AMPHIBOLE-CLINOPYROXENE-PHLOGOPITE CUMULATES IN CARBONATITE BRECCIAS IN THE KOLA ALKALINE PROVINCE: A RECORD OF THE EVOLUTION OF ALKALINE-ULTRAMAFIC MELTS IN INTERMEDIATE CHAMBERS

Abstract

The study of the amphibole-clinopyroxene-phlogopite macrocryst association in carbonatites from the Kola Alkaline Province is of key importance for deciphering the evolution of alkali–ultramafic melts in intermediate magma chambers and for elucidating the nature of the amphibole-bearing mantle source. This paper presents the results of a comprehensive petrographic, mineralogical (SEM, EMPA), geochemical (XRF, ICP-MS), and Sr-Nd isotopic (TIMS) investigation of multiphase dykes of amphibole carbonatites from the Kola Province (Niva River estuary, Kandalaksha town, Murmansk region).The studied dykes (378.6±3.8 Ma, Rb-Sr isochron age) represent multiphase bodies up to 1 m in thickness. The bulk volume of the dykes is composed of carbonatite breccias containing macrocrysts and intergrowths of clinopyroxene, phlogopite, and amphibole, with amphibole being the predominant phase. It has been established that the macrocrysts do not represent phenocrysts crystallized from the host melt, but rather constitute entrained cumulate material formed in intermediate crustal chambers at 5–7 kbar and ~1070 °C during the evolution of alkali–ultramafic melts and subsequently captured by the carbonatite melt.The observed zonation in the macrocrysts is not related to fractional crystallization but is of reaction origin. The presence of mineral populations with discrete compositions (characterized by sharp contacts and an absence of diffusion controlled zoning) indicates a short-term interaction between the original cumulates and new batches of high-Mg# and oxidized alkali–ultramafic melts immediately prior to entrapment by the carbonatite magma. The established Sr-Nd isotopic equilibrium between the macrocrysts and the carbonate matrix points to their genetic relationship and suggests that the carbonatite and alkali–ultramafic melts are products of the evolution of a single magmatic system.The model age T DM (Nd) of the carbonatite breccias, not exceeding 700 Ma, precludes a Paleoproterozoic (1.9–1.7 Ga) subduction-related metasomatized mantle as a direct source of the melts. The most likely source is represented by amphibole pyroxenite veins of the MARID type, formed in the lithospheric mantle during a Neoproterozoic (700–570 Ma) magmatic pulse in Fennoscandia.

The authors' abstract, as published at the source. Geodynamics & Tectonophysics, 2026 · DOI ↗