%0 Journal Article
%A Clarke, D. Barrie
%A Dorais, Michael
%A Barbarin, Bernard
%A Barker, Dan
%A Cesare, Bernardo
%A Clarke, Geoffrey
%A El Baghdadi, Mohamed
%A Erdmann, Saskia
%A Förster, Hans Jürger
%A Gaeta, Mario
%A Gottesmann, Bärbel
%A Jamieson, Rebecca A.
%A Kontak, Daniel J.
%A Koller, Friedrich
%A Leal Gomes, Carlos
%A London, David
%A Morgan, George B. VI
%A Neves, Luis J.P.F.
%A Pattison, David R.M.
%A Pereira, Alcides J.S.C.
%A Pichavant, Michael
%A Rapela, Carlos W.
%A Renno, Axel D.
%A Richards, Simon
%A Roberts, Malcolm
%A Rottura, Alessandro
%A Saavedra, Julio
%A Sial, Alcides Nobrega
%A Toselli, Alejandro J.
%A Ugidos, Jose M.
%A Uher, Pavel
%A Villaseca González, Carlos
%A Visonà, Dario
%A Whitney, Donna L.
%A Whilliamson, Ben
%A Woodard, Henry H.
%T Occurrence and Origin of Andalusite inPeraluminous Felsic Igneous Rocks
%D 2005
%@ 0022-3530
%U https://hdl.handle.net/20.500.14352/49592
%X Andalusite occurs as an accessory mineral in many types of peraluminousfelsic igneous rocks, including rhyolites, aplites, granites,pegmatites, and anatectic migmatites. Some published stabilitycurves for And = Sil and the water-saturated granite solidus permita small stability field for andalusite in equilibrium with felsic melts.We examine 108 samples of andalusite-bearing felsic rocks frommore than 40 localities world-wide. Our purpose is to determine theorigin of andalusite, including the T–P–X controls on andalusiteformation, using eight textural and chemical criteria: size—compatibility with grain sizes of igneous minerals in the same rock;shape—ranging from euhedral to anhedral, with no simple correlationwith origin; state of aggregation—single grains or clusters ofgrains; association with muscovite—with or without rims of monocrystallineor polycrystalline muscovite; inclusions—rare mineralinclusions and melt inclusions; chemical composition—andalusitewith little significant chemical variation, except in iron content(0.08–1.71 wt % FeO); compositional zoning—concentric, sector,patchy, oscillatory zoning cryptically reflect growth conditions;compositions of coexisting phases—biotites with high siderophyllite–eastonite contents (Alw ≈ 268 ± 007 atoms per formulaunit), muscovites with 0.57–4.01 wt % FeO and 0.02–2.85 wt % TiO2, and apatites with 3.53 ± 0.18 wt % F.Coexisting muscovite–biotite pairs have a wide range of F contents,and FBt = 1.612FMs + 0015. Most coexisting minerals havecompositions consistent with equilibration at magmatic conditions.The three principal genetic types of andalusite in felsic igneous rocksare: Type 1 Metamorphic—(a) prograde metamorphic (in thermallymetamorphosed peraluminous granites), (b) retrogrademetamorphic (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, T↑) associated with leucosomes in migmatites,(b) peritectic (water-undersaturated, T↓), as reaction rims ongarnet or cordierite, (c) cotectic (water-undersaturated, T↓) directcrystallization from a silicate melt, and (d) pegmatitic (watersaturated,T↓), associated with aplite–pegmatite contacts or pegmatiticportion alone; Type 3 Metasomatic—(water-saturated,magma-absent), spatially related to structural discontinuities inhost, replacement of feldspar and/or biotite, intergrowths withquartz. The great majority of our andalusite samples show one ormore textural or chemical criteria suggesting a magmatic origin. Ofthe many possible controls on the formation of andalusite (excessAl2O3, water concentration and fluid evolution, high Be–B–Li–P,high F, high Fe–Mn–Ti, and kinetic considerations), the two mostimportant factors appear to be excess Al2O3 and the effect ofreleasing water (either to strip alkalis from the melt or to reducealumina solubility in the melt). Of particular importance is theevidence for magmatic andalusite in granites showing no significantdepression of the solidus, suggesting that the And = Sil equilibriummust cross the granite solidus rather than lie below it. Magmaticandalusite, however formed, is susceptible to supra- or sub-solidusreaction to produce muscovite. In many cases, textural evidenceof this reaction remains, but in other cases muscovite maycompletely replace
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