Geochemistry and Conditions of Formation of the Precambrian Complexes

Abstract

A detailed description of the Pechenga complex rocks was given for the first time by Zagorodny et al. (1964), who adduced evidence in support of a theory of the geological integrity and synchronous formation of the northern and southern zones of the Pechenga structure. It was suggested that this structure originated as a result of the successive formation of four major rhythms which started their development with sedimentation and ended with volcanism. The chemical study of volcanites revealed some peculiar features, on the basis of which they can be regarded as products of the cyclic, multi-staged and complex process of basalt magma evolution in the direction from the spilite-keratophyric series enriched with alkaline elements to low-alkaline tholeiites. Basite-ultrabasite intrusions, including nickeliferous and andesitic extrusions, are considered to be differentiates of the same basalt lava, the only exception being that this lava intruded the rocks at the final stages of magmatic activity, i.e. during the inversion stage of the Pechenga structure formation.