Ma. Gornova et Zi. Petrova, Mantle peridotites of granulate-gneiss complex as fragments of Archean (?)ophiolites in the Biakal region (Russia), OFIOLITI, 24(2), 1999, pp. 223-238
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.