OCCURRENCE AND ORIGIN OF ANDALUSITE IN PERALUMINOUS FELSIC IGNEOUS ROCKS

Andalusite occurs as an accessory mineral in some peraluminous felsic igneous rocks, including rhyolite, granite, aplite, pegmatite, and migmatitic leucosomes. Texturally, andalusite can range from microns to metres in grain size, from euhedral to anhedral in shape, and from equilibrium to reaction relationships with coexisting minerals. Chemically, andalusite shows little major element variation, except in iron content (Fe₂O₃=0.0-1.9 wt%), but coexisting biotites have high siderophyllite-eastonite contents, coexisting muscovites have FeO=0.6-4.0 wt% and TiO₂=0.0-1.7 wt%, and coexisting apatites have F=3.5±0.4 wt%. Coexisting muscovite-biotite pairs have a wide range of fluorine contents, and F_biot=1.612*F_musc+0.015. If sillimanite also coexists, X^Sil_Al2SiO5/X^And_Al2SiO5=0.997-1.001. Combined textural and chemical criteria permit the definition of three types of andalusite in felsic igneous rocks [including the number of our samples in each category]: Type 1 Metamorphic - (a) prograde metamorphic (thermally metamorphosed peraluminous granites)[2], (b) xenocrystic (anhedral, many inclusions, spatial proximity to country rocks or xenoliths)[7], and (c) restitic (anhedral with inclusions of high grade metamorphic minerals)[3]; Type 2 Magmatic - (a) peritectic (water-undersaturated, T up, subhedral to euhedral, associated with leucosomes in migmatites)[4], (b) peritectic (water-undersaturated, T down, subhedral to anhedral, as reaction rims on garnet or cordierite)[5], (c) cotectic (water-undersaturated, T down, euhedral, grain size compatibility with host rock, few inclusions)[50], and (d) pegmatitic (water-saturated, T down, large subhedral to euhedral grains)[15]; and Type 3 Metasomatic - (water-saturated, magma-absent) [0]. The substantial number of Type 2 andalusites requires overlapping stability fields of andalusite and granite melt. Principal controls on the formation of these primary magmatic andalusites may include one, or more, of the following: high A/CNK, high Be-B-Li, high Fe-Mn, high F, water oversaturation with consequent alkali stripping, structural state of Al2SiO5, and type of growth (stable or metastable). All andalusite is susceptible to supra- or sub-solidus reaction with aqueous fluids to produce muscovite.