The Metamorphism of Fe-rich Pelites from Connemara, Ireland

Connemara pelites show progressive metamorphism from staurolite to upper sillimanite zones and possess low Mg/(Fe + Mg) values, typically 0.30 to 0.35 from about 100 analyses. As a consequence of their composition, many sillimanite zone pelites lack both muscovite and K-feldspar. Staurolite, garnet, biotite, muscovite, feldspars and iron ores have been microprobe analysed in 48 samples. Assemblages, textures and mineral compositions indicate that metamorphism followed a sequence of continuous and discontinuous reactions with systematic variations in mineral Mg/(Mg + Fe) as predicted by theory. Contrary to some common assumptions, most reaction takes place along divariant equilibria; univariant reactions are seldom reached because reactants such as chlorite or muscovite are first consumed along divariant curves. Pelite petrogenetic grids showing univariant curves can only indicate limits to natural assemblages; they typically do not show which reactions have actually taken place. Physical conditions of metamorphism have been calculated by a variety of means; temperatures range from 550° for the staurolite zone to 650° for the upper silimanite zone, with the first appearance of sillimanite near 580°. An early kyanite-staurolite metamorphism at pressures above about 5 kb was followed by a steepening of the thermal gradient leading to regional cordierite and andalusite. This was probably accompanied by uplift with pressures of around 4 kb for roeks near the sillimanite-in isograd.