Pyroxenites from mantle section of Voykar Ophiolite (Polar Urals) – pathways for melt and fluid migration

Late stages of formation of rocks from mantle section of Voykar Ophiolite were shown to take place in SSZ environment. Dunite bodies and pyroxenite veins mark different stages of melt migration. Best evidence for SSZ environment is preserved in mineral compositions from pyroxenite veins – high #Cr of spinel and LILE enrichments in Cpx and high-Al amphibole. Pyroxenite compositions display variability which is a function of their modal compositions, morphological features and spatial location. Minerals from orthopyroxenites usually have high mineral #Mg, are most depleted in incompatible elements, show low Al in pyroxenes and spinel (#CrSpl >0.6). Other group of pyroxenites with high mineral #Mg is represented by thin (cm-scale) clinopyroxenites and websterites and complex dunite-pyroxenite veins. Their major element compositions were equilibrated with adjacent harzburgites. Minerals from thicker websterites and clinopyroxenites have lower #Mg and subparallel boninite-like trace element patterns of Cpx. Cpx compositions across harzburgite-pyroxenite contacts show depletion in HREE (opposite to harzburgite-dunite contacts) and Zr and enrichment in LREE and Sr. High-Al amphibole from websterites display trace element patterns with different LREE/HREE and MREE/HREE ratios, which could reflect their formation from melt to fluid-like agent. Therefore, Voykar mantle section pyroxenite veins represent pathways for melts variously depleted in incompatible elements or in some cases fluids with boninite-like trace element patterns and elevated silica contents. Those melts were produced by melting of hybridized mantle source formed as a result of transformation of mantle olivine to orthopyroxene under the influence of slab-derived fluids/melts. Different areas of massif could be arranged in order of increasing HREE contents in Cpx from dominated type of pyroxenite which could reflect relative depth or degree of melt fractionation.