Mantle peridotites and pyroxenites of the Saramta Massif of the Precambrian Sharyzhalgai gneiss-granulite complex

The Sharyzhalgai gneiss-granulite complex, located in southwestern Prebaika lia, is an exposure of the Precambrian basement on the margin of the Siberi an Platform. We have studied ultramafics which exist within the complex as concordant lenses with tectonic contacts. The Saramta Massif (the largest s tructure here) is composed mainly of harzburgites and lherzolites cut by di kes of wehrlite, spinel websterite, and garnet websterite. Peridotites have porphyroclastic and mosaic-porphyroclastic textures and are of mantle orig in. Harzburgites and lherzolites have high MgO contents, with MgO/Sigma FeO of about 7 to 4.5. Harzburgites are extremely poor in Al2O3, CaO, TiO2, and Na 2O and rich in Cr, Ni, and Co. Lherzolites are compositionally proximal to model upper-mantle rocks. Rock forming minerals have high X-mg=Mg/(Mg + Fe) (0.91-0.93 in Ol, 0.908-0.931 in Opx, and 0.934 in Cpx). Orthopyroxenes ar e represented by enstatite, and clinopyroxenes by chrome-diopside, Reverse correlation between Cr/(Cr+Al) and Mg/(Mg+Fe) is observed in Cr-Al-spinels. Spinels in harzburgites have Cr/(Cr + Al) and Mg/(Mg + Fe) of 0.45 and 0.5 7, respectively, and are poor in Fe3+. Therefore, the Saramta peridotites c an be regarded as a mantle substratum depleted by partial melting. Spinel websterites belong to "green" Cr-diopsides and are regarded as fract ionated from komatiite melts, Garnet websterites differ from Ti-augite pyroxenites in lower contents of T iO2 and Al2O3 and higher SiO2. Orthopyroxenes are bronzites, and clinopyrox enes are Ti- and Al-low augites. Incongruent melting of spinel websterites by the reaction Cpx + Opx + (Ol + Sp) --> Mg-Ol + L + (Cr-Sp) produced a me lt and residual wehrlites with progressively increasing amount and Mg/(Mg Fe) of olivine and increasing Cr/(Cr + Al) of spinel without change In its amount. Later on, the melt crystallized in: situ, producing garnet webster ites with magmatic textures. Ultramafics formed under the spinel-facies con ditions at a pressure of about 15-20 kbar. Pyroxenes were recrystallized in subsolidus conditions at T similar to 1000 degrees C and constant pressure . Regional granulite-facies metamorphism at T similar to 700-800 degrees C and P similar to 5-6 kbar produced exsolution lamellae of spinels in pyroxe nes, kelyphitlc Sp + Opx + Pl rims on garnets, and zonality in Qpx from gar net websterites. Tectonitic residual peridotites of the Saramta Massif are similar to lower parts of ophiolite sections. They represent paleooceanic u pper mantle and can be regarded as fragments of Precambrian ophiolites.