The effects of ferromagnesian components on the paragonite‐muscovite solvus: a semiquantitative analysis based on chemical data for natural paragonite‐muscovite pairs

Abstract

ABSTRACT Chemical data for 139 natural paragonite‐muscovite (Pg‐Ms) pairs illustrate the effects of ferromagnesian components on the P‐T‐X topology of the Pg‐Ms solvus. The pairs were selected on the basis of: reasonably accurate knowledge of the P‐T conditions 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.