Bangong-Nujiang Suture Research Articles

An island arc origin of Jurassic plagiogranite in the Shiquanhe ophiolite, western Bangong Suture, Tibet: Zircon U–Pb chronology, geochemistry, and tectonic implications of Bangong Meso‐Tethys

The plagiogranites in ophiolites are minor in volume but can provide crucial information for the origin and tectonic evolution of ancient oceanic lithosphere. This paper presents the geochronology and geochemistry of a newly discovered plagiogranite in the Shiquanhe ophiolite, from the west end of the Shiquanhe‐Jiali ophiolite sub‐belt, Bangong Suture, central Tibet. Zircon U–Pb dating of two samples yields Middle Jurassic ages (167.4 ± 1.2 Ma and 167.5 ± 1.5 Ma). The plagiogranite has positive whole‐rock εNd(t) (4.2–4.9) and zircon εHf(t) (9.6–14.3) values, high Th/Nb ratios (0.6–2.8) but relatively low La/Nb ratios (0.9–9.9), indicating that it was possibly derived from a depleted mantle with the contribution of minor subducted sediments. The LREE‐enrichment but HREE‐flat patterns with negative Eu anomalies and negative Nb‐Ti anomalies resemble those of shear‐type plagiogranites, which mean that this rock was likely formed by partial melting of metabasite. Combined with the plagiogranite which does not exhibit chilled contacts against the Shiquanhe ophiolitic metabasite, suggests that the plagiogranite may have been derived from the associated ophiolitic metabasite. Geochemical calculating and modelling indicate that the plagiogranite was possibly produced by a low degree (<10%) partial melting of metabasite and replenished by minor sediments melts at low temperature (<800 °C) and low pressure (<0.1 GPa) conditions. The Shiquanhe plagiogranite, together with the contemporaneous Lagkorco plagiogranite in the same ophiolite sub‐belt, indicates that an intra‐oceanic island arc system was developed in the Bangong Meso‐Tethys during the Middle Jurassic.

Reworking of juvenile crust beneath the Bangong–Nujiang suture zone: Evidence from Late Cretaceous granite porphyries in Southern Qiangtang, Central Tibet

Crustal reworking in the collisional zones is commonly considered as an important process to result in continental crust differentiation and maturation. However, the reworking mechanism remains unclear. Here, a combined study of zircon SIMS U–Pb age, whole-rock major and trace element, Sr–Nd–Hf and zircon Hf–O isotopic geochemistry was carried out for the Duoma–Namuqie (DM–NMQ) granite porphyry intrusions, including DM, NMQ I, II and III intrusions, in southern Qiangtang, central Tibet. Zircon SIMS U–Pb dating yielded ages of 79–76 Ma, suggesting that they were formed in the later stage of Late Cretaceous. The DM granite porphyries are metaluminous to peraluminous with slightly low SiO2 (69.2–71.4 wt%) and high MgO (0.67–0.79 wt%) contents. They are depleted in high-field-strength elements (HFSEs) and enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), and show weakly negative Eu anomalies. The NMQ III granite porphyries show similar elemental features to the DM granite porphyries, but have relatively high SiO2 (69.8–72.2 wt%) and low MgO (0.25–0.28 wt%) contents. Both of them belong to I-type granites. The NMQ I and II granite porphyries are relatively differentiated with high SiO2 (76.4–77.9 wt%), low MgO (0.11–0.16 wt%) contents, and significantly negative Eu anomalies. Extremely low LREE contents and notable REE tetrad effect can be observed in the NMQ II granite porphyries. The NMQ I and II intrusions are fractionated I-type granites. The whole-rock Sr–Nd–Hf and zircon Hf–O isotope data exhibit an enriched trend from the DM granite porphyries ((87Sr/86Sr)i = 0.7049–0.7050, εNd(t) = 1.2–1.6, εHf(t)whole-rock = 9.9–12.0, εHf(t)zircon = 7.4–11.5, δ18O = 6.2–7.1‰) to the NMQ III granite porphyry ((87Sr/86Sr)i = 0.7058–0.7067, εNd(t) = −0.9 to −1.1, εHf(t)whole-rock = 7.0–7.4, εHf(t)zircon = 2.7–9.5, δ18O = 7.2–7.6‰) to the NMQ I and II granite porphyries (εNd(t) = −3.0 to −4.3, εHf(t)whole-rock = 5.3–6.6, εHf(t)zircon = 1.5–7.4, δ18O = 6.6–9.2‰). We suggest that the DM–NMQ granite porphyries were mainly derived by partial melting of the juvenile mafic lower crust with variable volumes of ancient crustal components involved in their magma sources. The way in which these granite porphyries were formed provides evidence for the crustal differentiation and maturation in a post-collisional extensional setting triggered by the far-field rollback of the subducted Neo-Tethys oceanic slab

Subduction erosion and crustal material recycling indicated by adakites in central Tibet

AbstractSubduction erosion is important for crustal material recycling and is widespread in modern active convergent margins. However, such a process is rarely identified in fossil convergent systems, which casts doubt on the importance of subduction erosion through the geological record. We report on ca. 155 Ma Kangqiong (pluton) intrusive rocks of a Mesozoic magmatic arc in the southern Qiangtang terrane, central Tibet. These rocks mainly consist of trondhjemites and tonalites and are similar to slab-derived adakites with mantle-like zircon oxygen isotope compositions (δ18O = 5.2‰–5.6‰), they display more evolved Sr-Nd isotopes and higher Th/ La relative to mid-oceanic ridge basalts from the Bangong-Nujiang suture, and they contain abundant amphibole and biotite. These characteristics indicate magma generation via H2O- fluxed melting of eroded forearc crust debris with subducted oceanic crust at 1.5–2.5 GPa and 700–800 °C. In addition, the intrusions are exposed <20 km north of the Bangong-Nujiang suture. Given the formation of adakites, narrow arc-suture distance, migration of the Jurassic frontal arc toward the continent interior, and other independent geological archives, we suggest that the hydrated forearc crust materials were removed from the overlying plate and carried into the mantle by subduction erosion. Our study provides the first direct magmatic evidence for a subduction erosion process in pre-Cenozoic convergent systems, which confirms an important role for such processes in subduction-zone material recycling.

Timing of the Meso-Tethys Ocean opening: Evidence from Permian sedimentary provenance changes in the South Qiangtang Terrane, Tibetan Plateau

Timing of the opening of the Meso-Tethys Ocean, represented by the Bangong–Nujiang Suture Zone on the Tibetan Plateau, remains controversial. Further research is required to understand the breakup of the northern Gondwana margin and the tectonic evolution of the Tethyan realm. In this study, we present petrography, U-Pb dating and Hf isotopic data for detrital zircons from Upper Carboniferous–Upper Permian strata in the South Qiangtang Terrane on the Tibetan Plateau. These data, together with data from previous literature, indicate a youngest detrital zircon age peak of ca. 550 Ma for Upper Carboniferous–Lower Permian strata. This is far older than the depositional age of ca. 300 Ma and indicates a source in the stable Gondwana Continent. Upper Permian strata yield younger ages (490–247 Ma) with peaks at ca. 460, 355, 290 and 260 Ma, indicating a source in the active South Qiangtang Terrane. Combined with the unconformity between the Lower and Upper Permian strata in the western South Qiangtang Terrane, we conclude that a significant change in sedimentary provenance occurred at 280–260 Ma. This provenance change might have resulted from the 300–279 Ma rifting magmatism on the northern Indian margin of Gondwana (e.g., South Qiangtang). The 300–279 Ma magmatism is interpreted to reflect the early stages of rifting, and the subsequent 280–260 Ma sedimentary provenance change is interpreted as the later stage, both of which established a complete Early–Middle Permian (300–260 Ma) rifting process that marks the opening of the Meso-Tethys Ocean.

Intra-continental boninite-series volcanic rocks from the Bangong-Nujiang Suture Zone, Central Tibet

The boninitic volcanic rocks from Rutog, western Bangong-Nujiang Suture Zone (BNSZ), are characterized by high MgO, SiO2 but low TiO2 contents and interpreted as the products of intra-oceanic subduction in earlier studies. Recently, we discovered new breccia-like boninitic volcanic rocks (mainly trachyandesite and trachyte) that overlie the Rutog ophiolitic peridotites in BNSZ. These volcanic rocks not only show typical boninitic petrographical features of porphyritic texture in which euhedral augite phenocrysts (ca. 5 vol%) are embedded in fine-grained groundmass composed of pyroxene, plagioclase, quartz and minor glass, but also display strong boninitic geochemical affinities with high Mg# (61.59–67.90), Al2O3/TiO2 (40.31–55.97) ratios and low TiO2 (0.26–0.35 wt%) contents. In addition, their rare earth elemental patterns show a slight right-dip pattern with (La/Gd)N values of ~1.28 and (Gd/Yb)N values of ~1.31. Given the evolutionary trend from magnesium-rich intermediate to acid series, the boninite-series are therefore used to describe the vlocanic rocks. Zircon U-Pb isotopic dating on these boninite-series volcanic rocks yielded an age of ca. 79.3 ± 2.0 Ma, after the closure of the Bangong-Nujiang Tethyan Ocean (BNTO) that occurred before 96 Ma. The newly recognized boninite-series volcanic rocks probably formed more than 20 Ma after the latest regional magmatism related to subduction process and thus the subduction setting is not suitable for these boninite-series volcanic rocks. The enrichment in compatible elements and depletion in high field elements indicate the Rutog boninite-series originated from a severely depleted mantle while several lines of evidence (e.g., the enrichment in light rare earth elements, large iron lithophile elements) imply that the mantle source has been modified by continental crust. Given the widespread Cretaceous high Sr/Y magmatic rocks within BNSZ, the Rutog boninite-series rocks may provide an example of the crust-mantle interaction as a result of the delamination in a partially thickened and cratonized convergent margin during the late Cretaceous within BNSZ, central Tibetan Plateau.

Tectonic Evolution of the Meso-Tethys Ocean: Insights from Geochemistry and Geochronology of the Jurassic ophiolitic complex in the Asa area, central Tibet

ABSTRACT Although voluminous Jurassic ophiolites have been reported along the Bangong–Nujiang Suture Zone (BNSZ), the tectonic setting of ophiolites and the Jurassic evolution history of the Meso-Tethys Ocean remain mysterious. This study presents petrological, geochronological and geochemical data for newly discovered Jurassic ophiolitic complex in the Asa area, the Shiquanhe–Namco Ophiolite sub-belt (SNO sub-belt) of BNSZ. Detailed geological investigations and zircon U-Pb dating reveal that the Jurassic ophiolitic complex are composed of a suite of Mid-Late Jurassic (165–158 Ma) ophiolite and Early Jurassic gabbros (ca. 190 Ma). The Early Jurassic gabbros show affinities with island arc tholeiitic (IAT) basalts, which were derived from depleted mantle sources modified by oceanic slab-derived fluids and formed in a forearc settings. The Mid-Late Jurassic ophiolite mainly consists of peridotites, cumulate gabbros, isotropic gabbros, plagiogranites and greenschists. Whole-rock geochemical and zircon Hf isotopic data suggest that the Mid-Late Jurassic ophiolite formed in an intra-oceanicisland arc setting. Together, the Asa ophiolitic complex document magmatic processes from forearc extension to the development of intra-oceanicisland arc, confirming the occurrence of intra-oceanicsubduction system within the Meso-Tethys during the Jurassic. We tend to regard that both northward intra-oceanicsubduction and ocean–continent subduction were initiated during the Early Jurassic in the Meso-Tethys. During Mid-late Jurassic, a northward intra-oceanicarc–back-arc basin system developed in the Meso-Tethys, along with a continental forearc spreading basin resulting from northward ocean-continental subduction beneath the South Qiangtang terrane, producing the complex Jurassic ocean structure of the Meso-Tethys.

Geophysical constraints on the nature of lithosphere in central and eastern Tibetan plateau

The structure and rheology of lithosphere are fundamental to understanding lithospheric deformation operating within the Tibetan plateau and hence holds significant implications for continental dynamics. Yet, some of these aspects remain less well constrained in Tibet. In this study we construct new models of isotropic-average shear-wave velocity and radial anisotropy from previously published Rayleigh-wave and Love-wave phase velocities at periods of 8–143 s. Integrating with other previously published geophysical data, we demonstrate that a weak mid-to-lower crust is pervasive within the plateau, as directly inferred from seismically low velocity and high conductivity at these depths along with high Moho temperature. When combining with the observation of positive radial anisotropy patterns (Vsh > Vsv), we speculate that this weak mid-to-lower crust could probably flow towards southeastern Tibet, whereas in the Qinling Orogen this crustal flow is absent. This weak mid-to-lower crust in Tibet could decouple upper crust and lower crust/upper mantle, accommodating active deformation in response to the Indian-Asian convergence. Additionally, our results show that the subduction front of Indian lithospheric mantle (LM) could reach to the Bangong-Nujiang-Suture (BNS) in central Tibet and to the Jingsha-River-Suture (JRS) in eastern Tibet. We find no expected high velocity beneath northern Tibet to support southward subduction of the Asian LM. Instead, the northern Tibet is characterized with a continuous low-velocity-anomaly from Moho to 200 km. Coupled with the observations of shallow Curie-point depths and high Moho temperature, we infer that the Tibetan upper mantle in the north is hotter than that in the south, probably resulting from mantle upwelling due to either the Indian LM subduction, or thickened Tibetan lithosphere delamination with some contributions of shear heating and radioactive heating from the thickened Tibetan crust. This study highlights the nature of lithosphere, associated dynamic processes and the overall deformation within the Tibetan plateau.

Timing of closure of the Meso-Tethys Ocean: Constraints from remnants of a 141–135 Ma ocean island within the Bangong–Nujiang Suture Zone, Tibetan Plateau

AbstractKnowledge of the timing of the closure of the Meso-Tethys Ocean as represented by the Bangong–Nujiang Suture Zone, i.e., the timing of the Lhasa-Qiangtang collision, is critical for understanding the Mesozoic tectonics of the Tibetan Plateau. But this timing is hotly debated; existing suggestions vary from the Middle Jurassic (ca. 166 Ma) to Late Cretaceous (ca. 100 Ma). In this study, we describe the petrology of the Zhonggang igneous–sedimentary rocks in the middle segment of the Bangong–Nujiang Suture Zone and present results of zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd isotope analysis of the Zhonggang igneous rocks. The Zhonggang igneous–sedimentary rocks have a thick basaltic basement (>2 km thick) covered by limestone with interbedded basalt and tuff, trachyandesite, chert, and poorly sorted conglomerate comprising limestone and basalt debris. There is an absence of terrigenous detritus (e.g., quartz) within the sedimentary and pyroclastic rocks. These observations, together with the typical exotic blocks-in-matrix structure between the Zhonggang igneous–sedimentary rocks and the surrounding flysch deposits, lead to the conclusion that the Zhonggang igneous–sedimentary rocks are remnants of an ocean island within the Meso-Tethys Ocean. This conclusion is consistent with the ocean island basalt-type geochemistry of the Zhonggang basalts and trachyandesites, which are enriched in light rare earth elements (LaN/YbN = 4.72–18.1 and 5.61–13.7, respectively) and have positive Nb–Ta anomalies (NbPM/ThPM > 1, TaPM/UPM > 1), low initial 87Sr/86Sr ratios (0.703992–0.705428), and positive mantle εNd(t) values (3.88–5.99). Zircon U–Pb dates indicate that the Zhonggang ocean island formed at 141–135 Ma; therefore, closure of the Meso-Tethys Ocean and collision of the Lhasa and Qiangtang terranes must have happened after ca. 135 Ma.

Geochemistry and geochronology of Pengco subduction‐related ophiolites, Tibet: Implications for Dongkaco microcontinent in the Bangong–Nujiang suture zone

The identification of a microcontinent in Bangong–Nujiang suture zone (BNSZ) is important to understanding the tectonic evolution of Bangong–Nujiang Tethys Ocean (BNTO). Subduction‐related ophiolite is one of the keys to discriminate a microcontinent in suture zones, which can be generated during the oceanic subduction beneath the microcontinent. Generally, ophiolites in the adjacent area of a microcontinent would show characteristics of a volcanic arc, while ophiolites in adjacent area of an ocean show characteristics of a fore‐arc. North Pengco ophiolites (NPOs) and South Pengco ophiolites (SPOs) were collected in the central section of BNSZ, and dated at 115.0–111.5 Ma by SHRIMP II. According to their trace element characteristics, NPOs were determined to have formed at a setting of subduction‐related volcanic arc and affected by slab‐derived fluids or melts. The diagram based on the Ce versus Ce/Y shows that NPOs originated from the partial melting of spinel peridotites. On the other hand, SPOs were generated from the partial melting of ultra‐depleted mantle at a setting of subduction‐related fore‐arc on the basis of extraordinarily low REEs summation and HFSEs (e.g., Zr, Ti, Nb and Ta), relatively high concentrations of compatible elements (e.g., Cr, Ni), large ratios of CaO/TiO2 (37.67–437.50) and Al2O3/TiO2 (20.78–424.50), which were also affected by slab‐derived fluids. Based on the relative location of NPOs and SPOs, it reasonably proposed that a northward subduction of a minor oceanic basin had occurred at the Early Cretaceous, trigged by the Dongkaco microcontinent (DMC). In reverse, these ophiolites provide evidence for the existence of DMC in BNTO. Combining with other subduction‐related ophiolites developed in BNSZ, BNTO might have comprised of several minor oceanic basins separated by some blocks like DMC in the central and west sections, rather than a unified ocean basin.

Cretaceous magmatic rocks in the Nyima area, North Tibet: Constraints for the tectonic evolution of the Bangong-Nujiang suture zone

Cretaceous magmatic rocks in the Nyima area, North Tibet: Constraints for the tectonic evolution of the Bangong-Nujiang suture zone

Genesis of Dingqing mantle peridotite in the eastern segment of the Bangong-Nujiang suture zone: Evidence from mineralogy and geochemistry of mantle peridotite from borehole ZK02

丁青蛇绿岩位于班公湖-怒江缝合带东段，是该缝合带出露面积最大的蛇绿岩。为查明岩体成因，在丁青东岩体中实施了一口165.19m的钻孔。除最顶部有约0.5m厚的第四系残坡积物外，其余均为地幔橄榄岩。结合显微镜鉴定将岩心划分出17个岩性单元层，岩性主要以方辉橄榄岩为主，夹少量纯橄岩和含铬铁矿纯橄岩。地幔橄榄岩中橄榄石的Fo变化于88.79~93.73，铬尖晶石的Cr#变化于44.33~81.66，揭示丁青地幔橄榄岩可能经历过约20%~40%的中高度部分熔融作用；全岩地球化学分析表明其具有富镁（MgO=45.98%~49.45%）、贫铝（Al2O3=0.19%~1.37%）和贫钙（CaO=0.28%~0.70%）的特点，属于熔融程度较高的地幔残余物质。岩石具有明显不同于阿尔卑斯蛇绿岩的轻稀土元素富集特征，指示区内地幔橄榄岩先经历了较强程度的部分熔融，后经历了俯冲消减过程中的流体交代。利用地幔橄榄岩中的铬尖晶石成分计算母熔体Al2O3含量对应的FeO/MgO值，与不同构造环境原始岩浆成分相比较，发现丁青地幔橄榄岩母熔体大多处于玻安岩中。纯橄岩氧逸度估算FMQ=-3.05~-0.71，方辉橄榄岩氧逸度FMQ=-3.89~+1.47，显示丁青地幔橄榄岩有俯冲作用的参与。通过丁青钻孔岩心的研究，提出丁青东岩体可能形成于俯冲带之上的弧前环境这一观点。

Origin and tectonic implications of boninite dikes in the Shiquanhe ophiolite, western Bangong Suture, Tibet

Abstract The Shiquanhe ophiolite, hosting newly discovered boninite dikes, is located in the western part of the Shiquanhe-Jiali ophiolite sub-belt (SJO sub-belt) in the Meso-Tethyan Bangong-Nujiang suture zone (BNSZ), Tibet. The boninite dike samples are plagioclase-free, have a porphyritic texture with pyroxene phenocrysts, and a microcrystalline groundmass. Characteristically they have high MgO (7.24–8.59 wt%), SiO2 (54.02–61.49 wt%), low TiO2 (0.15–0.24 wt%), CaO/Al2O3 (0.42–0.58) and are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs). They display slightly concave “spoon-shaped” REE pattern and positive anomalies in Zr and Hf as well as negative Nb, Ta, and Ti anomalies which are similar to the typical low-Ca boninite in Cape Vogel, Papua New Guinea and elsewhere. These characteristics indicate that the Shiquanhe boninite originated from a depleted mantle source replenished by subducted sediments after the extraction of tholeiitic melts. The mantle source of the Shiquanhe boninites is harzburgite, and its melting degree is estimated to be approximately 20–35%. About 87.5% of melts from the depleted harzburgite mixed with about 12.5% of molten subducted sediments, forming the Shiquanhe boninite dikes in a forearc environment. This study on newly discovered boninite dikes further provides crucial evidence of island arc affinity of SJO in the Bangong Meso-Tethys. It gives another example of the relatively rare examples of forearc ophiolites in the Precambrian to Phanerozoic orogenic belts around the Earth.

Subduction initiation triggered by accretion of a Jurassic oceanic plateau along the Bangong–Nujiang Suture in central Tibet

AbstractLower crusts of oceanic plateaus have been rarely studied and their nature remains unclear, due to low probability in preservation. Here we demonstrate that cumulates of the Pengco Complex in central Tibet represent the lower crust of an oceanic plateau generated at ~188 Ma. Mineral crystallization order and geochemistry support their formation via low‐P fractionation from anhydrous basaltic melts. The Pengco cumulates are more magnesium rich and more depleted in rare earth elements than the mid‐ocean ridge cumulates, supporting a mantle source more refractory than the depleted MORB mantle (DMM). However, the extrusive rocks outcropped in the Pengco Complex are younger (161–166 Ma) and have a subducted‐related genesis. Such a temporal and genetical decoupling between the lower and upper crustal rocks in the Pengco Complex resulted from the subduction re‐initiation that was triggered by the oceanic plateau‐continent collision at ~164 Ma.

Genesis of the Shangxu orogenic gold deposit, Bangong-Nujiang suture belt, central Tibet, China: Constraints from H, O, C, Si, He and Ar isotopes

Shangxu is an orogenic gold deposit in the Bangong-Nujiang suture zone, central Tibet, China, formed in the Early Cretaceous orogene, related to convergence and collision between the Qiangtang and Lhasa terranes. The mineralization at Shangxu is hosted by Jurassic turbidite sedimentary rocks of the Mugagangri Group, and is associated with a regional fault system. Hydrothermal minerals develop muscovite, carbonate, sulfides and chlorite. Hydrothermal fluids record three main hydrothermal stages based on mineral paragenesis. The earliest is barren quartz stage (H1), which is pre-ore. During the early mineralization quartz-pyrite stage (H2a), defined by massive quartz veins with minor euhedral pyrite and gold, hydrothermal fluids had a δ18Ofluid of 6.1–6.4‰, δD of −74 to −116‰, δ13CCO2 of −5.4 to −7.6‰, and δ30Si of −0.1‰. In the quartz-pyrite-sulfides stage (H2b), characterized by abundant quartz, granular pyrite, muscovite with minor chalcopyrite, galena, sphalerite and gold, fluids had a δ18Ofluid of 7–8.2‰, δD of −109 to −120‰, δ13CCO2 of −9.6‰ and δ30Si of −0.1 to −0.2‰. During the ankerite-sulfide stage (H3a), distinguished by abundant ankerite, muscovite, sulfides with minor quartz and chlorite, hydrothermal system had a fluid with δ18Ofluid of 4.9–5.3‰, δD of −125.3‰, δ30Si of −0.1‰, δ13CCO2 of −12.4‰ in quartz inclusion fluid and δ13CCO2 of −2.7‰ in ankerite. The calcite-sulfide stage (H3b) is characterized by calcite, sulfides, with minor quartz and chlorite. Quartz formed earlier than calcite from a fluid having a δ18Ofluid of 6.4‰, δD of −112.6‰, δ30Si of −0.1‰, and δ13CCO2 of −7.4‰, after which calcite precipitated from a hydrothermal fluid with δ18Ofluid of 5.5–9‰, δ13CCO2 of −0.5 to −2.8‰. Hydrothermal fluids in H2b pyrite have 3He/4He ratios of 0.27–0.42Ra and 40Ar/36Ar ratios of 313–372.The stable isotope composition of hydrothermal fluids from the Shangxu gold deposit is similar to that of typical orogenic gold deposits. The early stage (H1), methane-bearing fluids were probably sourced from the basin sediments, leading to precipitation of early barren quartz veins and siderite alteration. From the quartz-sulfide stage (H2) to carbonate-sulfide stage (H3), hydrothermal fluids were likely derived from dissolution of the marine carbonate cement from sedimentary host rocks during metamorphism at depth. The decreasing δD and δ13C of the ore-forming fluids from early to late suggests mixing with δ13C-depleted oxidized graphite in sedimentary rocks, meteoric waters or reation between δD-depleted organic matters. Generally, these fluids were likely generated at depth through prograde metamorphic devolatilization of hydrous minerals in the deeper equivalents of the sedimentary rocks of the Mugagangri Group. Sulfur and gold, by inference, likely originated from sedimentary/diagenetic pyrite or the metasedimentary rocks between the greenschist and amphibolite facies, and migrated with the metamorphic fluids. During transportation to the site of deposition, gold-bearing fluids variably reacted with country rocks.

The Ren Co MOR‐type Ophiolite in the North‐central Tibetan Plateau: A Fast‐spreading Ridge Segment of the Meso‐Tethys Ocean?

AbstractThe Meso‐Tethys Ocean is generally considered to have opened in the late Early Permian as a result of the Cimmerian continental block drifting away from the Gondwana supercontinent. This ocean is also termed the north branch of the Neo‐Tethys Ocean, and is now represented by the Bangong–Nujiang suture zone in the north‐central Tibetan plateau. However, it is still unknown for the evolutionary history for as such a huge ancient ocean basin.Ophiolites are remnants of oceanic lithosphere and preserve key information in rebuilding the evolutionary history of ancient oceans. In this study, we undertook detailed geological mapping for the Ren Co ophiolite in the middle part of the Bangong–Nujiang suture zone, and a typical Penrose‐type ophiolite sequences was newly documented in the Ren Co area. The Ren Co ophiolite comprises serpentinized peridotite, cumulate rock, gabbro/diabase, sheeted dike, massive and pillow lavas, and minor red chert. These rocks exhibit well‐preserved mantle and crust rock suites, and show close similarities to oceanic lithospheres at modern fast‐spreading ridges. Zircon U–Pb dating for gabbro and plagiogranite yielded ages of 169–147 Ma, which suggest that the Ren Co ophiolites were formed during the Middle to Late Jurassic. Harzburgite in the Ren Co area shows similar features to those of abyssal peridotite indicating the residues of the oceanic mantle. Mafic rocks (basalt, diabase and gabbro) of the Ren Co ophiolite show geochemical features similar to those of typical N‐MORB. Moreover, all samples have positive whole‐rock ∊Nd(t), and zircon ∊Hf(t) and mantle‐like δ18O values. These features suggest that these rocks were derived from a depleted mantle source, thus the Ren Co ophiolite was typical MOR‐type ophiolite. We suggest that the Ren Co ophiolite was formed in a fast‐spreading mid‐ocean‐ridge (MOR) setting, and they most likely represented the ridge segment of the Bangong‐Nujiang Meso‐Tethys Ocean. The Bangong–Nujiang Meso‐Tethys Ocean was a wide paleo‐ocean, and experienced continuous oceanic spreading, subduction, accretion before final Lhasa and South Qiangtang continental assembly.

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.

Petrogenesis of Late Early Cretaceous high-silica granites from theBangong–Nujiang suture zone, Central Tibet

High-silica granites are common in collisional orogenic belts. However, the petrogenesis of such granites and the geodynamic processes associated with their emplacement are debated. In this study, we present zircon U–Pb ages and Hf isotopes, and whole-rock geochemical and Sr–Nd isotopic compositions for biotite granites from the Jiang Co area, in the Bangong–Nujiang suture zone, central Tibet. Zircon U–Pb dating reveals that the Jiang Co granites were emplaced during the late Early Cretaceous (ca.114 Ma). These granites are characterized by high and variable SiO2 (69.7–78.2 wt%) and K2O (4.7–8.4 wt%), and low MgO (0.09–1.25 wt%) and P2O5 (< 0.1 wt%) contents. The samples are enriched in Rb, Th, U, and Pb, and depleted in Ba, Nb, Ta, Sr, Ti, and Eu. These geochemical features suggest that the Jiang Co granites underwent extensive fractional crystallization. The high and variable initial 87Sr/86Sr (0.7066 to 0.7095) ratios and consistent εNd(t) (–8.2 to –9.2) values, along with TNd2DM ages of 1.66 to 1.58 Ga, and negative to positive zircon εHf(t) (−7.5 to +1.8) values, imply that these granites were formed by partial melting of the mature ancient crustal materials with minor involvement of mantle materials. Along with contemporaneous magmatic activity, we suggest that the Jiang Co granites were mainly generated by crustal melting in a post-collisional setting resulting from slab breakoff after the Lhasa–Qiangtang collision, and subsequently underwent a high degree fractional crystallization.

Lithospheric structures of and tectonic implications for the central–east Tibetan plateau inferred from joint tomography of receiver functions and surface waves

SUMMARYWe present an updated joint tomographic method to simultaneously invert receiver function waveforms and surface wave dispersions for a 3-D S-wave velocity (Vs) model. By applying this method to observations from ∼900 seismic stations and with a priori Moho constraints from previous studies, we construct a 3-D lithospheric S-wave velocity model and crustal-thickness map for the central–east Tibetan plateau. Data misfit/fitting shows that the inverted model can fit the receiver functions and surface wave dispersions reasonably well, and checkerboard tests show the model can retrieve major structural information. The results highlight several features. Within the plateau crustal thickness is &amp;gt;60 km and outwith the plateau it is ∼40 km. Obvious Moho offsets and lateral variations of crustal velocities exist beneath the eastern (Longmen Shan Fault), northern (central–east Kunlun Fault) and northeastern (east Kunlun Fault) boundaries of the plateau, but with decreasing intensity. Segmented high upper-mantle velocities have varied occurrences and depth extents from south/southwest to north/northeast in the plateau. A Z-shaped upper-mantle low-velocity channel, which was taken as Tibetan lithospheric mantle, reflecting deformable material lies along the northern and eastern periphery of the Tibetan plateau, seemingly separating two large high-velocity mantle areas that, respectively, correspond to the Indian and Asian lithospheres. Other small high-velocity mantle segments overlain by the Z-shaped channel are possibly remnants of cold microplates/slabs associated with subductions/collisions prior to the Indian–Eurasian collision during the accretion of the Tibetan region. By integrating the Vs structures with known tectonic information, we derive that the Indian slab generally underlies the plateau south of the Bangong–Nujiang suture in central Tibet and the Jinsha River suture in eastern Tibet and west of the Lanchangjiang suture in southeastern Tibet. The eastern, northern, northeastern and southeastern boundaries of the Tibetan plateau have undergone deformation with decreasing intensity. The weakly resisting northeast and southeast margins, bounded by a wider softer channel of uppermost mantle material, are two potential regions for plateau expansion in the future.

Hydrothermal fluid evolution at the Tiegelongnan porphyry-epithermal Cu(Au) deposit, Tibet, China: Constraints from H and O stable isotope and in-situ S isotope

The Tiegelongnan porphyry-epithermal Cu (Au) deposit is located in the Duolong porphyry district, north of the Bangong-Nujiang suture zone, Tibet, China. Mineralization is hosted by Jurassic sedimentary sandstone, and several phases of diorite and granodiorite porphyry dikes intruded between 123 and 116 Ma. The hydrothermal alteration is characterized by alunite-kaolinite-dickite overprinting quartz-muscovite-pyrite and biotite alteration zones. Porphyry chalcopyrite-pyrite ± molybdenite (Stage 1) mineralization is associated with biotite alteration. Porphyry chalcopyrite-bornite (Stage 2), and covellite (Stage 3) mineralization is associated with quartz-muscovite-pyrite alteration formed at ~121 Ma. Epithermal mineralization, consisting of pyrite-alunite (Stage 4), chalcopyrite-bornite-digenite (Stage 5), and tennantite-enargite (Stage 6), is hosted by two pulses of alunite-kaolinite breccia and veins at ~116 Ma and ~112 Ma. The fluid composition related to muscovite, with average δ18O of 8.9‰ and δD of −56‰, indicates a magmatic water origin. Fluid δ18O composition in equilibrium with quartz veins decreases from 6.7 to 2.3‰, which are likely the results of the water-rock isotopic exchange. Quartz fluid inclusions δD values between −50 to −84‰ are partly lower than that obtained from muscovite alteration fluids, which may result from H fractionation during fluid inclusions decrepitation. Epithermal stage fluid composition equilibrium with alunite yield δ18O from −1.2 to 2.7‰ and δD from −71 to −51‰, n = 11, which is comparable to the fluid composition equilibrium with Type Ⅰ kaolinite (hosting ores) with δ18O between −2.5 and 2.9‰, and δD between −72 and −51‰. It suggests that alunite and Type Ⅰ kaolinite formed with mixing between magmatic and high altitude Cretaceous meteoric water. Late Types Ⅱ and III kaolinite (filling alunite and quartz veins) fluid δ18O and δD values plot along a mixing line between magmatic and low altitude Cretaceous meteoric water, probably following the erosion and plateau subsidence. Porphyry mineralization sulfide stage 1 chalcopyrite and pyrite yield δ34S values between −5.8 and 0.9‰, with an average fluid δ34SH2S = −2.5‰ (n = 10), whereas stage 2 chalcopyrite returns δ34S values from −8.7 to −3‰ with an average δ34SH2S = −5.6‰ (n = 5). The lower fluid δ34SH2S values during sulfides stage 2 compared to that of stage 1, suggest that the chalcopyrite-bornite mineralization formed under higher oxidation conditions than that of the chalcopyrite-pyrite mineralization. Alunite yields δ34S values from 11 to 18.3‰ (n = 8), and the associated sulfide stage 4 pyrite have varying δ34S values from −32.2 to 5.4‰. Disequilibrium S isotope in alunite-pyrite pairs was likely because of rapid cooling and retrograde S isotope exchange during later sulfides emplacement. Epithermal mineralization sulfide Stage 4 S-equilibrated pyrite (−14.9 to −9.5‰), Stage 5 chalcopyrite (−11.6 to −8.2‰), and Stage 6 enargite (−5.4 to −2.6‰) display increasing δ34S values suggesting epithermal fluid compositions evolve towards more reducing conditions.

Amphibole and whole-rock geochemistry of early Late Jurassic diorites, Central Tibet: Implications for petrogenesis and geodynamic processes

Arc magmas are generated by complex geological processes in subduction settings. Due to the complexity of source materials and geological processes, the genesis of arc magmatic rocks can be difficult to disentangle. Here, we report on diorites from the Kenama area of the middle-eastern part of the Bangong–Nujiang suture zone in central Tibet. New SIMS and LA–ICP–MS zircon UPb dating for three diorite samples indicates that they crystallized during the early Late Jurassic (ca. 161 Ma). The diorites can be divided into two groups based on whole-rock major element compositions and mineralogical characteristics: Group I diorites with relatively low SiO2 (47.6–53.4 wt%), and high MgO (4.97–9.10 wt%) and amphibole contents (50–54 vol%); and Group II diorites with slightly higher SiO2 (56.9–59.5 wt%), and low MgO (2.9–3.8 wt%) and amphibole contents (25–30 vol%). The Group I diorites exhibit relative enrichment in light rare earth element (LREE) ((La/Yb)N = 5.7–10.5) with slightly negative Eu anomalies, and are characterized by enrichment of large ion lithophile elements (LILEs; K, Rb, Ba, Th, and U), the depletion of high field strength elements (HFSEs; Nb, Ta, Zr, Hf, and Ti). The Group I diorites have relatively high initial 87Sr/86Sr ratios (0.7073–0.7094), variably negative εNd(t) values(−10.3 to −5.9), negative zircon εHf(t) values (−14.8 to–4.4), and slightly elevated δ18O values (6.5‰–7.3‰). The Group II diorites show similar trace element characteristics to Group I. They also have high initial 87Sr/86Sr ratios (0.7083–0.7087), uniformly negative εNd(t) values (−9.5 to −9.0), negative zircon εHf(t) values (−10.3 to −4.2), and elevated δ18O values (6.6‰–7.5‰). Amphiboles from Group I and Group II diorites have low Al2O3 contents (3.6–9.9 wt% and 5.6–8.5 wt%, respectively), and formed at similar P–T conditions (751–871 °C and 69–226 MPa and 745–832 °C and 77–162 MPa, respectively). On primitive mantle-normalized spider diagrams, these low-Al amphibole grains have slightly convex upward REE patterns with distinctly negative anomalies in Pb, Sr, Eu, Zr, Hf, and Ti, suggesting that amphiboles from both groups crystallized from the similar arc magmas after plagioclase and magnetite crystallization. These mineralogical, geochemical and isotopic characteristics suggest that both groups of Kenama diorites probably originated from an enriched lithospheric mantle metasomatized by subducted oceanic sediment-derived melts. Their parental magmas may have similar geochemical characteristics and underwent varying degrees of accumulation and fractional crystallization. The Group I diorites were likely generated by accumulation of amphibole and fractional crystallization of olivine, clinopyroxene, and plagioclase from mafic magmas, and the Group II diorites were formed by the fractional crystallization of clinopyroxene and plagioclase from mafic magmas that were geochemically similar to the Group I diorites. In combination with regional geology, in particular adjacent ophiolites, high-magnesian andesitic rocks and Cretaceous sedimentary rocks, we conclude that all of the Kenama diorites were probably generated in an early Late Jurassic arc setting related to the subduction of the Bangong–Nujiang Tethys oceanic lithosphere.