Ev. Sharkov et al., The structure of the layered complex of the Voikar ophiolite association (Polar Urals) as an indicator of mantle processes beneath a back-arc sea, GEOCHEM INT, 39(9), 2001, pp. 831-847
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.