Geochronology and Geochemistry of Ore-related Granitoids in the Giant Gariatong Rb Deposit, Tibet and Implications for Rb Metallogeny in China

Rubidium (Rb) deposits mostly occur in the South China and Central Asia orogenic belts and are often closely associated with highly differentiated granites. This study investigates a newly-discovered giant Rb deposit at Gariatong in the Central Lhasa terrane in Tibet. Detailed field studies and logging data revealed that the Rb mineralization mainly occurs in monzogranite and is related to greisenization. LA-ICP-MS U-Pb dating of zircon yielded ages of 19.1 ± 0.2 Ma and 19.0 ± 0.2 Ma for greisenized monzogranite and fresh monzogranite, respectively. The monzogranites are characterized as strongly peraluminous, with high contents of SiO2, Al2O3, K2O and Na2O as well as a high differentiation index. They are enriched in light rare earth and large ion lithophile elements with signi?cant negative Eu anomalies and depleted high field-strength elements. Petrological and geochemical features of these ore-related monzogranites suggest that they are highly fractionated S-type granites, derived from remelting of crustal materials in a post-collisional setting. The geochemistry of zircon and apatite points to a low oxygen fugacity of the ore-related monzogranite during the magma’s evolution. The discovery of the Gariatong Rb deposit suggests that the Central Lhasa terrane may be an important region for rare metal mineralization.     

Contrasting Regional Responses of Indian Summer Monsoon Rainfall to Exhausted Spring and Concurrently Emerging Summer El Ni?o Events

The inverse relationship between the warm phase of the El Ni?o Southern Oscillation(ENSO) and the Indian Summer Monsoon Rainfall(ISMR) is well established. Yet, some El Ni?o events that occur in the early months of the year(boreal spring) transform into a neutral phase before the start of summer, whereas others begin in the boreal summer and persist in a positive phase throughout the summer monsoon season. This study investigates the distinct influences of an exhausted spring El Ni?o(springtime)...     

Detrital Zircon Records of the Banxi Group in the Western Jiangnan Orogen: Implications for Crustal Evolution of the South China Craton

The Neoproterozoic evolution of the Jiangnan Orogen is important for understanding the tectonic history of South China. As a volcanic-sedimentary sequence developed in the Nanhua rift, the Banxi Group preserves the records of important magmatic and tectonic events linked to the assembly and breakup of the Rodinia supercontinent. In this study, we report the results from whole-rock major-and trace-element concentrations, with zircon LA-(MC)-ICP-MS U-Pb ages, trace elements and Lu-Hf isotopic comp...     

Petrological and Geochemical Constraints on the Protoliths of Serpentine‐Magnetite Ores in the Zhaoanzhuang Iron Deposit,Southern North China Craton

The uncommon Mg-rich and Ti-poor Zhaoanzhuang serpentine-magnetite ores within Taihua Group of the North China Craton(NCC) remain unclear whether the protolith was sourced from ultramafic rocks or chemical sedimentary sequences. Here we present integrated petrographic and geochemical studies to characterize the protoliths and to gain insights on the ore-forming processes. Iron ores mainly contain low-Ti magnetite(TiO_2 ~0.1 wt%) and serpentine(Mg#=92.42–96.55), as well as residual olivine(Fo=89–90), orthopyroxene(En=89–90) and hornblende. Magnetite in the iron ores shows lower Al, Sc, Ti, Cr, Zn relative to that from ultramafic Fe-Ti-V iron ores, but similar to that from metamorphic chemical sedimentary iron deposit. In addition, interstitial minerals of dolomite, calcite, apatite and anhydrite are intergrown with magnetite and serpentine, revealing they were metamorphic, but not magmatic or late hydrothermal minerals. Wall rocks principally contain magnesian silicates of olivine(Fo=83–87), orthopyroxene(En=82–86), humite(Mg#=82–84) and hornblende [XMg=0.87–0.96]. Dolomite, apatite and anhydrite together with minor magnetite, thorianite(Th-rich oxide) and monazite(LREE-rich phosphate) are often seen as relicts or inclusions within magnesian silicates in the wall rocks, revealing that they were primary or earlier metamorphic minerals than magnesian silicates. And olivine exists as subhedral interstitial texture between hornblende, which shows later formation of olivine than hornblende and does not conform with sequence of magmatic crystallization. All these mineralogical features thus bias towards their metamorphic, rather than magmatic origin. The dominant chemical components of the iron ores are SiO_2(4.77–25.23 wt%), Fe_2O_3 T(32.9–80.39 wt%) and MgO(5.72–27.17 wt%) and uniformly, those of the wall rocks are also SiO_2(16.34–48.72 wt%), Mg O(16.71–33.97 wt%) and Fe_2O_3 T(6.98–30.92 wt%). The striking high Fe-Mg-Si contents reveal that protolith of the Zhaoanzhuang iron deposit was more likely to be chemical sedimentary rocks. The distinct high-Mg feature and presence of abundant anhydrite possibly indicate it primarily precipitated in a confined seawater basin under an evaporitic environment. Besides, higher contents of Al, Ti, P, Th, U, Pb, REE relative to other Precambrian iron-rich chemical precipitates(BIF) suggest some clastic terrestrial materials were probably input. As a result, we think the Zhaoanzhuang iron deposit had experienced the initial Fe-Mg-Si marine precipitation, followed by further Mg enrichment through marine evaporated process, subsequent high-grade metamorphism and late-stage hydrothermal fluid modification.     

Dehydration reaction and isotope front transport induced by CO2 infiltration at Nuliyam, South India

The field relations from a quarry at Nuliyam, South India, illustrate dehydration of an amphibolite facies gneiss to granulite facies charnockite by CO₂ influx, over a scale of 30 m. Both the calc-silicate source of the fluids and the full extent of their penetration into the gneiss are preserved in a continuous section. Fluid flow is by a hydraulic fracture mechanism, but is thought to be pervasive. The sharp reaction front predicted by the continuum mechanical theory for advective fluid transport is not observed. The front spreading is on too large a scale for either diffusive or dispersive control and is due to local kinetic disequilibrium between the fluid and rock, although the divariant nature of the reaction may also have a limited effect. The time-integrated fluid flux varies from the instantaneous porosity at the fluid front to 20 vol. % adjacent to the calc-silicate. Carbon isotope budgets suggest that decarbonation of the calc-silicate by a Rayleigh fractionation process provides a sufficient source for the CO₂ influxing into the gneiss. Graphite abundances vary from 0.01 to 0.1% (by weight), it is principally derived by precipitation from the fluid and may be modelled from phase equilibria. Carbon isotope fronts coincide with the reaction front on the scale of sampling, although isotopic disequilibrium between graphite and inclusion-CO₂ also implies local fluid-rock disequilibrium.     

Early Palaeozoic high‐pressure granulites from the Dunhuang block,northeastern Tarim Craton: constraints on continental collision in the southern Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary collages in the world, and records a prolonged sequence of subduction‐accretion and collision processes. The Tarim Craton is located at the southernmost margin of the CAOB. In this study, the discovery of early Palaeozoic high‐pressure (HP) granulites from the Dunhuang block in the northeastern Tarim Craton is reported, and these rocks are characterized through detailed petrological and geochronological studies. The peak mineral assemblage of the HP mafic granulite is garnet + clinopyroxene + plagioclase + quartz + rutile, which is overprinted by amphibolite facies retrograde metamorphic assemblages. The calculated P–T conditions of the peak metamorphism are ～1.4–1.7 GPa and ～800 °C. The retrograde P–T conditions are ～0.7 GPa and ～700 °C. The metamorphic zircon grains from the HP mafic granulite show homogeneous CL‐images, low Th/U ratios and flat HREE patterns and yield a weighted mean ²⁰⁶Pb/²³⁸U age of 444 ± 5 Ma. The metamorphic zircon grains from the associated kyanite‐bearing garnet gneiss and garnet‐mica schist show a similar ²⁰⁶Pb/²³⁸U age of 429 ± 3 and 435 ± 4 Ma, respectively. The c. 440–430 Ma age is interpreted to mark the timing of HP granulite facies metamorphism in the Dunhuang block. The results from this study suggest that the Dunhuang block experienced continental subduction prior to the early Palaeozoic collisional orogeny between the northeastern Tarim Craton and the southern CAOB, and the Dunhuang area could be considered as the southward extension of the CAOB. It is suggested that the continental collision in the eastern part involving the Dunhuang block of the southern CAOB may have occurred c. 120 Ma earlier than in the western part involving the Tianshan orogen.     

Morphology, Chemistry and U-Pb Geochronology of Zircon Grains In Quartz Monzodiorite from the Sunzhuang Area, Fanshi County, Shanxi Province

The morphology, REE geochemistry and U-Pb geochronology of zircons from quartz monzodiorite in the Sunzhuang area, Fanshi County, Shanxi Province are presented in this study. The zircon crystals can be classified into four main types as: AB, L, S and P, and 24 subtypes such as AB4, AB5, L5, and S3. The maximum crystallization temperature of zircon was estimated as 850°C, with the minimum of 550°C. The peak temperatures of the zircon crystallization range from 650°C to 700°C. The abundances of Th and U in the zircon grains show large variation with the Th/U values 0.4. The Th and U values also show a positive correlation in most zircons. The REE abundance of zircon in the quartz monzodiorite ranges from 280.4 ppm to 2143 ppm with an average of 856.4 ppm. The chondrite normalized zircon REE patterns show two types, one is characterized by HREE enrichment and LREE depletion with positive Ce-anomaly and negative Eu-anomaly whereas the other is HREE enriched and LREE depleted with negative Eu-anomaly but without positive Ce-anomaly, and relatively flat patterns. The LA-ICP-MS U-Pb geochronology on the zircons yields a mean age of 133±0.87 Ma. Our data on zircon morphology, composition and U-Pb geochronology reveal that the parent magma of the quartz monzodiorite which was emplaced during late Yanshanian had a mixed crust-mantle source, with crustal components dominating. The magma is inferred to have been water rich and alkaline with initial high oxygen fugacity. Post-magmatic hydrothermal activity occurred under relatively reducing conditions which was conductive for gold precipitation in the Yixingzhai gold deposit.     

Two stages of granulite facies metamorphism in the eastern Himalayan syntaxis,south Tibet: petrology,zircon geochronology and implications for the subduction of Neo‐Tethys and the Indian continent beneath Asia

The eastern Himalayan syntaxis in southeastern Tibet consists of the Lhasa terrane, High Himalayan rocks and Indus‐Tsangpo suture zone. The Lhasa terrane constitutes the hangingwall of a subduction zone, whereas the High Himalayan rocks represent the subducted Indian continent. Our petrological and geochronological data reveal that the Lhasa terrane has undergone two stages of medium‐P metamorphism: an early granulite facies event at c. 90 Ma and a late amphibolite facies event at 36–33 Ma. However, the High Himalayan rocks experienced only a single high‐P granulite facies metamorphic event at 37–32 Ma. It is inferred that the Late Cretaceous (c. 90 Ma) medium‐P metamorphism of the southern Lhasa terrane resulted from a northward subduction of the Neo‐Tethyan ocean, and that the Oligocene (37–32 Ma) high‐P (1.8–1.4 GPa) rocks of the High Himalayan and coeval medium‐P (0.8–1.1 GPa) rocks of the Lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the Indian continent beneath Asia. Our results provide robust constraints on the Mesozoic and Cenozoic tectonic evolution of south Tibet.