Origin of Mesozoic ophiolitic mélanges in the western Yarlung Zangbo suture zone,SW Tibet

The petrogenesis and tectonic evolution of the Mesozoic ophiolitic mélanges in the western section of the Yarlung Zangbo suture zone (YZSZ) remain controversial. In this paper, we present the results of whole-rock geochemical and SrNd isotope analyses, zircon UPb ages and in situ LuHf isotopic data obtained from mafic rocks of the northern and southern sub-belts of the western YZSZ Mesozoic ophiolitic mélanges to help us understand these controversial issues. Diabases and dolerites from the northern sub-belt and gabbros from the southern sub-belt exhibit variable fore-arc basalt (FAB)-like geochemical compositions and have zircon UPb ages of ∼126.4–120.3 Ma. In addition, gabbro-diabases from the northern sub-belt have boninite series affinities and yield a zircon UPb age of ∼125.7 Ma. These results, along with previous studies on the YZSZ Mesozoic ophiolitic mélanges and the Gangdese arc, reveal that the western YZSZ Mesozoic ophiolites were likely generated over multiple stages in the epicontinental Gangdese fore-arc basin as the Yarlung Zangbo Neo-Tethyan Ocean subducted northward in front of the Lhasa terrane. The Early Cretaceous FAB-like and boninite series mafic rocks were formed by the reinitiation of subduction, which was followed by a retreat of the subduction zone and the creation of the fore-arc basin and strong hyperextension, accompanied by asthenosphere upwelling at ∼130–120 Ma. During this process, the upwelling asthenosphere underwent decompressional melting with limited penetration of slab-derived fluids and gave rise to the N-MORB (normal mid-ocean ridge basalt)-like basaltic magmas that intruded the overlying, previously generated depleted mantle as FAB-like gabbro, diabase and dolerite sills or dykes. Then, boninitic magmas represented by boninitic gabbro-diabases were generated by remelting the extremely depleted residual mantle source, which was metasomatized by a small amount of slab-derived fluids, following previous extractions of FAB-like magma.     

LA-ICP-MS UPb geochronology of wolframite by combining NIST series and common lead-bearing MTM as the primary reference material:Implications for metallogenesis of South China

Direct dating of W and WSn deposits by wolframite is more reliable relatively to gangue mineral and important for understanding their timing and genesis. However, such analysis still lacks of homogeneous wolframite standard recently. Due to containing considerable and variable common lead, and inhomogeneous in different grains, the wolframite sample of MTM, which is a promising candidate reference material proposed by previous studies, is not suitable as a primary standard for wolframite UPb dating by LA-ICP-MS using the normal normalization method as zircons. In this contribution, a modified normalization method is established for wolframite UPb dating, in which NIST612 or 614 and MTM are used for correction of PbPb and UPb ratios, respectively. Wolframite UPb dating are performed on the Langcun, Xihuashan, Piaotang, Shamai W or WSn deposits and the Baiganhu ore district, the obtained lower intercept ²⁰⁶Pb/²³⁸U ages are comparable with the ages from syngenetic molybdenite, cassiterite, muscovite and the genetically related granites, as well as wolframite by water vapor-assisted ns-LA-ICP-MS UPb dating method. The results of this analysis demonstrate that the robust age for W mineralization can be determined by LA-ICP-MS UPb dating of wolframite using this modified calibration method. Mineralization ages of 125–130 Ma by directly dating of metal minerals for the Langcun W, Jianfengpo Sn and large-size Xianglushan W deposits confirm that there exists an important WSn mineralization event in this period. The close temporal and spatial correlation indicates the granites and W-Cu-Mo-Pb-Zn-Sn mineralization have a genetic relationship with each other and are resulted from the same tectonic-magmatic-hydrothermal events during 140 to 120 Ma in South China.     

Evidence for the stability of the potential nuclear waste host,sphene, over geological time,from uranium-lead ages and uranium-series measurements

A sphene-based, glass-ceramic material is being investigated as a potential host for Canadian nuclear fuel recycle waste. The ability of sphene to retain certain radionuclides over geological time is examined using observed properties and age measurements of natural sphenes.The long-term stability of sphene is determined by comparing UThPb isotopic age data for sphene, from the literature, with ages determined for zircon and other minerals. Better internal agreement is found between UThPb sphene ages than for zircon ages indicating the resistance of sphene to daughter radionuclide loss over periods as long as 3000 Ma. Excellent agreement was observed between sphene ages and the estimated age of crystallization of the host rocks or vein infillings, except where subsequent metamorphim had reset the sphene age. Complete lead loss was generally found to accompany high-grade metamorphic conditions when temperatures exceeded ∼ 500°C.Stability over shorter periods of geological time was found for vein infilling and mineral separates of sphene using uranium-series disequilibrium analysis. Conditions of secular equilibrium in the decay chain ²³⁸U-²³⁴U-²³⁰Th-²²⁶Ra were observed for most of the 21 sphenes studied indicating that no radionuclide migration had occurred over periods as long as the last 1 Ma. Exceptions to this were four sphenes that had ²³⁴U deficiencies between 6 and 24% and two sphenes that showed ²²⁶Ra/²³⁰Rh disequilibrium. No correlation was found between the extent of radiation damage suffered by the sphenes and the presence of uranium-series disequilibrium. The presence of alteration products in three sphenes did correlate, however, with measured disequilibrium, indicating that some ²³⁴U and ²²⁶Ra had been lost or gained by these samples in the last 1 Ma and 8 ka, respectively.     

SmNd,RbSr and ThUPb zircon ages of syn- and post-tectonic granitoids from the Axum area of northern Ethiopia

SmNd, RbSr and ThUPb zircon ages for four syn-tectonic and two post-tectonic granitoids from the Axum area of northern Ethiopia are determined. Two of the syn-tectonic granitoids (the Azeho and Deset) are intrusions into structurally southeast facing, predominantly tholeiitic arc metavolcanics and associated metasediments situated west of the central steep zone in the area. The other two syn-tectonic granitoids (the Chila and Rama) are intrusions into structurally northwest facing metasediments and calc-alkaline metavolcanics at the eastern part of the steep zone. One of the post-tectonic granites (the Sibta Granite) occurs west of the central steep zone and the second (the Shire Granite) cuts the central steep zone. Preliminary geochemical data of all the granitoids show that they are enriched in large ion lithophile elements, depleted in high field strength elements and have 1-type characteristics, similar to calc-alkaline granitoids in subduction-related volcanic arc setting. The geochronological data indicate three discrete intrusive events: ∼800 Ma to the east of the central steep zone, ∼750 Ma to the west and a post-tectonic intrusion at around 550 Ma. The ∼-750 and ∼800 Ma ages of the intrusives are interpreted as minimum ages of arc magmatism in the respective blocks across the central steep zone, and the ∼550 Ma age of the post-tectonic granites records the final magmatic event in the region. The contrast of age across the central steep zone emphasises that this zone is a major structural element that might have played a significant role during the accretion of structurally and lithologically contrasting tectonostratigraphical blocks.     

Genesis of the stratiform Zhenzigou PbZn deposit in the North China Craton: RbSr and COSPb isotope constraints

Isotopes (RbSr, C, O, S, and Pb) were investigated from the Zhenzigou PbZn deposit in the Qingchengzi mineral field (QMF) of the North China Craton as an aid to determine the genesis of stratiform PbZn deposits in the Liao-Ji Rift. A step-dissolution RbSr age of 1798 ± 8 Ma with 206Pb/204Pb ratios of 17.7477–17.8527 were obtained from sphalerite. Sulfur isotopic ratios for pyrite (5–14.4‰), sphalerite (2.4–8.6‰), and galena (− 0.3–8.6‰) from Zhenzigou have a narrower range than those from the host Paleoproterozoic Dashiqiao Formation, and granite in the area. Calcite and limestone from ore and wallrocks at the deposit have similar C and O isotope compositions, with δ¹³C_PDB ranging from − 6.0 to − 2.3‰ and δ¹⁸O_SMOW from 9.8 to 13.7‰, which are similar to those of carbonatite and the mantle.Comprehensive analysis of the Pb isotopic composition of the sulfide from the Zhenzigou deposit and PbZn deposits in adjacent area show that the Pb originated from the upper crust and mixed with Pb from the mantle. Sulfur isotopes from Zhenzigou deposit indicate that the mineralization has a volcanic eruption source. The δ¹³C_PDB and δ¹⁸O_SMOW values indicate that the CO₂ originated from a mixed mantle, marine carbonate and organic source.Combined with the study of regional metallogenic background, this paper proposes that deposition of stratiform PbZn mineralization in the QMF began ca. 2052 Ma during development of the Liaoji Rift. The mineralization extended to ca. 1798 Ma prior to deformation associated with the Lvliang Movement, which dismembered the stratiform PbZn mineralization. The veined mineralization in the region cross-cuts the stratiform deposits and represents remobilized and redeposited deposits associated with the emplacement of Triassic plutons such as the Xinling and Shuangdinggou granites.     

Jurassic–Cretaceous granitoids and related tectono-metallogenesis in the Zapug–Duobuza arc,western Tibet

The Zapug–Duobuza magmatic arc (ZDMA), located along the southern edge of the south Qiangtang terrane in western Tibet, extends east–west for ~ 400 km. Small scattered granite and porphyry intrusions crop out in the ZDMA, but a large amount of granite may be buried by Late Cretaceous to Paleogene thrusting. Two stages of magmatism have been identified, at 170–150 Ma and 130–110 Ma. The widely distributed Middle–Late Jurassic granite intrusions in the ZDMA exhibit SrNd isotopic characteristics similar to those of ore-bearing porphyries in the Duolong giant CuAu deposit, and their εHf(t) values mostly overlap those of other porphyry CuMo deposits in the ZDMA and the Gangdese zone. The SrNdHf isotopic geochemistry suggests variable contributions of mantle and Qiangtang crustal sources, and indicates the presence of two new ore districts with potentials for CuAu, Fe, and PbZn ores, located in the Jiacuo–Liqunshan and Larelaxin–Caima areas. Except for the Duolong ore-forming porphyries, which show significant contributions of mantle components intruded into an accretionary mélange setting, the Early Cretaceous granites in other areas of the belt are of mostly crustal origin, from sources in Qiangtang felsic basement and Permo-Carboniferous strata, indicating the weak ore-forming potential of skarn-type Fe and PbZn deposits. The ephemeral but deep Bangong Co–Nujiang ocean in the Early Jurassic evolved into a shallow compressional marine basin in the Middle–Late Jurassic, possibly transitioning to northward flat subduction of oceanic crust at this time. The subducted slab broke off in the Early Cretaceous, initiating a peak in arc magmatism and metallogenesis at 125–110 Ma.     

External sulphur in IOCG mineralization: Implications on definition and classification of the IOCG clan

Although the sources of the ore metals remain problematic in most Iron-oxide Cu and Au (IOCG) deposits, external sulphur, either from surficial basinal brines and seawater (e.g., Central Andean and Carajás deposits) or from formation water and metamorphic fluids (e.g., the Cloncurry deposits), or introduced by magmatic assimilation of metasedimentary units (e.g., Phalaborwa), has been documented in many major Cu-rich IOCG centres. However, only the evaporite-sourced fluids yield diagnostically high δ³⁴S values (i.e., > 10‰), while sedimentary formation water or metamorphic fluids commonly have lower values and are less clearly distinguishable from magmatic fluids, as in the Cloncurry deposits in which the involvement of external fluids is revealed by other evidence, such as noble gas isotopes. On the basis of these arguments, IOCG deposits could be redefined as a clan of Cu (AuAgU) deposits containing abundant hypogene iron oxide (magnetite and/or hematite), in which externally-derived sulphur probably plays an important role for the Cu (AuAgU) mineralization. In this definition, all “Kiruna-type” magnetite deposits, hydrothermal iron deposits (e.g., skarn Fe deposits) and magnetite-rich porphyry CuAu and skarn CuAu deposits are excluded. Two subtypes of IOCG deposits are recognized on the basis of the predominant iron oxide directly associated with the Cu (Au) mineralization, whether magnetite or hematite. Neither magnetite- nor hematite-rich IOCG deposits show any preference for specific host rocks, and both range in age from Neoarchean to Pleistocene, within a broad tectonic environment.     

Evolution of unconformity-related MnFeAs vein mineralization,Sailauf (Germany): Insight from major and trace elements in oxide and carbonate minerals

The Sailauf MnFeAs vein mineralization, located in the Spessart district (central Germany), is characterized by complex hydrothermal carbonate and oxide assemblages. The mineralization is hosted by a Permian rhyolite body and is structurally related to the Variscan unconformity that separates Permian sedimentary rocks from the underlying Variscan crystalline basement. The hydrothermal vein system has been studied by optical microscopy, electron-microprobe and LA-ICPMS analysis of major and trace elements (including the REE). Four distinct mineralization stages that are characterized by diagnostic carbonate-oxide assemblages are identified, which are (1) pre-ore stage, (2) ore stage 1, (3) ore stage 2, and (4) the replacement stage. Hydrothermal carbonates show complex compositional trends in CaMn (Fe + Mg) space, and comprise calcite, Mn-calcite, FeMg calcite, Ca-rhodochrosite, and Ca-kutnahorite. Oxide assemblages are dominated by braunite and hematite, with minor amounts of manganite and hausmannite. The mineralization is enriched in a distinct suite of trace elements, including As, W, Pb, Zn and Cu. Analysis of the paragenetic evolution, in conjunction with the major and trace element data, allows to reconstruct the fluid evolution of the hydrothermal system. The first order change in mineralogy between the two main ore stages (Mn oxides and calcite evolving into hematite and Mn-rich calcite) records a pronounced shift in fluid pH and silica activity of the system. This interpretation is also supported by variations in the behavior of Ce in different carbonate generations. The late stage replacement carbonates relate to destabilization of the primary ore assemblages. The distinct geological setting immediately below the Permian unconformity, in conjunction with the mineralogical and chemical data, suggests that dynamic fluid mixing processes involving basement-derived brines and more shallow groundwaters were important in the formation of the Sailauf MnFeAs mineralization. The significant enrichment in the AsWPbZnCu element suite resembles that of other MnFe deposits, and points to felsic lithologies as the main metal source of the mineralization.     

Solubility of rare earth elements in carbonatite magmas,indicated by the liquidus surface in CaCO3Ca(OH)2La(OH)3 at 1 kbar pressure

The system CaCO₃Ca(OH)₂(CCCH) represents a synthetic analog to a carbonatite magma. Addition of a light rare earth (RE) component La(OH)₃ (LH) gives a simple analog of a rare earth carbonatite. Liquidus relations for two joins were studied at 1 kbar pressure; CHLH and (CC₅₅CH₄₅)LH. CHLH is binary with a eutectic at CH₇₉LH₂₁ and 710°C. Thejoin CC₅₅ + CH₄₅)LH has a liquidus piercing point between CC and LH, at (CC₅₅CH₄₅)₆₀LH₄₀ and 700°C. Combining the new results with known results for CCCH allows construction of a liquidus diagram for part of the join CCCHLH. A ternary eutectic between the primary liquidus fields for CH, CC and LH occurs near 610°C with estimated composition CC= 33%,CH= 47%,LH= 20%. The solubility of La(OH)₃ in the synthetic carbonatite magma increases with increase of CO₂/H₂O from 20% at the eutectic to 40% at the piercing point on the join (CC₅₅CH₄₅)LH. The solubility of La in synthetic carbonatite is high compared with that of silicates. P₂O₅, and S. The results show that REE can become concentrated to high levels by fractionation of carbonatites, as long as they are not removed by high temperature crystallization of apatite and monazite.     

The Earth as a multiscale quantum-mechanical system

Major features of the Earth's structure and dynamics originate in the contrast between the rigidity of SiO bonds and the softness of SiOSi linkages. Because this contrast results from orbital hybridization, a real understanding of bonding relies on ab initio quantum-mechanical principles. As investigated with first-principles interatomic potentials, the α–β transitions of SiO₂ polymorphs illustrate how soft SiOSi linkages give rise to dynamical structures at rather low temperatures and yield the low melting temperatures of SiO₂-rich minerals that are at the roots of SiO₂ enrichment in magmatic differentiation. The increasing concentration of alkalis throughout this process is another aspect that must also be studied in terms of molecular orbitals in relation with the presence of aluminum in tetrahedral coordination. Finally, calculations of noble gas solubility show that some important features can be treated with “hybrid” calculations when, in addition to quantum-mechanical effects, the energy needed to create a cavity in the silicate melt is dealt with in a classical manner.     

Genesis and geodynamic process of early Cretaceous intermediate–felsic batholith within the Chem Co zone,western Qiangtang and implications for Bangong–Nujiang Tethyan Ocean subduction

The complex evolutionary history of collision-related suture zones in the western Qiangtang is poorly understood, due to the lack of pivotal magmatic and tectonic records. The Chem Co intermediate-felsic batholith is situated in the westernmost section of Qiangtang, where it intruded into the Upper Paleozoic metamorphic sedimentary series and was cut by Late-Cenozoic active normal faults. Here, we present evidence from field observations, geochemistry, zircon UPb and Hf isotopes, and mica ⁴⁰Ar³⁹Ar analysis of these granitoid rocks. Zircon UPb ages ranging from 107.3 ± 1.4 to 123.7 ± 1.7 Ma reveal that this magmatism initiated in the Early Cretaceous. Mica ⁴⁰Ar³⁹Ar yield plateau ages from 99 to 118 Ma suggest rapid emplacement into the cold crust without disturbance by subsequent >320 °C tectono-thermal events. Major element geochemistry shows these granitic rocks are enriched in Al, depleted in Ca, lie mostly within the high–K calc–alkaline series and have peraluminous features. Mineral modes that lack amphibole but contain muscovite, together with the geochemical characteristics, indicate an S–type granite affinity. Furthermore, the rocks are strongly enriched in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and are moderately to strongly depleted in high field strength elements (HFSE) and heavy rare earth elements (HREE), thus indicating typical characteristics of arc-related magmatic rocks. Considering the petrogenesis, geochemical features, zircon UPb ages, negative zircon εHf(t) values, similarity with the southwestern Qiangtang intrusions, and the geological setting, we conclude that the Chem Co granite was derived from the anatexis of crustal lithosphere through a “soft” collision associated with the continuing northwards subduction of the Bangong–Nujiang Tethyan (BNT) Ocean. We propose that the final closure and termination of the western segment of the BNT Ocean was completed later than 107 Ma.     

Zinc and lead isotope signatures of the Zhaxikang PbZn deposit,South Tibet: Implications for the source of the ore-forming metals

Stable Zn isotopes may be applied to trace the source of ore-forming metals in various types of PbZn deposits. To test this application, Zn and Pb isotope systematics for sulfides and associated basement rocks as well as FeMn carbonates (gangue) from the Zhaxikang PbZn deposit in South Tibet have been analyzed. The basement in this region includes metamorphosed mafic to felsic rocks (dolerite, quartz diorite, rhyolite porphyry, pyroclastics and porphyritic monzogranite). These rocks have similar δ⁶⁶Zn values of 0.33 to 0.37‰, with an average value of 0.36 ± 0.03‰ (2σ), except for the more evolved porphyritic monzogranite that has a heavier value of 0.49‰. FeMn carbonates are present as hydrothermal veins and were probably precipitated from magmatic fluids. They have an average δ⁶⁶Zn value of 0.27 ± 0.05‰, which is slightly lighter than the basement rocks, possibly representing δ⁶⁶Zn isotopic compositions of the hydrothermal fluids. Sphalerite and galena have similar Zn isotopic compositions with δ⁶⁶Zn ranging from 0.03 to 0.26‰ and 0.21 to 0.28‰, respectively. Considering the Zn isotope fractionation factor between sphalerite and fluids of − 0.2‰ at ~ 300 °C as reported in literature, hydrothermal fluids from which these sulfides precipitated will have δ⁶⁶Zn values of ca. 0.39 ± 0.10‰, which are consistent with the values of basement rocks and the FeMn carbonates. This similarity supports a magmatic-hydrothermal origin of the Zhaxikang PbZn deposit. Both Pb and S isotopes in these sphalerite and galena show large variations and are consistent with being derived from a mixture of basement and sedimentary rocks in various proportions. Zn isotopic compositions of the sulfides significantly extend the range of regional basement rocks, suggesting that sedimentary rocks (e.g., shales) are also a significant source of Zn. However, the Zn isotopic compositions of sphalerite and galena differ from those of marine carbonates and those of typical SEDEX-type deposits (e.g. Kelley et al., 2009), confirming a magmatic-hydrothermal model. Combined with regional geological observations and the age constraints of ~ 20 Ma (Zheng et al., 2012, 2014), the results of our investigation indicate that the Zhaxikang PbZn deposit is most likely a magmatic-hydrothermal deposit.     

Rock–water interactions and pollution processes in the volcanic aquifer system of Guadalajara,Mexico, using inverse geochemical modeling

In order to understand and mitigate the deterioration of water quality in the aquifer system underlying Guadalajara metropolitan area, an investigation was performed developing geochemical evolution models for assessment of groundwater chemical processes. The models helped not only to conceptualize the groundwater geochemistry, but also to evaluate the relative influence of anthropogenic inputs and natural sources of salinity to the groundwater. Mixing processes, ion exchange, water–rock–water interactions and nitrate pollution and denitrification were identified and confirmed using mass-balance models constraint by information on hydrogeology, groundwater chemistry, lithology and stability of geochemical phases. The water–rock interactions in the volcanic setting produced a dominant NaHCO₃ water type, followed by NaMgCaHCO₃ and NaCaHCO₃. For geochemical evolution modeling, flow sections were selected representing recharge and non-recharge processes and a variety of mixing conditions. Recharge processes are dominated by dissolution of soil CO₂ gas, calcite, gypsum, albite and biotite, and Ca/Na exchange. Non-recharge processes show that the production of carbonic acid and Ca/Na exchange are decreasing, while other minerals such as halite and amorphous SiO₂ are precipitated. The origin of nitrate pollution in groundwater are fertilizers in rural plots and wastewater and waste disposal in the urban area. This investigation may help water authorities to adequately address and manage groundwater contamination.     

Lead isotopic compositions in fault gouges and their parent rocks: implications for ancient fault activities

This is the first study on Pb stable isotopes in fault gouges and their parent rocks. We analyzed the composition of Pb isotopes and contents of U and Pb in 10 pairs of fault gouges and their parent rocks collected along several active faults in central Japan. Thorium-232-²⁰⁸Pb ages of two fault systems were determined as pre-Tertiary, which are consistent with the data from KAr ages and geological considerations.Naturally, the²³⁵U²⁰⁷Pb system is of little use for dating because the magnitude of difference in²⁰⁷Pb/²⁰⁴Pb between gouges and parent rocks is too small. It is found that the²⁰⁶Pb/²⁰⁴Pb can indicate the contribution of²⁰⁶Pb resulting from excess supplies of²²⁶Ra and²²²Rn along the fault. The excess²⁰⁶Pb accumulation rate corresponds to the average²²²Rn concentration in soil gas or groundwater through geological time since the gouge formation. A comparison of Quaternary fault activity and estimated Tertiary activity reveals the characteristics of each fault system.     

Protoliths and tectonic implications of the newly discovered Triassic Baqing eclogites,central Tibet: Evidence from geochemistry,SrNd isotopes and geochronology

Geochemical, SrNd isotopic and geochronological methods were used to reveal the protoliths and geodynamic implications of the newly discovered Triassic Baqing eclogites, eastern Qiangtang terrane, central Tibet. We assessed the mobility of trace elements, and it turned out that high field strength elements (HFSEs) and rare earth elements (REEs) were immobile in the high-pressure (HP) metamorphic process and reliably employed to discriminate the protoliths of the Baqing eclogites. Whole-rock geochemical data (especially immobile elements) suggested both arc-related (negative NbTa anomalies, high light (L) REEs/heavy (H) REEs ratios) and mid-oceanic ridge basalt (MORB)-related (without negative NbTa anomalies, relatively low REEs contents) characteristics, indicating the contribution of different proportions of subduction material with a backarc basin origin. According to the SrNd isotopic ratios (initial ⁸⁷Sr/⁸⁶Sr ratios: 0.7078–0.7086; ε_Nd(t) values: −0.58 to +1.96), the protoliths of the Baqing eclogites originated from mantle which experienced continental crustal contamination in the subduction zone. Compared with nearby subduction-related magmatic rocks from eastern Qiangtang terrane, backarc basin was the most likely tectonic environment for the Baqing eclogite protoliths. The Jinsha Paleo-Tethyan Ocean southward subduction underneath East Qiangtang block (EQB) and rollback led to the formation of this backarc basin between the Late Permian and Early Triassic, and subsequent northward subduction of the backarc basin formed the Baqing eclogites in early Late Triassic (227–221 Ma).     

Time constraints on the closure of the Paleo–South China Ocean and the Neoproterozoic assembly of the Yangtze and Cathaysia blocks: Insight from new detrital zircon analyses

The South China Block was built up by the assembly of the Yangtze and Cathaysia blocks along the Neoproterozoic Jiangnan Orogenic Belt. The timing of the Jiangnan Orogeny remains controversial. The widespread orogeny–related Neoproterozoic angular unconformity that separates the underlying folded Sibao (ca.1000–820 Ma) and overlying Danzhou (ca.800–720 Ma) Groups was investigated. Six sedimentary samples, below and above the unconformity in three distal localities (Fanjingshan, Madiyi, and Sibao) yield detrital zircon with UPb ages ranging from 779 ± 16 Ma to 3006 ± 36 Ma, with a prominent peak at ca. 852 Ma. The youngest ages of 832 ± 11 Ma and 779 ± 16 Ma are revealed for the underlying Sibao and overlying Danzhou Groups, respectively. The detrital zircon UPb age relative probability plot of the Jiangnan Orogen matches well with those of the Yangtze and Cathaysia blocks since ca. 865 Ma. Integrating geological, geochemical and geochronological results, we suggest that the Paleo–South China Ocean began to subduct under the Yangtze block at ca. 1000 Ma, and was partly closed at ca. 865 Ma. Afterwards, the Yangtze and Cathaysia blocks initially collide at 865 Ma, forming the Jiangnan Orogen. This collision resulted in not only the folding of the Sibao Group, but also sediment deposition in a syn-collisional setting, which makes the upper part of the Sibao Group. The youngest S-type granite dated at ca. 820 Ma that intruded in the Sibao Group marks the late stage of the Jiangnan Orogeny.     

Origins and tectonic implications of Late Cretaceous adakite and primitive high-Mg andesite in the Songdo area,southern Lhasa subterrane,Tibet

Late Cretaceous igneous rocks in the southern Lhasa subterrane, Tibet, include primitive high-Mg andesites and adakites, which provide important constraints on the tectonic evolution of the Neo-Tethys Ocean. Here, we present detailed zircon UPb and Hf isotopic and whole-rock geochemical data for granodioritic and dioritic porphyry samples from the Songdo area in the southern Lhasa subterrane. Zircon UPb dating indicates that the granodiorite crystallized at 88 Ma, whereas the diorite yields ages of 68 and 66 Ma. The granodiorite has adakite-like geochemical characteristics, including high Sr (801–1005 ppm) and low Y (6.8–15.2 ppm) and Yb (0.6–1.3 ppm) concentrations, and high Sr/Y (62–145) and La/Yb (39–93) ratios. We infer that the adakitic granodiorites formed through partial melting of subducted oceanic crust. The dioritic porphyry has intermediate moderate SiO2 (53–58 wt%) and high MgO (5.6–8.2 wt%) contents, and high Mg# (66.4–69.5) values, and is therefore classified as a primitive high-Mg andesite that was derived from interaction between subducted sediment and mantle. The presence of coeval adakite and charnockite, as well as high-temperature granulite-facies metamorphism, indicates that mid-ocean ridge subduction occurred at 100–80 Ma, followed by a 10 Myr hiatus in magmatism and subsequent rollback of the Neo-Tethys slab at 68 Ma. These processes resulted in significant crustal growth within the Lhasa terrane.     

Geochronologic (ReOs) and fluid-chemical constraints on the formation of the Mesoproterozoic-hosted Nanisivik ZnPb deposit,Nunavut, Canada: Evidence for early diagenetic,low-temperature conditions of formation

The age and origin of the past-producing Nanisivik carbonate-hosted ZnPb deposit in Nunavut, Canada, have been controversial for decades. Various direct and indirect dating methods have produced results ranging from Mesoproterozoic to Ordovician in age, and previous studies of the mineralising fluids have suggested that the fluids were anomalously hot (> 150 °C). This study combines ReOs (pyrite) geochronology, in-situ sulphur isotope analysis, and fluid inclusion analysis to refine both the timing of mineralisation and the nature of mineralising fluids. ReOs pyrite analysis shows that the Nanisivik deposit formed ca. 1.1 Ga, broadly similar to the depositional age of the host rock and with the Grenville orogeny, making it one of few known Precambrian carbonate-hosted ZnPb deposits. In-situ sulphur isotope measurements from Nanisivik show a narrow δ³⁴S range of 27.54 ± 0.72, very similar to what has been reported before in bulk sample analyses. New fluid inclusion data show that the mineralising fluids were ~ 100 °C, which is not anomalous in the context of carbonate-hosted base-metal deposits. The fluids exhibit no significant spatial variation in homogenisation temperature in the 2-km-long ‘upper lens’ of the ore deposit, but recrystallisation and modification of fluid inclusions took place in the immediate vicinity of the cross-cutting ~ 720 Ma “mine dyke”. The deposit is broadly inferred to have formed during late Mesoproterozoic assembly of supercontinent Rodinia, when regional hydrostatic head developed under the influence of far-field stresses originating in the developing Grenville orogen. The Nanisivik deposit remains anomalous only in its age; most other aspects of this ore deposit are now shown to be quite typical for carbonate-hosted ore deposits.     

Granulites,CO2 and graphite

Externally derived, pure CO₂ that mixes with a carbon-(under)saturated C-O-H fluid in lower crustal granulites may result in graphite precipitation if the host-rock oxygen fugacity (f_O2^rock) is below the upper f_O2 limit of graphite. The maximum relative amount of graphite that can precipitate varies between a few mol% up to more than 25 mol%, depending on pressure, temperature, and host-rock redox state. The maximum relative amount of graphite that can precipitate from an infiltrating CO₂ fluid into a dry granulite (CO fluid system) varies between zero and a few mol%. Thermodynamic evaluation of the graphite precipitation process shows that CO₂ infiltration into lower crustal rocks does not always result in a carbon (super)saturated fluid. In that case, graphite precipitation is only possible if carbon saturation can be reached as a result of the reaction CO₂ → CO + ½ O₂. Graphite that has been precipitated during granulite facies metamorphic conditions can subsequently be absorbed by a COH fluid during retrograde metamorphism. It is also possible, however, that significant amounts of graphite precipitate from a COH fluid during retrograde metamorphism. This study shows that interpreting the presence or absence of graphite in granulites with respect to CO₂ infiltration requires detailed information on the P–T–f_O2^rock conditions, the relative amount of CO₂ that infiltrates into the rock, and whether H₂O is present or not.     

Geochemistry and isotope chemistry of Michigan Basin brines: Devonian formations

Detailed chemical and isotope analysis of 87 formation waters collected from six Devonian-aged units in the Michigan Basin are presented and discussed in terms of the origin of the dissolved components and the water. Total dissolved solids in these waters range from 200,000 to >400,000mg/1. Upper Devonian formations produce dominantly NaCaCl brine, while deeper formations produce CaNaCl water. Ratios of Cl/Br and Na/Br along with divalent cation content (MCl₂), indicate that these brines are derived from evapo-concentrated seawater. Other ion concentrations appear to be extensively modified from seawater values by water-rock reactions. The most important reactions are dolomitization, which explains the Ca content of the brines, and reactions involving aluminosilicate minerals. Stable isotope (δ¹⁸O and δD) compositions indicate that water molecules in the deeper formations are derived from primary concentrated seawater. Isotope enrichment by exchange with carbonates and perhaps gypsum cannot be discounted. Isotope values indicate water in the Upper Devonian formations is a mixture of seawater brine diluted with meteoric-derived water. Dilution has predominantly occurred in basin margins. Two scenarios are presented for the origin of the brines in the Devonian formations: (1) they originated when the Devonian sediments and evaporites were first deposited; or (2) they are residual brine liberated from the deeper Devonian and possibly Silurian salt deposits.