Gangdese Magmatic Belt Research Articles

Geochemical characteristics of the Nyemo intrusion and crust–mantle interactions in southern Gangdese, Tibet

AbstractThe Gangdese magmatic belt across the southern Tibetan Plateau is juxtaposed with the Indus–Yurlung Zangbo suture zone (IYS), and many mafic microgranular enclaves (MMEs) are exposed in the belt, thus providing a window for observing deep crust–mantle processes related to the Indo-Asian collision. The Nyemo intrusion is located in the middle part of the Gangdese magmatic belt and comprises host diorites with abundant MMEs. Compared with other parts of the Gangdese magmatic belt, the host rock of the Nyemo intrusion has a mineral composition similar to that of the MME, although differences are observed in chemical contents. To explore the genetic type of the MMEs and the deep processes of the Gangdese magmatic belt, the Nyemo intrusion is selected as the research object for this paper. Here, we report zircon U–Pb geochronological and whole-rock geochemical data for host diorites and MMEs, and electron probe data for hornblendes in diorites and MMEs, and combine mineralogy, petrology, petrogeochemistry and isotope geochemistry analyses. Research has shown that diorites in the Nyemo intrusion belong to the medium-K, metaluminous series. The light rare earth elements (LREEs) and heavy rare earth elements (HREEs) are significantly fractionated, and the LREE/HREE values are 5.77–7.71. The (87Sr/86Sr)i values of the diorites range from 0.704260 to 0.704287, and the εNd(t) values are from 3.73 to 4.17. The MMEs in the Nyemo intrusion have a limited range of SiO2 contents, are calc-alkaline with metaluminous affinity, and have relatively high contents of MgO (4.34–5.00 wt %) with Mg# (Mg2+/Mg2+ + Fe2+) values of 42.36–43.53, which is close to that of evolved basic magma. The contents of REEs vary from 108.87 to 120.59 ppm and show obvious Eu anomalies. The (87Sr/86Sr)i values of the MMEs range from 0.704680 to 0.704704, and the εNd(t) values are 0.35–3.74. The crystallization temperature of the hornblende in the diorite is 820 °C, the formation depth is 5.39 km, the oxygen fugacity is ΔNNO + 0.88 and the water content is 5.95 %. The crystallization temperature of the hornblende in the MMEs is 880 °C, the formation depth is 12.18 km, the oxygen fugacity is ΔNNO + 0.38 and the water content is 8.27 %. The Nyemo MMEs are formed by magma mingling, and originate from the partial melting of the depleted mantle, while the host diorite originates from partial melting of the juvenile crust with the addition of mantle material. The formation of the Gangdese magmatic belt is related to the Indo-Asian continental collision. The break-off of the subducted Neo-Tethyan oceanic plate triggered partial melting of the asthenosphere, which resulted in accumulation of the basaltic magma and then caused the partial melting of the juvenile crust with the addition of mantle material, thus forming a variety of granitic rocks and the large Gangdese magmatic belt.

Prolonged Neo-Tethyan magmatic arc in Myanmar: evidence from geochemistry and Sr–Nd–Hf isotopes of Cretaceous mafic–felsic intrusions in the Banmauk–Kawlin area

Cretaceous mafic–felsic intrusions are extensively distributed in the West Burma terrane (Myanmar), but their petrogenesis and tectonic setting still remain unclear. In this study, whole-rock geochemical and Sr–Nd as well as zircon U–Pb and Hf isotopic data for the Cretaceous mafic–felsic intrusions from the Banmauk–Kawlin area (northern Myanmar) are presented. Precise zircon U–Pb dating results indicate that they emplaced at Cretaceous (~ 110–90 Ma) and likely belong to an eastward extension of the coeval Gangdese magmatic belt in the southern Lhasa terrane (Tibet). The studied Cretaceous intrusions (SiO2 = 41.97 to 74.54 wt%) mostly have calc-alkaline and Na-rich characteristics, and strong enrichments in large ion lithophile elements (e.g., Cs, Rb, and K), depletions in Nb and Ta on primitive mantle-normalized diagrams, consistent with geochemical characteristics of arc-type magmas. Most of Cretaceous gabbroic and dioritic intrusions with relatively depleted mantle Sr–Nd–Hf isotopic compositions (87Sr/86Sri = 0.7041–0.7048, eNd(t) = 0.2–6.8, and eHf(t) = 4.3–15.1) and high Ba/La ratios were possibly derived from partial melting of mantle wedge metasomatized by slab-derived fluids and underwent a certain degree of fractional crystallization. Whereas Cretaceous granodiorite and granite probably formed by partial melting of juvenile arc lower crust on the basis of positive eNd(t)–eHf(t) values (− 2.4 to 6.6 and − 2.8 to 15.3) and low initial Sr isotopic ratios (87Sr/86Sri = 0.7045–0.7063). A wide range of Th/Nb ratios and Sr–Nd–Hf isotopic values in these intrusions suggests that their juvenile arc lower crust possibly formed by melting of slab-derived fluids and/or sediment melts metasomatized mantle. Overall, Cretaceous (~ 110–90 Ma) mafic–felsic intrusions in the West Burma terrane may have formed in an arc setting during subduction of the Neo-Tethyan oceanic lithosphere, suggesting a prolonged Neo-Tethyan magmatic arc system from southern Tibet to Southeast Asia.

Identification of a new source for the Triassic Langjiexue Group: Evidence from a gabbro-diorite complex in the Gangdese magmatic belt and zircon microstructures from sandstones in the Tethyan Himalaya, southern Tibet

AbstractConsiderable debate persists as to the Triassic paleogeographic framework of the Neotethys and the origin of the Late Triassic Langjiexue Group in the Tethyan Himalaya. Triassic magmatic rocks in the Gangdese belt and Late Triassic Langjiexue sediments play a pivotal role in addressing these issues. Geochronological, petrological, and geochemical analyses have been performed on the Middle Triassic gabbro-diorite complex (with crystallization ages of ca. 244–238 Ma) from the Gangdese belt. These plutonic rocks are characterized by relatively low MgO and high Al2O3 contents, calc-alkaline trends, and depletion of Nb, Ta, and Ti, resembling low-MgO high-alumina basalts or basaltic andesites. These plutonic rocks exhibit depleted whole-rock εNd(t) values of ∼+5 and zircon εHf(t) values peaking at ∼+14. These features resemble those of rocks in a subduction-related arc setting.We also completed detrital zircon U-Pb dating and microstructure analysis for the sandstones of the Langjiexue Group in the Tethyan Himalaya. Zircon grains with ages >300 Ma are dominated by preweathered and weathered surfaces as well as fairly rounded to completely rounded scales, indicating a high degree of polycyclicity. In contrast, 300–200 Ma ones are characterized by fresh surfaces and completely unrounded to poorly rounded scales, indicating nearby sources. Collectively, our data, combined with published results, support that the subduction initiation of the Neotethys began no later than the Middle Triassic. Arc-affinity magmatic rocks supplied some materials to the Langjiexue Group. This scenario sheds new light on the provenance of the Langjiexue Group and the Triassic paleogeography of the Neotethyan realm.

New constraints on the tectono-magmatic evolution of the central Gangdese belt from Late Cretaceous magmatic suite in southern Tibet

In the Gangdese magmatic belt of southern Tibet, widespread Late Mesozoic magmatism has been reported, although their petrogenesis and tectonic evolution remain ambiguous. Here we present new zircon U-Pb and geochemical data on a gabbro-diorite suite near the Namling County in the central Gangdese belt, that provide insights into the geodynamic processes in this region. Zircon U-Pb data from LA-ICP-MS and SHRIMP II analyses indicate that the granite crystallized at ca. 87 Ma, and the gabbro-diorite suite was emplaced at ca. 91–95 Ma, both suggesting an early Late Cretaceous magmatism in the central Gangdese belt. The gabbro-diorite suite displays wide ranges of SiO2 (57.21–44.25 wt%) and MgO contents (9.04–3.18 wt%), with high Al2O3 (21.47–16.14 wt%) and Na2O/K2O (1.10–5.84) values. Compared to these, the granite rocks show limited variations of SiO2 (76.97–76.40 wt%), high Na2O + K2O (8.84–8.04 wt%), K2O/Na2O (2.33–1.30) and Sr/Y (48.99–80.36), with low Mg# (30.77–20.39) values and significant Eu anomalies (Eu/Eu* = 1.20–0.24). The gabbro-diorite rocks are characterized by typical arc-type features including enrichment of light rare earth elements (LREE) (LaN/YbN = 11.52–5.06 > 1) and large ion lithophile elements (LILE) (e.g. Cs, K and Rb), but are depleted in high strength field elements (HSFE) (e.g. Nb, Ta, Ti and P). The granitic rocks are also enriched in LREE (LaN/YbN = 55.42–26.55 > 1) and LILE (e.g. Cs, Rb, K and Pb), but depleted in HSFE (e.g. Nb, Ta, Ti and P). The gabbro-diorite rocks show distinctly positive εNd(t) (+3.5 to +4.2) and εHf(t) values (+8.5 to +15.6) values with a restricted range, with relatively homogeneous and low (87Sr/86Sr)I ratios (0.704213–0.703820). The granitic rocks also show positive εNd(t) values (+4.1 to +4.4) and consistent (87Sr/86Sr)I ratios (0.703977–0.703180). Our data suggest that the gabbro-diorite rocks were mostly sourced from partial melting of the depleted mantle that was metasomatized by fluids released from the subducted Neo-Tethys oceanic slab. The granite was probably produced by partial melting of the juvenile crust with no participation of mantle material. We correlate the magmatism with tectonics associated with the roll-back of the Neo-Tethys oceanic slab during the early Late Cretaceous.

Petrogenesis, geochemistry and geological significance of Paleocene Granite in South Gangdese, Tibet

The Gangdese magmatic belt, located along the southern margin of the Lhasa terrane, plays a critical role in understanding the tectonic framework associated with the Indian–Asian slab collision. In this paper, a chronology of zircon U–Pb and geochemical analysis of the rock of the Cuobulaguo granitic mass in the southern of the Gangdese, Tibet, revealed a series of results. The results show that the LA-ICP-MS monzonitic granite zircon U–Pb ages are 61–59 Ma, which corresponds to the same period as the magmatic arc of the southern limit of Gangdese. In terms of geochemical composition, the granite is rich in ω(SiO2) 70.09% to 72.64%, with a high ω(Al2O3) 14.40% to 15.99%, a low ω(TiO2) 0.08% to 0.24%, ω(MgO) 0.41% to 0.76%, ω(Fe2O 3 T ) 6.82% to 29.9%, ω(P2O5) 0.07% to 0.12%, and ω(CaO) 1.06% to 1.75%. The granite mainly belongs to the high-K calc-alkaline series. The light rare earth element (LREE) content of skeletal granite is between 133.69 × 10−6 ~ 226.64 × 10−6 and the heavy rare earth element (HREE) content is between 17.36 × 10−6 and 32.11 × 10−6. LREE/HREE is between 5.05 and 7.83. It is enriched in light rare earth elements (LREE) and large ion lithophile elements, such as Rb, K, U, etc., depletes high field strength elements, such as P, Nb, and Ta, and has the geochemical composition of arc magmatic rocks. In addition, the aluminum saturation index (A/CNK = 1.06 to 1.11) of Cuobulaguo granite, belongs to I-type granite. The comprehensive analysis showed that with the beginning of the collision between the Indian–Asian continental, the subduction ocean plate was separated from the continental plate due to gravity, resulting in an increase in the asthenosphere, which made partial fusion of the lithospheric mantle. It invaded the bottom of the lower crust, which in turn induced a partial melting of the lower crust to form granite.

Petrogenesis of Late Cretaceous mafic enclaves and their host granites in the Nyemo region of southern Tibet: Implications for the tectonic-magmatic evolution of the Central Gangdese Belt

Late Triassic-Early Eocene Gangdese magmatic belt is over 2000 km long forming a significant component of the Lhasa Terrane, and is regarded as the ideal place to study the tectonic-magmatic evolution of the Neo-Tethys Ocean and the collision of the Indian and Asian continental plates. Recent studies elucidate the major Late Cretaceous magmatic events in southern Tibet, but the evolution of the region is still equivocal. In this paper, we report new results from systematic field work as well as new geochronological, petrological, geochemical, and zircon Lu-Hf isotope analyses of granite and contemporaneous mafic microgranular enclaves (MMEs) from the Nyemo region in southern Tibet. Both the host granite and MMEs are dated at ca. 87–84 Ma, suggesting that they are related to a coeval magmatic event. Zircon Hf isotopes (εHf(t) = +11.0 – +14.6) and Mg# numbers (50–52) indicate that the host granite is derived from partial melting of juvenile crust with the contribution of material from the mantle, whereas the MMEs are derived from partial melting of depleted mantle (refractory peridotite melts) as deduced from their zircon Hf isotopes (εHf(t) = +6.12 – +13.79). Geochemically, both the granite and MMEs are characterized by enriched light rare earth elements (LREE) and large-ion lithophile elements (LILE), but depleted high field-strength elements (HFSE), indicative of arc-related geochemical compositions. The host granite also has high Sr/Y (111–140), which are interpreted as proxies for magmas with high water contents, and their elevated (La/Yb)N ratios of 22–33 are proxies for a thickened continental crust. We propose that the Late Cretaceous (ca. 100–80 Ma) magmatism dated some 30 million years before the collision of the Indian and Eurasian plates is possibly related to the interaction between upwelling asthenospheric mantle and metasomatized lithospheric mantle during subduction and possible rollback of the Neo-Tethys Oceanic plate. The subducted slab presumably led to mantle magma underplating and potentially caused crustal thickening in southern Tibet. Hence, the rollback of the Neo-Tethys Oceanic plate represents a key feature in the tectonic evolution of southern Tibet during the early Late Cretaceous.

Geochemical and Isotopic Characterization of Middle Eocene Hybrid Magmatism in the Gangdese Belt (Tibet) and Its Ancient Indian Crustal Fingerprint

AbstractWe present new geochemistry, geochronology, whole-rock Sr-Nd-Pb, and zircon Hf isotopic data from the Zhongba pluton in the western part of the Gangdese magmatic belt (GMB) and discuss its ...

Zircon U–Pb geochronology and geochemistry of Early Jurassic granodiorites in Sumdo area, Tibet: Constraints on petrogenesis and the evolution of the Neo-Tethyan Ocean

The Gangdese magmatic belt is oriented E–W and located between the Indus–Yarlung Tsangpo and Bangong–Nujiang suture zones in Tibet. It records the subduction of the Neo-Tethyan oceanic crust, Indian–Eurasian collision, and uplift of the Tibetan Plateau. Here we present new zircon U–Pb geochronology, Hf isotopic compositions, and whole-rock geochemistry of the granodiorites in the Sumdo area of the Gangdese Belt. Our aim was to identify the petrogenesis of granitoid rcoks and to clarify the early-stage evolution of the Neo-Tethyan Ocean during the Late Triassic to Early Jurassic. The granodiorites yield zircon U–Pb ages of 193–201 Ma, indicating an Early Jurassic magmatic event. They are metaluminous, with medium- to high-K calc-alkaline geochemistry, and show characteristics of I-type granitoids. They show enrichment in light rare earth elements (LREEs) and depletion in high-field-strength elements (HFSEs) in primitive-mantle-normalized spider diagrams, indicating that their initial magma was generated in a subduction-related active continental margin. They have negative εHf(t) values (−11.3 to −4.1) and crustal model ages (TC DM) of 1.50–1.95 Ga. Hf isotopes and whole-rock geochemistry indicate that the granodiorites were derived from partial melting of ancient lower-crust material mixed with mantle material. Our data, in combination with published geochronological and geochemical data, show that the Late Triassic–Early Jurassic arc magmatism in the Gangdese magmatic belt was formed by the northward subduction of the Neo-Tethyan oceanic crust beneath the Lhasa Terrane.

Timing of Secondary Hydrothermal Alteration of the Luobusa Chromitites Constrained by Ar/Ar Dating of Chrome Chlorites

Chrome chlorites are usually found as secondary phases formed by hydrothermal alteration of chromite deposits and associated mafic/ultramafic rocks. Here, we report the 40Ar/39Ar age of chrome chlorites separated from the Luobusa massive chromitites which have undergone secondary alteration by CO2-rich hydrothermal fluids. The dating results reveal that the intermediate heating steps (from 4 to 10) of sample L7 generate an age plateau of 29.88 ± 0.42 Ma (MSWD = 0.12, plateau 39Ar = 74.6%), and the plateau data points define a concordant inverse isochron age of 30.15 ± 1.05 Ma (MSWD = 0.08, initial 40Ar/36Ar = 295.8 ± 9.7). The Ar release pattern shows no evidence of later degassing or inherited radiogenic component indicated by an atmospheric intercept, thus representing the age of the hydrothermal activity. Based on the agreement of this hydrothermal age with the ~30 Ma adakitic plutons exposed in nearby regions (the Zedong area, tens of kilometers west Luobusa) and the extensive late Oligocene plutonism distributed along the southeastern Gangdese magmatic belt, it is suggested that the hydrothermal fluids are likely related to the ~30 Ma magmatism. The hydrothermal fluid circulation could be launched either by remote plutons (such as the Sangri granodiorite, the nearest ~30 Ma pluton west Luobusa) or by a similar coeval pluton in the local Luobusa area (inferred, not found or reported so far). Our results provide important clues for when the listwanites in Luobusa were formed.

Late Triassic Granites From the Quxu Batholith Shedding a New Light on the Evolution of the Gangdese Belt in Southern Tibet

AbstractThe Gangdese magmatic belt formed during Late Triassic to Neogene in the southernmost Lhasa terrane of the Tibetan plateau. It is interpreted as a major component of a continental margin related to the northward subduction of the Neo‐Tethys oceanic slab beneath Eurasia and it is the key in understanding the tectonic framework of southern Tibet prior to the India‐Eurasia collision. It is widely accepted that northward subduction of the Neo‐Tethys oceanic crust formed the Gangdese magmatic belt, but the occurrence of Late Triassic magmatism and the detailed tectonic evolution of southern Tibet are still debated. This work presents new zircon U‐Pb‐Hf isotope data and whole‐rock geochemical compositions of a mylonitic granite pluton in the central Gangdese belt, southern Tibet. Zircon U‐Pb dating from two representative samples yields consistent ages of 225.3±1.8 Ma and 229.9±1.5 Ma, respectively, indicating that the granite pluton was formed during the early phase of Late Triassic instead of Early Eocene (47–52 Ma) as previously suggested. Geochemically, the mylonitic granite pluton has a sub‐alkaline composition and low‐medium K calc‐alkaline affinities and it can be defined as an I‐type granite with metaluminous features (A/CNK<1.1). The analyzed samples are characterized by strong enrichments of LREE and pronounced depletions of Nb, Ta and Ti, suggesting that the granite was generated in an island‐arc setting. However, the use of tectonic discrimination diagrams indicates a continental arc setting. Zircon Lu‐Hf isotopes indicate that the granite has highly positive ∊Hf(t) values ranging from +13.91 to +15.54 (mean value +14.79), reflecting the input of depleted mantle material during its magmatic evolution, consistent with Mg# numbers. Additionally, the studied samples also reveal relatively young Hf two‐stage model ages ranging from 238 Ma to 342 Ma (mean value 292 Ma), suggesting that the pluton was derived from partial melting of juvenile crust. Geochemical discrimination diagrams also suggest that the granite was derived from partial melting of the mafic lower crust. Taking into account both the spatial and temporal distribution of the mylonitic granite, its geochemical fingerprints as well as previous studies, we propose that the northward subduction of the Neo‐Tethys oceanic slab beneath the Lhasa terrane had already commenced in Late Triassic (∼230 Ma), and that the Late Triassic magmatic events were formed in an active continental margin that subsequently evolved into the numerous subterranes, paleo‐island‐arcs and multiple collision phases that form the present southern Tibet.

Hot Paleocene-Eocene Gangdese arc: Growth of continental crust in southern Tibet

The 1600 km-long Gangdese magmatic belt features extensive Paleocene–Eocene I-type intrusive rocks and coeval volcanic successions, which can be divided into Group I (~69–53 Ma), Group II (~53–49 Ma), and Group III (~49–43 Ma), corresponding to Neo-Tethyan slab rollback, Neo-Tethyan slab breakoff, and ongoing Indian-Asian collision, respectively. The magmas from these three groups show significant variations in geochemical and isotopic compositions, which provide the information of the growth of continental crust in southern Tibet. The most voluminous magmatism in the Gangdese belt occurred during ~53–49 Ma. High zircon saturation temperature (up to 800 °C) and Ti in zircon temperature (up to 980 °C) estimations suggest there is a period of thermal anomaly during ~53–49 Ma. Starting from ~53 Ma, magmas have increased K2O contents, and their zircons have decreased Th/U ratios, and Y and Yb contents. Zircons from Group II have the most heterogeneous Hf isotopic compositions (εHf(t) = −5.3 to 15.1). These are evident of ingress of asthenosphere mantle in the arc, extensive crustal melting, and magma mixing. Magma underplating during this time is the main mechanism for the growth of continental crust. With the Indian-Asian collision going on, the magmas in Group III show high Th/Y and La/Yb ratios and K2O contents, but significantly low Tzr and T(ti-zr) values (mostly below 750 °C). These features suggest the water-fluxed melting of early arc residues occurred in the late stage of growth of continental crust. The crust has been thickened and nearly mature at this stage. This study has great implication on understanding of growth of continental crust in orogenic belts.

The Jurassic Yeba Formation in the Gangdese arc of S. Tibet: implications for upper plate extension in the Lhasa terrane

ABSTRACTThe formation of the Jurassic Xiongcun porphyry copper deposit, within the Gangdese magmatic belt of southern Tibet, took place coincident with subduction of the Neotethyan oceanic lithosphere. Although the Yeba Formation (coeval and co-magmatic intrusions and volcanic rocks) in the eastern Gangdese magmatic belt formed at the same time as the Xiongcun deposit, the Yeba Formation is devoid of subduction-related porphyry Cu deposits. Here, we report detrital zircon U–Pb ages for sedimentary rocks of the Yeba and Bima Formations along with a summary of all published whole-rock geochemical data and zircon epsilon Hf isotopes for the volcanic rocks in the Yeba Formation. We use these data to re-evaluate the petrogenesis, magmatic source, and tectonic setting of the volcanic rocks. Detrital zircon ages from the sedimentary rocks reveal that the volcanic rocks of the Yeba and Bima Formations are the main source for their sedimentary interlayers. In addition, the Yeba sedimentary layers contain inherited zircons sourced from older country rocks whereas the Bima Formation is devoid of ancient zircons. Geochemically, the Yeba volcanic rocks resemble a bimodal volcanic suite. Geochemical data reveal that the Yeba volcanic rocks formed in a continental arc setting, while the mafic ones were derived from the partial melting of the mantle wedge above the subducting slab rather than from the melting of the descending oceanic slab. In addition, the mafic ensemble formed in a back-arc setting, indicative of subduction-related extension in the Lhasa terrane. In contrast, the felsic ensemble may originate from the melting of basaltic lower crust. Thus, the magma source and tectonic setting for the Yeba Formation are distinct from typical subduction-related Cu-hosted porphyry and volcanic rocks in a compressional setting.

Multiple sources of the Early Mesozoic Gouli batholith, Eastern Kunlun Orogenic Belt, northern Tibetan Plateau: Linking continental crustal growth with oceanic subduction

Orogenic belts have been among the most important locations to investigate the growth of continental crust (CC). The Eastern Kunlun Orogenic Belt (EKOB), which contains widespread Permian–Triassic granitoids, is volumetrically comparable to the Cenozoic Gangdese magmatic belt in the Tibetan Plateau and is an ideal region to investigate the mechanism of the Paleozoic–Mesozoic CC growth in this region. The Gouli batholith at the eastern end of the EKOB consists of the synchronous Xiangride granodiorite, Asiha quartz diorite (ca. 242Ma) and adamellite. The granodiorite and quartz diorite, both of which contain magmatic enclaves, show medium–high K, calc-alkalic and metaluminous signatures and have similar rare earth element and trace element patterns to those of bulk CC. Besides, the Xiangride granodiorite displays distinct adakitic signatures (average Sr/Y of 47). The Sr-Nd isotopic values for the different types of rocks are roughly similar ((87Sr/86Sr)i=0.708167–0.713553, εNd(t)=−6.8 to −5.3), while Hf isotopes are distinguishable, with εHf(t)granodiorite=0.3 to 5.1 and εHf(t)diorite=−1.6 to 0.7. These geochemical and petrographic signatures suggest that the granodiorite originated from the partial melting of subducting oceanic crust and terrigenous sediments, and the quartz diorite and enclaves formed via the mixing of slab-derived magma and enriched mantle-derived melt. Further comprehensive analyses of the spatial and temporal distribution of regional magmatic rocks, metamorphism and sedimentary facies reveal that the Gouli batholith and most of the Permian–Triassic granitoids in the EKOB formed during the subduction of the Paleo-Tethys Ocean instead of subsequent syn-collision setting. Thus, we contend that the Permian–Triassic CC growth of the EKOB occurred in a slab subduction setting and that both oceanic slab and subcontinental mantle significantly contributed to this process.

Syn-convergence extension in the southern Lhasa terrane: Evidence from late Cretaceous adakitic granodiorite and coeval gabbroic-dioritic dykes

Late Cretaceous (∼100-80Ma) magmatism in the Gangdese magmatic belt plays a pivotal role in understanding the evolutionary history and tectonic regime of the southern Lhasa terrane. The geodynamic process for the formation of the early Late Cretaceous magmatism has long been an issue of hot debates. Here, petrology, geochronology and geochemistry of early Late Cretaceous granodiorite and coeval gabbroic-dioritic dykes in the Caina region, southern Lhasa, were investigated in an effort to ascertain their petrogenesis, age of intrusion, magma mixing and tectonic setting. Zircon U-Pb dating of granodiorite yields 206Pb/238U ages of 85.8±1.7 and 86.4±1.1Ma, whilst that of the E-W trending dykes yields ages of 82.7±2.6 and 83.5±3.5Ma. Within error, the crystallization ages of the dykes and the granodiorite are indistinguishable. Field observations and mineralogical microstructures are suggestive of a magma mixing process during the formation of the dykes and the granodiorite. The granodiorite exhibits geochemical features that are in agreement with those of subduction-related high-SiO2 adakites. The granodiorite and dykes have relatively constant εNd(t) values of +2.2 to +4.9 and initial 87Sr/86Sr ratios (0.7045–0.7047). These similar characteristics are herein interpreted as an evolutionary series from the dykes to granodiorite, consistent with magma mixing process. Ti-in-zircon thermometer and Al-in-hornblende barometer indicate that the granodiorite and the dioritic dyke crystallized at temperatures of ca. 750 and 800°C, depths of ca. 6–10 and 5–9km, respectively. Taking into account the synchronous magmatic rocks in the Gangdese Belt and the coeval rifted basin within the Lhasa terrane, the granodiorite and dykes reveal an early Late Cretaceous syn-convergence extensional regime in the southern Lhasa terrane, triggered by slab rollback of the Neotethyan oceanic lithosphere.

The geochemical evolution of syncollisional magmatism and the implications for significant magmatic-hydrothermal lead–zinc mineralization (Gangdese, Tibet)

In addition to well-known subduction processes, the collision of two continents also generates abundant ore deposits, as in the case of the Tibetan Plateau, which is the youngest and most spectacular collisional belt on Earth. During the building history of the Gangdese magmatic belt, several magmatic flare-up events developed, however, significant magmatic-hydrothermal lead–zinc mineralization dominantly accompanied the magmatism during the syncollisional period (~65–41Ma). Based on integrated geochemical and isotopic data, we provide insights into the genesis and evolution of syncollisional magmas, and their implications for significant magmatic-hydrothermal lead–zinc mineralization. The Sr–Nd isotopic compositions of most syncollisional igneous rocks (87Sr/86Sr=0.7034–0.7123; εNd(t)=−9.0 to +1.8) indicate a mixing origin between mantle-derived basaltic magmas and ancient crustal melts, and fractional crystallization is a fundamental mechanism by which syncollisional magmas evolve towards intermediate to silicic compositions. Most lead–zinc mineralization-related plutons are high silica (76.14% wt.% SiO2 on average), high oxygen fugacity (average ΔFMQ +2.5) granites with highly evolved chemical signatures [average Eun/Eun*=0.33, high Rb/Sr (average=3.9)], and they represent the final products from primary magmas. Due to the contribution of ancient crustal melts to the genesis of mineralization-related parent magmas, the spatial distribution of Pb–Zn deposits within the northern Gangdese magmatic belt is controlled by the lithospheric architecture. In compressional environments, magmas have low evacuation efficiency and long magma chamber lifespan, which is favorable for basaltic parents evolved to high silica granites through sufficient fractional crystallization. This scenario contributes to our understanding of the significant magmatic-hydrothermal lead–zinc mineralization that occurred in the syncollisional period.

Early Jurassic intra‐oceanic arc system of the Neotethys Ocean: Constraints from andesites in the Gangdese magmatic belt, south Tibet

AbstractThe Gangdese magmatic belt is located in the southern margin of the Lhasa terrane, south Tibet. Here zircon U–Pb ages and Hf isotopic data, as well as whole‐rock geochemistry and Sr–Nd isotopes on andesites from the Bima Formation with a view to evaluating the history of the Gangdese magmatism and the evolution of the Neotethys Ocean. Zircon U–Pb dating yields an age of ca 170 Ma from six samples, representing the eruptive time of these volcanic rocks. Zircon Hf isotopes show highly positive εHf(t) values of +13 to +16 with a mean of +15.2. Whole‐rock geochemical and Sr–Nd isotopic results suggest that the magma source of these andesites was controlled by partial melting of a depleted mantle source with addition of continental‐derived sediments, similar to those in the southern arcs of the Lesser Antilles arc belt. In combination with published data, the volcanic rocks of the Bima Formation are proposed to have been generated in an intra‐oceanic arc system, closely associated with northward subduction of the Neotethyan oceanic lithosphere.

Late Triassic Intraoceanic Arc Aystem within Neotethys: Evidence from Cumulate Hornblende Gabbro in Gangdese Belt, South Tibet

The Neotethys plays an important role in shaping the Gangdese magmatic belt, southern Tibet. However, the initial time of spreading and subduction of the Neotethys remains contentious.In this study, a suite of late Triassic cumulate hornblende gabbro was identified in the southern margin of the Gangdese magmatic belt. The gabbro exhibits cumulate structure, with hornblende and plagioclase as the primary mineral phases. Isotopic data indicate a hydrous magma source derived from a depleted mantle wedge that has been modified by slab dehydration. Geochemical discriminations suggest that the gabbro was formed in an intraoceanic arc setting, with crystallization ages of ca. 220–213 Ma. Hornblende, hornblende‐lagioclase and ilmenite thermometers reveal that the crystallization temperature of 900–750 °C for the gabbro. Hornblende and hornblende‐plagioclase geobarometers yield an emplacement depth at ca. 14.5–19.5 km. This gabbro constitutes a line of evidence for an intraoceanic arc magmatism that is coeval with the counterparts in the southern Turkey, revealing an intraoceanic subduction system within the Neotethys from west to east in the Late Triassic and that the oceanization of the Neotethys was much earlier than previous expectation.

The origin and tectonic significance of the volcanic rocks of the Yeba Formation in the Gangdese magmatic belt, South Tibet

Zircon U-Pb geochronology, Hf isotope and whole-rock geochemistry were performed on the tuffs of the Yeba Formation in the Gangdese magmatic belt, South Tibet. The results are used to detail the age, source nature and tectonic processes that led to the formation of the Gangdese belt. Dating results indicate that the rhyolitic-andesitic tuffs were formed at 174-170 Ma. Positive and variable zircon e Hf(t) values of the rhyolitic tuffs reveal that the source was dominated by juvenile material, however, experienced crustal contamination. The basaltic tuffs have low HREEs, high contents of compatible elements (V and Cr) and no Eu anomaly. In contrast, the rhyolitic-andesitic tuffs show low compatible trace elements, depletion in Eu but enrichment in incompatible elements (Rb, Zr and Hf). According to the discrimination diagrams of P2O5-SiO2 and Th-Rb, the rhyolitic-andesitic tuffs show a close affinity to I-type granitoids. Moreover, these tuffs are marked by significant depletion in Nb, Ta and Ti, plotted in calc-alkaline field, and with the andesitic-rhyolitic tuffs falling into an active continental margin setting. We suggest that these tuffs of the Yeba Formation were probably generated in an active continental margin above the northward subduction of the Neo-Tethyan oceanic lithosphere.

Evidence of magma mixing identified in the Early Eocene Caina pluton from the Gangdese Batholith, southern Tibet

Abundant mafic enclaves are enclosed within the Quxu batholith in the eastern Gangdese magmatic belt, southern Tibet. This study focuses on the petrogenesis and affinity of these mafic enclaves with their host diorite-granodiorite, as well as their mechanism of formation and tectonic setting. Zircon U-Pb geochronological and Hf isotopic results demonstrate that the mafic enclaves and their hosts were emplaced at ~50Ma, with similar positive zircon εHf(t) values of ~9–12. Al-in-hornblende geobarometry suggests that the mafic enclaves and their hosts intruded at depths of 11–13km. The geochronological and geochemical data point to a magma mixing process resulting in the formation of the host rocks and mafic enclaves. Our conclusion is further supported by mesoscopic field and microscopic thin-section observations consistent with magma mixing. We argue that the emplacement of these rocks within the eastern Gangdese Belt was triggered by slab rollback of the subducted Neotethyan oceanic lithosphere during the early phases of the Indo-Asian collision.

The discovery of late Triassic mylonitic granite and geologic significance in the middle Gangdese batholiths, southern Tibet

The Gangdese magmatic belt, located in the southern margin of the Lhasa terrain and carrying significant copper poly-metallic mineralization, preserves important information related to the tectonics associated with the Neo-Tethys Ocean subduction, Indian-Asian collision and the crustal growth of southern Tibet. Here, we investigated the Namling batholiths in the central domain of the Gangdese magmatic belt and report for the first time the occurrence of late Triassic mylonitic granite, and present its petrologic, zircon U-Pb-Hf isotopes and the whole-rock chemistry dataset. Zircon LA-ICP-MS U-Pb dating from three mylonitic granitic samples yields crystallization ages of 209.4±1.1Ma, 206.7±1.1Ma and 212±1.1Ma, indicating a late Triassic magmatic event in the Gangdese magmatic belt. Geochemically, the mylonitic granite exhibits magnesian genesis and calcic and calc-alkalic features. The depletions of Nb, Ta and Ti and enrichments of LREEs together suggest that the magmas from which these rocks crystallized were generated in an active continental margin. Besides, all the analyzed samples also demonstrate highly positive εHf(t) values of 8.95–12.91 (mean value 10.84), corresponding to single stage model ages (tDM1) in the range of 326Ma–486Ma (mean 414Ma), attesting to crustal growth in southern Lhasa terrain. The mylonitic pluton are likely sourced from partial melting of the lower crustal components (amphibolite or basaltic stuff) that was heated and metasomatized by underplating basaltic magmas, and subsequently might undergo fractional crystallization and upper sediments contamination during parental magmas ascent. In light of the distribution of the early Mesozoic magmatic events and robust sedimentary evidence together suggest that the northward subduction of the Neo-Tethys oceanic crust beneath the Lhasa terrain should commence prior to late Triassic (∼210Ma).