Yarlung Zangbo Suture Zone Research Articles

Fore-arc metasomatism by hybrid slab fluids during subduction initiation: Sr–Mg–Ca isotopes of rodingite, western Yarlung Zangbo suture zone

Subduction zone metasomatism is critical for Earth's material exchanges, yet the details of slab dehydration, particularly deserpentinization beneath fore-arcs, remain poorly understood. Here, we present Sr–Mg–Ca isotopic data for Purang rodingites from the western Yarlung Zangbo suture zone (YZSZ), a remnant of the Neo-Tethys Ocean that was subducted during the collision between the Indian and Eurasian Plates. The rodingites, occurring as centimeter- to meter-sized veins and blocks in serpentinized harburgite, are dominated by amphibolite- to greenschist-facies minerals like tremolite, magnesiohornblende, and chlorite. Their cumulate textures and rare earth element patterns resemble troctolite or gabbronorite, presumably formed beneath a seafloor spreading center. The rodingites are enriched in large ion lithophile elements and depleted in high field strength elements, with higher Sr/Nb and Ba/La and lower Nb/U and Ce/Pb ratios than mid-ocean ridge basalts (MORB) and rodingites formed through seawater or serpentinizing fluid alteration of MORB-like protoliths. They also exhibit higher initial 87Sr/86Sr ratios (0.7067–0.7075) and elevated δ26Mg values (−0.22 ± 0.07 ‰ to −0.13 ± 0.02 ‰) compared to unaltered oceanic basalts, while their δ44/40Ca values (0.72 ± 0.02 ‰ to 0.87 ± 0.03 ‰) remain similar to MORB. These signatures point to a metasomatic process beyond seafloor alteration, suggesting modification of a MOR-derived protolith in the fore-arc mantle at slab depths of <40 km, driven by Sr- and Mg-rich fluids from clay-rich sediments and serpentinized mantle. The measured Sr, Mg and Ca isotope compositions can be reproduced by mixing a MORB-like protolith with hybrid fluids derived from varying proportions of sedimentary clays and serpentinized peridotite. Combined with previous studies on YZSZ metamorphic soles, we propose that this fore-arc metasomatism occurred during the early Cretaceous, concurrent with the incipient subduction of the Neo-Tethys oceanic rocks. These results highlight the significance of deserpentinization at shallow fore-arc mantle settings during subduction initiation, suggesting that slab dehydration is more complex than previously recognized.

The Petrogenesis and Geological Implications of the Sanggeda Gabbros, Southern Tibet: Insights from the Amphibole Crystal Population

Amphibole is an important mineral during the differentiation of arc magmas but rarely as a phenocryst in arc lavas or eruptive pyroclastic rocks. The Sanggeda complex, intruded into the ophiolite of the Indus–Yarlung Zangbo Suture Zone (IYZSZ), Zedong, southern Tibet, mainly consists of amphibole-rich, fine-grained, and porphyritic gabbros. The complex provides an opportunity to study the differentiation of arc magmas through amphibole crystals. Four distinct amphibole crystal populations can be recognized according to petrographic observations, EMPA, and LA–ICP–MS analysis. The first ones (Type 1) are fined-grained and euhedral, are crystallized during ascent, and are the product of the shallow emplacement of host magma. The second ones (Type 2) are euhedral, with slight negative Eu and Sr anomalies, and crystallize from an evolved magma that previously experienced plagioclase fractionation. Type 3 amphiboles have similar morphological characteristics to Type 2 but are without Eu and Sr anomalies. Type 4 crystals are shown as pseudomorphs, formed by the reaction–replacement between the clinopyroxene and melt. Type 1 crystals are autocrysts. Other amphiboles within host magma, whether presented as phenocrysts or cumulate nodules, are antecrysts. Based on the amphibole crystal population developed in the complex, in this study, a trans-crustal magma plumbing system is proposed, containing at least three magma reservoirs located at different crust depths: the shallow emplaced crust (~4.8 km), the mid-crust (~12.9 km), and the lower crust (~21.8–24.9 km). Early amphibole crystallization is an effective process to generate silicic residual melts. Gravity could help in that sense. Precursor amphibole and clinopyroxene can efficiently delaminate back into the mantle and promote the generation of silicic continental crust.

Paleoseamounts in Yarlung Zangbo suture zone reveal dynamic processes of the Early Cretaceous Neo-Tethys lithosphere

Dynamic processes of the oceanic crust of seamounts trapped in continental sutures are poorly understood. The discovery and analysis of seamount sequences in the Yarlung Zangbo suture zone allowed us to detect vertical lithospheric movements of the eastern Neo-Tethys Ocean. In this study, we used petrological, geochemical, facies, paleontological, and stratigraphic methods to study composite blocks of mafic and sedimentary rocks in the Zhongba area of Xizang, China. Radiolarian fossils indicate an (early) Early Cretaceous age. Petrofacies and geochemistry show that most basalts occur as pillow lava and are enriched in light rare earth elements and high field strength elements, which suggests an oceanic-island basalt tectonic setting. Sedimentary lithofacies are grouped into breccias, shales, cherts, and limestones (ooid and cortoid grainstone microfacies). Three types of contact between basaltic basements and caps were identified as regular, talus, and erosive. These results demonstrate that two more superposed seamounts exist in the western Yarlung Zangbo suture zone than was known previously, and they comprise at least eight seamount sequences of basalts to sediments. Cap lithofacies and seamount textures illustrate that overall, the Zhongba paleo-seamounts record multiple cycles of abrupt deepening to shallowing, with water depth variations of ∼3000 m to ∼5 m due to multiple episodes of quick subsidence and uplift of seamounts (lithospheric “ups and downs”). The model of oceanic bypass recurrence and plume rejuvenation is hypothesized to explain the dynamic subsidence and uplift. While plume activities produced the mafic basement and considerable buoyancy-related lift, oceanic crust bypassed drift, created distance from the plumes, and led to cooling, loss of buoyancy, and rapid subsidence. This model provides a new approach to interpret the evolution of oceanic crusts that have disappeared.

Early Triassic Volcanic Rocks in the Western Yarlung Zangbo Suture Zone: Implications for the Opening of the Neo‐Tethys Ocean

AbstractIn this study, zircon U‐Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone (YZSZ) in southern Xizang (Tibet) yielded an age of 247 ± 3 Ma. According to whole rock geochemical and Sr‐Nd‐Pb isotopic data, the Early Triassic samples could be divided into two groups: Group 1 with P‐MORB affinity, showing initial 87Sr/86Sr ratios of 0.70253–0.70602, εNd(t) values of 4.2–5.3, (206Pb/204Pb)t ratios of 16.353–18.222, (207Pb/204Pb)t ratios of 15.454–15.564, and (208Pb/204Pb)t ratios of 35.665–38.136; Group 2 with OIB affinity, showing initial 87Sr/86Sr ratios of 0.70249–0.70513, εNd(t) values of 4.4–4.9, (206Pb/204Pb)t ratios of 17.140–18.328, (207Pb/204Pb)t ratios of 15.491–15.575, and (208Pb/204Pb)t ratios of 36.051–38.247. Group 2 rocks formed by partial melting of the mantle source enriched by a former plume, and assimilated continental crustal material during melt ascension. The formation of Group 1 rocks corresponds to the mixing of OIB melts, with the same components as Group 2 and N‐MORBs. The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo‐Tethys in southern Xizang (Tibet).

Correction to “New palaeomagnetic results of Permian tectonic blocks in the western segment of the Yarlung Zangbo Suture Zone and their tectonic implications”

Correction to “New palaeomagnetic results of Permian tectonic blocks in the western segment of the Yarlung Zangbo Suture Zone and their tectonic implications”

Late Cenozoic Cooling History of the Xigaze Fore-Arc Basin along the Yarlung–Zangbo Suture Zone (Southern Tibet): New Insights from Low-Temperature Thermochronology

Abstract The Tibetan Plateau is currently the widest and highest elevation orogenic plateau on Earth. It formed as a response to the Cenozoic and is still ongoing collision between the Indian and Eurasian plates. The Xigaze fore-arc basin distributed along the Indus–Yarlung suture zone in southern Tibet preserves important information related to the late Cenozoic tectonic and topographic evolution of the plateau. In this study, apatite fission track (AFT) thermochronology was carried out on twelve sandstone samples from the middle segment of the Xigaze basin and additionally on four sedimentary rocks from the neighboring Dazhuka (Kailas) and Liuqu Formations. Inverse thermal history modeling results reveal that the fore-arc basin rocks experienced episodic late Oligocene to Miocene enhanced cooling. Taking into account regional geological data, it is suggested that the late Oligocene-early Miocene (~27–18 Ma) cooling recognized in the northern part of the basin was promoted by fault activity along the Great Counter thrust, while mid-to-late Miocene-accelerated exhumation was facilitated by strong incision of the Yarlung and Buqu rivers, which probably resulted from enhanced East Asian summer monsoon precipitation. Sandstone and conglomerate samples from the Dazhuka and Liuqu Formations yielded comparable Miocene AFT apparent ages to those of the Xigaze basin sediments, indicative of (mid-to-late Miocene) exhumation soon after their early Miocene burial (&amp;gt; ~3–4 km). Additionally, our new and published low-temperature thermochronological data indicate that enhanced basement cooling during the Miocene prevailed in vast areas of central southern Tibet when regional exhumation was triggered by both tectonic and climatic contributing factors. This recent and widespread regional exhumation also led to the formation of the high-relief topography of the external drainage area in southern Tibet, including the Xigaze fore-arc basin.

Record of an oceanic island in the Neo-Tethys Ocean (lowermost Cretaceous Xiazha Formation, Yarlung-Zangbo suture zone, South Tibet)

The Xiazha Formation recently identified in the Yarlung-Zangbo suture zone of southern Tibet has been considered as crucial evidence for early (pre-Cretaceous) subduction of Neo-Tethyan oceanic lithosphere. In this study, we carried out an integrated stratigraphic, petrographic, and geochemical study of the Xiazha Formation, which consists of abyssal chert, siliceous mudrocks, and calcareous turbidites deposited onto basaltic basement. Basalt geochemistry indicates alkali to tholeiitic magmas generated in a within-plate oceanic setting. The siliciclastic fraction contained in calcareous turbidites consists of plagioclase and mafic-rock fragments derived from the underlying basalts, and includes Cr-spinel grains with relatively high TiO2 and moderate Al2O3 concentrations pointing to an OIB source. Based on these data, and virtual lack of detritus of continental origin, we infer that the Xiazha Formation was deposited in oceanic island/plateau within the Neo-Tethyan domain. Its stratigraphy records an evolution from basaltic eruptions to the development of a carbonate platform surrounding the basaltic island, and finally to drowning of the atoll. The Xiazha Formation was not deposited in a subduction-related basin and, therefore, cannot be used as evidence for pre-Cretaceous initiation of the Yarlung subduction zone.

Timing and mechanism of opening the Neo-Tethys Ocean: Constraints from mélanges in the Yarlung Zangbo suture zone

Timing and mechanism of opening the Neo-Tethys Ocean: Constraints from mélanges in the Yarlung Zangbo suture zone

Timing of India-Asia collision and significant coupling between them around 51 Ma: Insights from the activation history of the Zhongba-Gyangze thrust in southern Tibet

Abstract Constraining the timing of tectonic coupling between converging plates is crucial for understanding the transition from continental subduction to continental collision. In the case of the India-Asia collision, thrusting of an accretionary complex onto the Indian continental margin provides the most direct temporal constraint on the early stages of continental collision, as it represents the most immediate upper-crustal fault system corresponding to plate coupling. Here, we used structural analysis combined with K-Ar dating and hydrogen isotopes of authigenic illite and muscovite to unravel the time-progressive development of the Zhongba-Gyangze thrust (ZGT), which represents a tectonic boundary fault in southern Tibet. Our results suggest that the ZGT evolved from its initiation as a single fault zone infiltrated by metamorphic fluids with high δD values (–47‰ to –55‰) at ca. 80 Ma to multiple deformation localization zones starting around 51 Ma. This latter phase is represented by the development of different generations of authigenic 1 M/1Md illite and significant input of meteoric fluids with δD values ranging from –71‰ to –98‰ through multiple episodes of brittle fault reactivation. A Late Cretaceous tectono-thermal event related to the subduction of a Neotethyan oceanic ridge may have been responsible for the formation of 2M1 illite/muscovite at ca. 80 Ma. The oldest (ca. 51 Ma) 1 M/1Md illite age coincides with the first major pulse of shortening in the upper plate after the initial India-Asia contact. Given the synchronous deceleration of India-Asia convergence, the ca. 51 Ma deformation pulse across the Yarlung-Zangbo suture zone demarcates strong coupling (i.e., the onset of continental collision) between India and Asia at this time.

Tracking multiple stages of serpentinization processes of the Yarlung Zangbo Suture Zone peridotites in southern Tibet: Implications for the tectonic evolution of the Neotethyan oceanic lithosphere

Serpentinites are key repositories of fluid-mobile elements (FMEs) in subduction zones and record significant information about the origin and geodynamic evolution of oceanic lithosphere. Here, we report on the structural textures and mineralogical compositions of different types of serpentinites collected from the central segment of the Yarlung Zangbo Suture Zone in southern Tibet and present their bulk-rock and mineral chemistry, and Sr isotopic compositions. The main textures include massive, scaly, and gouge serpentinites exposed in the Ngamring and Sangsang ophiolites. Bulk-rock Al2O3/SiO2 and spinel Cr# values suggest that the Ngamring serpentinites originally formed in an abyssal setting, whereas the Sangsang serpentinites developed initially in a forearc mantle. Both serpentinite assemblages were subsequently incorporated into a subduction plate interface as subducted serpentinites. Massive serpentinites preserve the geochemical fingerprint of original serpentinized fluids in mid-oceanic ridge to forearc settings, whereas sheared serpentinites with scaly and gouge textures are reset in their Sr isotopic compositions and FME ratios (i.e., Cs/U, Li/U, and Rb/U) due to their reactions with slab-derived fluids. Scaly and gouge types of the Ngamring serpentinites have lower 87Sr/86Sr values (87Sr/86Sr = 0.7081−0.7082) and higher alkali element−U ratios (i.e., Cs/U, Li/U, and Rb/U) than those of the massive serpentinite types (87Sr/86Sr = 0.7091−0.7096), which indicates that they interacted with fluids at a slab interface after their initial seafloor serpentinization. In contrast, the massive Sangsang serpentinites display lower 87Sr/86Sr values (87Sr/86Sr = 0.7041−0.7043, similar to those of the Yarlung Zangbo ophiolites) and higher alkali element−U ratios than those of the sheared serpentinites (87Sr/86Sr = 0.7063−0.7087). These findings point to the significant role of the increased influx of subducted sediment-derived fluids within subduction shear zones in further affecting the serpentinization fingerprint. This study demonstrates that serpentinites with different textural, geochemical, and isotopic features within the same suture zone may represent the serpentinization products in different tectonic environments during the seafloor spreading, subduction initiation, and subduction zone evolution of oceanic lithosphere.

Ultramafic pseudotachylytes in high-pressure metamorphogenic peridotite from Luobusha, Tibet: a record of crustal paleo-earthquakes

In this paper, we report an occurrence of ultramafic pseudotachylytes, providing fault-rock evidence of paleo-earthquakes, from the Luobusha ophiolite complex in the Yarlung Zangbo suture zone. The pseudotachylytes form hairline-thin aphanitic veinlets and vein networks bounded by micro-damage zones cutting through the host harzburgite, forming flow banding in some places. The pseudotachylyte veins are dominated by close-knit ultrafine-grained minerals consisting of olivine, orthopyroxene, serpentine, spinel, and magnetite, cemented by an extremely fine matrix. As the primary component of the pseudotachylyte veins, olivine occurs as microphenocryst showing zoning from core to rim and as irregularly shaped microlite immersed in the interstitial material. Zoned crystals of olivine developed with Mg-rich cores and more Fe-rich rims. Microlite diagnosis of crystallization in a quenched melt includes dendritic, skeletal, and poikilitic olivine crystals, which are typical of ultramafic pseudotachylyte. The olivine microlites contain higher amounts of Ca, Al, and Cr but a lower Ni content compared with the host harzburgite olivine. Irregularly shaped chrome-spinel crystals are chemically zoned as well, indicating an Fe-rich rim overgrowth. Ni sulfide droplets interspersing among the matrix imply melt occurrence. The presence of a micro-fibrous and micro-vesicular interstitial matrix also indicates a melting-related origin. Ultracataclastite veins associated with pseudotachylyte transecting serpentine are observed, which convey that heat was generated during rapid comminution and injection. The characteristic petrography, microtextures, and chemical inhomogeneities meet the criteria of ultramafic pseudotachylyte and reveal a mixed genesis via a combination of crushing and melting. The development of extremely tiny globular prograde serpentine inclusions (∼100 nm) in the olivine microlites is ascribed to the dehydration reaction of serpentine to olivine within the pseudotachylyte. The Luobusha metamorphogenic peridotite was subjected to serpentinization after having emplaced in the crust and subsequently to high-pressure metamorphism. The pseudotachylytes were generated in the crust after the high-pressure metamorphism and did not descend to a greater depth. Flash ultra-comminution associated with frictional heating may release fluids via localized heat-driven prograde reactions in the crust.

Lithofacies, geochemistry, and sequences of basalt and carbonate rocks of a Middle Permian composite seamount (central Yarlung Zangbo Suture Zone, Tibet): Implications to the incipient opening of the Neo-Tethys Ocean

Lithofacies, geochemistry, and sequences of basalt and carbonate rocks of a Middle Permian composite seamount (central Yarlung Zangbo Suture Zone, Tibet): Implications to the incipient opening of the Neo-Tethys Ocean

Genesis of Pyroxenite Veins in the Zedang Ophiolite, Southern Tibetan Plateau

AbstractUnderstanding the nature of parental melts for pyroxenite veins in supra‐subduction zone (SSZ) ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones. The Zedang ophiolitic massif in the eastern Yarlung–Zangbo suture zone in Tibet consists of mantle peridotites and a crustal section of gabbro, diabase, and basalt. Veins of two pyroxenite varieties cut the southern part of the Zedang massif. These pyroxenite rocks have different geochemical characteristics, where the first variety (type‐I) has relatively higher contents of SiO2 (51.82–53.08 wt%), MgO (20.08–23.23 wt%), and ΣPGE (3.42–13.97 ppb), and lower Al2O3 (1.59–2.28 wt%) and ΣREE (1.63–2.94 ppm). The second pyroxenite variety (type‐II) is characterized by SiO2 (45.44–49.61 wt%), MgO (16.68–19.78 wt%), Al2O3 (4.24–8.77 wt%), ΣPGE (14.46–322.06 ppb), and ΣREE (5.82–7.44 ppm). Pyroxenite type‐I shows N‐MORB‐like chondrite‐normalized REE patterns. Zircon U‐Pb ages of pyroxenite type‐I (194 ± 10 Ma), associated ophiolitic gabbro (135.3 ± 2.0 Ma), and plagiogranite (124.2 ± 2.3 Ma) evidently imply episodic evolution of the Zedang ophiolites. The mineralogical and geochemical characteristics of the investigated pyroxenites can be explained by subduction‐initiated hydrous melting of metasomatized sub‐arc mantle, later overprinted by sub‐slab mantle melting triggered by upwelling asthenosphere during the Jurassic–Early Cretaceous times. The geochemical variations in pyroxenite vein composition, coupled with age differences amongst the other ophiolite units, may correspond to intermittent emplacement of pyroxenite dikes and isotropic gabbroic intrusions where the geodynamic setting progressed from arc maturation and slab rollback to slab tearing and delamination.

New palaeomagnetic results of Permian tectonic blocks in the western segment of the Yarlung Zangbo Suture Zone and their tectonic implications

The composition and structure of the Yarlung Zangbo Suture Zone (YZSZ) provide important constraints on long‐standing disputes about the evolution of the Neo‐Tethys Ocean and continental collision. The Permian tectonic blocks (mainly composed of limestones and volcanic rocks) are widely distributed in the YZSZ. There are still different understandings about the origin of these blocks and their relationship with the evolution of the Neo‐Tethys Ocean. Here, we report the first reliable palaeomagnetic results of the Permian tectonic blocks in the Gyanyima and Mayum areas, the western segment of the YZSZ. Fossil records and our new zircon U–Pb results provide 254–252 Ma and 249 ± 2 Ma age constraints for the strata in the Gyanyima and Mayum sections, respectively. A total of 109 specimens were subjected to stepwise thermal demagnetizations. Among them, 83 specimens yielded interpretable results. After removing a recent geomagnetic field direction at low to intermediate temperature ranges, characteristic remanent magnetization (ChRM) directions can be isolated from the upper Permian–lower Triassic Gyanyima Formation (Fm). Unblocking temperature spectra of the ChRM indicate magnetite and haematite as the main carriers in the basalt and limestone/andesitic‐basalt samples, respectively. The ChRM passed the fold tests and a reversal test at 95% confidence level, indicating that it is most likely a primary remanence. The tilt‐corrected mean direction for 14 sites is D = 18.0°, I = −14.4°, k = 20.5, and α95 = 9.0°, which corresponds to a palaeolatitude of 7.3°S and a palaeopole at 48.3°N, 233.3°E with A95 = 8.1°. Combining the rock assemblage, palaeomagnetic, palaeontological, and geochronologic data, we conclude that these blocks were most likely seamounts formed in the low‐latitude area of the Neo‐Tethys Ocean. The palaeolatitude and origin of these blocks indicate that the evolution of the Neo‐Tethys Ocean was a complex process interspersed with the development of a series of seamounts and microblocks.

Detrital zircon U–Pb geochronology and fluvial basin evolution of the Liuqu Conglomerate within the Yarlung Zangbo Suture Zone: A critical geochronometer for the collision tectonics of the Tibetan-Himalayan Orogenic Belt

We present new U-Pb detrital zircon ages, depositional history and tectonic model for the Liuqu Conglomerate (LQC) in southern Tibet that represents a critical geochronometer for the collision history of the Tibetan-Himalayan Orogenic Belt. LQC is a ∼5 km–thick, late Mesozoic–Cenozoic molasse deposit occurring strictly within the Yarlung Zangbo Suture Zone (YZSZ) and is tectonically overlain to the north by the Cretaceous Xigaze ophiolite and to the south by the Mesozoic Tethyan Himalaya sequence. It consists of matrix- and clast-supported conglomerates with sandstone intercalations, and its matrix includes poorly to moderately sorted sandstone and mudstone. New U–Pb detrital zircon dating of LQC sandstones has revealed a youngest zircon age of 307 ± 13 Ma and an oldest zircon age of 3362 ± 51 Ma. The age spectrum of zircons displays a prominent peak of ∼935 Ma, two large peaks at ∼516 Ma and 1474 Ma, and two small clusters of ∼2429 Ma and ∼2772 Ma that point to East Gondwana as the likely provenance for the LQC depocenter. The LQC represents fluvial deposits of an axial river system, which developed in an orogen-parallel, transtensional accommodation space within the YZSZ, after the collision of the Late Jurassic-Early Cretaceous Trans–Tethyan arc–trench system with the northern edge of India in the latest Cretaceous. The Indian subcontinent with the accreted Tethyan ophiolites and the intra–suture LQC depocenter arrived at and collided with the active margin of Eurasia during the latest Oligocene (∼23 Ma). The LQC depocenter started receiving clastic material and zircons for the first time from the Gangdese Magmatic Belt and the Xigaze forearc basin to the north by ∼20 Ma. The ensuing continent–continent collision resulted in significant crustal uplift across the collision zone, and in the inversion and rapid exhumation of the LQC strata by the early–Middle Miocene. The depositional and exhumation history of the fluvial LQC formation within the YZSZ involved two discrete collision events during the evolution of the Tibetan-Himalayan Orogenic system.

Accretion Processes of Oceanic Crust in Slow‐spreading Ridges: Plagiogranite Perspective of the Xigaze Ophiolite, South Tibet

AbstractStructural and petrological data suggest that the Xigaze ophiolite from the Yarlung Zangbo Suture Zone (YZSZ) in south Tibet was a typical slow‐spreading ridge. A new field, geochemical, mineral, and U‐Pb zircon dataset of plagiogranite intrusions were used to constrain the dynamic processes of oceanic accretion in this slow‐spreading ridge. Plagiogranites mainly occur as dykes or intrusions intruded into the whole sequence of the ophiolite and have a similar orientation to the dolerite dykes developed in the late stage of detachment faulting. U‐Pb zircon ages of 122–123 Ma were obtained for two types of plagiogranites and associated dolerite dykes. Detailed geochemical and mineralogical examinations suggest that the plagiogranites are the product of low‐pressure (2–3 kbar) fractional crystallization of mid‐ocean ridge basalt‐like magma and unlikely to have been derived from the partial melting of hydrous gabbroic rocks. The complex cross‐cut relationship between the plagiogranites and ophiolite sequence reflects that they are controlled by small discontinued melt lenses rather than a big magma chamber under the ridge axis and reveals multiple injections during the oceanic crust accretion. The formation of plagiogranites possibly reflects the complex characteristic of oceanic accretion at slow‐spreading ridges, time‐dependent on structural (external) and magmatic (internal) processes.

Reconstruction of the South Qiangtang–Zhongba–Tethyan Himalaya continental margin system along the northern Indian Plate: Insights from the paleobiogeography of the Zhongba microterrane

• The Zhongba microterrane drifted away from the Indian Plate during the Early Permian. • The South Qiangtang–Zhongba–Tethyan Himalaya system developed along the Indian Plate. • The Lhasa Block was exotic to the northern Indian margin system during the Permian. Information regarding the paleobiogeography of the Qinghai–Tibet Plateau can improve our understanding of its tectonic evolution. The Zhongba microterrane has been recognized as a small stratigraphical terrane within the western Yarlung–Zangbo suture zone. Paleobiogeographically, the Zhongba microterrane exhibited a moderate relationship with the Tethyan Himalaya throughout the Permian because they shared several typical Gondwanan/bipolar cool-water brachiopods and bivalves. However, several Cathaysian warm-water brachiopods, bryozoans, fusulines and compound corals appeared in the Zhongba microterrane as early as the Late Cisuralian but were not present in the Tethyan Himalaya until the latest Lopingian, indicating that the Zhongba microterrane drifted away from the northern Indian Plate no later than the Late Cisuralian. Additionally, Early Triassic marine faunas observed in the Zhongba microterrane were characterised by Tethyan ammonoid genera, along with several cosmopolitan/widely distributed ammonoids and bivalves, but Gondwanan/bi-temperate elements observed in the Tethyan Himalaya were lacking in the Zhongba microterrane, suggesting that the Zhongba microterrane had been biogeographically decoupled from the northern Indian Plate during that time. Additionally, both the Zhongba microterrane and Tethyan Himalaya exhibited an abrupt faunal variation from benthic brachiopod- to nektonic ammonoid-dominated assemblages across the P/T boundary, which is thought to be related to harsh and inhospitable oceanic environmental conditions during the Permian–Triassic mass extinction. Our results, together with published paleontologic, paleomagnetic, magmatic, and sedimentary studies, indicated that the Lhasa Block was exotic to the South Qiangtang–Zhongba–Tethyan Himalaya continental margin system along the northern Indian Plate during the Permian.

Petrogenesis of mafic rocks from the Xigaze ophiolite, Southern Tibet: Insights into forearc extension induced by Neotethyan rollback

The Xigaze ophiolite in the central Yarlung Zangbo suture zone (southern Tibet) provides evidence of the tectonic processes that formed the Neotethyan Ocean between the Indian and Eurasian plates. In this study, we present zircon U Pb geochronological, mineralogical, and geochemical data for mafic rocks from the Xigaze ophiolite to determine their petrogenesis and tectonic environment. The crustal section of the Xigaze ophiolite includes mainly gabbro lenses, amphibolite dikes, dolerite sills or dikes, and basaltic lavas. The absence of a well-developed gabbroic crust and sheeted dike complexes in the ophiolite indicates slow to ultraslow-spreading conditions and a heterogeneous structure of the oceanic lithosphere, which is not atypical for layered ophiolites. Zircons from the amphibolite and dolerite dike reveal crystallization ages of 125.0 ± 1.8 Ma to 123.8 ± 1.3 Ma, respectively. Geochemically, all mafic rocks have mid-ocean ridge basalt (MORB)-like affinities with enrichment in large ion lithophile elements (LILEs, U) relative to high field strength elements (HFSEs, Nb, Ta and Ti) and light rare earth elements (LREEs). They also exhibit high εNd(t) values ranging from +8.4 to +8.8 at 206 Pb/ 204 Pb = 16.75–17.64. Moreover, chromite and clinopyroxene grains from the mafic rocks have a wide compositional spectrum and are mainly plotted in the supra-subduction zone (SSZ) fields. These features indicate that MORB-like magmatism was triggered by decompression melting of the upwelling asthenospheric mantle with minor mass transfer from the subducting slab. The Xigaze ophiolite lacks boninite, high‑magnesium andesites, island arc tholeiites, and calc-alkaline lavas, which indicates that it is incompatible with the complete Izu-Bonin-Mariana forearc sequence. Combined with previous studies, we speculate that the Xigaze ophiolite represents an ancient SSZ-type oceanic lithosphere formed in the Gangdese forearc during the Early Cretaceous, and the upper plate extension is linked to the southward retreat and rollback of the Neotethyan slab. Thus, the Xigaze and other Neotethyan SSZ ophiolites with no systematic change in composition with stratigraphic age provide new insights into the diversity of melt evolution in the forearc crust. • Amphibolite and dolerite dikes in the Xigaze ophiolite were formed during the Early Cretaceous. • The mafic rocks were derived from upwelling asthenosphere with minor slab input. • The Xigaze ophiolite originated from the Gangdese forearc setting. • The SIR model appears likely to be oversimplified for variable forearc ophiolites.

Isotope spatiotemporal analysis and prospecting indication based on GIS in Tibet

Tibet has received much attention as a pivotal area to explore the genesis of ore deposits. So far, a large number of researchers have conducted a lot of discussions on the geological conditions, genesis, distribution and scale of ore deposits in Tibet, and accumulated a plenty of isotopic test data. Isotopic data can reveal the geological changes of an area and analyze the metallogenic regularity. In this paper, combined with GIS (Geographic Information System) spatiotemporal analysis technology, Hf and Nd isotopic data mapping analysis has been carried out in Tibet. The mapping shows high zircons εHf(t) value and high whole rock εNd(t) value and low TDMc value on both sides of the Eastern Yarlung Zangbo Suture Zone and the Western Bangong-Nujiang Suture Zone. It indicates that the terranes in these two areas are relatively young and have typical characteristics of Cu and Au mineralization, which are favorable areas for the prospecting of Cu and Au. Combined with the geological fault characteristics of Tibet Plateau, the influence of terrane boundary on the deposit and the relevant characteristics of geochemical elements, three favorable targets for Cu and Au prospecting in Tibet Plateau are delineated by GIS analysis. The results show that the characteristics of isotopic data in Tibet Plateau are quite different in disparate terranes, and the isotopic analysis can quickly delineate the strategic prospecting targets. Therefore, using GIS technology for isotope analysis and metallogenic prediction provides important guiding significance for scientific prospecting.

Petrogenesis of the East Hoerba harzburgites, SW Tibet: Implications for melt stagnation in the lithospheric mantle of Neo-Tethys

In this study we present petrographic and compositional data for harzburgites from the eastern parts of the Hoerba Massif of the Yarlung-Zangbo Suture Zone (YZSZ), SW Tibet. These harzburgites are cross cut by semi-ductile mylonitic zones. The investigated rock samples contain aggregates of unstrained clinopyroxene with tapered edges. They also contain olivine with a high forsterite [Fo# = Mg/(Mg + Fe) × 100] content (89.3–91.8), orthopyroxene with a low Al 2 O 3 content (2.0–5.6 wt%), and clinopyroxene porphyroclasts with Cr 2 O 3 contents between 0.2 and 1.0 wt% and low lanthanide contents. Chromian spinel (Cr-spinel) in the East Hoerba harzburgites has low Cr# [100 × Cr/(Cr + Al)] and high Mg# [100 × Mg/(Mg + Fe)] values (23.7–39.2 and 62.2–76.3, respectively). These harzburgites have chondrite-normalized rare earth element (REE) patterns characterized by enrichments in the light (L-)REE and positive slopes from the middle to the heavy REE. Semi-quantitative geochemical modeling demonstrates that the bulk-rock compositions of the East Hoerba harzburgites differ from those of typical abyssal and forearc peridotites. Clinopyroxene trace element modeling suggests that the East Hoerba harzburgites initially underwent melting below a mature spreading center, followed by melting in the mantle wedge above a Cretaceous Neo-Tethyan subduction system. Enrichment of the harzburgite samples in LREE is interpreted as due to post-melting entrapment of small volumes of a LREE-rich melt in the lithospheric mantle. We suggest that the East Hoerba harzburgites are sub-oceanic peridotites that underwent melt depletion events and melt stagnation before their exhumation at the Neo-Tethyan seafloor. • The East Hoerba Massif is composed of variably depleted harzburgites. • They were formed from mantle depletion events and post-melting magma stagnation. • They were generated beneath a transitional arc–back-arc setting. • They do not represent fragments of old sub-continental lithospheric mantle.