Chemie Der Erde-Geochemistry最新文献

Recent studies have suggested the existence of a correlation between the allochthonous units of NW Iberia (Galicia-Trás-os-Montes Zone - GTMZ) with SW Iberia (Ossa-Morena Zone – OMZ) in the European Variscan Belt. Such proposed equivalence is based on lithostratigraphic, tectono-metamorphic, geochronological, and geochemical affinities between these two domains, and implies that both zones had the same Neoproterozoic and early Palaeozoic basins, recording the same magmatic episodes and enduring the same tectono-metamorphic processes related to the Variscan Orogeny. As so, in order to better understand the evolution of these domains during the Ediacaran-Cambrian period, a comprehensive geochemical, isotopic and geochronological analysis of the metasediments that make up the OMZ and the allochthonous units of the GTMZ was carried out. The results show that the metasediments of OMZ and the Lower Allochthonous units of GTMZ present similar geochemical and geochronological features, having been likely deposited near each other, although small, but specific differences between both domains suggest that they do not correspond to the same depocentre. However, no geochemical, isotopic or geochronological correlation can be established with any of these domains and the Upper Allochthonous units of GTMZ. These metasediments display quite exotic features and, as such, were likely deposited in a different narrower basin, located to the W of the basins where OMZ and Lower Allochthonous units of GTMZ were deposited, thus providing precise constraints on the paleogeographic reconstruction of these domains during the Neoproterozoic to Early Ordovician dismembering of Gondwana.

Recent studies have shown that multistage magmatic and metallogenic events in NE China were dominated by the Paleo-Asian Ocean tectonic regime and the Paleo-Pacific Ocean tectonic regime. However, outstanding questions remain on the petrogenesis and fertility of ore-causative magma in each metallogenic event. In this study, we report new geochronologic and geochemical data on ore-causative intrusions from the Tianbaoshan orefield in the east Jilin-Heilongjiang belt (EJHB), aiming to identify their petrogenesis, magma fertility, and their implications for the geodynamic evolution of the EJHB. Middle Permian Lishan ore-causative quartz monzodiorites with abundant mafic microgranular enclaves (MMEs) were emplaced at ca. 264.9 ± 2.6 Ma. Petrographic and geochemical characteristics (⁸⁷Sr/⁸⁶Sr_(i) = 0.7049–0.7053, ε_Nd(t) = 2.9–3.7, and ε_Hf(t) = 4.7–11.7) indicate a juvenile crust source injected by a slab-metasomatized mantle component. The Early Jurassic Beishan monzogranites (ca. 192.5 ± 1.8 Ma) were generated by the partial melting of the juvenile underplating basaltic lower crust with subsequent fractional crystallization in response to their highly evolved geochemical features, combined with their depleted Sr-Nd-Hf isotopic signature (I_Sr = 0.7032–0.7037, ε_Nd(t) = 2.8–3.3, and ε_Hf(t) = 7.2–12.2). Based on the zircon trace element geochemistry, we infer that the Middle Permian quartz monzodiorites had a high oxygen fugacity (Ce⁴⁺/Ce³⁺ = 40–216), potentially generating the Pb-Zn-Cu mineralization. Early Jurassic Beishan monzogranites had a lower magmatic oxygen fugacity (Ce⁴⁺/Ce³⁺ = 14–106), which may account for the Mo-dominated mineralization. A combination of this study and previous results corroborates that the Tianbaoshan orefield records the superposition of different tectonic regimes during metallogenesis.

Hydrothermally altered quartzofeldspathic gneisses, granites and pegmatites of the West Garo Hills, Shillong Plateau (eastern India) preserve alteration assemblages of monazite, xenotime and zircon comprising thorite and coffinite in the altered domains. Albitization of muscovite and K-feldspars, sericitization of K-feldspar and albite, alteration of biotite to hydrobiotite, and crystallization of tourmaline is suggestive of the involvement of acidic fluids. The microtextural relations and the mineral assemblages indicate prevalence of low-temperature (T < 300 °C) conditions during hydrothermal alteration. The light δ¹¹B values of tourmaline (–15.1 to –13.5 ‰) suggests derivation/interaction of the fluids from/with metapelitic rock and/or S-type granite. Low ‘inferred’ Fe³⁺/Fe²⁺ ratio in tourmaline (based on high Al content in Y-site, 0.16–0.56 apfu, avg. 0.36 apfu), estimated excess charge (0.22–0.47, avg. 0.35 apfu) and lack of any Fe–Al relation support a reduced nature of the hydrothermal fluid. Mass balance calculations reveal that Th and U required for the formation of thorite and coffinite, respectively were likely derived from monazite and xenotime, and zircon. Significantly lower Cl^─ contents in hydrobiotite (avg. 0.03 wt%) compared to the unaltered biotite (avg. 0.13 wt%) suggests release of Cl^─ to the hydrothermal fluid during alteration. Textural evidences of albitization together with Cl^─ release from biotite suggest increased Cl^─ concentrations in the fluid during hydrothermal alteration. Based on solubility calculations for various U and Th species, we propose that high Cl^─ content in the fluid aided mobilization of Th and U as ThCl₄⁰ and UCl₄⁰/UO₂Cl₂⁰ complexes from accessory radioactive phases under reducing conditions. These results suggest that significant mobility of U and Th can be achieved in acidic high salinity fluid even under reducing conditions. Thermodynamic calculations for the solubility of Th- and U-chloride complexes and stability of monazite/xenotime in a range of pH and temperature suggest that thorite and coffinite precipitation was the result of an increase in pH.

This study deals with petrology, textural, thermometry, and geochemical characterization of naturally magnetized Cr-V-Ti magnetite deposits within the layered mafic-ultramafic intrusions in Coorg massif of southern India using ore petrography and mineral/whole rock geochemistry. These deposits, also known as ‘lodestones’, occur as rhythmic layers within the lateritized host rock, with the underlying basement distinguished by the presence of charnockites and layered gabbro-anorthosites and pyroxenites, delineating a stratified intrusive setting. Lodestones exhibit complex mineral assemblage involving magnetite, ilmenite, ulvöspinel, spinel, corundum, hematite, goethite, pyrite, pyrrhotite, and amphiboles. The bulk rock chemistry of the lodestone is analogues to Fe-Ti magnetite iron ore deposits with elevated vanadium and chromium contents. Their resemblance to tholeiitic magma-type suggests their formation in a layered intrusive setting, evolved through multiple fractional crystallization under oxidizing conditions. Thermometric and fugacity calculations using different textural associations estimate the magmatic fractional crystallization stage at elevated temperatures (601–704 °C) and low fO₂ (−18.6 to −15.3), succeeded by the exsolution stage during subsolidus cooling at lower temperatures (379–540 °C) and high fO₂ (−38.2 to −22.1). The whole sequence of formation and evolution of lodestone encompasses primary magmatic crystallization, subsolidus re-equilibration, metamorphism, and secondary weathering. The study also suggests a genetic linkage of lodestone with the associated mafic-ultramafic units, depicting two possible magmatic processes either through slab melting and fractional crystallization associated with subduction or due to plume magmatism and associated rifting.

The Nagaland-Manipur ophiolite belt (NMOB) represents remnants of the Neo-Tethyan Ocean that evolved during the accretion of the Indian and Myanmar lithospheric plates. We studied clinopyroxenes of cumulate gabbro and basalt rocks from the northern section of the belt to decode the mechanism that controlled their petrogenesis and geotectonic setting. Clinopyroxenes (augite-diopside) exhibit compositional zoning and significant variations in their major and trace element contents. Zoned clinopyroxene from a single volcanic rock sample (2d-3-6-11) show higher concentrations of TiO₂ (1.44 to 5.13 wt%) and Al (0.128–0.359 apfu) and lower Si (1.658–1.889 apfu) compared to clinopyroxenes from the other mafic rocks. The mg# [100 × Mg/(Mg + Fe²⁺)] values of clinopyroxenes range between 60.1 and 85.7. The trace element contents and the chondrite-normalised REE patterns of clinopyroxenes imply diverse geochemical affinities of both depleted mantle typical of tholeiite rocks and a less depleted LREE-enriched melt for the Nagaland mafic rocks. Pressure and temperature of crystallisation were estimated using single clinopyroxene geothermobarometry and were found to vary between 1–8 kbar and 840–1179 °C, respectively. In discrimination diagrams, the zoned clinopyroxenes show an alkaline composition consistent with a within-plate environment. On the other hand, clinopyroxenes from the rest of the mafic samples show a sub-alkaline composition typical of magmas produced in arc-related settings. Our study indicates the existence of diverse magma sources in an island arc-to-back arc setting during the formation of magmatic rocks in the NMOB.

There existed an early Mesozoic tectonic regime transformation in the South China Block (SCB). Although it was believed that this transformation was closely related to the subduction of Paleo-Pacific Plate, the process and initial time of the subduction of the Paleo-Pacific Plate have been controversial for a long time. Based on the published Upper Triassic and Middle Jurassic succession detrital zircons geochronology from the southeastern margin of the SCB, the newly obtained Late Triassic to Middle Jurassic detrital zircons Hf isotope data, and the available magmatic age data and Hf isotope data, this study discussed the subduction of the Paleo-Pacific Plate. The Hf isotope composition of zircon grains selected from the Early Mesozoic strata of the SCB, spanning from the Late Triassic to the Middle Jurassic, exhibits a consistent temporal trend with that of the Triassic–Middle Jurassic magmatic rocks. Both display a transition from negative to positive ε_Hf(t) values, indicative of a gradually increasing contribution of mantle-derived materials from the Triassic to the Middle Jurassic. Zircon trace elements indicate that a magmatic arc appeared outside the southeastern margin of the SCB at 200–190 Ma and continued to develop into the Middle Jurassic, which may have been generated by the Paleo-Pacific Plate subduction. This study proposed that the Paleo-Pacific Plate subduction was confined to the southeastern coast of the SCB in the Late Triassic, and the subduction of the flat slab was halted by obstruction at the end of the Late Triassic. In the Early Jurassic, the Paleo-Pacific Plate began to subduction again, and arc-related magmatic rocks were formed along the coast of SCB. At the same time, due to the remote effect of the subduction of the Paleo-Pacific Plate, the weak tectonic belt existing in the Nanling area was reactivated, resulting in the Nanling area in the intraplate extensional setting. Subsequently, the continuous subduction of the Paleo-Pacific Plate led to the thickening of the crust along the SCB in the Middle Jurassic, and the Nanling area was still under in the extensional setting.

The Jiawula-Chaganbulagen (Jia-Cha) ore field is located in the part of the southern Erguna Block of Northeast China and has significant mineralization potential. It comprises three deposits: The Chaganbulagen hydrothermal vein-type PbZn (Ag) deposit in the southeast, the Jiawula hydrothermal vein-type PbZn (Ag) deposit in the northwest, and the blind porphyry Mo mineralization in between. Previous geochronological dates, including molybdenite ReOs, zircon UPb, sulfide RbSr, and sericite ArAr, indicate that the formation of hydrothermal molybdenite and the PbZn mineralization were coeval and that they originated from a porphyry-epithermal metallogenetic system that was related to Early Cretaceous magmatism. However, our comprehensive analysis of fluid inclusions and H-O-He-Ar isotopes reveals dissimilarities in the features and sources of the fluids that form three different deposits. Both the Jiawula deposit and porphyry Mo mineralization have three distinct types of fluid inclusions, which comprise liquid-rich two-phase aqueous inclusions (L-type), vapor-rich two-phase inclusions (V-type), and daughter mineral-bearing three-phase inclusions (S-type). The HeAr (³He/⁴He = 1.32–1.51 Ra; ⁴⁰Ar/³⁶Ar = 300 to 324) and HO (δ¹⁸O_{H2O =} − 13.4 ‰ to 4.1 ‰; δD₌ − 155.1 ‰ to −129.9 ‰) isotope analysis results suggest that the trap fluids originated from an H₂O-NaCl magmatic fluid system, which experienced Mo mineralization with high-temperature transformed into medium-temperature fluids that were primarily of meteoric water in the late-stage (Jiawula PbZn (Ag) deposit). The Chaganbulagen PbZn (Ag) deposit features L-type and V-type inclusions, along with aqueous‑carbonic inclusions (C-type). The results of Laser Raman analysis reveal that the gas phases of the fluid inclusions present in Chaganbulagen contain three different compositions: H₂O, CO_2, and CH₄. The HO isotope compositions (δ¹⁸O_H2O = −18.8 ‰ to −11.6 ‰; δD = −166 ‰ to −131 ‰) and HeAr (³He/⁴He = 0.034–0.041 Ra; ⁴⁰Ar/³⁶Ar = 296–298) in quartz of the Chaganbulagen PbZn (Ag) deposit indicate that the crustal fluids (MASW) were the main source of the ore-forming fluids, and belonged to a H₂O-NaCl-CO₂-CH₄ system, which had low-temperature and low-salinity characteristics. By combining previously published geochronological dates, we proposed a model to explain the difference in the evolution of the fluids.

The termination of the collision between West and East Gondwana and the Pan-African orogeny is somewhat regarded as an endorsement for the generation and intrusion of alkaline magmatism in the Arabian-Nubian Shield. However, the current ages of the Abu Hamar-Abu Kharif alkaline granites have changed this endorsing connection. The Gebel Abu Hamr and Gebel Abu Kharif plutons, exposed in the northern Egyptian Eastern Desert, are anorogenic rift-related granite (A-type) and possess characteristics indicative of a within-plate tectonic setting. Their UPb ages (616.6 ± 6.4 and 614.6 ± 3.6 Ma) are remarkable and provide insight into the initial phase of the anorogenic alkaline magmatism within the shield. The zircon UPb dating, geochemical data, and zircon geochemistry together suggest that the primary magma was formed by the upwelling of a short-lived mantle plume. These alkaline granites subsequently intruded into the crustal mesozone at a thickness of 20–30 km with average temperatures of ~680 °C. The structural framework of the studied plutons reveals a noticeable correlation between their emplacement and the Najd left-lateral fault system. These alkaline granite plutons host wolframite-bearing quartz veins. The in-situ UPb dating of wolframite yielded a weighted mean age of 603.7 ± 7.4 Ma. The trace and rare earth elements of wolframite, zircon, and the hosting alkaline granites share significant geochemical similarities. Nevertheless, the proximity of the UPb age of wolframite and the associated granites suggests an epigenetic origin.

Allanite, a member of the epidote supergroup, is a widespread rare earth element (REE)-rich accessory mineral in the late Archean Closepet batholith (Dharwar craton, India). It is commonly associated with titanite. Previously recognized shear zones served as preferential paths for magma and later fluids. As a response to hydrothermal activity, allanite exhibits complex alteration textures, geochemical features, and breakdown products that vary across the batholith. In the central zone, allanite displays the largest variations. It has decomposed into secondary allanite, bastnäsite, chlorite, thorite, calcite, pyrite, and galena. In the southern zone, magmatic allanite core is preserved. The alteration products in the marginal regions are limited to secondary allanite, bastnäsite, chlorite, thorite, and synchysite. The breakdown products and textural features of allanite in the northern intrusions differ strongly from those in the other zones of the Closepet batholith and are limited to secondary allanite and chamosite. However, nanoscale element remobilization at the interface between allanite and titanite is evident. The observed texture in allanite indicates a complete dissolution–reprecipitation process. The chemical variations and differences in alteration products after allanite indicate that the fluid composition changed along the Closepet granitoid. The fluids that altered the allanite were most likely F-, Cl-, and CO₂-rich and alkaline but eventually became acidic. When the chlorine-bearing fluids reached the northern zone, the concentrations or active contributions of CO₂, F and H₂S were very low. The alteration products (bastnäsite, chlorite, and thorite) indicate a rather low-temperature fluid.