Amphibolites with staurolite and other aluminous minerals: calculated mineral equilibria in NCFMASH

Amphibolite facies mafic rocks that consist mainly of hornblende, plagiocla se and quartz may also contain combinations of chlorite, garnet, epidote, a nd, more unusually, staurolite, kyanite, sillimanite, cordierite and orthoa mphiboles. Such assemblages can provide tighter constraints on the pressure and temperature evolution of metamorphic terranes than is usually possible from metabasites. Because of the high variance of most of the assemblages, the phase relationships in amphibolites depend on rock composition, in add ition to pressure, temperature and fluid composition. The mineral equilibri a in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH) model system demonstrate that aluminium content is critical in controlling the occurrence of assemb lages involving hornblende with aluminous minerals such as sillimanite, kya nite, staurolite and cordierite. Except in aluminous compositions, these as semblages are restricted to higher pressures. The iron to magnesium ratio ( X-Fe), and to a lesser extent, sodium to calcium ratio, have important role s in determining which (if any) of the aluminous minerals occur under parti cular pressure-temperature conditions. Where aluminous minerals occur in am phibolites, the P-T-X dependence of their phase relationships is remarkably similar to that in metapelitic rocks. The mineral assemblages of Fe-rich a mphibolites are typically dominated by garnet- and staurolite-bearing assem blages, whereas their more Mg-rich counterparts contain chlorite and cordie rite. Assemblages involving staurolite-hornblende can occur over a wide ran ge of pressures (4-10 kbar) at temperatures of 560-650 degrees C; however, except in the more aluminous, iron-rich compositions, they occupy a narrow pressure-temperature window. Thus, although their occurrence in 'typical' a mphibolites may be indicative of relatively high pressure metamorphism, in more aluminous compositions their interpretation is less straightforward.