Reconstruction of Lateral Rows of the Late Cambrian and Early Ordovician Active Continental Margin Structures in the Paleozoids of Northern Kazakhstan

Abstract

A comprehensive study of the Upper Cambrian and Lower Ordovician complexes of Northern Kazakhstan was carried out, their ages were substantiated, the structures and compositions of the rocks were investigated. It has been ascertained that the Upper Cambrian formations comprise coarse clastic strata, mafic alkaline effusive rocks and gabbro, while felsic volcanic rocks and granites are represented only by boulders in conglomerates. The Lower Ordovician rocks contain basalt-rhyolite series, felsic alkaline volcanic rocks, and granitoids. The lateral rows of structures of the active continental margin have been reconstructed for the Late Cambrian and Early Ordovician. In the Late Cambrian, the lateral series includes only the structures of the rear extension region, where complexes with the within-plate geochemical characteristics were formed. In the lateral series of the Early Ordovician structures, the frontal volcanic area with the island-arc volcanic rocks and the rear extension area with the intraplate felsic volcanic rocks and granites, were revealed. It is assumed that the differences in the lateral rows of structures may be associated with a change in the tectonic mode of the active continental margin at the Cambrian–Ordovician boundary, when the transform mode with no evidence of suprasubduction magmatism was replaced by convergent magmatism accompanied by the wide distribution of island-arc volcanic rocks.