The influence of Cr on the garnet–spinel transition in the Earth''s mantle: experiments in the system MgO–Cr2O3–SiO2 and thermodynamic modelling

The position of the transition from spinel peridotite to garnet peridotite in a simplified chemical composition has been determined experimentally at high pressures and high temperatures. The univariant reaction MgCr₂O₄+2Mg₂Si₂O₆=Mg₃Cr₂Si₃O₁₂+Mg₂SiO₄, has a negative slope in P–T space between 1200 °C and 1600 °C. The experimental results, combined with assessed thermodynamic data for MgCr₂O₄, MgSiO₃ and Mg₂SiO₄ give the entropy and enthalpy of formation of knorringite garnet (Mg₃Cr₂Si₃O₁₂). Thermodynamic calculations in simplified chemical compositions indicate that Cr shifts the garnet-in reaction to much higher pressures than previously anticipated. Moreover, in Cr-bearing systems a pressure–temperature field exists where garnet and spinel coexist. The width of this divariant field strongly depends on the Cr/(Cr+Al) of the system.     

平南幔源包体中橄榄石的显微构造研究及其意义

平南玄武岩中的尖晶石二辉橄榄岩包体的平衡温度为930~980℃, 平衡深度为59~74km, 包体中橄榄石的扭折带滑移系多为(010) [100], 但也有(001) [100]的滑移系类型; 斜方辉石的滑移系为(100) [001], 它们均为高温低应变速率下的滑移系, 说明该区的上地幔主要是在高温低应变速率条件下经历了塑性变形作用.橄榄石位错组态多样, 有自由位错、位错壁、位错弓弯、缠结、{110}滑移带, 反映了上地幔的塑性变形特征.根据位错壁的大小估算, 上地幔差异流动应力为24.5~42.1MPa, 流动速率为2.93×10-17~8.36×10-16s-1, 有效粘度为1.72×1023~2.80×1024 Pa·s, 特征与中国东部新生代上地幔较为一致, 反映同处于拉张环境.      

Geological and C–O–S–Pb–Sr isotopic constraints on the origin of the Qingshan carbonate-hosted Pb–Zn deposit,Southwest China

Located in the western Yangtze Block, the Qingshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining–Shuicheng anticline and its intraformational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.

The C–O isotope compositions of hydrothermal calcite and S–Pb–Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb–Zn deposit. δ¹³C_PDB and δ¹⁸O_SMOW values of calcite range from –5.0‰ to –3.4‰ and +18.9‰ to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO₂ in the hydrothermal fluid had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ³⁴S values of sulphide minerals range from +10.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20‰ to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.561 to 18.768, ²⁰⁷Pb/²⁰⁴Pb = 15.701 to 15.920, and ²⁰⁸Pb/²⁰⁴Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. ⁸⁷Sr/⁸⁶Sr ratios of sphalerite range from 0.7107 to 0.7136 and (⁸⁷Sr/⁸⁶Sr)_200Ma ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C–O–S–Pb–Sr isotope compositions of the Qingshan Pb–Zn deposit indicate a mixed origin of the ore-forming fluids and metals.     

Geology,ore facies and sulphur isotopes of the Koushk vent-proximal sedimentary-exhalative deposit,Posht-e-Badam Block,Central Iran

The Koushk zinc–lead deposit in the central part of the Zarigan–Chahmir basin, central Iran, is the largest of several sedimentary–exhalative (SEDEX) deposits in this basin, including the Chahmir, Zarigan, and Darreh-Dehu deposits. The host-rock sequence consists of carbonaceous, fine-grained black siltstone with interlayered rhyolitic tuffs. It corresponds to the upper part of the Lower Cambrian volcano-sedimentary sequence that was deposited on the Posht-e-Badam Block due to back-arc rifting of the continental margin of the Central Iranian Microcontinent. This block includes the late Neoproterozoic metamorphic basement of the Iran plate, overlain by rocks dating from the Early Cambrian to the Mesozoic. Based on ore body structure, mineralogy, and ore fabric, we recognize four different ore facies in the Koushk deposit: (1) a stockwork/feeder zone, consisting of a discordant mineralization of sulphides forming a stockwork of sulphide-bearing dolomite (quartz) veins cutting the footwall sedimentary rocks; (2) a massive ore/vent complex, consisting of massive replacement pyrite, galena, and sphalerite with minor arsenopyrite and chalcopyrite; (3) bedded ore, with laminated to disseminated pyrite, sphalerite, and galena; and (4) a distal facies, with minor disseminated and laminated pyrite, banded cherts, and disseminated barite. Carbonatization and sericitization are the main wall-rock alterations; alteration intensity increases towards the feeder zone. The δ³⁴S composition of pyrite, sphalerite, and galena ranges from?+6.5 to?+36.7‰. The highest δ³⁴S values correspond to bedded ore (+23.8 to?+36.7‰) and the lowest to massive ore (+6.5 to?+?17.8‰). The overall range of δ³⁴S is remarkably higher than typical magmatic values, suggesting that sulphides formed from the reduction of seawater sulphate by bacteriogenic sulphate reduction in a closed or semi-closed system in the bedded ore, whereas thermochemical sulphate reduction likely played an important role in the feeder zone. Sulphur isotopes, along with sedimentological, textural, mineralogical, and geochemical evidences, suggest that this deposit should be classified as a vent-proximal SEDEX ore deposit.     

Modelling of Ocean Shallow Convection under Fast Ice in the Arctic

Ocean convection in the Antarctic has been studied many times and has been revealed to be responsible for ice-cover reduction. In the Arctic, proof of that phenomenon has not been documented. It is believed that this phenomenon happens on a smaller scale in the Arctic when local circulation of deep warmer water melts and slows ice production. An example of this is the North Water (NOW) polynya in northern Baffin Bay. A polynya is an area of open water in an otherwise ice-covered area. As ice forms under the fast ice near the boundary of the polynya, ocean salts (brine) are ejected from the newly formed ice. This water, which has an increased concentration of salt, sinks and is replaced by warmer water from below, and this slows ice formation. In our study a coupled one-dimensional thermodynamic snow–fast ice model incorporating ocean heat flux input via a shallow convection model was used. Ice thickness was calculated using a thermodynamic model that included a current-induced entrainment model and a convection model to account for brine rejection during ice growth. Atmospheric observations from Grise Fiord and Thule and ocean profiles around the NOW polynya near these sites were used as input to the model. This purely thermodynamic study enables us to obtain ice thickness values that can be compared with qualitative observations. This modelling study compares two sites related to the NOW polynya. The results indicate that the shallow convection model simulates the reduction of fast ice near Thule but not near Grise Fiord.     

中国东北地区上地幔组成、结构及热状态

本文通过对中国东北地区新生代玄武岩中超镁铁质包体的研究,根据地质压力计所估算的包体形成深度、玄武岩浆及辉石岩浆起源的深度等,建立了中国东北地区上地幔组成模式:1.壳幔过渡带;2.尖晶石二辉橄榄岩层;3.含有痕量硅酸盐熔体的低速层;4.尖晶石方辉橄榄岩层;5.石榴石二辉橄榄岩层。与此同时,根据地质温度计与地质压力计推算资料,作者认为中国东北地区上地幔属大洋地温。与典型克拉通地温相比,低速层位置较浅,上地幔处于过热状态,这与大陆裂谷的发育有关。     

Mid–Late Triassic metamorphic event for Changhai meta-sedimentary rocks from the SE Jiao–Liao–Ji Belt,North China Craton: Evidence from monazite U–Th–Pb and muscovite Ar–Ar dating

The precise constraints on the timing of metamorphism of the Changhai metamorphic complex is of great importance considering the prolonged controversial issue of the north margin and the extension of the Sulu–Dabie HP–UHP Belt. While the monazite U–Th–Pb and muscovite ⁴⁰Ar/³⁹Ar techniques are widely accepted as two of the most powerful dating tools for revealing the thermal histories of medium–low grade metamorphic rocks and precisely constraining the timing of metamorphism. The Changhai metamorphic complex at the SE Jiao–Liao–Ji Belt, North China Craton consists of a variety of pelitic schist and Grt–Ky-bearing paragneiss, and minor quartzite and marble. Analyses of mineral inclusions and back-scattered electric (BSE) images of monazites, combined with LA–ICP–MS U–Th–Pb ages for monazites and ⁴⁰Ar/³⁹Ar ages for muscovites, provide evidence of the origin and metamorphic age of the Changhai metamorphic complex. Monazites separates from various Grt–Mus schists and Grt–Ky–St–Mus paragneisses exhibit homogeneous BSE images from cores to rims, and contain inclusion assemblages of Grt + Mus + Qtz ± Ctd ± Ky in schist, and Grt + Ky + St + Mus + Pl + Kfs + Qtz inclusions in paragneiss. These inclusion assemblages are very similar to matrix minerals of host rocks, indicating they are metamorphic rather than inherited or detrital in origin. LA–ICP–MS U–Th–Pb dating reveals that monazites of schist and paragneiss have consistent ²⁰⁶Pb/²³⁸U ages ranging from 228.1 ± 3.8 to 218.2 ± 3.7 Ma. In contrast, muscovites from various schists show slightly older ⁴⁰Ar/³⁹Ar plateau ages of 236.1 ± 1.5 to 230.2 ± 1.2 Ma. These geochronological and petrological data conclude that the pelitic sediments have experienced a metamorphic event at the Mid–Late Triassic (236.1–218.2 Ma) rather than the Paleoproterozoic (1950–1850 Ma), commonly regarded as the Precambrian basement for the Jiao–Liao–Ji Belt. Hence, the Changhai metamorphic complex should be considered as a discrete lithotectonic group.This newly recognized Mid–Late Triassic metamorphic event (236.1–218.2 Ma) for the Changhai metamorphic complex is coeval with the HP–UHP metamorphic event (235–220 Ma) for Sulu–Dabie rocks. This leads us to speculate that the metamorphism of the Changhai complex belt along the SE margin of the North China Craton was genetically related to the Mid–Late Triassic collision of the North China and South China cratons. By the same token, the Sulu–Dabie HP–UHP Belt may have extended through Yantai, and the southern Yellow Sea, and to the southern side of the Changhai metamorphic complex.     

S?o Francisco-Congo Craton break-up delimited by U-Pb-Hf isotopes and trace-elements of zircon from metasediments of the Ara?uaí Belt

Detrital zircon U-Pb geochronology combined with Hf isotopic and trace element data from metasedimentary rocks of the Aracuai Belt in southeastern Brazil provide evidence for break-up of the Congo-Sao Francisco Craton. The U-Pb age spectra of detrital zircons from metasediments of the Rio Doce Group(RDG) range from 900-650 Ma and define a maximum depositional age of ca. 650 Ma. Zircon trace element and whole rock data constrain an oceanic island arc as source for the deposition setting of the protoliths to the metasediments. Zircon ε_(Hf)(t) values from these rocks are positive between +1 and +15, supporting previous evidence of a Neoproterozoic extensional phase and oceanic crust formation in a precursor basin to the Aracuai Belt. Recrystallization of detrital zircon at ca. 630 Ma is compatible with a regional metamorphic event associated with terrane accretion to the Paleoproterozoic basement after transition from an extensional to a convergent regime. The juvenile nature, age spectra and trace element composition recorded in detrital zircons of metasediments from the Aracuai Belt correspond with zircons from metasedimentary rocks and oceanic crust remnants of other orogenic belts to its south. This suggests that rifting and oceanic crust formation of the entire orogenic system, the so-called Mantiqueira Province, was contemporaneous, most likely related to the opening of a large ocean. It further indicates that the cratonic blocks involved in the orogenic evolution of the Mantiqueira Province were spatially connected as early as 900 Ma.     

The upper Cenomanian–lower Turonian carbonate platform of the Preafrican Trough,Morocco: Biostratigraphic,paleoecological and paleobiogeographical distribution of ostracods

The upper Cenomanian–lower Turonian paleoenvironments of the Preafrican Trough carbonate platform is characterized by analyzing the structure of the ostracod assemblages and the information provided by other groups, and also by linking together the paleontological and geochemical data (detrital influx-redox-paleoproductivity proxies, δ¹³C curve). Two different domains (eastern and western) can be recognized on the platform during the late Cenomanian, before the onset of the OAE2. The western domain corresponds to a low-energy environment developed on a mid and/or outer ramp with hypoxic waters, low detrital influx and low paleoproductivity. The paleoecological assemblages show limited specific diversity but variable density. The ostracods are opportunistic and unspecialized (r strategists), being associated with Buliminidae, surface and intermediate-water planktonic foraminifera, and fishes. The eastern domain corresponds to an inner ramp and/or peritidal environment with oxic waters, low detrital influx and low paleoproductivity, developed in a higher energy environment with paleoecological assemblages showing high diversity but variable density. The ostracods are more specialized (K strategists), being represented by diverse and constant assemblages associated with diversified benthic foraminifera, calcareous sponges and echinoderms, as well as intermediate- and deep-water planktonic foraminifera. The onset of the OAE2 has no influence on the western ostracod assemblages, but leads to the decline of the ostracod fauna and the disappearance of the deep-water planktonic foraminifera in the eastern domain. During the early Turonian, after the OAE2, the platform becomes an outer ramp with increased paleoproductivity, but is associated with a decrease of taxonomic diversity in hypoxic waters. The ostracods are very sparse and unspecialized, associated with siliceous sponges, Buliminidae, surface-living planktonic foraminifera, fishes and pelagic crinoids. Marine paleobiogeographic communication is relatively easy across the carbonate platforms between the Preafrican Trough and other Moroccan regions, as well as between Morocco and different parts of the South Tethyan and East Atlantic margins belonging to the Cenomanian–Turonian South Tethyan Ostracod Province (STOP). Thirteen new species are described: Cytherella tazzouguertensis n. sp., Bairdiacypris chaabetensis n. sp., Bythocypris amelkisensis n. sp., Pontocypris tadighoustensis n. sp., Procytherura? elongatissima n. sp., Loxoconcha akrabouensis n. sp., Hemiparacytheridea sagittaemucronata n. sp., Rehacythereis errachidiaensis n. sp., Rehacythereis zizensis n. sp., Veenia (Nigeria) tardaensis n. sp., Veenia (Nigeria) mediacostarobusta n. sp., Xestoleberis? preafricanensis n. sp., and Xestoleberis circinatus n. sp.     

Geochronological and geochemical constraints on the Erguna massif basement,NE China – subduction history of the Mongol–Okhotsk oceanic crust

We present new geochronological and geochemical data for granites and volcanic rocks of the Erguna massif, NE China. These data are integrated with previous findings to better constrain the nature of the massif basement and to provide new insights into the subduction history of Mongol–Okhotsk oceanic crust and its closure. U–Pb dating of zircons from 12 granites previously mapped as Palaeoproterozoic and from three granites reported as Neoproterozoic yield exclusively Phanerozoic ages. These new ages, together with recently reported isotopic dates for the metamorphic and igneous basement rocks, as well as Nd–Hf crustal-residence ages, suggest that it is unlikely that pre-Mesoproterozoic basement exists in the Erguna massif. The geochronological and geochemical results are consistent with a three-stage subduction history of Mongol–Okhotsk oceanic crust beneath the Erguna massif, as follows. (1) The Erguna massif records a transition from Late Devonian A-type magmatism to Carboniferous adakitic magmatism. This indicates that southward subduction of the Mongol–Okhotsk oceanic crust along the northern margin of the Erguna massif began in the Carboniferous. (2) Late Permian–Middle Triassic granitoids in the Erguna massif are distributed along the Mongol–Okhotsk suture zone and coeval magmatic rocks in the Xing’an terrane are scarce, suggesting that they are unlikely to have formed in association with the collision between the North China Craton and the Jiamusi–Mongolia block along the Solonker–Xra Moron–Changchun–Yanji suture zone. Instead, the apparent subduction-related signature of the granites and their proximity to the Mongol–Okhotsk suture zone suggest that they are related to southward subduction of Mongol–Okhotsk oceanic crust. (3) A conspicuous lack of magmatic activity during the Middle Jurassic marks an abrupt shift in magmatic style from Late Triassic–Early Jurassic normal and adakite-like calc-alkaline magmatism (pre-quiescent episode) to Late Jurassic–Early Cretaceous A-type felsic magmatism (post-quiescent episode). Evidently a significant change in geodynamic processes took place during the Middle Jurassic. Late Triassic–Early Jurassic subduction-related signatures and adakitic affinities confirm the existence of subduction during this time. Late Jurassic–Early Cretaceous post-collision magmatism constrains the timing of the final closure of the Mongol–Okhotsk Ocean involving collision between the Jiamusi–Mongolia block and the Siberian Craton to the Middle Jurassic.