COMPOSITION OF PLUTONIC MUSCOVITE: GENETIC IMPLICATIONS

Muscovite (white mica) is the most common mineralogical indicator of strongly peraluminous composition in plutonic rocks and, by inference, in their parental magmas. Its presence has been used to constrain depth of crystallization; based upon experimental data, approximately 3 kbar (11 km) is commonly considered the minimum pressure at which primary igneous muscovite can crystallize. Recent suggestions that independent criteria require depths < 11 km for emplacement of some granites with texturally primary-looking muscovite, so that such mica would in fact be secondary, raise questions about the use of apparently primary muscovite as an indication either of depth or of magma composition. New data from 41 samples representing 16 plutons in North America and Europe are relevant to the paragenesis of muscovite in igneous rocks. Formulas of the analyzed micas are typically about K0.91Na0.07Fe0.203+Fe0.052+Mg0.10Ti0.03 Al2.55Si3.10O10(OH1.93F0.07), very slight trioctahedral substitution (2.00 to 2.04 octahedral cations). Primary- and secondary-looking grains are generally similar, but primary ones are richer in Ti, Na and Al and poorer in Mg and Si. Plutonic muscovite is so far from ideal KAl3Si3O10[OH](2) in composition that it is difficult to evaluate its paragenesis in terms of existing experimental data. The many additional components may enhance the stability field sufficiently to explain occurrences of primary muscovite at surprisingly shallow depths.