Pechenga ferropicrites and other early Proterozoic picrites in the eastern part of the Baltic Shield

Abstract

Ti-poor komatiites and Ti-rich picrites exist among the widely occurring primitive, MgO-rich volcanics in the early Proterozoic successions of the Kola Peninsula. Both have been encountered in the Imandra-Varzuga Belt, but only picrites are found in the Pechenga Complex. The picritic rocks are divided into two types. The first type, restricted to the Kolasjoki and Pilgujarvi suites and the Tominga series, is called ferropicrite due to its exceptionally high total iron content which usually exceeds 14% (FeOtot). The second type, found in the South Pechenga series and in the Umba and Panarechka suites, has more typical iron content and geochemically resembles Suisaarian picrites from southern Karelia. Ferropicrites occur as pillow lavas, lava breccias, tuffs, massive lavas, and up to 50 m thick, differentiated, spinifex-textured flows. In addition to high FeOtot contents, these rocks are characterized by low Al2O3 contents and by high concentrations of TiO2, Zr and other incompatible elements. They are strongly LREE-enriched and have immobile trace element characteristics similar to within-plate transitional or alkali basalts. It is shown that, with regard to both the major and trace element compositions, the parental magma of the Ni-bearing intrusions of the Pilgujarvi suite is similar to ferropicrite, suggesting that the emplacement of the intrusions took place contemporaneously with ferropicritic volcanism. The high incompatible element contents of the ferropicritic rocks rule out a genetic relationship with associated tholeiitic basalts. Proposed geotectonic settings for the Pechenga area, including an intracratonic rift zone, a passive continental margin, an oceanic island arc, a mature continental volcanic arc, and a collisional suture zone are discussed. The presence of Ti-rich picritic rocks does not support a subduction-related environment.