Petrography and geochemistry of the Devonian ultramafic lamprophyreat Sokli in the northeastern Baltic Shield (Finland)

Abstract

The Sokli complex in the northeastern Baltic Shield (Finland) forms a part of the extensive Devonian Kola Alkaline Province. The complex contains ultramafic lamprophyres occurring as dikes of millimetric to metric thickness. The Sokli ultramafic lamprophyres have petrographical and geochemical affinities with aillikite. High concentrations of Cr and Ni with low Al2O3 content of the Sokli aillikites indicate a strongly depleted harzburgitic source. However, compared to the kimberlites, the lower Cr and Ni contents and mg-number with weaker HREE depletion of the Sokli aillilkites imply a smaller proportion of garnet in the source and thus suggest a shallower melting depth of the source. In order to account for high concentrations of all incompatible elements and LREEs, with high volatile content (especially CO2), an additional enriched material is thought to have been incorporated into the Sokli aillikite source. An anomalous enrichment of K in the Sokli aillikite, compared to nearby ultrapotassic rocks and world-wide ultramafic lamprophyres, indicate a presence of K-rich phase (probably phlogopite) in the source mantle.