Abstract <p>The new petrochemical data on Lower Cambrian volcanites of the Oldynda Formation of the Uda‒Vitim island-arc system are presented. The high REE content, high degree of spectrum light part differentiation, enrichment in large-ion lithophile elements, and presence of an Nb-minimum in the studied volcanites were analyzed, confirming their island-arc origin. Transverse structural zoning of volcanites in the cross-strike of the Uda‒Vitim island-arc system was determined. For the first time the structural zoning has been traced by the degree of volcano differentiation, as well as by the mineral and chemical rock composition. The depths of the mineral subduction zone have been calculated. It is suggested that this zone is located in the area of the Eravna lakes within the Eravna structure, confirmed by a large gravity stage with a northeastern strike.</p>