Petrographic, geochemical, and isotope–geochemical studies of the Mesoproterozoic volcanic association (lava flows and the Valaam sill) in the Ladoga graben in the suture zone of the Karelian craton and Svekofennian orogenic complex are conducted. Ferrobasites of close composition and geological position in lava flows and the sill have different Nd isotopic compositions, differing by 5 units of ε Nd( t ) : = –4…–5 in ferrobasalts of lava flows and a very low-radiogenic Nd isotopic composition in mafic rocks of the sill (ε Nd ( t ) to –11 for ferrogabbros), and identical to the isotopic composition of its acidic rocks (ε Nd( t ) to –11 for granophyres). It is shown that in the crust, which is heterogeneous in age, the process of fractional crystallization along the tholeiitic trend combined with assimilation of melts from wall rocks of magma chambers could have been an effective mechanism for forming such “paradoxical” features of the Nd isotopic composition. The isotopic composition of sill rocks indicates the likely occurrence of Mesoarchean blocks in the lower crust of the Ladoga graben region.

Petrography, geochemistry and isotope geochemistry of the Mesoproterozoic volcanic rocks occurred as lava flows and the Valaam sill in the Ladoga rift, have been studied. The Ladoga rift is located in the area of thrusting of the Palaeoproterozoic Svekofennian orogenic complexes onto the Archean Karelian craton. Ferrobasites of close composition and geological position in lava flows and sill have different Nd isotopic composition, differing by 5 units εNd(t): –4…-5 in ferrobasalts of lava flows and a very low-radiogenic Nd isotopic composition in mafic rocks of the sill (εNd(t) to –11 for ferrogabbro), and identical to the isotopic composition of its acidic rocks (εNd(t) to –11 for granophyres). It is shown that in the crust, heterogeneous in age, the process of fractional crystallisation along the tholeiitic trend combined with assimilation of melt from wall rocks can be an effective mechanism for forming such “paradoxical” features of the Nd isotopic composition. The isotopic composition of sill rocks indicates the probable presence of Mesoarchean blocks in the lower crust of the Ladoga graben region.

Petrographic, mineralogical, geochemical, isotope-geochemical studies of granophyres and host ferrogabbro, quartz ferromontsogabbro, quartz montsodiorites, and quartz monzonites in the Mesoproterozoic Valaam sill in the Ladoga Graben on the Karelian craton have been carried out. The sill is poorly layered: ferrogabbros are common in the lower part of the sill, the middle part consists of quartz gabbro-monzonites and quartz monzonites, granophyres form a network of veins mainly in the upper part of the sill. Geochemical features of ferrogabbro, iron-rich composition of olivine and pyroxene, low Ca composition of plagioclase indicate evolution along the Fenner trend. Granophyres have petro- and geochemical characteristics of anorogenic alkaline granites, are characterised by negative Eu/Eu* = = 0.15–0.49 and REE distribution similar to those of granophyres of layered intrusives. All rocks of the sill are characterised by a similar isotopic composition of Sr (87Sr/86Sr)T= 0.7043–0.7066, andεNdvalues ranging from –9.6 to –11.2. Model calculations show that fractional crystallisation can lead the initial ferrogabbro melt into immiscibility. Ilmenite-magnetite-silicate microstructures have been identified in ferrogabbro and ferromontzogabbro from the sill; similar microstructures in layered intrusives are considered evidence for immiscibility of Fe-enriched and Si-enriched liquids (Holness et al., 2011; Dong et al., 2013). The segregation of the high-silica melt may have occurred in a crustal chamber at around 350 MPa and 960oC; the sill formation at around 70 MPa injected magma in the form of a crystalline mush through which acidic melt migrated. This melt underwent fractional crystallisation and reacted with host minerals. At the level of sill formation, it crystallised under supercooling into granophyre aggregates. The example of the Valaam sill shows that after fractionation according to the classical Fenner trend reaches the final composition – ferrogabbro, its continuation with a conjugate decrease in SiO2and Fe contents can be associated with incomplete separation and mixing of iron-rich melts and separated acidic melt. Such a mechanism can be realised during the formation of the mafic part of AMCG-type massifs.

We investigate relationships between melt sources for lamprophyres, kimberlites and other alkaline carbonate -rich melts by studying rocks of the Terskiy Coast (Northwest Russia) situated between the coeval Devonian Kola Alkaline Carbonatite (KACP) and Archangelsk Kimberlite Provinces (AKP). This study reports Sr-Nd-Pb isotope systematics of lamprophyres, carbonatites, turjaite and foidite dykes, Turij Mys massif turjaites, perovskite and garnet from the dykes, and the Ermakovskaya-7 kimberlite, the only kimberlite of the Terskiy Coast. Principal Components Analysis was used to classify the data by five isotopic variables into rock groups T1, T2 and T3. A factor analysis technique was then applied to new Sr-Nd-Pb data and the KACP literature data to portray the analyses as three clusters on a planar 2-dimentional array. The first of these clusters, T1 is characterized by high combined parameter f(Nd, Pb) > 0.5137 and moderately radiogenic (87Sr/86Sr)375. T1 rocks are mostly ultra-mafic and plot along the Kola Carbonatite Line in Sr-Nd space. Group T2 is recognized by the presence of a linear correlation between Sr isotope compositions and f(Nd, Pb); the extended span of epsilon Nd is combined with a moderately radiogenic (87Sr/86Sr)375, and this group includes the least radiogenic Pb compositions. T2 rocks are petrographically diverse, ranging from kimberlites to phonolites. T3 rocks form a cloud of points with elevated (87Sr/86Sr)375 > 0.7041 and widely varying f(Nd, Pb); they are mostly alkaline lamprophyres. All three groups of studied rocks record a contribution from the depleted mantle, while the mantle beneath the AKP kimberlites that was metasomatized just prior to eruption contributed to T1 rocks, and the Paleoproterozoic metasomatized lithospheric mantle contributed to T2 rocks. T3 rocks record 10% contamination by the lower crust, as recorded by the Sr-Nd isotopic shift between the whole rock and garnet compositions. A geographic position of Ermakovskaya-7, Zolotitsa and Mela kimberlites within the adjoining areas of the KACP and AKP stretched along the strike of the Lapland-Kola belt results in their distinct low epsilon Nd(375) signatures highlighting a local control on melt generation by the ancient metasomatized mantle. The repeated generation, over billions of years, of melts in spatially restricted areas explains the observed contribution of ancient fenitized crust and metasomatized mantle to magmas from large alkaline or kimberlite provinces.

The study reports major and trace element compositions of olivine in lamprophyre and related nephelinite dykes with well documented petrography and geochemistry from two areas in the Kola Alkaline Carbonatitic Province (KACP), Kandalaksha and Turij Mys. Variously zoned or homogeneous olivine (Fo82-88) occurs as phenocrysts, antecrysts and a groundmass phase in damtjernites, monchiquites and melanephelinites. Most olivine lies on two modelled Ni-Mg# trends formed via fractional crystallization for two distinct melt compositions, the higher Ni melt for Kandalaksha and the lower Ni, more evolved melt for Turij Mys. Contents of V and Sc in olivine indicate lower fO2 at QFM in the Kandalaksha melt and a 1 log unit higher fO2 in the Turij Mys melt. Trace element contents of olivine cannot constrain the mantle source for the parent melt because of its evolved, non-primary character indicated by low Mg# of the bulk rock(0.5-0.64) and olivine (0.81-0.87), and high Ca (1665-5325 ppm) and Mn (1193-2610 ppm) of the olivine. The study reaches this conclusion by comparing compositions of experimental mantle partial melts and their olivine with compositions of lamprophyres and lamprophyric olivine. Primary melts parental to KACP lamprophyres fractionated 24-26% of Fo89 before an ascent and crys-tallization of the lamprophyres in the lower crust. An analysis of global data for magmatic olivine in lamp-rophyres and kimberlites suggest the similarity of their initial primary melts followed by a diverging evolution. Trends of correlated Ni-Mg# in lamprophyres is controlled by olivine fractionation, unlike olivine kimberlite trends of the widely varying Ni at a constant Mg#. Globally, major and trace element chemistry of lamprophyre olivine resembles olivine from alkaline ultramafic massifs with carbonatites.

We investigated mantle eclogite and garnet pyroxenite xenoliths from the V. Grib kimberlite located in the Arkhangelsk diamond province. The eclogites in the lithospheric mantle beneath the Arkhangelsk province were strongly modified by metasomatic processes, which totally obliterated the primary features of protolith. Detailed studies of the xenoliths allowed us to distinguish the following metasomatic events: (1) early mantle metasomatism and (2) interaction with kimberlite melt. During the multiple early mantle metasomatism, primary clinopyroxene and garnet were replaced by metasomatic clinopyroxene, garnet, amphibole, calcite, and phlogopite under the influence of carbonated ultramafic melts. The impact of kimberlite melt caused the dissolution and recrystallisation of solid-phase inclusions and formation of melt pockets consisting of serpentine, chlorite, carbonate, spinel, perovskite, amphibole, recrystallized garnet, and clinopyroxene. En route to the surface in kimberlite melt, the xenoliths were disintegrated and primary garnet and clinopyroxene were metasomatized with increasing Ti and Cr contents, up to formation of high-Cr megacrysts. The garnet pyroxenites are represented by high-Ca, low-Mg and low-Ca, high-Mg types. It is shown that the high-Ca, low-Mg garnet pyroxenites can be the final products of the eclogite xenolith metasomatism by carbonated ultramafic melts. The low-Ca, high-Mg pyroxenites were derived through the interaction of a partial eclogite melt with depleted peridotites.

Evidence for liquid immiscibility between Fe-rich and Si-rich silicate melts in ferrobasalt of the Ladoga Graben in the Baltic Shield belonging to the Mesoproterozoic anorthosite–rapakivi granite (AMCG-type) association is reported. The liquid immiscibility microtextures are represented by emulsion of tiny globules (1–2 μm) or individual larger globules (10–20 μm) of finely crystallized iron-rich silicate glass embedded in interstitial Si-rich glass. Large globules show the core–shell and core–shell–corona microtextures indicating the immiscibility of the sulfide melt in them. The globules have a high FeO content (31.9 ± 1.6 wt %) and a low SiO2 content (39.5 ± 4.2 wt %). The felsic glass hosting globules contains 61.4 ± 7.1 wt % SiO2 and 8.1 ± 1.5 wt % FeO. The immiscibility could occur at temperatures below 1100°C.

This publication is a series of datasets that accompany a manuscript on petrology of lamprophyre dykes in the Kola Alkaline Carbonatite Province (N Europe) [1]. The datasets served as the basis for interpretation of melt crystallization for lamprophyric and carbonatitic dykes in the crust, to supplement many papers devoted to mantle sources and melting parameters of these rocks based on radiogenic isotopes and trace elements. The first dataset contains bulk major and trace element compositions of the dykes in three areas, Kandalaksha, Kandaguba and Turiy Mys, along with supplementary information on sampling locations and dyke characteristics. The second dataset represents photos, major and trace element plots illustrating extreme mineralogical and textural heterogeneity and layering observed in some dykes. The photographs include field photographs, optical and electron microscopy shots for thin sections in central and marginal parts of the dykes. They should be viewed together with Harker and REE diagrams illustrating the changes from dyke margins to dyke cores. The third and fourth datasets are output tables from Ryolite-MELTS software used to model crystal fractionation of the dykes from the parental melts and various P, T, fO2, XCO2, XH2O parameters. One table shows compositions of evolved melts, while the other lists crystallizing phases.

To provide new insights into the evolution of kimberlitic magmas, we have undertaken a detailed petrographic and mineralogical investigation of highly evolved carbonate–phlogopite-bearing kimberlites of the Kepino cluster, Arkhangelsk kimberlite province, Russia. The Kepino kimberlites are represented by volcanoclastic breccias and massive macrocrystic units within pipes as well as coherent porphyritic kimberlites within sills. The volcanoclastic units from pipes are similar in petrography and mineral composition to archetypal (Group 1) kimberlite, whereas the sills represent evolved kimberlites that exhibit a wide variation in amounts of carbonate and phlogopite. The late-stage evolution of kimberlitic melts involves increasing oxygen fugacity and fluid-phase evolution (forming carbonate segregations by exsolution, etc.). These processes are accompanied by the transformation of primary Al- and Ti-bearing phlogopite toward tetraferriphlogopite and the transition of spinel compositions from magmatic chromite to magnesian ulvöspinel and titanomagnetite. Similar primary kimberlitic melts emplaced as sills and pipes may be transitional to carbonatite melts in the shallow crust. The kimberlitic pipes are characterised by low carbonate amounts that may reflect the fluid degassing process during an explosive emplacement of the pipes. The Kepino kimberlite age, determined as 397.3 ± 1.2 Ma, indicates two episodes of ultramafic alkaline magmatism in the Arkhangelsk province, the first producing non-economic evolved kimberlites of the Kepino cluster and the second producing economic-grade diamondiferous kimberlites.

The study reports petrography, bulk major and trace element compositions of lamprophyric Devonian dykes in three areas of the Kola Alkaline Carbonatite Province (N Europe). Dykes in one of these areas, Kandalaksha, are not associated with a massif, while dykes in Kandaguba and Turij Mys occur adjacent (< 5 km) to coeval central multiphase ultramafic alkaline-carbonatitic massifs. Kandalaksha dyke series consists of aillikites - phlogopite carbonatites and monchiquites. Kandaguba dykes range from monchiquites to nephelinites and phonolites; Turij Mys dykes represent alnoites, monchiquites, foidites, turjaites and carbonatites. Some dykes show extreme mineralogical and textural heterogeneity and layering we ascribe to fluid separation and crystal cumulation. Melt evolution of the dykes was modelled with Rhyolite-MELTS and compared with the observed order and products of the crystallization. Our results suggest that the studied rocks were related by fractional crystallization and liquid immiscibility. Primitive melts of aillikites or olivine melanephelinites initially evolved at P = 1.5-0.8 GPa without a SiO2 increase due to abundant clinopyroxene crystallization controlled by the CO2-rich fluid. At 1-1.1 GPa the Turij Mys melts separated immiscible carbonatite melt, which subsequently exsolved late carbonate-rich fluids extremely rich in trace elements. Kandaguba and Turij Mys melts continued to fractionate at lower pressures in the presence of hydrous fluid to the more evolved nephelinite and phonolite melts. The studied dykes highlight the critical role of the parent magma chamber in crystal fractionation and magma diversification. The Kandalaksha dykes may represent a carbonatite - ultramafic lamprophyre association, which fractionated at 45-20 km in narrow dykes on ascent to the surface and could not get more evolved than monchiquite. In contrast, connections of Kandaguba and Turij Mys dykes to their massif magma chambers ensured the sufficient time for fractionation, ascent and a polybaric evolution. This longevity generated more evolved rock types with the higher alkalinity and an immiscible separation of carbonatites.

The study reports petrography, bulk major and trace element compositions of lamprophyric Devonian dykes in three areas of the Kola Alkaline Carbonatite Province (N Europe). Dykes in one of these areas, Kandalaksha, are not associated with a massif, while dykes in Kandaguba and Turij Mys occur adjacent (< 5 km) to coeval central multiphase ultramafic alkaline-carbonatitic massifs. Kandalaksha dyke series consists of aillikites - phlogopite carbonatites and monchiquites. Kandaguba dykes range from monchiquites to nephelinites and phonolites; Turij Mys dykes represent alnoites, monchiquites, foidites, turjaites and carbonatites. Some dykes show extreme mineralogical and textural heterogeneity and layering we ascribe to fluid separation. The crystallization and melt evolution of the dykes were modelled with Rhyolite-MELTS and compared with the observed order and products of crystallization. Our results suggest that the studied rocks were related by fractional crystallization and liquid immiscibility. Primitive melts of alkaline picrites or olivine melanephelinites initially evolved at P=1.5-0.8 GPa without a SiO2 increase due to abundant clinopyroxene crystallization controlled by the CO2-rich fluid. At 1-1.1 GPa the Turij Mys melts separated immiscible carbonate melt, which subsequently exsolved carbothermal melts extremely rich in trace elements. Kandaguba and Turij Mys melts continued to fractionate at lower pressures in the presence of hydrous fluid to the more evolved nephelinite and phonolite melts. The studied dykes highlight the critical role of the parent magma chamber in crystal fractionation and magma diversification. The Kandalaksha dykes may represent a carbonatite - ultramafic lamprophyres association, which fractionated at 45- 20 km in narrow dykes on ascent to the surface and could not get more evolved than monchiquite. In contrast, connections of Kandaguba and Turij Mys dykes to their massif magma chambers ensured the sufficient time for fractionation, ascent and a polybaric evolution. This longevity generated more evolved rock types with the higher alkalinity and an immiscible separation of carbonatites.

The study reports petrography, mineralogy and carbonate geochemistry and stable isotopy of various types of ocelli (silicate-carbonate globules) observed in the lamprophyres from the Chadobets Uplift, southwestern Siberian craton. The Chadobets lamprophyres are related to the REE-bearing Chuktukon carbonatites. On the basis of their morphology, mineralogy and relation with the surrounding groundmass, we distinguish three types of ocelli: carbonate-silicate, containing carbonate, scapolite, sodalite, potassium feldspar, albite, apatite and minor quartz ocelli (K-Na-CSO); carbonate–silicate ocelli, containing natrolite and sodalite (Na-CSO); and silicate-carbonate, containing potassium feldspar and phlogopite (K-SCO). The K-Na-CSO present in the most evolved damtjernite with irregular and polygonal patches was distributed within the groundmass; the patches consist of minerals identical to minerals in ocelli. Carbonate in the K-Na-CSO are calcite, Fe-dolomite and ankerite with high Sr concentration and igneous-type REE patterns. The Na-CSO present in Na-rich damtjernite with geochemical signature indicates the loss of the carbonate component. Carbonate phases are calcite and Fe-dolomite, and they depleted in LREE. The K-SCO was present in the K-rich least-evolved damtjernite. Calcite in the K-SCO has the highest Ba and the lowest Sr concentration and U-shaped REE pattern. The textural, mineralogical and geochemical features of the ocelli and their host rock can be interpreted as follows: (i) the K-Na-CSO are droplets of an alkali–carbonate melt that separated from residual alkali and carbonate-rich melt in highly evolved damtjernite; (ii) the Na-CSO are droplets of late magmatic fluid that once exsolved from a melt and then began to dissolve; (iii) the K-SCO are bubbles of K-P-CO2 fluid liberated from an almost-crystallised magma during the magmatic–hydrothermal stage. The geochemical signature of the K-SCO carbonate shows that the late fluid could leach REE from the host lamprophyre and provide for REE mobility.

To provide new insights into the origin and evolution of ultramafic lamprophyres (UMLs) and their mantle source, we examined two UML (aillikite and damtjernite) occurrences of different ages in the western portion of the Siberian Craton (Ilbokich and Chadobets). New age, mineral and rock geochemistry, along with Sr-Nd-Pb-C-O isotope data was obtained. Our new Pb-206/U-238 perovskite age (399 +/- 4 Ma) confirms the previously published Early Devonian age of the Ilbokich aillikite. RbSr isochron and 40Ar/39Ar dating yielded a Middle Triassic age (243 +/- 3 Ma and 241 +/- 1 Ma, respectively) for the Chadobets aillikites, indicating post-Trap emplacement of these rocks. Both UMLs are characterized by incompatible elements, including light rare earth element (LREE) enrichments (La is up to x200 chondrite concentration), and strong fractionation of REEs ((La/Yb)n: 33-84). Despite the close geochemical affinity of both UMLs, the Nd isotopic compositions of aillikites, as well as the Pb isotopic composition of Chadobets and Ilbokich UMLs, do not overlap and are distinctly different from each other. The initial Sr and Nd isotopic compositions of the Ilbokich UMLs fall in within a narrow Sr-87/Sr-86(0) range (0.7032-0.7042) and epsilon Nd(T) (4.03-3.97). Chadobets UMLs have a similar Sr isotopic signature (Sr-87/Sr-86(0): 0.7031-0.7043) and a more depleted Nd isotopic signature (epsilon Nd(T) 4.09-5.08). The initial Pb isotope compositions of the Chadobets UMLs are moderately radiogenic, ranging between Pb-206/Pb-204 = 18.4-19.0, Pb-208/Pb-204 = 38.3-38.8, and are characterized by a narrow Pb-207/Pb-204 ratio between 15.5 and 15.6. The Ilbokich Pb isotope compositions are less variable and range between Pb-206/Pb-204 = 18.0-18.4, Pb-208/Pb-204 = 37.8-38.4 and Pb-207/Pb-204 ratios between 15.5 and 15.6. The oxygen isotopic composition of carbonate from both UMLs is characterized by highly variable delta O-18 values from +12.1 and up to +20.5 parts per thousand (SMOW). The isotopic composition of delta C-13 values range from -1.3 parts per thousand to -7.1. Based on the minor impact of crustal contamination in both aillikites, it is inferred that their radiogenic isotope composition reflects a mantle source signature. The mantle source of the Chadobets aillikites is likely to include carbonatitic magma as a metasomatic agent. In contrast, phlogopite-rich metasomes within the lithospheric mantle could have contributed more significantly to the Ilbokich aillikites. These metasomes could be formed during the Caledonian orogeny, which did not only affect the southwestern boundary of the Siberian Craton, but also expanded to the craton interior. This study provides additional support for the evolution of the south-western portion of the Siberian SCLM, ranging from mantle containing phlogopite enrichment domains during the Early Devonian to hydrous-phase reduced mantle in the Triassic due to the thermal impact of the Siberian Traps. (C) 2020 Published by Elsevier B.V. To provide new insights into the origin and evolution of ultramafic lamprophyres (UMLs) and their mantle source, we examined two UML (aillikite and damtjernite) occurrences of different ages in the western portion of the Siberian Craton (Ilbokich and Chadobets). New age, mineral and rock geochemistry, along with Sr-Nd-Pb-C-O isotope data was obtained. Our new Pb-206/U-238 perovskite age (399 +/- 4 Ma) confirms the previously published Early Devonian age of the Ilbokich aillikite. RbSr isochron and 40Ar/39Ar dating yielded a Middle Triassic age (243 +/- 3 Ma and 241 +/- 1 Ma, respectively) for the Chadobets aillikites, indicating post-Trap emplacement of these rocks. Both UMLs are characterized by incompatible elements, including light rare earth element (LREE) enrichments (La is up to x200 chondrite concentration), and strong fractionation of REEs ((La/Yb)n: 33-84). Despite the close geochemical affinity of both UMLs, the Nd isotopic compositions of aillikites, as well as the Pb isotopic composition of Chadobets and Ilbokich UMLs, do not overlap and are distinctly different from each other. The initial Sr and Nd isotopic compositions of the Ilbokich UMLs fall in within a narrow Sr-87/Sr-86(0) range (0.7032-0.7042) and epsilon Nd(T) (4.03-3.97). Chadobets UMLs have a similar Sr isotopic signature (Sr-87/Sr-86(0): 0.7031-0.7043) and a more depleted Nd isotopic signature (epsilon Nd(T) 4.09-5.08). The initial Pb isotope compositions of the Chadobets UMLs are moderately radiogenic, ranging between Pb-206/Pb-204 = 18.4-19.0, Pb-208/Pb-204 = 38.3-38.8, and are characterized by a narrow Pb-207/Pb-204 ratio between 15.5 and 15.6. The Ilbokich Pb isotope compositions are less variable and range between Pb-206/Pb-204 = 18.0-18.4, Pb-208/Pb-204 = 37.8-38.4 and Pb-207/Pb-204 ratios between 15.5 and 15.6. The oxygen isotopic composition of carbonate from both UMLs is characterized by highly variable delta O-18 values from +12.1 and up to +20.5 parts per thousand (SMOW). The isotopic composition of delta C-13 values range from -1.3 parts per thousand to -7.1. Based on the minor impact of crustal contamination in both aillikites, it is inferred that their radiogenic isotope composition reflects a mantle source signature. The mantle source of the Chadobets aillikites is likely to include carbonatitic magma as a metasomatic agent. In contrast, phlogopite-rich metasomes within the lithospheric mantle could have contributed more significantly to the Ilbokich aillikites. These metasomes could be formed during the Caledonian orogeny, which did not only affect the southwestern boundary of the Siberian Craton, but also expanded to the craton interior. This study provides additional support for the evolution of the south-western portion of the Siberian SCLM, ranging from mantle containing phlogopite enrichment domains during the Early Devonian to hydrous-phase reduced mantle in the Triassic due to the thermal impact of the Siberian Traps. (C) 2020 Published by Elsevier B.V. To provide new insights into the origin and evolution of ultramafic lamprophyres (UMLs) and their mantle source, we examined two UML (aillikite and damtjernite) occurrences of different ages in the western portion of the Siberian Craton (Ilbokich and Chadobets). New age, mineral and rock geochemistry, along with Sr-Nd-Pb-C-O isotope data was obtained. Our new Pb-206/U-238 perovskite age (399 +/- 4 Ma) confirms the previously published Early Devonian age of the Ilbokich aillikite. RbSr isochron and 40Ar/39Ar dating yielded a Middle Triassic age (243 +/- 3 Ma and 241 +/- 1 Ma, respectively) for the Chadobets aillikites, indicating post-Trap emplacement of these rocks. Both UMLs are characterized by incompatible elements, including light rare earth element (LREE) enrichments (La is up to x200 chondrite concentration), and strong fractionation of REEs ((La/Yb)n: 33-84). Despite the close geochemical affinity of both UMLs, the Nd isotopic compositions of aillikites, as well as the Pb isotopic composition of Chadobets and Ilbokich UMLs, do not overlap and are distinctly different from each other. The initial Sr and Nd isotopic compositions of the Ilbokich UMLs fall in within a narrow Sr-87/Sr-86(0) range (0.7032-0.7042) and epsilon Nd(T) (4.03-3.97). Chadobets UMLs have a similar Sr isotopic signature (Sr-87/Sr-86(0): 0.7031-0.7043) and a more depleted Nd isotopic signature (epsilon Nd(T) 4.09-5.08). The initial Pb isotope compositions of the Chadobets UMLs are moderately radiogenic, ranging between Pb-206/Pb-204 = 18.4-19.0, Pb-208/Pb-204 = 38.3-38.8, and are characterized by a narrow Pb-207/Pb-204 ratio between 15.5 and 15.6. The Ilbokich Pb isotope compositions are less variable and range between Pb-206/Pb-204 = 18.0-18.4, Pb-208/Pb-204 = 37.8-38.4 and Pb-207/Pb-204 ratios between 15.5 and 15.6. The oxygen isotopic composition of carbonate from both UMLs is characterized by highly variable delta O-18 values from +12.1 and up to +20.5 parts per thousand (SMOW). The isotopic composition of delta C-13 values range from -1.3 parts per thousand to -7.1. Based on the minor impact of crustal contamination in both aillikites, it is inferred that their radiogenic isotope composition reflects a mantle source signature. The mantle source of the Chadobets aillikites is likely to include carbonatitic magma as a metasomatic agent. In contrast, phlogopite-rich metasomes within the lithospheric mantle could have contributed more significantly to the Ilbokich aillikites. These metasomes could be formed during the Caledonian orogeny, which did not only affect the southwestern boundary of the Siberian Craton, but also expanded to the craton interior. This study provides additional support for the evolution of the south-western portion of the Siberian SCLM, ranging from mantle containing phlogopite enrichment domains during the Early Devonian to hydrous-phase reduced mantle in the Triassic due to the thermal impact of the Siberian Traps. (C) 2020 Published by Elsevier B.V.

To provide new insights into the origin and evolution of kimberlitic magmas with different diamond concentrations from the Arkhangelsk diamond province in north-western Russia, we examined the major- and trace-element compositions of ilmenite from diamondiferous kimberlite of the Grib pipe and diamond-barren kimberlites from the Kepino cluster (Stepnaya and TsNIGRI–Arkhangelskaya pipes). Ilmenite from diamond-barren kimberlites shows lower Mg, Ti, Cr, Ni and Cu concentrations with increase in both Fe 3+ and Fe 2+ and Nb, Ta, Zr, Hf, Zn and V concentrations. The main differences between kimberlites with different diamond contents are the Nb and Zr concentrations and their correlation patterns with Mg and Cr concentrations. Ilmenite from the Grib kimberlite has Zr concentrations <110 ppm, whereas ilmenite from the Kepino kimberlites has Zr concentrations >300 ppm. Ilmenite crystallisation within the Grib kimberlite occurred under increasing oxygen fugacity ( f O 2 ), which may reflect assimilation of mantle peridotite by the kimberlitic magmas. Ilmenite from the Kepino kimberlites suggests its crystallisation under constant f O 2 , with the ilmenite composition being controlled by processes of fractional crystallisation of megacrystic minerals. These assumptions were confirmed with assimilation–fractional crystallisation calculations. On the basis of obtained data, we developed a model for the evolution of the kimberlitic magmas for both diamondiferous and barren kimberlites. The diamond-bearing kimberlitic magmas were generated under intense interaction of kimberlitic magmas with the surrounding lithospheric mantle. It may be that during early modification of the lithospheric mantle by kimberlitic magmas as well as with kimberlitic magmas’ local stretching and swift ascent, the capture of the mantle xenoliths was favoured over the crystallisation of phenocrysts. The formation of barren kimberlitic magmas may have occurred when the lithospheric mantle in the vicinity of ascending magmas was already geochemically equilibrated with them. It also is possible that the magma’s ascent slowed under conditions of dominantly compressive stresses with crystallisation of olivine and other megacrystic phases.

The paper is dedicated to the study of the Devonian magmatic association of the Eastern Azov region, which is a part of the Pripyat-Dnieper-Donets rift zone. The association includes gabbros, peridotites, pyroxenites, and lamprophyre dikes of the Pokrovo-Kireevsky massif (PKM) and picrites, picrobasalts, and basalts of the Anton-Taram Formation (ATF). Analysis of clinopyroxenes of different generations from the PKM micaceous gabbro and the ATF alkaline picrite provided insight into the mantle source of these rocks and the evolution of melts, which determined the close spatiotemporal association of kimberlites, basites, ultramafic rocks, including alkaline varieties. Clinopyroxenes from the micaceous gabbro are composed of Cpx1 (Mg # = 0.87–0.88) or Сpx2 (Mg # = 0.80–0.81) cores and Cpx3 external zones (Mg # = 0.70–0.76). Clinopyroxenes from alkaline picrite are composed of Сpx2 cores (Mg # = 0.80–0.84) and external Cpx3 zones (Mg # = 0.71–0.78). Clinopyroxenes in general have an upward concave multielement pattern, with enrichment in LREE, depletion in Ba, Nb, and HREE, Zr–Hf negative anomaly, and, additionally, negative Sr-anomaly in Cpx2 and Cpx3. The calculated equilibrium melts for Сpx2 from the micaceous gabbro are very close in composition to this gabbro, and those for Cpx2 from the alkaline picrite coincide in composition with this picrite, and in general are close to ATF picritic lavas. The high Mg# and Cr content in cores Cpx1 indicate that this mineral was derived from the earliest weakly differentiated magma close to the primary melt. The presence of a negative Zr–Hf anomaly in Cpx1 geochemical patterns at ZrPM 0.8) melts, which generated picrobasalts and lamprophyres. The geochemical similarity of the early Cpx1 cores with clinopyroxenes from ilmenite-bearing mantle metasomatites is consistent with the assumption that ultrahigh-Ti primary melts of the Eastern Azov lamprophyres were derived from carbonated ilmenite-bearing, and likely, phlogopite-bearing (PIC) peridotites.