Yarlung Zangbo Suture Zone Research Articles

Radiolarian Biochronology, Detrital Zircon Geochronological and Geochemical Constraints on Provenance and Depositional Environment of Cherts in the Southern Belt of the Western Yarlung Zangbo Suture Zone, Tibet

AbstractOphiolites in the southern belt (SB) occur as much larger peridotite massifs compared with those of the northern belt (NB), sporadically overlain by a thin layer of isotropic gabbro in the ...

Cenozoic retrogression and exhumation of the amphibolites in the eastern Gangdese Belt, SW China

Abstract The collision between India and Asia is marked by the Yarlung Zangbo suture zone, which can be traced from Tibet into northwest Yunnan. The suture zone provides a record of the subduction of Neotethyan oceanic lithosphere beneath the Lhasa Block, creating the Gangdese magmatic belt, prior to the India–Asia collision. This paper reports petrological, zircon U–Pb, and whole-rock elemental and isotope data for amphibolite from the extension of the Gangdese belt in northwestern Yunnan. Two metamorphic stages are identified in relict amphibolite-facies rocks preserved in granitic gneisses from the belt. They are early amphibolite-facies metamorphism (M1) and late amphibolite-facies retrograde metamorphism (M2). The assemblage of M1 comprises garnet, amphibole, and plagioclase that formed at 0.89–0.97 GPa and 658–700 °C. Dating of syn-metamorphic zircons assigns this event to the middle Eocene (44–38 Ma). Symplectites (corona) of amphibole, plagioclase, and quartz surrounding garnet formed during retrograde stage M2, at 30–29 Ma, under P–T conditions of 0.41–0.68 GPa and 549–716 °C. The amphibolites display geochemical characteristics similar to those of mid-ocean ridge basalt, with eNd(t) values of +1.8 to +8.9, and are inferred to have formed in a back-arc-basin setting, suggesting that they constitute part of the lithospheric mantle of the Tengchong terrane that lies to the east. The amphibolite-facies metamorphism was related to underplating of the Indian plate during the India–Asia collision, which led to clockwise rotation of eastern Tibet during the middle Eocene, generating amphibolite recrystallization and, subsequently, with ongoing convergence of the two plates, exhumation of the region during the middle Oligocene.

Fingerprints of the Kerguelen Mantle Plume in Southern Tibet: Evidence from Early Cretaceous Magmatism in the Tethyan Himalaya

AbstractEarly Cretaceous magmatism suggested to be related with the Kerguelen mantle plume has been reported in both the eastern and western Tethyan Himalayan terrane. Coeval magmatism (133–138 Ma) recorded by hypabyssal intrusive rocks have been recently discovered in the central Tethyan Himalaya (TH). The hypabyssal intrusions are dominated by OIB‐like basaltic rocks intruded by later porphyritic/ophitic intermediate rocks and are characterized by strongly light rare earth element enrichment and prominent Na‐Ta depletion and Pb enrichment. The basaltic rocks have low 143Nd/144Nd ratios ranging from 0.512365 to 0.512476 but relatively high 87Sr/86Sr ratios ranging from 0.708185 to 0.708966. The ∊Nd(t) ratios of the basaltic rocks are between −4.33 and −2.20 and initial 87Sr/86Sr ratios are 0.707807 to 0.708557. Geochemical data demonstrate that these rocks have experienced combined crustal assimilation and fractional crystallization processes. Magmatic zircons from the hypabyssal rocks exclusively have negative ∊Hf (t) values ranging from −0.7 to −12.7, suggestive of assimilation of crustal material. Zircons from these hypabyssal rocks have U‐Pb ages ranging from 130 to 147 Ma. Inherited zircons have U‐Pb ages from 397 to 2495 Ma. All the zircons are characterized by negative ∊Hf(t) values. The Jiding ocean island basalt (OIB)‐like magmatism is geochemically and geochronologically comparable with that in the western and eastern Tethyan Himalaya, indicating widespread OIB‐like magmatism in the northern margin of Greater India during the Cretaceous. Collectively, these rocks can be correlated with other early Cretaceous magmatism in western Australia and northern Antarctica. Considering the similarities, we suggest that the Jiding hypabyssal rocks are also genetically related to Kerguelen plume. Within the Yarlung Zangbo Suture Zone (YZSZ), there are also numerous occurrences of OIB‐like rocks derived from mantle sources different from those of N‐MORB‐like magmas. The OIB‐like magmatism in the YZSZ is nearly coeval with that in the TH, and the two are geochemically similar. We suggest that the OIB‐like magmatism in the Neo‐Tethyan ocean and the northern margin of Greater India may represent the dispersed fingerprints of the Kerguelen plume preserved in southern Tibet, China.

Remnants of a Middle Triassic Oceanic Lithosphere in the Western Yarlung Zangbo Suture Zone, Southern Tibet

AbstractDiscontinuous ophiolite suites along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet representing remnants of the Neo‐Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous. Older ophiolite or ophiolitic mélange (e.g. Triassic) to reveal the initial evolution the Neo‐Tethyan ocean within the YZSZ have rarely been documented so far. The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic sub‐belt, the Zhongba terrane and the southern ophiolitic sub‐belt. In this study, we document structural, petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub‐belt of the western YZSZ. Two lithos‐tectonic sub‐units, the southern ophiolitic complex and the northern ophiolitic mélange, are recognized in the Mayoumu massif in terms of structural pattern and petrology. LA‐ICP‐MS zircon U‐Pb dating of gabbro from the ophiolitic mélange yields an age of 243 ± 1 Ma with zircon ∊Hf(t) values of +7.9 to +13.2. Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zircon ∊Hf (t) values ranging from +10.8 to +15.0 and +12.3 to +15.4, respectively. Geochemically, rare earth elements (REEs) patterns show that these mafic intrusions are similar to those of the normal mid‐ocean‐ridge basalt (N‐MORB). Enrichment of fluid‐mobile elements (e.g. Rb and Ba) and depletion of Nb, and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab. Our data strongly confirms that the Neo‐Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least. The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo‐Tethyan ocean could be as a continuing process during 120–130 Ma. Emplacement and preservation of older ophiolitic recorder during evolution of the Neo‐Tethyan Ocean may be closely related to the occurrence of the Zhongba micro‐terrane within the western YZSZ.

Tectonic Evolution of Neotethys Ocean: Evidence of Ophiolites and Ocean Plate Stratigraphy from the Northern and Southern belts in the Western Yarlung Zangbo Suture Zone, Tibet

AbstractThe Yarlung Zangbo suture zone (YZSZ) separates Indian plate and its northern passive margin units to the south from Eurasian plate and its active continental margin units of Xigaze forearc basin and Gangdese batholith to the north (Xu et al., 2015; Yang et al., 2015). The western YZSZ in southern Tibet is divided by the Zhongba terrane into the northern belt (NB) and southern belt (SB). Ophiolites in the NB are dismembered as ophiolitic mélanges. Peridotite, cumulated gabbro, ocean plate stratigraphy (OPS) of seamount remnants and pelagic‐hemipelagic sequence as blocks in serpentinite matrix are mainly observed, from west to east, in Dajiweng, Baer, Kazhan, Cuobuzha, Zhalai, Gongzhu. Ophiolites in the SB are absent ophiolitic units of sheeted dikes and MORB‐like pillow lavas, occur as much larger peridotite massifs (i.e., Dongbo, 400 km2; Purang, 650 km2; Xiugugabu, 700 km2; Dangqiong, 300 km2) which are intruded by mafic dike swarms and overlain by volcanic sedimentary OPS (Liu et al., 2018). We propose that the SB mafic–ultramafic rocks and volcanic sedimentary OPS represent fragments of an early Cretaceous continental margin ophiolite whose magmatic evolution was influenced by 140–137 Ma plume magmatism (Liu et al., 2015; Zheng et al., 2019). Relics of Late Paleocene to very Early Eocene deep‐marine basin were developed in Saga and Gyirong (Ding, 2003; Li et al., 2018). In contract, the NB ophiolitic mélanges report a travel log of an oceanic plate ranging from Middle Triassic to Early Cretaceous.

Peridotites and Chromitites from the Dingqing Ophiolite in the Eastern Segment of Bangong–Nujiang Suture Zone, Tibet: Occurrence Characteristics and Classifications

AbstractThe Dingqing ophiolite is located in the eastern segment of the Bangong‐Nujiang suture zone. This suture zone is W–E trending parallel with the Yarlung–Zangbo suture zone and is an strategic area for exploring chromite deposits in China. The Dingqign ophiolite is distributed in near SE‐NW direction. According to the spatial distribution, the Dingqing ophiolite is sudivided into two massifs, including the East and the West massifs. The Dingqing ophiolite covers an area of nearly 600 km2. This ophiolite is composed of peridotite, pyroxenite, gabbro, diabase, basalt, plagiogranite and chert (Fig. 1).The peridotite is the main lithology of the Dingqing ophiolite. The peridotite covers about 90% of the total area of the Dingqing ophiolite. The Dingqing ophiolite is dominated by harzburgite with a small amounts of dunite. The Dingqing harzburgite displays different textures, such as massive, Taxitic, oriented and spherulitic textures (Fig. 2d–i). These four types of harzburgite occur in both the East and West massifs, especially in the Laraka area of the eastern part of the East massif. Dunites have different occurrences in the field outcrops, such as lenticular or strip‐shaped, thin‐shell and agglomerate varieties (Fig. 2a–c).On the basis of detailed field work, we have discovered 83 chromitite bodies, including 27 in the East massif and 56 in the West massif. According to the occurrence scale and quantity of the chromitite bodies, we have identified four prospecting areas, namely Laraka, Latanguo, Langda and Nazona. Chromitites in the Dingqing ophiolite show different textures, including massive, disseminated, veined and disseminated‐banded textures (Fig. 3). On the basis of the Cr# (=Cr/(Cr+Al)×100) of chromite, we have classified the Dingqing chromitite into high‐Cr, medium high chromium type, medium chromium type and low chromium type chromitite (Figs. 4, 5). Among them, low chromium type chromitite Cr# is extremely low, ranging from 9.23 to 14.01, with an average of 11.89; TiO2 content is 0.00% to 0.04%, and the average value is 0.01%, which may be a new output type of chromitite.These different types of chromitites have different associations/assemblages of mineral inclusions. The inclusions in high chromium type chromitite are mainly clinopyroxene and a small amount of olivine; medium high chromium chromitite are mainly amphibole, a small amount of clinopyroxene and phlogopite; while low‐chromium chromite rarely develops mineral inclusions, and micron‐sized clinopyroxene inclusions are common in olivines which are gangue mineral in it.These different types of chromite ore bodies have a certain correspondence with the field output, and may also restrict their genesis. This part will be further developed in the follow‐up work.

Comparison of Redox States between the Ultramafic Bodies of Xigaze and Luobusha Ophiolites, Tibet, China

AbstractThe tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity (fO2) (e.g., Ballhaus, 1993; Dare et al., 2009; Parkinson and Arculus, 1999). Here we present results of fO2 of chromites determined by Mössbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet.The fO2 values (−1.02∼0.04 log units against the FMQ buffer) and Cr# (22∼54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO2 values (0.04∼2.25 log units) and Cr# (73∼84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO2 values (−0.22∼2.19 log units) and Cr# (22∼82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO2 values (−0.3∼0.71 log units) and Cr# (16∼63%), both of which are similar to those of MORB‐like basalts, inferring that they form in an extensional setting. Both high‐Cr and high‐Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO2 values (e.g., Chashchukhin and Votyakov, 2009; Melcher et al., 1997; Quintiliani et al., 2006; Rollinson and Adetunji, 2015), while the distribution‐limited small chromitites and chromite dunites from Dazhuqu exhibit low fO2 values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.

The Boninite‐like Dolerites in the Xigaze Ophiolites, Tibet: Similar to the MORB‐like Dolerites

AbstractThe Early Cretaceous Xigaze ophiolites (XO) exposed along the central segment of the more than 2000 km long Yarlung Zangbo Suture Zone in southern Tibet, preserved the structure of the upper mantle and oceanic crust, is interpreted as a definite record of Neo‐Tethys oceanic lithosphere. Many evolution models of the XO have been proposed since the 1980s (Nicolas et al., 1981; Hébert et al., 2012; Maffione et al., 2015). However, the geodynamic environment of the XO whether at a MOR or SSZ remains controversial. As key evidence for the identification of the SSZ ophiolite, the boninite‐like dolerites from the XO are still controversial and poorly constraint (Chen et al., 2003; Bao et al., 2013; Dai et al., 2013). According to previous reports, dolerites were subdivided to MORB‐like type and boninite‐like type (Chen et al., 2003; Dai et al., 2013). Apart from high‐SiO2 (> 52 wt%), high MgO (>8 wt%) and low‐TiO2 (<0.5 wt%), we found the boninite‐like and MORB‐like dolerites are indistinguishable in outcrop, mineral and chemical. They are sills invaded into mantle sequence of the XO, with 50.35–56.80 wt% SiO2, 0.32–1.19 wt% TiO2, 7.39–8.89 wt%Fe2O3T, 5.50–10.42 wt% MgO and Mg# of 0.49–0.74. MORB‐like dolerites display trace element and REE patterns similar to those of most fore‐arc or/and back‐arc basalts from Izu‐Bonin‐Mariana (IBM), i.e. enrichment in large ion lithophile elements (LILEs, e.g. Cs, Rb, Ba, Pb, U) and depletion in high‐field strength elements (HFSE, Nb, Ta) (Fig. 1). Boninite‐like dolerites resemble the MORB‐like dolerites in trace elements and REE patterns, excepting for lower concentrations in REE, but distinct from the boninites discovered from the IBM or Troodos ophiolite (Fig. 1c, d). Th in both dolerites deviate from the MORB array with negative slopes on the Th/Yb vs. Nb/Yb diagram (Fig. 2a), suggesting constant subduction component added to the mantle source (Pearce et al., 1995). We chose depleted MORB source Mantle (DMM, Workman and Hart, 2005) as the starting composition computing immobile element contents for primary melts using the methods and partition coefficients of Shervais and Jean (2012). The results show that MORB‐like and boninite‐like dolerites can be explained by non‐batch fractional melting (≥15%) of a depleted spinel peridotite with few subduction‐derived components (Fig. 2b). Therefore, the boninite‐like dolerites from the XO cannot be regarded as a product of hydrous melting of hot and highly depleted mantle wedge fluxed by the slab‐derived fluid and the XO cannot be used as analogs for the subduction initiation.

Reconsideration of Neo‐Tethys Evolution Constrained from the Nature of the Dazhuqu Ophiolitic Mantle, Southern Tibet

AbstractThe 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 (Day et al., 2017; Parkinson and Pearce, 1998); 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 (Hellebrand et al., 2002) 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) (Meisel et al., 2001). 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 (Day et al., 2017) and the Oman southern massifs (Hangh⊘j et al., 2010) 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.

The embryonic Himalayan foreland basin revealed in the eastern Yarlung Zangbo suture zone, southeastern Tibet

Understanding of early foreland basin development is key to reconstruct the timing and nature of early collisional processes. Though the nature of the early Himalayan foreland basin system is well established in the central Himalaya range, it lacks analysis in the eastern Yarlung Zangbo suture zone. In this article, we investigate the region's lithological and structural evolution and analyze the geochemistry and geochronology of detrital zircons and bulk rocks in the Jiacha-Jindong region, southeastern Tibet. We show that: 1) The previously named Cretaceous Langxian Mélange comprises two separate stratigraphic units—the newly revised early Eocene (~56 Ma) Langxian unit and the Triassic Langjiexue Group; 2) The Langxian unit overthrusts the Oligocene to Miocene Dazhuka Formation and is overlain and/or underlain by the highly-deformed Langjiexue Group; 3) Shale-normalized REE + Y patterns of marbles with light REE depletion, La enrichment, and positive Y anomaly plus Sr isotope ratios 0.707481–0.707897, together demonstrate the Langxian unit was originally deposited in a marine environment; 4) ~1270 concordant detrital zircons from 20 rock samples indicate a bilateral provenance—from both the Gangdese arc and/or recycled forearc strata in the north and the Langjiexue Group in the south. The evidence illustrates that the Langxian unit records the early India-Asia collision and its protoliths could have been deposited in a distal foredeep basin to the embryonic eastern Himalaya orogeny. The simplest explanation is that it reflects an underfilled marine basin with sources from the early mountain range to the north and a region of forebulge erosion to the south.

Middle Jurassic radiolarians from Jinlang, Zedong, southern Xizang (Tibet), China

Moderately to well-preserved radiolarians have been extracted from nine greyish green and purplish-red bedded cherts in the mud-matrix mélange of the eastern Yarlung-Zangbo Suture Zone (YZSZ) at Jinlang Section, Zedong, southern Xizang (Tibet). Forty-two species belonging to 27 genera, including Middle Jurassic characteristic species such as Laxtorum (?) jurassicum Isozaki and Matsuda, Laxtorum (?) hichisoense Isozaki and Matsuda, Stichocapsa japonica Yao, Stichocapsa robusta Matsuoka, Parahsuum (?) magnum Takemura, Sella chrafatensis (El Kadiri), and Unuma typicus Ichikawa and Yao are recognized and three assemblages, Laxtorum (?) jurassicum Assemblage (Aalenian), Quarticella ovalis Assemblage (late Bajocian), and Stichocapsa robusta Assemblage (middle Bathonian) are established in ascending order. These assemblages can be well correlated to the Middle Jurassic Unitary Association (UA) Zones (Baumgartner et al., 1995) in west Tethys and coeval biozones in Japan and provide reliable age information for the Middle Jurassic stratigraphic correlation of the pelagic sediments along the YZSZ.

Genetic classification, distribution and ore genesis of major PGE, Co and Cr deposits in China: A critical review

Major platinum-group element (PGE), cobalt (Co) and chromium (Cr) deposits in China are commonly associated with mafic-ultramafic rocks regardless magmatic or hydrothermal origins. Nearly all PGE and half of Co resources in China are from magmatic Ni-Cu-(PGE) sulfide deposits hosted in mafic-ultramafic intrusions. Major resources of Cr are from podiform chromite deposits associated with the mantle sequences of ophiolites, including the Luobusha chromite deposit in Tibet and the Sartohay chromite deposit in Xinjiang. However, there is a lack of large PGE and chromite deposits related to layered mafic-ultramafic intrusions. Sedimentary rock-hosted stratiform Cu deposits and Ni-Co laterites are two dominant types of Co-bearing deposits among terrestrial Co resources and are sporadically distributed in China, although these are insignificant. Platinum-group elements from three Ni-Cu-(PGE) sulfide-bearing mafic-ultramafic intrusions that formed within-plate settings account for 74% of the total PGE reserves of China. Among these, the Jinbaoshan and Yangliuping intrusions are parts of the Emeishan large igneous province (LIP) in SW China, and the Jinchuan intrusion emplaced in a rifting margin of the southern North China Craton. The Jinbaoshan intrusion hosts the only PGE-dominated deposit in China, but has not yet been mined due to the difficulty for access. In the Jinchuan and Yangliuping Ni-Cu deposits, PGEs are by-products. The Jinchuan intrusion is by far the largest PGE producer in China. Cobalt resources are from deposits associated with mafic-ultramafic rocks in China, including Ni-Cu sulfide deposits, Ni-Co laterites, volcanogenic massive sulfide (VMS) deposits and magmatic Fe-Ti oxide deposits, which make up 41% of total Co reserves of China. Major Ni-Cu sulfide deposits in both within-plate and convergent margin settings contain economic Co resources, such as the Jinchuan intrusion and those in the Emeishan LIP, and the Xiarihamu intrusion in the East Kunlun orogenic belt, the Jianchaling intrusion in a Neoproterozoic subduction zone in the northern margin of the Yangtze block and those in the central Asian orogenic belt. Ni-Co laterites that contain economic Co resource have been only discovered locally in Hannan, Yunnan and Taiwan. The Derni Cu-Co deposit in the A’nyemaqen suture zone of the East Kunlun orogenic belt is a special type of VMS deposits closely associated with ophiolite suites, which is also very rare worldwide. Magmatic Fe-Ti oxide deposits in the Emeishan LIP and hydrothermal iron oxide deposits in the East Tianshan orogenic belt have likely potential Co resources. Podiform chromite deposits in China occur in ophiolites, such as the Sartohay, Hegenshan and Suolunshan ophiolites in the central Asian orogenic belt, the Dadao Erji and Yushi ophiolites in the Qilian-Qinling orogenic belt, the Dongqiao ophiolite in the Bangong-Nujiang suture zone and Luobusha ophiolite in the Yarlung-Zangbo suture zone. Understanding the mechanisms of PGE enrichment in major Ni-Cu-(PGE) sulfide deposits such as the Jinchuan and Jinbaoshan intrusions has been a focus of studies in the past. The dissolution and re-precipitation processes that may lead to the enrichment of PGE and Co during hydrothermal overprints on magmatic Ni-Cu sulfide deposits have gained popularity. The origin of podiform chromite deposits is generally thought to be triggered by melt-mantle interaction and melt mixing. But it appears that such a traditional thought is challenged by the discoveries of various high-pressure and hydrous minerals that are trapped in chromite. Given PGE, Co and Cr are all strategic and critical metals to China, it is very important to conduct new exploration projects near active mines, such as those in the Emeishan LIP and the Jinchuan intrusion. However, additional experimental and mineralogical studies on the enrichment processes of PGE, Co and Cr are desired to make breakthroughs in order to better understand the behaviors of PGE, Co and Cr in magmatic-hydrothermal systems. Main issues that are worthy to be closely examined in the future include (1) micro- to nano-scale occurrence of PGE, (2) enrichment mechanism and occurrence of Co in mafic-ultramafic rocks and related ore deposits, and (3) mobilization and enrichment processes of Cr in the formation of podiform chromite deposits.

Discovery of Vestige Sedimentary Archives of the India‐Asia Collision in the Eastern Yarlung Zangbo Suture Zone

AbstractThe lack of syncollisional sediments in the eastern Yarlung Zangbo suture zone (YZSZ) has impeded our understanding of the India‐Asia collision, particularly as to whether the collision was synchronous or diachronous. In this paper, we present evidence for the vestige sedimentary archives of the initial India‐Asia collision retained in the previously proposed “Cretaceous Langxian Mélange” in the eastern YZSZ, southern Tibet. Field investigations of marbles and intercalated meta‐siliciclastic rocks, together with analyses of petrography, cathodoluminescence image, geochronology, trace elements, and Nd isotopes show that (1) marbles occur as thin‐bedded to massive individual layers consistently dipping to the south at 50–80°, (2) marble and intercalated meta‐siliciclastic rocks exhibit the same deformational (foliations S0 and S1) and metamorphic (green‐schist facies) history with nearly same detrital‐zircon age spectra and εNd(0) values, (3) protoliths of the meta‐sedimentary rocks were sourced from both India and Asia and deposited during the early Eocene, and (4) protoliths of the marbles were probably precipitated in a marine environment. These lines of evidence suggest that the previously proposed mélange is indeed not the Cretaceous mélange, but the vestige of syncollisional sedimentary strata that was originally deposited in situ in a syncollisional foreland basin. The estimated collisional age of prior to ~56 Ma in the eastern YZSZ is almost consistent with the 60–56 Ma age estimated from the western‐central YZSZ and Himalaya, indicating a (quasi‐) synchronous initial India‐Asia collision at ~60–56 Ma.

Seismic reflection imaging of crustal deformation within the eastern Yarlung-Zangbo suture zone

Various models have been proposed to explain formational mechanisms for the Himalayan orogeny but uncertainties remain concerning uplift processes and deeper crustal structure of the orogen. Imaging of the orogen's deep structure can help resolve these uncertainties. This paper describes one migrated deep seismic reflection profile across the eastern Yarlung-zangbo suture (YZS) at 92° E, which provides a high resolution image of crustal structure that reveals (1) Yarlhashampo dome is divided into upper, middle and lower tectonic units by two detachments from rim, mantle to the core of dome, (2) one blind north-dipping reverse fault that offsets the South Tibet detachment (STD) developed along the northern flank of the Yarlhashampo dome, (3) a shift of the Main Himalayan thrust (MHT) dip angle from 18° into relatively gentle dips (6°) in the central part of the profile (18–20 s, t.w.t). Meantime, further inferring that (4) North-south compression event happened after ca.10 Ma in North Himalayas, (5) multi-stage duplexing as the primary mechanism for crustal thickening in the eastern YZS based on interpreted structures.

Compositional and geochronological signatures of metamafic dykes from the Sangsang peridotites, South Tibet: Evidence for magma-starved forearc rifting during Neo-Tethyan subduction re-initiation

In this study we present new mineral chemistry, whole-rock geochemical and zircon U–Pb geochronological data for 12 metamafic dykes in the mantle sequence of the Sangsang ophiolite in South Tibet (China). Modal analyses of these dykes gave averages of ~40%–65% plagioclase and ~35%–60% amphibole and small amounts of (igneous) clinopyroxene, epidote and opaque minerals. This mineral assemblage resembles that of typical orthoamphibolites. Nevertheless, due to the absence of foliation the investigated rocks are described as metamafic lithologies. These rocks have primitive mantle (PM)-normalized multi-element patterns with negative Nb and Ta anomalies as well as weak, negative Ti anomalies. In addition, they have initial 87Sr/86Sr ratios [(87Sr/86Sr)i] of 0.702844–0.703581, initial 143Nd/144Nd ratios [(143Nd/144Nd)i] of 0.512891–0.512959 and high εNd(t) values (+7.9 to +9.3). Uranium-Pb ages of magmatic zircons separated from the investigated metamafic dykes indicate that the parental melts of their protoliths intruded the Sangsang mantle at ~119.0–118.5 ​Ma.The metamorphic mineral assemblages recognized in the investigated dykes are suggestive of a retrograde metamorphic process, from (epidote-)amphibolite facies (~470–610 ​°C, ~1.9–4.3 ​kbar) and to prehnite-pumpellyite facies (≤280 ​°C, ≤ 3 ​kbar), active within a rift-produced oceanic lithosphere. Microtextural and geochemical data suggest that the protoliths of the dykes were most likely massive gabbros. Compositional data show that the parental magmas of the gabbroic protoliths were generated by melting of a depleted mantle (DM) source that had been weakly modified by fluids emanating from a subducted oceanic lithospheric slab. The age of the gabbroic protoliths is slightly younger than the existing ages for ophiolites from the central Yarlung-Zangbo Suture Zone (YZSZ) in the literature (~129–123 ​Ma). We, therefore, suggest that the gabbroic protoliths of the Sangsang metamafic dykes were formed in an incipient forearc setting during Neo-Tethyan subduction re-initiation (Aptian). Our tectonomagmatic model provides insights into the igneous accretion and post-solidification evolution of the oceanic lithosphere in South Tibet.

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.