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Abstract: Tectonic models for the evolution of the Himalayan orogen interpret the Greater Himalayan crystalline complex (GHC) to be the result of either thick-skinned thrusting involved Indian basement, thin-skinned thrusting involving exotic terranes, middle-crustal ductile flow, or wedge extrusion of the Indian crust during India–Asia collision. Two key pieces of information needed to test the validity of these models is the temporal–spatial distribution of, and the identification of the dynamic mechanisms involved in, regional melting under southern Tibet. Here, we document an Eocene–Oligocene melting event in southern Tibet, which forms a 150-km-long, NW–SE-trending granitoid belt along the Zedong–Lhunze traverse between the Indus–Yarlung suture (IYS) and the south Tibetan detachment (STD). U–Pb dating of magmatic zircons indicates that this granitoid belt youngs northward from ∼46Ma (in Lhunze) to ∼30Ma (in Zedong). 40Ar/39Ar dating of deformed biotite within 42–46Ma granitoids constrains the timing of shearing to ∼39–41Ma. The granitoid belt of southern Tibet is dominated by Eocene two-mica granites in the Tethyan Himalaya, with minor ∼30Ma granodiorites along the IYS and ∼35Ma granites in the Yelaxiangbo dome, where Indian mid-crustal rocks are exposed. The ∼35Ma granites are characterized by variable Na2O/K2O ratios (1.03–4.44), relatively high Sr concentrations, and high Sr/Y (14.0–126.3) and La/Yb (11.1–42.8) ratios, which distinguish these granitoids from Miocene leucogranites in the Himalaya. Comparison of the Sr–Nd isotopic compositions of these granites with mid-crustal amphibolites exposed in the Yelaxiangbo dome suggests that the granites were derived from melting of the amphibolites at ∼880°C and ∼10kbar. The ∼30Ma granodiorites and ∼42–46Ma two-mica granites are Na-rich and peraluminous, and are adakitic. They contain inherited Proterozoic zircons, and have a much wider range in ε Nd(t) of –14.9 to –2.5 and (87Sr/86Sr) i of 0.7062–0.7188, and have a Nd isotopic model age of 1486–1978Ma, indicating that these magmas were derived from a thickened Indian lower crust and were subsequently mixed with amphibolite-derived granite melts or were contaminated by the middle crust under southern Tibet. An apparent northward-younging age trend and shearing of the Eocene–Oligocene granitoids requires the southward migration of slices of middle crustal material, in which the Eocene granitoid magmas were emplaced and stored. Our data, along with structural, metamorphic, and intrusive histories of the Himalaya, lead us to propose a model for crustal anatexis and tectonic evolution of the Himalayan orogen, controlled by a number of large-scale events, such as slab break-off, buoyancy-driven uplift, lateral movement, and subsequent exhumation of slices of the subducted Indian crust during Indo-Asia collision at 55–40Ma. [Copyright &y& Elsevier]

Abstract: We describe the whole-rock geochemistry, biotite Ar–Ar and zircon U–Pb geochronology, and in situ Hf–O isotopic compositions of the Linzhi two-mica granitoid in the southeastern Lhasa terrane. The granitoids are calc-alkaline to high-K calc-alkaline in composition, and they have geochemical affinities with adakites. The whole-rock geochemistry, with high SiO2 and Al2O3 contents, low MgO, Cr, and Ni contents, low Mg# values, negative εNd(t) (−6.2 to −2.9), and low 87Sr/86Sr(i) (0.7059–0.7074), together with the in situ zircon Hf–O isotopic data (εHf(t) =−2.7 to 2.7; δ18O=7.2‰–8.8‰), indicates that the granitoid was derived mainly from the partial melting of a rejuvenated lower crust underneath the Lhasa terrane. This lower crust was dominated by ancient crustal material with the minor involvement of juvenile mantle inputs (<55%). SIMS zircon U–Pb analyses of two samples indicate that the crystallization of the Linzhi granitoid took place around 26Ma, implying that the crust of the Lhasa terrane was already thickened up to 50km prior to emplacement of the adakitic rocks. 40Ar/39Ar ages for biotites, and the presence of magmatic epidote, suggest the granitoid was intruded at mid-crustal depths, indicating that the eastern Lhasa terrane experienced a pulse of rapid uplift in the interval from 26 to 22Ma. The geochemical features of the Linzhi adakitic rocks are significantly different from those of Eocene adakitic rocks from the north Himalayan belt, and this suggests that we can rule out the possibility of crustal thickening in the eastern part of Lhasa terrane, caused by impingement of the Indian lower crust. Post-collisional (26–10Ma) adakitic rocks occur within an E–W-trending magmatic belt that runs parallel to the Yarlung–Tsangpo suture in the Lhasa terrane. These rocks have distinctive variations in geochemistry along the strike of the Lhasa terrane, indicating that the lower crust is isotopically heterogeneous from west to east along the terrane. These geochemical variations probably result from variable degrees of input, along strike, of juvenile mantle-derived melts into an old lower crust. The middle part of the Lhasa terrane is marked by a juvenile lower crust that contains large amounts of mantle-derived melt, underplated during Neo-Tethyan oceanic crust subduction. In contrast, the lower crust below the western and eastern parts of the terrane is relatively ancient. Therefore, crustal thickening caused by underplating of mantle-derived magmas could play an important role in the middle segment of the terrane, whereas crustal compression and shortening as a result of the collision of the Indian and Asian plates could be the major cause for crustal thickening in the western and eastern segments. [Copyright &y& Elsevier]

Abstract: The relationship between potassic felsic intrusions, cospatial and cotemporal with potassic mafic magmatism, and strike–slip displacement along the Ailao Shan-Red River shear zone in western Yunnan, SW China has been highly controversial. We report 22 new SHRIMP zircon U–Pb ages from 19 potassic felsic intrusions located as far as 150km east and 50km west of the shear zone to constrain temporal relationships. The results show that the felsic intrusions in western Yunnan were emplaced between 36.9±0.3Ma and 32.5±0.3Ma. This age range is significantly shorter than previous dating of 38Ma to 23Ma using K–Ar and Ar–Ar methods from 15 intrusions, indicating that the latter are reset or cooling ages. Potassic felsic intrusions are coeval with potassic mafic volcanic rocks (36.6Ma to 33.4Ma) in the same area, supporting a possible genetic link between the two end members. The dated intrusions occur over a 200km wide zone across the Ailao Shan-Red River shear zone; therefore they are unlikely genetically related to shearing. Rather, potassic mafic magmatism in western Yunnan may be related to delamination and melting of previously subduction-modified mantle lithosphere along the Mesozoic Jinsha suture reactivated in the Eocene–Early Oligocene, following the India-Asia continental collision; potassic felsic magmas were crustal melts hybridized with mafic liquids. Delamination in turn facilitated and localized initiation of the Ailao Shan-Red River shear zone at ca. 32Ma during continental extrusion. [Copyright &y& Elsevier]

• Fluids in the three skarn stages are characterized by complex volatile system, respectively. • Isotopic data showed a primary magmatic source, mingled with meteoric water at the late stages. • Boiling, temperature falling and f O 2 rising are the key to metal deposition at the retrograde stage. • f O 2 and temperature decreasing play role in metal precipitation at the late hydrothermal stage. Scheelite mineralization accompanied by traces of chalcopyrite occurs in skarn systems in the giant Zhuxi deposit in the Taqian area, Northeastern Jiangxi province, South China. Three paragenetic stages of skarn with ore deposition have been recognized: the prograde skarn, retrograde skarn and hydrothermal quartz-sulfide stages. Microthermometer data showed that the prograde skarn were formed at relatively high temperatures (450–>500 °C) and pressures (0.97–1.38 kbars) and low oxidation to weak reduction conditions. The Raman microprobe data demonstrated that the fluid of the stage was H 2 O–NaCl/KCl/CaCl 2 ± CH 4 /C 2 H 4 system. The retrograde skarn alteration formed at 1.24 kbars and relatively medium temperatures (280–320 °C), and the ore-forming fluid was H 2 O–NaCl/KCl/CaCl 2 system. During the hydrothermal quartz-sulfide stage, the fluids were methane-bearing aqueous, low-salinity (0.9–8.0 wt% NaCl eqv) and low homogeneous temperature (160–240 °C). Correspondingly, the minimum fluid pressures were evaluated between 75 and 252 bars. The H- and O-isotopic values from the prograde and retrograde stages implied that the mineralizing fluids were derived principally from magmatic water. The mineralizing fluids in the quartz-sulfide hydrothermal stages were mingled with magmatic water with some meteoric water as well as minor contributions of wall rocks. The sulfur and lead isotopes indicated that ore-forming materials including sulfur, lead, copper, zinc and iron in the Zhuxi deposit were derived from local magmatic and sedimentary sources. Geological, fluid inclusion and isotopic data supported that the Zhuxi deposit is a typical skarn-type W(Cu) deposit related to ilmenite-series granitic rocks and formed in a compressional tectonic environment. [ABSTRACT FROM AUTHOR]

In the world-class Gejiu Sn district, South China, tin mineralization is related to the Gejiu granitoids, which consist of porphyritic quartz monzonite, syenogranite, and porphyritic/equigranular biotite monzogranites. Zircon U Pb dating shows that these granitoids were emplaced at ca. 77.9–81.7 Ma. Close spatio-temporal associations, systematic element distribution patterns, and similar isotopic compositions, together reveal that they are consanguineous, and the porphyritic quartz monzonite represents relatively primitive magma. The occurrence of hornblende, high apatite Nd N /Nd N * (mostly > 1), and whole-rock evolutionary trends, collectively indicate that the Gejiu granitoids are fractionated I-type granites. Negative and fluctuant εHf(t) values (−13.3 to −3.8) reveal that their I-type affinities could be caused by a mixing source through partial melting of metasedimentary rocks with minor input of mantle-derived materials, which is supported by the presence of mafic microgranular enclaves (MMEs). Zircon and apatite elemental fingerprints, combined with Rayleigh fractional crystallization modeling, decipher a two-stage fractional crystallization of hornblende-dominated and feldspar-dominated. The negative correlation between whole-rock Fe 2 O 3 /FeO and Rb/Sr ratios suggests that the magmatic oxygen fugacity gradually decreases with fractional crystallization. High zircon saturation temperatures (∼ 835 °C) indicate that the additional heat from the mantle provided a higher melting temperature to trigger biotite breakdown reactions, which can effectively release Sn into the initial melt. Subsequently, tin in magma underwent a two-stage enrichment process under the conjunctive effect of the variation of fractionating phases and magmatic redox state. In the first stage, there is no significant Sn enrichment during hornblende, magnetite, and titanite fractionation, which is due to the compatibility of Sn with these Ti-bearing minerals under oxidized conditions. In the second stage, the dominant Sn-poor feldspar with limited Sn-sequestering biotite fractionation under more reduced conditions efficiently increases Sn concentration in the residual melt by a factor of ∼ 8, as demonstrated by systematic apatite geochemical data. The hysteretic crystallization of biotite prevents the premature sequestration of Sn and reserves adequate metals for eventual mineralization. Finally, fluid exsolution during the magmatic-hydrothermal transition loads magmatically enriched Sn into the hydrothermal system, leading to the formation of the Gejiu Sn district. [Display omitted] • The Gejiu granitoids are fractionated I-type granites. • Their magmatic oxygen fugacity gradually decreases with fractional crystallization. • They experienced a two-stage fractional crystallization process. • Feldspar-dominated fractionation results in significant enrichment of magmatic Sn. • Melt-fluid transition loads magmatically enriched Sn into the hydrothermal system. [ABSTRACT FROM AUTHOR]

[Display omitted] • Erdaokan is a magmatic-hydrothermal deposit genetically linked with bitumen. • Bitumen is of biogenic origin, and was sourced from the adjacent sedimentary strata. • Organic matter that formed bitumen significantly interacted with ore-forming fluids. The newly discovered Triassic Erdaokan Ag-Pb-Zn deposit having an ore reserve of 2964 kt, with an average grade of 517 g/t Ag, 0.6 wt% Pb and 0.4 wt% Zn, is the only known bitumen-bearing magmatic-hydrothermal Ag-Pb-Zn deposit in the northeastern segment of the Great Xing'an Range, NE China. Its genetic relationship with organic matter remains elusive. In this contribution, we present results from detailed geochemistry, isotope and biomarker studies on bitumen and discuss the genesis of bitumen and its relationship with Ag-Pb-Zn mineralization. The carbon isotope and biomarker analyses suggest a biogenic origin for the organic matter that formed bitumen. We infer that the organic matter was most likely sourced from the carbonaceous shale and limestone belonging to Niqiuhe Formation. Significant interaction between the organic matter and ore-forming fluid is indicated. The light n-alkanes and analogs, alkali elements, vanadium and nickel in the bitumen were removed by the ore-forming fluid, and a variety of metal elements entered into the bitumen. The thermochemical oxidation of methane induced by high-valence metal oxides and thermochemical sulfate reductions were the two main interaction mechanisms between the organic matter and ore-forming fluid, which made an effective contribution to metal precipitation in the Erdaokan Ag-Pb-Zn deposit. On this basis, we propose a genetic model for the genesis of the Erdaokan Ag-Pb-Zn deposit and other hydrocarbon-bearing magmatic-hydrothermal deposits which envisages a significant role played by the organic matter sourced from the sedimentary rocks in ore concentration. [ABSTRACT FROM AUTHOR]

Abstract: The Lhasa Terrane in southern Tibet has long been accepted as the last geological block accreted to Eurasia before its collision with the northward drifting Indian continent in the Cenozoic, but its lithospheric architecture, drift and growth histories and the nature of its northern suture with Eurasia via the Qiangtang Terrane remain enigmatic. Using zircon in situ U–Pb and Lu–Hf isotopic and bulk-rock geochemical data of Mesozoic–Early Tertiary magmatic rocks sampled along four north–south traverses across the Lhasa Terrane, we show that the Lhasa Terrane has ancient basement rocks of Proterozoic and Archean ages (up to 2870Ma) in its centre with younger and juvenile crust (Phanerozoic) accreted towards its both northern and southern edges. This finding proves that the central Lhasa subterrane was once a microcontinent. This continent has survived from its long journey across the Paleo-Tethyan Ocean basins and has grown at the edges through magmatism resulting from oceanic lithosphere subduction towards beneath it during its journey and subsequent collisions with the Qiangtang Terrane to the north and with the Indian continent to the south. Zircon Hf isotope data indicate significant mantle source contributions to the generation of these granitoid rocks (e.g., ~50–90%, 0–70%, and 30–100% to the Mesozoic magmatism in the southern, central, and northern Lhasa subterranes, respectively). We suggest that much of the Mesozoic magmatism in the Lhasa Terrane may be associated with the southward Bangong–Nujiang Tethyan seafloor subduction beneath the Lhasa Terrane, which likely began in the Middle Permian (or earlier) and ceased in the late Early Cretaceous, and that the significant changes of zircon εHf(t) at ~113 and ~52Ma record tectonomagmatic activities as a result of slab break-off and related mantle melting events following the Qiangtang–Lhasa amalgamation and India–Lhasa amalgamation, respectively. These results manifest the efficacy of zircons as a chronometer (U–Pb dating) and a geochemical tracer (Hf isotopes) in understanding the origin and histories of lithospheric plates and in revealing the tectonic evolution of old orogenies in the context of plate tectonics. [Copyright &y& Elsevier]

[Display omitted] • Bitumen Sm-Nd age of 234.6 Ma is interpreted as the hydrocarbon deposition timing. • The ore-forming age of Erdaokan is constrained at middle Triassic. • Bitumen Sm-Nd can be used to constrain the hydrothermal mineralization age. Dating of bitumen can potentially constrain both the timing of mineralization and hydrocarbon deposition in hydrocarbon-bearing hydrothermal deposits. Here we report for the first time direct Sm-Nd dating on bitumen from the Erdaokan Ag-Pb-Zn deposit, a newly discovered, large, hydrocarbon-bearing hydrothermal deposit with 2964 kt of ore and 1535 t Ag with an average grade of 517 g/t Ag, 0.6 wt% Pb and 0.41 wt% Zn in the Back-Arc basin of NE Great Xing'an Range, NE China. Dating results show that Sm-Nd isochron age of four bitumen samples is 234.6 ± 1.2 Ma and an isochron age that includes three galena and four bitumen samples is 234.9 ± 1.4 Ma. In order to verify these ages, we performed Rb-Sr dating on pyrite and galena from the same deposit and U-Pb dating of zircon from the adjacent diorite porphyrite dike, which yielded ages of 232.9 ± 2.3 Ma and 234.2 ± 2.8 Ma, respectively. The highly concordant ages from multiple techniques confirms that bitumen Sm-Nd isotopic dating is reliable, and that the ages obtained in our study represent the hydrocarbon deposition as well as mineralization timing in the deposit. We conclude that, in the Duobaoshan metallogenic belt, the Erdaokan Ag-Pb-Zn deposit is the first identified large, Middle Triassic, hydrocarbon-bearing silver-lead–zinc deposit and is of great significance to the regional metallogenesis research and prospecting work in the Great Xing'an Range. [ABSTRACT FROM AUTHOR]