Strong LREE Enrichment Research Articles

Discovery of carbonatite-syenite hosted REE-Nb mineralization in the Amaragh area, Gleibat Lafhouda (South Morocco): Hints toward an extended mineralized alkaline province

We report the results of a reconnaissance study of several small carbonatite and syenite bodies with very low topography (up to 1.5 m), that were recently discovered in the Archean basement in the Amaragh area near the Gleibat Lafhouda (GL) carbonatite, previously interpreted as a primary carbonatite. Two varieties of carbonatite have been identified: 1) brownish to greyish banded dolomite carbonatite, composed of dolomite as the main carbonate phase, and calcite, magnetite, apatite, monazite-(Ce), magnesite and minor bastnaesite-(Ce), 2) calciocarbonatite, mainly composed of calcite and several feldspar aggregates (the latter are interpreted as xenocrysts). The Amaragh syenites consist of small outcrops and show white to dark grey colors. The main outcrop is a nepheline syenite that contains coarse-grained rounded magnetites, rimmed by biotite and epidote within a feldspar and nepheline groundmass, suggesting fluid-mineral reactions. The other syenites consist mainly of alkali feldspar, albite, mica, amphibole and accessory apatite, epidote, and titanite. The host minerals of Nb and REE are pyrochlore and allanite. All syenite samples display a peraluminous composition and strong LREE enrichment with negative EuCN anomalies, except for two samples with a slightly positive EuCN anomaly, suggesting at least two types of differentiated syenites. All samples show strong postmagmatic and hydrothermal overprint, which is revealed by, for example, the occurrence of negative YCN and positive CeCN anomalies and mica-epidote mineral reaction rims between magnetite and feldspar. The close spatial association of the Amaragh carbonatite and syenite bodies with the GL carbonatites along with geochemical and mineralogical similarities and REE-Nb enrichment suggest a close petrogenetic relationship, pointing toward a single carbonatite-syenite alkaline complex that was probably formed in an extensional tectonic setting.

SHRIMP U-Pb zircon geochronology of the carbonatite-hosted REE deposit of Kamthai, Late Cretaceous polychronous Sarnu Dandali alkaline complex, NW India: Links to plume-related metallogeny and CO2 outgassing at the K-Pg boundary

The carbonatite plug at Kamthai in the Late Cretaceous polychronous (66–89 Ma) Sarnu- Dandali alkaline complex (SDAC), NW India, is India’s only known carbonatite-hosted rare- earth-element deposit. The SDAC intrudes the Neoproterozoic (∼750 Ma) Malani silicic igneous suite in the Aravalli raton. The Kamthai carbonatite is a calcio-carbonatite dominated by calcite along with barite, strontianite, quartz, zircon and REE-bearing minerals. Textural features suggest an open-system exchange of the magma chamber at varying depths of crystallization. REE patterns are smooth and characterized by strong LREE enrichment, as typical of carbonatites. SHRIMP dating of 12 spots on three magmatic zircons from the carbonatite yielded a concordia age of 66.1 ± 1.4 Ma and an indistinguishable average 206Pb/238U mean age of 68.4 ± 1.8 Ma. The obtained U-Pb ages unambiguously link the carbonatite emplacement to the Deccan Large Igneous Province, i.e. the Réunion hotspot near the K-Pg (Cretaceous-Paleogene) boundary in the polychronous set-up of the SDAC. These ages are also indistinguishable from the 68.53 ± 0.16 Ma magmatic pulse recorded from the nearby Mundwara polychronous alkaline complex and the 69.7 ± 0.2 Ma alkaline sills from the Tethyan suture zone in Pakistan, which are regarded to be earliest manifestations of the Réunion hotspot in the Indian sub-continent. Our results in the context of REE mineralization in the Kamthai deposit have also implications with respect to CO2 outgassing, induced by at least four near-synchronous carbonatitic eruptions in NW India, and to triggering the mass extinction near the K-Pg boundary.

Genesis of the Daping Gold Deposit in the Middle Xuefeng Mountain Area, Southern China: Constraints from Geochemistry, Fluid Inclusion, and H-O-S Isotope

The medium-sized Daping gold deposit is located in the middle Xuefeng Mountain area of Southern China with gold ores hosted in sericite phyllite, sericitolite, and mylonite. The auriferous quartz-carbonate-sulfide veins and adjacent alteration rocks are structurally controlled within the NE (northeast) shear zone with NE, NNE (north-east-east), and NW (northwest) trending at high inclination angles. The petrogeochemistry analysis results show that the gold ores are characterized by high content values of SiO2, S, and As and low content values of Al2O3 and Na2O and display strong enrichment of LREE with δEu values ranging from 0.54 to 0.75. Four stages of mineralization/alteration were identified: the first stage has mineral assemblages of quartz+pyrite+arsenopyrite±carbonate minerals, the second stage has mineral assemblages of quartz+polymetallic sulfide minerals (pyrite, arsenopyrite, chalcocite, galena, chalcopyrite, tetrahedrite)±chlorite±carbonate minerals, the third stage has mineral assemblages of quartz and carbonate minerals, and the supergene stage is characterized by limonite±patina which were formed by the oxidation of metal sulfides. Among them, the first stage and the second stage are the main gold mineralization stages. The ore-forming fluid inclusions in quartz are mainly composed of liquid phase (H2O) and gas phase (H2O and CO2), and based on the microthermometric analysis, the first metallogenic stage and second metallogenic stage yielded average homogenization temperature of 184.5 and 255.8°C and average salinity of 7.64 wt.% NaCl eqv. and 11.35 wt.% NaCl eqv., respectively. Thus, the ore-forming fluids belong to H2O-CO2-NaCl, medium-low temperature, and medium-low salinity fluid. The δ D H 2 O and δ 18 O H 2 O values of auriferous quartz are from -51‰ to 62‰ and from -1.44‰ to 5.42‰, respectively, indicating that the ore-forming fluids may belong to mixing fluids of the magmatic fluid and meteoric hydrothermal fluid. The values of δ34S of metal sulfides range from -0.94‰ to 1.98‰ (-0.131‰ in average), implying that sulfur may source from the concealed granite and/or basement metamorphic strata. The Daping gold deposit formed in the Indosinian period under the tectonic environment of compression between the Cathaysian plate and Yangtze plate and may belong to orogenic gold deposits.

Ca. 815 Ma intra-plate granitoids and mafic dykes from Emeishan pluton in the western Yangtze Block, SW China: A record of rifting during the breakup of Rodinia

The western Yangtze Block, South China is characterized by voluminous Neoproterozoic extension-related volcano-sedimentary sequences and intrusions (e.g. Emeishan granitoid intrusion) and spatially associated mafic dykes. The origin and tectonic setting of Emeishan granitoid intrusion and mafic dykes in the Yangtze Block is poorly study. Understanding the origin and geodynamic setting of dykes and granitoid intrusion are significant in deciphering thermal structure and composition of mantle, tectonic evolution as well as crustal growth processes in the Yangtze Block. SIMS Zircon U–Pb dating of Emeishan granitoid intrusion and mafic dykes suggests that the granitoids (ca. 818 Ma) and mafic dykes (ca. 814 Ma) are contemporaneous. Compositionally, the Emeishan granitoids are restricted and characterized by high SiO2 (69.11–70.50 wt%), Na2O + K2O (3.52–4.13%) and Al2O3 (14.16–14.47%), but low CaO (1.78–2.03%), MgO (0.72–0.91%) contents. They feature strong LREE-enrichment (La/YbN = 7.2–12.3) in conjunction with negative Eu anomalies. The mafic dykes have low SiO2 (48.09–48.65 wt%), K2O (1.94–2.12 wt%) and Na2O (1.64–1.75 wt%) contents, and high Mg# (61.67–62.26). On primitive mantle-normalized trace element plot, they are characterized by enrichment of LILEs and depletion of HFSEs. Both the mafic dykes and granitoids record similar zircon εHf(t) values (+2.2 to +12.1, +1.4 to +6.8), and zircon δ18O values (4.47‰ and 10.41‰, 4.49‰ and 12.05‰, respectively). The rifting-related geochemical signature of the Emeishan pluton and contemporaneous margin and intra-plate plume-related magmatism suggest that they were derived from mantle melting in extensional tectonic setting during mid-Neoproterozoic.

Genesis of the Daping Gold Deposit in the Middle Xuefeng Mountain Area, Southern China: Constraints from Geochemistry, Fluid Inclusion, and H-O-S Isotope

The medium-sized Daping gold deposit is located in the middle Xuefeng Mountain area of Southern China with gold ores hosted in sericite phyllite, sericitolite, and mylonite. The auriferous quartz-carbonate-sulfide veins and adjacent alteration rocks are structurally controlled within the NE (northeast) shear zone with NE, NNE (north-east-east), and NW (northwest) trending at high inclination angles. The petrogeochemistry analysis results show that the gold ores are characterized by high content values of SiO2, S, and As and low content values of Al2O3 and Na2O and display strong enrichment of LREE with δEu values ranging from 0.54 to 0.75. Four stages of mineralization/alteration were identified: the first stage has mineral assemblages of quartz+pyrite+arsenopyrite±carbonate minerals, the second stage has mineral assemblages of quartz+polymetallic sulfide minerals (pyrite, arsenopyrite, chalcocite, galena, chalcopyrite, tetrahedrite)±chlorite±carbonate minerals, the third stage has mineral assemblages of quartz and carbonate minerals, and the supergene stage is characterized by limonite±patina which were formed by the oxidation of metal sulfides. Among them, the first stage and the second stage are the main gold mineralization stages. The ore-forming fluid inclusions in quartz are mainly composed of liquid phase (H2O) and gas phase (H2O and CO2), and based on the microthermometric analysis, the first metallogenic stage and second metallogenic stage yielded average homogenization temperature of 184.5 and 255.8°C and average salinity of 7.64 wt.% NaCl eqv. and 11.35 wt.% NaCl eqv., respectively. Thus, the ore-forming fluids belong to H2O-CO2-NaCl, medium-low temperature, and medium-low salinity fluid. The and values of auriferous quartz are from -51‰ to 62‰ and from -1.44‰ to 5.42‰, respectively, indicating that the ore-forming fluids may belong to mixing fluids of the magmatic fluid and meteoric hydrothermal fluid. The values of δ34S of metal sulfides range from -0.94‰ to 1.98‰ (-0.131‰ in average), implying that sulfur may source from the concealed granite and/or basement metamorphic strata. The Daping gold deposit formed in the Indosinian period under the tectonic environment of compression between the Cathaysian plate and Yangtze plate and may belong to orogenic gold deposits.

Petrogenesis of Paleoproterozoic alkali-feldspar granites associated with alkaline rocks from the Trans-North China Orogen

Granitoids are the dominant component of the continental crust and can be used as probes for crustal formation and evolution. The origin of granites, especially ambiguous granites with alkaline affinities emplaced in orogenic settings, however, remains controversial. Here, we report the petrology, geochronology and geochemistry of Paleoproterozoic alkali-feldspar granites associated with carbonated alkaline rocks from the Trans-North China Orogen in the North China Craton. Corundum-bearing Al-rich gneisses are associated with the granites and occasionally contain granite pockets and veinlets. The rocks have high amounts of K2O (10–14 wt%) and low CaO (0.3–2.3 wt%) contents and show metaluminous to weakly peraluminous features. Moreover, they have low levels of total REE (18–100 ppm) and show strong LREE enrichment [(La/Yb)CN = 44–128] and positive Eu anomalies (δEu = 1.6–10.2). The zircons record a U–Pb age of 1809 ± 3 Ma and εHf(t) values ranging from −3.7 to −7.1, which are consistent with those in the gneisses (1827 ± 14 Ma, εHf(t) = −3.3 to −8.4). Inherited zircons occur in both rocks and show similar ages of ∼2.0 and ∼2.1 Ga. Considerably high δ18OSMOW ratios (9.9–10.9 ‰) in the zircons and low εNd(t) values (−4.0 to −6.2) of the granites were identified. The granite was generated by partial melting of pelitic rocks instead of differentiation of mantle-derived magmas, whilst the gneiss represents the residuum. The carbonated alkaline magmas arising from the mantle provided sufficient heat for anatexis of the crust. Granitoid rocks showing alkaline affinity in orogenic belts may not be the fractionation products of mantle-derived melts but may have formed from reworked sedimentary materials.

Mantle-like Hf[sbnd]Nd isotope signatures in ~3.5 Ga greenstones: No evidence for Hadean crust beneath the East Pilbara Craton

The geological setting of extensive ~3.5 Ga greenstone sequences in the Pilbara Craton (Australia) has been controversial for many years. Models range from a purely oceanic origin to emplacement over a pre-existing, possibly Hadean, sialic basement. Here we present new isotopic (NdHf) and trace element data for a geographically extensive suite of komatiitic-tholeiitic metavolcanic rocks which comprise much of the Pilbara Supergroup. Most of the samples have flat to mildly enriched normalised trace element patterns, often associated with a slight NbTa depletion. A small subset of samples, represented throughout the stratigraphy, show strong LREE enrichment and pronounced HFSE anomalies (low Nb/Th, high La/Nb and Zr/Y) either requiring a different magma source or the addition of an enriched component (possibly pre-existing continental crust). Despite this compositional diversity, the SmNd and LuHf data for all samples define ~3.5 Ga isochrons and yield homogeneous initial ratios (εNd = 1.15 ± 0.19; εHf = 2.29 ± 0.65) suggesting a magma source — or sources — slightly more depleted than the putative primitive (chondritic) mantle but consistent with recent observations of other primordial, deep mantle reservoirs. Published SmNd data for the Dresser Formation obtained during the Pilbara Drilling Project appear to be affected by alteration processes and are considered unreliable for the estimation of an accurate initial isotopic composition.The data for the enriched samples are inconsistent with assimilation of known examples of locally exposed continental crust as a mechanism responsible for their trace element characteristics. If an alternate crustal contaminant is invoked it cannot be much older than the basaltic magmatism itself. As a consequence, we see no evidence for an extensive Hadean (>4.0 Ga) sialic basement in the region.

GEOCHEMICAL CHARACTERISTICS OF THE UMAT TRACHYTIC PLUG: IMPLICATIONS FOR THEIR ORIGIN

The Umat trachytic plug is located in Bokkos area, Plateau State, part of Kurra sheet 189 (NE), North Central Nigeria. This study presents the geochemical results carried out on some samples of the Umat Trachytic Plug. The Umat trachytic hill extruded the Precambrian Basement granite-gneiss complex and in places, the Pan African Granites (Older Granites), Younger Granites Complexes and the Cenozoic volcanoes. A total of 10 rocks samples from the trachytic hill were analyzed for major, trace and rare earth elements using XRF and ICPMS respectively. Major, trace and rare element composition ratios and plots show that they have the same mantle source. They are alkali in nature which have been emplaced within plate volcanism comparable with Ocean Island Basalts as opposed to Mid-Oceanic-Ridge basalts which are tholeiitic. On the TAS classification diagram, the rocks expressed variation in their composition ranging from phonolite to tephriphonolite-trachyandesite. This is consistence with their trace element ratios such as Zr/Y; Ba/Nb, Ba/La, Zr/Nb, Th/Nb and La/Nb which are higher than those of primitive mantle and MORB but similar to those observed in the ocean island basalts. The average Zr/Y, Ba/Nb, Ba/La ratios are in agreement with a derivation related to OIB like mantle source. Similarly, their LREEs are highly incompatible in the mantle-melt system and LREE ratios should be close to those of the mantle source under batch melting condition. On chondrite-normalized REE patterns, the rocks exhibit strong LREE enrichment with (La/Yb) La/Ce ratios varying from 9.29 to 31. These ratios are higher than those of NMORB (0.5), EMORB (1.3) and OIB (12), indicating LREE enrichment which may be interpreted as due to low degrees of partial melting or reflecting an enriched fertile mantle source.

Composition, pressure, and temperature of the mantle source region of quaternary nepheline-basanitic lavas in Bitlis Massif, Eastern Anatolia, Turkey: A consequence of melts from Arabian lithospheric mantle

The Quaternary (0.66–0.63 Ma) nepheline basanites (ne - basanite) are the firstly observed volcanic products of Arabia-Eurasia collision on Bitlis Pötürge Massif. They composed of clinopyroxene, olivine, Ti-magnetite, Cr spinel, and nepheline. The forsterite compositions of olivines range between 73 and 83%, calcic clinopyroxenes show modest variations in Wo48–57-En37–45-Fs5–7 and nephelines occur as minor minerals within the networks of other groundmass minerals. They are characterized by low SiO2 (40.16–41.96 wt%), high MgO contents (8.54–9.73 wt%) and similar SrNd isotopic compositions with Arabian Plate volcanics. Mineral and whole rock thermobarometry yield crystallization pressure and a temperature range between 8 and 20 kbar and 1301 °C – 1035 °C respectively. Lavas have high Mg-number (>0.58), high Cr and Ni contents and strong LREE enrichment but depletion in Rb, K, and Pb. Trace elements together with Sr isotopic compositions inferred negligible assimilation of the local upper crustal material. The calculated average pressure and temperature of mantle melting for ne-basanites is 2,85 kbar and 1353 °C respectively. FC3MS (wt% FeO/CaO-3*MgO/SiO2) parameter and melting models using REE data reveal Çatak basanites are products of amphibole and phlogopite bearing metasomatised lithospheric mantle in garnet stability field. They seem to originated from depths of ~ 85 km which corresponds to the base of the Arabian lithosphere in the region.

The petrogenesis of albitized Early-Permian trachyandesites from Świerki quarry (Lower Silesia, Poland) - constraints on spilitization supported by mineralogical and geochemical data

Trachyandesites from Świerki quarry (Lower Silesia, Poland), related to the post-collisional extension-related magmatic activity developed within the Intra-Sudetic Basin during Early-Permian times, are well-known for the occurrence of mineral associations typical of spilites (low-grade metamorphic rocks affected by Na-metasomatism). This paper deals with the reconstruction of the evolution of these rocks and puts constraints on the spilitization mechanism and its role in redistributing main and trace elements based on cathodoluminescence microscopy and spectroscopy combined with electron microprobe analysis. Our study reveals that the trachyandesites were strongly affected by low-temperature albitization, which occurred during the post-magmatic stage of the rock formation. The replacement of primary green luminescent andesine-labradorite (An39-50Ab47-58Or2–3) by weakly-luminescent to dark-brown luminescent secondary albite (Ab∼99 mol.%) was related to the CO2-induced, crystallographically-controlled, interface-coupled dissolution-reprecipitation mechanism, triggered by hydraulic fracturing of fluids and/or contractional cooling forces. The replacive albite forms isolated and homogenous patches along polysynthetic twinning planes of andesine-labradorite and/or within intergranular fractures. It is closely associated with such minerals as calcite, chlorite and kaolinite, and exhibits enhanced microporosity. Moreover, the low-temperature alteration was accompanied by physicochemical fluctuations (alkaline-acidic) during rock formation as revealed by two generations of fluoroapatite: (1) violet-pink (REE3+-activated) overprinted by (2) yellowish (Mn2+-activated) luminescent domains. We also report that primary (green luminescent) plagioclases (andesine-labradorite) are rimmed by bright-blue luminescent Ab-rich (An2-7Ab34-61Or32–64) alkali-feldspars followed by dull-blue luminescent Or-rich alkali-feldspars (An2Ab38-44Or54–60). The bright-blue luminescent crystals contain higher amounts of BaO (∼0.59 wt.%) and TiO2 (∼0.08 wt.%) than the dull-blue luminescent domains (∼0.12 and 0.03 wt.%, respectively). Hence, the difference in CL intensity in blue wavelengths are ascribed to the presence of both Al-O−-Al centres, related to a coupled KSi-BaAl substitution, and Al-O-Ti bridges. Not only the geochemical affinities of the rocks are peculiar to those reported for within-plate basalts, but also reveal similarities with arc-related geotectonic environments. Trachyandesites sourced from enriched subcontinental lithospheric mantle source owing to the strong LREE and LILE enrichment relative to Nb and Ta, while the presence of positive ZrHf anomalies coupled with low Nb/U ratios points to the involvement of crustal material during magma evolution. The spilitization has predominately resulted in the increase of Cs and decrease of Sr concentration among strongly-altered samples, while its influence on REE distribution was insignificant.

REE mineralization in the Bayan Obo deposit, China: Evidence from mineral paragenesis

Preliminary mineralogical and geochemical studies have been carried out on dolomite marble drill cores from the Bayan Obo REE deposit in China. Three types of apatites and four types of monazites have been identified based on textural features: Type 1 apatite occurs as grains with minor monazite (Type 1 monazite) on its border; Type 2 apatite veinlet shows clusters of assemblages with abundant bastnäsite and parisite at the rim; Type 3 apatite has a linear array associated with fluorite and bastnäsite veinlets. Type 2 monazite occurs as clusters intergrowing with parisite and fluorite. Type 3 and 4 monazites occur as polymineralic (fluorite and bastnäsite) and monomineralic veinlets, respectively. These four types of monazites have similar LREE composition but variable Y content (Y2O3 ranging from below determination limits to 0.7 wt%). The three types of apatites also show different REE content and distribution patterns, ranging from high REE abundance (∑REE + Y: 27243–251789 ppm) and strong LREE enrichment [(La/Yb)CN ∼101] in Type 1, less LREE enrichment [(La/Yb)CN ∼8] in Type 2 to relatively low REE abundance (∑REE + Y: 4323–11175 ppm) but high REE fractionation [(La/Yb)CN ∼58] in Type 3. The primary apatite has high Sr (5461–6892 ppm) and REE content, implying a carbonatite origin. The late-stage apatites (Types 2 and 3) show different Sr and REE abundances. Significant differences in their Sr composition (6189 ± 573, 6041 ± 549 and 3492 ± 802 for Types 1–3 samples, respectively) and Y/Ho ratio (20.9 ± 0.11, 19.5 ± 0.17 and 17.4 ± 0.37, respectively) indicate that the three types of apatites may have crystallized from different metasomatic fluids. Multi-stage metasomatism resulted in remobilization and redeposition of primary REE minerals to form the Bayan Obo REE deposit.

Petrogenesis and tectonic implication of Paleoproterozoic granites and granulites in the Fengzhen area of North China craton

The North China Craton (NCC) is one of the oldest cratons in the world that preserves the records of many important geological events in the early evolution of the Earth. However, the early Precambrian tectonic evolution history of the NCC is still controversial. Here we report Paleoproterozoic leucogranites, garnet granites and granulites in the Fengzhen area that constitutes a junction between the Khondalite Belt and the Trans-North China Orogen (TNCO). The leucogranites have high SiO2 content (75–85wt%), and magmatic zircon U-Pb age of 2199±3 Ma. Their zircons have slightly various εHf(t) values (−0.45 to 1.41) with TDM model ages of 2.5–2.6Ga. The granites show strong LREE enrichment with La/Ybcn ratio of 12–176, and have high initial Nd isotope (εNd(t)=3.5–4.5). Most of them display positive Eu anomaly, excluding fractional crystallization contribution on the high Si feature. The leucogranite was produced by melting of Late Archean felsic TTGs (tonalite–trondhjemite–granodiorite) and addition of juvenile crust. Magmatic zircons yield the garnet granite age of 2002±3Ma. The zircons show various εHf(t) values (−5.8 to 7.1) and TDM model ages (2.1–2.6Ga). Low initial εNd(t) isotope value (−4.2 to −8.3) was determined in the granites. The granulites are high pressure metamorphic rocks with a peak pressure of 12.0±0.8 kbar, and have similar metamorphic age of 1967±10Ma to the garnet granites. This indicates that the Fengzhen area underwent 2-Ga inter-continental orogeny, in which the melting of sedimentary rock formed the garnet granites. Similarly, sodic TTG melting to form K-rich leucogranites also needs high pressure. However, the leucogranite age is obviously older than the peak metamorphic timing of continent–continent collision. This implies that the Fengzhen area may have underwent arc-continental collision at 2.2Ga.

GEOCHEMISTRY AND TECTONIC SETTING OF ECLOGITE PROTOLITHS FROM KECHROS COMPLEX IN EAST RHODOPE (N.E. GREECE)

Eclogites and partially amphibolitized eclogites from the metamorphic Kechros complex in East Rhodope are studied in order to provide the geodynamic framework for the origin of their protoliths. Geochemical evidence from whole rock major and trace element concentrations shows two distinct protolith groups. The low-Fe-Ti eclogites (Charakoma locality) have low-TiO2 content (<0.67 wt%), negative Nb anomalies, positive Sr anomalies, small negative Zr and Hf anomalies and variable enrichments in LILE (e.g. Rb and Ba). The REE patterns are characterized by strong LREE enrichment (LaN/YbN=5.45-5.81), HREE depletion (GdN/YbN=1.60-1.63) and HREE abundance within the rangeof 9-10 × chondrite. The high-Fe-Ti eclogites (Kovalo and Virsini locality) have variable Sr contents, small to moderate LILE enrichment, HREE`s similar to MORB values and absence of Nb anomalies. The REE patterns of the Kovalo and Virsini eclogites are characterized by LREE depletion and relative flat MREE HREE patterns at approximately 20-30 × chondrite concentrations. Our results suggest that the protoliths of the Low-Ti eclogites show a continental rifting tectonic environment. In contrast, the protoliths of the High-Ti eclogites indicate formation of their protoliths by partial melting in an extensional oceanic environment.

Petrological and geochemical features of the early Paleozoic granitic gneisses and iron ores in the Tianhu iron deposit, Eastern Tianshan, NW China: Implications for ore genesis

This paper reports whole-rock geochemical, zircon U–Pb and Hf isotopic data for ore-hosted granitic gneisses, mineral compositions of oxides, and sulfur isotopic data for sulfides in iron ores from the Tianhu deposit, central part of the Eastern Tianshan. Our results can provide crucial constraints on the genesis of granitic gneisses and early Paleozoic tectonic setting of the Eastern Tianshan. LA-ICP-MS U–Pb dating on magmatic zircons yielded weighted mean 206Pb/238U ages of 463 to 438Ma, interpreted as the crystallization ages of the granitic protoliths and the formation ages of the Tianhu Group. Zircon U–Pb age of ore-hosted granitic gneiss (ca. 459Ma) can provide reliable constrains on upper limit for iron mineralization age in the Tianhu deposit. Geochemical characteristics suggest that the protoliths of the Tianhu granitic gneisses are metaluminous to weakly peraluminous high-K calc-alkaline granitic rocks, exhibiting typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Zircon Hf isotopic compositions show a positive trend from ~463 to ~438Ma, indicating that ~460Ma magmas came from both ancient and juvenile sources, whereas ~438Ma magmas involved more juvenile material. Some early Paleozoic granitoids were recently identified in the Eastern Tianshan with the ages between ca. 475 and ca. 425Ma. The formation of these early Paleozoic granitoids was in response to subduction processes, suggesting that subduction of Junggar Ocean probably began in the Early Ordovician and lasted until Late Silurian.Pyrite and pyrrhotite in iron ores have δ34SCDT values from +4.6 to +15.7‰, which are consistent with the marine source, but inconsistent with the magmatic source or those involved evaporites in skarn iron deposit. Geological, geochemical, and isotopic data suggest that the Tianhu iron ores were formed by volcano-sedimentary processes in a subduction environment during the early Paleozoic time, and Tianhu is a metamorphosed volcano-sedimentary iron deposit, rather than a skarn iron deposit as previously proposed.

Neoproterozoic subduction-accretionary tectonics of the South Qinling Belt, China

The Neoproterozoic magmatism in the South Qinling Belt and northern margin of the South China Block is important key to understanding the tectonic evolutionary history of the South China Block, and to reconstruct the tectonics of the supercontinent Rodinia. The Xiaomoling magmatic complex in the South Qinling Belt consists mainly of basalts in the Mogouxia area, gabbros and diorites in the Chishuigou, Lengshuigou and Qianyouhe areas. The basalts in the Mogouxia are characterized by depletion of LREEs and LILEs, and non- differentiation of HFSEs, suggesting their deviation from a slightly depleted MORB mantle source. Taken together with geochemical and geological data, the basalt in the Mogouxia area is interpreted to have derived from a subduction-related back-arc basin setting. The gabbros from both Chishuigou and Lengshuigou sections display clear LREE, Th, and LILEs enrichment, and depletion of Nb, Ta and Ti, and highly differentiation in HFSEs, which are similar to that of a typical Island Arc Basalts. The diorites from both Chishuigou and Qianyouhe areas display strong LREE enrichment, depletion of Nb, Ta, P and Ti, enrichment of LILEs, and flat distribution pattern of HFSEs in the primitive mantle-normalized trace element pattern. According to their geochemistry, both gabbros and diorites from different localities of the Xiaomoling magmatic complex were mostly likely formed in an island-arc tectonic setting. The zircons concentrated from the gabbro and diorite yield LA-ICPMS U-Pb ages of 753±4Ma and 843±5Ma, respectively, representing their formation ages. Integrated all the available geological, geochemical and geochronological data, we propose that the Xiaomoling magmatic complex represents the northernmost of the accretionary belt to the north of the South China Block, while the Hannan-Huangling Neoproterozoic intrusions mark the southernmost of the accretionary belt. Therefore, the most of southern Qinling represents a wide subduction and accretionary tectonic belt in Neoproterozoic times. Our new geochronology and previously published reliable ages estimate a long-lived subduction during ca. 850–700Ma at the present South Qinling area, to the north of the South China Block.

Latest Carboniferous closure of the Junggar Ocean constrained by geochemical and zircon U–Pb–Hf isotopic data of granitic gneisses from the Central Tianshan block, NW China

Once situated between the Central Tianshan and Junggar terranes during Paleozoic time, the Junggar Ocean was a major southern branch of the Paleo-Asian Ocean. Thus, when and how it was closed are essential in understanding the final assembly of the Central Asian Orogenic Belt. However, the exact closure time of the Junggar Ocean remains unresolved due to the lack of reliable timing of collision-related regional metamorphism. This paper reports whole-rock geochemical and zircon U–Pb and Lu–Hf isotopic data for granitic gneisses from the northern margin of the Central Tianshan block, which can provide crucial constraints on the final closure of the Junggar Ocean. Mineral assemblages and geochemical suggest that the protoliths of the Central Tianshan gneisses are weakly peraluminous high-K calc-alkaline I-type granites, possessing typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Negative Eu anomalies (δEu=0.46–0.81) and highly variable zircon Hf isotope compositions indicate various amounts of residual plagioclase in the source and crustal contamination during magma formation. LA-ICPMS U–Pb dating on magmatic-type zircons (72%), characterized by euhedral to subhedral shapes, concentric oscillatory zoning, high Th/U ratios (0.30–2.05) and large ranges of εHf(t) values (−3.4 to +8.7; up to 6 epsilon units in each sample), yields consistent weighted mean 206Pb/238U ages of ca. 325–320Ma, interpreted as the crystallization ages of the granitic protoliths. Geochemical signatures and dominantly positive zircon εHf(t) values reveal that the protoliths were emplaced in a continental arc setting, pinpointing the development of a late Early to early Late Carboniferous continental arc system on the northern margin of the Central Tianshan block, probably related to the southward subduction of the Junggar oceanic plate. Meanwhile, younger ages at ca. 303–301Ma were obtained on recrystallized zircon-rims and unzoned/homogeneous zircons (25%) with narrow ranges of εHf(t) values (−1.8 to +7.2; <3 epsilon units in each sample), considered as the timing of a later tectonothermal event, which was associated with strong deformation that formed the gneissosity of the granitic gneisses. Combined with previous studies, we suggest that this event most likely resulted from the arc–continent collision/accretion between the island arcs (e.g. Bogda, Dananhu) in the Junggar Ocean and the Central Tianshan block during the final closure of the Junggar Ocean, which led to the final assembly of the Eastern Tianshan during the latest Carboniferous time.

Geochemical characteristics and new eruption ages of ruby-related basalts from southeast Kenya

Two ruby-related basaltic fields were recently discovered in the southeast region of Kenya, exposed in the Nguu and Ngulai Hills vicinities. These fields host abundant deep-seated xenoliths, including corundum-bearing granulites. The basalts are alkaline affinity having compositions from foidrite to basanite. The Ngulai basalts have a wider range of SiO2 (38.2 wt.%–44.8 wt.%) covering those of the Nguu basalts (38.7 wt.%–42.3 wt.%). This overlapping behavior also holds for other major oxides and trace elements, e.g., Al2O3, Na2O, K2O, Cr, Ni, Rb and Ga. The overall OIB-like incompatible patterns with strong K depletion and slight spike of Ti enrichment signatures imply low degrees of partial melting of the upper mantle region source induced under a mantle plume-related process. The K-depletion signature indicates a residual K-bearing phase still retained in the source domain. Chondrite-normalized REE patterns exhibiting strong LREE enrichment without Eu anomalies suggest that plagioclase fractionation is insignificant. New 40Ar/39Ar ages indicate eruption events occurred during the Pleistocene times, which are around 2 Ma for the Ngulai basalts and 0.9 to 1.6 Ma for the Nguu basalts. Clinopyroxene-basalt thermobarometric calculations yield the equilibrium P-T ranges of ∼8-29 kbar and 1 200–1 450 °C.

The Geodynamic Context of the Cenozoic Volcanism of the Cap-Vert Peninsula (Senegal)

The Cenozoic alkaline volcanism of the Senegalo-Mauritania sedimentary basin presents an episodic long-lasting volcanic activity from the Eocene-Oligocene boundary up to the Quaternary. Two volcanic episodes are usually distinguished on stratigraphical grounds: a Miocene one and a Quaternary one separated by a period of quiescence of several million years corresponding to the main phase of lateritic weathering. The Tertiary lavas are highly silica-undersaturated alkaline rocks ranging from nephelinites to basanites. They contain nepheline and fassaite-type clinopyroxene in their mineralogy. The Quaternary lavas are more evolved with Hy-normative in comparison to the Tertiary ones; they are composed of basanites and medium to coarser-grained dolerites. The distribution of the REE and other incompatible elements are typical of alkaline lavas with generally strong LREE enrichment without significant Eu anomaly. The Cenozoic Cap-Vert lavas have OIB (oceanic islands basalts)-affinities as shown by their relatively radiogenic Nd and unradiogenic Sr characters closed to the HIMU-OIB. This suggests a HIMU-type end member in the magma sources which is fairly comparable to those erupted in the Canaries and Cape Verde archipelagoes during the same period.

Kimberlite, lamproite, ultramafic lamprophyre, and carbonatite relationships on the Dharwar Craton, India; an example from the Khaderpet pipe, a diamondiferous ultramafic with associated carbonatite intrusion

Are kimberlites, lamproites, ultramafic lamprophyres and carbonatites genetically associated or not? There are strong opinions for and against any relationship. The 20Ha Khaderpet pipe, discovered by Rio Tinto Exploration, is an unusual diamondiferous carbonatite–kimberlite clan rock (KCR) association in the Anumpalle Cluster of the Wajrakarur Kimberlite Field in the Dharwar Craton of Peninsular Indian. The Khaderpet pipe has a discrete sovite phase intrusive into KCR breccia, not noticed elsewhere in the Wajrakarur kimberlite field. Petrographically, the KCR is a clast-supported lithic breccia and crystal lithic tuff, with occasional pelletal lapilli. Clasts show a weak horizontal imbrication. The overall appearance of the tuffs and breccias is suggestive of terminal-blocked, vent accumulations that formed by under pressure, with spallation of country rock causing an abundance of granitoid debris. The sovite phase has up to 95% calcite, occasionally showing flow textured polycrystalline laths set in a minor saponite matrix. There are mineralogical gradations from an olivine-rich ultramafic to a calcite-dominant rock resembling pure carbonatite. Rare REE mineral phases in the carbonatite include allanite and other REE-rich unidentified mineral phases. Xenocrystic high pressure phases in both ultramafic and carbonatite include mantle-derived diamonds, lherzolitic-, eclogitic- and subcalcic-pyrope, Ti-poor andradite, chrome diopside, picrochromite and picroilmenite. Extensive metasomatism in the form of reddening of country rock feldspars by hematite, introduction of green chlorite, and saponitic alteration of breccia clasts and the ultramafic phase is common. The chemistry of the Khaderpet ultramafic component, suggests that the KCR is transitional between kimberlite and ultramafic lamprophyres, like certain other pipes in the Wajrakarur Kimberlite Field, with strong enrichment in LREE, CaO and CO2. However, low MgO (~13%) and high CaO (~10%) values are more typical of ultramafic lamprophyres such as alnoite or aillikite. The significant carbonate content and the presence of accessory Ti-andradite would lead to the KCR being classified as an ultramafic lamprophyre under the IUGS classification. The Khaderpet carbonatite component shows extreme enrichment in REE approaching that of world-average carbonatite. Given the chemical and petrological characteristics of a carbonatite, along with the associated and widespread aureole of metasomatism, this phase is considered to be a late-stage fractionation product of the Khaderpet diamond-bearing ultramafic magma.

Geology and Geochemistry of Archaean Felsic Metavolcanic Rocks of the Eastern Part of the Kolar Greenstone Belt, Dharwar Craton, India: Implications for their Petrogenesis and Geodynamic Setting

Abstract: In the Kolar greenstone belt of the Dharwar craton, felsic metavolcanics are encountered prominently in its eastern region around Surapalli and Marikoppa. These felsic volcanic rocks are essentially homogeneous and their bulk mineralogy is almost the same. They consist of phenocrysts of quartz and feldspar, set in a fine-grained quartzo-feldspathic groundmass. They are calc-alkaline rhyolite in composition, and are characterized by high SiO2 (av. 75.74 wt.%), moderate Al2O3 (av. 11.84 wt.%), Na2O (av. 3.55 wt.%), K2O (av. 3.26 wt%) contents and low Mg# (av. 6.07), Cr (av. 8 ppm), Ni (av. 8 ppm), Sr (av. 331 ppm.), Y (av. 7 ppm), Yb (av. 0.87 ppm) and Nb/Ta (av. 6.40) values, suggesting Tonalite-Trondhjemite-Granodiorite (TTG) affinity for these felsic volcanics. They are strongly fractionated [(La/Yb)N- =14.41 -48.70] with strong LREE enrichment [(La/Sm)N = 2.50-3.59] and strong HREE depletion [(Gd/Yb)N = 1.34 - 2.77] with positive Eu anomaly. The regional geological set-up, petrographic and geochemical characteristics suggest that these felsic volcanics probably were derived by partial melting of a subducting basalt slab at shallow depth without much involvement of mantle wedge in an island arc geodynamic setting.