Eastern Gangdese Belt Research Articles

Petrogenesis and Metallogenesis of Late Cretaceous Adakites in the Nuri Large Cu-W-Mo Deposit, Tibet, China: Constraints from Geochronology, Geochemistry, and Hf Isotopes

The Gangdese metallogenic belt in Tibet is an important polymetallic metallogenic belt formed during the subduction of the Neo-Tethys Ocean and subsequent India–Asia collision. Adakitic rocks are widely distributed in this belt and are considered to be closely related to porphyry–skarn Cu-Mo polymetallic mineralization. However, the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt remain controversial. In this study, we focus on the quartz diorite in the Nuri Cu-W-Mo deposit along the southern margin of the eastern Gangdese belt. LA-ICP-MS zircon U-Pb dating yields a Late Cretaceous age of 93.6 ± 0.4 Ma for the quartz diorite. Whole-rock geochemistry shows that the quartz diorite possesses typical adakitic signatures, with high SiO2, Al2O3, and Sr contents, but low Y and Yb contents. The relatively low K2O content and high MgO, Cr, and Ni contents, as well as the positive zircon εHf(t) values (+6.58 to +14.52), suggest that the adakites were derived from the partial melting of the subducted Neo-Tethys oceanic slab, with subsequent interaction with the overlying mantle wedge. The Late Cretaceous magmatic flare-up and coeval high-temperature granulite-facies metamorphism in the Gangdese belt were likely triggered by Neo-Tethys mid-ocean ridge subduction. The widespread occurrence of Late Cretaceous adakitic intrusions and associated Cu mineralization in the Nuri ore district indicate a strong tectono-magmatic-metallogenic event related to the Neo-Tethys subduction during this period. This study provides new insights into the petrogenesis and geodynamic setting of the Late Cretaceous adakites in the Gangdese belt, and has important implications for Cu polymetallic deposit exploration in this region.

Geology, geochemistry, and geochronology of the Zegulangbei deposit in the Jiama ore district: Implications for a polycentric, complex porphyry mineralization system model

Jiama is a typical giant-scale collisional-type porphyry mineralization system of in the Gangdese metallogenic belt, Tibet. The Zegulangbei deposit is located in the northern part of the ore district, which was a recent discovery. In the Zegulangbei deposit, intrusions have developed in multiple stages and led to the formation of complex porphyry intrusions (2# porphyries), of which granodiorite porphyry and Ⅱ-quartz diorite porphyry occurred related to mineralization. Hydrothermal alteration surrounding the 2# porphyries is characterized by potassic, phyllic, and chlorite–sericite alteration, with minor propylitization, argillization and tourmalinization. Furthermore, veinlet-disseminated copper and molybdenum mineralization mainly occurred within the potassic and phyllic alteration zones. Partial copper mineralization developed within the chlorite–sericite alteration zone. The mineralization is comprised of porphyry Cu–Mo, hornfels Cu–Mo and skarn Cu–polymetallic ore bodies. Hydrothermal vein system developed well, including 5-staged veins (M-, A-, B-, C-, and D-type vein). The similar ages of intrusive rocks (15.8–14.5 Ma, U–Pb), biotite and sericite alteration minerals (15.04–15.02 Ma, 40Ar–39Ar), and molybdenite (14.9 Ma, Re–Os) indicate that the intermediate–felsic magmatic rock emplacement triggered the hydrothermal alteration and mineralization in the Zegulangbei deposit. Geochemistry and Sr–Nd–Pb isotopes of the intrusions in Zegulangbei are consistent with those of the intrusions in the Main deposit (1# porphyries) and the Miocene adakites that developed in the eastern Gangdese belt, implying they were generated from the partial melting of the thickened phlogopite-bearing juvenile crust that had been reworked by diapirs of the arc magma during subduction. The evidence presented in this study suggests that Zegulangbei developed a complete porphyry Cu–polymetallic mineralization system, which was related to the same magma chamber in the upper crust that caused the mineralization in the Main deposit.

Paleomagnetism of late Cretaceous dykes in the Gangdese belt: New constraints on the position and structure of the southern margin of Asia prior to the India-Asia collision

We report paleomagnetic data from late Cretaceous diorite dykes that sub-vertically intrude granodiorites in the eastern Gangdese belt near the city of Lhasa. Our research goals are to provide further constraints on pre-collisional structure of the southern margin of Asia and the onset of the India-Asia collision. Magnetite is identified as the main magnetic carrier in our study. The magnetite shows no evidence of metamorphism or alteration as determined from optical and scanning electron microscope observations. A strong mineral orientation is revealed by anisotropy of magnetic susceptibility analysis both for the intruded dykes and the country rocks. We interpret this AMS fabric to have formed during intrusion rather than deformation. Fifteen of 23 sites yield acceptable site mean characteristic remanences with dual polarities. A scatter analysis of the virtual geomagnetic poles suggests that the mean result adequately averaged paleosecular variation. The paleomagnetic pole from the Gangdese dykes yields a paleolatitude of 14.3°N ± 5.8°N for the southern margin of Asia near Lhasa. The paleolatitude corresponds to an in-between position of the Lhasa terrane during ~130-60 Ma. Furthermore, the mean declination of the characteristic remanent magnetization reveals a significant counterclockwise rotation of 18° ± 9° for the sampling location since ~83 Ma. In the light of tectonic setting of the dykes, the strike of the southern margin of Asia near Lhasa is restored to trend approximately ~310°, which is compatible with the hypothesis that the southern margin of Eurasia had a quasi-linear structure prior to its collision with India.

A tale of an orbicule in the eastern Gangdese belt of southern Tibet: Petrographic, geochemical, and submagmatic structural perspectives on its formation

Owing to their intriguing appearance and unusual occurrence, as well as their significance in addressing magmatic processes, orbicules have attracted attention from geologists all over the world. We discovered an orbicule-bearing boulder in the Linzhi region, near the Eastern Himalayan Syntaxis, which is the first report of orbicules in the Gangdese magmatic belt, southern Tibet. The orbicules are composed of dioritic shells around monzodioritic cores, in a dioritic matrix. They typically have a single shell, composed of elongate and radially oriented hornblendes, with interstitial plagioclase. Zircon laser ablation−inductively coupled plasma−mass spectrometry U-Pb dating yields an indistinguishable age of ca. 28.8 Ma for both the orbicule core and matrix, indicating approximately coeval crystallization. Zircon Lu-Hf and whole-rock Sr-Nd isotopes reveal that the original magmas of both the orbicule matrix and orbicule core were relatively juvenile (ƐHf(t) = +2.2 to +6.0 for matrix and +1.2 to +3.9 for core; ƐNd(t) = −1.68 to −1.46 for matrix and −3.65 to −2.52 for core), similar to reported Oligocene−Miocene post-collisional adakites in the Gangdese belt. Zircon and hornblende geochemistry reveals that both the orbicule matrix and cores were generated from highly oxidized, high oxygen fugacity magmas. Hornblende composition further indicates that the original magmas of the matrix and the orbicule cores were water-rich, with H2O contents greater than 4 wt%. In combination with the whole-rock geochemistry, these results suggest that the orbicules were formed in a post-collisional setting, from magma generated by partial melting of the juvenile thickened lower crust of the Lhasa terrane triggered by convective removal of the Lhasa lithospheric root. The preferred orientation of euhedral igneous minerals in the matrix and the alignment and compaction of orbicules define a magmatic to submagmatic foliation. This, together with the subsolidus deformation microstructures exhibited by minerals within the orbicules, indicates that some flow or movement of orbicules took place under magmatic to submagmatic conditions, as the subsolidus plastic orbicules were jostled and transported within a hypersolidus liquid magma.

Middle Permian and Late Triassic mafic magmatism in Nyemo area of the eastern Gangdese belt, Tibet, and its constraints on the subduction initiation of the Neo-Tethys

Middle Permian and Late Triassic mafic magmatism in Nyemo area of the eastern Gangdese belt, Tibet, and its constraints on the subduction initiation of the Neo-Tethys

Petrogenesis and Tectonic Implications of the Latest Cretaceous Intrusive Rocks from the Eastern Gangdese Belt, Southeast Tibet

AbstractThe latest Cretaceous magmatic activity in the eastern segment of the Lhasa terrane provides important insights for tracking the magma source and geodynamic setting of the eastern Gangdese batholith, eastward of eastern Himalayan Syntaxis. Detailed petrological, geochemical and geochronological studies of the intrusive rocks (monzodiorites and granodiorites) of the eastern Gangdese batholith are presented with monzodiorites and granodiorites giving zircon U–Pb crystallization dates of 70–66 Ma and 71–66 Ma with εHf(t) values of –4.8 to +6.2 and –1.9 to +5.3, respectively. These rocks are metaluminous to weakly peraluminous I‐type granites showing geochemically arc‐related features of enrichment in LREEs and some LILEs, e.g., Rb, Th, and U, and depletion in HREEs and some HFSEs, e.g., Nb, Ta, and Ti. The rocks are interpreted to be derived from partial melting of mantle material and juvenile crust, respectively, which are proposed to be triggered by Neo‐Tethyan slab rollback during northward subduction, with both experiencing ancient crustal contamination. The studied intrusive rocks formed in a transitional geodynamic setting caused by Neo‐Tethyan oceanic flat subduction to slab rollback beneath the eastern Gangdese belt during the latest Cretaceous.

Geochronology and petrogenesis of Eocene gabbros and granitic rocks of the eastern Gangdese belt, southern Tibet: Implications for the timing of India-Asia collision

The Palaeocene-Eocene (66–41 Ma) hosts the most intense stage of extensive magmatism in the Gangdese belt of southern Tibet. However, the magma sources and tectonic setting of this magmatic belt have been a matter of discussion for decades. In this study, we conducted zircon geochronological and geochemical studies on Eocene gabbroic rocks and granitic rocks from the Jiacha area in the eastern Gangdese belt. Laser ablation-ICP-MS zircon U–Pb dating suggests almost synchronous crystallization ages of 50–47 Ma for both rock types. Geochemically, the gabbroic rocks have low total rare earth elements and high positive zircon εHf(t) values (from +8.4 to +15.0), indicating that the parental magma was derived from an isotopically depleted mantle. The granitic rocks are magnesian calc-alkalic to calcic granites that are characterized by variable zircon εHf(t) values (from +3.0 to +10.8), indicating that the magmas were generated by differentiation of a mantle-derived magma with variable contributions from ancient crustal materials. Furthermore, an enlarged database of 66–41 Ma plutonic rocks and coexisting Linzizong volcanic rocks reveals an evolutionary trend from mafic to silicic rocks. The zircon Hf and Nd isotopic compositions become more evolved with increasing whole-rock SiO2. The whole-rock and isotope geochemistry of the 66–41 Ma plutonic rocks and the Linzizong volcanic rocks indicate an origin and evolution of these rocks in an active continental margin setting. Therefore, we argue that the Gangdese belt was associated with the subduction of the Neo-Tethyan Oceanic plate throughout the Early Eocene, thereby implying that the initial India-Asia collision occurred after ~ 41 Ma.

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.

Geochemistry and zircon trace elements composition of the Miocene ore‐bearing biotite monzogranite porphyry in the Demingding porphyry Cu‐Mo deposit, Tibet: Petrogenesis and implication for magma fertility

Demingding is a poorly studied post‐collisional porphyry Cu‐Mo deposit, located in the eastern part of the Gangdese porphyry copper belt. In this study, we present LA‐ICP‐MS zircon U‐Pb dating, whole‐rock geochemistry, and zircon trace elemental data for the mineralization related biotite monzogranite porphyry in Demingding. Zircon U‐Pb dating shows that the weighted mean 206Pb/238U ages of the Miocene biotite monzogranite porphyry is 20.36 ± 0.46 Ma (MSWD = 2.6, n = 21). The Miocene biotite monzogranite porphyry is characterized by high SiO2, K2O, and Al2O3 contents and shows adakite‐like signatures. These rocks have high Sr/Y ratios, fractionated REE patterns with low HREE abundances, enrichment in LILE compared with HFSE, and have (87Sr/86Sr)i values of 0.7059 to 0.7062, εNd(t) values of −2.35 to −1.67 and (206Pb/204Pb)i ratios of 18.50–18.55. These features are similar to other Miocene adakite‐like intrusions in the Gangdese belt. We propose that the biotite monzogranite porphyry shares the same petrogenesis with other Miocene ore‐bearing adakite‐like intrusions in the Eastern Gangdese belt and was derived from remelting of the subduction‐modified, eclogitized Tibetan lower crust and mixing with hydrous mafic magmas from metasomatized Tibetan mantle. The zircons from biotite monzogranite porphyry have only minimal negative Eu anomalies (EuN/EuN* > 0.3), and exhibit large ratios of Ce4+/Ce3+(average value of 113), 10,000*(EuN/EuN*)/Y (3.94–8.14, >1), Ce/Nd (9–58, Average value of 30), (Ce/Nd)/Y (0.008–0.127, >0.003), and low value of Dy/Yb (0.15–0.24, <0.3), indicating that the biotite monzogranite porphyry is relatively hydrous and oxidized. The biotite monzogranite porphyry, therefore, is believed to be a relatively fertile intrusion with certain exploration potential.

Tectonic erosion and crustal relamination during the India-Asian continental collision: Insights from Eocene magmatism in the southeastern Gangdese belt

Understanding the processes of tectonic erosion and crustal relamination during continental collision has important implications for the growth and differentiation of the continental crust. The discrepancy in isotopic compositions between the pre- and syn-collision magmatic rocks from the Gangdese belt in south Tibet provides an opportunity for studying these processes during the India-Asian collision. The Nyingchi granites and Confluence hornblende gabbros from the eastern Gangdese belt have zircon U-Pb ages of ca. 50 Ma. The Nyingchi granites have high Sr/Y and (Dy/Yb)N ratios, indicating that the magma was generated under eclogite-facies conditions. Their Sr-Nd-Pb-Hf isotopic compositions require significant incorporation of ancient supracrustal materials from the Gangdese belt and the Indian continent. The Confluence hornblende gabbros display arc-like trace element patterns but have enriched Sr-Nd-Pb-Hf isotopic compositions compared with those from the Jurassic-Cretaceous arc magmatic rocks, indicating significant input of ancient components into their mantle sources. The occurrence of the Cenozoic felsic metamorphic rocks in the lower crust of the Gangdese belt allows us to propose that the Nyingchi high Sr/Y granites were derived from partial melting of relaminated crustal materials which were removed from the Gangdese belt by tectonic erosion and the subducted Indian continent. The Confluence gabbros were sourced from lithospheric mantle which was metasomatized by inputs from relaminated crustal materials derived from the Gangdese belt and the subducted Indian continent. The estimated tectonic erosion rate is 150–188 km3/km/my, indicating significant crustal loss occurred during continental collision. Our study demonstrates that tectonic erosion and crustal relamination play an important role in the refinement of the composition of continental crust during continental collision.

Miocene porphyry copper deposits in the Eastern Tethyan orogenic belt: Using Sr, O isotopes and Sr/Y ratios to predict the source of ore-related and ore-barren magmas

There are many Miocene collision- and subduction-related porphyry Cu deposits in the Eastern Tethyan orogenic belt. However, the petrogenesis of the ore-related magmas and the mechanism of metal enrichment remain controversial. Here, we present a geochemical study and comparison of the major Miocene ore-related porphyries and coeval ore-barren magmatic rocks in the Urumieh-Dokhtar magmatic arc in Iran and the Chagai belt in Pakistan, and the Eastern Gangdese belt of the Himalaya. The results show that ore-related porphyries are characterized by relatively depleted mean Sr isotopic compositions (0.704217–0.706859 in the Urumieh-Dokhtar magmatic arc; 0.705052–0.706588 in the Eastern Gangdese belt; 0.705252–0.706708 in the Chagai belt), but higher maximum Sr/Y ratios (40–149 in Urumieh-Dokhtar; 79–178 in Eastern Gangdese; 52–178 in Chagai). In contrast, the coeval ore-barren magmatic rocks yield relatively enriched, mean Sr isotopic compositions (0.704722–0.707653 in Urumieh-Dokhtar; 0.704701–0.707802 in Eastern Gangdese; 0.705982–0.707856 in Chagai), but lower maximum Sr/Y (2–88 in Urumieh-Dokhtar; 61–136 in Eastern Gangdese; and 19–35 in Chagai). A deep crustal hot zone model provides a robust explanation of these data. Melts derived from a deeper melt source in juvenile mafic lower crust, where mantle-derived materials are common, have a greater potential to yield a higher tonnage of metallic Cu, such as the Sar-Cheshmeh deposit in Iran, which is derived from a deep melt zone, and characterized by a low radioactive mean Sr isotopic value of 0.704851, but a huge tonnage of metallic Cu of 7.2Mt and a high maximum Sr/Y value of 117 (representative of a deeper depth of the melt zone), whereas the Dalli deposit in Iran derived from a relatively shallow melt zone has a more radioactive mean Sr isotopic value of 0.706859, but a smaller amount of metallic Cu of 0.04Mt and a lower maximum Sr/Y ratio of 40. Conversely, re-melting the contact between newly-formed and pre-existing crust where mantle-derived materials are absent will produce ore-barren rocks. Additionally, the ore-barren rocks with low Sr/Y ratios (<20) should be derived from a shallower melt zone in the crust.

Constraints on the origin of adakites and porphyry Cu-Mo mineralization in Chongjiang, Southern Gangdese, the Tibetan Plateau

Chongjiang is a low-grade porphyry Cu deposit, located in the Gangdese belt, south Tibet. The petrogenesis and geodynamic settings of the Miocene intrusions associated with the deposit remain controversial. This study presents new results on in situ zircon Hf-O isotopic compositions and U-Pb ages, whole rock major and trace elements, and Sr-Nd isotopes for the adakitic intrusions from Chongjiang deposit. The ore-bearing biotite monzogranite porphyry has adakitic characteristics, with enriched large-ion-lithophile elements (LILE) and light rare earth elements (LREE), and depleted in high-field-strength elements (HFSE), P and Ti. LA-ICP-MS zircon U-Pb dating indicates that the ore-bearing and barren adakites were emplaced at 14.9±0.3Ma and 12.9±0.3Ma, respectively. The porphyry is characterized by relatively high initial 87Sr/86Sr ratios (0.7059 to 0.7066), and negative whole-rock εNd(t) values (−3.8 to −2.6). Zircon δ18O is slightly higher than mantle values (5.0 to 7.2‰), with varied εHf(t) (−1.0 to 7.6). Most of the in situ zircon Hf-O isotopic data plot in a binary mixing trend between MORB and lower continental crust-derived melts. These results indicate contributions from mixing of a mantle-like source (e.g., slab melts) with continental crust. Interestingly, most of the samples plot in the field defined by Dabie adakites (representing partial melting of the lower continental crust), with several samples near/in the circum-Pacific adakite field (representing partial melting of subducted oceanic slabs), which seemingly indicates that Chongjiang adakites mostly formed through partial melting of lower continental crust, with a small amount derived from oceanic slab melts. These may be plausibly explained by plagioclase retention in the thickened Tibetan continental crust, which lowers Sr contents in the magmas during crustal assimilation. Such a model is supported by other adakite discrimination diagrams, which all point towards slab melting. Crustal contamination can compellingly explain the low grade of the Chongjiang deposit. Considering the temporal-spatial distribution of porphyry Cu deposits, geochemical characteristics and high oxygen fugacity, we propose that the subducting Ninetyeast Ridge probably played a critical role in controlling the formation of Miocene adakites and porphyry copper deposits in the eastern Gangdese belt.

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.

Adakite-like geochemical signature produced by amphibole-dominated fractionation of arc magmas: An example from the Late Cretaceous magmatism in Gangdese belt, south Tibet

Late Cretaceous (~106–76Ma) adakite-like intrusive rocks in the middle-eastern Gangdese belt occur in an E–W trending belt paralleling the Indus–Yarlung suture, south Tibet. Their petrogenesis and geodynamic processes have been a subject of debate. We report here U–Pb zircon ages, geochemical and Sr–Nd–Hf isotopic data for adakite-like intrusive rocks as well as the normal arc rocks (gabbros and gabbroic diorites) in the middle Gangdese belt. LA-ICPMS U–Pb zircon analyses yielded an identical age of ~88Ma for two adakite-like rocks, which are slightly younger than the gabbro and gabbroic diorite (ca. 94–90Ma). Both the adakite-like rocks and the normal arc rocks have similar whole-rock Sr–Nd and zircon Hf isotope compositions, indicating that they have been derived from a common source. Similarly, the adakite-like and normal arc intrusive rocks in the eastern Gangdese belt also show similar Sr–Nd–Hf isotope compositions. In the middle-eastern Gangdese belt, the >85Ma Late Cretaceous intrusive rocks consist of a magma series from gabbro to granodiorite, including both normal arc rocks and adakite-like rocks. These rocks overlap in space and time that conform to a normal arc differentiation trend. In terms of major and trace elements, they also show a clear evolution from the normal arc magmatic into adakitic field. Thus, we suggest that these >85Ma Late Cretaceous intrusive rocks were ultimately derived from melting of the hydrated mantle wedge and the adakite-like rocks can be generated in normal arc magmas by amphibole-dominated fractionation. Taking into accounting for the spatial and temporal distribution of the Cretaceous magmatic rocks in the Lhasa terrane, we prefer a model of early Late Cretaceous rollback following Early Cretaceous low-angle oceanic slab subduction. At intermediate pressure and H2O-rich conditions, fractionation of amphibole changes the major and trace element compositions of arc magmas, and will efficiently drives basaltic composition to andesitic composition in arc magmas.

Zircon U–Pb age and Sr–Nd–Hf–O isotope geochemistry of the Paleocene–Eocene igneous rocks in western Gangdese: Evidence for the timing of Neo-Tethyan slab breakoff

Northward Neo-Tethyan oceanic lithosphere subduction beneath southern Tibet in the Mesozoic–Early Cenozoic produced continental arc magmas in the ~1600km-long Gangdese belt. The most voluminous magmatism occurred in the Paleocene–Eocene, and is characterized by extensive I-type calc-alkaline to high-K calc-alkaline Linzizong volcanic rocks, and coeval plutons. These rocks have been extensively studied in the eastern Gangdese belt (east of ∼89°E), but few data exist from the western Gangdese belt. New data for eleven samples of these rocks, combined with existing data from the literature, show that they are similar to the eastern Gangdese belt rocks, with relative depletions in Nb, Ta, P, and Ti, and enrichments in Rb, Ba, Th, U, K, Pb, Zr, and Hf on a primitive mantle-normalized trace element diagrams, typical of continental arc-related igneous rocks. However, compared to the east, western Gangdese igneous rocks range to higher K2O contents (up to 6.1wt.%), higher (87Sr/86Sr)i ratios (up to 0.7151), and lower εNdi values (down to −8.1), suggesting that an evolved crustal source was involved in arc magmatism.The Gangdese arc magmatism lasted to ~80Ma, and has a gap or quiescent period afterwards. Starting at ~69Ma, the arc magmatism initiated and shifted southward from ~30.5°N to ~29.5°N in southern Tibet with an abrupt change of India-Asia convergence rate. The magmas through the whole Gangdese belt at ~69–53Ma are characterized by intermediate εNdi values (−0.6 to +4.0), positive εHfi values (+3.8 to +7.1), intermediate δ18O values (+5.0‰ to +6.5‰), and low Th/Y and La/Yb ratios (<20). These magmas were likely derived from the mantle with crustal contamination by MASH process in response to Neo-Tethyan slab rollback as proposed by previous studies.The Sr–Nd–Hf–O isotopic compositions of Linzizong volcanic rocks and coeval intrusions are relatively homogeneous during ~69–53Ma, but show a wide range at ~53–50Ma. Zircon temperature values from these rocks are distinctively high during the latter period and whole rock Th/Y and La/Yb ratios increased significantly after ~50Ma. We suggest that the high temperature and heterogeneous geochemistry of these magmas at ~53–50Ma reflect Neo-Tethyan slab breakoff during India-Asia collision, which triggered asthenospheric mantle upwelling. Mantle derived magmas resulted in extensive heating, induced crustal melting, and generated magmas with variable isotopic compositions. Tibetan crust was significantly thickened after ~50Ma.

Increasing Magmatic Oxidation State from Paleocene to Miocene in the Eastern Gangdese Belt, Tibet: Implication for Collision-Related Porphyry Cu-Mo Au Mineralization

In the Gangdese belt of southern Tibet, Paleocene-Eocene magmas record the final stage of Neo-Tethyan subduction and are associated with few economic porphyry deposits. In contrast, magmas formed during later stages of the India-Asia collision in the Oligo-Miocene are associated with several large porphyry Cu-Mo ± Au deposits, especially in the eastern part of the belt (~89°E–93°E). In a previous study, we showed that these Oligocene-Miocene magmas were more hydrous than the earlier Paleocene-Eocene magmas. In this study, we show that the later magmas were also more oxidized. Paleocene-Eocene rocks from the eastern Gangdese belt are characterized by low zircon Ce4+/Ce3+ ratios (mostly 50; 32.3–141.9, average = 74.3 ± 30.1, n = 33) and higher f O2 values (ΔFMQ 0.8–2.9, average = 1.8 ± 0.8, n = 6; estimated from magnetite-ilmenite pairs). Estimates of magmatic oxidation state from amphibole compositions also show an increase from ΔFMQ 1.2 to 2.1 (average = 1.6 ± 0.2, n = 40) in the Paleocene-Eocene to 2.0 to 2.8 (average = 2.5 ± 0.2, n = 58) in the Miocene. Sparse whole-rock Fe3+/Fe2+ ratios show the same general trend. The amphibole results are systematically shifted to higher ΔFMQ compared to data from magnetite-ilmenite pairs, but their trend is internally consistent. The higher oxidation states (and water contents) of Miocene igneous rocks from the eastern Gangdese belt may explain their unique association with large porphyry deposits in Tibet, because it has been shown from other studies that the potential for hydrous calc-alkaline magmas to transport Au is maximized near ΔFMQ 1.0, and Cu at higher f O2 (ΔFMQ > 1.0). In comparison, the somewhat less oxidized and less hydrous Paleocene- Eocene magmas would have been less fertile for the formation of such deposits. We suggest that these differences reflect the tectonomagmatic evolution of the Cenozoic Gangdese collisional orogen, from early collision-related magmatism in the Paleocene-Eocene to late collisional magmatism in the Miocene. Asthenospheric upwelling following slab breakoff in the Miocene caused interaction between mantle-derived magmas and previously subduction modified Tibetan lithosphere. The resultant evolved magmas had high oxidation states and water contents, which are favorable properties for the subsequent formation of magmatic-hydrothermal porphyry Cu-Mo ± Au deposits.

Increased Magmatic Water Content--The Key to Oligo-Miocene Porphyry Cu-Mo Au Formation in the Eastern Gangdese Belt, Tibet

The eastern Gangdese magmatic belt (between ~88°E and 94°E) in southern Tibet preserves a series of Cenozoic collision-related igneous rocks that have formed since the start of the India-Asia collision at ~55 to 50 Ma. The Paleocene-Eocene intrusive rocks have intermediate [La/Yb]N ratios (average = 12.0 ± 8.6, n = 150, range = 1.15–64.0), intermediate to low Sr/Y ratios (mostly 40), and weak or absent Eu anomalies (Oligocene: average Eun/Eu* = 0.99 ± 0.27, n = 35, range = 0.67–1.58; Miocene: average Eun/Eu* = 0.95 ± 0.13, n = 93, range = 0.44–1.46), and their mineralogy consists mainly of plagioclase, quartz, and amphibole as phenocrysts. These geochemical and mineralogical characteristics suggest that the Paleocene-Eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase, whereas the Oligo-Miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement. Oligocene and Miocene magmatic rocks crop out as small-volume intrusions and are associated with several large- to giant-sized porphyry Cu-Mo ± Au deposits, as well as numerous smaller porphyry and skarn deposits. In contrast, earlier, more voluminous Paleocene-Eocene magmatism is only known to be associated with three small porphyry deposits. Erosional loss of subvolcanic porphyry systems from the older Paleocene-Eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution, because exhumation rates after 55 Ma were relatively low and coeval Linzizong volcanic rocks are extensively preserved in the Gangdese belt. Instead, we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits. The Paleocene-Eocene magmas were generated during the onset of collision between India and Asia and were triggered by rollback of the Neo-Tethyan oceanic slab. This magmatism records the final stage of subduction of oceanic lithosphere beneath southern Tibet, and the relatively low water contents of these magmas may reflect final dehydration of the remnant Neo-Tethyan slab. After oceanic slab breakoff at ~40 to 38 Ma, the Greater India slab began to subduct beneath the Gangdese belt until hard collision with the Indian continent at ~35 Ma and breakoff of the Greater India slab at ~25 to 10 Ma. This last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction-modified, hydrated Tibetan lithosphere. The resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism, giving rise to a suite of postsubduction porphyry-type ore deposits in the Oligo-Miocene.

Origin and evolution of multi-stage felsic melts in eastern Gangdese belt: Constraints from U–Pb zircon dating and Hf isotopic composition

This integrated study of whole rock geochemistry, zircon U–Pb dating and Hf isotope composition for seven felsic rocks from the Nyingchi Complex in eastern Himalayan syntaxis has revealed a complex magmatic history for the eastern Gangdese belt. This involves multiple melt sources and mechanisms that uniquely identify the tectonic evolution of this part of the Himalayan orogen. Our U–Pb zircon dating reveals five stages of magmatic or anatectic events: 165, 81, 61, 50 and 25Ma. The Jurassic granitic gneiss (165Ma) exhibits εHf(t) values of +1.4 to +3.5. The late Cretaceous granite (81Ma) shows variable εHf(t) values from −0.9 to +6.2, indicating a binary mixing between juvenile and old crustal materials. The Paleocene granodioritic gneiss (61Ma) has εHf(t) values of +5.4 to +8.0, suggesting that it originated from partial melting of a juvenile crustal material. The Eocene anatexis is recorded in the leucosome, which has Hf isotopic composition similar to that of the Jurassic granite, indicating that the leucosome could be derived from partial melting of the Jurassic granite. The late Oligocene biotite granite (25Ma) shows adakitic geochemical characteristics, with Sr/Y=49.3–56.6. The presence of a large number of inherited zircons and negative εHf(t) values suggest that it sourced from anatexis of crustal materials. In contrast to the Gangdese batholiths that are mainly derived from juvenile crustal source in central Tibet, the old crustal materials play an important role for the magma generation of the felsic rocks, suggesting the existence of a crustal basement in the eastern Gangdese belt. These correspond to specific magmatic evolution stages during the convergence between India and Asia. The middle Jurassic granitic gneiss resulted from the northward subduction of the Neo-Tethyan oceanic slab. The late Cretaceous magmatism is probably related to the ocean ridge subduction. The Paleocene–Eocene magmatism, metamorphism and anatexis are interpreted to result from roll-back and break-off of the subducted Neo-Tethyan slab that occurred in the early stage of the India–Asian collision, respectively. The late Oligocene adakitic rocks resulted from the break-off of the subducted Indian continental crust starting at ~25Ma.