Subsolidus fluid driven post-magmatic mineral formation in peridotite and ijolite rocks from the Kovdor and Iivaara complexes (Kola Alkaline Province)

Rocks exposed in alkaline magmatic complexes and associated rare metal deposits are a product of multistage formation. Petrographic, mineralogical and geochemical characteristics suggest that post-magmatic metasomatic mineral reactions triggered by high flux of magmatic volatiles are very common. Release of magmatic fluids and rock permeability are pressure dependent, which may exert a depth control on metasomatic reactions altering intrusion related vertical magmatic zonation along the intrusive conduit. Fenites attest to fluid release to the wall rocks. High permeability in the contact to the wall rock causes intensive fluid-rock interaction and metasomatic products observed across the contact at a given exposure level are the integrated product of fluid flux with a significant density driven vertical advective component. Alkaline rocks may thus reflect input from various mantle and crustal sources situated at lower levels of the intrusive system.

Here, we present results of a mineralogical and isotope geochemical investigation of samples from the complexes of Kovdor and Iivaara (Kola Alkaline Province), which are considered to represent different levels in both intrusive alkaline systems. Peridotitic and ijolitic rocks show substantial subsolidus mineral reactions. In both cases mineralogical as well as Sr and Nd isotopic signatures suggest a complex and multistage post-magmatic metasomatic evolution mainly driven by magmatic fluids, in which, however, input from the wall rocks across the contact into the complex cannot be ruled out.