of these strata in the Xigaze area, southern Tibet, including the Qiuwu and Dazhuka Formations, was studied by integrated stratigraphic, sedimentologic, petrographic, and geochemical techniques. Palynological data and detrital zircon U-Pb ages indicate that the Qiuwu Formation was deposited during the latest Oligocene to the earliest Miocene (most probably ∼23 Ma), while the overlying Dazhuka Formation was deposited during 23–18 Ma. The Qiuwu Formation was deposited in a deltaic setting, and detritus was entirely derived from the Gangdese magmatic arc in the north. The Dazhuka Formation, in contrast, was deposited in mainly braided river environments and contains clasts derived from both the Gangdese arc in the north and the Himalayan orogen in the south. Clasts derived from the south first occur at the base of the Dazhuka Formation and increase in abundance upsection to become predominant at the top of the formation. This indicates active Early Miocene uplift and accelerated erosion of the Himalayan belt. Paleocurrent data from the Dazhuka Formation show westward axial sediment transport, which together with mixed provenance from both sides of the basin indicates that a paleo-Yarlung-Zangbo River running parallel to the suture zone initiated at the very start of the Miocene, although with flow direction opposite to that of the present. Flow reversal and establishment of the modern eastward-flowing course must have occurred later on in the Neogene, possibly initiating rapid uplift and focused erosion of the Namche-Barwa syntaxis. Basin subsidence at the close of the Paleogene and subsequent development of a major longitudinal paleo-Yarlung-Zangbo took place contemporaneously with initiation of the South Tibetan Detachment System and Main Central Thrust farther to the south, probably reflecting onset of the “hard collision” phase of the Himalayan orogeny.

Late Cretaceous evolution of the Coqen Basin (Lhasa terrane) and implications for early topographic growth on the Tibetan Plateau

The tectonic evolution of the Lhasa terrane (southern Tibetan Plateau) played a fundamental role in the formation of the Tibetan Plateau. However, many uncertainties remain with regard to the tectonic and paleogeographic evolution of the Lhasa terrane prior to the India-Asia collision. To determine the early tectonic processes that controlled the topographic evolution of the Lhasa terrane, we analyze the Cretaceous strata exposed in the Coqen Basin (northern Lhasa subterrane), which comprises the Langshan and Daxiong Formations. The Langshan Formation unconformably overlies the volcanic rocks of the Lower Cretaceous Zelong Group and consists of similar to 80 m of Orbitolina-bearing limestones, which were deposited in a low-energy, shallow marine environment. Micropaleontological analysis indicates that the Langshan Formation in the Coqen Basin was deposited from late Aptian to early Cenomanian times (ca. 113-96 Ma). The overlying Daxiong Formation (similar to 1700 m thick) consists of conglomerate, coarse sandstone, and siltstone with interbedded mudstone, and represents deposits of alluvial fans and braided rivers. The Daxiong Formation was deposited after the early Cenomanian (ca. 96 Ma) and accumulated until at least ca. 91 Ma, indicating accumulation rates of greater than 0.3 km m.y.(-1). By combining paleocurrent data, sandstone petrology, detrital zircon U-Pb ages, and Hf isotope analysis, we demonstrate that the Daxiong Formation was derived from Lower Cretaceous volcanic rocks and pre-Cretaceous strata in the northern Lhasa subterrane. During Late Cretaceous time, two thrust systems with opposite vergence were responsible for transforming the northern Lhasa subterrane into an elevated mountain range. This process resulted in the evolution from a shallow marine environment (Langshan Formation) into a terrestrial depositional environment (Daxiong Formation) on the southern margin of the northern Lhasa subterrane. Given the regional paleogeographic context, we conclude that the Daxiong Formation in the Coqen Basin records local crustal shortening and flexure resulting in foreland basin development on the southern margin of the northern Lhasa subterrane, which implies early topographic growth of the northern Lhasa subterrane in southern Tibet prior to the India-Asia collision.

Constraining the timing of the India-Asia continental collision by the sedimentary record

Placing precise constraints on the timing of the India-Asia continental collision is essential to understand the successive geological and geomorphological evolution of the orogenic belt as well as the uplift mechanism of the Tibetan Plateau and their effects on climate, environment and life. Based on the extensive study of the sedimentary record on both sides of the Yarlung-Zangbo suture zone in Tibet, we review here the present state of knowledge on the timing of collision onset, discuss its possible diachroneity along strike, and reconstruct the early structural and topographic evolution of the Himalayan collided range. We define continent-continent collision as the moment when the oceanic crust is completely consumed at one point where the two continental margins come into contact. We use two methods to constrain the timing of collision onset: (1) dating the provenance change from Indian to Asian recorded by deep-water turbidites near the suture zone, and (2) dating the age of unconformities on both sides of the suture zone. The first method allowed us to constrain precisely collision onset as middle Palaeocene (59±1 Ma). Marine sedimentation persisted in the collisional zone for another 20–25 Ma locally in southern Tibet, and molassic-type deposition in the Indian foreland basin did not begin until another 10–15 Ma later. Available sedimentary evidence failed to firmly document any significant diachroneity of collision onset from the central Himalaya to the western Himalaya and Pakistan so far. Based on the Cenozoic stratigraphic record of the Tibetan Himalaya, four distinct stages can be identified in the early evolution of the Himalayan orogen: (1) middle Palaeocene-early Eocene earliest Eohimalayan stage (from 59 to 52 Ma): collision onset and filling of the deep-water trough along the suture zone while carbonate platform sedimentation persisted on the inner Indian margin; (2) early-middle Eocene early Eohimalayan stage (from 52 to 41 or 35 Ma): filling of intervening seaways and cessation of marine sedimentation; (3) late Eocene-Oligocene late Eohimalayan stage (from 41 to 25 Ma): huge gap in the sedimentary record both in the collision zone and in the Indian foreland; and (4) late Oligocene-early Miocene early Neohimalayan stage (from 26 to 17 Ma): rapid Himalayan growth and onset of molasse-type sedimentation in the Indian foreland basin.

Mineralogical characteristics and geological significance of Albian (Early Cretaceous) glauconite in Zanda, southwestern Tibet, China

Abstract Early Cretaceous glauconite from the Xiala section, southwestern Tibet, China, was investigated by petrographic microscopy and scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, and electron probe microanalysis (EPMA). The investigations revealed that the glauconite in both sandstones and limestone is highly evolved. The glauconite in sandstone is autochthonous, but in limestone it may be derived from the underlying glauconitic sandstone. Based on analyses of the depositional environments and comparisons of glauconite-bearing strata in Zanda with sequences in adjacent areas, we conclude that the glauconitization at Zanda was probably associated with rising sea levels during the Late Albian, which represent the final separation of the Indian continent from the Australian-Antarctic continent. After the separation of the Indian continent from the Australian- Antarctic continent, cooling of the Indian continent resulted in subsidence and northward subduction of the Indian plate. A gradually rising sea level in Zanda, located along the northern margin of the Indian continent, was the cause of the low sedimentation rate. Continued transgression resulted in the occurrence of the highly evolved glauconite in this area.

Intralesional copper wire retention and pingyangmycin injection: an effective combinational therapy for complex venous malformation in soft tissue.

Objectives: Complex venous malformations (VMs) may extensively involve the soft tissue. The treatment remains a challenge till now. Here we introduce a combinational therapy of copper wires and pingyangmycin (bleomycin A5, PYM). Methods: Copper wires were retained in VMs by repeated penetration with a straight needle. Subsequently, PYM solution was injected into the lesion. Eight to 10 days later, copper wires were removed. The dressing was changed every day until the puncture pores healed. Magnetic resonance imaging scanning was performed to observe the change of VMs. Results: From January 2001 to December 2011,56 patients were treated. During the follow-up period, most of the VMs shrunk obviously. The symptoms were relieved or disappeared. The complications included local pain, temporary paraesthesia and moderate fever, which disappeared quickly after the removal of copper wires. Conclusions: This combinational therapy is a safe and effective approach for the complex VMs in soft tissue.