Crystallization conditions of dunites in the Konder platiniferous alkaline-ultramafic massif of the southeastern Aldan Shield

Abstract

The investigation of melt inclusions in Cr spinels yielded direct information on the physicochemical parameters of the magmatic processes responsible for the formation of the Konder platiniferous alkaline-ultrabasic massif (southeastern Aldan Shield). The comparative analysis of the composition of the chromites containing the inclusions revealed that the Cr spinels from the Konder dunites differ significantly from their counterparts in the ultrabasic complexes of ophiolites and the modern oceanic crust. In terms of their composition and REE distribution, the clinopyroxene microcrystals from the Konder chromites are significantly different from the pyroxenes in the basic-ultrabasic ophiolite complexes of associations and identical to the minerals from the Kytlym platiniferous massif (the Urals). With respect to the distribution of the major components, the high-magnesian inclusions are identical to alkaline biotite-pyroxene picrites, testifying to the active participation of ultrabasic (picritic) alkaline magmatic systems in the formation of the dunites in the Konder Massif. The results of the ion probe investigation of the inclusions indicate a high water content (up to 0.54 wt %) in the melts. The data on the distribution of the rare and rare-earth elements in the inclusions suggest that the dunites of the Konder Massif crystallized with the involvement of water-saturated magmas at minimal temperatures of about 1230°C. Such temperatures are consistent with the earlier estimates for the melt inclusions in the olivine of the Konder Massif.