Parental Melts of Neoarchean Subalkaline Magmatic Rocks of the Keivy Structure (Kola Peninsula)

Abstract

The Neoarchean subalkaline magmatism of the Keivy structure is manifested by the emplacement of the latite–monzonite–granite (LMG) volcano-plutonic association. The measured ¹⁴³Nd/¹⁴⁴Nd ratio in rocks is 0.511061–0.511365; the values of model ages fall within the interval of 3.14–3.21 Ga with εNd₍₂₆₇₀₎ varying from 2.2 to –1.3. The initial ¹⁷⁶Hf/¹⁷⁷Hf ratios in zircon are within the range of 0.280950–0.281069, and composition data points make up a field in the region of the evolutionary trend of the chondritic uniform reservoir. As based on isotope and geochemical data, the protoliths of all rocks are assumed to be basic. In terms of the εHf(T) value, the data points of the zircon composition fall within the domain of the crust with ¹⁷⁶Lu/¹⁷⁷Hf = 0.015 and an age of 3.0–3.3 Ga. This suggests that the parental melts were probably formed predominantly from material that spent a long time in the crust. The parental melts of the LMG association presumably formed as a result of the melting of metasomatically altered basic rocks during the intrusion into the lower crust of basaltic plutons, which were parental for the rocks of the dyke suite and gabbrolabradorite intrusions in the upper crust.