Geochemistry and mineralogy of kimberlites from the Arkhangelsk Region, NW Russia: evidence for transitional kimberlite magma types

The Arkhangelsk kimberlite province (AKP) is situated in the north of the Baltic Shield within the buried southeastern portion of the Kola–Kuloi craton. It forms part of the extensive Devonian magmatic event of the northern Baltic Shield and Kola Peninsula. Two main groups of kimberlites can be distinguished within the province: (1) kimberlites from the diamondiferous Zolotitsa field that have geochemical and isotopic affinities with Group 2 kimberlites and lamproites; (2) diamond-poor Ti–Fe-rich kimberlites from other Arkhangelsk fields that have geochemical and isotopic affinities with Group 1 kimberlites. However, the Zolotitsa and Ti–Fe-rich kimberlites have mineralogical characteristics that are not typical for their respective assigned kimberlite group classifications. Both groups of Arkhangelsk kimberlites are apparently transitional to Group 1 kimberlites, Group 2 kimberlites and lamproites as they are defined elsewhere in the world. An associated kimberlite from the Mela Sill Complex has strong affinities with carbonatites. The low Al2O3, high Ni and Cr contents, and high Mg# in both groups of kimberlites indicate strongly depleted lherzolitic–harzburgitic mantle sources. Trace element patterns show a variable enrichment of incompatible elements and strong LREE enrichment. However, kimberlites from the Zolotitsa field have overall lower trace element abundances and less steep REE patterns, suggesting a higher degree of partial melt and/or a less enriched source compared to that of the Ti–Fe-rich kimberlites. A calciocarbonatite of the Mela Sill Complex has trace element and REE patterns typical of other carbonatites closely associated with kimberlites. 87Sr/86Sri and 143Nd/144Ndi isotope compositions of the Arkhangelsk kimberlites and carbonatite reveal that at least two mantle sources are required to explain the isotopic variation: (1) most of the Zolotitsa and Mela kimberlites and the Mela carbonatite are derived from an ancient enriched lithospheric source (EMI); (2) the Ti–Fe-rich kimberlites are derived from a plume-related asthenospheric mantle source with an isotopic composition close to Bulk Earth. Present-day Pb isotope compositions reveal that the Zolotitsa kimberlites have values close to Group 1 kimberlites. However, the Ti–Fe-rich kimberlites generally have slightly more radiogenic Pb isotope values.