The structure of the layered complex of the Voikar ophiolite association (Polar Urals) as an indicator of mantle processes beneath a back-arc sea

The vertical cross section and evolution of the major mineral phases were s tudied in the Layered complex of the Late Devonian Voikar ophiolite associa tion. The complex consists of two megarhythms separated by a transitional z one. The megarhythms are similar in structure and are characterized by a gr adation from ultrabasic rocks at the bottom to gabbroids at the top. The Up per megarhythm differs from the Lower one owing to the occurrence of orthop yroxene in the gabbroids. The Transitional zone between the megarhythms has the reverse sequence (grading upsection from basic to ultrabasic rocks). B ased on this study and previous isotopic data [1, 2], we concluded that the parental magmas of the Lower megarhythm were derived from a slightly deple ted mantle source, similarly to rocks of the ultrabasic complex of this ass ociation. The magmas of the Upper megarhythm were derived from a more deple ted mantle and were contributed by an ancient material from a subducted pla te or mixed with subduction-related magmas. We suggest that the Layered com plex represents a well-preserved fragment of the lower crust in the back-ar c setting. This crustal domain originated from intrusions of newly formed m agmas along the crust-mantle boundary. The formation of the lower crust was accompanied by the spreading of the mantle plume head, which resulted in t he shearing of mantle peridotites and the overlying newly formed hot cumula tes. In terms of geodynamics, the first stage of the formation of the Voika r ophiolites was related to the opening of a back-arc basin, while the seco nd stage reflected the initiation and evolution of a subduction zone.