Mantle peridotites of granulate-gneiss complex as fragments of Archean (?)ophiolites in the Biakal region (Russia)

The Sharyzhalgay block, located in the south-western Baikal region, is an e xposure of the Precambrian basement at the margin of the Siberian Platform. It chiefly consists of granitoids (80%) that host relic granulite facies r ocks (20%). Metamorphism under granulite facies conditions took place about 2.4-2.5 Ga ago. Petrological acid geochemical data on the granulites sugge st protoliths of predominantly MORB-like basalt (similar to 70%) and minor sedimentary rocks. Granulites also Include ultramafic rocks. Relics of prot ogranular texture, high X-Mg in rocks and rock-forming minerals, occurrence of depleted harzburgites, mantle compositional trend of spinels, REE patte rns, and thermobarometry data allow us to interpret the provenance of Saram ta peridorites as mantle material depleted by partial melting in the stabil ity field of spinel. Peridotites are cut by dikes of layered wehrlite, spin el websterite and garnet websterite. Spinel webjterites have a chemistry an d mineralogy of Cr-Di series. Their origin inferred to be the result of min eral segregations from percolating melt within the upper mantle. Garnet web sterites differ from Al-augite pyroxenites by lower contents of TiO2 and Al 2O3, and higher SiO2. Incongruent melting of Cr-Di spinel websterites on th e scheme Cpx+Opx+Sp-->Mg-richer Ol+Cr-richer Sp+L produced melts and restit e wehrlites. Later on the melt crystallized ill situ into garnet websterite s with magmatic textures. Regional granulite-facies metamorphism at tempera tures of 700-800 degrees C and a pressure of 5-6 kb produced exsolution lam ellae of spinel in pyroxene, kelyphite Sp+Opx+Pl rims on garnet, and zoning of orthopyroxene in garnet websterite. Tectonitic restite harzburgites wit h ultramafic dikes are correlative with the mantle parts of ophiolite secti ons. Ail these facts allow us to interpret relic granulite-facies rocks as fragments of Precambrian ophiolites.