Geochronology, mineral inclusions, and geochemistry of zircons in eclogitized gabbronorites in the Gridino area, Belomorian province

Abstract

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 the genesis of zircons in their various populations and, correspondingly, the age of certain geological events (magmatic crystallization of the gabbroids, their eclogitization, and overprinted retrograde metamorphism). The 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. The zircons of group I are xenogenic and have a Meso- to Neoarchean age. Mineral inclusions in them (quartz, apatite, biotite, and chlorite) are atypical of gabbroids, and the geochemistry of these zircons is very diverse. The zircons of group II 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 were formed during the late magmatic crystallization of the gabbroids at temperatures of 1150–1160°C, and their U-Pb age of 2389 ± 25 Ma corresponds to this process. The eclogite mineral assemblages crystallized shortly after the magmatic process, as follows from the fact that the marginal portions of the prismatic zircons contain clinopyroxene inclusions with elevated contents of the jadeite end member. The zircons of group III 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 an U-Pb age of 1911 ± 9.5 Ma, which corresponds to the age of the overprinted amphibolite-facies metamorphism.