PHYSICOCHEMICAL CONDITIONS OF THE DEVELOP MENT OF THE TERTIARY TRACHYBASALT-PHONOLITE FORMATION IN THE TALYSH ZONE (AZERBAIJAN)

It has been established by mineral thermobarogeochemistry that the mag mas initial of the trachybasalt-phonolite series in the Talysh zone we re crystallized according to Bowen's scheme at the following temperatu res and order of phenocryst formation: O1 (>1350 degrees C) --> Cpx (1 280-1170 degrees C) --> Ap (1240-1030 degrees C) --> P1 (1190-1035 deg rees C). The fluid phase at the time of olivine phenocryst crystalliza tion was composed of 90 mol. % CO2 and 10 mol. % N-2, while at the sta ge of pyroxene and plagioclase formation it was 100 mol. % N-2. A dras tic change in fluid composition is related to a discontinuity in time and place of phenocryst formation: olivine crystallization in deep-sea ted conditions, and formation of pyroxene and plagioclase in shallow d epth of the Earth's crust. A microprobe study of silicate melt inclusi ons has shown that the evolution of the initial magma proceeded by mea ns of differentiation and fractionation of minerals. In the process of crystallization the derivative melts were enriched in SiO2 (up to 64% ), Al2O3 (up to 21%), alkalies (up to 10-11 wt.%) and were depleted in femic components (totalled to few per cent). Residual rhyolite-dacite alkali-enriched melts (67-73% SiO2, 14-17% Al2O3, 4-8% alkalies) appe ared in the final stages of magma evolution. The presence of alkalies brings us back to the problem of the mantle, <<basalt>>, origin of som e siliceous rocks, as well as of the possibility to overcome an <<impa ssable>> barrier between quartz-normative acid melts and alkaline leuc ite-bearing rocks.