Petrogenesis of metasomatic rocks in the fenitized zones of the Ozernaya Varaka alkaline ultrabasic complex, Kola Peninsula

This paper reports the results of an investigation of metasomatic alterations in basement gneisses within the contact aureole of the Ozernaya Varaka alkaline ultrabasic massif. Based on mineralogical and geochemical data, the metasomatic rocks were classified, variations in major and trace elements were evaluated along a cross-section through the contact zone, the mobility of components during the metasomatic transformation of rocks was estimated, and the gain or loss of components was quantified. The obtained results provided insight into the trends of changes in mineral assemblages, the mechanisms of migration of fluid flows, and, most importantly, the scale of input or removal of major and a number of trace components at different stages of the evolution of the metasomatic system within the contact aureole and border zones. It was shown that a fluid phase was released directly from a magma chamber filled with alkaline ultrabasic melt, which is indicated by the high He-3/He-4 value (5.9 x 10(-6)) of the fluid, corresponding to 50% of the mantle component. It was shown that the release of a considerable amount of fluid phase related to the alkaline ultra-basic melt was not accompanied by a significant input of high field strength and rare earth elements. In contrast, the fluids of the subsequent carbonatite stage were enriched in volatile components (F, Cl, P, and S), which resulted in the migration of REE into the fenitized zones and removal of Ta, Zr, and Hf from it. The low He-3/He-4 value of this fluid suggests that its noble gases were contaminated by a crustal component, and solutions percolated along fracture systems and shatter zones from the root of the magmatic system. During the final postmagmatic stage, alkaline metasomatism was caused mainly by aqueous fluids and was accompanied by the removal of all rare earth elements and most of the high field strength elements (Nb, Ta, Zr, and Hf). A comparison of the fenitized zone of the Ozernaya Varaka complex with those of the Khibiny and Lovozero agpaitic syenite massifs showed that the metasomatic reworking of gneisses in the multiphase alkaline ultrabasic complex occurred in several stages and was accompanied by the input and removal of components, whereas the single-stage fenitization in the agpaitic syenite plutons involved minor amounts of heterogeneous water-fluoride fluids and resulted in extensive migration of rare earth and high-field strength elements in the contact zone.