Alkaline Complex Research Articles

Magmatic origin and petrogenesis characterization of syenite rock from Pakkanadu alkaline complex, Southern Granulite Terrain, India: Implication on emplacement and petrogenetic history

The present study mainly focused on understanding the magmatic origin and petrogenesis characterization based on the Petrography, major, trace and Rare Earth Element (REE) signatures in the alkaline syenite from Pakkanadu alkaline carbonatite complex. The alkaline plutons from South Indian granulite terrain are intruded along with Archaean epidote-hornblende gneisses. The study area was carbonatite complexes of Tamil Nadu and is characterized by a group of rock associations Carbonatite-Syenite-Pyroxenite - Dunite. From Harker various patterns Pakkanadu alkaline complex syenite showed increasing trends of SiO2, Al2O3, Na2O + K2O opposite to decreasing order of CaO, Fe2O3, MgO, TiO2, P2O5 and MnO trend, suggest fractionation of clinopyroxene, hornblende, sphene, apatite and oxide minerals and feldspar that ruled the fractionation. The concentration of trace elements enriched in Large Ion lithophile elements (LILE) (Ba, Sr, and Rb) elements and High Field Strength Elements (HFSEs) indicated that the dyke intrusion by differentiation of magma from a mantle source. Rare earth element (REE) distribution of Light rare earth element (LREE) enriched and High rare earth element (HREE) depleted pattern show strongly fractionated pattern with moderate Eu anomalies. Plots of tectonic discrimination diagrams of Pakkanadu samples fall in the field of syn-COLG field to the VAG syn- COLG field. For the first time, this type of study was carried out in the study region in a detailed manner. The present study significantly exposed the petrography, petrogenesis and magmatic origin process in the Pakkanadu alkaline carbonatite complex.

Onset of the anorogenic alkaline magmatism in the Nigerian Younger Granite province: Constraints from the Daura and Dutse complexes

The Younger Granite province of Niger and Nigeria exemplifies a classical anorogenic alkaline ring granite belt with southward age migration from ca. 470 Ma to 140 Ma. However, the precise age transition from the older Nigerien Younger Granite (NrYG) to the younger Nigerian Younger Granite (NaYG) is poorly constrained. The Daura and Dutse complexes, located at the northernmost tip of the NaYG province, represent a temporal and spatial link between the NrYG and NaYG provinces. The Daura complex is composed of amphibole syenite and biotite granite, while the Dutse complex consists of fayalite syenite and arfvedsonite aegirine granite. Similar to other alkaline rocks of the NrYG and NaYG provinces, the Daura and Dutse samples all chemically show typical A1-type granite features, such as high alkalis (K2O + N2O = 8.33–11.27 wt%.), Ga/Al ratios, Zr + Nb + Ce + Y contents (527–2345 ppm) and FeOt/(FeOt+MgO) ratios (0.03–0.70) as well as elevated HFSEs. The NdHf and Pb isotopic systematics show that all the syenite-granites yield εNd (t) = −2.9 to +1.5, ɛHf (t) = −3.8 to +16.7, and 206Pb/204Pb = 17.818 to 19.555, suggestive of magmas derived from a slightly depleted mantle source with OIB imprints. A reduced to slightly oxidized redox state is invoked for the parental melt based on estimations from mineral chemistry. The Daura amphibole syenite is composed of >60% alkali feldspar, show high Eu/Eu* (>1), Ba/Zr (>3.5) values and cumulate textures, resembling an alkali feldspar cumulate rock. The other alkaline granitic rocks were likely generated from variable fractional crystallization of the residual melt with distinct crustal contaminations. Zircon UPb dating reveal that the Daura amphibole syenite and biotite granite were emplaced at 324 ± 1 Ma and 321 ± 3 Ma, respectively, whereas the Dutse granitoids showed episodic intrusive periods of ca. 264 Ma and 232 Ma. All these results predate other anorogenic alkaline complexes of the NaYG and update the oldest age (ca. 214 Ma in previous references) recorded in the alkaline granite province. To the north, it connects well with ages from the Zinder complex (~330 Ma) in the southernmost NrYG province, reflecting a significant link between the NrYG and NaYG provinces. From a broad view, a linear age migration from north to south in the Niger-Nigeria Younger Granite province remains well preserved, although temporal and spatial intervals exist in local region. This emplacement pattern can be traced to the periodic and repeated reactivation of lithospheric shear zones and transcurrent faults.

SHRIMP U–Pb ages, mineralogy, and geochemistry of carbonatite–alkaline complexes of the Sillai Patti and Koga areas, NW Pakistan: Implications for petrogenesis and REE mineralization

Carbonatites and related alkaline igneous rocks are important hosts of rare earth elements (REEs). Carbonatite–alkaline complexes in NW Pakistan represent rare examples of carbonatite emplaced in a collisional orogenic belt. However, the precise age, magmatic source, and petrogenesis of carbonatites emplaced in collisional settings and associated REE mineralization remain uncertain. The present study focuses on the origin and REE mineralization potential of alkaline–carbonatite complexes exposed in the Koga and Sillai Patti (SP) areas, NW Pakistan.The Koga carbonatites occur as minor plug-like intrusions in syenites. SHRIMP U–Pb dating of zircons from the Koga syenites, which are spatially associated with carbonatites, yield weighted mean ages of 283.6 ± 1.7(1σ), 281.9 ± 1.6(1σ), 282.6 ± 1.6(1σ), and 306.5 ± 3.7 Ma (1σ). The Koga syenites are characterized by high SiO2 (62.79–65.58 wt%) and Al2O3 (16.92–18.55 wt%) and low TiO2 (0.10–0.59 wt%) contents and are enriched in alkalis (Na2O > K2O). Sr and Nd isotopes from the Koga carbonatite complex are characterized by positive εNd(t) values (+3.10 to + 3.47) and low (87Sr/86Sr)i values (0.70233–0.70382), which are consistent with rift-related carbonatites.The SP carbonatites yield high total REE contents (1768–1920 ppm) and are enriched in light REEs (LREEs) relative to heavy REEs (HREEs; LREE/HREE = 20.0–20.7). The results of Sr–Nd isotope analyses show that the SP carbonatite has high Sr and low Nd isotopic ratios with negative εNd(t) values ranging from − 1.71 to − 2.21, variable 207Pb/206Pb ratios, and anomalous Pb–Sr and Pb–Nd patterns, suggesting Pb contamination during ascent of the primary carbonatite melts. Values of δ18OSMOW and δ13CPDB (‰) for the SP carbonates vary from + 14.45‰ to + 18.12‰ and from − 6.57‰ to − 9.70‰, respectively, which can be attributed to low-temperature alteration.On the basis of data for bulk-rock trace elements and Sr–Nd–Pb–C–O isotopes, it is concluded that the Koga carbonatite–alkaline complex was formed during the rifting and break-up of the northern margin of Gondwanaland. In contrast, carbonatite of the SP carbonatite–alkaline complex was probably generated by partial melting of sub-continental lithospheric mantle in a post-collisional tectonic setting. Although the SP carbonatite contains abundant REEs, it does not host significant REE mineralization owing to the lack of a mature sequence of evolutionary phases and to possible phosphate mineral saturation in the primary magmas, which caused REE depletion in the later carbonatitic magmas through fractional crystallization. Therefore, we conclude that carbonatites that are relatively rich in apatite but poor in fluorite and barite may not be prospective for REE mineralization.

Textures and Chemical Compositions of Nb-Bearing Minerals and Nb Mineralization in the Shuangshan Nepheline Syenite Pluton, East Qinling, China

The Shuangshan alkaline complex located in the Henan province of China is a newly discovered, potentially giant niobium (Nb) deposit. A variety of Nb-bearing minerals including pyrochlore, zircon, and titanite have been identified in this deposit. Distinct textural and chemical differences of pyrochlore and zircon indicate that both have different origins. The magmatic pyrochlore and zircon both have euhedral grains with small sizes. On the other hand, hydrothermal pyrochlore is mainly intergrown on the edge or inside of hydrothermal zircon in the form of an aggregate. Compared with magmatic pyrochlore, the contents of F, Ca, and Na in hydrothermal pyrochlore are obviously high. The texture and composition of hydrothermal pyrochlore and zircon indicate that Ca-bearing hydrothermal alteration resulted in the migration of Nb from Nb-bearing zircon and the reprecipitation of Nb to form aggregate pyrochlore. However, the quantitative calculation shows that the amount of Nb migrated from zircon is very small. Therefore, this study suggests that hydrothermal alteration plays a certain role in the redistribution of Nb, but the enrichment of Nb is limited.

Phosphate Rocks: A Review of Sedimentary and Igneous Occurrences in Morocco

Phosphate rocks are a vital resource for world food supply and security. They are the primary raw material for phosphoric acid and fertilizers used in agriculture, and are increasingly considered to be a potential source of rare earth elements. Phosphate rocks occur either as sedimentary deposits or igneous ores associated with alkaline rocks. In both cases, the genesis of high-grade phosphate rocks results from complex concentration mechanisms involving several (bio)geochemical processes. Some of these ore-forming processes remain poorly understood and subject to scientific debate. Morocco holds the world’s largest deposits of sedimentary phosphate rocks, and also possesses several alkaline complexes with the potential to bear igneous phosphate ores that are still largely underexplored. This paper summarizes the main geological features and driving processes of sedimentary and igneous phosphates, and discusses their global reserve/resource situation. It also provides a comprehensive review of the published data and information on Moroccan sedimentary and igneous phosphates. It reveals significant knowledge gaps and a lack of data, inter alia, regarding the geochemistry of phosphates and basin-scale correlations. Owing to the unique situation of Moroccan phosphates on the global market, they clearly deserve more thorough studies that may, in turn, help to constrain future resources and/or reserves, and answer outstanding questions on the genesis of phosphates.

LA–SF–ICP-MS U–Pb baddeleyite and zircon geochronology applied to Cretaceous alkaline rocks of the São Paulo state– Brazil

The laser ablation sector field inductively coupled plasma mass spectrometry (LA–SF–ICP-MS) U–Pb geochronological method is frequently used to date minerals, in particular zircon. In turn, baddeleyite is the ideal geochronometer when it comes to silica-undersaturated rocks, such as alkaline rock types. In this study, we describe a series of protocols to separate baddeleyite crystals and date them by the LA–SF–ICP-MS method in the Isotopic Geology Laboratory of the Geoscience Institute (University of Campinas), starting from sample crushing and milling and the calibration of the Wilfley table. The method includes the use of Kovdor and Phalaborwa primary and secondary reference materials, respectively. Laser operation conditions were spot size of 25 μm and ablation time of 60 s (20 for local blank and 40 for baddeleyite signal), at a fluence of 20 L per minute. Cretaceous alkaline rocks associated with the Paraná-Etendeka magmatic province were analyzed, comprising Jacupiranga, Pariquera-Açu, and Cananéia intrusions, all related to the Guapiara lineament. The results yielded concordant baddeleyite ages of 115–120 Ma for Jacupiranga carbonatite, around 110 Ma for Pariquera-Açu shonkinite, and 81 ± 2 Ma and 83 ± 0.4, respectively for baddeleyite and zircon from the Cananéia syenite. Despite the difficulties in baddeleyite separation, the LA–SF–ICP-MS U–Pb ages have proven to be robust for the young alkaline complexes related to the Serra do Mar Province, such as those occurring along the São Jerônimo-Curiuva Lineament, avoiding the concerns about thermal events that could reset, e.g. K–Ar and Ar–Ar systems. Field relations show a hierarchy of intrusive pulses and our data show that, temporarily, the Jacupiranga Complex is active in a younger time interval, 115 to 120 Ma, than previously established.

Mineralogy and genesis of soils developed from ultrabasic and alkaline rocks of Lages Alkaline Complex, Brazil

In the Plateau of Lages, intrusions of alkaline and ultrabasic rocks occur, rocks of rare occurrence in Brazil, whose soils derived from them are practically unknown. The study aimed to evaluate the chemical composition, mineralogy, genesis, and classification of the soils developed from the rocks Nepheline Sienite (P1), Volcanic Breccia (P2), Kimberlitic Breccia (P3), Phonolitic Tephrite (P4), and Phonolite (P5). After the morphological description (field) and the collection of soil samples, chemical analyses of the soils, mineralogical of the clay fraction and the total elements of the soils and the corresponding rocks that gave rise to them were carried out. Three Humudepts (Cambissolos), a Hapludolls (Chernossolo), and a Kandiudox (Nitossolo) were identified. Except for the P3 profile, the others are acidic soils, with high levels of exchangeable aluminum, poor in basic cations, with low sum and base saturation, showing a high degree of weathering and strong leaching. In the clay fraction, the presence of 2:1 clay minerals predominated with kaolinite with interlayered hydroxy-Al polymers. The P3 profile showed a slightly acidic reaction, high sum and base saturation, and practically zero exchangeable aluminum, a situation compatible with mineralogy, where trioctahedral vermiculite predominated, revealing that it was a young, slightly weathered soil.

Supercritical fluids as recorded in quartz megacrysts of the Late Jurassic porphyritic granitic dyke in the giant Dongping gold deposit, Northern China

The giant Dongping gold deposit, hosted in Devonian alkaline complex, is the first deposit of this kind discovered in China. In this deposit, a newly found porphyritic granitic dyke that contains abundant quartz megacrysts was studied. The dyke occurs along a NE-SW striking shear zone and has gold mineralization within the shear zone. Orthoclase from porphyritic granitic dyke has good plateau ages of 144.3 ∼ 148.2 Ma and isochronal ages of 145.4 ∼ 146.7 ± 1.53 Ma, similar to zircon U-Pb ages of a previous study for the porphyritic dyke. This indicates that the porphyritic granitic dyke is coeval with the adjacent Late Jurassic Shangshuiquan K-feldspar-rich granitic pluton. Plentiful primary fluid inclusions in those quartz megacrysts have homogenization temperatures (300 °C to 400 °C) similar to those in Au-bearing quartz veins of the Dongping deposit, with lower salinities of 5.0–19.0 wt%NaCl equiv. A small number of melt inclusions which can be classified as melt-fluid inclusions were seen in quartz megacrysts together with fluid inclusions. It is suggested that the initial fluids related to the formation of porphyritic granitic dyke could be H2O-silicate supercritical fluids. We propose that H2O-silicate supercritical fluids containing Au were originated from Late Jurassic K-feldspar-rich granitic magmas and might further leach Au from country rocks of alkaline complex. These fluids, which migrated along NE-SW shear zone that controlled the porphyritic granitic dyke, replaced alkaline complex, formed granitic dyke by replacement related to granitization, and, later, Au-bearing quartz veins.

Short-lived alkaline magmatism related to the Réunion plume in the Deccan Large Igneous Province: inferences from petrology, 40 Ar/ 39 Ar geochronology and palaeomagnetism of lamprophyre from the Sarnu-Dandali Alkaline Igneous Complex

Abstract Available geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Réunion mantle plume based on the 40 Ar/ 39 Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali Complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40 Ar/ 39 Ar age determinations of the phlogopites from this lamprophyre reveal two distinct ages of 65.44 ± 1.5 Ma and 68.17 ± 1 Ma. However, palaeomagnetic results show a VGP at 32.31° N and 298.52° E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at c. 65.44 ± 1.5 Ma. Analysed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of c. 68.17 ± 1 Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an OIB-type enriched (metasomatized) mantle source with an involvement of phlogopite. This finding suggests that the pre-Deccan ages of c. 68–69 Ma reported previously may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at c. 68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short duration (between 67–65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Réunion (Deccan) plume.

Rare earth element precipitation induced by non-redox transformation of magnetite to hematite: Microtextural and geochemical evidence from the Kamthai carbonatite complex, western India

Hydrothermal rare earth element (REE) precipitation in carbonatite is generally attributed to increase in pH or decrease in temperature when REE-bearing acidic fluids interact with carbonates. Here, we document microtextures comprising intergrowth of bastnäsite, hydroxyl-parisite, röntgenite, synchysite, and pyrochlore, with calcite that pseudomorphically replaces patches of hematite with cellular boxwork-type structure, in calciocarbonatites from the Kamthai alkaline complex in western India. The nature of the REE mineralization grades from proximal bastnäsite-dominated to distal hydroxyl-parisite dominated in the boxworks. The microtextural relations and trace element chemistry of hematite, magnetite and calcite, and C-O isotope composition of carbonate are suggestive of extensive low-temperature hydrothermal alteration of the carbonatites. The hematite boxwork structure and the REE mineral-calcite intergrowths often have squarish outlines and are interpreted to have pseudomorphed primary magnetite during fluid-rock interaction. We propose a new mechanism of REE precipitation in magnetite-rich carbonatites involving influx of acidic hydrothermal fluids, which scavenged the REE and other trace elements from magmatic carbonates and apatite. These acidic fluids were responsible for the protonation of magnetite and leaching out of Fe2+, converting them to hematite through a non-redox transformation. The reaction results in 32% volume reduction for every mole of magnetite consumed, generating significant rock porosity, which further aided and abetted fluid-rock reaction and hydrothermal alteration. More importantly, it consumed proton, which increased the fluid pH triggering precipitation of bastnӓsite-group minerals (including bastnӓsite, parasite, röntgenite, synchysite). Close to the magnetite-hematite reaction front, fluor-dominated bastnӓsite-group minerals (mainly bastnӓsite) appear, while away from such fronts, mineralization was enriched in hydroxyl-bastnӓsite group minerals (mainly parisite) as a consequence of decreasing activity of F‐ and/or increasing activity of OH‐ with the progress of magnetite-to-hematite transformation.

Petrogenetic evolution of the Zhuopan potassic alkaline complex, western Yunnan, SW China: Implications for heterogeneous metasomatism of lithospheric mantle beneath Simao and western Yangtze block

Potassic alkaline rocks are instrumental in unveiling the composition and geodynamic evolution of mantle lithosphere. The Zhuopan alkaline complex, located in the western Simao block, SW China, exhibits features of high potassium, enrichment in LILEs, and depletion in HFSEs. These same elemental features and SrNd isotopic compositions are also evident in the coeval clinopyroxenitic and syenitic rocks, indicating a comagmatic origin. In this paper, we present new data and discuss the petrogenesis of these rocks. We show that magmas parental to the Zhuopan alkaline complex are derived from highly enriched clinopyroxene-rich lithospheric mantle. These alkaline magmas emplaced in shallow crustal level and experienced crustal assimilation during ascent and through magma chamber processes. Pyroxenite is of cumulitic origin dominated by clinopyroxene and apatite. It formed at an early stage and received more effective crustal contamination. Pyroxene syenite, syenite and nepheline syenite are compositional mixture of variably evolved melt with incompletely segregated liquidus phases (e.g., clinopyroxene, biotite, alkali-feldspar, apatite, nepheline). An important observation is that the Dy/Yb ratio, LILE and LREE abundances of the Zhuopan complex are significantly higher than those of the contemporaneous mafic volcanic rocks in the western Yangtze block, reflecting different sources and source histories. The lithospheric mantle beneath Simao and western Yangtze blocks must have been heterogeneously metasomatized by oceanic subduction-related fluids and melts. Particularly, the mantle source for the Zhuopan alkaline rocks exhibits feature of carbonatite metasomatism. This is further supported by our modeling where melts derived from marine carbonate play a crucial role.

Autometassomatic and hydrotermal processes in the crystallization and recrystallization of calcite, Floresta Azul Alkaline Complex, NE Brazil

The syenitic intrusion of the Floresta Azul Alkaline Complex is characterized by the presence of intense fluid activity in late stages of crystallization, leading to the formation of sodalitites and to the genesis of REE-rich accessory minerals. Calcite is a common mineral in the nepheline syenite of the complex, its formation being derived from CO3 enrichment during fractional crystallization of phonolitic magma. Calcite may be found as white, pink to brown anhedral crystals which occur in the interstices of essential mineralogy and are commonly associated with cancrinite and siderophyllite. Calcite shows high contents of the rhodochrosite, siderite and strontionite molecules and the REE are present in its structure. From scanning electronic microscopy studies, with backscattered electron detector and a dispersive energy spectrometer, it has been possible to identify and characterize a number of textures and minerals related to calcite. The cooling of calcite promotes the formation of a rare REE-carbonate, the carbocernaite. This mineral occurs as exolutions, being distributed disseminated in calcite with irregular crystals and with acicular and oriented crystals. The interaction with hydrothermal fluids resulted in the another unusual REE-carbonate, the ancylite. That results from the recrystallization of calcite, mainly at its borders. A leaching process also occurred at the hydrothermal stage, leading to remobilization of Fe-Mn-Sr which precipitation as siderite, ankerite and strontianite. Therefore, calcite is an extremely important mineral in the syenitic intrusion of the Floresta Azul Complex, being possible to describe the autometasomatic and hydrothermal stages with their genesis, compositional variation, present textures and mineralogical associations.

Melting of metasomatically enriched lithospheric mantle – Constraints from Pan-African monzonites (Damara Orogen, Namibia)

The 560.8 ± 2.4 Ma old Okamutambo Pluton is the easternmost intrusion of the syn-collisional Otjimbingwe Alkaline Complex (OAC; Damara Belt, Namibia) and consists of mafic to intermediate alkaline rocks that belong to the monzodiorite-syenite series. The studied samples are potassic (K2O/Na2O > 1) and have moderately high magnesium (MgO: 5.7–2.6 wt%), nickel (Ni: 66–26 ppm), and chromium (Cr: 223–62 ppm) concentrations. LILE (Ba: 2425–1243 ppm; Sr: 1359–941 ppm) and HFSE (Zr: 447–202 ppm, Nb: 32.8–16.4 ppm, Hf: 4.7–9.4 ppm, Ta: 1.2–2.0 ppm) contents are also high. Strontium and Nd isotope data reveal the existence of two magmatic suites indicating a multi-source origin. Group 1 monzonites-quartz monzonites have moderately evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7066 to 0.7073; initial εNd: −3.5 to −5.0) and radiogenic Pb isotope ratios (206Pb/204Pb: 17.65–18.02; 207Pb/204Pb: 15.62–15.67; 208Pb/204Pb: 38.19–38.32). In contrast, group 2 monzodiorites-syenites display more evolved Sr and Nd isotopic compositions (initial 87Sr/86Sr: 0.7088 to 0.7090; initial εNd: −6.7 to −7.1) but similar Pb isotope ratios (206Pb/204Pb: 17.63–17.82; 207Pb/204Pb: 15.64–15.66; 208Pb/204Pb: 38.21–38.37). Differentiation involved AFC processes in group 1 monzonites-quartz monzonites whereas group 2 monzodiorites-syenites were modified by fractional crystallization. Although second-order processes were operative, high total alkali contents, incompatible trace element concentrations in excess of bulk crustal values, and evolved isotopic compositions of the most primitive samples are source-controlled and provide insight into the origin of the parental magmas. With reference to experimental data from the literature, it is inferred that the Okamutambo alkaline rocks represent evolved melts that were generated through melting of enriched lithospheric mantle (phlogopite-lherzolite). The observed negative Nb-Ta and Ti anomalies and positive Pb anomalies in primitive mantle-normalized trace element patterns are in line with a mantle source that contains a recycled crustal component. Isotopic compositions indicate that mantle enrichment is an ancient feature that might be linked to Proterozoic subduction. At 561 Ma, the geodynamic regime in the Damara Orogen was mainly characterized by compression during convergence and continental collision between the Congo and Kalahari cratons which is difficult to reconcile with the generation of the OAC as alkaline magmatism is commonly associated with extensional tectonic regimes. The OAC is, however, associated with a major suture zone that may have been involved in localized transtensional tectonics during oblique flat subduction and thus enabled the generation and ascent of mantle-derived alkaline melts.

Mineralogy and geochemistry of a giant agpaitic magma reservoir: The Late Cretaceous Poços de Caldas potassic alkaline complex (SE Brazil)

The Late Cretaceous (~78 Ma) Poços de Caldas massif is the largest alkaline complex in Brazil and the second in the world by extension (>800 km2). It is considered the westernmost outcrop of the Cabo Frio magmatic lineament, in the northern sector of Serra do Mar potassic-ultrapotassic igneous province, central-eastern Brazil. The outcropping rocks are peralkaline phonolites (~80%) and nepheline syenites (~15%) with rarer (<5%) basic-ultrabasic rocks (leucite basanites, basanites, basalts and lamprophyres). The phonolites have different levels of volatile (F and Cl) and other trace elements, which tend to decrease with increasing evolution, due to removal of F-, Mn-, HFSE- and REE-rich accessory phases accompanying cotectic alkali feldspar and nepheline. The geochemical variability of titanite, eudialyte, F-disilicates, aenigmatite, lamprophyllite, clinopyroxene, amphibole and other phases indicate the effects of removal of accessory phases and the presence of independent liquid lines of descent in the various intrusive pulses. The initial Sr−Nd isotopic ratios of the basic-ultrabasic lavas [87Sr/86Sri = 0.70440–0.70498; εNdi = −3.7 to −1.2] are within the range of the other Late Cretaceous-Paleogene rocks of the northern sector of Serra do Mar, indicating a highly metasomatized K-rich lithospheric source. Nepheline syenites and phonolites have SrNd isotopes largely overlapping with that of the basic-ultrabasic lavas (87Sr/86Sri = 0.70503–0.70540 and εNdi = −3.9 to −2.5 for nepheline syenites; 87Sr/86Sri = 0.70511–0.70527 and εNdi = −3.4 to −3.2 for phonolites). Prolonged fractional crystallization processes dominated by clinopyroxene removal from melts with compositions resembling those of the basic-ultrabasic compositions can produce residual liquids qualitatively comparable with those represented by the phonolites and nepheline syenites.

Petrogenesis and U–Pb zircon dating of the Chaitma Alkaline Complex from the southern margin of the Central Indian Tectonic Zone: geodynamic implications

Abstract In this study, we constrain the petrogenesis and U–Pb zircon age of a newly discovered alkaline complex, christened the Chaitma Alkaline Complex at the southern margin of the Central Indian Tectonic Zone in central India. The Chaitma Alkaline Complex comprises syenites and gabbros, emplaced coevally, and show features consistent with magma mixing. Geochemically, syenites are potassic–ultrapotassic (K 2 O/Na 2 O: 0.79–3.42), and contain high Ba ( c. 800–2700 ppm) and Sr ( c. 1400–3200 ppm). They show enrichment of the light rare earth elements (LREEs) relative to the heavy rare earth elements (HREEs) (La/Yb: 32–103) and do not display any Eu anomaly. Based on their geochemical signatures, such as low MgO (&lt;0.87 wt%), Ni (8–16 ppm) and Cr (7–44 ppm) contents and prominent Zr–Hf negative anomaly, the syenites are inferred to have been derived by partial melting of a carbonated/metasomatized thickened lower crustal source. The coeval gabbros are undersaturated in silica (41–44 wt%), with relatively high total alkalis (Na 2 O + K 2 O: 3.7–5.1 wt%), Fe 2 O 3 (17–19 wt%), P 2 O 5 (3.1–4.9 wt%), Sr (1600–3400 ppm) and Ba (300–3500 ppm) contents. These have low MgO (&lt;4.8 wt%), Ni (13–30 ppm) and Cr (18–84 ppm). Their chemistry is interpreted to be the result of interaction with the syenitic magma. These geochemical characters along with the high LREE/HREE ratio, negative trough in Nb–Ta, Zr–Hf, Ti, Sr and Rb, and positive spike of Pb in a multielement diagram, and enrichment of LILEs over HFSEs indicate their derivation from a metasomatized subduction-modified garnet–peridotite mantle source. Our study indicates that syenites and gabbros of the Chaitma Alkaline Complex were formed from genetically unrelated parental magmas derived from distinct sources. U–Pb dating of zircon yielded a magmatic emplacement age of 1626 ± 15 Ma for the syenites. The Chaitma Alkaline Complex was presumably formed during a short period of crustal extension in the midst of a protracted period of continent–continent collision and granulite-grade metamorphism ( c. 1.71–1.58 Ga) at the southern margin of the Central Indian Tectonic Zone.

Mantle sources and magma evolution in Europe's largest rare earth element belt (Gardar Province, SW Greenland): New insights from sulfur isotopes

Alkaline igneous complexes are often rich in rare earth elements (REE) and other metals essential for modern technologies. Although a variety of magmatic and hydrothermal processes explain the occurrence of individual deposits, one common feature identified in almost all studies, is a REE-enriched parental melt sourced from the lithospheric mantle. Fundamental questions remain about the origin and importance of the mantle source in the genesis of REE-rich magmas. In particular, it is often unclear whether localized enrichments within an alkaline province reflect heterogeneity in the mantle source lithology (caused by prior subduction or plume activity) or variations in the degree of partial melting and differentiation of a largely homogeneous source. Sulfur isotopes offer a means of testing these hypotheses because they are unaffected by high temperature partial melting processes and can fingerprint different mantle sources. Although one must be careful to rule out subsequent isotope fractionation during magma ascent, degassing and crustal interactions.Here, we present new S concentration and isotope (δ34S) measurements, as well as a compilation of major and trace element data, for a suite of alkaline magmatic units and crustal lithologies from the Mesoproterozoic Gardar Province. Samples span all phases of Gardar magmatism (1330–1140 Ma) and include regional dykes, rift lavas and the alkaline complexes Motzfeldt and Ilímaussaq, which represent two of Europe's largest REE deposits. We show that the vast majority of our 115 samples have S contents >100 ppm and δ34S of −1 to 5‰. Only 8 samples (with low S contents, <100 ppm) show evidence for crustal interactions, implying that the vast majority of Gardar melts preserve the S isotopic signature of their magma source. Importantly, samples from across the Gardar Province have δ34S above the canonical mantle range (≤−1.4‰) and therefore require recycled surface S in their mantle source. Elevated δ34S values are explained by a period of Andean-style subduction and mantle metasomatism which took place ∼500 Ma before rift onset and are also supported by trace elements signatures (e.g. Ba/La) which match modern subduction zones.Comparing the various generations of Gardar magmas, we find that δ34S values, large ion lithophile elements (K, Ba, P) and selective incompatible elements (Nb and light REE) are particularly enriched in the Late Gardar dykes, alkaline complexes and clusters of silica-undersaturated dykes spatially associated with the alkaline complexes. These data indicate that subduction-related metasomatism of the Gardar mantle was spatially heterogeneous, and that alkaline complexes are sourced from localized mantle domains highly enriched in 34S, REE, alkalis and volatiles (particularly, F). Since alkalis and volatiles play an essential role in driving extreme differentiation of alkaline melts and fluids, we suggest the co-location of these species plus incompatible metals at high concentrations in the lithospheric mantle is a critical first-step in the genesis of a world-class alkaline REE deposit. S isotopes are powerful tools for identifying enriched mantle domains and the sources of mineralized alkaline igneous bodies.

Mapping of potential rare earth deposits in the Schiel alkaline complex using sentinel-2B multispectral sensor

In this study, the images of Sentinel-2B multispectral sensor are used to map minerals associated with rare earth elements (REE) in an attempt to discover new potential targets for REE deposits in the Schiel Alkaline Complex of South Africa. Spectral Information Divergence (SID) classification was applied to Sentinel-2B images to map areas containing potential monazite and hematite. Generally, the accuracy of SID classification is warranted by selecting appropriate threshold values, which are normally user-defined. In this study, a simple thresholding approach is proposed; this approach requires at least one area, with a confirmed existence of a mineral being sought, to be used as a reference point for selecting optimum threshold value. The aim of this approach is to determine the lowest threshold value at which mineral classification in a reference point starts. The smaller the threshold value, the more likely the classified pixels represent minerals in question in the ground. Following this approach, a value of 0.02 and 0.03 was used as the maximum acceptable threshold value for mapping REE-bearing mineral (monazite) and iron-rich mineral (hematite), respectively. The proposed approach was validated through field sampling and geochemical analysis of samples collected at potential REE deposit targets, including one rock sample (S3) collected in the area void of minerals of interest, based on SID classification. The analytical results show that the samples collected at anomalous mineral clusters have relatively high REE contents that are well beyond the average crustal abundance and S3, thus suggesting the viability of the proposed approach.

Alkaline magmatism on Neo‐Tethyan extensional domains: Evidences from the Gejiu complex in Yunnan, China

The Gejiu alkaline complex (GAC) within the western part of the Youjiang Basin provides a window to investigate the evolution in the junction of Cathaysia, Yangtze and Indochina blocks. Here, we investigate the GAC in terms of their petrology, zircon U–Pb geochronology, whole‐rock geochemistry, and Sr–Nd isotopic data to gain insights into the origin and evolution of the alkaline magma. The GAC is lithologically composed of alkali syenites and feldspathoid syenites, in which some were altered into sericite syenites. Zircon U–Pb dating of the alkali syenites yielded an age of 85.03 ± 0.47 Ma, which is slightly older than the feldspathoid syenites. The alkali syenites and feldspathoid syenites are silica‐saturated and silica‐undersaturated, respectively, and are characterized by high alkalinity with K2O + Na2O of 11.55–17.08 wt% and Al2O3 of 18.57–22.49 wt%, low MgO of 0.11–1.39 wt%, weakly negative Eu anomalies, enrichments of LILEs such as Th and U, HFSEs like Zr and Hf but depletion of Ba, Sr, Nb, Ta, P, Ti, strongly fractionated LREEs to HREEs. Their uniform Sr–Nd isotope composition with initial 87Sr/86Sr = 0.708802–0.710571 and εNd(t) = −7.1 to −6.6 indicates that they were products of a homologous magma. They crystallized at c. 810–956°C and have a relatively high magmatic oxygen fugacity. Our geochemical and isotopic data proved that the GAC magma was derived from the low‐degree partial melting (&lt;10%) of a phlogopite‐bearing‐enriched mantle that was metasomatized by subducting sediments and originated possibly from the spinel and garnet transition zone at a depth of 60–80 km and a pressure of about 1.8–2.4 Gpa. The primary magma experienced protracted two‐stage crystal fractionation of clinopyroxene+amphibole and biotite+K‐feldspar, forming alkali syenites and feldspathoid syenites. Crustal contamination plays a negligible role in their formation. Considering previous tectonic studies, it was therefore proposed that the GAC formed in an extensional tectonic setting related to the Neo‐Tethyan tectonic regions during the Late Cretaceous.

On the participation of natural salts in alkaline magmatism. Article 3. Genetic aspects of the model of salt-alkaline interactions

Research subject. An analysis of regional and global geological material characterizing the spatio-temporal relationships between alkaline magmatic and saline complexes allowed the author to propose and justify a new geological-genetic model of alkaline magmatism. This model considers saline complexes, located along the paths of the upward movement of deep magmas, as additional sources of alkaline and volatile components.Materials and methods. Three articles are devoted to the discussion and justification of this model. Two articles were devoted to geological aspects of the problem. The prerequisites and signs of the participation of ancient saline complexes in alkaline magmatism were characterized. It was shown that the presence of saline rocks in the deep zones of the earth's crust along the paths of the upward movement of deep magma flows is a geologically natural and common phenomenon. Natural alkaline-salt associations (spatio-temporal combinations of alkaline and salt objects) were indicated; their tectonic types were distinguished. A global overview of their different-age analogues (neo- and paleogeodynamic) was given.Results and discussion. The collected data made it possible to evaluate older (than magmas) salt-bearing complexes (deeply buried in the substrate) as a possible important and active participant in the ontogenesis of alkaline complexes, to give a positive assessment of the geological aspects of the “magma halocontamination” model and salt-magmatic interactions; to formulate the main geological-genetic provisions of this model.Conclusion. This article focuses on the discussion of the genetic aspects of the proposed model with an assessment of the probable role and significance of various halophilic components in the formation of alkaline magmas and their features. For this purpose, the similarity features in the spatial and quantitative distribution of halophilic and foydaphilic components in salt and alkaline rocks are considered; the probable role of various halophilic components in the formation of alkaline specialization of magmas, in the emergence of a rich set of unusual features of alkaline rocks (material, structural, morphological, etc.) is discussed. The probability of participation of the complex of paragenic (non-salt) members of the halophilic community (dolomites, anhydrites, black shales and associated ore components) in the interaction with hot magma is estimated. A comparative analysis of some basic provisions of the model under consideration with other geological-genetic models of alkaline petrogenesis is performed. The advantages of this model and its predictive capabilities are evaluated. A number of ideas have been proposed by the author for the first time, thus requiring further elucidation.

B–Sr–Nd–Pb isotopic constraints on the origin of the Maoniuping alkaline syenite–carbonatite complex, SW China

The Maoniuping giant rare earth element deposit is genetically related to a Cenozoic alkaline syenite–carbonatite complex. However, the origin of the alkaline complex and the genesis of the Maoniuping deposit are still unclear. In this study, the origin of the Maoniuping complex was constrained through B–Sr–Nd–Pb isotopic data for fresh/unaltered alkaline syenites. Our results indicate that the Maoniuping alkaline syenite has arc-like trace element patterns with elevated initial 87Sr/86Sr (0.706040–0.706541), 206Pb/204Pb (18.00–18.32), 207Pb/204Pb (15.53–15.60), and 208Pb/204Pb (38.25–38.78) ratios, and relatively low εNd(t) values (–4.92 to –2.66), suggesting derivation by partial melting of fertile mantle source. It also has higher B contents (1.06–7.65 ppm) and heavier δ11B values (−5.38‰ to − 2.46‰) than the upper mantle, indicating modification of the mantle source by interaction with carbonatitic melts/fluids derived from marine sediments and altered oceanic crust within the subducting Indian slab. Our results, together with recently acquired seismic tomographic data, indicate that hybridisation in the mantle source was likely induced by subduction input of marine sediments to the mantle transition zone. Boron isotopes provide a novel indicator of the deep recycling of subducted crustal materials in the sources of alkaline magmas.