Distribution of Al between coexisting micas in metamorphic rocks from the Central Alps

In 61 pairs of coexisting biotites and muscovites from the Central Alps total Al scatters considerably, but in both series a gradual increase is noticed with increasing metamorphic grade. The ratio Al _Mu ^tot /Al _Bi ^tot remains virtually constant (1.61 average for greenschist facies, 1.57 for amphibolite facies). Tetrahedral Al varies little in biotites and increases in muscovites-phengites with rising metamorphic grade; accordingly the ratio Al _Mu ^IV /Al _Bi ^IV increases slightly with grade. Far the best control of metamorphism is evidenced by octahedral Al. In the muscovite series, and still more pronounced in the biotite series, Al^VI increases with increasing metamorphic grade. Consequently 1 $$K_D = \frac{{Al_{Mu}^{VI} }}{{Al_{Bl}^{VI} }}$$ decreases from 14 to 3. A map (Fig. 6) representing the regional distribution of the K_D values locates a 100 km long and 23 km broad central zone with low K_D. The outline of this central core almost coincides with the isograds anorthite-diopside-calcite and labradorite-pyroxene-hornblende of the Tertiary regional metamorphism; with some deviations this core also agrees with the zone in which phenomena of partial anatexis are observed. The K_D values of micas from anateotic pegmatites agree with those of associated gneisses and schists. The study demonstrates that in the course of progressive regional metamorphism equilibrium has been approached to an unexpected extent and that the two micas coexisted in a strict sense.