MANTLE PERIDOTITES OF GRANULITE - GNEISS COMPLEX AS FRAGMENTS OF ARCHEAN (?) OPHIOLITES IN THE BAIKAL REGION (RUSSIA)

Abstract

The Sharyzhalgay block, located in the south-western Baikal region, is an exposure of the Precambrian basement at the margin of the Siberian Platform. It chiefly consists of granitoids (80%) that host relic granulite facies rocks (20%). Metamorphism under granulite facies conditions took place about 2.4-2.5 Ga ago. Petrological and geochemical data on the granulites suggest protoliths of predominantly MORB-like basalt (~70%) and minor sedimentary rocks. Granulites also include ultramafic rocks. Relics of protogranular texture, high XMg in rocks and rock-forming minerals, occurrence of depleted harzburgites, mantle compositional trend of spinels, REE patterns, and thermobarometry data allow us to interpret the provenance of Saramta peridotites as mantle material depleted by partial melting in the stability field of spinel. Peridotites are cut by dikes of layered wehrlite, spinel websterite and garnet websterite. Spinel websterites have a chemistry and mineralogy of Cr-Di series. Their origin inferred to be the result of mineral segregations from percolating melt within the upper mantle. Garnet websterites differ from Al-augite pyroxenites by lower contents of TiO2 and Al2O3, and higher SiO2. Incongruent melting of Cr-Di spinel websterites on the scheme Cpx+ Opx+ Sp ® Mg-richer Ol+ Cr-richer Sp+L produced melts and restite wehrlites. Later on the melt crystallized in situ into garnet websterites with magmatic textures. Regional granulite-facies metamorphism at temperatures of 700-800°C and a pressure of 5-6 kb produced exsolution lamellae of spinel in pyroxene, kelyphite Sp+Opx+Pl rims on garnet, and zoning of orthopyroxene in garnet websterite. Tectonitic restite harzburgites with ultramafic dikes are correlative with the mantle parts of ophiolite sections. All these facts allow us to interpret relic granulite-facies rocks as fragments of Precambrian ophiolites.