METAMORPHISM AND METASOMATISM OF RESTITIC ULTRAMAFIC ROCKS OF THE OLKHON TERRANE ( <i>Western Cisbaikalia</i> )

Small bodies of restitic ultramafic rocks composed of dunite and harzburgite and enclosed in gneisses of the Olkhon composite terrane (West Baikal Area) are described. The estimates of the P-T conditions of metamorphism of the ultramafic rocks generally correspond to the metamorphic parameters of the host gneiss and amphibolite. The restitic rocks include unique aluminous ultramafic rocks composed of forsterite, enstatite, and chromium-free spinel. The latter are distinguished from restitic rocks by high contents of Al2O3 (up to 23 wt.%) at “peridotite” concentrations of magnesium (25–37 wt.% MgO) and silicon (30–42 wt.% SiO2). It is assumed that these rocks are the products of high-temperature (Max T = 730–790 °C) metasomatism of dunite and harzburgite. As shown by comparing the compositions of restitic and aluminous ultramafic rocks, the metasomatic process involves many elements, including those considered to be weakly mobile. The input elements are Al, Ti, V, Zr, and rare earth elements; the output elements are Mg, Si, Cr, and Ni. At the same time, there are no possible magmatic sources of metasomatizing fluids. It is assumed that host felsic gneiss served as a source of fluid at high-temperature metamorphism. The impact of metasomatizing fluids on aluminosilicate rocks, occurring as small fragments in restitic ultramafic rocks, strongly enriched them with alumina (up to 50 wt.% Al2O3) and formed specific mineral associations with corundum and sapphirine.