Zircon U–Pb ages and Sr–Nd isotope ratios for the Sirstan granitoid body,NE Iraq: Evidence of magmatic activity in the Middle Cretaceous Period

The Sirstan granitoid (SG), comprising diorite and granodiorite, is located in the Shalair Valley area, in the northeastern part of Iraq within the Sanandaj–Sirjan Zone (SSZ) of the Zagros Orogenic Belt. The U–Pb zircon dating of the SG rocks has revealed a concordia age of 110 Ma, which is interpreted as the age of crystallization of this granitoid body during the Middle Cretaceous. The whole-rock Rb–Sr isochron data shows an age of 52.4 ± 9.4 Ma (MSWD = 1.7), which implies the reactivation of the granitoid body in the Early Eocene due to the collision between the Arabian and Iranian plates. These rocks show metaluminous affinity with low values of Nb, Ta and Ti compared to chondrite, suggesting the generation of these rocks over the subduction zone in an active continental margin regime. The SG rocks are hornblende-bearing I-type granitoids with microgranular mafic enclaves. The positive values of ?_Nd (t = 110 Ma) (+0.1 to +2.7) and the low (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7044 to 0.7057) indicate that the magma source of the SG granitoids is a depleted subcontinental mantle. The chemical and isotope compositions show that the SG body originated from the metasomatic mantle without a major role for continental contamination. Our findings show that the granitoid bodies distributed in the SSZ were derived from the continuous Neo-Tethys subduction beneath the SSZ in Mesozoic times and that the SSZ was an active margin in the Middle Cretaceous.     

Density structure of the cratonic mantle in Southern Africa: 2. Correlations with kimberlite distribution,seismic velocities,and Moho sharpness

We present a new regional model for the depth-averaged density structure of the cratonic lithospheric mantle in southern Africa constrained on a 30′ × 30′ grid and discuss it in relation to regional seismic models for the crust and upper mantle, geochemical data on kimberlite-hosted mantle xenoliths, and data on kimberlite ages and distribution. Our calculations of mantle density are based on free-board constraints, account for mantle contribution to surface topography of ca. 0.5–1.0 km, and have uncertainty ranging from ca. 0.01 g/cm³ for the Archean terrains to ca. 0.03 g/cm³ for the adjacent fold belts. We demonstrate that in southern Africa, the lithospheric mantle has a general trend in mantle density increase from Archean to younger lithospheric terranes. Density of the Kaapvaal mantle is typically cratonic, with a subtle difference between the eastern, more depleted, (3.31–3.33 g/cm³) and the western (3.32–3.34 g/cm³) blocks. The Witwatersrand basin and the Bushveld Intrusion Complex appear as distinct blocks with an increased mantle density (3.34–3.35 g/cm³) with values typical of Proterozoic rather than Archean mantle. We attribute a significantly increased mantle density in these tectonic units and beneath the Archean Limpopo belt (3.34–3.37 g/cm³) to melt-metasomatism with an addition of a basaltic component. The Proterozoic Kheis, Okwa, and Namaqua–Natal belts and the Western Cape Fold Belt with the late Proterozoic basement have an overall fertile mantle (ca. 3.37 g/cm³) with local (100–300 km across) low-density (down to 3.34 g/cm³) and high-density (up to 3.41 g/cm³) anomalies. High (3.40–3.42 g/cm³) mantle densities beneath the Eastern Cape Fold belt require the presence of a significant amount of eclogite in the mantle, such as associated with subducted oceanic slabs.We find a strong correlation between the calculated density of the lithospheric mantle, the crustal structure, the spatial pattern of kimberlites, and their emplacement ages. (1) Blocks with the lowest values of mantle density (ca. 3.30 g/cm³) are not sampled by kimberlites and may represent the “pristine” Archean mantle. (2) Young (< 90 Ma) Group I kimberlites sample mantle with higher density (3.35 ± 0.03 g/cm³) than the older Group II kimberlites (3.33 ± 0.01 g/cm³), but the results may be biased by incomplete information on kimberlite ages. (3) Diamondiferous kimberlites are characteristic of regions with a low-density cratonic mantle (3.32–3.35 g/cm³), while non-diamondiferous kimberlites sample mantle with a broad range of density values. (4) Kimberlite-rich regions have a strong seismic velocity contrast at the Moho, thin crust (35–40 km) and low-density (3.32–3.33 g/cm³) mantle, while kimberlite-poor regions have a transitional Moho, thick crust (40–50 km), and denser mantle (3.34–3.36 g/cm³). We explain this pattern by a lithosphere-scale (presumably, pre-kimberlite) magmatic event in kimberlite-poor regions, which affected the Moho sharpness and the crustal thickness through magmatic underplating and modified the composition and rheology of the lithospheric mantle to make it unfavorable for consequent kimberlite eruptions. (5) Density anomalies in the lithospheric mantle show inverse correlation with seismic Vp, Vs velocities at 100–150 km depth. However, this correlation is weaker than reported in experimental studies and indicates that density-velocity relationship in the cratonic mantle is strongly non-unique.     

First results of GPS measurements on the Ulaanbaatar geodynamic testing area

First results of the analysis of GPS measurement data obtained from 18 sites of two local networks in the vicinity of Ulaanbaatar (Mongolia) for the period 2010–2015 have been presented. Horizontal velocity vectors are consistent with each other in the ITRF2014 system and with the velocities of the IGS permanent station ULAB. The sites move in the E–SE direction at a rate of 25–30 mm/yr, with the displacement azimuth averaging 105°. With respect to Eurasia, the vectors for most of the sites are slighly turned to the south, but they are still of SE orientation with the azimuth range of 130°–150° and magnitude values of 2–4 mm/yr. Relative horizontal velocities tend to decrease towards southeast that determines a zonal distribution of different type of relative strain patterns. The western part of the Ulaanbaatar network is characterized by the W–E oriented extension with the elongation rate ε₁ = 12–16 × 10^–8 yr^–1. The shortening NW–SE trending strain with calculated value ε₂ = 22.4 × 10^–8 yr^–1 characterizes the network’s eastern part. The highest values of the maximum shear strains (ε_max = 10–14 × 10^–8 yr^–1) form an extended area in the center of the testing ground, which is elongated in the northeastern direction, conformably with the strike of the major geologic structures. The strain distribution pattern of the Emeelt network located within the eponymous seismogenic structures is characterized by the crustal elongation (5 × 10^–6 yr^–1) trending SE–NW and less pronounced shortening in the SW–SE directions.The axial part of the fault crossing the network in the NW direction exhibits maximum deformations.     

Helium isotopes on the Macdonald seamount (Austral chain): constraints on the origin of the superswell

We present new helium isotope data from the Macdonald seamount (Austral chain). The helium isotopic ratio varies from ⁴He/³He=45 000 (R/Ra=16.0) to 200 170 (R/Ra=3.6). The helium content is between 1.5×10^?8 and 1.1×10^?5 ccSTP/g. These helium results show clearly the presence of primitive mantle material in the source of the Austral chain. Macdonald has the lowest ⁴He/³He ratio among the Polynesian submarine volcanoes, except Hawaii (Loihi). The simplest explanation for the primitive helium signature is the presence under Macdonald of a mantle plume that derives either from the 670 km or 2900 km boundary layers, or, eventually, from the top of a large mantle dome resulting from a stratified two-layer convection. This plume contains less-degassed material with low ⁴He/³He ratio. To cite this article: M. Moreira, C. Allègre, C. R. Geoscience 336 (2004).     

The early terrestrial crust

Recent geochemical evidence based on the ¹⁴⁶Sm–¹⁴²Nd system and Hadean zircons shows that the Earth's mantle experienced depletion approximately 100 Ma after the formation of the solar system, and possibly even before (earlier than 30 Ma), due to the extraction of a crust enriched in incompatible elements. Depending on the model ¹⁴²Nd abundance assumed for the Bulk Earth, the early crust may have been stored in the deep mantle, or may have been remixed in the mantle with a timescale of ∼1 Ga. If the Earth is considered to have a ¹⁴²Nd composition identical to that of ordinary chondrites, then it implies that the early crust (or the enriched reservoir) is now present at the core–mantle boundary and has remained isolated from the rest of the Earth for the past 4.5 Ga. If the primordial crust had a basaltic composition, then it is unlikely that this enriched reservoir remained isolated for more than 4.5 Ga due to entrainment; yet, there is no signature of this reservoir in hotspot lavas that should sample this enriched reservoir. In contrast, if the Bulk Earth has an Nd isotopic composition slightly distinct from that of chondrites, then there is no need to invoke a hidden reservoir and the early crust must have been remixed by mantle convection prior to the formation of the modern continents.     

Mesozoic lithospheric mantle of the northeastern Siberian craton (evidence from inclusions in kimberlite)

Several thousand clinopyroxene, garnet, and phlogopite inclusions of mantle rocks from Jurassic and Triassic kimberlites in the northeastern Siberian craton have been analyzed and compared with their counterparts from Paleozoic kimberlites, including those rich in diamond. The new and published mineral chemistry data make a basis for an updated classification of kimberlite-hosted clinopyroxenes according to peridotitic and mafic (eclogite and pyroxenite) parageneses. The obtained results place constraints on the stability field of high-Na lherzolitic clinopyroxenes, which affect the coexisting garnet and decrease its Ca contents. As follows from analyses of the mantle minerals from Mesozoic kimberlites, the cratonic lithosphere contained more pyroxenite and eclogite in the Mesozoic than in the Paleozoic. It virtually lacked ultradepleted harzburgite-dunite lithologies and contained scarce eclogitic diamonds. On the other hand, both inclusions in diamond and individual eclogitic minerals from Mesozoic kimberlites differ from eclogitic inclusions in diamond from Triassic sediments in the northeastern Siberian craton. Xenocrystic phlogopites from the D’yanga pipe have ⁴⁰Ar/³⁹Ar ages of 384.6, 432.4, and 563.4 Ma, which record several stages of metasomatic impact on the lithosphere. These phlogopites are younger than most of Paleozoic phlogopites from the central part of the craton (Udachnaya kimberlite). Therefore, hydrous mantle metasomatism acted much later on the craton periphery than in the center. Monomineral clinopyroxene thermobarometry shows that Jurassic kimberlites from the northeastern craton part trapped lithospheric material from different maximum depths (170 km in the D’yanga pipe and mostly < 130 km in other pipes). The inferred thermal thickness of cratonic lithosphere decreased progressively from ~ 260 km in the Devonian-Carboniferous to ~ 225 km in the Triassic and to ~ 200 km in the Jurassic, while the heat flux (Hasterok-Chapman model) was 34.9, 36.7, and 39.0 mW/m², respectively. Dissimilar PT patterns of samples from closely spaced coeval kimberlites suggest different emplacement scenarios, which influenced both the PT variations across the lithosphere and the diamond potential of kimberlites.     

New 40Ar/39Ar constraints for the “Grande Nappe”: The largest rhyolitic eruption from the Mont-Dore Massif (French Massif Central)

Since the 1960s, an early explosive activity in the Mont-Dore Massif is associated with a major pyroclastic rhyolitic eruption (5–7 km³) known as the “Grande Nappe” (GN). This event, linked to the formation of a 6-km-diameter cryptic caldera named “Haute Dordogne”, was before our investigation dated by ⁴⁰Ar/³⁹Ar at 3.07 ± 0.04 Ma. Our new single-crystal laser fusion ⁴⁰Ar/³⁹Ar dates obtained on two outcrops of the GN (Rochefort-Montagne and Ludières) questioned several hypotheses made concerning this “landmark” event of the Mont-Dore Massif history. We demonstrate that: (1) the GN rhyolitic eruption has occurred much later than previously estimated (i.e. 2.77 ± 0.02–0.07 Ma full external uncertainties); (2) the correlation made between the Vendeix rhyolitic complexes (intra-caldera position) dated back to 2.74 ± 0.04 Ma and the GN is proposed; (3) xenocryst contamination could be very high (i.e. 70% for the Rochefort-Montagne GN outcrop) and explains the noticeable older age obtained previously; (4) a link between the GN eruption and the formation of a caldera is questionable; the hypothesis of a northward-oriented blast channeled eastward toward the paleo-Allier River is thus proposed.     

Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite,Democratic Republic of Congo,and the Changle alkali basalt,China

Oxygen isotope signatures of ruby and sapphire megacrysts, combined with trace-element analysis, from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China, provide clues to specify their origin in the deep Earth. At Mbuji-Mayi, pink sapphires have δ¹⁸O values in the range 4.3 to 5.4‰ (N = 10) with a mean of 4.9 ± 0.4‰, and rubies from 5.5 to 5.6‰ (N = 3). The Ga/Mg ratio of pink sapphires is between 1.9 and 3.9, and in rubies, between 0.6 and 2.6. The blue or yellow sapphires from Changle have δ¹⁸O values from 4.6 to 5.2 ‰, with a mean of 4.9 ± 0.2‰ (N = 9). The Ga/Mg ratio is between 5.7 and 11.3. The homogenous isotopic composition of ruby suggests a derivation from upper mantle xenoliths (garnet lherzolite, pyroxenite) or metagabbros and/or lower crustal garnet clinopyroxenite eclogite-type xenoliths included in kimberlites. Data from the pink sapphires from Mbuji-Mayi suggest a mantle origin, but different probable protoliths: either subducted oceanic protolith transformed into eclogite with δ¹⁸O values buffered to the mantle value, or clinopyroxenite protoliths in peridotite. The Changle sapphires have a mantle O-isotope signature. They probably formed in syenitic magmas produced by low degree partial melting of a spinel lherzolite source. The kimberlite and the alkali basalt acted as gem conveyors from the upper mantle up to the surface.     

Lithospheric petrology of the eastern Arabian Plate: Constraints from Al-Ashkhara (Oman) xenoliths

Mafic granulite and spinel lherzolite xenoliths from Cenozoic alkaline basalts near Al-Ashkhara, eastern Oman, have been selected for a systematic mineralogical, geochemical and Sr–Nd–Pb isotopic study. This is the only place in E Arabia where samples of both lower crust and upper mantle can be examined. Lower crustal xenoliths consist of two mineralogically and chemically distinct groups: gabbronorite (subequal abundances of ortho- and clino-pyroxene and plagioclase) and plagioclase pyroxenite (dominant pyroxene and subordinate plagioclase). Temperature estimates for lower crustal xenoliths using the two pyroxene geothermometer (T-Wells) yield 810–865 °C. The mineral assemblage (spinel–pyroxene–plagioclase) and Al content in pyroxene indicate that plagioclase-bearing xenoliths equilibrated at 5–8 kbar (13 and 30 km depth) in the lower crust. εNd and ⁸⁷Sr/⁸⁶Sr calculated at 700 Ma for Al-Ashkhara lower crustal xenoliths (+ 6.4 to + 6.6; ⁸⁷Sr/⁸⁶Sr = 0.7028 to 0.7039) are consistent with the interpretation that juvenile, mafic melts were added to the lower crust during Neoproterozoic time and that there was no discernible contribution from pre-Neoproterozoic crust. Upper mantle xenoliths consist of both dry and hydrous (phlogopite-bearing) lherzolites. These peridotites are more Fe-rich than expected for primitive mantle or melt residues and probably formed by pervasive circulation of melts that have refertilized pre-existing mantle peridotites. Mineral equilibration temperatures range from 990 to 1070 °C. Isotopic compositions calculated at 700 Ma are εNd = + 6.8 to + 7.8 and ⁸⁷Sr/⁸⁶Sr = 0.7016 to 0.7025, indicating depleted upper mantle. Pb isotopic compositions indicate that the metasomatism was relatively recent, perhaps related to Paleogene tectonics and basanite igneous activity. Nd model ages for the spinel peridotite xenoliths range between 0.59 and 0.65 Ga. The xenolith data suggest that eastern Arabian lower crust is of hotspot origin, in contrast to western Arabian lower crust, which mostly formed at a convergent plate margin. Geochemical and isotopic differences between lower crust and upper mantle indicate that these are unrelated, possibly because delamination replaced the E Arabian mantle root in Neoproterozoic time.     

Paleoclimatic implications of hydrogen and oxygen isotopic compositions of Cretaceous–Tertiary kaolins in the Douala Sub-Basin,Cameroon

Increased interest in paleoenvironmental studies is a result of climatic changes occurring at present and predicted for the future. Such studies could be done using the stable isotope compositions (δ²H and δ¹⁸O) of kaolins, which provide knowledge on the paleoenvironmental conditions prevailing during the time of kaolinisation. In this study, the stable isotopic compositions of clay-size fraction of kaolins occurring in Cretaceous and Tertiary Formations of the Douala Sub-Basin in Cameroon are presented, with the aim of reconstructing the paleoenvironmental conditions of the Sub-Basin. To achieve this, the clay-size fraction (< 2 μm fraction) of 8 kaolinite-rich samples were analysed for their δ²H and δ¹⁸O compositions, and results were reported as part per mil (‰) relative to the SMOW standard. The δ¹⁸O values of kaolins found in the Cretaceous–Tertiary Formations of the Douala Sub-Basin varied between +18.2 and +21.0‰, whereas the δ²H values varied between –69 and –53‰. Nine of the eleven samples plotted on the right of the supergene–hypogene line. Five of these nine samples plotted very close to the kaolinite line, which represents the composition of kaolinite in equilibrium with meteoric water at 20 °C; suggesting a supergene weathering origin of these kaolins. The determination of the temperature of kaolinisation yielded mean formation temperatures of 22 ± 2 °C and 27 ± 6 °C for Cretaceous and Tertiary kaolins, respectively. Excluding the two samples falling in the hypogene field, averages of kaolinisation temperatures were 20 and 25 °C during the Cretaceous and Tertiary periods, respectively. These temperatures are slightly below the present mean annual temperature in Douala (27 °C), thereby suggesting that the climate was becoming warmer from the Cretaceous to the Present. Therefore, Douala had a cooler and rainy climate during the Cretaceous, and the climate is gradually becoming hotter and more humid, favouring the refinement of existing kaolins and the kaolinisation of kaolin-forming minerals in the Sub-Basin.     

Dating the giant Zhuxi W–Cu deposit (Taqian–Fuchun Ore Belt) in South China using molybdenite Re–Os and muscovite Ar–Ar system

The recently discovered Zhuxi W–Cu ore deposit is located within the Taqian–Fuchun Ore Belt in the southeastern edge of the Yangtze Block, South China. Its inferred tungsten resources, based on new exploration data, are more than 280 Mt by 2016. At least three paragenetic stages of skarn formation and ore deposition have been recognized: prograde skarn stage; retrograde stage; and hydrothermal sulfide stage. Secondly, greisenization, marmorization and hornfels formation are also observed. Scheelite and chalcopyrite are the dominant metal minerals in the Zhuxi deposit and their formation was associated with the emplacement of granite stocks and porphyry dykes intruded into the surrounding Carboniferous carbonate sediments (Huanglong and Chuanshan formations) and the Neoproterozoic slate and phyllites. The scheelite was mostly precipitated during the retrograde stage, whereas the chalcopyrite was widely precipitated during the hydrothermal sulfide stage. A muscovite ⁴⁰Ar/³⁹Ar plateau age of about 150 Ma is interpreted as the time of tungsten mineralization and molybdenite Re–Os model ages ranging from 145.9 ± 2.0 Ma to 148.7 ± 2.2 Ma (for the subsequent hydrothermal sulfide stage of activity) as the time of the copper mineralization. Our new molybdenite Re–Os and muscovite ⁴⁰Ar/³⁹Ar dating results, along with previous zircon U–Pb age data, indicate that the hydrothermal activity from the retrograde stage to the last hydrothermal sulfide stage lasted up to 5 Myr, from 150.6 ± 1.5 to 145.9 ± 1 Ma, and is approximately coeval or slightly later than the emplacement of the associated granite porphyry and biotite granite. The new ages reported here confirm that the Zhuxi tungsten deposit represents one of the Mesozoic magmatic–hydrothermal mineralization events that took place in South China in a setting of lithospheric extension during the Late Jurassic (160–150 Ma). It is suggested that mantle material played a role in producing the Zhuxi W–Cu mineralization and associated magmatism.     

Remarkably uniform oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble,with special reference to Vietnamese deposits

Oxygen isotope systematics for co-existing pairs of gem-spinel and calcite in marble from Vietnam and other worldwide deposits have been determined in order to characterize the O-isotope fractionation between calcite and spinel. In Vietnam, the Δ¹⁸O_cc–sp (= 3.7 ± 0.1‰ for six samples from the An Phu and Cong Troi deposits) is remarkably constant. The combination of these data with those obtained on calcite–spinel pairs of Paigutan (Nepal, n = 2), Ipanko (Tanzania, n = 1), and Mogok (Myanmar, = 2) are also consistent with an overall Δ¹⁸O_cc–sp of 3.6 ± 0.3‰ for all the spinel samples (n = 11). The straight line correlation δ¹⁸O_cc = 0.96 δ¹⁸O_sp + 4.4 is excellent despite their worldwide geographic spread. The increment method of calculating oxygen isotope fractionation gave a geologically unreasonable temperature of formation for both minerals at 1374 °C when compared to temperatures obtained by mineral assemblage equilibrium of these marble type deposits, between 610 and 750 °C. The constant Δ¹⁸O_cc–sp reflects a constant temperature for this amphibolite facies assemblage, whose current best estimate is calculated at 620 ± 40 °C, but unquantified uncertainties remain.     

Coexistence of the moderately refractory and fertile mantle beneath the eastern Central Asian Orogenic Belt

Relative to the North China Craton, the subcontinental lithospheric mantle (SCLM) beneath the Central Asian Orogenic Belt is little known. Mantle-derived peridotite xenoliths from the Cenozoic basalts in the Xilinhot region, Inner Mongolia, provide samples of the lithospheric mantle beneath the eastern part of the belt. The xenoliths are predominantly lherzolites with minor harzburgites, and can be subdivided into three groups, based on the REE patterns of clinopyroxenes. Group 1 peridotites (LREE-enriched), with low modal Cpx (3–7%), high Mg^# in olivine (> 90.6) and Cr^# in spinel (> 43.8), low whole-rock CaO + Al₂O₃ contents (1.62–3.22 wt.%) and estimated temperatures of 1043–1126 °C, represent moderately refractory SCLM that has experienced carbonatite-related metasomatism. Group 2 peridotites (LREE-depleted), with high modal Cpx (9–13%), low Mg^# in olivine (< 90.6) and Cr^# in spinel (< 20.0), high whole-rock CaO + Al₂O₃ contents (4.93–6.37 wt.%) and estimated temperatures of 814–970 °C, show affinity with Phanerozoic fertile SCLM that has undergone silicate-related metasomatism. Group 3 peridotites (convex-upward REE patterns), show wide ranges of olivine-Mg^# (88.4–90.6), spinel-Cr^# (11.5–47.6), and modal Cpx (3–14%) that overlap Groups 1 and 2. Their spinels have high TiO₂ contents (> 0.41 wt.%), implying involvement of reactions between melt and peridotites. The estimated temperatures of Group 3 (1033–1156 °C) are similar to those of Group 1. We suggest that the pre-existing moderately refractory lithospheric mantle (i.e., Group 1) beneath the eastern part of the Central Asian Orogenic Belt was strongly penetrated by upwelling asthenospheric material, and the cooling of this material produced fertile lithospheric mantle (i.e., Group 2). The present lithospheric mantle of this area consists of interspersed volumes of younger fertile and older more refractory lithosphere, with the fertile type dominating the shallower levels of the mantle.     

Reduction of buried oxidized oceanic crust during subduction

The relationship among subducted oxidized oceanic crust and oxidation state of the subarc mantle, and arc magmas is one of the important aspects to evaluate convergent margin tectonics. However details of the oxidized mass transferred from buried oceanic crust to the overlying subarc mantle wedge remain obscure. Here we investigate the Songduo eclogites from south Tibet formed by the subduction of the paleo-Tethyan oceanic crust, and identify an abrupt decrease in pyrope and increase in almandine contents from the mantle to rim of garnet grains. This is coupled with a decrease in the Fe^3 + content of epidote and Fe^3 +/(Fe^2 ++ Fe^3 +) ratios from garnet core to rim domains, as well as speciation of calcite, a new mineral phase, in the rock matrix. Minor sulfates occur only as inclusions in garnet core domains, whereas sulfides are confined to the matrix as an accessory mineral phase. Aegirine augite occurs as relics or inclusions in garnet and omphacite. These features clearly suggest that oxidized components, Fe^3 + and S^6 +, were reduced as Fe^2 + and S^2 −, respectively, at the subduction zone. Thermodynamic modeling in the P–T-log₁₀f_O2 space using updated Perplex_X programs further revealed that the Songduo eclogites experienced oxygen fugacity variation of up to 8 log₁₀ units, with decreasing pressure. Petrological observations further suggest that the strong redox processes took place, after breaking of garnet, during the initial exhumation of the eclogites. CO₂ and minor sulfur are subsequently transferred from the cold oceanic subduction zone to the overlying mantle wedge, partially released by arc volcanoes to atmosphere. Our study presents a case of C and S recycling between the Earth's exterior and interior.     

Refertilization of lithospheric mantle beneath the Yangtze craton in south-east China: Evidence from noble gases geochemistry

The Yangtze craton (YC), in eastern China, is one of the oldest cratons in the world and is characterized by a complex tectonic and geodynamic evolution. This evolution regards most of the eastern China craton, which since Mesozoic time has undergone significant thinning (> 200 km) of Archean lithosphere. This thinning favored the refertilization of the old refractory subcontinental lithospheric mantle (SCLM) by the upwelling of younger fertile asthenosphere. Whether this feature is localized only beneath certain areas of eastern China or is a more widespread characteristic of the mantle, including the YC, is a matter of debate.In order to constrain the history of the YC SCLM, we have measured the He- and Ar-isotopic compositions of fluid inclusions hosted in mantle xenoliths in the Lianshan area, which is part of the poorly investigated YC in south-east China. We also report new mineral chemistry and trace element compositions of clinopyroxenes from the same suite of samples, for comparison with noble gases. Two distinct types of xenoliths can be identified: Type 1, characterized by mantle-like He-isotopic (³He/⁴He) ratios (up to 9.1 Ra), represents fragments of a fertile lithospheric mantle; Type 2, showing ³He/⁴He values in the SCLM range (³He/⁴He < 7 Ra), represents shallow relicts of a refractory mantle. The patterns of rare-earth elements as well as the Y and Yb concentrations in the clinopyroxenes normalized to primitive mantle (Y_N and Yb_N, respectively) indicate that fractional partial melting might have affected the local mantle by < 3% in Type 1 and up to 20% in Type 2 xenoliths from Lianshan, respectively. The range of ⁴He/⁴⁰Ar* (⁴⁰Ar* is corrected for atmospheric contamination) ranges from 4.9 × 10^− 4 to 3.6 × 10^− 1, which is below the typical production ratio of the mantle (⁴He/⁴⁰Ar* = 1–5); this range is however compatible with this fractional partial melting. The variable ³He/⁴He and ⁴He/⁴⁰Ar* values in Lianshan xenoliths suggest that the local mantle source was also influenced by kinetic fractionation, possibly triggered by metasomatic melts. Metasomatism associated with carbonatitic melts, together with fluxing by CO₂-rich fluids, have permeated the mantle beneath Lianshan, generating the observed decoupling between noble gases and trace elements. The interpretative framework is also applicable for other mantle xenoliths from eastern China, indicating that the refertilization of the SCLM by ascending mantle-like melts is common also to YC, which can be identified using noble gases.     

Précision de l'estimation des précipitations au Sahel selon la densité du réseau d'observation pluviométrique

Many earlier studies have shown the very large spatial variability of rainfall in the Sahel at all time steps, from the event to the season. Often, the meteorological network in these countries is sparse, with one to five rain gauges per 10 000 km². It is thus difficult to calculate accurate estimates of the mean rainfall over such a large area. To improve the knowledge of Sahelian systems and the spatial distribution of rainfall, a dense network was set up in an area of 16 000 km² in southwestern Niger between 1991 and 1996. The aim was to calculate accurate rainfall spatial means over an area of 12 000 km² at different time steps (from the season to the ten-day period). With the spatialisation method used (kriging), it was possible to calculate curves of estimation errors of mean rainfall versus the rain-gauge network density. Operational abacuses of the standard estimation error as a function of the spatial mean of rainfall and the network density are proposed.     

Noble-metal mineralization of the Adycha-Taryn metallogenic zone: Geochemistry of stable isotopes,fluid regime,and ore formation conditions

The regional geologic setting of the Adycha-Taryn metallogenic zone, one of the areas most productive for noble-metal mineralization in northeastern Russia, is discussed. The intricate metallogenic history of the zone and the prolonged geodynamic activity of its ore-hosting structures are documented. Different types of mineralization, such as hydrothermal-metamorphogenic, gold-bismuth, gold-quartz, gold-antimony, and silver-antimony, are described. New data on the isotopic compositions of oxygen in quartz, sulfur in sulfides, and oxygen and carbon in carbonates from different mineralization types are presented. The early metamorphogenic quartz beyond the ore zones has δ¹⁸O = + 20.1 ± 2.0‰. At the gold-bismuth deposits, the δ¹⁸O values of quartz are within the narrow range of + 12.5 ± 0.4‰. Quartz from the gold-quartz mineralization shows much wider variation in δ¹⁸O values, from + 14.2 to + 19.5‰. A similar range (δ¹⁸O = + 16.1 to + 19.2‰) is observed for the gold-antimony mineralization. Cryptograined quartz from the silver-antimony mineralization is enriched in light oxygen isotopes (δ¹⁸O = -3.2 to + 4.7‰). The following δ³⁴S values (‰) have been established in sulfides of mineralization of different types: gold-bismuth -3.7 to -2.2 (Apy) and -6.7 to -6.8 (Py); gold-quartz -2.1 to + 2.4 (Apy), -6.6 to + 5.4 (Py), and -6.1 to + 4.2 (St); gold-antimony -2.0 to + 1.6 (Apy), -3.5 to + 2.1 (Py), and -5.3 to + 0.2 (St); and silver-antimony -2.0 to -1.9 (Apy), -2.2 ± 0.1 (Py), and -5.7 to -5.6 (St). The δ¹³C and δ¹⁸O values are contrasting in the studied types of mineralization, varying respectively from -6.9 to -5.9‰ and from + 2.1 to + 5.7‰ (gold-bismuth), from -9.1 to -6.1‰ and from + 12.4 to 18.7‰ (gold-quartz), from -12.1 to -9.5‰ and from + 15.0 to + 16.3‰ (gold-antimony), from -11.6 to -11.1‰ and from + 1.5 to + 4.7‰ (silver-antimony). Metamorphogenic calcites are rich in both heavy C (-1.1 to -1.7‰) and heavy O (+ 20.3 to + 20.5‰) isotopes. Microthermometric study and crush-leach analysis of fluid inclusions have revealed differences in the composition of ore-forming fluids and formation conditions for different types of mineralization. The isotopic compositions of O, C, and S of mineral-forming fluids suggest a significant input of magmatic fluids to the formation of gold-bismuth and gold-antimony deposits, the contribution of metamorphic fluids increases at gold-quartz deposits, and meteoric water is involved in the formation of silver-antimony deposits.     

Helium and carbon isotope variations in Liaodong Peninsula,NE China

Chemical and C–He isotopic compositions have been measured for N₂-rich hydrothermal gases from the Liaodong (abbreviation of East Liaoning Province) Peninsula from which the oldest crustal rocks in China with ⩾3.8 Ga outcrop. With the exception of one sample containing tritogenic ³He and atmospheric ⁴He in Liaoyang, the observed ³He/⁴He ratios from 0.1 Ra to 0.7 Ra indicate 1–8% helium from mantle, 92–98% from crust and 0.1–0.8% from atmosphere. Despite the lack of Quaternary volcanism, such ³He/⁴He ratios suggest, together with geophysical evidences, the existence of intrusive magmas that contain mantle helium and heat within the Liaodong middle-lower crust. The ³He/⁴He ratios are high along the NE-trending Jinzhou faults and gradually decrease with the increase of distance from the faults. Such a spatial distribution suggests that the mantle helium exsolves from magmatic reservoir in the middle-lower crust, becomes focused into the root zones of Jinzhou faults, and subsequently traverses the crust via permeable fault zones. When transversely migrated by groundwater circulation in near surface, mantle helium with high ³He/⁴He ratio may have been further diluted to the observed values by addition of radiogenic helium produced in the crust. This pattern shows strong evidence that the major faults played an important role on mantle-derived components transport from mantle upwards.     

Long-lived,stationary magmatism and pulsed porphyry systems during Tethyan subduction to post-collision evolution in the southernmost Lesser Caucasus,Armenia and Nakhitchevan

The composite Meghri–Ordubad and Bargushat plutons of the Zangezur–Ordubad region in the southernmost Lesser Caucasus consist of successive Eocene to Pliocene magmatic pulses, and host two stages of porphyry Cu–Mo deposits. New high-precision TIMS U–Pb zircon ages confirm the magmatic sequence recognized by previous Rb–Sr isochron and whole-rock K–Ar dating. A 44.03 ± 0.02 Ma-old granite and a 48.99 ± 0.07 Ma-old granodiorite belong to an initial Eocene magmatic pulse, which is coeval with the first stage of porphyry Cu–Mo formation at Agarak, Hanqasar, Aygedzor and Dastakert. A subsequent Oligocene magmatic pulse was constrained by U–Pb zircon ages at 31.82 ± 0.02 Ma and 33.49 ± 0.02 Ma for a monzonite and a gabbro, and a late Miocene porphyritic granodioritic and granitic pulse yielded ages between 22.46 ± 0.02 Ma and 22.22 ± 0.01 Ma, respectively. The Oligo-Miocene magmatic evolution broadly coincides with the second porphyry-Cu–Mo ore deposit stage, including the major Kadjaran deposit at 26–27 Ma.Primitive mantle-normalized spider diagrams with negative Nb, Ta and Ti anomalies support a subduction-like nature for all Cenozoic magmatic rocks. Eocene magmatic rocks have a normal arc, calc-alkaline to high-K calc-alkaline composition, early Oligocene magmatic rocks a high-K calc-alkaline to shoshonitic composition, and late Oligocene to Mio-Pliocene rocks are adakitic and have a calc-alkaline to high-K calc-alkaline composition. Radiogenic isotopes reveal a mantle-dominated magmatic source, with the mantle component becoming more predominant during the Neogene. Trace element ratio and concentration patterns (Dy/Yb, Sr/Y, La/Yb, Eu/Eu*, Y contents) correlate with the age of the magmatic rocks. They reveal combined amphibole and plagioclase fractionation during the Eocene and the early Oligocene, and amphibole fractionation in the absence of plagioclase during the late Oligocene and the Mio-Pliocene, consistent with Eocene to Pliocene progressive thickening of the crust or increasing pressure of magma differentiation. Characteristic trace element and isotope systematics (Ba vs. Nb/Y, Th/Yb vs. Ba/La, ²⁰⁶Pb/²⁰⁴Pb vs. Th/Nb, Th/Nb vs. δ¹⁸O, REE) indicate that Eocene magmatism was dominated by fluid-mobile components, whereas Oligocene and Mio-Pliocene magmatism was dominated by a depleted mantle, compositionally modified by subducted sediments.A two-stage magmatic and metallogenic evolution is proposed for the Zangezur–Ordubad region. Eocene normal arc, calc-alkaline to high-K calc-alkaline magmatism was coeval with extensive Eocene magmatism in Iran attributed to Neotethys subduction. Eocene subduction resulted in the emplacement of small tonnage porphyry Cu–Mo deposits. Subsequent Oligocene and Miocene high-K calc-alkaline and shoshonitic to adakitic magmatism, and the second porphyry Cu–Mo deposit stage coincided with Arabia–Eurasia collision to post-collision tectonics. Magmatism and ore formation are linked to asthenospheric upwelling along translithospheric, transpressional regional faults between the Gondwana-derived South Armenian block and the Eurasian margin, resulting in decompression melting of lithospheric mantle, metasomatised by sediment components added to the mantle during the previous Eocene subduction event.     

Ediacaran Águas Belas pluton,Northeastern Brazil: Evidence on age,emplacement and magma sources during Gondwana amalgamation

Ediacaran syenogranites from the Águas Belas pluton, Borborema Province, Northeastern Brazil were investigated in this work. The studied granitoids show high SiO₂, Fe# [FeO / (FeO + MgO)], total alkalis (K₂O + Na₂O) and BaO contents and medium Sr and low Nb contents. They show gentle fractionated rare earth patterns with discrete Eu negative anomalies. Major and trace element data point to chemical features of transitional high-K calc-alkaline to alkaline post-collisional magmatism. Structural data coupled with geochronological data suggest that NNE–SSW-trending sinistral movements at shear zones were initiated at ca. 590 Ma and have activated E–W pre-existing structures at the current crustal level. The synchronism of these shear zones allowed the dilation to generate the necessary space for the emplacement of the Águas Belas pluton.U–Pb SHRIMP zircon data show a cluster of ages around 588 ± 4 Ma which is interpreted as the crystallization age. Some zircon grain cores yielded ages within 2060–1860 Ma and 1670–1570 Ma intervals. Oxygen isotope compositions of zircon grains with distinct ages were measured using SHRIMP techniques. Twenty three analyses in the same zircon spots previously analyzed for U–Pb show δ¹⁸O values ranging from 5.79‰ to 10.30‰ SMOW. This large range of values results from variations both between grains and within grains (core–mantle/rim), and is interpreted as the result of mixing of components with distinct oxygen isotope compositions. The U–Pb zircon ages and the δ¹⁸O values associated with Paleoproterozoic Nd T_DM model ages suggest that the protolith of these granitoids involved a mantle component (Paleoproterozoic lithospheric mantle), Paleoproterozoic and Mesoproterozoic igneous rocks. Interactions with Mesoproterozoic or Neoproterozoic supracrustal rocks, may have occurred during the intrusion. The resulting magma evolved through biotite and K-feldspar fractionation.