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

Abstract

Coexisting white micas and plagioclase were studied by electron microprobe (EMP), and transmission and analytical electron microscopy (TEM-AEM) in greenschist- to amphibolitegrade metabauxites from Naxos. The TEM-AEM studies indicate that sub-micron scale (O·01-1·0 μm thick) semicoherent intergrowths of margarite, paragonite and muscovite are common up to lower amphibolite conditions. If unrecognized, such small-scale mica interlayering can easily lead to incorrect interpretation of EMP data. Muscovite and paragonite in M2 greenschist-grade Naxos rocks are mainly relics of an earlier high-pressure metamorphism (M1). Owing to the medium-pressure M2 event, margarite occurs in middle greenschist-grade metabauxites and gradually is replaced by plagioclase + corundum in amphibolite-grade metabauxites. The margarite displays minor IVAl3VI(Fe3+,Al) Si-3VI□-1 and considerable (N,aK) SiCa-1Al-1 substitution, resulting in up to 44 mol % paragonite and 6 mol % muscovite in solution. The compositional variation of muscovite is mainly described by VI(Fe2+,Mg)SiVIAl-1VIAl-1 and VI(Fe3+Al-1) exchanges, the latter becoming dominant at amphibolite grade. Muscovite is significantly richer in Fe than margarite or paragonite. Ca-Na-K partitioning data indicate that margarite commonly has a significantly higher N/(N + K + Ca) value than coexisting muscovite or plagiodase. Exceptions are found in several greenschist-grade rocks, in which M1-formed muscovite may have failed to equilibrate with M2 margarite. The sluggishness of K-rich micas to recrystallize and adjust compositionally to changing P-T conditions is also reflected in the results of muscovite-paragonite solvus thermometry. Chemical data for Ca-Na micas from this study and literature data indicate that naturally coexisting margarite-paragonite pairs display considerably less mutual solubility than suggested by experimental work. The variable and irregular Na partitioning between margarite and muscovite as observed in many metamorphic rocks could largely be related to opposing effects of pressure on Na solubility in margarite and paragonite and/or non-equilibrium between micas.