Petrogenesis of Devonian lamprophyre and carbonatite minor intrusions, Kandalaksha Gulf (Kola Peninsula, Russia)

Abstract

Minor magmatic intrusions (dykes and explosion pipes) of lamprophyric and carbonatitic compositions occur on several islands in the Gulf of Kandalaksha (White Sea, Kola Peninsula, Russia). The lamprophyre dykes yielded K-Ar ages of 368 ± 15 Ma and 360 ± 16 Ma, similar to the majority of alkaline rocks from the Kola Alkaline Province. Mineralogical data (presence of perovskite and sodalite, absence of amphibole phenocrysts) and geochemical data (low SiO 2 and A1 2O 3, high MgO) indicate an ultramafic lamprophyre affinity for the investigated silicate rocks. The lamprophyres contain a wide variety of xenoliths including hornblende- and biotite-rich cumulate ultramafic rocks. The carbonatite intrusions have ferrocarbonatite affinities and one of them contains xenoliths of coarse-grained Si-rich calciocarbonatites, together with abundant hornblendites and glimmerites which resemble those in the lamprophyres. The calciocarbonatite xenoliths themselves contain fragments of mica- and hornblende-rich rocks. 40Ar- 39Ar ages on phlogopite and amphibole from calciocarbonatite and hornblende-rich cumulate xenoliths are between 386 ± 1.0 Ma and 395.6 ± 4.4 Ma, indicating an early Devonian age and suggesting a close relationship between the calciocarbonatite xenoliths and the ultramafic cumulate xenoliths. Thus, the ferrocarbonatite host magma may have disrupted an older calciocarbonatite-hornblendite-glimmerite intrusion at depth and incorporated xenoliths from it. The presence of hornblendite and glimmerite xenoliths with similar parageneses and identical ages in both the ultramafic lamprophyres and ferrocarbonatites suggests a close relationship between the ferrocarbonatite and lamprophyric magmas. REE patterns of the lamprophyric dykes and calciocarbonatite xenoliths show strong similarities, indicating a petrogenetic relationship. The ultramafic lamprophyres have REE patterns which are indistingishable from the contemporaneous kimberlites and melilitites from the nearby Terskii Bereg area, south Kola Peninsula. Age-corrected Sr and Nd isotope compositions demonstrate that the calciocarbonatite xenoliths have close affinities with other Devonian carbonatites from Kola and Karelia, whereas the lamprophyres are similar to the Kola kimberlites and melilitites. Differences between the Sr and Nd isotopic ratios of the silicate and carbonatite magmas throughout the Kola Alkaline Province are probably due to different mantle-source components. The Kola carbonatites mainly show a depleted mantle signature whereas the lamprophyres, melilitites and kimberlites were derived from a more enriched mantle. However, some degree of assimilation of lower continental crust and late-stage hydrothermal alteration of the silicate magmas may also have occurred.