METAMORPHISM AND METASOMATISM OF RESTITE ULTRAMAFIC ROCKS OF THE OLKHON TERRANE
Sklyarov E.V.1, Lavrenchuk A.V.2,3, Pushkarev E.V.4, Scherbakov Yu.D.5
1Institute of the Earth crust SB RAS, Irkutsk
2Institute of Geology and Minaralogy SB RAS, Novosibirsk
3Novosibirsk State University, Novosibirsk
4Institute of Geology and Geochemistry UB RAS, Ekaterinburg
5Institute of Geochemistry SB RAS, Irkutsk
Keywords: dunite, harzburgite, Al-rich ultramafics, clinopyroxene-anorthite rocks, sapphirine, corundum, Olkhon terrane, West Baikal Area.
Abstract
Small bodies of restitic ultramafic rocks composed of dunite and harzburgite and enclosed to gneisses of the Olkhon composite terrane (West Baikal Area) are described. P-T parameters of metamorphism of the ultramafic rocks correspond to metamorphic conditions of the embedding gneisses and amphibolite. Peculiar Al-rich ultramafic rocks, composed of forsterite, enstatite and Cr-free spinel are found in restite dunite and harzburgite. They contain high concentration of Al2O3 (up to 23 wt.%) and MgO (25-37 wt.%), low silica (30-42 wt.% SiO2) and very low CaO (less than 2 wt.%). We regard these rocks as products of syn-metamorphic high-temperature metasomatism (Max T=730-790°С) of the dunite and harzburgite. Comparison of the restitic and high-Al ultramafic rocks shows that in metasomatic process took part elements usually regarded low mobile - Al, Ti, V, Zr, REE, (input) и Mg, Si, Cr, Ni (output). There are no possible magmatic source of metasomatic fluids. We suppose that metamorphic felsic gneiss were the source of fluids in high-temperature metamorphic conditions. The impact of metasomatizing fluids on small fragments of aluminosilicate rocks in ultramafic rocks led to an even more significant enrichment of them with alumina (up to 50 wt.% Al2O3) and occurrence of specific mineral associations containing corundum and sapphirine.
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