Yarlung Tsangpo Suture Zone Research Articles

Sensitive High Resolution Ion Micro‐Probe U‐Pb Zircon Geochronology and Geochemistry of Mafic Rocks from the Pulan‐Xiangquanhe Ophiolite, Tibet: Constraints on the Evolution of the Neo‐tethys

Abstract:The Pulan‐Xiangquanhe ophiolite in the western Yarlung Tsangpo suture zone of Tibet is investigated for its geochemistry, geochronology, and tectonic implications in detail. Sensitive high resolution ion micro‐probe zircon U‐Pb dating reveals that diabases in the ophiolite from the three locations of Xugugab, Mapam Yum Co and La'nga Co are dated at 122.3 ± 2.5 Ma, 118.8 ± 1.8 Ma and 120.5 ± 1.9 Ma, respectively. These early Cretaceous mafic rocks have Na2O+K2O, rare earth element patterns, trace elemental spider diagram and other geochemical fingerprints of typical mature back‐arc basin affinity. Therefore, the Pulan‐Xiangquanhe ophiolite formed under a mature back‐arc basin environment, which was a product of this intra‐oceanic subduction system. A suprasubduction system could have existed in the southern margin of Eurasia, which involved both intra‐oceanic and continent‐ward subductions. Extension dominated the southern margin of the Eurasian continent during the early Cretaceous.

Detrital zircon U–Pb ages along the Yarlung-Tsangpo suture zone, Tibet: Implications for oblique convergence and collision between India and Asia

Age-dating of detrital zircons from 22 samples collected along, and adjacent to, the Yarlung-Tsangpo suture zone, southern Tibet provides distinctive age-spectra that characterize important tectonostratigraphic units. Comparisons with data from Nepal, northern India and the Lhasa and Qiangtang terranes of central Tibet constrain possible sources of sediment, and the history of tectonic interactions. Sedimentary rocks in the Cretaceous–Paleogene Xigaze terrane exhibit strong Mesozoic detrital zircon peaks (120 and 170 Ma) together with considerable older inheritance in conglomeratic units. This forearc basin succession developed in association with a continental volcanic arc hinterland in response to Neotethyan subduction under the southern edge of the Eurasia. Conspicuous sediment/source hinterland mismatches suggest that plate convergence along this continental margin was oblique during the Late Cretaceous. The forearc region may have been translated > 500 km dextrally from an original location nearer to Myanmar. Tethyan Himalayan sediments on the other side of the Yarlung-Tsangpo suture zone reveal similar older inheritance and although Cretaceous sediments formed 1000s of km and across at least one plate boundary from those in the Xigaze terrane they too contain an appreciable mid-Early Cretaceous (123 Ma) component. In this case it is attributed to volcanism associated with Gondwana breakup. Sedimentary overlap assemblages reveal interactions between colliding terranes. Paleocene Liuqu conglomerates contain a cryptic record of Late Jurassic and Cretaceous rock units that appear to have foundered during a Paleocene collision event prior the main India–Asia collision. Detrital zircons as young as 37 Ma from the upper Oligocene post-collisional Gangrinboche conglomerates indicate that subduction-related convergent margin magmatism continued through until at least Middle and probably Late Eocene along the southern margin of Eurasia (Lhasa terrane). Although the ages of detrital zircons in some units appear compatible with more than one potential source with care other geological relationships can be used to further constrain some linkages and eliminate others. The results document various ocean closure and collision events and when combined with other geological information this new dataset permits a more refined understanding of the time–space evolution of the Cenozoic India–Asia collision system.

Adakitic rocks from slab melt-modified mantle sources in the continental collision zone of southern Tibet

Major-trace element and Sr–Nd–Pb isotopic data are presented for newly discovered adakitic rocks in the western Gangdese belt, southern Tibet. The Miocene (26–10 Ma) adakitic rocks from the southern Tibetan continental collision zones exhibit distinct differentiation trends typical of arc magmas. These rocks display geochemical affinities similar to those of Cretaceous (136–80 Ma) adakitic rocks derived from the partial melting of subducted Neotethyan slab in southern Tibet. The whole rock geochemical and isotope characteristics of the post-collision adakitic rocks reveal that their magmas likely originated from an upper mantle region previously metasomatized by slab melts during the Cretaceous subduction event. An interesting observation is that the E–W trending belt of adakitic rocks along the Yarlung Tsangpo suture zone occupies a fore-arc position, reflecting a geotectonic setting compatible with the genesis of adakitic magmas. The widespread occurrences of Cretaceous adakitic rocks in this region are interpreted to testify to the former location of the adakite-metasomatized mantle. Our favoured interpretation is that the spatial isotopic variation in the post-collision adakitic rocks is mostly linked to a westward increase in sediment input in the Tibetan mantle region. Under the framework of our paleo-subduction model, a slab break-off event that initiated at around 25 Ma would have allowed an asthenospheric upwelling beneath southern Tibet, which was instrumental in generating the post-collision adakitic magmatism in southern Tibet.

Probing the basement of southern Tibet: evidence from crustal xenoliths entrained in a Miocene ultrapotassic dyke

Abstract: A variety of felsic and mafic granulites and ultramafic rocks occur as xenoliths within a 12.7 Ma ultrapotassic dyke intruding Xigaze flysch immediately to the north of the Yarlung–Tsangpo suture zone in southern Tibet. Garnet–clinopyroxene–plagioclase–quartz thermobarometry on mafic granulite xenoliths gives temperatures of 1130–1330 °C and pressures between 22 and 26 kbar indicating equilibration in the high-pressure and ultrahigh-temperature granulite field and defining a geotherm of c . 16 °C km −1 . Ultramafic xenoliths consist mainly of hornblende and biotite, probably of restitic crustal rather than mantle origin, and attained peak metamorphic conditions of 920–1130 °C and 17–24 kbar, whereas felsic granulites equilibrated at 870–900 °C at an inferred pressure of 17 kbar. In situ U–(Th)–Pb laser ablation inductively coupled plasma mass spectrometry dating of zircons shows that protoliths may include Proterozoic basement rocks, Late Cretaceous calc-alkaline tonalites of the Gangdese batholith root and/or remnants of a Neo-Tethyan oceanic arc. Certain zircons from a felsic granulite and an ultramafic xenolith have mean 206 Pb/ 238 U ages of 16.8 ± 0.9 Ma and 15.6 ± 0.6 Ma, respectively, and monazites from a micaceous xenolith yielded a mean 208 Pb/ 232 Th age of 14.4 ± 0.4 Ma. These results show that the southern Tibet basement reached a thickness of c . 80 km by 17–14 Ma at the latest and has remained unchanged until the present day. Supplementary material: Mineral analyses used for pressure–temperature estimations and results of 40 Ar/ 30 Ar laser single-grain fusion experiments are available at http://www.geolsoc.org.uk/SUP18325 .

Cretaceous-Tertiary Carbonate Platform Evolution and the Age of the India-Asia Collision along the Ladakh Himalaya (Northwest India)

ABSTRACT The India-Asia collision resulted in the formation and uplift of the Himalaya and the enhanced uplift of the Tibetan plateau. The transition from marine to continental facies within the Indus–Yarlung Tsangpo suture zone and along the northern margin of the Indian plate provides the most accurate method of dating the closure of the Tethys Ocean separating the Indian and Asian plates. Other indirect methods of dating the collision, such as paleomagnetism, dating the UHP metamorphism along the north margin of India, dating the youngest subduction-related granites along the southern margin of Asia, and dating the postorogenic Indus Molasse Group deposits within the suture zone, cannot provide such a precise or reliable age of collision. Ophiolite obduction onto the Indian passive margin occurred during the latest Cretaceous and predated initial collision of the two continental plates. Unconformities occur beneath the Late Maastrichtian Marpo Formation and beneath the Danian Stumpata Formation on the ...

Tectonic implications of felsic tuffs within the Lower Miocene Gangrinboche conglomerates, southern Tibet

Felsic tuffs are interbedded with the Gangrinboche conglomerates adjacent to the Yarlung Tsangpo suture zone in southern Tibet. Laser Ar/Ar dating of mineral separates indicates they are of Early Miocene age. Such tuffs are most likely an eruptive manifestation of geochemically indistinguishable coeval felsic (adakitic) intrusions that are widely reported across southern Tibet. The considerable lateral (E–W) extent of the Gangrinboche conglomerates and their depositional setting indicates sediment accumulation in an overall N–S compressional regime and thereby places important constraints on the tectonic setting in which magmatism initiated. The conglomerates were folded and truncated during back-thrusting associated with development of the north-directed Great Counter thrust. As N–S trending rifts associated with E–W extension of the Tibetan Plateau cut both the conglomerates and this thrust system it can be inferred that post-collisional volcanism is unlikely to have been genetically linked to later E–W extension. Early Miocene slab break-off beneath Tibet provides a model that appears to be consistent with petrogenesis of the associated magmatic suite, which requires a lowermost crust or lithospheric mantle generation, molasse accumulation, and uplift and emplacement of North Himalayan gneiss domes.

Initial uplift of the Tibetan Plateau and environmental implications

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp.142

Miocene collision-related conglomerates near Dazhuqu and Xigaze, Yarlung Tsangpo suture zone, Tibet

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 112

Radiolarian biostratigraphic constraints on the generation of the Nidar ophiolite and the onset of Dras arc volcanism: Tracing the evolution of the closing Tethys along the Indus – Yarlung-Tsangpo suture

The Nidar ophiolite in Ladakh Himalaya is a substantial fragment of oceanic lithosphere preserved along the western continuance of the Indus - Yarlung-Tsangpo suture zone. It was generated in a supra-subduction zone setting and is linked in development to the Dras intra-Tethyan volcanic arc. Earlier reported Hauterivian - Aptian radiolarians from the volcano-sedimentary succession atop the ophiolite were used to constrain the age of the ophiolite and duration of arc volcanism. Re-assessment of radiolarian taxonomy and biostratigraphy reveals an upper Barremian to upper Aptian range for the Nidar volcano-sedimentary section, the same as for the marine sedimentary cover atop the Dazhuqu ophiolite in the southern Tibet. The lower limit of its stratigraphic range places a minimum temporal constraint on the Nidar ophiolite generation in the late Barremian (ca 126 Ma). This compares well with radiometric age for the ophiolite (124± 1 Ma). The sedimentary successions of both ophiolites bear resemblance in the record of associated arc volcanism. Dras arc volcanism in the Nidar section began in the early Aptian. The Nidar and Dazhuqu ophiolites are interpreted as distal chronological equivalents developed in association with the same intra-Tethyan subduction system. Parallels between the evolution of this Cretaceous intra-Tethyan subduction system and other systems in the region remain uncertain.

Shoshonites in Southern Tibet Record Late Jurassic Rifting of a Tethyan Intraoceanic Island Arc

Detailed field mapping combined with a petrologic and geochemical investigation of the Zedong terrane within the Yarlung Tsangpo suture zone provides insights to the evolution of now mostly subducted portions of Tethys during the Late Jurassic. The terrane is dominated by volcanic rocks of shoshonitic affinity, which were erupted in a submarine oceanic island arc setting. The volcanic island arc was built on a basement of oceanic crust, and the shoshonites locally overlie a thin section of pillowed island arc tholeiites and red ribbon–bedded radiolarian cherts. Geochemistry of the shoshonites suggests that their development occurred in a setting analogous to that of Late Miocene to Early Pliocene Fiji and was associated with an arc rifting. We speculate that this event may have been a far‐field response to developments associated with Gondwana breakup.

Bainang Terrane, Yarlung–Tsangpo suture, southern Tibet (Xizang, China): a record of intra-Neotethyan subduction–accretion processes preserved on the roof of the world

The Bainang terrane, an intra-oceanic island arc subduction complex into which Tethyan oceanic rocks were accreted during the Cretaceous, is preserved within the Yarlung–Tsangpo suture zone of Tibet. The lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central Tethyan domain from Late Triassic to mid-Cretaceous time. These rocks are preserved within a south-verging imbricate thrust stack of thin (≪1 km thick) northward younging tectonic slices. Five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts. The northern tract, which accumulated on the north side of Neotethys, was probably separated from its southern counterpart by a mid-ocean ridge. Detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract. Oceanic plate stratigraphy of the northern tract records its northward travel and mid-Cretaceous (late Aptian) approach towards a south-facing intra-oceanic subduction zone. Rocks in the southern tract developed closer to the Indian subcontinent and experienced thermotectonic subsidence and Mid-Jurassic basic alkaline intraplate magmatism. They were probably accreted late in the Cretaceous. Variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion. The overall tectonostratigraphy of the terrane reflects its development in a remote intra-oceanic setting.

Conglomerates record the tectonic evolution of the Yarlung-Tsangpo suture zone in southern Tibet

Abstract The histories of individual conglomeratic units along the Yarlung-Tsangpo (River) suture zone in southern Tibet reflect significant phases in the Mesozoic to Cenozoic tectonic evolution of this area. Several temporally distinct conglomerate units are recognized along the suture, and their detailed examination permits analysis of the collision between India and Asia. Upper Jurassic to Lower Cretaceous conglomerates crop out within the Sangri Group along the southern Lhasa terrane. They are dominated by limestone and andesitic volcanic cobbles derived entirely from the Lhasa terrane. These rocks have experienced amphibolite facies metamorphism, and exhibit a strong penetrative regional foliation. Thick successions of the Palaeocene Liuqu Conglomerate crop out within the suture from Xigaze to Lhaze. They contain detritus sourced from intra-oceanic terranes associated with the suture zone, as well as clasts of Indian affinity, while Lhasa and Xigaze terrane-derived material is notably absent. These conglomerates record an early suture zone event prior to India-Asia collision. Uppermost Oligocene to Lower Miocene ‘Gangrinboche facies’ conglomerates crop out on the southern edge of the Lhasa terrane along the length of the suture. Several correlative units within this facies exhibit broadly similar stratigraphic histories. A basal depositional contact upon an eroded Lhasa terrane surface is ubiquitous with initial clast derivation from the north. Up-section, the first arrival of coarse-grained, suture-zone and India-derived clasts, is abrupt. These southerly derived clasts predominate by the top of most sections. An areally restricted succession of gently dipping Late Neogene ultramafic breccias unconformably overlies folded Liuqu Conglomerate near Quanggong. Other Neogene sediments are extensive west of Mount Kailas. Deposition of coarse clastic sediments is presently continuing along the length of the Yarlung Tsangpo. Discrimination and detailed investigation of each of these units will improve our understanding of the evolution of the India-Asia collision.

Precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the Dazhuqu terrane, Yarlung–Tsangpo suture zone, Tibet

Well-preserved, abundant radiolarians provide high-precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the Yarlung–Tsangpo suture zone in southern Tibet. Dazhuqu terrane ophiolites were generated in an intra-oceanic supra-subduction zone setting within a relatively short (<10 Ma) interval from late Barremian to late Aptian. Accumulation of sediments upon the newly generated ophiolite initially occurred in a series of discrete rift-controlled sub-basins associated with various spreading centres. An increasing flux of arc-derived volcaniclastic sediment up-section indicates nearby volcanic arc activity. The Dazhuqu terrane developed in an intra-oceanic setting within Tethys where it was isolated from any continental influence.

The Gangdese thrust: a phantom structure that did not raise Tibet

ABSTRACTDetailed field investigations do not support the existence of a ‘Gangdese thrust’ along the Yarlung Tsangpo suture zone in southern Tibet. A relationship where Lhasa terrane rocks are thrust southwards over components of this zone was not observed over 2000 km of the suture. On the contrary, at the type locality of this ‘Gangdese thrust’, Miocene conglomerates unconformably overlie an eroded surface of Lhasa terrane rocks. Interpretations that invoke Late Oligocene – Early Miocene south‐directed thrusting on a ‘Gangdese thrust’ as a mechanism for uplift of the Tibetan Plateau must therefore be reassessed.

Stratigraphic and sedimentological constraints on the age and tectonic evolution of the Neotethyan ophiolites along the Yarlung Tsangpo suture zone, Tibet

Abstract Ophiolitic rocks distributed along the Yarlung Tsangpo suture zone in southern Tibet are the few remaining fragmentary remnants of many thousands of kilometres of the ocean space that formerly existed between India and Eurasia. Portions of mid-Jurassic and mid-Cretaceous intra-oceanic island arcs can be recognized amongst those rocks that have been studied in detail. Complete suprasubduction zone ophiolite successions are preserved in the Dazhuqu terrane, which crops out both east and west of Xigaze. Radiolarians in inter-pillow cherts and immediately overlying sedimentary rocks indicate a Barremian ophiolite generation event. Palaeomagnetic data show that this ophiolite formed at equatorial latitudes south of the Lhasa terrane before its south-directed emplacement onto the northern margin of India. Highly refractory ultramafic rocks in the Luobusa ophiolite appear to be of Mid-Jurassic age and are potentially related to intra-oceanic island arc remnants in the nearby Zedong terrane. Ophiolitic massifs along the suture in western Tibet are thrust southwards onto northern India and record Late Jurassic ocean-floor development. Miocene north-directed back-thrusting associated with India-Asia collision has further complicated interpretation of regional geology. The ophiolitic rocks of the Yarlung Tsangpo suture zone provide evidence for the former existence of multiple oceanic island arc segments within Neotethys and suggest that consumption of the oceanic space between India and Asia was more complicated than has been predicted by existing models.

New constraints on the India–Asia collision: the Lower Miocene Gangrinboche conglomerates, Yarlung Tsangpo suture zone, SE Tibet

Lower Miocene conglomerates crop out along the length of the Yarlung Tsangpo suture zone on the southern margin of the Lhasa terrane. These conglomerates, known by various local names, are correlated herein as the Gangrinboche conglomerates. All units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the Lhasa terrane is ubiquitous. At most localities the tops of sections are either removed by erosion or truncated by north-directed thrusts. These conglomeratic molasse units developed in response to the India/Asia collision and record aspects of its development. In all units initial clast derivation was from the Lhasa terrane on the northern margin of the Yarlung Tsangpo suture zone. Up-section the first appearance of clasts derived from terranes within the suture zone and the northern margin of India, all of which lie to the south of any outcrops of Gangrinboche conglomerates, is observed. Although these units were previously thought to be Eocene, analysis of fossil and structural constraints indicates Early Miocene deposition. As development of the Gangrinboche conglomerates records a significant phase in the evolution of the India–Asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models.

New insights into the evolution of the Yarlung Tsangpo suture zone, Xizang (Tibet), China

Detailed investigations along the Yarlung Tsangpo suture zone, Tibet result in the following conclusions. (1) Arc, forearc, and subduction complex elements of at least one intra-oceanic island arc which once lay within Tethys and accreted to India prior to its collision with Asia are preserved within the suture. (2) Collision-related tectonic melange is widespread. (3) Temporallydistinct conglomeratic molasse units were developed; they are related to different collision events. (4) Improved radiolarian fossil data place constraints on timing of the development of intra-oceanic terranes and melange formation. (5) A simple unconformable contact occurs between Upper Oligocene–Lower Miocene conglomerates and the southern Lhasa terrane; the socalled south-directed Gangdese thrust does not exist. These new results suggest the former existence of an intra-oceanic subduction system within Tethys and indicate that considerable revision of existing models for the evolution of the Yarlung Tsangpo suture zone is required.

The Zedong terrane: a Late Jurassic intra-oceanic magmatic arc within the Yarlung–Tsangpo suture zone, southeastern Tibet

An overturned sequence of igneous and volcaniclastic rocks crops out along the Yarlung–Tsangpo suture zone (YTSZ), in southeastern Tibet. These rocks are remnants of an intra-oceanic island arc, the Zedong terrane, that once lay between India and Asia. 40Ar/ 39Ar dating and U–Pb ion microprobe analyses reported here reveal that this arc was active during the Jurassic. U–Pb dating of zircon from a dacite breccia from the middle portion of the Zedong terrane, yields an age of 161±2.3 Ma (1 σ). 40Ar/ 39Ar dating of hornblende from a cross-cutting andesite dyke yields the youngest age of 152.2±3.3 Ma (1 σ). 40Ar/ 39Ar results from hornblende from andesite dykes and an andesite breccia from the upper portion yield a mean age of 156±0.3 Ma (1 σ). Additional U–Pb and 40Ar/ 39Ar dating of zircon and hornblende from quartz diorite yields a mean age of 156.8±0.8 Ma (1 σ). Geochronological data reported here and other published work indicate that the intra-oceanic subduction system to which the Zedong terrane belonged was active from at least Late Jurassic to Early to mid Cretaceous.

Paleogene island arc collision-related conglomerates, Yarlung–Tsangpo suture zone, Tibet

Abstract Coarse clastic rocks in the Liuqu Conglomerate, formed in both terrestrial and subaqueous settings, record a Paleogene phase in the tectonic evolution of Tibet. Facies changes are commonly abrupt with rapid changes in clast types, grain size and stratal patterns. Sediments were derived from the leading (northern) edge of the Indian margin and a Late Jurassic–Cretaceous intraoceanic island arc that lay within Tethys. The coarse clastic sedimentary rocks of the Liuqu conglomerates are extremely proximal, but are locally offset relative to their original source terranes. They record aspects of the history of collision between these terranes and are interpreted to have been deposited in oblique–slip basins that developed along the zone of collision. The absence of clasts derived from terranes to the north of the Yarlung–Tsangpo suture suggests that basins associated with deposition of the Liuqu Conglomerate developed prior to the final collision between India and Asia.