PARENTAL MELTS IMPRINTED IN PLUTONIC ROCKS OF THE VOYKAR OPHIOLITE, POLAR URALS: EVIDENCES FROM CLINOPYROXENE GEOCHEMISTRY

Abstract

The plutonic rocks (dunites, wehrlites, clinopyroxenites and gabbronorites) of the Voykar ophiolite are associated with mantle tectonites and a diabase dike complex. Clinopyroxenes of the plutonic rock units were analyzed for trace elements. The abundances of moderately incompatible elements (heavy rare-earth elements and Y) in these clinopyroxenes display two distinct levels (relatively enriched and depleted) with no crystal fractionation trends.enriched clinopyroxenes occur both in ultramafics and in gabbronorite. In the ultramafic plutonic rocks they have Mg# = 89 and high Cr content (>3800 ppm), while in the gabbronorites they have Mg# <77 and low Cr abundance (<1000 ppm). The presumable silicate liquid ( melt-1 ) equilibrated with the enriched clinopyroxenes should be only slightly depleted in incompatible elements relatively to MORB. The melt-1 migration through the hot mantle restite resulted in metasomatic wehrlite and diopside-bearing dunite with enriched clinopyroxenes. In upper horizons of the plutonic section the melt-1 formed gabbronorite bodies.depleted clinopyroxenes occur in wehrlite and clinopyroxenite and have Mg# = 89-85 with intermediate Cr abundances (1300-3600 ppm). They were equilibrated with a melt-2 significantly depleted in incompatible elements relatively to MORB, with marked positive Sr and negative Zr anomalies. The olivine+chromite and olivine+clinopyroxene cumulates are related to this melt. The melt-2 incompatible-element pattern closely matches with that of least evolved Voykar dikes and suggests a supra-subduction type magma.clinopyroxene geochemistry reinforces a supra-subduction nature of the Voykar magmatic complexes. The parental melts imprinted in the plutonic rocks show a geochemical evolution of mantle source from slightly depleted relatively to those of MORB through strongly depleted with high Sr/Zr, i.e. large-ion lithophile / high-field strength elements ratios.specific features of the Voykar magmatism are: (1) discrete and compositionally contrast magma influx into the lower crust of a supra-subduction spreading zone; (2) continuous melt percolation through aggregates of accumulating crystals resulted in limited variations within distinct depleted and enriched HREE abundances in clinopyroxene.