Composition of rock-forming minerals in the Kivakka layered massif, northern Karelia, and systematic variations in the chemistries of minerals in the rhythmic layering subzone

Abstract

Data obtained on the compositions of rock-forming minerals from the Kivakka layered intrusion indicate that the #Fe of olivine increases up the vertical section of the intrusion, and the #Mg of the pyroxenes and the anorthite contents of the plagioclase simultaneously decrease, which reflects the fractionation of the parental melt. Emphasis is placed onto the compositional variability of minerals in the layered unit. A method is developed for the reliable calculation of the composition of pyroxenes before their exoslution (separately for ortho-and clinopyroxene). The composition of minerals in the zone of intercalating melano-, meso-, and leucocratic norites remains practically unchanged, which is generally consistent with the results of mathematical simulations: compositional variations of the bronzite (#Mg) and plagioclase (An content) during the crystallization of the bronzite-plagioclase cotectic were no greater than 5 mol %. The variations in the concentrations of trace elements in the cumulus and intercumulus clinopyroxene occurred to be the most informative. It was determined that the Cr concentration in the cumulus augite is statistically significantly lower than that in the intercumulus augite, which is at variance with fractionation laws and led us to suggest that the rocks with cumulus pyroxenes were formed by a melt with a higher degree of fractionation than those of the intercumulus melts of the adjacent layers.