NH4-bearing micas in poly-metamorphic Alpujárride micaschists and gneisses from the central zone of the Betic Cordillera (Spain): tectono-metamorphic and crystal-chemical constraints

The content and distribution of NH 4 -bearing micas in micaschists and paragneisses of the Alpujárride complex (Betic Cordillera, Spain) are interpreted on the basis of textures, metamorphic history and crystal-chemical constraints. NH 4 is present in important amounts in early micas, recording their older diagenetic-to-metamorphic history. NH 4 was inherited by micas formed in successive metamorphic events. At similar metamorphic grade, annite is enriched in NH 4 relative to muscovite with a fractionation of 4:1. Maximum NH 4 contents were estimated in golden annite from medium-grade schists and in retrogressive annite-chlorite mixed-layers (~1.0 wt.%). A Ti-NH 4 avoidance gives rise to associated chemical changes, such as the enrichment in Mg and in VI Al. These data suggest that NH 4 can be a decisive factor in chemical equilibrium and element partitioning between coexisting phases, thus affecting to the commonly used thermobarometers. Estimation of the P-T conditions from texturally different muscovite-annite pairs reveals the superposition of several stages of mica growth, under different geothermal gradients.