Occurrence and Origin of Andalusite in Peraluminous Felsic Igneous Rocks

Abstract

Andalusite occurs as an accessory mineral in many types of per aluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and anatectic migmatites. Some published stability cunes for And = Sil and the water-saturated granite solidus permit a small stability field for andalusite in equilibrium with, felsic melts. We examine 108 samples of andalusite-bearing felsic rocks from more than 40 localities worldwide. Our purpose is to determine the origin of andalusite, including the T-P-X controls on andalusite formation, using eight textural and chemical criteria: sizecompa tibility with grain sizes of igneous m inera ls in the same rock; shape-ranging from euhedral to anhedral, with, no simple correla tion with, origin; state of aggregation-single grains or clusters of grains; association with, muscovite-with, or without, rims of mono crystalline or polycrystalline muscovite; inclusions-rare mineral inclusions and melt inclusions; chemical composition-andalusite with, little significant chemical variation, except in iron content (0-08-1-71 wt. °/o FeO); compositional zoning-concentric, sec tor, patchy, oscillatory zoning cryptically reflect growth, conditions; compositions of coexisting phases-biotites with. high, siderophy llite-eastonite contents (AT ~2-68 ± 0-07 atoms per formula unit), muscovites with 0-57-4-01 wt % P'eO and 0-02-2-85 wt % TiOg, and apatites with. 3-53 ± 0-18 wt % F. Coexisting muscovite-biotite pairs have a wide range of F contents, and FSt = 1-612FAIs + 0-015. Most coexisting minerals have compositions consistent with, equilibration at. magmatic conditions. The three principal genetic types of andalusite in felsic igneous rocks are: Type 1 Metamorphic-(a) prograde metamorphic (in ther mally metamorphosed peraluminous granites), (b) retrograde metamorphic (inversion from sillimanite of unspecified origin), (c) xenocrystic (derivation from local country rocks), and (d) restitic (derivation from source regions); Type 2 Magmatic-(a.) peritectic (water-undersaturated, TJ) associated with, leucosomes in migma tites, (b) peritectic (water-undersaturated, T^J, as reaction rims on garnet, or cordierite, (c) cotectic (water-undersaturated, T j direct, crystallization from a silicate melt, and (d) pegmatitic (watersaturated, T^J, associated with, aplite-pegmatite contacts or peg matitic portion alone; Type 3 Metasomatic-(water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement, of feldspar and/or biotite, intergrowths with, quartz. Tie great, majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin. Of the many possible controls on the formation of andalusite (excess AfOy,, water concentration and fluid evolution, high. Be-B-LiP , high. F, high. Fe-Mn-Ti, and kinetic considerations), the two most, important, factors appear to be excess Af03 and the effect, of releasing water (either to strip alkalis from the melt, or to reduce alumina solubility in the melt). Of particular importance is...

Andalusite and Na- and Li-rich cordierite in the La Costa pluton, Sierras Pampeanas, Argentina: textural and chemical evidence for a magmatic origin

“…3a for illustrative purposes. The compositions are very similar to those reported for other andalusites crystallized from a magma (D'Arnico et al 198 1;Gordillo et al 1985;Kawakami 2002;Clarke et al 2005), i.e. Al 2 0 3 between 62.90 and 64.26 wt%, and small amounts of Fe203 (0.42-0.51 wt%).…”

“…The high ASI values (1.27-1.42; Table 2) of the Santa Cruz igneous facies suggest that the granitic magma was saturated in alurnina, making Gordillo et al 1985 ;Clarke 1995 ;Dahlquist et al 2005;Clarke et al 2005 and references therein). In addition, Patiño Douce (1992) has shown that the activity coefficient of Al in silicic melts is a function of the alkali content of the melt, whereby alkali-rich a Mg Na Fig.…”

“…3 a, b BSE images with corresponding sketches (c, d) showing the location of elec1ron microprobe spots for andalusite, Na-rich cordierite (Tables 3, 5), muscovite from the Santa Cruz igneous facies and textural relationships among them. According to Clarke et al (2005), textura! relationships like those shown here are in di cativ e of subsolidus replacement of muscovite after andalusite and Na-and Li-rich cordierite that resulted in a polycrysta1line muscovite pseudomorph.…”

“…Moreover, textural and chemical evidence presented here points to a primary origin for sorne of the muscovites in the Santa Cruz igneous facies. It is well known that addition of P, Li, F and B to peraluminous magmas depresses the granite solidus to lower temperatures at any given pressure (Clarke et al 2005 and reference therein). The presence of sorne tourmaline (ca.…”

“…whether they crystallized from the magma or were trapped as xenocrysts (e.g. Flood and Shaw 1975 ;Clarke et al 1976Clarke et al , 2005Bellido and Barrera 1979;Phillips et al 198 1;Allen and Barr 1983 ;Weber et al 1985 ;Dimer and Dupraz 1986;Georget and Fourcade 1988 ;Clarke 1995 ;Erdrnann et al 2004;Gottesmann and Forster 2004;Dahlquist et al 2005). We have focused here on the petrography, mineral chemistry, and the bulk major and trace element composition of the Santa Cruz igneous facies of the La Costa peraluminous granite pluton in the province of La Rioja, NW Argentina, which contains Na-and Li-rich cormerite, andalusite and muscovite.…”