Yarlung Zangbo Suture Research Articles

Multistage origin of dunite in the Purang ophiolite, southern Tibet, documented by composition, exsolution and Li isotope characteristics of constituent minerals

Abstract. The Purang ophiolite, which crops out over an area of about 650 km2 in the western Yarlung–Zangbo suture zone, consists chiefly of mantle peridotite, pyroxenite and gabbro. The mantle peridotite is comprised mainly harzburgite and minor dunite. Locally, the latter contains small pods of chromitite. Pyroxenite and gabbro occur as veins of variable size within the peridotite; most of them strike northwest, parallel to the main structure of the ophiolite. Three types of dunite occur in the Purang ophiolite: dunite that envelopes podiform chromitite (1) and lenses of dunite with either Cr-rich spinel (2) or Cr-poor spinel (3) in a harzburgite host. The constituent minerals of dunite envelopes around podiform chromitite are similar in composition to those of transition-zone dunite (Fo91.01−91.87 in olivine; Cr/(Cr+Al) (Cr#) =41.5–47.0 and Mg/(Mg+Fe2+) (Mg#) =58.9–63.0 in Cr-spinel). Forsterite contents in olivine decrease from type 2 lenses with Cr-rich spinel (91.9–93.0) to type 1 dunite enveloping chromitite (91.7–93.7) to type 3 lenses with Cr-poor spinel (95.3–96.0). Similarly, Cr# in spinel decreases from type 2 (66.9–67.9) to type 1 (41.5–47.0) to type 3 (19.8–20.6). In addition, Al2O3 in clinopyroxene is highest in type 2 (3.48–5.24 wt %) and decreases to type 1 (1.56–3.29 wt %) and type 3 (0.78–0.86 wt %). Olivine in type 1 dunite enveloping podiform chromitite has Li concentrations and δ7Li values of 1.48–1.71 ppm and 6.19 ‰–7.98 ‰, respectively. Type 2 dunite lenses with Cr-rich spinel contain olivine with Li =0.98–1.64 ppm and δ7Li =6.77 ‰–10.99 ‰. The type 3 dunite lenses with Cr-poor spinel show the highest values of Li =0.94–1.40 ppm and δ7Li =10.25 ‰–14.20 ‰. Exsolution lamellae of clinopyroxene and magnetite occur as oriented intergrowths in olivine of type 3 dunite lenses with Cr-poor spinel. We suggest that the Purang ophiolite developed during two main stages of formation. In the first stage, abyssal peridotites formed in a mid-ocean-ridge environment. During the second stage, hydrous high-Mg boninitic melts were produced by high degrees of partial melting in a supra-subduction zone mantle wedge, which reacted with peridotite to form type 2 dunite pods with high-Cr# spinel. At lower degrees of partial melting in the same mantle wedge, Al-rich melts were produced, which reacted with peridotite to form type 3 dunite pods that contain low-Cr# spinel. These Al-rich melts were also relatively rich in Ti4+, Ca2+ and Fe3+, which were incorporated into the olivine structure by appropriate substitutions. During cooling, these elements exsolved as lamellae of magnetite and clinopyroxene.

Early Cretaceous subduction initiation beneath southern Tibet caused the northward flight of India

Collision between the Indian and Eurasian plates formed the ~2500 ​km long Yarlung Zangbo Suture Zone and produced the Himalaya mountains and Tibetan plateau. Here we offer a new explanation for tectonic events leading to this collision: that the northward flight of India was caused by an Early Cretaceous episode of subduction initiation on the southern margin of Tibet. Compiled data for ophiolites along the Yarlung Zangbo Suture Zone show restricted ages between 120 ​Ma and 130 ​Ma, and their supra-subduction zone affinities are best explained by seafloor spreading in what became the forearc of a north-dipping subduction zone on the southern margin of Tibet. The subsequent evolution of this new subduction zone is revealed by integrating data for arc-related igneous rocks of the Lhasa terrane and Xigaze forearc basin deposits. Strong slab pull from this new subduction zone triggered the rifting of India from East Gondwana in Early Cretaceous time and pulled it northward to collide with Tibet in Early Paleogene time.

Mineralogy and Geochemistry of the High‐Cr Podiform Chromitite from the Cuobuzha Ophiolite, Yarlung Zangbo Suture Zone, Western Tibet, China: Implication for its Origin

AbstractThe Cuobuzha high‐Cr chromitites in the western segment of Yarlung Zangbo Suture Zone of Tibet are mainly hosted in the harzburgites as massive type, which are characterized by high concentrations of platinum group elements (PGE) ranging from 380 to 577 ppb, and low Pd/Ir ratios (<0.1). In mid‐ocean ridge basalts (MORB)‐normalized spidergrams, chromites of the Cuobuzha chromitites are depleted in Al, Ga, V, Mg and Zn, and enriched in Mn and Cr, sharing similar patterns with those of ophiolitic boninites in the Bonin and Thetford Mines. Approximately 20 platinum group mineral (PGM) grains were discovered from the samples, including laurite, erlichmanite, Os‐Fe alloy, cuproiridsite, and irarsite. The PGM assemblages indicate that sulfur fugacity was initially low enough to allow the precipitation of Os‐Fe alloy and increased thereafter, with the fall in temperature. Primary Fe‐Ni and Fe‐Cr alloys, which are stable in a highly reduced environment, occur as inclusions within chromites or clinopyroxenes. Calculated results show that the parental magma has an intimate affinity with boninites. Based on our observations, a model is proposed wherein the Cuobuzha chromitites contain high‐pressure and low‐pressure chromites. Low‐pressure chromites were formed via reaction between boninitic melts and peridotites, during which the high‐pressure chromites hosting highly reduced minerals were mobilized by melts and were reallocated to podiform chromitites.

The latest Jurassic protoliths of the Sangsang mafic schists in southern Tibet: Implications for the spatial extent of Greater India

Field observations, petrology, zircon geochronology, whole-rock geochemistry, and Sr-Nd isotopes were used to reveal the lithology, age, and tectonic setting of the protoliths of the Sangsang mafic schists in the Yarlung Zangbo Suture Zone (YZSZ), central southern Tibet. The mafic schists occur as exotic blocks within the accretionary complex of the YZSZ. Relic amygdaloidal features indicate the schist protoliths were volcanic rocks. The mineral assemblage mainly comprises riebeckite + magnesioriebeckite + chlorite + sericite + albite + relic clinopyroxene. The youngest group of zircon ages constrains the formation time of the protoliths to 149.2 ± 2.2 Ma (i.e., latest Jurassic). Abundant Paleozoic and older zircons suggest the protolith volcanic rocks were erupted onto a continental terrane. Whole-rock geochemical and Sr-Nd isotopic data indicate the protoliths were of ocean island basalt affinity. The mafic schists mostly have high-Ti, alkaline, basaltic compositions with 43.57–46.93 wt% SiO2, 3.27–7.24 wt% Na2O + K2O, and 4.04–4.69 wt% TiO2. The schists are enriched in light rare earth elements relative to heavy rare earth elements, and have (La/Yb)N = 5.85–8.53 and small positive Ta and negative Zr-Hf anomalies. (87Sr/86Sr)i values vary from 0.7044 to 0.7055, while (143Nd/144Nd)i ranges from 0.512670 to 0.512727, with εNd(t) values of +4.4 to +5.5. The protoliths of the mafic schists were probably formed in a within-plate extension setting associated with mantle plume upwelling and melting of continental lithosphere. This setting was related to the Late Jurassic continental breakup of Argoland off the northern margin of east Gondwana, and thus marked the paleoposition of the northern edge of Greater India before the breakup of India from east Gondwana during Early Cretaceous. The N-S extent of Greater India at that time was ~2400 km. This further indicates how the India–Eurasia continental convergence has been accommodated.

The Late Jurassic Zedong ophiolite: A remnant of subduction initiation within the Yarlung Zangbo Suture Zone (southern Tibet) and its tectonic implications

The Zedong ophiolite is the largest ophiolite massif east of Dazhuqu in the Yarlung Zangbo Suture Zone in the southern Tibetan Plateau. However, its age, geodynamic setting and relationship to the Xigaze ophiolite remain controversial. New zircon U–Pb ages, whole-rock geochemical and Nd–Pb isotopic data from ophiolitic units provide constraints on the geodynamic and tectonic evolution of the Zedong ophiolite. U–Pb zircon geochronology of dolerite lavas and late gabbro–diabase dikes yield weighted mean ages of 153.9 ± 2.5 Ma and 149.2 ± 5.1 Ma, respectively. Strong positive εNd(t) and positive Δ7/4Pb and Δ8/4Pb values indicate derivation from a highly depleted mantle source with an isotopic composition similar to that of the Indian MORB-type mantle. The geochemistry of ophiolitic lavas and early dikes are analogous to typical island arc tholeiites whereas late dikes are similar to boninites. The geochemistry of these rock types suggests multi-stage partial melting of the mantle and gradually enhanced subduction influences to the mantle source through time. Combined with the MORB-like 162.9 ± 2.8 Ma Luobusha ophiolitic lavas, we suggest that the Luobusha lavas, Zedong lavas and early dikes originated in an infant proto-arc setting whereas late dikes in the Zedong ophiolite originated in a forearc setting. Together, they represent a Neo-Tethyan subduction initiation sequence. The Late Jurassic intra-oceanic proto-arc to forearc setting of the Zedong ophiolite contrasts with the continental margin forearc setting for the Xigaze ophiolite, which suggests a laterally complex geodynamic setting for ophiolites along the Yarlung Zangbo Suture Zone.

Record of Early-Stage Rodingitization from the Purang Ophiolite Complex, Western Tibet

Rodingitization, commonly coupled with serpentinization of ultramafic rocks, bears significant information for fluid-rock interactions and element transfer from sea-floor to subduction zone environments. Numerous outcrops of rodingites are exposed along the Yarlung Zangbo suture zone (YZSZ) of southern Tibet, providing us an excellent opportunity to probe the petrogenetic processes, and unravel their implications for regional tectonic evolution. Several studies have been performed on rodingites from the eastern to central portions of the YZSZ, whereas limited work has ever been conducted on rodingitized rocks from the western segment of the YZSZ, precluding a comprehensive understanding of this lithological type. In this paper, we present detailed studies of petrology, mineral, whole-rock geochemistry and phase equilibrium modeling on a suite of newly recognized rodingites within the Purang ophiolite massif in the southwestern part of the YZSZ. The rodingites have a major metasomatic mineral association of chlorite, clinozoisite, amphibole and minor amounts of plagioclase, representing products of an early-stage rodingitization. They generally present compositions of low SiO2 (48.89 wt.%.53.57 wt.%), Fe2O3T (3.77 wt.%.5.56 wt.%), Na2O (1.31 wt.%.1.93 wt.%), Al2O3 (4.78 wt.%.8.84 wt.%), moderate CaO (9.69 wt.%.11.23 wt.%), and high MgO (24.11 wt.%.26.08 wt.%) concentrations with extremely high Mg# values [Mg#=100×Mg/(Mg+Fe2+) molar] of 89.92. Bulk-rock recalculation reveals that the rodingites have a protolith of mantle-derived olivine gabbro or gabbronorite. They have low rare earth element compositions (ΣREE=2.4 ppm.6.5 ppm) and are characterized by flat LREE and slightly enriched HREE patterns with positive Eu anomalies; they also exhibit positive anomalies in Sr, U and Pb and negative anomalies in high-field strength elements, including Nb, P and Ti, suggesting for a subduction-zone imprinting. Phase equilibrium modeling shows that the rodingitization did take place at P<2 kbar and T= ~350.400 °C, consistent with low greenschist facies conditions. Taking into account of all these petrological and geochemical features, we propose that the rodingites record evidence of early-stage fluid-rock interactions between olivine gabbroic rocks and Ca-rich fluids, which may have derived from weakly serpentinized ultramafic country rocks. Although this process may initially have occurred in a mid-ocean ridge setting, an obvious overprinting by supra-subduction zone fluids in a fore-arc environment is recognized.

New Discovery of Radiolarians in the Pomulong Mélange, Middle part of the Yarlung Zangbo Suture Zone, Tibet

New Discovery of Radiolarians in the Pomulong Mélange, Middle part of the Yarlung Zangbo Suture Zone, Tibet

Origin of Mesozoic ophiolitic mélanges in the western Yarlung Zangbo suture zone, SW Tibet

The petrogenesis and tectonic evolution of the Mesozoic ophiolitic mélanges in the western section of the Yarlung Zangbo suture zone (YZSZ) remain controversial. In this paper, we present the results of whole-rock geochemical and SrNd isotope analyses, zircon UPb ages and in situ LuHf isotopic data obtained from mafic rocks of the northern and southern sub-belts of the western YZSZ Mesozoic ophiolitic mélanges to help us understand these controversial issues. Diabases and dolerites from the northern sub-belt and gabbros from the southern sub-belt exhibit variable fore-arc basalt (FAB)-like geochemical compositions and have zircon UPb ages of ∼126.4–120.3 Ma. In addition, gabbro-diabases from the northern sub-belt have boninite series affinities and yield a zircon UPb age of ∼125.7 Ma. These results, along with previous studies on the YZSZ Mesozoic ophiolitic mélanges and the Gangdese arc, reveal that the western YZSZ Mesozoic ophiolites were likely generated over multiple stages in the epicontinental Gangdese fore-arc basin as the Yarlung Zangbo Neo-Tethyan Ocean subducted northward in front of the Lhasa terrane. The Early Cretaceous FAB-like and boninite series mafic rocks were formed by the reinitiation of subduction, which was followed by a retreat of the subduction zone and the creation of the fore-arc basin and strong hyperextension, accompanied by asthenosphere upwelling at ∼130–120 Ma. During this process, the upwelling asthenosphere underwent decompressional melting with limited penetration of slab-derived fluids and gave rise to the N-MORB (normal mid-ocean ridge basalt)-like basaltic magmas that intruded the overlying, previously generated depleted mantle as FAB-like gabbro, diabase and dolerite sills or dykes. Then, boninitic magmas represented by boninitic gabbro-diabases were generated by remelting the extremely depleted residual mantle source, which was metasomatized by a small amount of slab-derived fluids, following previous extractions of FAB-like magma.

Metamorphic Petrology of Clinopyroxene Amphibolite from the Xigaze Ophiolite, Southern Tibet: P-T Constraints and Phase Equilibrium Modeling

The clinopyroxene amphibolite from the Bailang terrane is located in the central section of the Yarlung Zangbo suture zone (YZSZ), southern Tibet. The study of it is expected to provide important clues for the subduction of the Neo-Tethyan Ocean below the Asian Plate and thus for better understanding of the development of the India-Asia collision zone. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of the clinopyroxene amphibolite within a serpentinite melange, four overprinted metamorphic stages are established. They are the first metamorphic record of M1 stage indicated by a relict assemblage of plagioclase+clinopyroxene+amphibole, an early M2 stage characterized by an assemblage of medium-grained clinopyroxene+amphibole+plagioclase+quartz as well as rutile inclusion in titanite, which is formed during burial process, an isobaric cooling M3 stage which is characterized by an assemblage of clinopyroxene+amphibole+plagioclase+titanite, and a decomposing retrograde stage M4, which is represented by the amphibolite+plagioclase symplectite+titanite+ rutile+quartz. By applying the THERMOCALC (versions 6.2 and 6.3) technique in the NCFMASHTO system, the P-T conditions estimated from M1 to M4 stages are ca. 8.6 kbar/880 °C, 10.8-13.4 kbar/800-840 °C, 12.7-13.2 kbar/650-660 °C and <11.2 kbar/640 °C, respectively. The mineral assemblages and their P-T conditions define a counterclockwise P-T path for the clinopyroxene amphibolite of the Xigaze ophiolite, suggesting that the rocks underwent a cooling process during burial from magmatic protolith, and a decompressing stage after the pressure peak metamorphic conditions, which implies that the Bailang terrane of the Xigaze ophiolite may have experienced subduction/collision-related tectonic processes. The peak metamorphism reaches to the transitional P-T conditions among amphibolite facies, granulite facies and eclogite facies with a burial depth of 30–40 km. After exhumation of the ophiolitic unit to the shallow crustal levels, the clinopyroxene amphibolite exposes to a high fO2 condition on the basis of the stable epidotebearing assemblage in the T-MO2 diagrams. A late subgreenschist facies overprinting subsequently occurs, the relevant mineral assemblage is prehnite+albite+chlorite+epidote+quartz.

High‐ and low‐Cr chromitite in a Tibetan Ophiolite: Evolution from Mature Subduction System to Incipient Forearc in the Neo‐Tethyan Ocean

AbstractThe microstructures, major‐ and trace‐element compositions of minerals and electron backscattered diffraction (EBSD) maps of high‐ and low‐Cr# [spinel Cr# = Cr3+/(Cr3++Al3+)] chromitites and dunites from the Zedang ophiolite in the Yarlung Zangbo Suture (South Tibet) have been used to reveal their genesis and the related geodynamic processes in the Neo‐Tethyan Ocean. The high‐Cr# (0.77‐0.80) chromitites (with or without diopside exsolution) have chromite compositions consistent with initial crystallization by interaction between boninitic magmas, harzburgite and reaction‐produced magmas in a shallow, mature mantle wedge. Some high‐Cr# chromitites show crystal‐plastic deformation and grain growth on previous chromite relics that have exsolved needles of diopside. These features are similar to those of the Luobusa high‐Cr# chromitites, possibly recycled from the deep upper mantle in a mature subduction system. In contrast, mineralogical, chemical and EBSD features of the Zedang low‐Cr# (0.49‐0.67) chromitites and dunites and the silicate inclusions in chromite indicate that they formed by rapid interaction between forearc basaltic magmas (MORB‐like but with rare subduction input) and the Zedang harzburgites in a dynamically extended, incipient forearc lithosphere. The evidence implies that the high‐Cr# chromitites were produced or emplaced in an earlier mature arc (possibly Jurassic), while the low‐Cr# associations formed in an incipient forearc during the initiation of a new episode of Neo‐Tethyan subduction at ∼130‐120 Ma. This two‐episode subduction model can provide a new explanation for the coexistence of high‐ and low‐Cr# chromitites in the same volume of ophiolitic mantle.

Petrogenesis and Tectonic Significance of the Cuobuzha Peridotite, Yarlung Zangbo Suture Zone, China

AbstractThe Yarlung Zangbo suture zone (YZSZ) in southern Tibet includes the remnants of Neo‐Tethyan oceanic lithosphere and marks a major suture between the Indian plate to the south and the Lhasa terrane of Tibet to the north. The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of the western YZSZ comprises mainly clinopyroxene (cpx)‐rich and depleted harzburgites. Spinels in the cpx‐harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx‐harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), yielding Re‐depletion model ages (TRD) ages from 1.8 to 1.7 Ga, indicating that the Cubuzha mantle underwent at least one ancient melt extraction event ca. 1.8‐1.7Ga; whereas the cpx‐harzburgites have suprachondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb), indicating subsequent addition of Re following the last partial melting event that occurred during mid‐ocean ridge melt evolution processes. Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid‐oceanic ridge–type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx‐harzburgites formed from low‐degree partial melting of a primitive mantle source, and they were subsequently modified by melt‐rock interactions in a mid‐oceanic ridge environment. The depleted harzburgites, however, were produced by remelting of the cpx‐harzburgites, which later interacted with mid‐oceanic ridge basalt– or island‐arc tholeiite–like melts, possibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neo‐Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.

Kalaymyo Peridotite Massif in the Indo‐Myanmar Ranges (western Myanmar): Its Mineralogy, Geochemistry and Tectonic Implications

AbstractMesozoic ophiolites crop out discontinuously in the Indo‐Myanmar Ranges in NE India and Myanmar, and represent the remnants of the Neotethyan oceanic lithosphere. These ophiolites in the Indo‐Myanmar Ranges are the southern continuation of the Neotethyan ophiolites occurring along the Yarlung Zangbo Suture Zone in southern Tibet farther northwes, as indicated by their coeval crystallization ages and geochemical compositions. The Kalaymyo ophiolite is located in the central part of the Indo‐Myanmar Ranges (Myanmar). The Kalaymyo ophiolite are composed of olivine (Fo = 89.8–90.5), orthopyroxene (En86‐91Wo1‐4Fs8‐10; Mg#=89.6–91.9), clinopyroxene (En46‐49Wo47‐50Fs3‐5; Mg# = 90.9–93.6) and spinel (Mg# = 67.1–78.9; Cr# = 13.5–31.5), and have relatively homogeneous whole‐rock compositions with Mg# of 90.1–90.8 and SiO2 (41.5–43.65 wt.%), Al2O3 (1.66–2.66 wt.%) and CaO (1.45–2.67 wt.%) contents. They display Light Rare Earth Element (LREE)‐depleted chondrite‐normalized REE patterns and show a slight enrichment from Pr to La. The Kalaymyo peridotites are characterized by Pd‐enriched chondrite‐normalized PGE patterns with superchondritic (Pd/Ir)CN ratios (1.15–2.36). Their calculated oxygen fugacities range between QFM–0.57 and QFM+0.90. These features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees (5–15%) of partial melting at a mid‐ocean‐ridge environment. The observed enrichment in LREE and Pd was a result of their reactions with enriched MORB‐like melts, percolating through these already depleted, residual peridotites. The Kalaymyo and other ophiolites in the Indo‐Myanmar Ranges hence represent mid‐ocean ridge–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward migrating subduction–accretion system along the eastern margin of India.

Deep‐seated lithospheric geometry in revealing collapse of the Tibetan Plateau

Abstract The Tibetan Plateau's enigmatic collapse, which was indicated based on extensional faulting in the interior of the plateau and large eastward-directed strike-slip faults, has inspired intensive geologic inquiry and debate. Interaction of the subducting Indian plate with the overlying Asian lithosphere is one factor that may be affecting the plateau. A more detailed image of the subducting Indian plate can help constrain its role, but regional high-resolution seismic data have not been widely available at a relevant scale. Here we present an integrated interpretation based on two types of seismic datasets: (1) two N-S oriented deep seismic-reflection profiles that cross the Yarlung-Zangbo suture, and (2) two E-W receiver function profiles that cross the Xainza-Dingjye and Nyima-Tingri rifts, respectively. These images reveal different crustal-scale structures in the dominant collision zone between the western and central regions, as well as distinctly offset Moho discontinuities across the N-S trending rift grabens in southern Tibet. These crustal variations, combined with previous tomographic studies in Tibet, outline an easterly tilt of the subducting Indian slab, along which crust-mantle decoupling occurred. Together with the spatio-temporal distribution of synchronous potassium-rich volcanics exposed within the grabens in southern Tibet, this offset Moho structure beneath the grabens lends support to the hypothesis that eastward steepening of the subducting Indian slab was accompanied by slab tearing beneath each north-south striking rift. This overall crustal geometric variation suggests a stepwise flattening of the torn Indian slab from west to the east, a process that brought easterly migration of gravitational instability to the overriding Tibetan crust that drove collapse of the Tibetan Plateau once gravitational instability reached a critical level to the east.

Primary discussion about tectonic deformation characteristics in thev east Yarlungzangbo suture and subsurface structure of the Yarlhashampo dome as revealed by a deep seismic reflection profile

Primary discussion about tectonic deformation characteristics in thev east Yarlungzangbo suture and subsurface structure of the Yarlhashampo dome as revealed by a deep seismic reflection profile

Petrogenesis of lherzolites from the Purang ophiolite, Yarlung-Zangbo suture zone, Tibet: origin and significance of ultra-high pressure and other ‘unusual’ minerals in the Neo-Tethyan lithospheric mantle

ABSTRACTThe Purang ultramafic massif, located in the Yarlung-Zangbo Suture Zone (YZSZ) of the Tibetan Plateau, consists mainly of harzburgites and minor lherzolites. The spinel-bearing lherzolites of the NW part of the massif display a granular texture, consisting of large olivine and pyroxene crystals with curvilinear grain boundaries. These lherzolites contain chromian spinel (Cr-spinel) of low Cr# [100 × Cr/(Cr +Al) = 24.7–30.2], enstatite with high Mg# [100 × Mg/(Mg + Fe2+) = 90.0–91.2] and relatively high Al2O3 content (3.3–4.1 wt%), and diopside with high Mg# (90.2–93.3) and Al2O3 content (4.6–5.0 wt%). These compositions are analogous to those of spinel and pyroxenes from residual peridotites. However, the Purang lherzolites show U-shaped primitive mantle (PM)-normalized rare earth element (REE)-profiles, which are not consistent with a potential origin as melting residues. The high LREE contents and positive Ti anomalies shown by the investigated lherzolites coupled with the low TiO2 content of their mineral constituents imply that these rocks possibly stored LREE- and Ti-bearing arc-related melts/fluids in their groundmass.A mineral assemblage composed of diamond, super-reduced [(SuR) moissanite, native Cr] and crustal-derived minerals (zircon, corundum, rutile), has been separated from the Purang lherzolites. Uranium-Pb geochronological dating of zircons yielded an age range between 1718 and 465 Ma, indicating that they represent ancient crustal material delivered into the upper mantle via previous subduction events. Diamonds and old zircons (± crustal minerals) were carried to shallow mantle levels by asthenospheric magmas produced during a slab rollback-induced decompression melting process. The recovery of SuR minerals is consistent with fluid percolation and crystallization of alteration-related minerals in the lithospheric parts of a (hydrated) mantle wedge, resulting in the formation of highly reduced micro-environments.

Reconsideration of Neo-Tethys evolution constrained from the nature of the Dazhuqu ophiolitic mantle, southern Tibet

The nature (i.e., sub-oceanic, sub-arc or sub-continental) of ophiolitic mantle peridotites from the eastern Neo-Tethyan domain in southern Tibet has been hotly debated. This uncertainty limits our understanding of the history and evolution of the eastern Neo-Tethys Ocean. Here we present petrological, geochemical and Re–Os isotopic data for the mantle peridotites from the Dazhuqu ophiolite in the central segment of the Yarlung Zangbo suture zone, southern Tibet. Samples collected include both spinel lherzolites and spinel harzburgites. The lherzolites have spinel Cr# [Cr/(Cr + Al), ~ 0.3–0.4] comparable to those of typical abyssal peridotites. In contrast, the harzburgites have spinel Cr# (~ 0.3–0.7) overlapping with the ranges of both abyssal and fore-arc peridotites; two samples have spinel Cr# higher than 0.6, which is probably ascribed to intense melt–rock interactions. Clinopyroxene trace element modeling indicates that the Dazhuqu mantle peridotites have experienced 0–6% garnet-facies melting followed by 10–18% melting in the spinel stability field. This is similar to the degree of garnet-facies melting inferred for many abyssal peridotites and implies deep initial melting (> 85 km), which distinguishes the Dazhuqu mantle peridotites from fore-arc peridotites (commonly < 80 km in origin). The Dazhuqu peridotites have unradiogenic 187Os/188Os of 0.11836–0.12922, which are commonly lower than the recommended value of primitive upper mantle (PUM). All but one samples yield relatively younger Re depletion ages (TRD = 0.06–0.81 Ga) with respect to the only one sample having an older TRD age of 1.66 Ga. Re–Os isotopes and highly siderophile element (HSE) compositions of the Dazhuqu peridotites are similar to those of abyssal peridotites and the Oman southern massifs but are distinct from non-cratonic sub-continental lithospheric mantle (SCLM) xenoliths and sub-arc mantle. We emphasize the similarity between the Dazhuqu and Oman ophiolites, both representing Neo-Tethyan oceanic lithosphere and implying ridge–trench collision.

Coexistence of MORB- and OIB-like dolerite intrusions in the Purang ultramafic massif, SW Tibet: A paradigm of plume-influenced MOR-type magmatism prior to subduction initiation in the Neo-Tethyan lithospheric mantle

The Yarlung Zangbo Suture Zone (YZSZ) of South Tibet is divided by the ZhongbaZhada terrane into two subparallel ophiolitic belts in its western end. The peridotite massifs of the southern belt tectonically overlie the Tethyan Himalaya sequence. The Purang peridotite body in this belt is intruded by two groups of dolerite dikes, providing significant compositional, geochronological, and isotopic information about the melting history of the Neo-Tethyan mantle. U-Pb ages of zircons separated from dolerites show that peridotites of West Purang were intruded by an early generation of dikes at 138.5 +/- 2.0 Ma (Valanginian). These dolerites show ocean island basalt (OIB)-type normalized multi-elemental profiles and Sr-Nd isotopic signatures [(La/Yb)(N )= 13-16], high initial Sr-87/Sr-86 ratios (0.70598-0.70765), and low epsilon(Nd)(t) values (-2.6 to -2.3). Zircons separated from this group of dolerites have slightly radiogenic epsilon(Hf)(t) values (+2.6 to +4.6). The next generation of dolerite dikes intruded the East Purang peridotites between 124.5 +/- 2.5 Ma and 124.4 +/- 3.2 Ma (Aptian). These East Purang dolerites show normal mid-ocean ridge basalt (N-MORB)-type normalized multi-element patterns [(La/Yb)(N) = 0.6-0.9] with noticeable negative Nb and Th (+/- Ti) anomalies, and have high Sr-87/Sr-86((i)) (0.70295-0.70618) and high epsilon(Nd)(t) values (+7.7 to +9.2). Zircons separated from the East Purang dolerites show strongly radiogenic epsilon(Hf)(t) values (+3.5 to +17.0).Semiquantitative geochemical modeling demonstrates that the parental magmas of West Purang dolerites were generated from 5 %-10 % polybaric partial melting of a deep-seated juvenile asthenospheric source enriched by plume-type components. In contrast, the parental melts of East Purang dolerites were derived from more than 20% melting of a juvenile spinel-bearing MORB-type mantle source that was modified by subduction-related melts/fluids to a minor extent. A possible tectono-magmatic model for the petrogenesis of the Purang ophiolitic massif could be linked to incipient continental rifting and subsequent oceanic seafloor spreading associated with decompression upwelling of an asthenospheric source contaminated by plume-type components. This plume-proximal seafloor spreading-system was succeeded by the initiation of Neo-Tethyan intra-oceanic subduction close to the active continental margin of Eurasia during the Early Cretaceous.

The burial and exhumation history of the Liuqu Conglomerate in the Yarlung Zangbo suture zone, southern Tibet: Insights from clumped isotope thermometry

The Liuqu Conglomerate, exposed along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet, archives not only the process of the India-Asia collision, but also the exhumation history of the India-Asia collision zone. The burial and exhumation history of the Liuqu Conglomerate thus could potentially provide clues to the India-Asia collision and orogeny. Low-temperature thermochronometric data indicate that maximum burial temperatures of the Liuqu Conglomerate ranged from 80 to 110 °C; however, the exact burial history is not well constrained. Here, we examine the burial conditions of the Liuqu Conglomerate by applying clumped isotope thermometry to paleosol carbonates. Extensive microcrystalline recrystallization indicates that the samples have been diagenetically altered during burial. Moreover, the clumped isotope temperatures (ranging from ∼48 to ∼97 °C) are clearly above Earth surface conditions, indicating the original climatic information was overwritten by alteration at higher burial temperatures. Nevertheless, the calculated diagenetic water δ18O values represent maximum meteoric water values that can be used to broadly constrain the paleoelevation. The highest clumped isotope temperature implies that the maximum burial temperature of the Liuqu Conglomerate was at least ∼97 °C. We conclude that the Liuqu Conglomerate was buried as deeply as 3.7–4.3 km during the latest Paleocene to Eocene, and then exhumed during the Miocene (∼10–12 Ma). Incision of the paleo-Yarlung River was likely responsible for erosion and exhumation of the Liuqu Conglomerate, which suggests that the drainage system of Paleo-Yarlung River was similar to that of today and supports the model of outward growth of the Tibetan Plateau.

Geochemistry and geochronology of dolerite dykes from the Daba and Dongbo peridotite massifs, SW Tibet: Insights into the style of mantle melting at the onset of Neo-Tethyan subduction

This study reports on compositional, whole-rock SrNd isotopic, and zircon UPb geochronological data for dolerite dykes in the Daba and Dongbo ultramafic massifs of the southwest Yarlung–Zangbo Suture Zone (YZSZ), SW Tibet. The 121 ± 2 Ma dolerite dykes from the Daba peridotite exhibit primitive mantle (PM)-normalised multi-element patterns with (La/Yb)N = 0.43–0.72 and negative Nb anomalies. They have high initial 87Sr/86Sr ratios (87Sr/86Sr(i) = 0.707197 to 0.707879) and high εNd(t) values (+7.4 to +7.9). The 125 ± 2 Ma dolerite intrusions within the Dongbo peridotite show PM-normalised trace element profiles [(La/Yb)N = 0.65–0.84] that are characterised by apparent negative Nb anomalies and moderate negative Ti (±Y) anomalies. They also have high 87Sr/86Sr(i) ratios (0.706108–0.706793) and high εNd(t) values (+7.8 to +8.2). Semi-quantitative Sm/Yb vs. La/Sm modelling demonstrates that the parental magmas of the investigated dykes were derived from 10%–20% (cumulative) melting of a (broad) mantle source region that had a spinel-bearing normal mid-ocean ridge basalt (N-MORB)-like lherzolitic composition. The geochemical and isotopic data indicate that the composition of the inferred mantle source was influenced by minor inputs of subducted crustal material. The petrogenesis of the Daba and Dongbo massifs can be linked to the upwelling of an asthenospheric source that caused continental rifting and subsequent seafloor spreading that was followed by the subduction initiation adjacent to a passive margin during the Early Cretaceous (~130–120 Ma). The results provide a more detailed and perhaps more elegant hypothesis for the tectonomagmatic evolution of the southwestern YZSZ “ophiolitic” peridotites after their accretion beneath a Neo-Tethyan marginal basin.

The Moho structure beneath the Yarlung Zangbo Suture and its implications: Evidence from large dynamite shots

The Yarlung Zangbo Suture (YZS) is the collisional front between the Indian and Eurasian plates. The deep structure beneath the YZS has long been a focus of research. Its particular tectonic location and formation time provide a natural laboratory for the study of ongoing continental collisional processes. Lateral variations in the Moho discontinuity provide evidence of basic processes governing plate tectonic evolution. The depth and geometry of the Moho provides first-order information for restoration of complex geodynamic systems. This paper presents a studying result of the Moho, both adjacent to and beneath the YZS, obtained using five large dynamite shots along two profiles. These five large shots were combined to construct two single-fold profiles (YZS-A and YZS-B) and obtain high-resolution images of the Moho beneath the YZS. Both profiles show that continental crust thickness beneath the YZS is twice the global average of continental crust, and the depth of the Moho along profile YZS-B is deeper than YZS-A. YZS-A (81°E) indicates a relatively flat Moho without offset related to continent-continent convergence beneath the collision zone. Conversely, a Moho structure with offset and overlapping reflections was imaged beneath profile YZS-B (88°E). Significant lateral variations in the geometry of the Moho discontinuity along the YZS may indicate that the Indian lithosphere is thrusting beneath Tibet at a different angle from west to east.