Subalkali and Li-F granites of the central Cis-Sayan region: Rb-Sr age andSr isotopic composition

Results of Rb-Sr isotopic study of subalkali and alkali granitic rocks from the central Cis-Sayan region are reported. The isochron for the rocks of t he Bugul'ma, Sopkol, Pravaya Kamenka, and Tarta plutons gave an age of 427 +/- 7 Ma and initial Sr isotope ratio of 0.70476 +/- 14. Regardless of the tectonic setting of the granitic rocks, they have a low initial isotope rat io indicative of the origin by melting of crustal rocks under the effect of mantle-derived melts. The Li-F granite of the Khoroi pluton and dikes cutt ing the Bugul'ma pluton have the Rb-Sr isochron age of 403 +/- 5 Ma and the initial Sr isotope ratio of 0.7102 +/- 4 Ma. A difference in Sr isotopic c omposition, geochemistry, and age between the Khoroi and Bugul'ma granitic rocks prevent us from their affiliation to a genetically related associatio n of one intrusive complex.