New isotopic and geochemical data from the Palaeoproterozoic Pechenga Greenstone Belt, NW Russia: Implication for basin development and duration of the volcanism

The North Pechenga Group of the Pechenga Greenstone Belt comprises a thick supracrustal succession developed within a time interval of ca. 500 Ma in the early part of the Palaeoproterozoic Era. It records several important global events leading to the onset of modern-style Earth system, but the temporal evolution of the belt is still insufficiently established. New Nd isotope data clearly indicate an abrupt change from a strong crustal signature in the lower two volcanic units to a depleted-mantle like signature in the upper two volcanic units, which conforms with the coeval shift of the volcanism from subaerial to submarine, indicating a significant thinning of the lithosphere due to continental rifting. The timing of the change in the geotectonic regime can be bracketed between 2058 +/- 6 Ma (U-Pb data) and 2018 +/- 54 Ma (Sm-Nd data). In situ ion microprobe analyses of zircon grains from a thin felsic tuff member in the upper part of the succession (Pilgujarvi Volcanic Formation) produced an age of 1988 +/- 3 Ma, which is within error the same as has been reported previously for the ferropicritic magmatism that produced the Pechenga Ni-Cu ores. It is concluded that the upper 3-km-thick volcanic pile dominated by tholeiitic basalts and minor ferropicrites was generated within a short time span unresolvable with current geochronological tools. Geochemical and isotopic data suggest that the felsic tuffs were generated from a ferropicritic parental magma via extensive fractional crystallisation, and the presence of gneiss xenoliths in the tuffs reveals that this happened in a magma chamber located in the underlying Archaean basement. Thus, although the mafic subaqueous volcanism of the upper part of the belt does not bear Nd isotopic or trace element evidence for contamination with ancient crustal material, the rifting of the basement did not yet proceed to a total break-up of the Archaean craton. (C) 2014 Elsevier B.V. All rights reserved.