Petrogenesis of synorogenic high-temperature leucogranites (Damara orogen,Namibia): Constraints from U–Pb monazite ages and Nd,Sr and Pb isotopes

Two suites of leucogranites were emplaced at 508 ± 5.9 Ma in the Okombahe District of the Damara belt (Namibia) synchronous with the peak of regional high-temperature metamorphism. The Sr (⁸⁷Sr/⁸⁶Sr_init: 0.707 to 0.711), Nd (εNd_init: − 4.5 to − 6.6), and Pb isotopic (²⁰⁶Pb/²⁰⁴Pb: 18.51–19.13; ²⁰⁷Pb/²⁰⁴Pb: 15.63–15.69; ²⁰⁸Pb/²⁰⁴Pb: 38.08–38.66) compositions indicate that these peraluminous S-type granites were derived from mid- to lower-crustal rocks, which are slightly different to the metapelitic rocks into which they intruded. Since the leucogranites are unfractionated and show no evidence for assimilation or contamination, they constrain the temperature and pressure conditions of their formation. Calculated Zr and LREE saturation temperatures of ca. 850 °C indicate high-temperature crustal melts. High Rb/Sr and low Sr/Ba ratios are consistent with biotite dehydration melting of pelitic source rocks. Qz–Ab–Or systematics reveal that melting and segregation for the least fractionated samples occurred at ca. 7 kbar corresponding to a mid-crustal level of ca. 26 km. However, there is no evidence for a mantle component that could have served as a local heat source for crustal melting. Therefore, the hot felsic magmas that formed close to the time of peak metamorphism are the result of long-lasting high temperature regional metamorphic conditions and intra-crustal collision.     

Petrogenesis of the Late Jurassic Laomengshan rhyodacite (Southeast China): constraints from zircon U–Pb dating,geochemistry and Sr–Nd–Pb–Hf isotopes

Extensive magmatism occurred in southeast China during Late Jurassic time, forming large-scale granitic and volcanic rocks associated with non-ferrous, rare earth and rare, radioactive metal deposits. The Shuikoushan Pb–Zn–Au orefield is a typical example located in Hunan Province. This study reports LA-ICP-MS zircon U–Pb ages, whole-rock chemistry, and Sr–Nd–Pb isotopic compositions, and in situ Hf isotopic geochemistry of zircons from the Laomengshan rhyodacite in the Shuikoushan Pb–Zn–Au orefield. Zircon U–Pb dating yields a weighted average age of 156.7 ± 1.6 Ma for the intrusion of the rhyodacite. The rhyodacite samples are mainly shoshonitic series, having metaluminous to weakly peraluminous A/CNK values ranging from 0.96 to 1.09, with moderately high magnesium content (Mg# = 42.4–47.5). Samples display high (⁸⁷Sr/⁸⁶Sr)_i values (0.71165–0.71176), low ε_Nd(T) values (?10.7 to ?10.3), old Nd model ages (T_DM = 1.73–1.86 Ga), and relatively homogeneous Pb isotopic compositions [(²⁰⁶Pb/²⁰⁴Pb)_i = 18.365–18.412, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.663–15.680, and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.625–38.666]. The zircons exhibit enriched ε_Hf(T) values (?16.22 to ?9.86) and old two-stage Hf model ages (T_DM2 = 1.82–2.22 Ga). All the above data indicate that the Laomengshan rhyodacite originated from melting Palaeoproterozoic basement, perhaps contaminated by subordinate mantle melts. Intense extension and thinning of the continental lithosphere during Late Jurassic time resulted in melting of upwelling asthenosphere, and mafic mantle melts interacted with and melted Palaeoproterozoic lower crust, thus forming the Laomengshan rhyodacite.     

Geochemistry of major and trace elements and Pb–Sr isotopes of a weathering profile developed on the Lower Cambrian black shales in central Hunan,China

This paper reports a geochemical study on the major and trace elements and Pb–Sr isotopes of a weathering profile developed in the Lower Cambrian black shales in central Hunan (China). Six weathering horizons were identified and sampled vertically throughout the profile. The chemical composition of the profile consists of variable concentrations of the major elements Fe₂O₃, FeO, MnO, MgO, CaO, Na₂O, and P₂O₅ and of less variable concentrations of SiO₂, TiO₂, Al₂O₃, and K₂O. The chemical change caused by weathering is estimated by mass-balance calculations, and the results show that the element mobility is characterised by substantial loss of SiO₂, FeO, CaO, K₂O, Na₂O, LOI, Cr, V, Ba, Cs, Rb, Sr, U, and Th, and moderate loss of Al₂O₃, MgO, Fe₂O₃, Ni, Cu, Pb, Tl, Sn, Sc, Ge and REE (Y). The high field strength elements TiO₂, Sn, Sc, U, Ga, Ge, Zr, Hf, Nb, and Ta were immobile during weathering. The chemical changes and the Pb–Sr isotopic data suggest that four types of chemical reactions occurred: the oxidation of sulphide minerals (e.g., pyrite) and organic carbon (OS), the dissolution of less resistant clinochlore-Ia, calcite, and P-bearing minerals (DL), the dissolution of detrital albite and microcline (DA), and the transformation of clay (TC) minerals (e.g., muscovite and illite–smectite). These chemical reactions then led to two stages of geochemical processes, an early stage of chemical differentiation and a later stage of chemical homogenisation. The chemical differentiation dominated by the OS, DL, and DA reactions, led to the leaching of mobile elements (e.g., MgO, Na₂O, K₂O, P₂O₅, Sr, and REE) and the redistribution of some less mobile elements (e.g., SiO₂ and Al₂O₃). In contrast, the chemical homogenisation, which was caused by TC reactions, led to the leaching of both mobile and less mobile elements from the system and ultimately transformed the weathered black shales into soil. Soils derived from black shales in South China might result from the above two geochemical processes.     

Emplacement ages and sources of kimberlites and related rocks in southern Africa: U–Pb ages and Sr–Nd isotopes of groundmass perovskite

Groundmass perovskite has been dated by LA-ICPMS in 135 kimberlites and related rocks from 110 localities across southern Africa. Sr and/or Nd isotopes have been analysed by LA-MC-ICPMS in a subset of these and integrated with published data. The age distribution shows peaks at 1,600–1,800, 1,000–1,200, 500–800 and 50–130 Ma. The major “bloom” of Group I kimberlites at ca 90 ± 10 Ma was preceded by a slow build-up in magmatic activity from ca 180 Ma. The main pulse of Group II kimberlites at 120–130 Ma was a distinct episode within this build-up. Comparison of the isotopic data with seismic tomography images suggests that metasomatized subcontinental lithospheric mantle (SCLM) with very low ε _Nd and high ⁸⁷Sr/⁸⁶Sr, (the isotopic signature of Group II kimberlites) was focused in low-Vs zones along translithospheric structures. Such metasomatized zones existed as early as 1,800 Ma, but were only sporadically tapped until the magmatic build-up began at ca 180 Ma, and contributed little to the kimberlitic magmas after ca 110 Ma. We suggest that these metasomatized volumes resided in the deep SCLM and that their low-melting point components were “burned off” by rising temperatures, presumably during an asthenospheric upwelling that led to SCLM thinning and a rise in the ambient geotherm between 120 and 90 Ma. The younger Group I kimberlites therefore rarely interacted with such SCLM, but had improved access to shallower volumes of differently metasomatized, ancient SCLM with low ⁸⁷Sr/⁸⁶Sr and intermediate ε _Nd (0–5). The kimberlite compositions therefore reflect the evolution of the SCLM of southern Africa, with metasomatic-enrichment events from as early as 1.8 Ga, through a major thermal and compositional change at ca 110 Ma, and the major kimberlite “bloom” around 90 Ma.     

Source components of the Gran Canaria (Canary Islands) shield stage magmas: evidence from olivine composition and Sr–Nd–Pb isotopes

The Canary Island primitive basaltic magmas are thought to be derived from an HIMU-type upwelling mantle containing isotopically depleted (NMORB)-type component having interacted with an enriched (EM)-type component, the origin of which is still a subject of debate. We studied the relationships between Ni, Mn and Ca concentrations in olivine phenocrysts (85.6–90.0 mol.% Fo, 1,722–3,915 ppm Ni, 1,085–1,552 ppm Mn, 1,222–3,002 ppm Ca) from the most primitive subaerial and ODP Leg 157 high-silica (picritic to olivine basaltic) lavas with their bulk rock Sr–Nd–Pb isotope compositions (⁸⁷Sr/⁸⁶Sr = 0.70315–0.70331, ¹⁴³Nd/¹⁴⁴Nd = 0.51288–0.51292, ²⁰⁶Pb/²⁰⁴Pb = 19.55–19.93, ²⁰⁷Pb/²⁰⁴Pb = 15.60–15.63, ²⁰⁸Pb/²⁰⁴Pb = 39.31–39.69). Our data point toward the presence of both a peridotitic and a pyroxenitic component in the magma source. Using the model (Sobolev et al. in: Science 316:412–417, 2007) in which the reaction of Si-rich melts originated during partial melting of eclogite (a high pressure product of subducted oceanic crust) with ambient peridotitic mantle forms olivine-free reaction pyroxenite, we obtain an end member composition for peridotite with ⁸⁷Sr/⁸⁶Sr = 0.70337, ¹⁴³Nd/¹⁴⁴Nd = 0.51291, ²⁰⁶Pb/²⁰⁴Pb = 19.36, ²⁰⁷Pb/²⁰⁴Pb = 15.61 and ²⁰⁸Pb/²⁰⁴Pb = 39.07 (EM-type end member), and pyroxenite with ⁸⁷Sr/⁸⁶Sr = 0.70309, ¹⁴³Nd/¹⁴⁴Nd = 0.51289, ²⁰⁶Pb/²⁰⁴Pb = 20.03, ²⁰⁷Pb/²⁰⁴Pb = 15.62 and ²⁰⁸Pb/²⁰⁴Pb = 39.84 (HIMU-type end member). Mixing of melts from these end members in proportions ranging from 70% peridotite and 30% pyroxenite to 28% peridotite and 72% pyroxenite derived melt fractions can generate the compositions of the most primitive Gran Canaria shield stage lavas. Combining our results with those from the low-silica rocks from the western Canary Islands (Gurenko et al. EPSL 277:514–524, 2009), at least four distinct components are required. We propose that they are (1) HIMU-type pyroxenitic component (representing recycled ocean crust of intermediate age) from the plume center, (2) HIMU-type peridotitic component (ancient recycled ocean crust stirred into the ambient mantle) from the plume margin, (3) depleted, MORB-type pyroxenitic component (young recycled oceanic crust) in the upper mantle entrained by the plume, and (4) EM-type peridotitic component from the asthenosphere or lithosphere above the plume center.     

Sediment recycling and crustal growth in the Central Asian Orogenic Belt: Evidence from Sr–Nd–Hf isotopes and trace elements in granitoids of the Chinese Altay

Felsic igneous rocks are common constituents of volcanic arcs, and contain valuable information about subduction-related magmatism. In this study we investigate nine granitoids with S-type volcanic arc affinity from the Chinese Altay, emplaced from 507 to 391 Ma in an active subduction zone during the early–middle Paleozoic. These granitoids are characterized by moderate to high SiO₂ contents (61.01–75.30 wt.%), moderate total alkalis (Na₂O + K₂O, 3.43–7.64 wt.%), and high Al₂O₃ contents (13.29–17.18 wt.%). Negative εNd(t) values (− 6.1 to − 1.0), the wide range of εHf(t) values (− 7.0 to + 9.0), and enrichment of LILEs such as Pb, Th and U, all suggest that the granitoids were probably derived from the partial melting of subducting oceanic sediments and the associated mantle wedge. This inference is further supported by the Nd-isotope data. The high initial ⁸⁷Sr/⁸⁶Sr ratios (0.703963–0.719428), low Ba/Th ratios (7.00–118.93), and uniformly negative εNd(t) values (− 6.1 to − 1.0) indicate that slab-derived aqueous fluids were vital in generating the initial magma of these granitoids, and assimilation played only a minor role. Our data demonstrate that residual zircon retains a substantial amount of Hf during the partial melting of oceanic sediments, therefore, Hf may not be an effective tracer for the input of recycled sediments. We conclude that sediment recycling played an important role in the generation of arc magmatism and the growth of the Central Asian Orogenic Belt (CAOB).     

Origin of the Yinshan epithermal-porphyry Cu–Au–Pb–Zn–Ag deposit,southeastern China: insights from geochemistry,Sr–Nd and zircon U–Pb–Hf–O isotopes

The Yinshan deposit is a large epithermal-porphyry polymetallic deposit, and the timing and petrogenesis of ore-hosting porphyries have been hotly debated. We present new results from geochemical, whole-rock Sr–Nd and zircon U–Pb–Hf–O isotopic investigations. Zircon U–Pb data demonstrate that the quartz porphyry, dacitic porphyry, and quartz dioritic porphyry formed at ?172.2 ± 0.4 Ma, ?171.7 ± 0.5 Ma, and ?170.9 ± 0.3 Ma, respectively. Inherited zircon cores show significant age spreads from ?730 to ?1390 Ma. Geochemically, they are high-K calc-alkaline or shoshonitic rocks with arc-like trace element patterns. They have similar whole-rock Nd and zircon Hf isotopic compositions, yet an increasing trend in ?_Nd(t) and ?_Hf(t) values typifies the suite. Older (inherited) zircons of the three porphyries display Hf compositions comparable to those of the Jiangnan Orogen basement rocks. In situ zircon oxygen isotopic analyses reveal that they have similar oxygen isotopic compositions, which are close to those of mantle zircons. Moreover, a decreasing trend of δ¹⁸O values is present. We propose that the ore-related porphyries of the Yinshan deposit were emplaced contemporaneously and derived from partial melting of Neoproterozoic arc-derived mafic (or ultra-mafic) rocks. Modelling suggests that the quartz porphyries, dacitic porphyries, and quartz dioritic porphyries experienced ?25%, ?10%, and ?10% crustal contaminations by Shuangqiaoshan rocks. Our study provides important constraints on mantle–crust interaction in the genesis of polymetallic mineralization associated with Mesozoic magmatism in southeastern China.     

Intermittent mixing processes occurring before Plinian eruptions of Popocatepetl volcano,Mexico: insights from textural–compositional variations in plagioclase and Sr–Nd–Pb isotopes

Volcán Popocatépetl has explosively erupted in Plinian style at least five times in the last 23,000 years. Extreme deviations in composition and the occurrence of dissolution features in plagioclase and pyroxene, and the occasional presence of xenocrysts of Cr-rich Fe–Ti oxides and Mg-rich olivines and pyroxenes indicate that magma mixing has been a major process affecting the magmatic system. The nearly invariant composition of the erupted products (andesitic–dacitic) suggests, however, that mixing is not acting alone and must be balanced by assimilation and/or crystallization. To investigate the magmatic processes that have modified the Plinian magmas, textural and compositional variations and growth rates in plagioclase were used to approximate the frequency of mixing events affecting each magma. Systematic analysis of Sr, Nd, and Pb isotopes was carried out on plagioclase, pyroxene, and pumice matrix glass to constrain the extent of assimilation of upper crustal rocks. Additionally, a series of phase equilibrium experiments were carried out to constrain the depth where such mixing and assimilation occurred. We find that magma was stored at one of two different depths beneath Popocatépetl with magma mixing acting in both reservoirs. Mixing frequency and the relative impact on mineral compositions and textures has varied with time. Assimilation of calcareous rocks underneath Popocatépetl has not been pervasive and does not contribute significantly to the evolution of the Plinian magmas. The similar compositions of magmas with diverse mixing histories suggest that fractional crystallization, and possibly assimilation of deep crust, takes place at depth and that intermediate magmas ascend into the upper crust already differentiated.     

Metallogenesis and ore-forming time of the Changtuxili Mn–Ag–Pb–Zn deposit in Inner Mongolia: Evidence from C–O–S isotopes and U–Pb geochronology

This paper reports new geochronological (U–Pb) and isotope (C, O, and S) data to investigate the timing of mineralization and mode of ore genesis for the recently discovered Changtuxili Mn–Ag–Pb–Zn deposit, located on the western slopes of the southern Great Hinggan Range in NE China. The mineralization is hosted by intermediate–acidic lavas and pyroclastic rocks of the Baiyingaolao Formation. Three stages of mineralization are identified: quartz–pyrite (Stage I), galena–sphalerite–tetrahedrite–rhodochrosite (Stage II), and quartz–pyrite (Stage III). δ¹³C and δ¹⁸O values for carbonate from the ore vary from −8.51‰ to −4.96‰ and 3.97‰ to 15.90‰, respectively, which are indicative of a low-temperature alteration environment. δ³⁴S_V-CDT values of sulfides range from −1.77‰ to 4.16‰ and show a trend of equilibrium fractionation (δ³⁴S_Py ​> ​δ³⁴S_Sp ​> ​δ³⁴S_Gn). These features indicate that pyrite, sphalerite, and galena precipitated during the period of mineralization. The alteration mineral assemblage and isotope data indicate that the weakly acidic to weakly alkaline ore-forming fluid was derived largely from meteoric water and the ore-forming elements C and S originated from magma. During the mineralization, a geochemical barrier was formed by changes in the pH of the ore-forming fluid, leading to the precipitation of rhodochrosite. On the basis of the mineralization characteristics, new isotope data, and comparison with adjacent deposits, we propose that the Changtuxili Mn–Ag–Pb–Zn deposit is an intermediate-to low-sulfidation epithermal deposit whose formation was controlled by fractures and variability in the pH of the ore-forming fluid. The surrounding volcanic rocks yield zircon U–Pb ages of 160−146 ​Ma (Late Jurassic), indicating that the mineralization is younger than 146 ​Ma.     

Metamorphic growth and recrystallization of zircon: Distinction by simultaneous in-situ analyses of trace elements,U–Th–Pb and Lu–Hf isotopes in zircons from eclogite-facies rocks in the Sulu orogen

Simultaneous in-situ analyses of trace elements, U–Th–Pb and Lu–Hf isotopes were carried out on distinct domains of zircons in ultrahigh-pressure (UHP) eclogite-facies metamorphic rocks from the main hole of the Chinese Continental Scientific Drilling (CCSD) in the Sulu orogen. For the first time, trace elements are directly linked to Lu–Hf isotopes in metamorphic zircons with reference to their U–Pb dates. This enables methodological integration to distinguish four types of metamorphic zircon: solid-state, replacement and dissolution recrystallizations of protolith zircons, and new growth from the aqueous fluid. Metamorphically grown zircons are characterized by concordant U–Pb ages for the metamorphism, flat HREE patterns typical of the garnet effect, low contents of REE (especially HREE), Y, Nb + Ta and Th + U, high contents of Hf, low (Lu/Gd)_N, Lu/Hf and Th/U (< 0.1) ratios, and elevated ¹⁷⁶Hf/¹⁷⁷Hf ratios relative to solid-state recrystallized zircons. This suggests the effects of both garnet and fluid on the growth of metamorphic zircons. In contrast, metamorphic recrystallization has reset the U–Th–Pb isotope system of protolith zircons to different extents, depending on the extents of fluid action during metamorphism. Solid-state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons, which are characterized by discordant U–Pb ages close to or below the protolith age, steep MREE–HREE patterns typical of magmatic origin, high contents of trace elements and their ratios, and low ¹⁷⁶Hf/¹⁷⁷Hf ratios. On the other hand, dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant U–Pb ages for the metamorphism, steep MREE–HREE patterns, lowered contents of trace elements such as REE, Th, U, Y, Nb, Ta and Ti relative to the protolith zircons, and almost unchanged Hf isotope ratios. Replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between. While the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of HREE with flat pattern and the low contents of trace elements, the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of MREE to HREE without the flat HREE pattern. Therefore, the simultaneous in-situ analyses of metamorphic zircons have the advantage over single-term analyses in making distinction between the new growth and the different types of recrystallization.     

Genesis of the Huangshaping W–Mo–Cu–Pb–Zn polymetallic deposit in Southeastern Hunan Province,China: Constraints from fluid inclusions,trace elements,and isotopes

The Huangshaping polymetallic deposit is located in southeastern Hunan Province, China. It is a world-class W–Mo–Pb–Zn–Cu skarn deposit in the Nanling Range Metallogenic Belt, with estimated reserves of 74.31 Mt of W–Mo ore at 0.28% WO₃ and 0.07% Mo, 22.43 Mt of Pb–Zn ore at 3.6% Pb and 8.00% Zn, and 20.35 Mt of Cu ore at 1.12% Cu. The ore district is predominantly underlained by carbonate formations of the Lower Carboniferous period, with stocks of quartz porphyry, granite porphyry, and granophyre. Skarns occurred in contact zones between stocks and their carbonate wall rocks, which are spatially associated with the above-mentioned three types of ores (i.e., W–Mo, Pb–Zn, and Cu ores).Three types of fluid inclusions have been identified in the ores of the Huangshaping deposit: aqueous liquid–vapor inclusions (Type I), daughter-mineral-bearing aqueous inclusions (Type II), and H₂O–CO₂ inclusions (Type III). Systematic microthermometrical, laser Raman spectroscopic, and salinity analyses indicate that high-temperature and high-salinity immiscible magmatic fluid is responsible for the W–Mo mineralization, whereas low-temperature and low-salinity magmatic-meteoric mixed fluid is responsible for the subsequent Pb–Zn mineralization. Another magmatic fluid derived from deep-rooted magma is responsible for Cu mineralization.Chondrite-normalized rare earth element patterns and trace element features of calcites from W–Mo, Pb–Zn, and Cu ores are different from one another. Calcite from Cu ores is rich in heavy rare earth elements (187.4–190.5 ppm), Na (0.17%–0.19%), Bi (1.96–64.60 ppm), Y (113–135 ppm), and As (9.1–29.7 ppm), whereas calcite from W–Mo and Pb–Zn ores is rich in Mn (> 10.000 ppm) and Sr (178–248 ppm) with higher Sr/Y ratios (53.94–72.94). δ¹⁸O values also differ between W–Mo/Pb–Zn ores (δ¹⁸O = 8.10‰–8.41‰) and Cu ores (δ¹⁸O = 4.34‰–4.96‰), indicating that two sources of fluids were, respectively, involved in the W–Mo, Pb–Zn, and Cu mineralization.Sulfur isotopes from sulfides also reveal that the large variation (4‰–19‰) within the Huangshaping deposit is likely due to a magmatic sulfur source with a contribution of reduced sulfate sulfur host in the Carboniferous limestone/dolomite and more magmatic sulfur involved in the Cu mineralization than that in W–Mo and Pb–Zn mineralization. The lead isotopic data for sulfide (galena: ²⁰⁶Pb/²⁰⁴Pb = 18.48–19.19, ²⁰⁷/²⁰⁴Pb = 15.45–15.91, ²⁰⁸/²⁰⁴Pb = 38.95–39.78; sphalerite: ²⁰⁶Pb/²⁰⁴Pb = 18.54–19.03, ²⁰⁷/²⁰⁴Pb = 15.60–16.28, ²⁰⁸/²⁰⁴Pb = 38.62–40.27; molybdenite: ²⁰⁶Pb/²⁰⁴Pb = 18.45–19.21, ²⁰⁷/²⁰⁴Pb = 15.53–15.95, ²⁰⁸/²⁰⁴Pb = 38.77–39.58 chalcopyrite: ²⁰⁶Pb/²⁰⁴Pb = 18.67–19.38, ²⁰⁷/²⁰⁴Pb = 15.76–19.90, and ²⁰⁸/²⁰⁴Pb = 39.13–39.56) and oxide (scheelite: ²⁰⁶Pb/²⁰⁴Pb = 18.57–19.46, ²⁰⁷/²⁰⁴Pb = 15.71–15.77, ²⁰⁸/²⁰⁴Pb = 38.95–39.13) are different from those of the wall rock limestone (²⁰⁶Pb/²⁰⁴Pb = 18.34–18.60, ²⁰⁷/²⁰⁴Pb = 15.49–15.69, ²⁰⁸/²⁰⁴Pb = 38.57–38.88) and porphyries (²⁰⁶Pb/²⁰⁴Pb = 17.88–18.66, ²⁰⁷/²⁰⁴Pb = 15.59–15.69, ²⁰⁸/²⁰⁴Pb = 38.22–38.83), suggesting Pb²⁰⁶-, U²³⁸-, and Th ²³²-rich material are involved in the mineralization. The Sm–Nd isotopes of scheelite (ε_Nd(t) = − 6.1 to − 2.9), garnet (ε_Nd(t) = − 6.8 to − 6.1), and calcite (ε_Nd(t) = − 6.3) from W–Mo ores as well as calcite (ε_Nd(t) = − 5.4 to − 5.3) and scheelite (ε_Nd(t) = − 2.9) from the Cu ores demonstrate suggest more mantle-derived materials involved in the Cu mineralization.In the present study we conclude that two sources of ore-forming fluids were involved in production of the Huangshaping W–Mo–Pb–Zn–Cu deposit. One is associated with the granite porphyry magmas responsible for the W–Mo and then Pb–Zn mineralization during which its fluid evolved from magmatic immiscible to a magmatic–meteoritic mixing, and the other is derived from deep-rooted magma, which is related to Cu-related mineralization.     

Petrogenesis of the Bao'anzhai granite and associated Mo mineralization,western Dabie orogen,east-central China: Constraints from zircon U–Pb and molybdenite Re–Os dating,whole-rock geochemistry,and Sr–Nd–Pb–Hf isotopes

Recently, many Mo deposits genetically related to emplacement of Early Cretaceous granites have been found in the Dabie–Qinling belt. A typical intrusion that combines magmatism and metallogenesis, the Bao'anzhai granite, yields a zircon ²³⁸U–²⁰⁶Pb age of 123.2 ± 1.1 Ma and a molybdenite Re–Os isochron age of 122.5 ± 2.7 Ma. This granite is characterized by high silica and alkali, but low Mg, Fe, and Ca. It is enriched with light rare earth elements (REEs) and large-ion lithophile elements (LILEs, Rb, K, Th, U) but depleted of heavy REEs, high field strength elements (HFSEs, Nb, Ta, Ti, and Y), and Sr. This high-K granite has medium initial ⁸⁷Sr/⁸⁶Sr ratios (0.706518–0.707116) and low initial Pb isotopic ratios [(²⁰⁶Pb/²⁰⁴Pb)_i, 16.423–16.699; (²⁰⁷Pb/²⁰⁴Pb)_i, 15.285–15.345; (²⁰⁸Pb/²⁰⁴Pb)_i, 37.335–37.633], and is characterized by low ?_Nd(t) and ?_Hf(t) values (?14.92 to??14.22 and??21.67 to??19.19, respectively). These data indicate that this pluton is a high-K calc-alkaline fractionated I-type granitite. It was generated by partial melting of the Yangtze lower crust, which is probably similar to Neoproterozoic TTG-like magmatic rocks at the north Yangtze Block under a non-thickened lower crust environment (<35 km). The ores also have low radiogenic Pb isotopes (²⁰⁶Pb/²⁰⁴Pb, 16.592–17.674; ²⁰⁷Pb/²⁰⁴Pb, 15.300–15.476; ²⁰⁸Pb/²⁰⁴Pb, 37.419–37.911) and low Re content in molybdenite (5.693–10.970 ppm), suggesting a crustal magmatic source for the metallogenic minerals in the Mo deposit.     

A porphyry-skarn metallogenic system in the Lesser Xing’an Range,NE China: Implications from U–Pb and Re–Os geochronology and Sr–Nd–Hf isotopes of the Luming Mo and Xulaojiugou Pb–Zn deposits

The Luming porphyry Mo deposit and the Xulaojiugou skarn Pb–Zn deposit are located in the southeast Lesser Xing’an Range, NE China. They are about 15 km apart, and are both related to monzogranite. Mo orebodies in the Luming deposit are hosted within the medium- to fine-grained monzogranite, while Pb–Zn orebodies in the Xulaojiugou deposit are hosted by the contact zone between the medium-grained monzogranite and the marbles of the early Cambrian Qianshan Formation.LA-ICP-MS zircon U–Pb dating of the ore-related monzogranite in the Luming deposit yields crystallization age of 180.7 ± 1.6 Ma, and the medium-grained and porphyritic monzogranites from the Xulaojiugou deposit yield crystallization ages of 181.2 ± 1.1 Ma and 179.9 ± 1.0 Ma, respectively. Analyses of seven molybdenite samples from the Luming deposit display Re–Os isochron age of 177.9 ± 2.6 Ma. These results indicate that the mineralization in the Luming and Xulaojiugou deposits occurred at about 181–178 Ma. These two deposits are genetically linked and belong to a porphyry-skarn metallogenic system. Combined with the previously reported geochronological data for ore deposits in adjacent areas, we consider that the early Jurassic is an important epoch for Mo and Pb–Zn mineralization in the Lesser Xing’an Range.The monzogranites from the Luming and Xulaojiugou deposits are enriched in and Rb, Th, U, Pb and light rare earth elements (LREEs), and are depleted in Ba, Nb, Ta, P, Ti and Eu. They have positive ε_Hf(t) values of 1.0–4.0 with two-stage Hf model ages (T_DM2) of 868–1033 Ma. Whole-rock Sr and Nd isotopes show restricted ranges of initial compositions, with (⁸⁷Sr/⁸⁶Sr)_i between 0.706346 and 0.707384 and ε_Nd(t) between −3.5 and −1.8. These data indicate that their primary magmas originated from the partial melting of a depleted lithospheric mantle which had been metasomatized by subducted slab-derived fluids/melts. The early Jurassic magmatic–metallogenic events in the Lesser Xing’an Range are interpreted as a response to the subduction of the Paleo-Pacific Plate.     

Two pulses of mineralization and genesis of the Zhaxikang Sb–Pb–Zn–Ag deposit in southern Tibet: Constraints from Fe–Zn isotopes

Zhaxikang is one large Sb–Pb–Zn–Ag deposit located in the North Himalaya of southern Tibet. To date, the genesis of this deposit still remains controversial. Here, we present new pyrite Fe and sphalerite Zn isotopic data for the first three stages of mineralization, Fe–Zn isotopic data for Mn–Fe carbonate that formed during the first two stages of mineralization, and Zn isotopic data for the slate wall rocks of the Jurassic Ridang Formation to discuss the genesis of the Zhaxikang deposit. The overall δ⁵⁶Fe and δ⁶⁶Zn values range from −0.80‰ to 0.43‰ and from −0.03‰ to 0.38‰, respectively. The δ⁵⁶Fe values of Mn–Fe carbonates are lighter than those of associated pyrite in six mineral pairs, indicating that the iron carbonates are preferentially enriched in light Fe isotopes relative to pyrite. The sphalerite has lighter δ⁶⁶Zn values than associated Mn–Fe carbonates in three mineral pairs.The δ⁵⁶Fe values of pyrite that formed during the first three stages of mineralization gradually increase from stage 1 (−0.33‰ to −0.09‰) through stage 2 (−0.30‰ to 0.19‰) to stage 3 (0.16‰–0.43‰). In comparison, the sphalerite that formed during these stages has δ⁶⁶Zn values that gradually decrease from stage 1 (0.16‰–0.35‰) through stage 2 (0.09‰–0.23‰) to stage 3 (−0.03‰ to 0.22‰). These data, in conjunction with the observations of hand specimens and thin sections, suggest that the deposit was overprinted by a second pulse of mineralization. This overprint would account for these Fe–Zn isotopic variations as well as the kinetic Rayleigh fractionation that occurred during mineralization. The temporally increasing δ⁵⁶Fe and decreasing δ⁶⁶Zn values recorded in the deposit are also coincident with an increase in alteration, again supporting the existence of two pulses of mineralization. The δ⁵⁶Fe values of the first pulse of ore-forming fluid were calculated using theoretical equations, yielding values of −0.54‰ to −0.34‰ that overlap with those of submarine hydrothermal solutions (−1‰ to 0‰). However, the δ⁵⁶Fe values of the stage 3 pyrite are heavier than those of typical submarine hydrothermal solutions, which suggests that the second pulse of mineralization was probably derived from a magmatic hydrothermal fluid. In addition, the second pulse of ore-forming fluid has brought some Fe and taken away parts of Zn, which results the lighter δ⁶⁶Zn values of sphalerite and heavier δ⁵⁶Fe values of pyrite from the second pulse of mineralization. Overall, the Zhaxikang deposit records two pulses of mineralization, and the overprint by the second pulse of mineralization causes the lighter δ⁶⁶Zn values and heavier δ⁵⁶Fe values of modified samples.     

Petrogenesis of adamellites from eastern Shandong Province: geochronological,geochemical, and Sr–Nd–Pb isotopic evidence

Geochronology, geochemistry, and whole-rock Sr–Nd–Pb isotopes were studied on a suite of Mesozoic adamellites from eastern China to characterize their ages and petrogenesis. Sensitive high-resolution ion microprobe U–Pb zircon analyses were done, yielding consistent ages of 123.2 ± 1.8 to 122.1 ± 2.1 Ma for the samples. These rocks belong to the alkaline magma series in terms of K₂O + Na₂O contents (8.45–9.58 wt.%) and to the shoshonitic series based on their high K₂O contents (5.23–5.79 wt.%). The adamellites are further characterized by high light rare earth element contents [(La/Yb)_N = 14.96–45.99]; negative Eu anomalies (δEu = 0.46–0.75); positive anomalies in Rb, Th, Pb, and U; and negative anomalies in Sr, Ba, and high field-strength elements (i.e. Nb, Ta, P, and Ti). In addition, all of the adamellites in this study display relatively low radiogenic Sr [(⁸⁷Sr/⁸⁶Sr)_i = 0.7081–0.7089] and negative ?_Nd(t) values from –16.70 to –17.80. These results suggest that the adamellites were derived from low-degree partial melting of an enriched lithospheric mantle below the North China Craton (NCC). The parent magmas likely experienced fractional crystallization of potassium feldspar, plagioclase and Fe–Ti oxides (e.g. rutile, ilmenite, and titanite), apatite, and zircon during the ascent of alkaline rocks without crustal contamination.     

Paleoproterozoic crustal evolution of the Tarim Craton: Constrained by zircon U–Pb and Hf isotopes of meta-igneous rocks from Korla and Dunhuang

The Tarim Craton is one of three large cratons in China. Presently, there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis, especially an in situ Lu–Hf isotope analysis of zircon. In this study, Precambrian basement rocks from the Kuluketage and Dunhuang Blocks in the northeastern portion of the Tarim Craton have been analyzed for combined in situ laser ablation ICP-(MC)-MS zircon U–Pb and Lu–Hf isotopic analyses, as well as whole rock elements, to constrain their protoliths, forming ages and magma sources. Two magmatic events from the Kuluketage Block at ∼2.4 Ga and ∼1.85 Ga are revealed, and three stages of magmatic events are detected in the Dunhuang Block, i.e., ∼2.0 Ga, ∼1.85 Ga and ∼1.75 Ga. The ∼1.85 Ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings, suggesting that the Kuluketage and Dunhuang Blocks are part of the uniform Precambrian basement of the Tarim Craton. Zircon Hf model ages of the ∼2.4 Ga magmatism indicate that the crust of the Tarim Craton may have been formed as early as the Paleoarchean period. The ∼2.0 Ga mafic rock from the Dunhuang Block was formed in an active continental margin setting, representing an important crustal growth event of the Tarim Craton in the mid-Paleoproterozoic that coincides with the global episode of crust formation during the assembly of the Columbia supercontinent. The ∼1.85 Ga event in the Kuluketage and Dunhuang Blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the Columbia supercontinent, while the ∼1.75 Ga magmatism in the Dunhuang Block resulted from a mixture of the reworked Archean crust with juvenile magmas and was most likely related to a post-collisional episode.     

U–Pb–Hf isotopes and trace elements of metasomatic zircon delimit the evolution of neoproterozoic Capané ophiolite in the southern Brasiliano Orogen

The Capané ophiolite is a fragment of oceanic lithosphere obducted into the Ediacaran Porongos fold and thrust belt, southern Brasiliano Orogen. A studied rodingite blackwall contained in serpentinite has metasomatic zircon that displays multiple U–Pb ages from Tonian to Cryogenian (793 ± 0.9, 757 ± 2.1, 715 ± 2.2 Ma). The ages are interpreted as corresponding to multiple alteration events in the mantle. Multiple U–Pb–Hf isotopes and trace element analyses on the same crystals by laser ablation were controlled by backscatered electron images. Hf isotopes indicate zircon origin from a depleted mantle (εHf = +15 to +10.7), and trace elements point to an oceanic origin. The Capané ophiolite thus marks the evolution of the Adamastor ocean during the Tonian and Cryogenian, a significant result for the reconstruction of Rodinia and Gondwana supercontinents.     

Magma mixing and the generation of isotopically juvenile silicic magma at Yellowstone caldera inferred from coupling 238U–230Th ages with trace elements and Hf and O isotopes in zircon and Pb isotopes in sanidine

The nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time. We examine compositional heterogeneity in the youngest (~170 to 70 ka) post-caldera volcanism at Yellowstone caldera, the Central Plateau Member (CPM) rhyolites, as a case study. We compare ²³⁸U–²³⁰Th age, trace-element, and Hf isotopic data from zircons, and major-element, Ba, and Pb isotopic data from sanidines hosted in two CPM rhyolites (Hayden Valley and Solfatara Plateau flows) and one extracaldera rhyolite (Gibbon River flow), all of which erupted near the caldera margin ca. 100 ka. The Hayden Valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the CPM reservoir. The Gibbon River flow hosts one zircon population that is compositionally distinct from Hayden Valley flow zircons. The Solfatara Plateau flow contains multiple sanidine populations and all three zircon populations found in the Hayden Valley and Gibbon River flows, demonstrating that the Solfatara Plateau flow formed by mixing extracaldera magma with the margin of the CPM reservoir. This process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs. More generally, Hf isotopic data from the CPM zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post-caldera magmatic system and demonstrate that the sources contributing magma to the CPM reservoir were heterogeneous in ¹⁷⁶Hf/¹⁷⁷Hf at ca. 100 ka. Thus, the limited compositional variability of CPM glasses reflects homogenization occurring within the CPM reservoir, not a homogeneous source.