The effects of ferromagnesian components on the paragonite‐muscovite solvus: a semiquantitative analysis based on chemical data for natural paragonite‐muscovite pairs
- 1 November 1994
- journal article
- Published by Wiley in Journal of Metamorphic Geology
- Vol. 12 (6) , 779-788
- https://doi.org/10.1111/j.1525-1314.1994.tb00059.x
Abstract
Chemical data for 139 natural paragonite‐muscovite (Pg‐Ms) pairs illustrate the effects of ferromagnesian components on theP‐T‐Xtopology of the Pg‐Ms solvus. The pairs were selected on the basis of: reasonably accurate knowledge of theP‐Tconditions of formation; evidence for close approach to equilibrium at peak metamorphic conditions; exclusion of pairs in which paragonite contains more than 5 mol% margarite; and exclusion of pairs from polymetamorphic rocks that contain more than one set of cogenetic Pg‐Ms pairs. Graphical analysis reveals considerable scatter in the data; nevertheless, it is evident that the muscovite limb of the solvus shifts markedly toward end‐member muscovite with increasing pressure from approximately 7 kbar to 21 kbar. This shift is attributed to a pressure‐induced increase of the ferromagnesian content of muscovite, which increases the size of the XII alkali site ‐ to the effect that K is more readily accommodated than Na. The data also suggest that the paragonite limb of the solvus migrates slightly toward end‐member paragonite with increasing pressure. Broadening of the Pg‐Ms solvus with increasing pressure reflects increasingly nonideal Na‐K mixing as the phengite content of muscovite increases. Due to the wide scatter of data for Pg‐phengitic‐Ms pairs, it is concluded that, at the present time, Pg‐Ms solvus thermometry is only viable for quasibinary Pg‐Ms pairs.Keywords
This publication has 39 references indexed in Scilit:
- Buffering in the assemblage staurolite-aluminium silicate-biotite-garnet-chloriteJournal of Metamorphic Geology, 1991
- High-pressure metamorphism in gneisses and pelitic schists in the East Rhodope zone (N. Greece)Tschermaks Mineralogische und Petrographische Mitteilungen, 1989
- Fluid heterogeneities and hornblende stability in interlayered graphitic and nongraphitic schists (Tauern Window, Eastern Alps)Contributions to Mineralogy and Petrology, 1987
- Margarite-paragonite-muscovite assemblages from the low-grade metapelites of the Tono metamorphic aureole, Kitakami Mountains, Northeast Japan.The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists, 1985
- Garnet and associated minerals in the southern margin of the Menderes Massif, southwest TurkeyGeological Magazine, 1984
- Mineral chemistry of regional chloritoid assemblages in the Chlorite Zone, Lycian Nappes, South-west TurkeyMineralogical Magazine, 1984
- Petrology of pelitic schists in the oligoclase‐biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high‐pressure intermediate type of metamorphic beltJournal of Metamorphic Geology, 1983
- High Water Activities in the Generation of High Pressure Kyanite Eclogites of the Tauern Window, AustriaThe Journal of Geology, 1979
- On the ?late? formation of paragonite and its breakdown in pelitic rocks of the southern Damara Orogen (Namibia)Contributions to Mineralogy and Petrology, 1978
- Chemismus und phasenpetrologische Untersuchungen der Gesteine aus der Eklogitzone des Tauernfensters, sterreichTschermaks Mineralogische und Petrographische Mitteilungen, 1977