LIGHT AND HEAVY MINERAL PETROGRAPHY AND GEOCHEMISTRY OF PALEOPROTEROZOIC SANDSTONES DRILLED DURING THE FAR-DEEP PROJECT – DECIPHERING THE PALEOPROTEROZOIC SEDIMENTARY PROVENANCE OF THE BALTIC SHIELD

Most of the known Archean and Paleoproterozoic crustal blocks are relatively well studied. However, little is known about the provenance of Paleoproterozoic sedimentary rocks of the northern Baltic Shield. The Fennoscandian Arctic Russia – Drilling Early Earth Program (FAR-DEEP) targeted the Archean-Paleoproterozoic transition preserved in the Pechenga and Imandra-Varzuga Greenstone belts of the Kola Province, and the Onega Basin of the Karelian Province. Several formations representing the stratigraphic succession of these greenstone belts were analysed to constrain provenance areas and source rocks of sampled sand- and siltstones using different analytical methods. Next to U-Pb geochronology on detrital zircons, heavy mineral analysis and light mineral petrography was used to unravel the provenance of the detritus accumulated in the Paleoproterozoic basins. The studied sandstones vary between quartz-rich and quartz-poor with average light mineral compositions mainly of Q₈₄F₁₄L₂, and, in the Kuetsjärvi Volcanic Formation, subordinately of Q₁₃F₁₃L₇₄. The data indicate a main derivation from exhumed continental crustal blocks with some contribution from arc type upper crust. Heavy minerals are mainly represented by stable mineral assemblages of rutile, zircon and tourmaline. Labile minerals like amphiboles and pyroxenes are rare. The geochemical compositions of rutiles and tourmalines, determined using a micro probe, indicate a bimodal provenance from mainly felsic and subordinately mafic upper crustal units. Furthermore, tourmaline compositions point to metasediments as well as magmatic sources, both overprinted during regional metamorphism. Amphiboles were mainly derived from contact metamorphic rocks probably linked to intrusive activity in the Archean greenstone belts. Zr-in-rutile thermometry data reflect metamorphic temperatures indicating predominantly eclogite to amphibolite facies conditions and subordinately granulite facies conditions. The analysed samples mirror the tectonic evolution of the Baltic Shield from cratonic generation to rift processes and the development of a passive continental margin. The results indicate the dominance of short transport paths and variable upper crustal sources for rocks from the Kola and Karelian provinces.