REE Patterns Research Articles

REE + Y distribution in Tremadocian shelly phosphorites (Toolse, Estonia): Multi-stages enrichment in shallow marine sediments during early diagenesis

Sedimentary phosphorites in Cambrian-Ordovician boundary beds in Estonia are among Europe's most extensive phosphate rock reserves, with a tonnage of approximately three billion tonnes. In addition, they are potential sources of rare earth elements and yttrium (REE + Y). The ore consists of sandstone rich in phosphatic brachiopod fragments deposited in a shallow marine peritidal environment of the Baltic Paleobasin. A detailed geochemical and mineralogical investigation was conducted on the northern part of the Toolse deposit based on three drill core cross-sections. The REE sequestration model was developed based on the correlation of the sedimentary facies of the members of the Kallavere formation. Analyses were conducted on 140 whole-rock samples and 14 discrete black shale samples. Inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence (XRF) and X-ray diffraction (XRD) measurements were performed for each sample. Textural analyses were conducted with field-emission scanning electron microscopy (FE-SEM), and quantitative wavelength dispersive spectrometer (WDS) analysis of major and trace elements was performed with an emission electron probe microanalyser (EPMA).The PAAS normalised REE + Y patterns were relatively homogeneous throughout the deposit except for the cerium and yttrium contents. The REE + Y contents of the phosphorites indicate a distinctive ‘bell-shaped’ pattern enriched with middle-REE (MREE) enriched. In the lower part of the deposit, the ∑REE + Y concentration is enriched up to 12-fold, compared to the average shale, reaching up to 1234 ppm. LREE enrichment is observed in these horizons, with a 9-fold peak at neodymium. In terms of absolute values, the most prevalent REEs are, in order, Ce, Y, Nd, and La, which respectively reach maximum mean values of 296 ppm, 248 ppm, 164 ppm and 135 ppm.The profiles indicate multistage uptake during transport, deposition, and early diagenesis of brachiopod detritus. The sedimentation was associated with a steep redox gradient in the porewater in a coastal environment during the beginning of marine transgression. The near-shore upwelling of oxygen-poor deeper water rich in dissolved nutrients and Mn, combined with the input of lithogenic Fe, helped create favourable conditions for REE sequestration. Positive Y anomalies indicate an initial uptake of REE + Y by hydroxyapatite through Ca substitution in an oxic environment near the sediment–water interface. The sediment burial induced the formation of carbonate fluorapatite (CAF) and a shift to the REE adsorption uptake mechanism, reinforced by the significant intercrystalline porosity of the shells. The transition to a suboxic environment below the surface layers resulted in the reductive dissolution of Mn-(oxyhydr)oxides and the release of LREE in the porewater system, resulting in an overprint of the original REE pattern to the point of inducing positive Ce anomalies. A late enrichment in MREE occurred during early diagenesis as a result of desorption of REE from the Fe-(oxyhydr)oxides and organic-rich particles under relatively anoxic conditions.

Narsarsukite in peralkaline granites from the Papanduva Pluton, Graciosa Province, south Brazil: Insights from textural and compositional features

AbstractWe report textural and compositional data for the titanosilicate narsarsukite [Na2(Ti,Fe3+)Si4(O,F)11] in peralkaline granites from the Papanduva Pluton in Graciosa Province, south-southeastern Brazil. Two distinct narsarsukite generations, one late magmatic and the other post-magmatic, were identified on the basis of textural and compositional features. The magmatic generation consists of larger, euhedral to subhedral variably zoned crystals and late poikilitic intergrowths between narsarsukite and albite laths, representing the crystallisation of the latest melt pockets. The post-magmatic generation forms smaller, typically fibrous crystals and irregular aggregates that occur interstitially or replace the primary mafic minerals, particularly arfvedsonite. Compositions of narsarsukite from the Papanduva Pluton cover most of the compositional range described in known occurrences. The magmatic generation is enriched in Zr and depleted in Al. The Fe3+ and Al contents show a positive correlation for the magmatic crystals, but a negative correlation for the post-magmatic narsarsukite. The Al/Fe3+ ratios are higher in the post-magmatic crystals and can be used to discriminate between the two generations. The compositional variations are controlled mainly by the heterovalent substitution (Ti, Zr)4+ + O2– = (Al, Fe)3+ + F1–, and are compatible with (OH)1– ions in the O site. Narsarsukite has significantly higher concentrations of mid REE+Y and HFSE and very low to absent contents of LREE and LILE relative to the host-rocks. The average REE pattern of the magmatic crystals is highly fractionated, consistent with the expected strong preference for the smaller HREE. The post-magmatic crystals that replace arfvedsonite have flatter patterns, with higher concentrations of LREE and MREE. Their compositions suggest a fluid phase relatively rich in HFSE, REE, Y, Pb, Th and U. Narsarsukite is a common phase in strongly peralkaline granites such as the Papanduva Pluton and the well-known Strange Lake Complex, and should be considered a diagnostic mineral of highly peralkaline SiO2-oversaturated rocks containing rare Ti and Zr mineral assemblages.

Origin of Erzgebirge ultrahigh‐pressure garnetite: Formation from a basaltic protolith by serpentinization‐assisted metasomatism?

AbstractErzgebirge ultrahigh‐pressure (UHP) garnet peridotite includes scarce layers of garnet pyroxenite, nodules of garnetite and, very rarely, of eclogite. Peridotite‐hosted eclogite shows the same subalkali‐basaltic bulk rock composition, mineral assemblage and peak conditions as gneiss‐hosted eclogite present in the same UHP unit. Garnetite has considerably more Mg, moderately enhanced Ca and Fe and significantly lower contents of Na, Ti, P, K and Si than eclogite, whereas Al is very similar. In addition, the compatible trace elements (Ni, Co, Cr, V) are elevated and most incompatible elements (Zr, Hf, Y, Sr, Rb and rare Earth elements [REE]) are depleted in garnetite relative to eclogite. In contrast to other large ion lithophile elements (LILEs), Pb (+121%) and Ba (+83%) are strongly enriched. The REE patterns of garnetite are characterized by depletion of light and heavy REE and a medium REE hump indicative of metasomatism, features being absent in eclogite. An exceptional garnetite sample shows an REE distribution similar to that of eclogite. Garnetite is interpreted to have formed from the same, but metasomatically altered, igneous protolith as eclogite. Except for Ba and Pb, the chemical signature of garnetite is explained best by metasomatic changes of its basaltic protolith caused by serpentinization of the host peridotite. Garnetite is chemically similar to basaltic rodingite/metarodingite. Although rodingite is commonly more enriched in Ca, there are also examples with moderately enhanced Ca matching the composition of Erzgebirge garnetite. Limited Ca metasomatism is attributed to the preservation of Ca in peridotite during hydrous alteration. This can be explained by incomplete serpentinization favouring metastable survival of the original clinopyroxene. In this case, most Ca is retained in peridotite and not available for infiltration and metasomatism of the garnetite protolith. This inescapable consequence is supported by the fact that clinopyroxene is part of the garnet peridotite UHP assemblage, which would not be the case if Ca had been removed from the protolith prior to high‐pressure metamorphism. The enrichment of compatible elements in garnetite is attributed to decomposition of peridotitic olivine (Ni, Co) and spinel (Cr, V) during serpentinization. Enrichment of Ba and Pb contrasts the behaviour of other LILEs and is ascribed to dehydration of the serpentinized peridotite (deserpentinization). This requires two separate stages of metasomatism: (1) intense chemical alteration of the basaltic garnetite precursor, together with serpentinization of peridotite at the ocean floor or during incipient subduction; and (2) prograde metamorphism and dehydration of serpentinite during continued subduction, thereby releasing Pb–Ba‐rich fluids that reacted with associated metabasalt. Finally, subduction to >100 km and UHP metamorphism of all lithologies led to formation of garnetite, eclogite and garnet pyroxenite hosted by co‐facial garnet peridotite as observed in the Erzgebirge.

Geology, geochemistry, fluid inclusion microthermometry and depositional environment of the Early Cretaceous Meymeh sub-seafloor replacement sideritic-ankeritic iron deposit, Malayer-Esfahan Metallogenic Belt, Iran

Meymeh sideritic-ankeritic iron deposit in the eastern part of Malayer-Esfahan Metallogenic Belt (MEMB) is formed in the sedimentary-volcanic rocks of the Early Cretaceous sequence. Ore mineralization in the study area based on stratigraphic location and type of host rocks divided in two ore horizons. Lower ore horizon located in the dolomitic-sandy limestone (Kc3) unit, which upper ore horizon is formed in thin-bedded limestone (Km) rocks. Petrographic studies indicates that mineralization comprises three ore facies: stockwork, bedded and massive ore facies. The most important primary minerals are siderite, ankerite, ferroan-dolomite, pyrite, pyrolusite, barite and minor chalcopyrite. The most frequent textures in the ore zones include laminae, replacement, vein-veinlet, massive and banded. Dolomitization and silicification are the main wall rock alteration styles; alteration intensity increases towards the ore zones. Based on relationships between ore minerals and rock forming minerals, ore mineralization (hypogene and supergene) in the Meymeh deposit formed during three main stages: fine-grained Fe-carbonate bands are intricately interlayered with dolomite beds. Sideritic-ankeritic bands exhibit classic sedimentary textures, such as laminations and bedding, indicative of a syn-sedimentary to early diagenesis origin. Coarser-grained stage two siderites and ankerites show breccia and vein-veinlet textures, and are considered to have formed by replacement during burial diagenetic sub-seafloor fluid flow. In stage three, siderite and ankerite were converted to secondary iron oxides such as oxide/hydroxide Fe minerals during meteoric water flow through the inverted normal and thrust faults and uplift. The primary two-phase fluid inclusions in the quartz-2 and ankerite-2 minerals that have been investigated from the ore mineralization section of the Meymeh deposit are homogenized at temperatures between 110.3 to 226.9 °C. Salinities of the primary fluid inclusions range from 3.39 wt.% to 14.77 % NaCl eq. This finding suggests that hydrothermal brine fluid mixing with seawater could be the primary mechanism that prompted ore formation. The similarity of REE patterns between siderite and ankerite in different ore facies and host rock carbonates indicates their derivation from the same ore fluids. The Meymeh deposit is considered a typical case of sedimentary hydrothermal diagenetic sideritic-ankeritic mineralization, in which minerals deposited when hydrothermal fluid was released from anoxic to suboxic water columns.

Monazite record of assimilation and differentiation processes in the petrogenesis of Himalayan leucogranites

The Himalayan leucogranites have received particular attention as they record the continental crust-building processes of a collisional orogenic system. Deciphering the magmatic evolution processes and potential sources is essential for understanding the origin of the Himalayan leucogranites. In-situ monazite Th-Pb ages, trace element geochemistry, and Nd isotope ratios have been determined with whole-rock geochemical data for the leucogranites and their country rocks in the Yadong area, southern Tibet. Monazites in most of the studied leucogranites are marked by multiple age populations with variable REE ratios and wide ranges of Nd isotope ratios, reflecting a complex history of magmatic processes. Abundant inherited monazites are identified in the Yadong leucogranites, and most of them are excluded from the source regions due to their distinguished Nd isotope compositions from the magmatic monazites. The inherited monazites share close affinities in geochronological dates and geochemical compositions with those in the country rocks, strongly suggesting a major origin of the country-rock contamination. This highlights the contamination-assimilation processes in the genesis of the Yadong leucogranites. It is worth noting that xenocrystal monazites from the metasedimentary country rock are less preserved in the leucogranites than those from the granitoid country rock, which probably reflects the different assimilation reactions for different types of country rock. Assimilation of the metapelitic rocks released low content of xenocrystic monazite and high volume of partial melt into the leucogranites, whereas assimilation of the granitoid rocks released high content of xenocrystic monazite. In addition to the contamination-assimilation processes, the uniform Nd isotope data with large variations in REE ratios for magmatic monazites in some leucogranite samples suggest a subsequent closed-system fractional crystallization process during the magma evolution. Besides, some magmatic monazites in the muscovite granites show a significant tetrad effect in the Chondrite-normalized REE patterns, indicating that the magma system has evolved into an aqueous fluid-available environment. This supports an important role of the magmatic differentiation process in the formation of the Yadong leucogranites. Taken together, a petrogenic model involving AFC (assimilation and fractional crystallization) processes is proposed, and the geochemical variations of the Yadong leucogranites are mainly attributed to the post-source processes. It is suggested that the rock geochemistry of the Himalayan leucogranites often dose not directly reflect the nature and the petrogenetic processes in the source regions.

Origin of heavy rare earth element enrichment in carbonatites

Heavy rare earth elements (HREE) are currently in high demand for use in high technology, renewable energy and low-carbon transport, but they are the least abundant in nature. Carbonatites are the primary source of REE; however, they are dominated by light REE (LREE). It remains unknown whether carbonatites have the potential to form economic HREE mineralization. Here we report a xenotime-bearing carbonatite in the Bachu REE deposit, northwestern Tarim Large Igneous Province (TLIP), China, and infer the origin of HREE in carbonatites. The rocks evolved from dolomite to calcite carbonatites, and their HREE content correspondingly increased. Both types of rocks have similar monazite U-Pb ages (ca. 300 Ma), and are older than the major eruption of flood basalt of the TLIP, and associated alkaline complexes. They contain higher εNd(t) (2.4–4.1) and lower initial Sr isotopic ratios [(87Sr/86Sr)i = 0.7036–0.7041] than the basalts but similar values to those of younger alkaline rocks. The carbonatites are inferred to be directly derived from low-degree melting of lithospheric mantle sources induced by a deep-seated mantle plume. Calcite carbonatites contain characteristic xenotime, which is associated with burbankite, sulfates, and minor quartz. The rock-forming calcite shows high HREE abundance and flat REE patterns (La/Ybcn = 0.3–2.1). Apatite and LREE minerals in calcite carbonatites also have a higher HREE content (e.g., Y2O3 up to 2 wt%) than those in dolomite rocks. This finding indicates that the early dolomite carbonatite underwent strong fractionation of dolomite and LREE minerals, resulting in HREE and alkali enrichment in the evolved calcite rocks. High amounts of alkalis further enhance the solubility of REE, particularly HREE, in the residual melts. Silica assimilation from the country rocks facilitates the HREE mineralization by sequestering alkalis. Therefore, HREE enrichment in carbonatites may require substantial fractional crystallization of initial melts as well as alkali conservation during ascent.

Controls of sedimentary facies and sealevel fluctuation on dolomitization: The Lower Cambrian Longwangmiao Formation in Sichuan Basin, China

The Lower Cambrian Longwangmiao Formation is a significant deep carbonate (dolomite) reservoir that has great hydrocarbon potential in the Sichuan Basin, South China. However, the origin of the Longwangmiao dolomite and the mechanism of dolomitization remain a debate that needs better understanding due to its crucial importance in determining reservoir quality. In the current study, a multi-technique approach of petrography, geochemistry (C- and O-isotopes, major and trace elements including rare earth elements), and microthermometry has been applied to reconstruct the mechanisms of dolomitization. Petrographic examinations revealed three types of matrix dolomite: (1) near-micritic (dolomicrite), fabric retentive texture (Md1, 10–50 μm), (2) finely to medium crystalline dolomite, subhedral to euhedral crystals (Md2, 60–120 μm), and (3) medium to coarsely crystalline dolomite, subhedral to nonplanar crystals (Md3, 100–300 μm). The similarity of PAAS-normalized REE patterns of the investigated dolomites to that of their precursor lime mudstone and their δ13C (C1 = 0.3 ± 0.3‰, Md1 = −1.3 ± 0.6‰, Md2 = −0.6 ± 1.6‰, and Md3 = 0.3 ± 1.5‰) and δ18O (C1 = −9.4 ± 0.2‰, Md1 = −7.8 ± 0.8‰, Md2 = −8.7 ± 0.5‰, and Md3 = −8.9 ± 0.6‰) values suggest that the parent dolomitizing fluids of Md1, Md2, and Md3 dolomites were likely modified seawater. The significant enrichment of Na (1266 ± 1316 ppm) and K (4649 ± 3199 ppm), tight structure and presence of moldic pores in Md1 and its slightly elevated δ18O values suggest that it formed in a near-surface (i.e., mesosaline to penesaline) environment from slightly evaporated seawater, which is consistent with its Sr/Ca molar ratios (0.0075−0.0090). The Md2 dolomite is crosscut by stylolites and has a coarser grain size, higher Mn (295 ± 172 ppm), NdSN/YbSN (1.33 ± 0.06) and U (0.49 ± 0.11 ppm), and lower Ce/Ce* (0.83 ± 0.02) values than Md1. This suggests a shallow to mid-burial dolomitization with possible downwards seepage-reflux of Mg2+-rich evaporated brines. The Md3 dolomite has the largest crystal size and mosaic contacts, lowest Ce/Ce* (0.81 ± 0.02), and highest U (0.64 ± 0.15 ppm) and Mn (487 ± 240 ppm) values, which imply a deeper dolomitization environment than Md2. Thus, it precipitated likely in a mid-to deep burial setting at a higher temperature (Th = 120−135 °C), from the residual Mg2+-rich fluids stored in the strata pores. Considering the regional geology, we suggest the sedimentary facies and paleogeomorphology controlled the spatial distribution of early dolomites, while sealevel fluctuations influenced the initial time and scale of dolomitization.

Origin, dynamics, and chemical evolution of garnet-bearing leucogranitic magma, Eastern Desert of Egypt: Controls on the rare-metal enrichment in the A-type magmatism

Egypt hosts numerous rare-metal granites, i.e., highly evolved granites enriched in rare metals (Ta, Nb, Be, Sn, Zr, Th, and REE). However, the processes involved in the rare-metal enrichment are not fully understood. We present new data on the textural characteristics and chemical composition of rare-metal mineralization associated with microgranite dikes in the Ras Abdah area of the Egyptian Eastern Desert. These dikes are garnet-bearing leucogranites (GLG) composed of perthitic alkali-feldspars and quartz. When compared to other Egyptian A-type granites, microgranite dikes are alkaline rocks with particularly higher HREE contents. Zircon, huttonite, fergusonite (Y), and Fe-Ti-Zn oxides (magnetite, Zn-bearing ilmenite and pyrophanite) are largely associated with the altered domains, which are also enriched in Nb, Zr, Y, Ta, Th, and REE. However, similarities between the chondrite-normalized REE patterns of the altered and unaltered domains of the GLG dikes favor the hypothesis of a unique magmatic signature. Moreover, the chemical and textural features of rare-metal minerals indicate that the alteration of primary minerals was caused by deuteric fluids or aqueous residual melt exsolved from the parental granitic magma (autometasomatism). Garnet compositions are rich in the spessartine component (up to 84 %), which is typical of garnet in highly evolved pegmatitic rocks. Furthermore, garnet exhibits no major element zoning but shows chemical fluctuations in trace element concentrations, suggesting correspondingly abrupt changes in melt composition due to sequential magma pulsing. This magma emplacement may cause crystal nucleation and oscillatory crystallization followed by magmatic segregation. Overall, parental magma type, dike injection, and magmatic-hydrothermal processes all play a role in the unusual enrichments of rare metals.

Tectonic Transition from Passive to Active Continental Margin of Nenjiang Ocean: Insight from the Middle Devonian-Early Carboniferous Granitic Rocks in Northern Great Xing’an Range, NE China

Northeast China occupies the majority of the eastern Central Asian Orogenic Belt, which mainly consists of continental blocks and accretionary terranes. The Devonian was a tectonic quiet period in the NE China region due to a lack of tectono-magmatism, but the tectonic background of this period has been unclear, especially for the Hegenshan-Heihe Suture between Xing’an and Songliao accretionary terranes, which represents the Paleozoic Nenjiang Ocean (a branch ocean of the eastern Paleo-Asian Ocean). Here we report granitic rocks from the Woluohe area, Northern Great Xing’an Range, NE China, to constrain the tectonic process of the transition from the Devonian quiet period to the Early Carboniferous active tectonic period. Three granitic rock samples produce zircon U-Pb ages of 389 Ma, 368 Ma, and 351 Ma, belonging to the Middle and Late Devonian and Early Carboniferous, respectively. They have high Si, Al, K, and Na contents, but with low Mg, Fe, and Ti contents, together with positive Hf isotopic features and low molar Al2O3/(MgO+FeOT) ratios, we suggest that they were derived from partial melting of lower crustal igneous rocks. Meanwhile, the narrow major element variation at odd with the fractionation process and their negative Nb and Ta anomalies imply the obvious contribution of crustal. Comprehensive tectonic setting analysis shows all samples are in calc-alkali magmatic series with rightward fractionated REE and trace element patterns that are enriched in LREE and LILE and depleted in HREE and HFS, indicating a subduction-related magmatic arc setting. Considering the regional tectonic setting and the small scale of the Devonian plutons, we suggest a limited subduction tectonic setting during the quiet period of the northern Great Xing’an Range, which might indicate the beginning of an initial northwestward subduction of the Nenjiang Oceanic lithosphere beneath the Xing’an Accretionary Terrane in the Middle Devonian, accelerated subduction in the Late Devonian, and bidirectional subduction in the Early Carboniferous.

Xenocrystic zircon in Central New York State Kimberlites record Neoproterozoic disturbance of Grenville Basement rocks

Zircon grains from two central New York State (USA) Mesozoic kimberlite dikes intruding Middle and Upper Devonian sedimentary rocks were analyzed for UPb and LuHf isotopes, and trace elements, and were derived from different parts of the continental crust and/or upper mantle. Zircon from the Six Mile Creek (SMC) kimberlite has variable physical properties and “felsic” chondrite normalized REE patterns, and yields dates (321–1732 Ma) compatible with entrainment of sedimentary xenoliths during accent through the upper crust. In contrast, zircon from the Taughannock Creek (TAG) kimberlite have near concordant UPb dates that are distributed between intercept ages of 572 ± 25 and 960 ± 20 (2σ) Ma (MSWD = 0.61) with homogeneous LuHf systematics and 176Lu/177Hf ratios <0.0001. Examination in CL reveal TAG zircon grains vary in internal zoning and structures, but they are geochemically and isotopically similar. Their evolution includes early Neoproterozoic (ca. 950 Ma) granulite-facies metamorphism in equilibrium with garnet growth (flat to negative sloping HREE patterns and Ti-in-zircon temperatures >900 °C) during the final assemblage of the supercontinent Rodinia, followed by subsequent Pb-loss, resulting in a discordant array and decoupling of the UPb and LuHf isotopic systems prior to entrainment. Concordant zircon UPb dates, anchoring the lower intercept, yield an age of 596 ± 7 (2σ) Ma for the timing of Pb-loss. This most likely occurred in a zone of rifting and mafic magmatism beneath Central New York and Pennsylvania during the opening of the Iapetian Ocean.

Multi-episodes of pre-Cenozoic bimodal magmatism in the Himalaya in response to arc-back-arc settings

The Himalayan orogen preserves multi-episodes of pre-Cenozoic magmatic history, but remains a subject of debate for their tectonic settings, limiting a better understanding of the Himalaya litho-tectonic structure. This study demonstrates the protolith nature of granitic gneisses and garnet amphibolites collected from three areas (Kangmar, Cona, and Arun Valley) in the east-central Himalaya. Then we investigated the bulk-rock major and trace elements, Sr-Nd isotopes, zircon U-Pb ages, and Hf isotopes. The garnet amphibolites are classified into sub-alkaline basalt and show tholeiite affinity. Garnet amphibolites from the Kangmar and Cona areas show Paleozoic (512–450 Ma) and Neoproterozoic (803 ± 8 Ma) protolith ages, respectively. Geochemically, they display similar MORB-like trace element features and enrich in large ion lithophile elements (LILEs: Rb, Th, U, Sr, Pb) and Ta, Ti, and mantle-like Sr-Nd isotopes with initial 87Sr/86Sr of 0.667787–0.705088 and εNd(t) of 2.9–5.0. The high Th/Yb and Ba/Th ratios and varied εHf(t) (−22.3–11.0) indicate subduction-related material contributions. The geochemical affinity to MORB and slab contributions is associated with a back-arc basin setting. Besides, garnet amphibolites from Arun Valley yield a protolith age of 1869 ± 10 Ma and are divided into two groups. Group I exhibits enriched LREE patterns [(La/Yb)N = 5.99–6.44] and shows wide εNd(t) of −1.3 to 7.1, variable Th/Ta and Nb/La, and high Zr/Y ratios. These features suggest the protolith of Group I might be continental flood basalts. In comparison, Group II exhibits MORB-like REE patterns, enrichments of LILEs, and depletions of high field strength elements (HFSEs) similar to island arc basalt. Thus, the geochemical affinity to MORB and arc-like features indicates they are metamorphosed from back-arc basalts. As for the granitic gneisses, the Kangmar and Cona samples give a Paleozoic age range of 552–438 Ma, due to a protracted (∼110 Myr) magmatism. The Kangmar granitic gneisses were metamorphosed from arc-related I-type granites, by contrast, the Cona granitic gneisses belong to A-type granites formed in a back-arc setting. These Paleozoic granitic gneisses show similar εHf(t) of −7.1–−2.5 and TDM2 of 1229–1923 Ma, indicating they were derived from partial melting of Meso- to Paleoproterozoic crust. Furthermore, the Arun Valley granitic gneisses give an 1819 ± 6 Ma protolith age and show arc features with enriched LILEs and depleted HFSEs. The wide εHf(t) of −5.3–13.2 indicates the varying degree of mantle contributions resulting from slab break-off or roll-back. Therefore, the Himalaya witnessed bimodal magmatism during Paleoproterozoic, Neoproterozoic, and Paleozoic related to Andean-type orogeny. The multi-episodes of Andean-type magmatism contribute to the formation of the Cenozoic Himalayan architecture.

Geochemical constraints on the provenance and tectonic setting of metasedimentary rocks from the South Delhi Supergroup, NW India: implications for tectonic evolution of western margin of the Aravalli orogen

ABSTRACT The provenance and tectonic setting of the Stenian (<1200 to 1015 Ma) metasedimentary sequences of the South Delhi Supergroup, NW India, are least understood and thus debatable. The results of this study show that the protoliths of the metasedimentary rocks received detritus dominantly from felsic sources, particularly from Mesoproterozoic granitoids exposed in northern and eastern India. The weathering indices suggest low to moderate weathering in the source areas and that the sediment compositions reflect mixing of provenance components having different compositions. Overall, the REE patterns of the South Delhi rocks are similar with variable negative Eu anomalies and absolute ∑REE contents. The low REE abundances in the quartzites are the consequence of quartz dilution during hydraulic sorting. The tectonic multidimensional discrimination diagrams suggest a passive margin setting for the South Delhi metasedimentary rocks, which is in consonance with the detrital zircon age spectra and regional geology of the western margin of Aravalli orogen, NW India.

Carboniferous back-arc bimodal rocks in West Kunlun during northward subduction of the Paleo-Tethys

ABSTRACT The widespread occurrence of Carboniferous Wuluate Formation volcanic rocks in the West Kunlun is closely related to the evolution of the Paleo-Tethys but has received little attention. In this study, we conducted detailed fieldwork on the Wuluate Formation in West Kunlun and performed zircon U‒Pb dating, whole-rock major and trace element analysis, whole-rock Sr-Nd isotope analysis, and in situ zircon Hf isotope analyses. The Wuluate Formation is characterized by a bimodal volcanic suite. Zircon U‒Pb dating indicates that the rhyolites erupted at ca. 348–353 Ma, which is coeval with the zircon U‒Pb dating result of 354 Ma for the basalt. The basalts show a tholeiitic magmatic evolution trend with flattened REE patterns and depleted whole-rock Sr-Nd and zircon Hf isotopes, indicating geochemical affinity to back-arc basin basalt (BABB). Trace element modelling results indicate that the primitive basaltic compositions fit well with the products of the DMM + sediments + H2O source under melting conditions of asthenospheric temperature and thin lithosphere thickness, and the REE modelling results show that the basalts underwent 40% to 95% crystallographic differentiation of olivine, monazite, hornblende, plagioclase, and magnetite. The silicic rocks are calcic to calcic-alkaline and meta/peraluminous. They exhibit flattened REE patterns and slightly enriched zircon Hf isotopes, which could originate from Precambrian metaigneous sources of West Kunlun. The Kungai bimodal suite is likely to have formed in a back-arc setting corresponding to the northward subduction of Paleo-Tethys. The subduction of Paleo-Tethys had already occurred, and a back-arc basin had developed before the Early Carboniferous.

Role of colloids in the transfer and dispersion of trace elements into river waters through a former mining district

Characterizing the different chemical forms of trace elements in surface inland waters of mining regions is very important to understand the mechanisms underlying metal(loid) transport, their accumulation and toxicity, especially when waste storage areas are closely linked to the hydrographic network. This study focuses on the oldmining district of Salsigne in the Orbiel Valley (France), which was at one time the largest gold mine in Europe and the world’s largest arsenic mine. Our objective was to better understand the factors governing the transport of As and to investigate the role of colloidal matter as vector of major (Al, Ca, Fe, Mg) and trace (As, Pb, REE) elements in waters.A field sampling campaign was carried out during high water season to collect samples from the Orbiel River and its tributaries. In situ (ultra)filtrations at different cut-off thresholds (0.22 μm, 0.025 μm, 100 kDa and 3 kDa) were performed and physicochemical parameters (conductivity, pH, alkalinity, Dissolved Organic Carbon: DOC, anions, major cations, and metal(loid)s concentrations) of river water were assessed in all fractions.Results showed decreasing concentrations of DOC, Fe, Al and REE in the conventionally dissolved fraction (< 0.22 μm) from upstream to downstream. Arsenic concentration increased from upstream to downstream, especially at the exit of the old mining zone, and was mainly present in the truly dissolved fraction. However, arsenic in the colloid fraction was about 40% upstream and showed a clear decrease when going downstream. Upstream, Fe and Al were mostly present in the colloidal fraction (around 90%, but decreased to 30% downstream for Al). REE patterns suggested that colloidal organic matter played a significant role in metal transport. This was also inferred from the greater enrichment in middle REE as compared to light and heavy REE; this difference in enrichment progressively disappeared with decreasing cut-off filter size. The concentration variations along the length of the river were found to be a combined effect of the changes in lithology and input of contaminants from storage areas or alluvial table.

Diagnostic REE patterns of magmatic and hydrothermal apatite in the Zhuxi tungsten skarn deposit, China

Apatite is increasingly used as a tracer for petrogenetic and hydrothermal processes. We studied the cathodoluminescence (CL) texture and composition of igneous and hydrothermal apatite from the Zhuxi biotite granite and its associated hydrothermal tungsten system in the Late Mesozoic Jiangnan porphyry-skarn tungsten belt in South China. Both magmatic and hydrothermal apatite is anhedral to subhedral, with up to >1 mm grain size. Igneous apatite has relatively constant ΣREE + Y contents in the 2700–6500 ppm range (3970 ± 1003 ppm; n = 52). Hydrothermal apatite in exoskarn has lower ΣREE + Y contents in the range from <1 ppm to ∼1500 ppm, i.e. a strongly heterogeneous REE distribution (101 ± 253 ppm; n = 68). The ΣREE + Y contents correlate positively with CL brightness in both magmatic and hydrothermal apatite.The REE patterns of magmatic apatite display patterns similar to the host rock (Zhuxi biotite granite): generally flat ((La/Yb)N = 1.19–8.06), with moderately negative Eu anomalies (EuN/EuN* = 0.11–0.51), slightly positive Ce anomalies (CeN/CeN* = 1.05–1.12), and chondritic Y/Ho ratios (25–36, average = 30). The hydrothermal apatite is much different: downward-convex (MREE depleted) patterns ((La/Yb)N = 2.48–211), slightly to strongly positive Eu anomalies (EuN/EuN* = 1.11–540), slightly positive to moderately negative Ce anomalies (CeN/CeN* = 0.40–1.16), and non-chondritic Y/Ho ratios (40–152, average = 84). Since the hydrothermal samples are located outside of the pluton, excluding the influence of feldspar on Eu content, the differences of the Ce and Eu anomalies between magmatic and hydrothermal apatite from the retrograde skarn suggest that the retrograde skarn formed in a more oxidized environment than the granite.The low and variable contents of REEs, low Th/U, and non-chondritic Y/Ho ratios can be diagnostically used to distinguish hydrothermal from magmatic apatite, and the MREE-depleted REE pattern may be a common feature of apatite from retrograde tungsten skarn deposits.

Geochemistry of Neoproterozoic Nagod Limestones from the Girgita Mine, Bhander Group, Madhya Pradesh, India

Vindhyan limestone of the Bander Group (Late Neoproterozoic) in the Maihar area, Satna district, is a part of an anticlinal fold and it forms an outlier in the study area. Geochemical analysis reveals that the proportion of Rare Earth Elements (REEs; La-Lu) in limestones varies between 14.8 ppm and 49.5 ppm. The Eu and Ce anomalies (normalized against Post Archean Australian Shale; PAAS) range from ~ 0.53 to 0.69 and ~ 0.57 to 1.07, respectively. The results of this study revealed that the limestones were principally controlled by the admixture of detrital materials, i.e., (i) The low values of REEs (ii) high values of Y/Ho (iii) positive correlation between REEs and Si, Al, Ti, V, Co, Ni, and Nb, and (iv) slightly negative correlation between CaO and REEs. Further, the variations in Ce anomaly may be due to the influx of detrital materials and positive Ce anomaly was attributable to the shallow marine depositional condition. The distribution of REEs (Chondrite normalized values) illustrates different patterns in the Nagod limestone due to the degree of differentiation in terms of LREEs and HREEs and Eu anomaly. The variations in elemental concentrations and the REE patterns revealed the mixing of detrital materials in the Nagod limestones.

Granulite facies metasedimentary rocks: Insight into the formation of the East Kunlun continental arc

The metamorphism and dynamic processes of the Early Paleozoic continental margin arc in the East Kunlun orogen are still unclear. This study investigated the metasedimentary rocks with granulite facies in the Qingshuiquan area in the eastern section of the East Kunlun orogen. The kyanite is not found in the pelitic granulite. The phase equilibrium modeling indicates a middle-pressure type with metamorphic conditions of 0.7 to 0.8 GPa and 800 to 840°Cfor pelitic granulite. UPb dating of zircons from metasedimentary rocks yielded metamorphic ages of 490-480 Ma and detrital zircons ages of 1700-1500 Ma. REE patterns of the metasedimentary rocks are enriched LREEs and distinct negative Eu anomalies, the εNd values vary from −1 to −16. These results imply that the Precambrian supracrustal rocks were buried into the middle-lower crust level and granulite facies metamorphism occurred in Late Cambrian-Early Ordovician.Synchronous (520-480 Ma) magmatic rocks in Qingshuiquan and adjacent areas indicate that subduction of the Proto-Tethyan oceanic crust led to the formation of arc magmatic rocks. Meanwhile, it induced some sedimentary rocks to underthrust or return flows into the middle-lower crust of the upper plate and underwent granulite facies metamorphism.

Multiple pulses of fluids involved in the formation of carbonatite-related REE deposits as revealed by fluorite

Formation of carbonatite-related REE deposits generally associated with hydrothermal fluids exsolved from carbonatite melts. Deciphering the nature and evolution of the fluids is critical for establishing ore-formation models, and was mostly benefited from multi-analytical investigation of specific minerals presenting in multi-generation veins. However, there is a risk that the minerals in specific-generation veins not always precipitated solely from the single-stage fluids particularly for a dynamic mineralizing system. In order to evaluate this potential risk, this study conducted a combined textural, in situ trace elemental and fluid inclusion investigation of fluorite from multiple generations of veins in the Muluozhai carbonatite-related REE deposit, SW China. The veins dominate the mineralization system of the deposit, and include four generations/stages: fenitization (Stage I), pre-REE (Stage II), early REE (Stage III) and late REE (Stage IV) stages in which the ore minerals are all dominated by bastnäsite. Fluorite grains are widely present in these generations of veins, but high-resolution CL imaging reveal that most of them display complex internal textures composed commonly of core, mantle and rim domains from center outward. These domains exhibit different CL-intensities that are well correlated with chemical compositions and even types of fluid inclusions they hosted. This study confirmed that the “core-mantle-rim” textures of fluorite are essentially responses to interaction of early grains (i.e., represented by cores) with multi-stage fluids through dissolution-reprecipitation processes. Trace elemental and fluid-inclusion microthermometric results show that the cores (also for mantles or rims) in fluorite from Stages I to III veins have comparable REE concentrations and patterns and homogenization temperatures of fluid inclusions, strongly suggesting that the core-mantle-rim fluorite grains in these veins are originally formed at Stage I, and were variably captured or metasomatized by fluids of Stage II or III during development of these generations of veins. Similar core-mantle-rim fluorite grains are also present in Stage IV veins, and thus are constrained to be also “xenocrysts” formed in a similar manner. They are surrounded or even locally disturbed by a number of euhedral and/or slightly oscillatory fluorite grains which are supposed to be formed from a different manner. On the basis of their close association with bastnäsite and relatively high homogenization temperatures of fluid inclusions they host, we proposed that these euhedral fluorite grains are products of Stage IV fluids.Clear understanding of the internal textures and paragenesis of fluorite allows us to propose a genetic model for the Muluozhai deposit that the REE mineralization in Stages III and IV involves likely different pulses of carbonatite-related REE-rich fluids, and REE deposition was possibly mainly triggered by incursions of external fluids (e.g., meteoric water). This study also highlights that the phenomenon regarding presences of early-stage “xenocrysts” in late-generation veins of carbonatite-related vein systems may be more common than previously thought, and thus investigation in a textural context is critical for deciphering ore genesis.

Fertile mantle domains with different metasomatic features beneath the eastern North China Craton: Transformation from variably auto-metasomatized asthenospheric upwelling

The eastern North China Craton is known for replacement of refractory lithospheric mantle by fertile one relevant to the (Paleo-) Pacific subduction. However, the formation mechanism of the fertile mantle domain remains enigmatic, thus hindering profound understanding of the associated lithosphere-asthenosphere interaction. To explore the deep dynamic processes, we present petrology, in-situ element and Sr isotopic data on spinel peridotite xenoliths entrained by the Cenozoic basalt in the Qingyuan county, Liaoning Provence.The Qingyuan mantle xenoliths can be categorized into two groups based on textural characteristics and mineral compositions. Group A peridotites consist of lherzolites containing patch of fine-grained aggregates and have low olivine Fo (100×Mg/(Mg + Fe) ≤ 89, atomic ratios) and mineral major-element composition trends deviating from residual mantle. The coarse-grained clinopyroxenes outside the patch and the patch-hosted fine-grained clinopyroxenes and amphiboles display similar light rare earth element (LREE)-enriched patterns and comparable 87Sr/86Sr ratios (0.7035–0.7039, 0.7040–0.7062 and 0.7029–0.7033, respectively). These characteristics suggest that Group A peridotites may represent fragments of lithospheric mantle strongly modified by melt impregnation. In contrast, Group B xenoliths, typically lherzolites, have protogranular texture without the patches of fine-grained aggregates. The olivines have moderate Fo (ca. 90–91) and mineral major-element compositions highly resembling melting residua. The melting is estimated to occur at about 1370–1590 °C and 2–4 GPa, corresponding to mantle potential temperatures of around 1450–1550 °C. The clinopyroxenes show variably enriched REE patterns and depleted 87Sr/86Sr ratios (0.7020–0.7044). These peridotites are thus interpreted as fragments of residual lithospheric mantle transformed from small-scale asthenospheric upwelling. Diffusion modeling of trace elements in clinopyroxenes suggests ubiquitous Mesozoic-Cenozoic metasomatism in the studied peridotites.Combining with other Cenozoic basalt-borne peridotite xenoliths from the eastern NCC and associated tectonic setting, we conclude that the fertile lithospheric mantle domains with different metasomatic features may be transformed from variably auto-metasomatized asthenospheric upwelling. That is, the subduction-related small-scale upwelling not only leaves behind residual domains with weak metasomatism but also produces mantle regions strongly infiltrated by the decompressional melts. This model links the mantle domains of different formation mechanisms to coherent geodynamic processes and emphasizes the complex mantle processes associated with the fluctuated lithosphere-asthenosphere boundary.

LA-ICP-MS U-Pb dating, chemical composition and evolution of oscillatory zoned andradite from the Chengmenshan Cu-Au deposit, Jiangxi province

The Chengmenshan Cu-Au deposit is a typical porphyry-skarn deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYRMB), developing abundant oscillatory zoned andradite. However, the physiochemical conditions, hydrothermal evolution and age of skarn formation is unknown. This paper presents BSE image, EMPA point analyses, LA-ICP-MS point analyses and mapping, U-Pb isotope data for andradite from the Chengmenshan Cu-Au deposit. From core to rim, three generations of andradite could be identified (Grt-1, Grt-2, Grt-3). Grt-1 (Adr73-88) exhibit compositional variations as oscillatory zoning. Grt-2 (Adr93-100) is almost pure andradite, with smooth surface. Grt-3 (And71-93) occur as halo surrounding earlier andradite, with broken surface. Incorporation of REE and U into all generations of andradite mainly follows coupled YAG-type substitution mechanism and the coupled substitution of [U4+]VIII + 2[Fe3+, Al3+]Ⅳ – [Ca2+] – 2[Si4+]Ⅳ in the dodecahedral position, respectively. Their associated intrusions, mineral assemblages and consistent positive Eu anomalies indicate an acidic, oxidizing and low salinity forming conditions. From core to rim, the contents of REE and HFSE are decreasing, and the REE patterns evolve from flatted, through LREE-enriched, HREE-depleted, to REE-depleted. The chemical composition and evolution of oscillatory zoned andradite suggest that the fluids in Chengmenshan deposit experienced the sequences of pore fluid-mixed, through magmatic-hydrothermal dominated, to meteoric water influx. In this process, the temperature, pH and fluid compositions varied. U contents are closely correlated with REE contents, especially LREE contents. The andradite samples yield Tera-Wasserburg lower intercept 206Pb/238U ages of 140.1 ± 2.6 Ma (MSWD = 1.8), representing the age of Cu-Au mineralization. These results indicate the evolutionary characteristics of hydrotheraml fluids and precisely date the Cu-Au skarn formation, which may serve as a useful indicator in the exploration of similar skarn deposits and point selection for U-Pb dating.