PETROLOGICAL FEATURES OF THE MAKSYUTOV AND THE ATBASHI ECLOGITE-GLAUCOPHANE-SCHIST COMPLEXES WITHIN THE URAL-MONGOLIA FOLD BELTS

According to Miyashiro's concept of paired metamorphic belts, the Maksyutov and the Atbashi eclogite-glaucophane-schist complexes (Russia) belong to the united Ural-Mongolia Fold Belt, but are related to its different branches – outside and inside, respectively. Hence, our main task was to study the metamorphic evolution of these regions in comparison on the basis of petrological, experimental, and theoretical data on mineral equilibria. Preliminary geochemical data obtained for eclogite samples from the Maksyutov complex indicate that the eclogite protolith most likely corresponds to the island arc basalt rather than MORB. The prominent negative Eu anomaly in these rocks suggests that plagioclase was a subliquidus phase during the basalt formation, which contradicts the hypothesis of the magmatic origin of the eclogitic garnet. On the other hand, the REE spectra of rock samples do not provide an answer to the question if the complexes were formed under identical geological and tectonic conditions. In this study, we attempted to use the REE distribution in rock-forming minerals to solve this problem. We used the analytical data on the rock-forming mineral compositions and geothermobarometry database to restore the P-T history of the eclogite-glaucophane-schist complexes. The “clockwise” P-T path with a low geothermal gradient (about 10 deg. per km), typical for collision zones, was determined for the first metamorphic stage of the Atbashi complex. The extensive studies of both Maksyutov and Atbashi clinopyroxenes by X-ray powder diffraction and Moessbauer spectroscopy revealed several structural modifications in the Jd-Aeg-Di solid solution, which have different elastic properties. More detailed information was obtained on the local structure of these modifications. The studied omphacites have the perfect structure, characterized by local cation ordering in M1 and M2 sites (apparently, the sp. gr. P2/n), which could be saved under changing metamorphic conditions. These studies resulted in corrections in clinopyroxene thermodynamic properties used in thermobarometry. As a result, allochemical metamorphic conditions were determined for both eclogite-glaucophane-schist complexes, but magmatic origin is suggested for the Maksyutov complex only. This study was supported by the Russian Foundation for Basic Research, project nos. 05-05-64561 and 05-07-90318.