EXSOLUTION MICROSTRUCTURES IN NH4-BEARING MUSCOVITE AND ANNITE IN GNEISSES FROM THE TORROX AREA, BETIC CORDILLERA, SPAIN

Abstract

Variably sized lamellae of annite within muscovite crystals and lamellae of muscovite within annite crystals from gneissic rocks of the Torrox area in the Betic Cordillera, Spain, are interpreted as formed by exsolution of NH 4 -bearing micas present in the earliest metamorphic assemblage. The NH 4 -rich white mica, identified only in the overlying schists, was formed during the first diagenetic-to-metamorphic episode, with N derived from the maturation of organic matter. Initially contained in white mica, the NH 4 was redistributed between white mica and annite, probably at conditions of increasing metamorphism. In the current assemblage of gneisses, NH 4 is preferentially concentrated in annite, especially in the composite Ms+Ann grains. This assemblage, which consists of muscovite in homogeneous grains + muscovite in exsolved grains + annite in homogeneous grains + annite in exsolved grains, was probably derived from the lower-temperature four-mica assemblage: muscovite + NH 4 -rich white mica + annite + NH 4 -bearing annite. Exsolution as temperature increased explains the presence of NH 4 -rich micas in the overlying schists and the presence of exsolved grains in gneisses. The exsolution process occurred at ~450°C and ~3 kbar, physical conditions characterizing the first metamorphic episode affecting this formation.