THE MINDYAK PALAEOZOIC LHERZOLITE OPHIOLITE, SOUTHERN URALS: GEOCHEMISTRY AND GEOCHRONOLOGY
The Mindyak ophiolite, Southern Urals, preserves metamorphic garnet- and amphibole-bearing blocks, along with a diverse assemblage of ocean crust-derived clasts, in a serpentinite tectonic breccia in contact with its spinel-plagioclase/spinel lherzolite mantle and wehrlite/clinopyroxenite transition zone sequences. On the basis of petrological and geochemical data we suggest that the mantle section preserves variably depleted lherzolites generated by a two-stage process involving melting followed by magma-mantle interaction in a supra-subduction zone setting. The garnetiferous rocks are interpreted as high-temperature metamorphosed, rodingitized ocean crust gabbros that subsequently underwent later stage low-temperature retrogression. Previously determined radiometric U-Pb and Pb-Pb dates of 410–415 Ma, at the Silurian-Devonian boundary, for the peak metamorphic event affecting the metagabbros are confirmed with new Sm-Nd radiometric data. The complex, multistage evolution of the garnet metagabbros and their preservation in the Mindyak lherzolitic ophiolite massif give insights into processes from plate divergence and ocean formation, to plate convergence and, ultimately, to preservation in the Uralian orogeny.