Occurrence and origin of andalusite in peraluminous felsic igneous rocks

CLARKE, D.B.; DORAIS, M.; BARBARIN, B.; BARKER, D.; CESARE, B.; CLARKE, G.; EL BAGHDADI, M.; ERDMANN, S.,; FORSTER, H.; GAETA, M.; GOTTESMANN, B.; JAMIESON, R.A.; KONTAK, D.J.; KOLLER, F.; LEAL GOMES, C.; MORGAN VI, G.B.; NEVES, L.J.P.F.; PATTISON, D.R.M.; PEREIRA, A.J.S.C.; PICHAVANT, M.; RAPELA, C.W.; RENNO, A.D.; RICHARDS, S.; ROBERTS, M.; ROTTURA, A.; SAAVEDRA, J.; SIAL, A.N.; TOSELLI, A.J.; UGIDOS, J.M.; UHER, P.; VILLASECA, C.; VISONA, D.; WHITNEY, D. L.; WILLIAMSON, B.; WOODWARD, H.H.

JOURNAL OF PETROLOGY

Oxford University Press

Año: 2005 vol. 46 p. 441 - 472

0022-3530

Andalusite occurs, normally as an accessory mineral, in many types peraluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and migmatites. We have examined the textural relations, mineral assemblages, and mineral compositions of 102 samples of andalusite-bearing felsic rocks from more than 40 localities world-wide. Texturally, andalusite in felsic igneous rocks can range in size from microns to metres, in shape from euhedral to anhedral, in relation to other grains as either equilibrium or reaction relationships, but inclusion relationships provide little useful information. Chemically, andalusite shows little significant chemical variation, except in iron content (FeO = 0.0 – 1.7 wt. %), although distinctly zoned Fe-rich cores probably represent rapid disequilibrium growth from melts. The compositions of coexisting biotites have high siderophyllite-eastonite contents, coexisting muscovites have FeO = 0.57 – 4.01% and TiO2 = 0.0 – 1.7%, and coexisting apatites have F = 3.47 ± 0.38%. Coexisting muscovite-biotite pairs have a wide range of F contents, and Fbiot = 1.612*Fmusc + 0.015. A few samples contain coexisting andalusite and sillimanite with XSilAl2SiO5/XAndAl2SiO5 = 0.997 – 1.001. A combination of textural and chemical criteria permits characterization of three principal types of andalusite in felsic igneous rocks: Type 1 Metamorphic: (a) prograde metamorphic (in thermally metamorphosed peraluminous granites), (b) xenocrystic (generally anhedral, many inclusions, spatial proximity to country rocks and/or pelitic xenoliths), and          (c) restitic (theoretically anhedral with inclusions of high grade metamorphic minerals); Type 2 Magmatic: (a) peritectic (water-undersaturated, T­), subhedral to euhedral, associated with leucosomes in migmatites, (b) peritectic (water-undersaturated, T¯), subhedral to anhedral, as reaction rims on garnet or cordierite, (c) cotectic (water-undersaturated, T¯), euhedral, grain size compatibility with host rock, few inclusions, and (d) pegmatitic (water-saturated, T¯), large subhedral to euhedral grains, associated with aplite-pegmatite contacts or pegmatitic portion alone; and Type 3 Metasomatic: (water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement of feldspar and/or biotite, intergrowths with quartz. Considering all criteria, we conclude that andalusites in our sample sets have the following origins:  Type 1a Metamorphic – prograde (2 samples); Type 1b Metamorphic – xenocrystic (7 samples); Type 1c Metamorphic – restitic (3 samples); Type 2a Magmatic – peritectic T↑, water undersaturated (4? samples); Type 2b Magmatic -peritectic, T↓, water undersaturated (5 samples); Type 2c Magmatic - cotectic, T↓, water undersaturated (50 samples); Type 2d Magmatic - cotectic, T↓, water undersaturated (15 samples); and Type 3 Metasomatic - (0 samples). The principal controls on the formation of primary magmatic andalusite include: high A/CNK (inherited from source rocks, or the result of fractional crystallization), high Be-B-Li, high F, water saturation with consequent alkali stripping, high Fe-Mn, structural state of Al2SiO5, and whether stable or metastable growth. Andalusite formed by any of these mechanisms is highly susceptible to supra-or sub-solidus reaction with water to produce muscovite. In many cases, textural evidence of this reaction remains, but in other cases muscovite may completely replace andalusite leaving little of no evidence of its former existence. We conclude that andalusite forms primarily as a result of high A/CNK, and that source rock composition, fractional crystallization, and alkali element stripping by a supercritical aqueous fluid phases are the principal mechanisms for reaching andalusite saturation in felsic magmas.     Keywords: andalusite, granite, aplite, pegmatite, rhyolite, xenocryst, metamorphic, magmatic, metasomatic