Negative Europium Anomalies Research Articles

Constraints of magmatic differentiation on epithermal mineralization at Dongan, NE China: Insights from zircon geochronology, elements and Sr–Hf–Nd isotope geochemistry

Low-sulfidation (LS) epithermal deposits formed under relatively low temperature and pressure conditions are generally associated with volcanic activity. To better understand magmatic processes prior to epithermal mineralization, field geology, petrology, UPb zircon geochronology, whole-rock major, and trace element geochemistry along with HfNd isotopic geochemistry of the cogenetic volcanic rocks at Dongan were assessed. The Early Cretaceous (109.8 to 107.0 Ma) volcanic suite consists of high-K calc-alkaline andesitic-rhyolitic tuff and granite porphyry dikes or/and stocks. Arc magma affinity characterizes the suite with enrichments in large-ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depletions in high field intensity elements (HFSEs) and heavy REEs (HREEs). The extrusive andesitic rocks showing enriched mantle features (εNd(t) = + 0.76 to +1.83, εHf = − 3.4 to +3.7, TDM2 = 933 to 1124 Ma) are the result of the hybridization between the mantle and lower crust melts with the contamination of upper crustal materials during eruption or ascending of melts. The intrusive granite porphyry (εNd(t) = +3.81 to +3.9, εHf = −0.3 to +3.7, TDM2 = 901 to 1187 Ma) is geochemically comparable with andesitic rocks and was likely formed by the fractional crystallization of plagioclase and amphibole minerals from cooling andesitic melts at middle–upper crustal levels. Note that the rhyolitic magmas show distinct features of lower εNd(t) (−3.05 to – 2.78), εHf (−0.6 to +1.6) and total REE values (54.78 to 70.09 ppm), older TDM2 (1464 to 1662 Ma) and obvious negative europium anomalies (0.34 to 0.37), suggesting an origin of upper crustal remelting (compositionally equivalent to water-poor felsic granulite or tonalite) that results from the heating of the underlying andesitic magmas. In summary, the sequential emplacement of the ore-related andesitic and granitic melts is attributed to the magmatic processes of the active magma chamber in a continental extensional setting, and the metal-rich bubbles focused at the tops of this chamber due to prolonged differentiation are released to produce epithermal veins at shallower levels (< 1.5 km).

ХАРАКТЕР РАСПРЕДЕЛЕНИЯ РЕДКОЗЕМЕЛЬНЫХ ЭЛЕМЕНТОВ В МЕЗОЗОЙСКИХ ОСАДОЧНЫХ ПОРОДАХ НИЖНЕГО ПРИАМУРЬЯ

The article presents original data on the contents of rare earth elements (REE) and characterizes their distribution in Mesozoic sedimentary rocks of the Lower Amur region. The author compares various parameters of the studied rocks with similar parameters in the standards of the same rocks. The stratigraphic divisions made on the M-54-I sheet of the 1:200,000 scale State Geological Map belong to the Amur (or Chayatyn) subzone (SFPZ) of the West Sikhote-Alin Structural-Formation Zone (SFZ). Among them, the Zhorminsky (K1 zm) strata, Silasinsky (K 1-2 sl) and Utitsky (K2 ut K2) formations are distinguished. The stratons are composed of sandstones and siltstones in different ratios. They also contain conglomerates, tuffs of different composition, and andesites. The REE content in siltstones is higher than in sandstones. Higher concentrations of elements were found in the studied rocks relative to the rocks of the Caucasian Folded System (CC). Compared to the aleuropes of the Russian platform, the sandstones of Priamurye subzone have less REE content, and the siltstones have more. Compared to PAAS (post-Archean Australian shale), the REE content is less. Chondrite normalized REE contents. The trend of the charts is negative. The same behavior of elements is characteristic of the distribution spectra in the studied sandstones, siltstones, RAAS, aleuropesstones of the Russian platform, and rocks of the Caucasian SS. In most of the rocks presented, the LRSE/TRSE is higher than 8, this indicates that the sedimentation basin of detrital material has a predominantly acidic composition. There is a significant negative europium anomaly – from 0.34 to 0.71 in sandstones, and from 0.47 to 0.75 in siltstones. It also indicates the acidic rocks erosion. Consequently, a set of parameters determines the rocks acidic composition in the demolition areas.

Rare-Earth Elements as Natural Tracers for In Situ Remediation of Groundwater.

The utility of rare-earth elements (REEs) as natural geochemical tracers for the analysis of groundwater remediation was examined in several example permeable reactive barriers (PRBs). The PRBs utilize zero-valent iron and organic carbon plus limestone mixtures for contaminant treatment. Zero-valent iron removed REEs from groundwater to below detection levels (2-4 ng/L) and subsequent rebound of REE concentrations in regions down-gradient of the treatment zones was not observed. In addition, REE concentrations within and down-gradient of an organic carbon/limestone PRB were significantly reduced to <1% of influent levels. Thus, REEs are sensitive tracers for evaluating the interaction of groundwater with materials placed in the subsurface for contaminant remediation. Analysis of geochemical tracers for understanding in situ remediation becomes important in situations where down-gradient contaminant concentrations fail to decrease within expected timeframes. The field data indicated that increased solid-phase partitioning of REEs occurred with increasing pH and heavy REEs were preferentially removed compared to light REEs in ZVI systems. In the organic carbon PRB, unexpected negative europium anomalies were observed, revealing new information about redox conditions within the treatment zone. REE concentrations and shale-normalized profiles can be used as natural tracers to better understand in situ technologies for groundwater remediation.

Magmatic volatiles episodically flush oceanic hydrothermal systems as recorded by zoned epidote

Circulation of seawater at oceanic spreading centers extracts heat, drives rock alteration, and transports leached metals to shallower levels of the crust, where they may precipitate and form ore deposits. Crystallization of the lower crust, may exsolve and introduce magmatic volatiles into the seawater-dominant system. However, the role of magmatic volatiles added to the hydrothermal system, including pathways of these fluids are lesser known. Here we present coupled in-situ strontium isotope and rare earth element data of distinct domains in epidote, a common hydrothermal mineral throughout the Troodos ophiolite, to track magmatic fluid input and flow. Epidote crystal domains characterize three distinct strontium isotope-rare earth element signatures—suggesting sequential growth from magma-derived fluids (0.704, negative europium anomalies), rock-buffered fluids (0.7055, positive europium anomalies) and seawater-derived fluids (0.7065, negative cerium anomalies). Epidote records episodic fluxing of magmatic fluids from plagiogranites, through epidosites in the upflow zone and into metal ore deposits.

Geochemistry, zircon U-Pb geochronology and Hf-O isotopes of the Late Mesozoic granitoids from the Xiong'ershan area, East Qinling Orogen, China: Implications for petrogenesis and molybdenum metallogeny

The petrogenesis and genetic relationship of molybdenum mineralization in the Late Mesozoic granitoids from the Xiong’ershan area remains controversial. This study presents integrated whole-rock geochemistry, zircon U-Pb geochronology and Hf-O isotopes for Late Mesozoic ore-bearing granite porphyry stock (Leimengou granite porphyry) and ore-barren plutons (Haoping and Jinshanmiao plutons) in the Xiong’ershan area of the East Qinling Orogen Belt (EQOB). The ore-barren granitoids are high-K subalkaline, metaluminous, and show negative correlations between Al2O3, MgO, TiO2, MnO, CaO, FeOT and P2O5 with SiO2. However, the Leimengou granite porphyry lacks distinctive linear correlation between major oxides with SiO2 except Al2O3, K2O and Na2O. These granitoids are enriched in LREE over HREE with slightly negative europium anomalies. Most samples show enrichment of Rb, Ba, Th, U, K and Pb, and depletion of Nb, Ta, P and Ti, mostly overlapping the ranges of the Taihua Group. Zircon U-Pb ages from the Haoping pluton, Jinshanmiao pluton and Leimengou granite porphyry are 130.9 ± 0.4 Ma, 127.7 ± 0.7 Ma and 131.5 ± 0.9 Ma, respectively. The Hf-O isotopic features of zircon from the Leimengou granite porphyry (δ18Ozircon = 5.03‰ − 5.86‰, εHf(t) = −24.3 to −14.4) are markedly similar with the Haoping pluton (δ18Ozircon = 5.17‰ − 5.61‰ and εHf(t) = −26.6 to −21.4). Zircon grains from the Jinshanmiao pluton have unique δ18Ozircon ranging from 6.01‰ to 8.90‰ and εHf(t) from −15.6 to −12.2. The co-variations of whole-rock geochemistry and Hf-O isotopic features in the samples suggest that Haoping pluton and Leimengou granite porphyry originated from mixture of partial melts of the Taihua Group with mantle-derived material, whereas the Jinshanmiao pluton originated from mixture of Xiong’er Group and mantle-derived material, with minor input of Taihua Group. The zircon grains from the Leimengou granite porphyry have significant higher Ce4+/Ce3+ (mean 547) and δEu (mean 0.65) ratios than those in the ore-barren Haoping pluton (mean 207 and 0.54, Ce4+/Ce3+ and δEu, respectively) and the Jinshanmiao pluton (mean 235 and 0.62, respectively). The main controlling factors for porphyry-type Mo mineralization are likely the oxygen fugacity (fO2) of the magma and input of mantle-derived material in the Xiong'ershan area. The geochemical and isotopic data show that the emplacement of these intrusions in the EQOB occurred in the post-collision settings during 132–127 Ma.

Early Cretaceous bimodal magmatism related epithermal mineralization: A case study of the Gaosongshan gold deposit in the northern Lesser Xing’an Range, NE China

Low-sulfidation (LS) gold mineralization is partially associated with bimodal volcanism in origin. Sources of ore-mineralized fluids and relevant dynamic processes remain poorly understood. To better understand these issues, a study including deposit geology, petrology, zircon U–Pb dating, and element and Sr–Nd–Hf isotope geochemistry of bimodal volcanic rocks at Gaosongshan was conducted. The bimodal volcanic rocks are composed of andesitic rocks (basalt andesite–andesite–trachyandesite–andesitic pyroclastic) of the Meifeng Formation and overlying rhyolite lava of the Fuminhe Formation cut by equivalent porphyry stocks, which contain pyrite-bearing granular-textured quartz cavity with a sugary appearance, with trapped single-phase magmatic fluids. The andesitic suite formed at approximately 106–108 Ma, prior to eruption of the younger rhyolitic magmas (105.6 ± 1.7 Ma; n = 13, MSWD = 1.2). The andesites show calc-alkaline affinities, which is a subduction-related characteristic, and show moderate La/YbN (7.91–11.36) and weakly negative europium anomalies (0.80–1.02), indicating a source in the asthenospheric mantle with varying contamination by both subduction and continental materials. A-type rhyolitic rocks are geochemically comparable to the andesitic suite, and their higher La/YbN (13.01–30.05) and lower Eu/Eu* (0.66–0.73) indicate evolved products of andesitic magmas. The estimated oxygen fugacity of dry (＜1.2% water) rhyolite lava is FMQ −1.87, whereas rhyolite porphyry is more oxidized (FMQ + 0.12) and water rich (2.5–2.8%) with probably being water saturated at 2.0–2.4 km depths. Therefore, it is assumed that the essential constraints of bimodal magmatism on epithermal gold mineralization are sequential processes from ascending and gradual differentiation of crustal-level mantle-derived basaltic magmas to releases of hydrothermal solutions from the silica-rich magma reservoir at shallower levels. Most importantly, the oxygen fugacity of and the fluid fluxes from dry rhyolitic melts are critical restricting parameters for the limited reserves at Gaosongshan; thus, this research contributes objective evidence for investigations of deposit genesis and prospecting targets.

Mining Activities and the Chemical Composition of R. Modonkul, Transbaikalia

The Dzhidinsky orefield is located in the Zakamensky district of Buryatia. It is characterized by a high concentration of mineralization in a small area. The ores of the Dzhidinsky economic deposits are complex. This ore field includes Pervomaiskoye Mo, Kholtosonskoe W, and Inkurskoe W deposits. The catchment basin of the river Modonkul is located in the Dzhidinsky orefield. Mineral resource industry waste is a real threat to the city Zakamensk. Currently, the waste of deposits is located at the floodplain terrace and the watershed of the river Modonkul. In this work, the impact of the drainage mine and tributary Inkur on the formation chemical composition of the river Modonkul is studied. The 80 samples of water and suspended matter were taken from a surface of 0–0.5 m on seven sites. Physical and chemical parameters were measured at the sampling sites, and chemical composition was analyzed in the laboratory. In the natural background of the river, the major cations are, in decreasing order, Ca2+ &gt; Mg2+ &gt; Na+ + K+, and major anions are, also in decreasing order, HCO3− &gt; SO42− &gt; Cl−. Along the river, the chemical type of water changes from bicarbonate to sulfate across the sulfate-bicarbonate or bicarbonate-sulfate class and from calcium-magnesium across sodium-calcium-magnesium reverse calcium-magnesium group. Total dissolved solids (TDS) increase by a factor of 4.0–4.7 between the upper and mouth of the river. The dissolved and suspended loads were studied in the mixing zone of acid mine and river water. The different hydrochemistry characteristics may result from Fe, Al, and Mn hydroxide precipitation. The pH values and dissolved oxygen cycles could change the uptake of heavy metals on suspended iron and manganese oxides, and the mechanism of removing it in water. River particulates have the potential of regulating heavy metal inputs to aquatic systems from pollution. The ore elements or the heavy metals are removed from the water in two ways: by the runoff of Modonkul and the sedimentation of suspended material at the bottom. The inflow of mine water into the Modonkul river leads to the rare earth elements (REEs) composition with negative cerium and positive europium anomalies.

Subduction–collision processes and crustal growth in eastern Dharwar Craton: Evidence from petrochemical studies of Hyderabad granites

The granite batholiths of eastern Dharwar Craton, which are showing intrusive relationship with TTGs, exposed in the eastern part of Telangana state at University of Hyderabad, Gachibowli (9.30 km2), are studied for their petrographic and geochemical characteristics compared with their counterparts in EDC and evaluated their petrogenesis. These are predominantly microcline and quartz with subordinate plagioclase, exhibiting intergranular and perthitic textures. Geochemically, they are strongly peraluminous to slightly metaluminous in nature with high Alumina Saturation Index (ASI) ranging from 0.86 to 1.11 indicating the role of plagioclase in their genesis. Their alkali-calcic to alkalic nature, narrow range of Modified Alkali-Lime Index (MALI; Na2O+K2O −CaO), and low Fe-number reflect their similarities with the I-type Cordilleran granites. Prominent negative Europium anomalies, high Sr, Rb, Rb/Sr and low Sr/Y ratios indicate moderate to low pressure partial melting of pre-existing TTG with residual plagioclase in the source. We suggest, the melting of older TTGs through crustal anataxis process formed these granites and the sanukitoid melts supplied the required heat for the melting of TTG to evolve into granites. The genesis of these granites supports reworking of older crust, crustal differentiation during syn-collisional stage and marks the stabilization of continental crust in the Dharwar Craton during the Neoarchean time.

Highly fractionated Himalayan leucogranites and associated rare-metal mineralization

The formation of the Himalaya was associated with the exhumation of high-grade metamorphosed rocks of the Higher Himalayan sequence (HHS) complex, which underwent amphibolite-, granulite- to eclogite-facies metamorphism and anatexis. Occurring along the Himalayan mountain crest in the south and the Lhagoi Kangri mountain in the north, the Himalayan leucogranites contain varying proportions of biotite (<5%), muscovite, tourmaline, and garnet, and are typically equigranular in texture with variable structures from foliated to massive. These peraluminous granites have been interpreted as purely crust-derived melts, without any input from the mantle, and may therefore record the timing of continental collision. However, they were emplaced between 44 and 7 Ma, much later than the timing of collision between India and Asia (c. 60 Ma). Although typically interpreted as products of in situ partial melting of the HHS during subduction and exhumation, we suggest that the Himalayan leucogranites underwent intense crystal fractionation, as recorded by crystal layering, sedimentary-like cross-bedding, and the occurrence of pegmatites with varying grain sizes. Geochemically, these leucogranites are low in siderophile but high in lithophile elements, with significant negative europium (Eu) anomalies, non-chondritic Nb/Ta and Zr/Hf ratios, and rare earth element (REE) tetrad effects. Many of the leucogranites and pegmatites contain rare-metal minerals, such as beryl and chrysoberyl; columbite–tantalite, tapiolite, and pyrochlore–microlite; rutile and fergusonite; and zinnwaldite, lepidolite, spodumene, and petalite, supporting an origin involving a high degree of magmatic fractionation. We suggest that the primary magma was generated through partial melting of subducted Indian crust due to input of heat from depth. The generation of magma triggered the exhumation of the Indian crust and subsequent formation of the Himalayan mountain chain. The magma ascended with the Indian crust along the South Tibetan Detachment System (STDS), during which time it underwent significant fractionation. Anatectic melts generated from exhumed Indian crust through decompression melting are distinct from the leucogranites, indicating that exhumation of the Indian crust had a limited contribution to the generation and evolution of the leucogranites. Extensional faulting facilitated a high degree of magma fractionation and rare-metal mineralization.

Provenance and tectonic setting of the Eocene Nanka Sandstone, Anambra Basin, Nigeria

The Anambra Basin is a Cretaceous intracratonic basin, trending NE–SW in the southern part of Nigeria. The basin is believed to have been formed during the Santonian tectonic episode, which affected the entire Benue Trough. Inorganic geochemical data of the Nanka Sandstone was applied in studying its tectonic setting and provenance. The sandstones have fractionated chondrite-normalised rare earth element (REE) patterns, enriched light rare earth elements (LREE) and nearly horizontal REE with negative europium (Eu) anomalies. These patterns as well as a wide range of LREE/HREE (heavy rare earth elements) ratios and negative Eu anomalies suggest a felsic and/or mixed rock type source. A provenance discriminant function diagram utilising major oxides indicates a quartzose recycled sedimentary source. A plot of La/Th vs. Hf suggests a mixed felsic/basic source; a plot of Cr/Th and Th/Sc indicates contributions from felsic and basic end members, suggesting a mixed-source provenance. A Th/Co vs. La/Sc diagram, V-Ni-Th*10 ternary plot and Th/Co vs. La/Co suggest mixed source rock. Provenance investigations allude to sediment derivation from felsic, intermediate and mafic components and probably inputs from recycled older sediments. Tectonic discrimination based on major and trace elements indicates a passive margin setting.

Sediment chemistry and detrital zircon record in the Bosque and Paseo del Mar coastal areas from the southwestern Gulf of Mexico

Abstract In this study, we analyzed the mineralogy and bulk chemical composition of 20 sediment samples, as well as trace, rare earth element (REE) concentrations and U–Pb ages of 400 detrital zircons retrieved from the Bosque (PB) and Paseo del Mar (PM) coastal sediments, southwestern Gulf of Mexico, to investigate the provenance. The XRD study revealed that the PB sediments are associated with quartz, muscovite, cordierite, albite, and anorthite. The PM sediments are associated with calcite, aragonite, and biogenic materials, which derived probably within the coastal environment. The weathering indices like Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA) revealed a moderate intensity of weathering for the PB sediments. The chondrite normalized REE patterns of the PB and PM sediments are with negative and positive europium anomalies (Eu/Eu*), suggesting that the sediments were derived by the combination of felsic and mafic source rocks. The Th/U ratios (>0.3) of the PB and PM zircons indicated an igneous origin. The chondrite normalized REE patterns of the PB and PM zircons are depleted in low REE (LREE) and enriched in heavy REE (HREE) with positive cerium and negative europium anomalies. A wide variation in chondrite normalized REE patterns of zircons with respect to ages was observed. The PB is dominated by Proterozoic (~551–2500 Ma) and Paleozoic zircons (~445–480 Ma; Ordovician), whereas PM is dominated by Cenozoic zircons (~5–21 Ma; Miocene), indicating that the zircons were derived from different source terranes. The Chiapas Massif (CMC) and Chuacus complexes are identified as the potential source areas, which contributed Proterozoic zircons to PB. On the other hand, Cuicateco terrane (CUI) and Tuxtla Volcanic Field (TVF) are considered as the potential source areas, which supplied significant number of Cenozoic zircons to the PM coastal area. The zircon U–Pb ages and the bulk sediment geochemistry together indicate that the source terranes, which supplied sediments to the PB and PM coastal areas, consist of mafic and felsic rocks. The results of this study further illustrated the importance of littoral currents in distributing sediments, although rivers are considered as a carrier and agent of distributing sediments along the coastal areas.

Geochemistry of Rare Earth Elements in Lower Gondwana Coals of the Talchir Coal Basin, India

This study investigated the concentration, distribution, and modes of occurrences of rare earth elements in coal from the Talchir coal basin, Odisha State. This basin comprises two coal bearing Formations i.e. Barakar and Karharbari. Samples, collected from one borehole, were analyzed by X-ray diffraction, X-ray fluorescence, electron microprobe and inductively coupled plasma mass spectrometry (ICP-MS). The coals are medium to high volatile, high ash and low sulphur. The Rare Earth Elements and Yttrium (REY) of the 34 coal samples varies from 29.6 ppm to 179.4 ppm with an average value of 91.0 ppm. The average value of total REY of the Barakar Formation (53.6 ppm) is considerably lower than the average value of Karharbari Formation (127.4 ppm). The average of the ratio of light to heavy rare earth elements (LREE/HREE) is also much higher in the Karharbari Formation (8.1) than in the Barakar Formation (4.4). Coals from both Formations show negative europium anomalies while only coals from the Karharbari Formation show a positive cerium anomaly. These differences indicate differences in depositional conditions between the coals of the two Formations within the coal basin. Through EPMA analysis, REY-bearing phosphates are found to be present in this coal in clay minerals. Monazites are found in Mg aluminosilicate matrix as dispersed minerals which may be detrital and monazite-bearing chlorite indicates that the monazites are formed by hydrothermal alteration during coalification. Ag, Ba and Zr are found as replacements or substitution in the REY phosphates.

Geochemistry and provenance of Neoproterozoic metasedimentary rocks from the Togo structural unit, Southeastern Ghana

Neoproterozoic metasedimentary rocks of greenshist facies, consisting predominantly of quartzites, phyllites, and phyllonites, occur in the Akwapim range of the Togo Structural Unit (TSU) in Ghana. The geochemistry of the phyllites were studied to determine their provenance and tectonic setting. The major element analysis was carried out by the Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and trace elements, including REEs, by the Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) method. The studied metasedimentary rocks have SiO2 and Al2O3 contents comparable to that of average Neoproterozoic upper crust. The metasedimentary rocks are strongly depleted in CaO, Na2O, and Sr and enriched in K2O, Ba and Rb with respect to average Neoproterozoic upper crust, reflecting K addition during diagenesis. Zr and Hf concentrations are significantly above Neoproterozoic upper crustal values. Chondrite-normalized rare earth element (REE) patterns are characterised by fractionated light-REE (LREE) (average LaN/SmN = 3.68), significant negative europium anomaly (average Eu/Eu* = 0.61) and fairly flat heavy-REE (HREE) (average GdN/YbN = 1.3). The geochemical data, particularly the high La/Sc, Th/Sc, La/Co, Th/Co, Zr/Sc, La/Th ratios, Eu/Eu* values, and high Zr and Hf concentrations, suggest that the metasedimentary rocks of the Togo Structural Units in the Akwapim range were derived mainly from recycled sedimentary sources. Comparison of geochemical signatures of the studied metasedimentary rocks with those of the Paleoproterozoic Birimian rocks suggests that the felsic materials in the Togo Structural Units (TSU) could not have been derived predominantly from the Birimian rocks, implying more distal sources. The studied metasedimentary rocks exhibit provenance characteristics similar to that of the Kwahu/Bombouaka Group of the Voltaian Basin, suggesting derivation from the same source, probably in the Amazon Craton. The metasedimentary rocks exhibit geochemical characteristics indicative of sediments derived from a passive continental margin. This inference supports previous studies that indicate that prior to the Pan-African Dahomeyide orogenic event, the southern margin of the West African Craton (WAC) was under passive margin settings.

Trace elements and REE enrichment at Seboah Hill, SW Egypt

Abstract Seboah Hill - a small body of peralkaline granite (&lt; 0.1 km2) in south-western Egypt containing aegirine minerals ± magnesiohornblende ± riebeckite, cut by dikes of riebeckite-aegirine rhyolite, and exhibiting high radioactivity in veins of K-feldspar-aegirine-chalcedony-quartz ± trace hematite ± trace goethite was sampled and analyzed using inductively coupled plasma methods. Whole-rock chemical compositions of 5 granite, 3-rhyolitedike and 10 radioactive vein samples are presented. Of special significance is the enrichment of trace elements and rare earth elements (REE) in the radioactive veins. These include up to 6081 ppm Zr, 4252 ppm Ce, 1514 ppm Nd, 1433 ppm La, 1233 ppm Nb, 875 ppm Y, 388 ppm Pr, 350 ppm Th, 222 ppm Sm, 189 ppm Gd, 159 ppm Dy, 153 ppm Hf, 83 ppm Er, 76 ppm Yb and 58 ppm U. The chondrite-normalized patterns of REE in all samples show only limited variation and have negative europium (Eu) anomalies. These findings suggest that the sources of the REE are genetically related. Values of the Eu anomalies vary from 0.38-0.41 for the radioactive veins, 0.39-0.53 for the granite and 0.31-0.44 for the rhyolite dikes. Eu variations are consistent for different paragentic stages.

Differentiating Lithogenic Supplies, Water Mass Transport, and Biological Processes On and Off the Kerguelen Plateau Using Rare Earth Element Concentrations and Neodymium Isotopic Compositions

Distributions of dissolved rare earth element (REE) concentrations and neodymium isotopic compositions (expressed as eNd) of seawater over and off the Kerguelen Plateau in the Southern Ocean are presented. The sampling took place during the austral spring bloom in October-November 2011 (KEOPS2 project, GEOTRACES process study) and aimed to further the investigations of the KEOPS1 austral summer study in terms of sources and transport of lithogenic material, and to investigate the impact of local biogeochemical cycles on the REE distributions. The REE signature of the coastal eastern Kerguelen Islands waters was characterized by negative europium anomalies (Eu/Eu*) and negative eNd in filtered samples. By contrast, the unfiltered sample showed a positive Eu/Eu* and more radiogenic eNd. These distinct signatures could reflect either differential dissolution of the local flood basalt minerals or differential leaching of local trachyte veins. The dissolved Kerguelen coastal REE patterns differ from those observed close to Heard Island, these latter featuring a positive Eu/Eu* and a less radiogenic eNd (Zhang et al., 2008). These differences enabled us to trace the transport of waters (tagged by the Kerguelen REE signature) 200 km downstream from the coastal area, north of the Polar Front. Northward transport of the central Plateau shallow waters, enriched by both local vertical supplies and lateral advection of inputs from Heard Island, was also evident. However, the transport of Kerguelen inputs southeastward across the Polar Front could not be discerned (possibly as a result of rapid dilution or scavenging of REE signatures), although evidence for such transport was found previously using Ra isotopes (Sanial et al., 2015). Comparison of the REE patterns at stations sampled prior, during and at the demise of the bloom revealed diverse fractionations, including production of significant lanthanum and europium anomalies, which are tentatively ascribed to chemical reactions with various inorganic and biogenic phases, including surface coatings, barite crystals, and biogenic silica.

Elemental and Sr-Nd isotopic geochemistry of Mesoproterozoic sedimentary successions from NE Lesser Himalaya, northern India: Implications for Proterozoic climate and tectonics

The sedimentation history of northeastern Himalayan Proterozoic basins is poorly known. In the current study, we examine major, trace, rare earth elements (REEs) and Sr-Nd isotope geochemistry of pelites and quartzites from Mesoproterozoic metasedimenatary sequences of Bomdila and Rupa Groups, NE Himalaya in order to understand provenance, tectonic setting and paleoweathering conditions of deposition. The salient geochemical parameters, such as, high SiO2 and Al2O3 concentrations, relatively low MgO values, typical enrichment of immobile incompatible elements (Al2O3/TiO2 = 32.2, Th/Sc = 2.1), fractionated REE patterns (La/Ybn = 28) and a strong negative europium anomaly (Eu/Eu∗ = 0.3–0.7) indicate dominant felsic source for Bomdila sediments. Bomdila sediments show significantly radiogenic εNd0.95Ga values (−10.25 to −17.90, avg. −15.24) compared to Bundelkhand craton (avg. −26) and Singhbhum craton (avg. −30) and yield much younger TDM ages (2.3–2.7 Ga) compared to Archean Bundelkhand (3.2–3.5 Ga) and Singhbhum cratons (3.4–3.9 Ga). These results clearly rule out any significant detritus contribution from old cratonic sources. εNd and TDM data of Mesoprterozoic Bomdila sediments envelop the signature of Lower Proterozoic Inner Lesser Himalayan (ILH) terrane. Prominent U-Pb age peaks at 1702, 1651, ∼1400, 1248 and 1118 Ma in detrital zircons indicate major contribution from Late Paleoroterozoic to Late Mesoproterozic sources. However, initial Nd isotope ratios and TDM do not favour any juvenile inputs. We argue that ILH source terrain may have been extensively reworked during Late Paleoroterozoic to Late Mesoproterozic time and could have been part of a long lived orogenic belt.. Severe depletion of Ca, Na and Sr in Bomdila sediments indicate intense chemical weathering of the source region, and can be attributed to warm and humid climatic conditions, a conclusion which is consistent with the CO2-rich atmosphere during Mesoproterozoic period.

Chemical and Sr-Nd compositions and 40Ar/39Ar ages of NW-trending dolerite dikes of Burkina Faso: Evidence for a Mesoproterozoic magmatism in the West African Craton

The Paleoproterozoic basement of the northeastern part of the Leo-Man craton is intruded by generally NW-trending dikes. These regional scale dikes extend over 1000 km in Burkina Faso, Mali and Niger. We present chemical and Sr-Nd isotope compositions, as well as 40Ar/39Ar ages of these dikes with the following strikes N98°–N112°, and N114°–N124° in NE Burkina Faso. Field relationships show that the dikes are posterior to all other rock types dated between 2.26 Ga and 2.0 Ga. Chemical data indicate that the dikes are continental flood basalts and composed of low-Ti (TiO2 ≤ 2 wt.%) sub-alkaline basalts and andesites. They exhibit a minor negative Europium anomaly (0.86–0.99) and slightly fractionated REE patterns ((La/Yb)N = 2.5–9.1; YbN = 9.5–19.9). The ratios of Th/Ta (1.3–11.4) and Ce/Pb (5.2–58.5) suggest a varying crustal assimilation of the dike magmas during ascent in the continental crust for all studied samples. Calculated P-T conditions indicate that the magma reached temperatures of 1285 °C (calculated from olivine compositions) and pressures of 6.9 kbar (calculated for pyroxene minerals). Calculated initial 87Sr/86Sr (0.70040–0.70260) and ɛNd(t) = +2.1 to −3.5 at 1575 Ma, also point to a crustal contamination with the most primitive samples showing TDM values of 1946 Ma and 2154 Ma. The low values of La/Ba (<0.2) and Nb/La (<1.0), contrast with the low Th/Nb (<0.9), and suggest a lithospheric mantle or subduction-modified mantle as possible source for the dikes. Sr-Nd data, Mg# and Nb-Ta-Zr-Y-Th-Tb-Yb compositions further suggest that the most primitive samples were emplaced in a none orogenic setting and their magmas were subjected to variable crustal contaminations. Literature and the present whole rock 40Ar/39Ar age determinations show that the dikes were emplaced during a widespread Mesoproterozoic magmatism between 1.6 Ga and 1.2 Ga, and were affected by a thermal event causing the argon systematics resetting, best constrained by the date of sample KK1 (1236 ± 20 Ma, 40Ar/36Ar = 294 ± 13, MSWD = 2.2). Contemporaneous 1590–1570 Ma extensive magmatism is reported in other crustal blocks in Baltica (Sveconorw-Goth, svecofennian) NW Laurentia (Slave craton, Yukon), and Australia (Gawler craton), and together with the 1575 studied dikes, are related to the breakup of the supercontinent Nuna.

Geochemistry and U-Pb zircon geochronology of Iwo quartz potassic syenite, southwestern Nigeria: Constraints on petrogenesis, timing of deformation and terrane amalgamation

Petrography, whole rock geochemistry, and in-situ U-Pb zircon geochronology on quartz potassic syenite in Iwo area, southwestern Nigeria have been performed. The results indicate that the syenite was syn-tectonically emplaced during the Pan-African orogeny and shows heterogeneous deformation. On the basis of geochemical data, the rock is defined as high potassic calc-alkaline derived from metasomatised mantle with slight crustal contamination. The small negative europium (Eu) anomalies (Eu/Eu∗ = 0.87) displayed by the rock could be an indication of separation of amphibole and pyroxene. In-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry U-Pb zircon dating yielded a Concordia age of 642 ± 6 Ma which could be taken as that of emplacement and deformation. The relative position of the Iwo quartz potassic syenite in respect to the other undeformed syenite bodies in southwestern Nigeria and the main Pan-African suture zone suggest amalgamation of at least four terranes or micro blocks in the southwestern Nigeria during the Neoproterozoic time. The emplacement and deformation of the Iwo quartz potassic syenite were synchronous with basin closure around Ilesha-Ife and subsequent collision of two terranes/micro blocks. The collision resulted from a west-dipping subduction related tectonics which was different from the main Pan-African subduction zone at the margin of the West African craton. The amalgamation of these terranes contributed to the accretion of the Gondwanaland in the southern part of the Trans Saharan fold belt.

Oxygen isotope and trace element geochemistry of zircons from porphyry copper system: Implications for Late Triassic metallogenesis within the Yidun Terrane, southeastern Tibetan Plateau

It is intriguing to investigate the generation of the magmatic system and the geochemical signatures that distinguish the ore-bearing ones from barren ones, especially by examining the oxygen isotopic and trace element compositions of zircon. The oxygen isotopic composition and trace element abundances of zircons from the mineralized Pulang porphyries and the coeval Disuga and Lannitang andesites of the Yidun Terrane, eastern Tibetan Plateau, have been determined by secondary ion mass spectrometry (SIMS) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) in this study, respectively. Analyzed zircons have relatively high but invariant δ18O values (~5.8‰–6.9‰) that are indicative of derivation from homogeneous mantle or mantle-derived magmas together with contribution of mature crustal materials. We propose a model for the Late Triassic magmatism in the Yidun Terrane involving magmas derived from partial melting of subduction-metasomatized mantle peridotites that subsequently experienced melting–assimilation–storage–homogenization (MASH) processes. The low calculated Ti-in-zircon temperatures (644°C–863°C, generally between 640°C and 690°C) for these zircons may be caused by the lower solidus temperature for zircon in hydrous magmas, and partly controlled by the crystallization of amphibole. Zircon grains from porphyry copper system display small negative europium anomalies (mostly Eu/Eu*>0.5), contrasting with pronounced negative Eu anomalies in zircons (mostly Eu/Eu*<0.5) associated with magmato-hydrothermal quartz-vein Cu-Mo-W/Mo-W deposits. Calculated REE compositions of the melt that was in equilibrium with zircons from porphyry copper system display depletion in the middle rare earth elements (MREE) without negative Eu anomaly, while that of zircons from magmato-hydrothermal quartz-vein Cu-Mo-W/Mo-W system show depletion in the MREE with negative Eu anomaly. It is speculated that the parental melts of zircon have decreasing magmatic oxidation state in the following sequence: porphyry copper system>magmato-hydrothermal quartz-vein Cu-Mo-W deposit>magmato-hydrothermal quartz-vein Mo-W deposit. This study suggests that zircon trace element geochemistry probably bears significant implications for elucidating the origin of the arc magmatism and guiding exploration strategies for porphyry copper deposits.

Petrology and geochemistry of charnockites (felsic ortho-granulites) from the Kerala Khondalite Belt, Southern India: Evidence for intra-crustal melting, magmatic differentiation and episodic crustal growth

The Kerala Khondalite Belt (KKB) of the southern India encompasses volumetrically significant magmatic components. Among these, orthopyroxene-bearing, felsic ortho-granulites, popularly known as charnockites in Indian context, constitute an important lithology. In contrast to the well-known phenomena of arrested charnockitization, the geochemical characteristics and petrogenesis of these ortho-granulite suites remain poorly studied, leaving geodynamic models envisaged for the KKB highly conjectural. In this paper, we try to bridge this gap with detailed results on orthopyroxene-bearing, felsic ortho-granulites spread over the entire KKB and propose a new petrogenetic and crustal evolution model.Based on geochemical characteristics, the orthopyroxene-bearing, felsic ortho-granulites (charnockites sensu lato) of KKB are classified into (1) tonalitic (TC), (2) granitic (GC), and (3) augen (AC) suites. Members of the TC follow sodic (characterized by decreasing CaO/Na2O), whereas those of the GC and AC follow calc-alkaline trends of differentiation. Geochemical patterns of the TC resemble those of the Archaean tonalite–trondhjemite–granodiorite (TTG) suites, with slightly magnesian character (average Mg#=33), moderate LREE (average LaN=154), low HREE (average YbN=6) and Y (1–53ppm; average 11ppm). The TC is also characterized by positive to slightly negative europium anomalies (Eu/Eu*=0.7 to 1.67). The GC and AC suites, on the other hand, resemble post-Archaean arc-related granites. The GC displays ferroan nature (average Mg#=22), low to moderate degrees of REE fractionation (average [La/Yb]N=34.84), high contents of Y (5–128ppm; average 68), and low Sr/Y (1–98) ratios. Significant negative Eu anomalies (Eu/Eu*=0.18–0.91; average 0.50) and low Sr (65–690ppm) are also noted in the GC. Similar chemical characteristics are shown by the AC, with ferroan nature (average Mg#=21), low to moderate degrees of REE fractionation (average [La/Yb]N=26), high contents of Y (71–99ppm; average 87), and low Sr/Y (average 2) ratios with significant negative Eu anomalies (Eu/Eu*=0.03–0.31; average 0.23) and low Sr (average 160ppm) contents. The protoliths of the TC are interpreted as being derived from partial melting of thickened oceanic-arc crust composed of Archaean mafic composite source rocks (i.e., eclogite and/or garnet amphibolite) with a garnet amphibolite residue. Geochemical features of the GC, such as high Rb/Sr (average 1.80) and Ba/Sr ratios (average values >6), are considered as evidence for crustal reworking in their genesis, suggesting remelting of a quartzo-feldspathic (TTG) source, within the plagioclase stability fields.The geochemical features of the felsic ortho-granulite suite, substantiated with published geochronological data on members of the TC, GC, and AC suites, suggest a four-stage crustal evolution of the KKB. The first stage is marked by the formation of an over-thickened oceanic-arc. Zircon Hfc model ages of the TC and GC suites constrain the time of this juvenile magmatic crust-forming event as Meso- to Neoarchaean (2.8 to 2.6Ga). The second stage corresponds to the production of TTG magmas by melting of the over-thickened oceanic-arc crust, subsequent to basaltic underplating during Palaeoproterozoic (ca. 2.1Ga). The third stage was initiated by a transition in subduction style from shallow to steep due to continent-arc accretion. This stage is marked by the formation of granitic magmas through partial melting of the TTG crust and their differentiation into GC and TC. The zircon crystallization ages (1.89 and 1.85Ga) of the GC indicate arc accretion occurred during the Palaeoproterozoic. The fourth stage of crustal evolution is correlated with the Mesoproterozoic (~1.5Ga) emplacement of megacrystic K-feldspar granites (protoliths to the AC and augen gneisses). The distinct petrography, geochemistry and crystallization ages of the AC suggests recurrence of megacrystic, high-K calc-alkaline granitoids as the product of final phases of crustal-remelting marking subduction cessation. All these magmatic events are fairly well correlated with the major episodes of crustal growth observed in the once contiguous continental fragments (Sri Lanka and Madagascar) and worldwide events (2.7, 1.9, and 1.2Ga) implying similar episodic nature in the lower crustal evolution of the KKB.