A peculiar Ms-Pg textural association in a chloritoid-bearing micaschist recording a multistage P-T path

A peculiar textural association of muscovite, paragonite and chloritoid occ urs in a micaschist bearing muscovite and garnet porphyroblasts, from the G reiner Unit (Eastern Alps, Italy). Three different tectonometamorphic stage s can be recognised in this rock: 1) the early development of S-1 preserved only as relic within garnet porphyroblasts; 2) a syn-kinematic crystallisa tion of chloritoid and muscovite along an early foliation S-2; 3) the devel opment of a crenulation cleavage (S-3) associated with the growth of chlori te, muscovite and paragonite along S-3 and porphyroblastic garnet and musco vite. Both garnet and muscovite porphyroblasts show inclusion trails of chl oritoid, quartz and ores, which are parallel to S-2. Furthermore, the musco vite porphyroblasts include paragonite crystals which are parallel to S-2 a nd display a rectangular to elongate "blocky shape" in thin section, The el ongation is nearly perpendicular to the muscovite (001) basal plane (but wi th the same cleavage traces as those of the muscovite), An HRTEM/SAED/AEM study of the muscovite/paragonite single crystals reveals two different situations: 1) sharp boundaries for the "blocky shaped" para gonite crystal within muscovite porphyroblasts with the angle of the (001) basal plane across the boundary being that required for adjusting the diffe rent basal spacing; 2) the presence of K-rich paragonite micro-domains with in Na-rich muscovite porphyroblasts; both are 2M(1) polytypes and in struct ural continuity, with an angle between [00l]*paragonite and [00l]*muscovite that varies from 0 to 5 degrees in order to accommodate the layer-thicknes s misfit. This results in a "tweed texture", suggesting a further decomposi tion inside the muscovite-paragonite solvus. This peculiar muscovite-paragonite textural association is a microstructura l relic of a multistage metamorphism which affected these rocks with eviden ce of an isothermal increase of pressure due to the reaction: Fe-rich Cld Na-rich Ms + Qtz --> Phe-rich Ms + Pg + Chl +/- Mg-rich Cld + H2O.