Paleoproterozoic Eclogites in the Salma Area, Northwestern Belomorian Mobile Belt: Composition and Isotopic Geochronologic Characteristics of Minerals and Metamorphic Age

摘要

The paper represents results of a comprehensive geochemical and isotopic-geochemical (SIMS) study of eclogites from the northwestern part of the Belomorian Belt (Salma eclogites). A detailed fieldwork was carried out at the quarry of the Kuru-Vaara deposit of ceramic pegmatite in the northwestern part of the study area, in which tonalite–trondhjemite gneisses include bodies and blocks of eclogite and Grt–Aug eclog-ite-like clinopyroxenite and are cut across by numerous pegmatite veins. The least altered types of the Grt–Cpx rocks selected for our further research included: (1) widespread massive homogeneous fine-grained Grt–Omp eclogite that replaced gabbro and contained symplectites of Pl + low-Na–Cpx around omphacite and Pl–Hbl kelyphite rims around Grt; and (2) coarse-grained eclogite-like Grt–Hbl–Aug clinopyroxenite beds up to 20 cm thick in the central parts of high-Mg metaultrabasites, which are mostly tremolite–actinolite schists. The REE patterns of garnet, clinopyroxene, and amphibole from the eclogites confirm that they crystallized simultaneously, under a high pressure, and in the absence of plagioclase. Local U–Pb dates of the zircons and their geochemistry are at variance with the earlier hypothesis that the eclogite metamorphism occurred in the Archean. The eclogites and Grt–Hbl–Aug clinopyroxenite were determined to contain zircons of Svecofen-nian age (approximately 1900 Ma), which show all geochemical characteristics of classic eclogitic zircons andoccur either as individual crystals or as rims around Archean magmatic zircons from the primary gabbroids.