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In this paper, new main and trace elements and isotopic data are presented for 14 coarse-grained eclogite xenoliths from the V. Grib kimberlite pipe in the central part of the Arkhangelsk Diamondiferous Province. Based on reconstructed whole rock MgO content, this suite is divided into high-MgO and low-MgO varieties. Eclogitic groups have a similar range of variations in the trace element compositions of garnet, clinopyroxene and reconstructed whole rock. All eclogites show positive Eu anomalies in garnet and Sr anomalies in the whole rock. The negative correlation between the Mg#, Sr/Lu ratio and HREE in a whole rock points to upper and lower oceanic crustal rocks as a protolith for eclogites with high and low whole rock HREEs, respectively. Low-MgO eclogites with higher whole rock HREEs have the basaltic upper oceanic crustal protolith, whereas the protoliths of eclogites with lower whole rock HREEs could be of gabbroic composition from the lower oceanic crust. High-MgO eclogites could represent MgO-rich portions of oceanic crustal rocks: picritic/MgO basalt portions in the upper oceanic crust and troctolite portions in the lower oceanic crust. The Sr and Nd isotope compositions suggest a complex history of eclogites during their residence in the lithospheric mantle. Similarities in the Nd isotope compositions and two-point Sm-Nd isochron ages are evidence for re-equilibration of the Sm-Nd isotope system between the eclogite garnet and clinopyroxene via a pre-kimberlite thermal event at 396 ± 24 Ma. The subset of clinopyroxenes from four eclogites has a Sr isotope composition that plots on the isochron at an age of 2.84 Ga, which reflects the time of the subduction event and emplacement into the lithosphere and corresponds to the time of the Belomorian Eclogite Province of Baltic Shield formation. [ABSTRACT FROM AUTHOR]

The two world’s largest complexes of highly alkaline nepheline syenites and related rare metal loparite and eudialyte deposits, the Khibina and Lovozero massifs, occur in the central part of the Kola Peninsula. We measured for the first time in situ the trace element concentrations and the Sr, Nd and Hf isotope ratios by LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) in loparite, eudialyte an in some other pegmatitic minerals. The results are in aggreement with the whole rock Sr and Nd isotope which suggests the formation of these superlarge rare metal deposits in a magmatic closed system. The initial Hf, Sr, Nd isotope ratios are similar to the isotopic signatures of OIB indicating depleted mantle as a source. This leads to the suggestion that the origin of these gigantic alkaline intrusions is connected to a deep seated mantle source—possibly to a lower mantle plume. The required combination of a depleted mantle and high rare metal enrichment in the source can be explained by the input of incompatible elements by metasomatising melts/fluids into the zones of alkaline magma generation shortly before the partial melting event (to avoid ingrowth of radiogenic isotopes). The minerals belovite and pyrochlore from the pegmatites are abnormally high in 87Sr /86Sr ratios. This may be explained by closed system isotope evolution as a result of a significant increase in Rb/Sr during the evolution of the peralkaline magma. [ABSTRACT FROM AUTHOR]

Covering a vast area of the northern Siberian platform are the Siberian flood basalts (SFB), which make up one of the world’s largest magmatic provinces. Along the northeastern margin of the SFB province lies the Maymecha-Kotuy alkaline-ultramafic complex, consisting of a large volume of alkaline lavas, numerous dykes, and the Guli massif together with numerous other, smaller alkaline plutons. The genetic link between the SFB and the Maymecha-Kotuy complex continues to be a subject of active debate. Although the rocks in both units have essentially the same age close to the Permian-Triassic boundary, questions remain as to the relative order of emplacement and the contributing source materials of each lithology. This study builds upon earlier petrologic, geochemical, and isotopic work to further an understanding of the relationship between SFB and alkaline rocks. A whole-rock U-Pb age of 250 ± 9 Ma was determined for the Guli massif, which lies within the range of ages previously reported for the SFB. The Pb isotopic composition of the Guli rocks plot mainly in the lower portion of the OIB field, and dunite and carbonatite extend downward into the MORB field suggesting for them a more depleted source than the one that produced the SFB. The combined Pb, Sr, and Nd isotopic systematics of the SFB and the Guli alkaline rocks enable the identification of several discrete source components. The first component dominates many of the Guli rocks and is characterized by low 87Sr/86Sr (0.7031 to 0.7038), high εNd (+5.35 to +3.97), and relatively unradiogenic Pb (206Pb/204Pb = 17.88–18.31; 207Pb/204Pb = 15.38–15.46; 208Pb/204Pb = 37.33–37.70), which we associate with the depleted (MORB source) mantle. The second component representing most of the SFB demonstrates a notable chemical and isotopic uniformity with 87Sr/86Sr values of 0.7046 to 0.7052, εNd values of 0 to +2.5, and an average Pb isotopic composition of 206Pb/204Pb = 18.3, 207Pb/204Pb = 15.5, and 208Pb/204Pb = 38.0. This component, making up the majority of SFB, is speculated to be a relatively primitive lower mantle plume with a near-chondritic signature. Contamination by upper and lower continental crustal material, designated as components 3 and 4, is postulated to explain the isotopic characteristics of some of the higher SiO2 Guli rocks and SFB. Finally, metasomatic processes associated with the invasion of the Siberian super-plume add a fifth component responsible for the extreme enrichment in rare-earth and related elements found in some Guli rocks and SFB. [ABSTRACT FROM AUTHOR]