CORIERITE IN FELSIC IGNEOUS ROCKS - A SYNTHESIS

Cordierite is a characteristic mineral of many peraluminous felsic ign eous rocks. A combination of T-P-X parameters, which overlap the stabi lity conditions for felsic magmas, control its formation. Critical amo ng these parameters are relatively low T, low P, and typically high (M g+Fe2+), Mg/Fe2+, A/CNK, a(Al2O3), and fO2. Spatial and textural infor mation indicate that cordierite may originate in one of three principa l ways in felsic igneous rocks: Type 1 Metamorphic: (a) xenocrystic (g enerally anhedral, many inclusions, spatial proximity to country rocks and pelitic xenoliths); (b) restitic (generally anhedral, high-grade metamorphic inclusions); Type 2 Magmatic: (a,b) peritectic (subhedral to euhedral, associated with leucosomes in migmatites or as reaction r ims on garnet); (c) cotectic (euhedral, grain size compatibility with host rock, few inclusions); (d) pegmatitic (large subhedral to euhedra l grains, associated with aplite-pegmatite contacts or pegmatitic port ion alone); and Type 3 Metasomatic (spatially related to structural di scontinuities in host, replacement of feldspar and/or biotite, intergr owths with quartz). Of these, Type 2a (peritectic) and Type 2c (cotect ic) predominate in granitic and rhyolitic rocks derived from fluid-und ersaturated peraluminous magmas, and Type 2d (pegmatitic) may be the m ost common type in fluid-saturated systems.