An EMP and TEM--AEM Study of Margarite, Muscovite and Paragonite in Polymetamorphic Metabauxites of Naxos Cyclades, Greece) and the Implications of Fine-scale Mica Interlayering and Multiple Mica Generations

Coexisting white micas and plagioclase were studied by electronmicroprobe (EMP), and transmission and analytical electron microscopy(TEM—AEM) in greenschist- to amphibolite-grade metabauxitesfrom Naxos. The TEM—AEM studies indicate that sub-micronscale (0.01–1.0 µm thick) semicoherent intergrowthsof margarite, paragonite and muscovite are common up to loweramphibolite conditions. If unrecognized, such small-scale micainterlayering can easily lead to incorrect interpretation ofEMP data. Muscovite and paragonite in M₂ greenschist-grade Naxosrocks are mainly relics of an earlier high-pressure metamorphism(M₁). Owing to the medium-pressure M₂ event, margante occursin middle greenschist-grade metabauxites and gradually is replacedby plagioclase + corundum in amphibolite-grade metabauxites.The margarite displays minor ^IVAl_{3 VI}(Fe³⁺, Al) Si_-3 ^VI-_-1 andconsiderable (Na, K) SiCa_-1Al_-1 substitution, resulting in upto 44 mol% paragonite and 6 mol % muscovite in solution. Thecompositional variation of muscovite is mainly described by^VI(Fe²⁺, Mg) Si ^VI Al_-1^VI Al_-1 and _VI(Fe₃₊Al^-1) exchanges, thelatter becoming dominant at amphibolite grade, Muscovite issignificantly richer in Fe than margarite or paragonite. Ca—Na—Kpartitioning data indicate that margarite commonly has a significantlyhigher Na/(Na+ K+Ca) value than coexisting muscovite or plagioclase.Exceptions are found in several greenschist-grade rocks, inwhich M₁-formed mussovite may have failed to equilibrate withM₂ margarite. The sluggishness of K-rich micas to recrystallizeand adjust composidonally to changing P-T conditions is alsoreflected in the results of mus-covite-paragonite solvus thermometry.Chemical data for Ca—Na micas from this study and literaturedata indicate that naturally coexisting margarite—paragonitepairs display considerably less mutual solubility than suggestedby experimental work. The variable and irregular Na partitioningbetween margarite and muscovite as observed in many metamorphicrocks could largely be related to opposing effects of pressureon Na solubility in margarite and paragonite and/or non-equilibriumbetween micas. KEY WORDS: Ca—Na—K mica; margarite; metabauxite; Naxos; sub-micron-scale mica interlayering