MINERAL CHEMISTRY OF ULTRAMAFIC MASSIFS IN THE SOUTHERN URALIDES OROGENIC BELT (RUSSIA) AND THE PETROGENESIS OF THE LOWER PALAEOZOIC OPHIOLITES OF THE URALIAN OCEAN

Abstract

Ophiolites of the southern Uralides arc-continent collisional orogen include fertile mantle lherzolites and minor harzburgites in the Nurali and Mindyak massifs located along the Main Uralian Fault suture of the East European craton margin and the Magnitogorsk island arc. We present the first in situ analyses of pyroxerte from Nurali spinel +/- plagioclase-bearing lherzolites and harzburgites and Mindyak spinel lherzolites and harzburgites. Based on the trace element signatures of pyroxene, the Nurali peridotites are divided into: Group 1, consisting of plagioclase-bearing spinel lherzolites with moderately to extremely light rare earth element (LREE)-depleted clinopyroxenes, consistent with <= 8% fractional melting followed by impregnation by incremental to mid-ocean ridge basalt (MORB)-like melts; Group 2, formed by a spinel peridotite with strongly LREE- to middle REE (MREE)-depleted to enriched clinopyroxenes that testify to re-equilibration with large volumes of melt of tholeiitic affinity; Group 3, consisting of amphibole-bearing spinel harzburgites that underwent pervasive percolation of alkali-enriched melts or fluids. Clinopyroxenes from the Mindyak peridotites are strongly depleted and re-equilibrated by reactive porous flow of infiltrating tholefific melts. Two alternative petrogenetic models are proposed. In Model I the peridotites derive from oceanic lithosphere generated by mid-ocean ridge processes and affected by refertilization via melt percolation. In Model 2 the peridotites were subcontinental lithospheric mantle that experienced deep-seated magmatism and sub-solidus re-equilibration prior to the opening of the Uralian Ocean, and interacted with melts generated in the asthenospheric mantle by extension-related decompression partial melting during the opening of the Uralian Ocean. In both models the final pre-orogenic events are related to the subduction of the Uralian oceanic fithosphere and to mantle wedge processes, notably intrusion of gabbro-diorite at c. 400 Ma into the Moho sections.