Quartz Assemblage Research Articles

Sapphirine + quartz assemblage from the Irumide Belt, northern Malawi: Implications for Mesoproterozoic ultrahigh-temperature metamorphism related to Rodinia assembly

The Irumide Belt sensu lato situated between the Bangweulu Block to the northwest and the Neoproterozoic Mozambique–Zambezi Belt to the southeast is a Mesoproterozoic orogenic belt that developed probably during the amalgamation of the supercontinent Rodinia. In this study, we present new petrological, geothermobarometric, and geochronological data of pelitic granulites and related rocks from the Jenda area in northern Malawi, and evaluate the timing and pressure–temperature (P–T) conditions of high-grade metamorphism. To the best of our knowledge, we are the first to report the occurrence of sapphirine + quartz association in pelitic granulite from the Irumide Belt sl. which provides a robust evidence of peak ultrahigh-temperature (UHT) metamorphism. The sapphirine occurs as poikiloblastic grains with rounded quartz inclusions in the absence of any retrograde minerals. The mineral phase equilibrium modeling constrains the peak UHT conditions of the pelitic granulites as 950–1000 °C and 7–8.5 kbar for sapphirine-bearing and ∼6 kbar and > 950 °C for sapphirine-free samples. These conditions are consistent with the results of ternary-feldspar geothermometry (900–1000 °C at 8 kbar). From the stability of rutile, we estimate a prograde pressure of >9 kbar, and the occurrence of retrograde cordierite and biotite suggests that the rocks went through P–T conditions of ∼6–7 kbar/∼775–825 °C, indicating a clockwise P–T path and defining high-pressure and UHT conditions. In-situ monazite Th–U–Pbtotal geochronology of the sapphirine-bearing rock yielded a weighted-mean age of 1022 ± 10 Ma which is considered to mark the timing of peak metamorphism. Sapphirine-free granulites also gave consistent ages of 1049 ± 13 Ma and 1048 ± 10 Ma, which are also comparable with published ages. We thus infer that the Irumide Belt sl. underwent regional high-pressure to UHT metamorphism at ca. 1.05 to 1.02 Ga possibly related to the main collisional event of the Bangweulu Block with an unknown craton or magmatic arc during the assembly of Rodinia supercontinent.

Onset and tempo of ignimbrite flare-up volcanism in the eastern and central Mogollon-Datil volcanic field, southern New Mexico, USA

The Cenozoic ignimbrite flare-up (40–18 Ma) generated multiple volcanic fields in the southwestern United States and northern Mexico resulting from asthenospheric mantle upwelling after removal of the Farallon slab. The correlation of tuffs to one another and to source calderas within these volcanic fields is essential for determining spatiotemporal patterns in volcanism and magma geochemistry, which have been used to deduce migration of the Farallon slab at depth and associated mantle melting. However, the correlation of Eocene–Oligocene tuffs in the southwestern U.S. is difficult because of post-emplacement erosion and faulting. This study focuses on spatiotemporal patterns of the initial episode of ignimbrite flare-up activity (ca. 36.5–33.8 Ma) in the Mogollon-Datil volcanic field in south-central New Mexico, USA. We show that alkali feldspar major and trace element geochemistry is an effective tool for correlating tuffs when combined with high-precision, single-crystal 40Ar/39Ar geochronology and bulk-rock geochemistry. Using these data, we correlate several tuff units and differentiate other tuffs that have the same eruption age but very different geochemistry, and we conclude that there was a broadly northwestward migration in volcanism over time. The new tuff correlations are used to investigate spatiotemporal variations in magma geochemistry, erupted volumes, and recurrence intervals during the initial episode of Mogollon-Datil volcanic field volcanism. Early-erupted tuffs restricted to the eastern Mogollon-Datil volcanic field share similarities with western U.S. topaz rhyolites, which suggests that the silicic magmas were generated by partial melting of mafic lower crustal rocks. We also find differences in the compositions, crystallinities, and mineral assemblages between the early- and late-erupted tuffs. The early-erupted tuffs tend to have single-feldspar mineralogies, lower feldspar Or contents, large negative Eu anomalies, and low-whole–rock Ba concentrations. Conversely, late-erupted tuffs have two feldspar plus quartz assemblages, lesser Eu anomalies, higher whole-rock Ba concentrations, and feldspars have higher Or contents. Thus, we suggest that for some of the early eruptions, after magmas underwent crystal fractionation in the crust, the silicic melt largely separated from the crystalline mush prior to eruption, whereas late-erupted tuff magmas underwent crystal fractionation at near the eutectic minimum and were remobilized and erupted with a larger proportion of their crystalline mush. Using our new ages, correlations, and previously published data, we find that the initial phase of Mogollon-Datil volcanic field volcanism produced at least 15 eruptions between 36.5 Ma and 33.8 Ma, with a minimum total erupted volume of ~1350 km3 and an average recurrence interval of 170 k.y. However, eruptions were generally smaller in volume (most <15 km3) than in other coeval fields, and most eruptions (n = 11) occurred in the first 1.2 m.y. (ca. 36.5–35.3 Ma) of activity. Altogether, our work sheds new light on variations in the composition, timing, and migration of volcanism during the initial phase of Mogollon-Datil volcanic field activity and highlights the utility of feldspar geochemistry in both “fingerprinting” tuffs and elucidating magma evolution.

Crystals and melt inclusions record deep storage of superhydrous magma prior to the largest known eruption of Cerro Machín volcano, Colombia

Abstract Cerro Machín, a volcano located in the northern segment of the Andes, is considered one of the most dangerous volcanoes in Colombia with an explosive record that involves at least five plinian events. Prior studies focused on the last dome-building eruption have suggested the presence of a water-rich mid-crustal magma reservoir. However, no direct volatile measurements have been published and little work has been completed on the explosive products of the volcano. Here, we study the largest known eruption of Cerro Machín volcano which occurred 3600 yr BP producing dacitic pyroclastic fall deposits that can be traced up to 40 km from the vent. Lapilli pumice clasts have a mineral assemblage of plagioclase, amphibole, quartz, and biotite phenocrysts, with accessory olivine, Fe-Ti oxides, and apatite. The occurrence of Fo89-92 olivine rimmed by high Mg# amphibole and the established high-water contents in the magma imply the presence of magma near or at water saturation at pressures >~500 MPa. Measurements of up to 10.7 wt% H2O in melt inclusions hosted in plagioclase and quartz in the 3600 years BP eruption products support the idea that Cerro Machín is a remarkably water-rich volcanic system. Moreover, this is supported by measurements of ~103 – 161 ppm H2O in plagioclase phenocrysts. The application of two parameterizations of water partitioning between plagioclase and silicate melt allows us to use our water in plagioclase measurements to estimate equilibrium melt water contents of 5 ± 1 – 11 ± 2 wt% H2O, which are in good agreement with the water contents we measured in melt inclusions. Results of amphibole geobarometry are consistent with a magma reservoir stored in the mid-to-lower crust at a modal pressure of 700 ± 250 MPa, corresponding to a depth of ~25 km. Minor crystallization in the shallow crust is also recorded by amphibole barometry and calculated entrapment pressures in melt inclusions. Amphibole is present as unzoned and zoned crystals. Two populations of unzoned amphibole crystals are present, the most abundant indicate crystallization conditions of 853 ± 26 °C (1 se; standard error), and the less abundant crystallized at an average temperature of 944 ± 24 °C (1 se). Approximately 18% of the amphibole crystals are normally or reversely zoned, providing evidence for a minor recharge event that could have been the trigger mechanism for the explosive eruption. Plagioclase crystals also show normal and reverse zoning. The moderate Ni concentrations (<1600 μg/g) in the high-Fo olivine xenocrysts suggest that Cerro Machín primary magmas are generated by inefficient interaction of mantle peridotite with a high-silica melt produced by slab melting of basaltic material. Some sediment input is also suggested by the high Pb/Th (>2.2), Th/La (0.3 – 0.4), and low La/Th (<13; relative to mantle array) ratios. Whole rock chemistry reveals heavy rare earth element (HREE) depletion and Sr enrichment that likely formed during the crystallization of garnet and amphibole in the upper part of the mantle or lower portion of the crust, promoting the formation of water-rich dacitic magma that was then injected into the middle-to-lower crust. Textural and compositional differences in the crystal cargo that erupted during dome-building and plinian events support the idea that large volumes of magma recharge lead to effusive eruptions, while only small recharge events are needed to trigger plinian eruptions at Cerro Machín.

Corundum-quartz metastability: the role of silicon diffusion in corundum

The synthesis of the Al2SiO5 polymorphs kyanite, sillimanite and andalusite in a pure Al2O3–SiO2–H2O (ASH) system has long been known to be impeded. In order to decipher individual aspects of the reaction: corundum + SiO2aq, which repeatedly fails to produce thermodynamically stable Al2SiO5, we conducted experiments within the stability fields of kyanite and sillimanite (500–800 ℃; 0.2–1 GPa) with the aim of forming reaction coronas on corundum. Results showed that metastable corundum + quartz assemblages form persistently in pure ASH, even in Al2SiO5 seeded experiments, despite the presence of catalyzing fluid and evidence of fast reaction kinetics. Coronas on corundum spontaneously formed when additional components (Na, K, N, and Mg) were added to the experiment. In a similar experiment with baddeleyite (ZrO2) instead of corundum in silica saturated water, a zircon corona formed readily. This implies that nucleation and growth of Al2SiO5 is obstructed under conditions of Al and Si saturation in aqueous fluid, while both corundum and quartz saturated aqueous fluid are willing participants in other reactions towards stable corona formation. Instead of Al2SiO5 precipitation, an unexpected fluid-aided silica diffusion process into corundum was documented. The latter included the formation of nanometer wide hydrous silicate layers along the basal plane of the corundum host, which enhanced the silica diffusion rate drastically, leading to silica supersaturation in the host mineral, and ultimately to precipitation of quartz inside corundum. We conclude that the natural metastable assemblage of quartz and corundum is not necessarily the result of dry or fluid absent conditions, given that the aqueous fluid in experiments does not promote Al2SiO5 formation, but rather seems to support the formation and preservation of a metastable assemblage.

SIMS U-Pb dating of baddeleyite-zircon pairs in mafic rocks to determine the ages of magmatic mineralization and hydrothermal remobilization in a Ni-Cu-(PGE) deposit

Ni-Cu-(PGE) sulfide deposits associated with mafic–ultramafic rocks are the most important sources of Ni and platinum-group elements (PGE) in the world. These deposits also contain significant amounts of Cu. Such deposits are generally considered to have formed by multiple stages of primary magmatic concentration and secondary hydrothermal remobilization. However, constraining the ages of Ni-Cu-(PGE) sulfide deposits has proven challenging, particularly those that have experienced post-magmatic hydrothermal overprinting and remobilization. The lack of robust geochronology data has hindered understanding of the geological controls on this important type of mineralization. The Chibaisong Ni-Cu-(PGE) sulfide deposit in the North China Craton is characterized by disseminated and net-textured ores formed by magmatic processes and overprinted by hydrothermal Cu-rich vein stockwork. The Cu-rich vein stockwork is fracture-controlled, and contains a mineral assemblage of Ni-Cu sulfides (chalcopyrite, pyrrhotite, pentlandite), amphibole and quartz. Chalcopyrite in the hydrothermal stockwork is relatively depleted in Ni but enriched in Ag and Cd compared with disseminated magmatic chalcopyrite, typical of hydrothermal superimposed ores as documented elsewhere. We identified baddeleyite-zircon pairs associated with Ni-Cu sulfides in hydrothermal superimposed ores of the Chibaisong deposit, in which the baddeleyite is unequivocally of primary igneous origin whereas the zircon represents the replacement product of baddeleyite during hydrothermal alteration. In situ SIMS U-Pb dating of baddeleyite and zircon yielded weighted mean 207Pb/206Pb ages of 2181 ± 21 Ma and 1892 ± 33 Ma, respectively, interpreted as the timing of magmatic mineralization and subsequent hydrothermal remobilization of the Ni-Cu-(PGE) deposit. Our results explain the large discrepancies in sulfide Re-Os ages previously obtained from this deposit. This study demonstrates that on the basis of detailed BSE imaging, in situ SIMS U-Pb dating of baddeleyite-zircon pairs can provide robust age constraints for both magmatic mineralization and hydrothermal remobilization of Ni-Cu-(PGE) sulfide deposits hosted in mafic–ultramafic intrusions, although further studies are needed to test whether the formation of such deposits commonly entails multiple stages of hydrothermal remobilization. Since baddeleyite or baddeleyite-zircon pairs are common accessory minerals in Ni-Cu-(PGE)-bearing mafic–ultramafic rocks, the dating approach employed in this study may be applicable to similar deposits elsewhere.

Shallow storage of the explosive Earthquake Flat Pyroclastics magma body, Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand: evidence from phase-equilibria experiments

Rhyolitic tuffs range widely in their crystal contents from nearly aphyric to crystal-rich, and their crystal cargoes inform concepts of upper crustal magma reservoirs. The Earthquake Flat pyroclastics (Okataina Volcanic Center, Taupo Volcanic Zone, New Zealand) are 10 km3 of rhyolitic tuffs with abundant (~ 40 vol.%) plagioclase and quartz, minor biotite, hornblende, and orthopyroxene, and accessory Fe-Ti oxides, apatite, and zircon, set in high-silica rhyolitic glass. Major minerals form large, euhedral phenocrysts and abundant glomerocrysts with few disequilibrium textures excepting some faintly resorbed quartz. Plagioclase phenocrysts have thick rims of nearly constant composition near An30, and hornblende is weakly zoned or unzoned. The abundant and texturally complex mineral assemblage contrasts with the nearby (~ 25 km), nearly synchronous, but more voluminous and crystal-moderate rhyolite tuffs from Rotoiti caldera. New H2O-saturated phase-equilibria results on the erupted Earthquake Flat melt (glass) determine its co-saturation with the partial phenocryst assemblage of plagioclase, quartz, biotite, and Fe-Ti oxides at: 140 MPa, 755 ºC. These closely approximate the conditions of the pre-eruptive magma body assuming it was saturated with nearly pure H2O and at an fO2 of ~ Ni–NiO. Absence of hornblende and orthopyroxene from the synthesized assemblages may result from those minerals being in a peritectic reaction relation with melt to produce biotite, so they would not grow from the liquid used as starting material. Experimental results on Rotoiti rhyolite (Nicholls et al. 1992) show that the two bodies resided at similar pressures, temperatures, and fO2s. Lower crystal abundance of the Rotoiti tuffs may result from slight compositional differences. We interpret that the Earthquake Flat pyroclastics were sourced from the crystal-rich periphery of a mushy reservoir system with the Rotoiti occupying a more melt-rich central location. Uncertain is whether this was a single intrusion zoned continuously in crystallinity, or discrete adjacent intrusions, but our results illustrate and quantify complexities of magma storage across relatively short distances.

Origin of Early Triassic Hornblende Gabbro from the Yunkai Massif, South China: Constraints from Mineral and Bulk-Rock Geochemistry

The early Triassic (~250 Ma) hornblende gabbro from the Tengxian area of Yunkai Massif, South China, contains a mineral assemblage of clinopyroxene, hornblende, biotite, plagioclase, K-feldspar and quartz and accessory apatite, and zircon and ilmenite. Based on mineral association and crystallization sequence, two generations of the mineral assemblage have been identified: clinopyroxene + plagioclase + apatite (zircon) in Generation I and ilmenite + hornblende + biotite + K-feldspar + quartz in Generation II. The high crystallization temperature (T = 999–1069 °C) of clinopyroxene and its coexistence with labradorite (An = 52–58) indicate that Generation I crystallized in a basaltic magma, while the hornblende’s relatively low crystallization temperature (T = 780–820 °C) and coexistence with K-feldspar and quartz suggest that Generation II formed in an evolved alkaline melt. The mineralogical records are likely attributed to pulsed intrusion of the late-stage evolved magma into a crystal mush, like in Generation I. The bulk-rock geochemical data include a sub-alkaline affinity, arc-type trace element features, and highly enriched Sr-Nd-Pb-Hf isotopic compositions, consistent with the isotopic records from the accessory minerals, e.g., the very high δ18O values in both zircon and apatite and significantly negative εHf(t) in zircon. The combined mineral and bulk-rock geochemical data suggest that the primary magma for the Tengxian hornblende gabbro was derived from a mantle wedge that had been metasomatized by voluminous subducted terrigenous sediment-derived melts in response to subduction of the Paleo-Tethys Ocean.

Two episodes of mineralization in the Baolun high-grade lode gold deposit, Hainan Island, South China: Insight from LA-ICP-MS analysis of hydrothermal rutile

The Baolun is the largest gold deposit on Hainan Island, South China, with proven reserves of over 80 t Au at an average grade of 10.3 g/t. Two economic types of gold mineralization coexist in the long-lived fault system: 1) Au-bearing sulfide quartz veins (Va), dominated by pyrite, pyrrhotite, and arsenopyrite; and 2) Au-Bi assemblage quartz veins (Vb), dominated by bismuthinite, maldonite, and native bismuth. The Baolun district has a long history of tectonic reactivation, involving crustal compression and extension, magmatic activity, multiple episodes of fluid flow, and hydrothermal mineralization. Consequently, there exists no generally accepted model for the precise timing and source of the Baolun gold deposits. In this study, two different rutile grains (Rt-1 and Rt-2) were recognized separately in the Va veins and Vb veins based on paragenetic sequence. In situ U-Pb dating indicates that Rt-1 and Rt-2 yielded two group ages of 248.3 ± 2.8 Ma (MSWD = 0.8) and 78.2 ± 8.4 Ma (MSWD = 1.4), respectively. The Rt-1 contains 632.0–6051.2 ppm W, 459.1–2169.1 ppm Zr, 1215.0–8267.5 ppm Nb, 107.3–1229.6 ppm Ta, 19.0–79.2 ppm Hf, 160.2–311.8 ppm Sn, 1371.6–3500.7 ppm V, 325.3–3813.9 ppm Cr, 0.6–22.7 ppm Sb, 0.1–38.8 ppm Y, with Ca below the detection limit. The Rt-2 contains 704.9–4390.2 ppm W, 1.8–168.5 ppm Zr, 1264.6–4062.9 ppm Nb, 108.1–453.2 ppm Ta, 0.3–6.9 ppm Hf, 6.5–32.1 ppm Sn, 14.3–311.0 ppm V, 29.4–167.1 ppm Cr, 0.6–27.3 ppm Sb, 56.3–38.8 ppm Y, and 548.7–2750.3 ppm Ca. Obviously, LA-ICP-MS trace element analysis revealed significant discrepancies in composition between Rt-1 and Rt-2, suggesting that they precipitated from distinct fluid systems. Combining mineral studies, field observations, structural analysis, and geologic evolution, the results of this study indicate that the Baolun gold deposit formed during two episodes of mineralization. The Early Triassic gold mineralization is clearly classified as an orogenic type. The ore-forming materials were liberated by the deformation and metamorphism during the Indosinian orogeny of South China, which was triggered by the closure of the Paleotethys Ocean. The Late Cretaceous Au-Bi mineralization belongs to a magmatic-hydrothermal origin, which was deposited under an extension setting associated with the post-subduction of the paleo-Pacific plate. The new mineralization model proposed provides significant implications for future exploration strategies.

U–Pb LA-ICP-MS and Re–Os dating of wolframite and molybdenite: Constraints on multiple mineralization and cooling history (40Ar/39Ar) for the magmatic–hydrothermal system at Borralha, northern Portugal

The Borralha area (Iberian Massif, Portugal) reflects a long-lived and multiphase magmatism (ca. 316 to 300 Ma), characterized by three types of granite emplacement during two different extensional events (E1 and E2) and one compressional event (C3) within the Variscan orogeny. The W (Cu) mineralization styles correspond to veinlets and quartz veins (i.e., São António and Santa Helena), and disseminations in the Santa Helena breccia pipe. Molybdenite coexists with an Fe-bearing wolframite–quartz assemblage in the Venise breccia pipe and quartz veins (i.e., São José and São Paulo).The geochemistry and geochronology of wolframite and molybdenite samples were studied by electron probe microanalysis (EPMA), in-situ U–Pb laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) and Re–Os thermal ionization mass spectrometry (TIMS). The wolframite yields a U–Pb age (sample WBV1) of 315.2 ± 4.7 Ma (2σ), representing the first recognized episode of W deposition in the Iberian Massif. Wolframite samples (WBV1, WB1, and WB4) have a ferberite composition with a hübnerite/ferberite ratio of < 29. High amounts of Nb (up to 3703 ppm) with low Ta (up to 123 ppm) and REEs (up to 11.4 ppm) and conversely, low amounts of Nb (up to 471 ppm) with high Ta (up to 190 ppm) and REEs (up to 53.7 ppm) were measured in wolframite samples. The REEs normalized patterns of the wolframite samples record a mixed signature. Large and positive Eu/Eu* and Er/Er* anomalies were identified (sample WBV1), whereas negative Ce/Ce* and Nd/Nd* anomalies were observed in samples WB1 and WB4.Molybdenite Re–Os dating yields absolute ages ranging from 305.1 to 303.8 ± 2.2 Ma (2σ), which are coeval with the emplacement of E2 biotite granite (Penedos; I-type; <305 Ma). Low amounts of Re (<1 ppm), W, Co, Ni, As, Cu, Bi, Fe, Mn V and Se (<0.005 wt%) were detected in the molybdenite structure.Constraints between the timing of W- and Mo-deposition and a protracted cooling history of the host rocks are confirmed by 40Ar/39Ar white mica dating. The 40Ar/39Ar ages obtained for secondary white micas collected from Mo veins (Venise breccia pipe; 277 ± 3 and 287 ± 4 Ma 2σ) and W (Cu) dissemination (Santa Helena breccia pipe; 287 ± 2 and 279.0 ± 6 Ma 2σ) record the isotopic resetting by later hydrothermal overprints and cooling /exhumation history after E2 Gerês biotitic granite emplacement.The W-ore deposition did occurred during the final E1 extensional tectonic stage (315 Ma) and E2 extensional tectonic stage for Mo-ore deposition, where a temporal offset was needed for the adjustment of tectonic conditions after the C3 compressional and E2 extensional stage started.

U−Pb geochronology and microstructural analysis of apatite within a Proterozoic crustal-scale shear zone, Outer Hebrides, NW Scotland

U–Pb apatite geochronology is increasingly recognized as a valuable tool for constraining the age of mid-crustal ductile shear zones. The crustal-scale Outer Hebrides Fault Zone (OHFZ) within the Laurentian foreland of the Scottish Caledonides has long been of uncertain age and tectonic significance. Earliest deformation within the OHFZ was associated with top-to-the-NW ductile thrusting that formed a belt of greenschist facies mylonites within host Archean−Paleoproterozoic basement gneisses. Previous estimates for the timing of thrusting vary between c . 1600 Ma and c . 430 Ma. The mylonitic fabrics are defined by a recrystallized assemblage of quartz + albite/oligoclase + sericite + actinolite + epidote + apatite ± calcite, consistent with deformation temperatures of 400–500°C and within the range of reported closure temperatures for Pb diffusion in apatite. U–Pb (LA−ICP−MS) dating of two texturally distinct apatite grain types within the mylonites has yielded ages mostly in the range c . 1100–900 Ma. The OHFZ is therefore interpreted as a Grenville–Sveconorwegian structure that formed during the tripartite collision of Laurentia, Baltica, and Amazonia and the assembly of Rodinia. Supplementary material : U–Pb isotopic data (Table S1), trace element data (Table S2) and laser ablation spot images are available at https://doi.org/10.6084/m9.figshare.c.7084925

P–T conditions of metamorphic and hydrothermal events at Tick Hill gold deposit, Mount Isa, Queensland, Australia

Tick Hill gold deposit is a unique gold mineralisation style in the Mary Kathleen Fold Belt in the Mount Isa Inlier. The gold at Tick Hill is generally pure without silver and was formed during two discrete metamorphic-deformation events (D1 and D3). Early gold was observed as inclusions or coarse grains hosted within D1 peak-metamorphic diopside, scapolite and hornblende from the garnet–biotite–hornblende (tschemakite)–plagioclase (andesine)–quartz assemblage. Late gold is closely associated with bismuth selenide, chlorite, albite, sericite and K-feldspar, and formed during D3. The syn-D1 garnet–plagioclase–hornblende–quartz–biotite assemblage was used to constrain the pressure and temperature (P–T) conditions of metamorphism and mineralisation using the garnet–plagioclase–hornblende–quartz barometer and the hornblende–plagioclase, garnet–biotite and garnet–hornblende thermometers. The results show that peak metamorphism at Tick Hill reached P–T conditions of 6.0–7.6 kbar and 720–760 °C. These P–T conditions together with gneissic and migmatisation textures recorded in different rock types at the Tick Hill area indicate that the peak metamorphism preserved in the area occurred at the amphibolite–granulite facies, compared with the amphibolite facies at the southern Mary Kathleen and Eastern Fold Belt. The D3 chlorite, which formed during stage 2 to stage 4 mineralisation events, displays a wide range of compositions reflecting a gradual retrograde temperature change from ∼380 °C to ∼130 °C. The pressures during D3 could not be reliably determined, but the presence of various Bi-selenides suggests that towards the waning stages of D3, the rocks may have been exhumed to a pressure less than 1 kbar.

Metamorphosed Plagiogranite Veins In Salma Eclogites, Belomorian Eclogite Province

The Salma subduction eclogites of the Meso-Neoarchean Belomorian Eclogite Province have compositions of N-MORB oceanic crust protoliths and contain felsic veins of plagiogranite compositions. Felsic veins consist of metamorphic minerals sush as garnet, quartz and kyanite, the earliest high-pressure inclusion of phengite and zoisite in garnet. Veins have geochemistry characteristics that are typical for island arc granites or adakites. They are compositionally comparable with eclogite-bearing tonalite-trondhjemite-granodiorite (TTG) gneisses, modern adakite, and Archaean andesite association of the Vedlozero–Segozero greenstone belt. The origin of these felsic veins can be represented as a product of melting of fluid-modified oceanic mafic slab at PT-conditions of eclogite metamorphism.Geothermometry and pseudosection calculations of metamorphic conditions in felsic veins, for eclogitic mineral assemblage of pyropic garnet, kyanite, high-silica phengite and quartz gave high pressures conditions (ca. 16–22 kbar) at relatively low temperatures (ca. 600–650 °C); for secondary decompression plagioclase ± biotite coronas between quartz, kyanite and garnet gave 14–11 kbar at 650–700 °C. This estimate corresponds “warm” subduction, which is most consistent with the parameters of the modern subduction of the base of the Cascadia plate . Mineral assemblages in eclogites, that included the felsic veins, give 15–11 kbar at 750–700 °C. Thermodynamic modelling indicates that the eclogites hosting the felsic veins did not reach partial melting conditions under the estimated PT conditions: exposure temperatures to the mafic rocks must have been significantly higher. A probable explanation for high-pressure parageneses in felsic veins may be the formation of plagiogranites as a result of the melting of the mafic slab under higher temperature and pressure conditions than were measured in this work. Thus, the high-pressure paragenesis of garnet, phengite, zoisite, and kyanite reflects the path of rock exhumation through eclogitic conditions.Zircon from the felsic veins gave several Archean and Paleoproterozoic ages ∼ 2.87, 2.78, ∼2.70 and, ∼1.9 Ga. The oldest age gave the fluid-modified oceanic zircon (∼2.9 Ga) from Fe-Ti eclogites directly near the veins. The oldest igneous zircons in the veins are represented by a population with the age of ∼ 2.87 Ga and correspond to the igneous age of some TTG gneisses, which do not contradict a hypothesis of the plagiogranite formation during the melting of the mafic slab in the subduction zone. A Neoarchean zircon population of ∼ 2.78 Ga is often present in eclogitized mafic rocks and surrounding TTG gneisses which can be associated both with the formation of TTG gneiss protoliths in the subduction zone (?) and with the recrystallization of older zircon as a result of fluid or thermal impact. The ɛHf of the older population (∼2.87 Ga) is + 3 to + 4, indicating a juvenile origin; the ∼ 2.78 Ga population has similar Hf-isotope compositions, suggesting that their ages reflect resetting (non-zero Pb loss) of the older zircons. The Sm-Nd model TDM ages of Grt-Ky veins have Meso-Neoarchean values (3.0–2.75 Ga) with positive εNd = 1.8–3.8 that may indicate a juvenile nature of sources of the plagiogranite melt. The eclogitized rocks of the Belomorian province generally underwent the granulite facies overprint at ∼ 2.74–2.64 Ga ago that is coincides with the age of granulite facies metamorphism of regional and global rank. The Paleoproterozoic evolution of the Salma eclogites and the East European Craton as a whole: at 2.5–2.4 Ga and 2.0–1.9 Ga with formation of the Lapland–Mid-Russia–South Baltia intracontinental and Svecofennian accretionary orogens was terminated in Paleoproterozoic time at ∼ 1.87–1.7 Ga. These age values correspond to closure of Pb-Pb and Sm-Nd isotopic systems of rock-forming metamorphic minerals of the felsic veins.

Age of Carlin-type gold mineralization in the Youjiang basin, South China: Constraint from hydrothermal zircon geochronology in the Badu dolerite-hosted gold deposit

Selecting a robust chronometer is crucial to determine the primary mineralization age of the deposits that have witnessed multiple hydrothermal events. The Youjiang basin of South China hosts the world’s second-largest Carlin-type gold province. However, the age of the Carlin-type gold mineralization remains highly debated, partly due to the presence of multiple hydrothermal events that may have disturbed the primary isotopic system with low durability. Hydrothermal zircon is a robust chronometer that can survive multiple hydrothermal events. To this end, in this study, we report in-situ secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb dating results of hydrothermal zircon identified in carbonate–sericite–sulfide–altered dolerite from the Badu Carlin-type gold deposit to constrain the timing of gold mineralization.The hydrothermal zircons exhibit subhedral to anhedral morphologies, sector cathodoluminescence zonings, and contain gold-related mineral assemblages of arsenian pyrite, quartz, sericite, and carbonate. Primary two-phase fluid inclusions also commonly occur in the hydrothermal zircons. Compositionally, these zircons feature relatively flat light rare-earth element (REE) patterns, high La contents, low (Sm/La)N values, and small Eu and Ce anomalies, which are different from the magmatic counterparts of the Badu pluton but consistent with those of hydrothermal zircons reported worldwide. These features indicate that the hydrothermal zircons are probably paragenetically associated with the gold mineralization. Consequently, the U–Pb ages of ca. 140 Ma of these zircons, which are much younger than those of the magmatic zircon in the fresh dolerite, can represent the timing of primary gold mineralization of Badu. Our new data suggests that there truly was an episode of gold mineralization during the Early Cretaceous in the Youjiang basin, which may have been driven by large-scale lithospheric extension in the interior of South China. This study demonstrates that hydrothermal zircon could be a robust clock for investigating the timing of magmatic rock-hosted Carlin-type gold deposits.

Metallogenic model of the Eocene Santa María and Antares Zn-Pb(-Ag) skarn deposits, Velardeña Mining District, Durango, Mexico

The Santa María and Antares Zn-Pb(-Ag) skarn deposits in the Velardeña Mining District are located in central–NW Mexico. They lie 470 m apart along the contact between Oligocene felsic intrusions and Cretaceous limestones, and were developed during prograde, retrograde, post-ore (Santa María), and late stages. Firstly, the prograde stage was formed by fluids at ~ 600 °C and 15 wt% NaCl equiv., and consists of garnet + wollastonite ± clinopyroxene and biotite ± K-feldspar assemblages. Secondly, the retrograde/ore stage was formed by fluids at 300–500 °C with salinities of 20–30 wt% CaCl2 (Santa María) and > 40 wt% NaCl equiv. (Antares). It comprises assemblages of chlorite, amphibole, epidote, calcite, scapolite, quartz, sericite, adularia, fluorite, and muscovite associated with sphalerite, pyrite, galena, pyrrhotite, arsenopyrite, chalcopyrite, and Pb-Bi-Sb sulfosalts. Thirdly, the post-ore stage was formed by fluids at ~ 400 °C and 20–30 wt.% CaCl2 and comprises poorly mineralized calcite veins. Fourthly, the late stage was formed by fluids at < 300 °C and 20–30 wt.% CaCl2 (Santa María) and ~ 15 wt% NaCl equiv. (Antares), and crystallized tetrahedrite-group minerals and pyrite + marcasite. δ18Ofluid between ~ 14‰ and 23‰ at Santa María and between ~ 12‰ and 17‰ at Antares show a less-modified magmatic affinity for mineralizing fluids at Antares; δ13Cfluid between 0‰ and –6‰ register recycling of sedimentary C. Moreover, sulfides with δ34SVCDT between –3‰ and 2‰ reveal a magmatic source for S. Altogether, these data suggest that, at Santa María, magmatic-derived fluids actively interacted with the wall rocks, whereas at Antares the fluid-rock interaction was milder. In both deposits, metal deposition was triggered by the cooling and neutralization of ore-bearing fluids with carbonate rocks. Our 40Ar/39Ar dates for adularia of ca. 37.5 Ma place the deposits within the Eocene–early Miocene metallogenetic epoch of central–NW Mexico, during which other world-class skarn-epithermal systems were emplaced (e.g., Concepción del Oro and Mazapil-Peñasquito).

Non-Ceramic Site of Shamanka 8 on the Southern Coast of Baikal Lake

Purpose. The southern coast of lake Baikal is a territory known in Siberian archaeology for its Kitoi cemeteries. Besides burials of the Shaman capes there are some Neolithic and Bronze age complexes at Circum–Baikal railway. In 2012 it the first non-ceramic site was discovered in South Baikal, which was named Shamanka 8. This paper presents the assemblage of this complex and discusses its chronological and cultural features.Results. The Shamanka 8 site is located on the top of the third hill of the Shaman cape. The cultural layer lies under the Holocene Optimum sediments. The site stratigraphic structure is characterized by compression. There is no bone fragments and ceramic. For lithic knapping, mainly local raw material (quartz) were used. The quartz assemblage has such features as poor formal standardization, high degree of flake fragmentation, cores with the orientation of fracture plane relative to the longitudinal axis, bipolar reduction. All of these specifics are explained by uneven fracture characteristics and bad workability of quartz, as well as its breakage patterns. Some artifacts, including single non-quartz tool, suggest that there may be a connection between the Shamanka 8 site and the industries of the Final Paleolithic sites in the Northern Baikal region and the Irkutsk region. Quartz small knife (?), carinated end-scraper and tubular core have analogies in the 2nd cultural layer of the Kurla II site, dated by 13.5 uncal kya. Similarly dated quartz components of Nirikan I and flint tool-kit of Verkholenskaya Gora I include other cultural link types.Conclusion. The similarity in the morphology of some tools with Final Paleolithic counterparts and stratigraphic position of finds allow us to assume the Final Sartan age of the Shamanka 8 site and dates it to the Bølling – Allerød warming. However, in the absence of direct radiocarbon dating, an Early Holocene/Early Neolithic attribution of the site is also possible. Further studies may help resolve this issue.

Eocene Kohat Formation suitability as an aggregate from Kohat Range, Sub-Himalayas, Pakistan, based on outcrop, petrographic and geotechnical properties

This paper evaluates the physicochemical and mechanical characteristics of the Limestone of the Kohat Formation. The Eocene Kohat limestone is generally exposed in the Kohat, Hangu, and Bannu districts. Kohat Formation limestone is well exposed throughout the Northern Kohat fold and thrust belt at Togh Bala village. It is cream to grey, hard, compact, well-bedded and fossiliferous. Kohat Formation has been identified into two lithofacies which are from base to top calcareous shale lithofacies with large benthic foraminifera and limestone lithofacies. The faunal assemblage indicates that the Kohat Formation was deposited in an inner to middle shelf environment. The petrographic analyses of samples of the Kohat Formation depict that it principally consists of calcite (60-90%) with a small to trace assemblage of dolomite (0-3%), ungraded quartz (0-1.5%), opaque minerals (0-1.2%) and fossils (8-40%). Aggregate physical properties like water absorption (0.88%), specific gravity (2.69), soundness (2.55%), Los Angeles abrasion value (23.06%), aggregate crushing value (11.56%), aggregate impact value (12.68%), flaky index (9.90%), elongation index (12.76%) and coating and stripping values (96.27%) conform to available standards. Our results indicate that Kohat Limestone is safe in terms of Alkali Carbonate Reactivity (ACR) and Alkali-Silica Reactivity (ASR). The facies, thin section study, geochemical, and geotechnical understanding of Kohat Formation indicate that these aggregate deposits are potential aggregate sources for ballast (railway), road, building material, concrete, mortar and other mega projects for connecting Asia and Europe.

The trace-element compositions of amphibole, magnetite and ilmenite as potential exploration guides to metamorphosed Proterozoic Cu–Zn±Pb±Au±Ag volcanogenic massive sulfide deposits in Colorado, USA

AbstractOrthoamphibole, clinoamphibole and magnetite are common minerals in altered rocks associated spatially with Palaeoproterozoic volcanogenic massive sulfide (VMS) deposits in Colorado, USA and metamorphosed to the amphibolite facies. These altered rocks are dominated by the assemblage orthoamphibole (anthophyllite/gedrite)–cordierite–magnetite±gahnite±sulfides. Magnetite also occurs in granitoids, banded iron formations, quartz garnetite, and in metallic mineralisation consisting of semi-massive pyrite, pyrrhotite, chalcopyrite, and sphalerite with subordinate galena, gahnite and magnetite; amphibole also occurs in amphibolite. The precursor to the anthophyllite/gedrite–cordierite assemblages was probably the assemblage quartz–chlorite formed from hydrothermal ore-bearing fluids (~250° to 400°C) associated with the formation of metallic minerals in the massive sulfide deposits.Element–element variation diagrams for amphibole, magnetite and ilmenite based on LA-ICP-MS data and Principal Component Analysis (PCA) for orthoamphiboles and magnetite show a broad range of compositions which are primarily dependent upon the nature of the host rock associated spatially with the deposits. Although discrimination plots of Al/(Zn+Ca) vs Cu/(Si+Ca) and Sn/Ga vs Al/Co for magnetite do not indicate a VMS origin, the concentration of Al+Mn together with Ti+V and Sn vs Ti support a hydrothermal rather than a magmatic origin for magnetite. Principal Component Analyses also show that magnetite and orthoamphibole in metamorphosed altered rocks and sulfide zones have distinctive eigenvalues that allow them to be used as prospective pathfinders for VMS deposits in Colorado. This, in conjunction with the contents of Zn and Al in magnetite, Zn and Pb in amphibole, ilmenite and magnetite, the Cu content of orthoamphibole and ilmenite, and possibly the Ga and Sn concentrations of magnetite constitute effective exploration vectors.

Contrasting Styles of Lower Crustal Metamorphism from a Granulite Suite of Rocks from Angul, Eastern Ghats Belt, India: Implications for the India–Antarctica Correlation

AbstractThe present work is focussed on a suite of high-grade rocks including mafic granulite, aluminous granulite, khondalite, charnockite, and augen gneiss along with medium-grade rocks like olivine-bearing metanorite, gabbro, and porphyritic granite of the Angul domain at the northern margin of the Proterozoic Eastern Ghats Province (EGP). Based on the petrological and geothermobarometric data, two distinct metamorphic events have been identified. The imprints of the earlier event (MA1) are preserved in the mafic granulite, aluminous granulite, khondalite, augen gneiss, and fine-grained charnockite, but those are best preserved in mafic granulite and aluminous granulite. In mafic granulite, orthopyroxene + clinopyroxene + plagioclase ± garnet+ ilmenite ± quartz assemblage was stabilised at the peak MA1 conditions, whereas the peak MA1 assemblage is represented by Fe3+-garnet + hematite + magnetite + cordierite + K-feldspar + plagioclase + sillimanite + quartz + melt in aluminous granulite. Phase equilibria modelling and thermobarometric data suggest the P–T conditions of &amp;gt;850°C, 7 to 8 kbar for this event. The retrograde metamorphism (MA1R) involved minor decompression (down to ~5 kbar) and subsequent cooling to form biotite- and hornblende-bearing mineral assemblages in aluminous granulite and mafic granulite, respectively. Texturally constrained monazite (U–Th–total Pb) and zircon (U–Pb) data from the former rock suggest ca. 1200 Ma age of the MA1 metamorphism, which was associated with granitic and charnockitic magmatism as constrained from oscillatory-zoned zircon domains in the augen gneiss and fine-grained charnockite. The rock ensemble was affected by a younger metamorphic event (MA2), which is texturally characterised by partial replacement of hornblende (developed during MA1R) to orthopyroxene ± clinopyroxene + plagioclase ± ilmenite + melt assemblage in mafic granulite. Moreover, biotite of aluminous granulite has undergone dehydration melting to produce garnet + cordierite-bearing assemblage. Garnet in the above assemblage did not form as porphyroblastic phase and overgrew the MA1 garnet. The MA2 event followed a counterclockwise P–T trajectory, causing heating (up to 800°C) with associated loading (from 4.0 to 5.8 kbar) along the prograde path. Monazite U–Th–total Pb data from aluminous granulite and khondalite suggest MA2 ages of 987 ± 12 and 975 ± 16 Ma, respectively. Fine-grained charnockite and augen gneiss also recorded the imprints of MA2 event by developing thin to thick sector-zoned overgrowth yielding group ages of 979 ± 12 and 982 ± 29 Ma, respectively. Zircon overgrowth in mafic granulite formed at 962 ± 13 Ma. The MA2 event coincides with the crystallisation of coarse-grained charnockite at 983 ± 22 Ma and porphyritic granite at 960 ± 10 Ma. Geochronological data, thus, indicate that the Angul domain underwent the MA2 metamorphism and associated magmatism at ca. 990 to 960 Ma. The apparent absence of MA1 event (~1200 Ma) in the greater part of the EGP and its dominance in the Angul domain suggest that the latter was possibly an exotic block at ca. 1200 Ma and joined with the rest of the EGP only after ca. 960 Ma. We further suggest that the metamorphic history of the Angul domain is strikingly different from the rest of the EGP, but matches well with that of the Prydz Bay region of the East Antarctica.

Effects of Metasomatism on Granite-Related Mineral Systems: A Boron-Rich Open Greisen System in the Highiş Granitoids (Apuseni Mountains, Romania)

Greisenization is typically linked with highly fractionated granites and is often associated with hydrothermal vein systems. Late to postmagmatic metasomatic processes involve the enrichment of volatile components such as boron and halogens as well as several metallic elements. The purpose of this study is to reveal the main metasomatic effects and paragenetic sequences of the related mineralizations in Highiş granitoids, Romania. In a natural outcrop, more than 30 samples were collected from granitoids, felsic veins, and country rocks. We carried out a detailed mineralogical and petrological characterization of carefully selected samples using X-ray powder diffractometry, electron microprobe analysis, and microscopic methods together with K–Ar ages of whole rocks and K-bearing minerals. Several characteristic features of albitization, sericitization, tourmalinization, epidotization, and hematitization were recognized in the studied samples. Crystallization of quartz, K-feldspar, and magnetite represents the first stage during the magmatic-hydrothermal transition. The mineral assemblage of albite, sericite, schorl, and quartz originates from the early and main stages of greisenization. While the subsequent mineral assemblages, which predominantly include dravite, specular hematite, and epidote, are closely related to the late vein-depositing stage. We propose that the study area could belong to a boron-rich open greisen system in the apical portion of Guadalupian A-type granite. Based on a new hypothesis, the previously published Permian crystallization ages (between ~272 Ma and ~259 Ma) could be homogenized and/or partially rejuvenated during the hydrothermal mineralization processes due to uraniferous vein minerals. Additionally, the Highiș granite-related system suffered a Cretaceous thermal overprint (between ~100 Ma and ~96 Ma). The results may help to understand the evolution of highly evolved granite intrusions worldwide and improve our knowledge of the effect of hydrothermal mineralization processes on the emplacement ages.

Petrogenesis of the Weiling beryl-bearing granitic pegmatite – A giant LCT-type pegmatite in the Northern Wuyi area, South China

The Weiling granitic pegmatite is a super-large (ca. 9.20 million tons) LCT-type pegmatite body in the Northern Wuyi area of South China, with the typical mineral assemblage quartz, plagioclase, K-feldspar and muscovite as well as tourmaline (schorl-dravite), garnet (almandine-spessartine) and beryl. This overall unzoned pegmatite with locally layered texture has economic amounts of the rare metal mineral beryl and ultrapure quartz, and porcelain clay in its uppermost weathering parts. The 40Ar/39Ar dating of pegmatitic muscovite yielded a precise age of 405.33 ± 3.38 Ma, indicating this beryl-bearing pegmatite body was formed in the Early Devonian rather than in the Jurassic-Cretaceous as previously thought. Boron and Nd isotope data demonstrate that the pegmatitic melts have solely crustal sources and are most likely derived from partial melting of the ambient metasedimentary rocks from the Zhoutan Formation. Geochemical characteristics of pegmatitic minerals muscovite, beryl, tourmaline, garnet and K-feldspar and the absence of coeval fertile granites around this pegmatite suggest that the Weiling pegmatite most likely evolved from anatectic melts that did not experience significant fractionation. The pegmatitic melts were generated by low degrees of muscovite dehydration partial melting of local metasedimentary rocks rich in fluxing elements and rare metals. These melts experienced limited differentiation during ascent in the crust. The beryl mineralization formed in the medium-temperature (222–357 ℃), low-salinity (3.3–10.9 wt% NaCl equiv.) and high-density (0.62–0.86 g/cm3) H2O-NaCl-KCl-CO2-N2 system. These temperatures are much lower than typical granitic pegmatites, possibly reflecting late fluids derived from the pegmatitic magma and experienced post-crystallization alteration. The Weiling pegmatite represents a Be-bearing pegmatite that formed by partial melting of local metasedimentary rocks.