Consequences of Channelized and Diffuse Melt Transport in Supra-subduction Zone Mantle: Evidence from the Voykar Ophiolite (Polar Urals)

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Abstract

The well-preserved, 6 km thick mantle section of the Voykar ophiolite in the Polar Urals contains numerous dunite bodies as well as dunite and pyroxenite veins within the host harzburgites. These rocks provide evidence of a composite asthenosphere–lithosphere history of partial melting, plastic deformation, multi-stage melt migration and melt–rock interaction. We investigated the petrology and geochemistry of multiple samples of the different mantle lithologies to define the sequence of mantle melting and melt migration events, as well as the composition of the percolating melts. Spinel harzburgites sampled far from dunite bodies and pyroxenite veins have fairly homogeneous bulk-rock, olivine and Cr-spinel compositions and are interpreted as residues after 14–16% of partial melting, most probably at a mid-ocean ridge. Near the contacts with the dunite bodies and pyroxenite veins, spinel peridotites demonstrate distinct compositional changes marking different stages of melt migration in a supra-subduction environment. At the earliest stage, which probably took place in the lithosphere–asthenosphere boundary of the forearc mantle at temperature between 1050 and 1200°C and a pressure of 1–1·7 GPa, the dunite bodies formed as a result of stress-driven focused melt flow. The latest stage melts moved in cracks under a conductive cooling regime within the lithospheric mantle section when it was horizontally displaced towards the trench. The trace element composition of the melts that migrated through the mantle section during dunite formation have geochemical characteristics like those of high-Ca boninites. The role of the slab-derived component progressively increased through time and late-stage, pyroxenite-forming melts were conspicuously rich in SiO2 and H2O. These low-viscosity melts impregnated the surrounding harzburgites, modifying or obliterating their primary composition.

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