EQUILIBRIUM RELATIONS OF PROGRADE METAMORPHIC MINERAL ASSEMBLAGES - STABLE ISOTOPE STUDY OF ROCKS OF THE DAMARA OROGEN, FROM NAMIBIA

δ18O of quartz, biotite, muscovite, garnet, ilmenite or magnetite, K-feldspar, and δD of biotite from prograde metamorphic pelites of the Damara Orogen have been analyzed. The samples were taken from one stratigraphic unit which is exposed in the various stages from low-grade up to high-grade metamorphism with anatexis. Using the fractionation curves experimentally investigated up to now, it can be shown that equilibrium has been reached among the metamorphic assemblages in the low-grade and lower medium-grade metamorphism only. In the high-grade rocks only a partial equilibrium exists between those phases formed at the specific high-grade isogrades, i.e., K-feldspar, garnet, magnetite, or ilmenite, but not between these and the main reacting phase biotite. Biotite in equilibrium with these minerals is found only in anatectic rocks where new biotite crystallized from the melt. From this it is concluded that the18O/16O ratio of biotite is fixed at the time of crystallization and its initial composition is preserved when the temperature increases, unless the biotite recrystallizes. The isotopic temperatures derived from mineral pairs formed at specific isograds are in excellent agreement with temperatures derived from petrological investigations. The hydrogen isotoperatio of biotites decreases with increasing grade of metamorphism. © 1979 Springer-Verlag.