Sources of Terrigenous Material in Formation of Metasedimentary Rocks of the Archean Complex Cored by the Kola Superdeep Borehole

摘要

The basement of the Paleoproterozoic Pechenga structure was cored by the Kola superdeep borehole SD-3 at the depths of 6842-12 262 m. It consists mainly of alternating strata of metavolcanic dacite-plagiorhyodacite rocks and high-alumina gneisses; the protoliths of these rocks corresponded to siltstones, graywackes, arkoses, polymictic sandstones, and silt-pelitic and pelitic argillites. Resulting from the examination of zircons from metaterrigenous rocks of the 1st, 3rd, and 9th strata of the SD-3, the detrital, anatec-tic, metamorphogenic, and contact-metasomatic genetic types have been identified. Detrital zircons include several age groups. The most homogeneous, i.e., comparable to zircons from tonalite gneisses (bottoms of the SD-3 section) and from analogous rocks from surrounding rocks, zircons have appeared to be those from gneisses of the deepest 9th stratum. The data on the age of these zircons, along with a poor rounding of the grains, signifies formation of the host gneisses' protoliths owing to washing-out and redeposition of material. Widening of alimentation areas, which supplied terrigenous material into sedimentation basins, took place during formation of alumina gneisses of the 3rd and especially 1st strata of the section. Detrital zircons of the 1st stratum are characterized by a good rounding of crystals and a broad age spectrum (from 2.79 to >3.1 Ga). By composition, they are close to zircons from Neoarchean tonalite gneisses of the SD-3 borehole and its surroundings, and gneisses of the Kola Series; however, they differ from zircons of the most ancient granitoids from the north of the Baltic Shield by the higher content of common lead.