Light Rare Earth Element Depletion Research Articles

Trace element variations in mussels' shells from continent to sea: The St. Lawrence system, Canada

Rare Earth Elements (REE) and several trace elements abundances in mussel's shells collected along the St. Lawrence River, the Estuary, and the Gulf of St. Lawrence (EGSL) reveal coherent chemical variations, with a sharp contrast between freshwater and seawater bivalves. In freshwater mussel's shells, Rare Earth Elements and Y (REY) patterns are rather flat. Their Mn and Ba concentrations are higher than those of EGSL mussel shells, which are much richer in Sr. Shale-normalized REY abundances in mussel's shells from the EGSL show positive anomalies in La and Y and well-marked negative anomalies in Ce, reflecting those of seawater. Prince Edward Island shells show light REE depletion relative to PAAS, positive La and Y anomalies, and negative Ce anomalies. Our data confirm the lack of detectable Gd pollution in the St. Lawrence River and in the EGSL, as well as Pb pollution at the mouth of the Saguenay Fjord and near Rimouski.

Nd isotope and element composition of shales from the Parnaíba Basin, NE Brazil: Insights for provenance shifts in a cratonic basin

Sm-Nd and Sr isotope compositions of 24 samples of fine-grained clastic rocks from eastern Parnaíba basin, NE-Brazil, are presented along with major and trace element compositions of 19 of these samples. Samples were collected from outcrops of the Paleozoic Canindé (Pimenteiras, Longá and Poti) and Balsas (Piaui Formation) groups, and from the Jurassic Pastos Bons Formation. Discrimination diagrams based on Th, Sc, La and Zr indicate sedimentary provenance from felsic dominant sources and high degrees of crustal recycling. PAAS-normalized REE diagrams show that the Pimenteiras and Longá Formations display flat patterns with slight REE enrichment, and initial εNd and 87Sr/86Sr around the average composition of the upper continental crust, respectively between −8.9 and −14.6 and between 0.711 and 0.760, indicating provenance from old Precambrian basement. The Nd TDM model ages of the Pimenteiras between 1.32 Ga and 2.00 Ga indicate heterogeneity of source areas during the Devonian, and TDM ages around 1.6 Ga in the Longá Formation samples suggest intense recycling of older sediments. The Poti formation samples yield TDM ages of 2.30 Ga and 2.32 Ga, respectively, suggesting tectonic reactivation and new input from Archean-Paleoproterozoic basement complexes. The Pastos Bons Formation samples show lower Th-Sc ratios, light-REE depletion, and less negative initial εNd, indicating contribution from mafic rocks, probably from the underlying Mosquito Formation intraplate tholeiitic basalts and dolerites.

Melting processes, cooling rates, and tectonic settings of the Neo-Tethyan mantle: the in-situ mineral chemical record

Peridotites in the Yarlung Zangbo ophiolite (YZO) have the potential to elaborate the mantle dynamics that operated below the Neo-Tethyan oceanic crust. Here, we measured major and trace element concentrations of minerals (e.g., clinopyroxene, orthopyroxene, olivine, and spinel) in YZO harzburgites and lherzolites to trace the origin and evolution of the YZO. The rare earth element (REE) patterns of clinopyroxenes in lherzolites and harzburgites plot within the field of abyssal and forearc peridotites, respectively, indicating that the latter experienced a higher degree of partial melting. Based on the relative extents of light (LREE) and heavy (HREE) REE depletions in clinopyroxene, we classified the lherzolites into two groups (I and II). The group I clinopyroxenes are depleted in HREEs and enriched in LREEs and MREEs compared to those of group II. Meanwhile, the REE patterns of harzburgite clinopyroxenes are identical to that of group II, although they display lower REE concentrations. Models based on REE abundances in clinopyroxene suggest that group I lherzolites experienced ~8–11% partial melting in the spinel domain, whereas group II lherzolites formed after 5% melting in the garnet domain followed by 6–9% and 9–16% partial melting in the spinel domain, respectively. These high degrees of melting may suggest that the harzburgites formed in a supra-subduction zone (SSZ) setting. Surprisingly, the REE-in-two-pyroxenes thermometer (TREE) shows that the YZO peridotites record high-temperature cooling at 991–1218 °C which is lower than the conditions below mid-oceanic ridges (MORs), but compatible with recent reports of high closure temperatures in forearc environments. Furthermore, our TREE results constrain the cooling rate of the YZO peridotites to ~0.01–0.5 °C/yr, overlapping with rates reported for SSZ, MOR, and abyssal peridotites. By integrating the petrological, geochemical, and thermal features, we tentatively interpret the group I lherzolites accreted in an Early Cretaceous forearc during subduction initiation, whereas the group II lherzolites and the harzburgites formed by further melting of the earlier lherzolites in a SSZ setting.

Fractional crystallization of garnet in alkali basalts at >1.8 GPa and implications for geochemical diversity of Large Igneous Provinces

A suite of lavas from northwest Mull, British Palaeocene Igneous Province, exhibit major and trace element characteristics that are best explained by very high pressure (>1.8 GPa) crystallization of an assemblage comprising aluminous clinopyroxene (Al-Cpx) plus garnet. The resulting series of consanguineous magmas are mildly Si-undersaturated. The trace element effects of this crystallizing assemblage are manifested in increasing light rare earth element (LREE) enrichment and heavy REE (HREE) depletion with decreasing whole-rock MgO, a predictable consequence of the bulk distribution coefficient (D) of this assemblage being > > 1 for HREE but <<1 for the LREE. Early crystallization of Al-Cpx and replacement of plagioclase feldspar by garnet in the crystallizing assemblage also results in increasing or near constant Al2O3 abundances in cotectic compositions with MgO < ∼7 wt%. Garnet as opposed to plagioclase crystallization is also reflected in the incompatibility of Sr and compatibility of Y resulting in very high Sr/Y lavas (up to 100) with high Sr (∼1200 ppm) and low Y contents (∼12 ppm). Lavas also have high Zr/Y (up to 30). Major element constraints suggest the crystallizing assemblage comprised ∼40% garnet and 60% Al-Cpx.Magmas that fractionated at ∼1.8 GPa rose to the surface without interacting with the continental crust. Immediately underlying basalts and picrites show evidence of crustal contamination and assimilated fusible portions of the crust, therefore effectively lining the plumbing system and allowing later magmas to rise to the surface without crustal contamination. A local change in tectonic regime from extension to passive appears to be linked to a change from low pressure to very high-pressure crystallization of magmas. Whilst evidence for garnet fractionation in continental flood basalts is very rare, this paper provides a characterization of its geochemical consenquences.

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 &gt;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.

Genesis and depositional environment of the Carboniferous Baishanquan iron deposit in Eastern Tianshan, Northwestern China

Submarine volcanic-hosted iron deposits are the most important iron deposit in Northwestern China. However, their origin remains controversial. Here, we present detailed studies of the petrology, geochronology, geochemistry and FeSi isotopes of iron ores from Baishanquan deposit in Eastern Tianshan, Northwestern China. Similar to Precambrian banded iron formations (BIFs), the Baishanquan deposit is hosted in a volcanic-sedimentary succession, the iron ores are characterized by banding with alternate iron-rich and silica-rich layers, and the mineral assemblage is dominant by magnetite, quartz and Fe-silicates. The iron ores show distinct light rare earth elements (REE) depletion [(La/Yb)PAAS = 0.24–0.47], no significant Eu anomalies (Eu/Eu⁎ = 0.88–1.16), relatively high Y/Ho ratios (30–34), and δ30Si values in quartz from the iron ores ranging from −0.80 to 1.67‰. These data indicate mixed seawater and low-temperature submarine hydrothermal source for both the iron and silica. The absence of negative Ce anomalies suggests that the depositional environment was characterized by low oxygen conditions similar to modern oxygen minimum zones. Positive δ56Fe values in most magnetite (−0.45 to +1.02‰) suggest partial Fe(II) oxidation of the iron ores. Interestingly, the iron ores also contain relatively high Al2O3, CaO, MgO, and Zr concentrations, indicating that the sedimentary basin was subject to periodic clastic input. Indeed, zircon LA-ICP-MS UPb dating of interlayered dacite indicates that the Baishanquan volcanic-sedimentary rocks and associated iron ores were deposited at 327.3 ± 2.8 Ma (N = 25, MSWD = 0.52). This is significant because the appearance of these iron deposit provides direct evidence that marine ferruginous conditions – at least locally - may have still been present at Carboniferous period.

Geochemical Characteristics of Garnet from Zinc–Copper Ore Bodies in the Changpo–Tongkeng Deposit and Its Geological Significance

The Changpo–Tongkeng tin polymetallic deposit in Dachang, Guangxi, is a world-class, superlarge, polymetallic tin deposit consisting of lower skarn zinc–copper ore bodies and upper tin polymetallic ore bodies. Garnet is the main gangue mineral in the skarn zinc–copper ore bodies and has a granular texture. Based on hand specimens and microscopic observations, the existing garnet can be divided into two generations: an early generation (Grt I) and a late generation (Grt II). The results of electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in situ microanalysis show that the contents of SiO2 and CaO in the garnets from the two generations present limited variations, while the FeOT and Al2O3 contents vary significantly, indicating the grossular–andradite solid solution series (Gro29–82And12–69). Compared with Grt I (Gro72And25), Grt II (Gro39And59) is Fe-enriched and oscillatory zoning is developed. The total rare earth element (REE) contents in the two generations of garnet are relatively low, showing light rare earth element (LREE) depletion and heavy rare earth element (HREE) enrichment patterns. Grt II has higher REE content than Grt I and exhibits significant negative Eu anomalies (δEu = 0.18–0.44). The contents and variation characteristics of the major and trace elements in the two generations of garnet suggest that there were variable redox conditions and water/rock ratios in the hydrothermal system during the crystallization process of garnet. In the early stage, skarnization was in a relatively closed and low-oxygen fugacity system, with hydrothermal diffusion metasomatism being dominant, forming homogeneous Grt I lacking well-developed zoning. In the late stage of skarnization, the oxygen fugacity of the ore-forming fluids increased, with infiltration metasomatism being dominant, forming Grt II with well-developed oscillatory zoning. The contents of Sn, As, W, In, and Ge in the garnets are relatively high and increase with the proportion of andradite. Sn in zinc–copper ore bodies mainly exists in the form of isomorphic substitution in garnet, which may be the main reason for the lack of tin ore bodies during the skarn stage. This paper compares the trace element contents in garnets from domestic skarn deposits. The results indicate that the Sn content and δEu in garnet can be used to evaluate the tin-forming potential of skarn deposits.

Whole Rock, Mineral Chemistry during Skarn Mineralization-Case Study from Tongshan Cu-Mo Skarn Profile

Studying the activation, migration and precipitation processes of ore-forming elements is essential for understanding the genesis and mechanisms of skarn deposits. A typical skarn profile formed by the intrusion of Yanshanian granodiorite into the Carboniferous carbonate strata was studied. The profile is highly consistent with the classic skarn profile, ranging from the intrusion, weak alteration belt, skarn belt (inner and outer skarn belt) and mineralization belt (mainly characterized by Cu mineralization) to the surrounding marble without being affected by late-stage low-temperature or supergene weathering alteration. Whole-rock data show that the major and trace elements exhibit relatively small changes in the granodiorite and inner skarn, but huge variation in the boundary between the inner and outer skarn; Na, Al, Ti and Sr show significant decreases, while Fe, Mg, Zn, V and Ni show significant increases. The elemental content in the outer skarn is 10–100 times or more higher than that in the marble, but the elements such as Ca, Sr and Cs diluted from the marble. During the migration process from the inner skarn to the outer skarn, some elements (such as K, Rb and Ba) were depleted in the inner, but not enriched in the outer, indicating that they may migrate to farther locations. Grossularite developed in the inner skarn, with light rare earth element (LREE) depletion and heavy REE enrichment, as well as positive and negative anomalies of Eu (δEu = 0.42–3.95). Andradite developed in the outer skarn, with zonation development, light REE enrichment, and heavy REE depletion and a positive Eu anomaly (δEu = 0.36–46.83). Some negative Eu anomalies appear at the edges of garnets in the outer skarn, indicating fluctuations in fO2 during the late skarn process. A positive correlation between Fe3+ and REE3+ in the garnets from the inner skarn, as well as between Al3+ and REE3+ from the outer skarn indicated that there are different YAG substitution mechanisms of REE between the inner and outer skarn. Low garnet REE contents and highly variable Y/Ho ratios in outer skarn suggest that the significant fluctuations in REEs may be primarily controlled by water-rock interactions. Considering the whole-rock major and trace element contents, as well as the trace element features of garnet, we found that whole-rock Na, Al, Ti and Sr elements, garnet Ti, Zr and Nb elements exhibit significant differences between the inner and outer skarn. These characteristics can be used to distinguish the boundary between the rock body and carbonate during the skarnification process.

Heterogeneous Subcontinental Lithospheric Mantle below the South Margin of the Siberian Craton: Evidence from Composition of Paleoproterozoic Mafic Associations

Abstract —Paleoproterozoic mafic associations of the Irkut block from the Sharyzhalgai uplift are gabbro-dolerite dikes and small gabbronorite and monzodiorite massifs, which formed at 1.87–1.84 Ga and were coeval with granitoids and basite intrusions of the South Siberian magmatic belt (SSB). All the Paleoproterozoic mafic associations of the Irkut block are characterized by the presence of biotite and alkali feldspar, enrichment in K2O, LILE, Th, and light REE, highly fractionated multielement spectra with sharp Nb and Ti depletion, and extremely low εNd(T) from –5.1 to –10.1. In these compositional features, they are similar to mafic complexes in the central and eastern parts of the SSB (the Baikal uplift and the western Aldan shield). Their geochemical and isotopic characteristics did not result from crustal contamination but point to derivation from the subcontinental lithospheric mantle (SCLM) enriched by reaction with felsic subduction-related and OIB-like mafic melts formed at a low degree of melting. The geochemically contrasting Paleoproterozoic gabbronorites in the Onot block of the Sharyzhalgai uplift are marked by depletion in K2O, Ba, LILE, Th, and light REE, weak depletion in Nb, and higher εNd(T) from –0.3 to –1.4. The gabbronorites indicate not only an increase in the contribution of a depleted source to their genesis but also the heterogeneity of the subcontinental lithospheric mantle below the south margin of the Siberian Craton. The formation of enriched SCLM domains throughout the South Siberian belt was mainly the result of Archean subduction-related metasomatic processes. The wide distribution of Paleoproterozoic mafic complexes with subduction geochemical signatures and negative εNd(T) on most early Precambrian cratons is due to global change in the composition and an increase in the heterogeneity of the subcontinental lithospheric mantle toward the end of the Archean.

Petrogenesis and tectonic setting of Early Cretaceous mafic dykes in the North Qinling Orogenic Belt, central China: constraints on the lithospheric lower crust delamination

Late Mesozoic mafic dykes, which are widely developed in the North Qinling Orogenic Belt (NQOB), include abundant geodynamic information. This paper describes the mafic dykes that intrude the Late Jurassic granite in the Dayu and Kuyu areas, and reports important petrological constraints for the late Mesozoic tectonic transition from compression to extension in the NQOB. Three zircon U–Pb results show that the minimum ages of the mafic dykes are 139.8 ± 1.4 Ma, 137.4 ± 1.7 Ma and 133.4 ± 0.9 Ma, indicating that the emplacement age of the Dayu and Kuyu mafic dykes is 140–133 Ma. Petrogeochemical analyses suggest that the mafic dykes belong to the high-K calc-alkaline shoshonite series with low SiO2 (46.93–56.73 wt%), MgO (1.88–9.10 wt%) and TiO2 (1.17–1.82 wt%), and high Al2O3 (13.98–17.46 wt%), TFe2O3 (7.81–10.92 wt%) and K2O (1.28–4.78 wt%). The mafic dykes are enriched in large ion lithophile elements (e.g. Rb, Ba, K, La, Sr) and depleted in high-field-strength elements (e.g. Nb, Ta, Zr, Ti). These samples have the right-sloping chondrite-normalised rare earth element patterns, which suggest light rare earth element enrichment and heavy rare earth elements depletion with no obvious Eu anomalies (δEu = 0.94–1.11). The I Sr, ε Nd(t), ε Hf (t) and T DM2(crust) values are 0.7056–0.7060, −10.60 to −5.98, −14.1 to −2.8, and 1382.4 ± 25.1 to 2081.9 ± 47.6 Ma, respectively. Both elemental and isotopic geochemistry show that the formation of Dayu and Kuyu mafic dykes is due to the partial decompression melting of previously enriched lithospheric mantle during a delamination process. The mafic dykes have undergone fractionation crystallisation of Mg–Fe phase minerals during magma ascent, accompanied by some crustal contamination. Combined with the regional tectonic setting, we suggested that the NQOB experienced intra-continental extension during the Early Cretaceous. KEY POINTS Early Cretaceous (140–133 Ma) mafic dykes have been discovered in the middle part of the North Qinling Orogenic Belt. The remote effect of the Paleo-Pacific Plate subduction has reached the middle of the North Qinling Orogenic Belt. The North Qinling Orogenic Belt entered the extensional stage in the Early Cretaceous (140–133 Ma).

Rare earth element signatures of Doushantuo cap dolostones capture an increase in oxygen in the anoxic Ediacaran ocean

The Rare Earth Element (REE) systematics of the post-Marinoan cap dolostones reflect the marine redox conditions and chemistry in the immediate aftermath of the snowball Earth. Rare earth elements and yttrium (REY) compositions in the Doushantuo cap dolostones that directly overlie Nantuo glacial diamictites in south China are determined from the inner shelf to the slope. In general, shale-normalized REY patterns (REYSN) of the cap dolostones show significant fractionations that are characterized by light REE depletion, slight middle REE enrichment relative to the light and heavy REEs, positive Eu anomalies, and slightly super-chondritic Y/Ho ratios. These dolostones, however, show no significant shale-normalized negative Ce anomalies. Such REYSN patterns are consistent with an extensive anoxic Ediacaran ocean in the shallow-to-deep water columns. The REE concentrations and normalized patterns in the cap dolostones vary spatially and stratigraphically. When compared to the shelf edge and the slope, cap dolostones from the inner shelf are distinguished by uniformly less fractionated REYSN patterns with an absence of Eu anomalies, and suppressed light REE depletion and MREE enrichment. These marked differences in the REYSN patterns indicate significant continental influences on the seawater chemistry within the restricted inner shelf (lagoon) during glacial meltdown. The temporal trends in the REY dataset of the Doushantuo cap dolostone reflect shifts in marine redox conditions. Dolostones from two thick shelf margin sections (5 m and 7 m thick, respectively) show almost identical stratigraphic trends in the REY concentrations, and the Ce, Y, and middle REE anomalies. The overall stratigraphic decrease in the Ce anomaly, accompanied by the increase in the Y anomaly and decrease in the middle REE anomaly towards the top of the rock unit, suggests that dissolved oxygen was increasing in the shallow water column during the deposition of the cap dolostones. The REE data in this study reveal that the oxygenation of the Ediacaran oceans started in the immediate aftermath of the snowball Earth (635–632 Ma).

Rare Earth Element Deposits in Mongolia

In Mongolia, rare earth element (REE) mineralization of economic significance is related either to the Mesozoic carbonatites or to the Paleozoic peralkaline granitoid rocks. Carbonatites occur as part of alkaline silicate-carbonatite complexes, which are composed mainly of nepheline syenites and equivalent volcanic rocks. The complexes were emplaced in the Gobi-Tien Shan rift zone in southern Mongolia where carbonatites usually form dikes, plugs or intruded into brecciated rocks. In mineralized carbonatites, REE occur mainly as fluorocarbonates (bastnäsite, synchysite, parisite) and apatite. Apatite is also present in the carbonatite-hosted apatite-magnetite (mostly altered to hematite) bodies. Alkaline silicate rocks and carbonatites show common geochemical features such as enrichment of light REE but relative depletion of Ti, Zr, Nb, Ta and Hf and similar Sr and Nd isotopic characteristics suggesting the involvement of the heterogeneous lithospheric mantle in the formation of both carbonatites and associated silicate rocks. Hydrothermal fluids of magmatic origin played an important role in the genesis of the carbonatite-hosted REE deposits. The REE mineralization associated with peralkaline felsic rocks (peralkaline granites, syenites and pegmatites) mainly occurs in Mongolian Altai in northwestern Mongolia. The mineralization is largely hosted in accessory minerals (mainly elpidite, monazite, xenotime, fluorocarbonates), which can reach percentage levels in mineralized zones. These rocks are the results of protracted fractional crystallization of the magma that led to an enrichment of REE, especially in the late stages of magma evolution. The primary magmatic mineralization was overprinted (remobilized and enriched) by late magmatic to hydrothermal fluids. The mineralization associated with peralkaline granitic rocks also contains significant concentrations of Zr, Nb, Th and U. There are promising occurrences of both types of rare earth mineralization in Mongolia and at present, three of them have already established significant economic potential. They are mineralization related to Mesozoic Mushgai Khudag and Khotgor carbonatites in southern Mongolia and to the Devonian Khalzan Buregtei peralkaline granites in northwestern Mongolia.

Geochemistry, zircon U-Pb chronology and Hf isotope composition of the Heishan’gou iron deposit in the Bikou Terrane, central China: Implication for the genesis of the Yudongzi banded iron formations

The Yudongzi banded iron formations (BIFs) are widely spread in the Neoarchean Yudongzi Group of the Bikou Terrane, Central China. Their sedimentary and metamorphic ages, however, remain controversial, either their forming or reworking processes. In this contribution, we conducted an integrated study of zircon U-Pb-Hf isotope geochemistry, as well as mineral and bulk-rock geochemistry on the Heishan’gou iron deposit, the largest iron deposit of the Yudongzi BIFs, to constrain their formation and metamorphism ages, source of ore-forming material and tectonic setting. Mineral chemistry analysis indicates that the amphibolite, host-rock of the Heishan’gou deposit suffered amphibolite facies metamorphism. LA-ICP-MS U-Pb dating of zircon grains from orthometamorphic host-rocks suggests that the Heishan’gou deposit was formed at the early Neoarchean (2781 ± 43 Ma for amphibolite and 2776 ± 43 Ma for plagioclase-hornblende schist), and metamorphosed at ca. 2480 ± 26 Ma and 1818 ± 44 Ma, respectively. The low contents of aluminum, titanium and high field strength elements in BIFs suggest that they are detritus-free marine chemical precipitates. The rare earth elements (REE) and Y patterns of BIFs display seawater-like signatures with light REE depletion, and positive La and Y anomalies. Meanwhile, their positive Eu anomalies suggest that submarine hydrothermal fluid might also be involved in the precipitation of BIFs. Combined with zircon Hf isotope data and geochemical compositions of amphibolites, it is proposed that the Yudongzi BIFs should be formed at back-arc basin setting. Compared with the representative Archean continent nucleus of the Yangtze Block (Kongling Complex), the Yudongzi Complex experienced distinct crustal evolutionary history. It indicates that the Yudongzi Complex might represent the accretionary block combined to the Yangtze continental nucleus later than the Paleoproterozoic.

A tool to distinguish magmatic from secondarily recrystallized carbonatites—Calcite/apatite rare earth element partitioning

Abstract Crustal geochemical signatures in carbonatites may arise from carbon recycling through the mantle or from fluid-mediated interaction with the continental crust. To distinguish igneous from fluid-mediated processes, we experimentally determined rare earth element (REE) partitioning between calcite/melt and apatite/melt at subvolcanic emplacement conditions (1–2 kbar, 750–1000 °C). Our data allow modeling of calcite-apatite (Cc/Ap) partition coefficients (D), representing a new tool to bypass the previously required but largely unknown carbonatite melt composition. Experimentally determined magmatic calcite/apatite REE patterns are flat, as is ~0.75, and they show a slight U-shape that becomes more pronounced with temperature decreasing from 1000 to 750 °C. Application to texturally well-equilibrated natural Ca-carbonatites and calcite-bearing nephelinites shows that some calcite-apatite pairs follow this pattern and, hence, confirm the magmatic nature of the carbonates. values of other mineral pairs range from 10−2 to 10−3, which, together with a substantial light REE depletion in the calcite, is interpreted as fluid-mediated light REE removal during secondary calcite recrystallization. Calcite/apatite REE distributions are well suited to evaluate whether a carbonatite mineralogy is primary and magmatic or has been affected by secondary recrystallization. In this sense, our tool provides information about the sample's primary or secondary nature, which is essential when assigning isotopic crustal signatures (in Ca, C, or Sr) or REE patterns to related geologic processes.

Mineralogical Characteristics of Biotite and Chlorite in Zuluhong Polymetallic Deposit: Implications for Petrogenesis and Paragenesis Mechanisms of the Tungsten and Copper

The Zuluhong quartz-vein-type polymetallic deposit, located in the Alatau area of Western Tianshan, China, is a particular and typical tungsten deposit associated with copper. This paper presents major and trace element analyses of magmatic and altered (i.e, chloritized) biotite from the deposit, in order to identify the source of the magmas and characterize the mineralization physical-chemical condition. Magmatic biotite is Fe-rich and has high Rb/Ba ratios (0.27–9.14), indicative of extensive differentiation of granite. Moreover, magmatic biotite has total rare earth element (∑REE) contents that are 5–10% of the whole-rock contents, shows slight light REE depletion, and negative Ce anomalies. Magmatic biotite is enriched in some large-ion lithophile elements (LILE; e.g., Rb and K) and depleted in some high-field-strength elements (HFSE; e.g., Th and Nb). These geochemical features, coupled with geological evidence, indicate that the Zuluhong intrusion is a highly fractionated I-type granite derived from lower crustal melting. During ore formation, magmatic biotite was partially to totally altered to chlorite due to interaction with ore-forming fluids. The temperature and oxygen fugacity decreased during alteration. The mineralization in the Zuluhong polymetallic deposit can be divided into at least two stages. In the early stage, quartz-vein-type wolframite mineralization formed from Si- and volatile-rich fluids that were derived from fractionated granitic magma. In the later stage, W–Cu ores formed as metal sulfides were dominated by chalcopyrite. The later ore-forming fluids experienced a decrease in temperature and oxygen fugacity as they reacted (i.e, chloritization and lesser silicification) with reducing wall rocks around the contact zone of the intrusion.

Dissolved rare earth elements in the Pearl River Delta: Using Gd as a tracer of anthropogenic activity from river towards the sea

Growing demands for high-tech uses of rare earth elements (REE) result in their releases into the aquatic environment, but local anthropogenic sources and water chemistry reactions may vary in aquatic systems. In this study, we quantified the yttrium and REEs (REY) in the surface waters of the Pearl River Delta (PRD), Southern China, and investigated their sources and processes controlling the dissolved REY distributions from rivers towards the sea. Spatial variations of dissolved REY concentrations (<0.4 μm) were observed in the surface water due to both natural and anthropogenic sources. Salt-induced flocculation removed colloidal REY from water during estuarine mixing, particularly at low salinity. In contrast, a significant increase of dissolved REY concentration with salinity suggested the occurrence of REY desorption from particles during estuarine mixing. The PAAS-normalized REE patterns (i.e., filtrates <0.4 μm and <3 kDa) showed a significant light REE (LREE) depletion relative to heavy REE (HREE) in dissolved fractions, indicating reactive LREE adsorption onto particles. Positive Gd anomalies in most waters revealed significant impacts from human activities, such as effluents from wastewater treatment plants in the PRD. In particular, the anthropogenic Gd contributed 90 % of the total dissolved Gd along the Pearl River and was conservatively mixed with saltwater in the estuary via the Humen outlet. Positive Sm anomalies were also observed in the PRD, indicating other point sources related to industrial activity. With the increase of urbanization and high demand for green technology, the REY anomalies could be a good indicator of human activities on assessing their fluxes and impacts on the aquatic systems.

Petrogenesis of the Alubaogeshan intrusion in the Maodeng–Xiaogushan area, southern great xing'an range, NE China: Implications for magma evolution and tin–polymetallic mineralization

• The Alubaogeshan intrusion formed at ca. 134 Ma and belongs to transitional I–A 2 -type granite . • The Alubaogeshan intrusion was formed by partial melting of a Neoproterozoic juvenile crust. • The syenogranite porphyry evolved from the porphyritic monzogranite. • Fluid–melt interaction is crucial for W mineralization. • The magma evolutionary degree influences the size of the Sn–polymetallic deposits in the SGXR. The Alubaogeshan intrusion, consisting of porphyritic monzogranite in the central part and syenogranite porphyry in the edge, is genetically related to the Maodeng–Xiaogushan Sn–polymetallic deposit. This paper aims to investigate the genetic link between these two granite phases and the relationship between magma evolution and Sn mineralization. LA-ICP-MS zircon U–Pb dating of the porphyritic monzogranite and syenogranite porphyry yielded ages of 134 ± 1.0 Ma and 133 ± 0.5 Ma, respectively. The Alubaogeshan intrusion has high SiO 2 and total alkali (Na 2 O + K 2 O) contents, with low Al 2 O 3 , Fe 2 O 3 , FeO, and MgO contents, belonging to high-K calc-alkaline metaluminous–weakly peraluminous granites. Chondrite-normalized rare earth element (REE) patterns are characterized by light REE enrichment and heavy REE depletion, with extremely negative Eu anomalies. In addition, the intrusion was also enriched in Rb, Th, U, K, Zr, Hf, and light REE and strongly depleted in Ba, Sr, P, and Ti. Mineralogical and geochemical characteristics, together with high zircon saturation temperature, indicate that the Alubaogeshan intrusion belongs to transitional I–A 2 -type granites. Compared with the porphyritic monzogranite, the syenogranite porphyry reveals higher SiO 2 content and lower TiO 2 , Al 2 O 3 , Fe 2 O 3 , FeO, CaO, MgO, and P 2 O 5 contents, with more obvious negative anomalies of Eu, Ba, Sr, P, and Ti. The temporal and spatial relationship and geochemical characteristics of the two rocks indicate that the syenogranite porphyry evolved from the porphyritic monzogranite. The modeling results show that the fractional crystallization of amphibole, plagioclase, K-feldspar, biotite, apatite, monazite, and allanite occurred during the magma evolution. The granitic magma differentiation plays a key role in the formation of the Maodeng–Xiaogushan Sn–polymetallic deposit. The ε Hf ( t ) values of zircon range from 2.58 to 7.08, and the two-stage Hf model ages range from 619 Ma to 850 Ma, implying that the Alubaogeshan intrusion was predominantly derived from partial melting of a Neoproterozoic juvenile crust. The TE 1,3 values of the ore-related granites from W–(Sn) deposits are generally higher than 1.1, suggesting that fluid–melt interaction is crucial for W mineralization. The degree of magmatic evolution is one of the controlling factors for the size of Sn–polymetallic deposits in the southern Great Xing′an Range.

Geochemical characteristics of mafic intrusive rocks from the Naga Hills Ophiolite, north‐east India: Constraints on petrogenesis and tectonic setting

The Naga Hills Ophiolite (NHO) of Late Jurassic to Eocene age is one of the well‐exposed domains of Tethyan ophiolites, located in the northern margin of the Indo‐Myanmar Orogenic Belt, north‐east India. It is composed of suites of tectonite peridotite and cumulative ultramafic, mafic intrusive‐extrusive rocks, felsic intrusive, radiolarian, and marine pelagic sediments. The mafic intrusives of the NHO exhibit rhythmic to cryptic layering and isotropic in nature. Based on their mineralogy, the mafic intrusives are identified as gabbro, gabbronorite, hornblende gabbro, and olivine gabbro. The gabbros are adcumulate and mesocumulate and essentially composed of plagioclase feldspar, clinopyroxene, orthopyroxene, hornblende, with minor olivine, serpentine, chlorite, and magnetite. These mafic intrusive rocks show affinity to tholeiitic parental magma and fractionated trends in the multi‐elemental diagram, with negative anomalies at Zr, P, Ce and positive anomalies at Sr, Nb, and Ti suggesting that the magmatism was influenced by subduction zone components. The majority of the samples represent light rare earth elements (LREE) depletion (LaN/SmN = 0.28–0.86), except two samples that display LREE enrichment (LaN/SmN = 2.11–3.03), with negligible negative Eu anomaly. Petrogenetic modelling suggests 5%–20% partial melting of a depleted mid‐ocean ridge basalt‐type mantle source, within the spinel lherzolite stability field, as well as low degree partial melting (&lt;10%) of an enriched mantle source probably metasomatized by sediment‐melt from a subducted slab. Therefore, based on mineralogical and geochemical data, we propose that the mafic intrusive of the NHO were generated in the intra‐oceanic arc system within the Neotethys Ocean with the active participation of possible wet basaltic melt from spinel‐lherzolite source‐mantle, subsequently modified by melts derived from sediments in a subduction zone.

Weathering stage and topographic control on rare earth element (REE) behavior: New constraints from a deeply weathered granite hill

Rare earth elements (REE) are sensitive tracers of rock-water interactions and soil formation processes. In this study, we investigated the REE concentration and distribution patterns for both bulk samples and acid leachable phase for a series of regolith and soil profiles developed on a deeply weathered granite hillslope in a subtropical climate. The exogenous input had little impact on the REE distributions on the studied hillslope. The REE distribution in the profiles was controlled by changes in the physical and chemical environmental parameters and mineral transformation during weathering. However, the specific mechanisms differed among the different weathering stages and profiles located at different positions along the slope. Among the environmental parameters, the profile redox state and acidity were the dominant factors controlling the REE behavior, with organic materials as the main regulators. A strong positive relationship between chemical index of alteration (CIA) and La/Yb was observed for the regolith samples, but the La/Yb ratio decreased upward from a depth of 120 cm (the bottom depth of the soil horizon) as the CIA value increased, suggesting a preference for light REE (LREE) fixation in kaolinite formed during the dissolution of feldspar; however, a conversion occurred during the subsequent pedogenesis period. The hilltop soil profile was characterized by the strongest LREE depletion and the highest positive Ce anomaly (~7.31–14.28), compared with other profiles down the hillslope (~0.84–2.78). The strongest LREE depletion on the hilltop profile JLN-S4 suggested a preference for LREE release resulting from the transformation of kaolinite to gibbsite, and the highest Ce anomaly indicates that Ce was fixed in a more oxidized environment within a well-drained profile. The hill foot profile JLN-S1 had the highest LREE leaching rate, but also the lowest heavy REE (HREE) leaching rate, which was ascribed to its lowest erosion rate, lowest pH value, and reductive conditions in a more hydromorphic zone down the hill. The releasing rates of REE calculated by a mass balance model at different positions along the catena ranged from 1395 to 14,379 μg m−2y−1. All the observed REE fractionation regimes and weathering rate variations along the hillslope illustrated that topography-regulated hydrologic condition played a critical role in the release and migration of REE during granite weathering.

Contrasting melt percolation and melt-rock reactions in the Parece Vela back-arc oceanic lithosphere, Philippine Sea: A mineralogical perspective

The accretion and evolution of oceanic lithosphere beneath back-arc spreading ridges remain less understood compared with mid-oceanic ridges (MOR). Here, we investigate mantle peridotites and gabbros recovered from the southernmost side of the Parece Vela back-arc basin, Philippine Sea. The peridotites contain multiple types of clinopyroxenes (Cpx) and amphiboles (Amp) which record contrasting melt percolation and melt-rock reaction processes. Type-1 Cpx shows primary texture and light rare-earth elements (LREE)-depleted pattern, which can be reproduced by ~8–10% fractional melting of depleted MORB mantle. Type-2 Cpx occurs in the deformed peridotites (SY224-1280-2a & -2b) and shows less depleted LREE at similar middle to heavy REE (M-HREE) contents compared with type-1 Cpx. Such features reflect re-equilibration with MORB-type percolating melts, which is further evidenced by elevated TiO 2 and Cr# in coexisting spinels and crystallization of associated MORB-type gabbros. Type-3 Cpx occurs in the undeformed peridotites (SY225-1255-1 & -2) and is texturally replaced by type-1 Amp. They show variably enriched large ion lithophile elements (LILE) and LREE at similar M-HREE contents compared with type-1 Cpx, which reflect an open-system (flux) melting and reaction process with a melt flux enriched in LILE and LREE. Type-1 Amp is almost magnesio-hornblende and shows trace-element pattern characteristic of supra-subduction amphiboles, which supports the hydrous melt that formed type-3 Cpx probably originated from subducted slab. In contrast, type-2 Amp occurs in a leucocratic vein in sample SY225-1255-1 and is characterized by nearly flat M-HREE pattern at 600–200 × CI, LREE depletion and marked negative Eu anomaly. These features are consistent with crystallization from an amphibole-saturated melt formed by extreme differentiation of MORB-type melts. Our data, together with previous observations, suggest co-occurrence of hydrous MORB-type and slab-derived melt percolations beneath the Parece Vela spreading ridge, which may affect the chemical evolution of back-arc oceanic lithosphere different from MOR. • Multiple types of clinopyroxenes and amphiboles occur in the PVB peridotites. • They reflect open-system melting and reaction with hydrous MORB and slab melts. • Hydrous open-system mantle processes are features of back-arc oceanic lithosphere.