THE STRUCTURE OF THE LAYERED COMPLEX OF THE VOIKAR OPHIOLITE ASSOCIATION (POLAR URALS) AS AN INDICATOR OF MANTLE PROCESSES BENEATH A BACK-ARC SEA

Abstract

The vertical cross section and evolution of the major mineral phases were studied in the Layeredcomplex of the Late Devonian Voikar ophiolite association. The complex consists of two megarhythms sepa-rated by a transitional zone. The megarhythms are similar in structure and are characterized by a gradation fromultrabasic rocks at the bottom to gabbroids at the top. The Upper megarhythm differs from the Lower one owingto the occurrence of orthopyroxene in the gabbroids. The Transitional zone between the megarhythms has thereverse sequence (grading upsection from basic to ultrabasic rocks). Based on this study and previous isotopicdata [1, 2], we concluded that the parental magmas of the Lower megarhythm were derived from a slightlydepleted mantle source, similarly to rocks of the ultrabasic complex of this association. The magmas of theUpper megarhythm were derived from a more depleted mantle and were contributed by an ancient materialfrom a subducted plate or mixed with subduction-related magmas. We suggest that the Layered complex rep-resents a well-preserved fragment of the lower crust in the back-arc setting. This crustal domain originated fromintrusions of newly formed magmas along the crust–mantle boundary. The formation of the lower crust wasaccompanied by the spreading of the mantle plume head, which resulted in the shearing of mantle peridotitesand the overlying newly formed hot cumulates. In terms of geodynamics, the first stage of the formation of theVoikar ophiolites was related to the opening of a back-arc basin, while the second stage reflected the initiationand evolution of a subduction zone.