Chemical and isotopic compositions of zircons from an eclogitized gabbronorite dike at the Gridino village, Northern Karelia

A comprehensive (mineralogical, geochronological, and geochemical) study of zircons from an eclogitized gabbronorite dike was carried out in order to identify reliable indicators (mineralogical and geochronological) of genesis of the zircons in their various populations and, correspondingly, ages of certain geological events (magmatic crystallization of the gabbroids, their eclogitization, and overprinted retrograde metamorphism). Three populations of zircons separated from two rock samples comprised xenogenic, magmatic (gabbroic), and metamorphic zircons, with the latter found exclusively in the sample of retrograded eclogitized gabbroids. Group I zircons are xenogenic and have a Meso- to Neoarchean age. Mineral inclusions in them (quartz, apatite, biotite, and chlorite) are atypical of gabbroids, and geochemistry of these zircons is very diverse. Group II zircons contain mineral inclusions of ortho- and clinopyroxene and are distinguished for their very high U, Th, Pb, and REE concentrations and Th/U ratios. These zircons formed during the late magmatic crystallization of the gabbroids at temperatures of 1150-1160°C, and their U-Pb age 2389±25 Ma corresponds to this process. Eclogite mineral assemblages crystallized shortly after the magmatic process, as follows from the fact that marginal portions of prismatic zircons contain clinopyroxene inclusions with elevated contents of the jadeite end-member. Group III zircons contain rare amphibole and biotite inclusions and have low Ti, Y, and REE concentrations, low Th/U ratios, high Hf concentrations, contain more HREE than LREE, and have U-Pb age 1911±9.5 Ma, which corresponds to age of overprinted amphibolite-facies metamorphism.