Occurrence and Origin of Andalusite in Peraluminous Felsic Igneous Rocks

32 Pages·2004·2.53 MB·English

JOURNALOFPETROLOGY PAGE1of32 doi:10.1093/petrology/egh083
Journal of Petrology Advance Access published November 24, 2004
Occurrence and Origin of Andalusite in
Peraluminous Felsic Igneous Rocks
D. BARRIE CLARKE1*, MICHAEL DORAIS2, BERNARD BARBARIN3,
DAN BARKER4, BERNARDO CESARE5, GEOFFREY CLARKE6,
MOHAMED EL BAGHDADI7, SASKIA ERDMANN1, HANS-JU¨RGEN
FO¨ RSTER8, MARIO GAETA9, BA¨RBEL GOTTESMANN8,
REBECCA A. JAMIESON1, DANIEL J. KONTAK10, FRIEDRICH
KOLLER11, CARLOS LEAL GOMES12, DAVID LONDON13,
GEORGE B. MORGAN VI13, LUIS J. P. F. NEVES14, DAVID R. M.
PATTISON15, ALCIDES J. S. C. PEREIRA14, MICHEL PICHAVANT16,
CARLOS W. RAPELA17, AXEL D. RENNO18, SIMON RICHARDS19,
MALCOLM ROBERTS20, ALESSANDRO ROTTURA21, JULIO
SAAVEDRA22, ALCIDES NOBREGA SIAL23, ALEJANDRO J.
TOSELLI24, JOSE M. UGIDOS25, PAVEL UHER26, CARLOS
VILLASECA27, DARIO VISONA`5, DONNA L. WHITNEY28,
BEN WILLIAMSON29 AND HENRY H. WOODARD30
1DEPARTMENTOFEARTHSCIENCES,DALHOUSIEUNIVERSITY,HALIFAX,NS,CANADAB3H3J5
2DEPARTMENTOFGEOLOGY,BRIGHAMYOUNGUNIVERSITY,PROVO,UT84602,USA
3LABORATOIREMAGMASETVOLCANS,UNIVERSITEE(cid:1) BLAISEPASCAL,5RUEKESSLER,F63038
CLERMONT-FERRANDCEDEX,FRANCE
4DEPARTMENTOFGEOLOGICALSCIENCES,UNIVERSITYOFTEXAS,AUSTIN,TX78712,USA
5DIPARTIMENTODIMINERALOGIAEPETROLOGIA,UNIVERSITA` DIPADOVA,I-35137PADOVA,ITALY
6SCHOOLOFGEOSCIENCES,UNIVERSITYOFSYDNEY,SYDNEY,N.S.W.2006,AUSTRALIA
7LABORATOIRED’EXPLORATIONETGESTIONDESRESSOURCESNATURELLES,DEE(cid:1)PARTEMENTDESSCIENCESDE
LATERRE,FACULTEE(cid:1) DESSCIENCESETTECHNIQUES,BENIMELLAL,MOROCCO
8GEOFORSCHUNGSZENTRUMPOTSDAM,D-14473POTSDAM,GERMANY
9UNIVERSITA` DEGLISTUDIDIROMALASAPIENZA,DIPARTIMENTODISCIENZEDELLATERRA,PIAZZALEALDO
MORO5,00185ROME,ITALY
10NOVASCOTIADEPARTMENTOFNATURALRESOURCES,POBOX698,HALIFAX,NS,CANADAB3J2T9
11DEPARTMENTOFGEOLOGICALSCIENCE,UNIVERSITYOFVIENNA,A-1090VIENNA,AUSTRIA
12DEPARTAMENTODECIEˆNCIASDATERRA,UNIVERSIDADEDOMINHO,4710-057BRAGA,PORTUGAL
13SCHOOLOFGEOLOGYANDGEOPHYSICS,UNIVERSITYOFOKLAHOMA,NORMAN,OK73019-0628,USA
14DEPARTAMENTODECIENCIASDATERRA,UNIVERSIDADEDECOIMBRA,3000-272COIMBRA,PORTUGAL
15DEPARTMENTOFGEOLOGYANDGEOPHYSICS,UNIVERSITYOFCALGARY,CALGARY,AB,CANADAT2N1N4
16INSTITUTDESSCIENCESDELATERRED’ORLEE(cid:1)ANS(ISTO,UMR6113),45071ORLEE(cid:1)ANSCEDEX2,FRANCE
17CENTRODEINVESTIGACIONESGEOLO´GICAS,644CALLENO.1,1900LAPLATA,ARGENTINA
*Corresponding author. Telephone: [phone]. Fax: (902) 494-
6889.E-mail:[email protected] JournalofPetrology#OxfordUniversityPress2005;allrightsreserved
JOURNALOFPETROLOGY
18INSTITUTEOFMINERALOGY,FREIBERGUNIVERSITY,D-09596FREIBERG,GERMANY
19SCHOOLOFGEOSCIENCES,UNIVERSITYOFNEWCASTLE,NEWCASTLE,N.S.W.,AUSTRALIA
20THECOUNCILFORGEOSCIENCE,POBOX5347,PORTELIZABETH6065,SOUTHAFRICA
21DIPARTIMENTODISCIENZEDELLATERRAEGEOLOGICO-AMBIENTALI,UNIVERSITA` DIBOLOGNA,
40126BOLOGNA,ITALY
22INSTITUTODERECURSOSNATURALESYAGROBIOLOGIA,CSIC,37071SALAMANCA,SPAIN
23NEG-LABISE,DEPARTMENTOFGEOLOGY,FEDERALUNIVERSITYOFPERNAMBUCO,RECIFE,PE50670-000,BRAZIL
24UNIVERSIDADNACIONALDETUCUMAN,FACULTADCIENCIASNATURALES,INSTITUTOSUPERIOR
CORRELACIO´NGEOLO´GICA,4000SANMIGUELDETUCUMAN,ARGENTINA
25DEPARTAMENTODEGEOLOGIA,FACULTADDECIENCIAS,37008SALAMANCA,SPAIN
26DEPARTMENTOFMINERALDEPOSITS,FACULTYOFNATURALSCIENCES,THECOMENIUSUNIVERSITY,
MLYSKADOLINAG,84215BRATISLAVA,SLOVAKIA
27DEPARTAMENTODEPETROLOGIAYGEOQUIMICA,FACULTADDECC.GEOLOGICAS,UNIVERSIDAD
COMPLUTENSE,28040MADRID,SPAIN
28DEPARTMENTOFGEOLOGYANDGEOPHYSICS,UNIVERSITYOFMINNESOTA,MINNEAPOLIS,MN55455,USA
29DEPARTMENTOFMINERALOGY,THENATURALHISTORYMUSEUM,LONDONSW75BD,UK
30DEPARTMENTOFGEOLOGY,BELOITCOLLEGE,BELOIT,WI53511,USA
RECEIVEDAUGUST4,2003;ACCEPTEDSEPTEMBER22,2004
Andalusite occurs as an accessory mineral in many types of per- (c)xenocrystic(derivationfromlocalcountryrocks),and(d)restitic
aluminousfelsicigneousrocks,includingrhyolites,aplites,granites, (derivation from source regions);Type 2 Magmatic—(a)peritectic
pegmatites, and anatectic migmatites. Some published stability (water-undersaturated, T") associated with leucosomes in migma-
curvesforAnd¼Silandthewater-saturatedgranitesoliduspermit tites,(b)peritectic(water-undersaturated,T#),asreactionrimson
asmallstabilityfieldforandalusiteinequilibriumwithfelsicmelts. garnet or cordierite, (c) cotectic (water-undersaturated, T#) direct
We examine 108 samples of andalusite-bearing felsic rocks from crystallization from a silicate melt, and (d) pegmatitic (water-
morethan40localitiesworld-wide.Ourpurposeistodeterminethe saturated, T#), associated with aplite–pegmatite contacts or peg-
origin of andalusite, including the T–P–X controls on andalusite matitic portion alone; Type 3 Metasomatic—(water-saturated,
formation, using eight textural and chemical criteria: size— magma-absent), spatially related to structural discontinuities in
compatibilitywithgrainsizesofigneousmineralsinthesamerock; host, replacement of feldspar and/or biotite, intergrowths with
shape—rangingfromeuhedraltoanhedral,withnosimplecorrela- quartz. The great majority of our andalusite samples show one or
tion with origin; state of aggregation—single grains or clusters of moretexturalorchemicalcriteriasuggestingamagmaticorigin.Of
grains;associationwithmuscovite—withorwithoutrimsofmono- the many possible controls on the formation of andalusite (excess
crystalline or polycrystalline muscovite; inclusions—rare mineral Al O ,waterconcentrationandfluidevolution,highBe–B–Li–P,
2 3
inclusions and melt inclusions; chemical composition—andalusite highF,highFe–Mn–Ti,andkineticconsiderations),thetwomost
with little significant chemical variation, except in iron content important factors appear to be excess Al O and the effect of
2 3
(0(cid:2)08–1(cid:2)71 wt % FeO); compositional zoning—concentric, sec- releasing water (either to strip alkalis from the melt or to reduce
tor, patchy, oscillatory zoning cryptically reflect growth conditions; alumina solubility in the melt). Of particular importance is the
compositions of coexisting phases—biotites with high siderophy- evidenceformagmaticandalusiteingranitesshowingnosignificant
llite–eastonite contents (Aliv (cid:3)2(cid:2)68 (cid:4) 0(cid:2)07 atoms per formula depressionofthesolidus,suggestingthattheAnd¼Silequilibrium
unit), muscovites with 0(cid:2)57–4(cid:2)01 wt % FeO and 0(cid:2)02– must cross the granite solidus rather than lie below it. Magmatic
2(cid:2)85 wt % TiO , and apatites with 3(cid:2)53 (cid:4) 0(cid:2)18 wt % F. andalusite, however formed, is susceptible to supra- or sub-solidus
2
Coexistingmuscovite–biotitepairshaveawiderangeofFcontents, reaction to produce muscovite. In many cases, textural evidence
and F ¼ 1(cid:2)612F þ 0(cid:2)015. Most coexisting minerals have of this reaction remains, but in other cases muscovite may
Bt Ms
compositions consistent with equilibration at magmatic conditions. completely replace andalusite leaving little or no evidence of its
Thethreeprincipalgenetictypesofandalusiteinfelsicigneousrocks former existence.
are: Type 1 Metamorphic—(a) prograde metamorphic (in ther-
mally metamorphosed peraluminous granites), (b) retrograde
metamorphic (inversion from sillimanite of unspecified origin), KEYWORDS:andalusite; granite; magmatic; origin; xenocrystic
2
CLARKEetal. ANDALUSITEINPERALUMINOUSFELSICIGNEOUSROCKS
INTRODUCTION
Purpose
Andalusiteoccursasanaccessorymineralinawiderange
of felsic peraluminous {A/CNK ¼ molar [(Al O )/
2 3
(CaO þ Na O þ K O)] > 1} extrusive and intrusive
2 2
igneousrocks.Thepurposesofthiscontributionare:
(1) topresenttexturalobservationsandchemicaldata
from a wide range of andalusite-bearing felsic igneous
rocks, including fine-grained glassy volcanics, anatectic
leucosomes, fine-grained aplites, medium- to coarse-
grained granitoids, and very coarse-grained granite
pegmatites;
(2) to discover the criteria (mineral assemblages,
textures, chemical partitioning, and phase equilibrium
constraints) for distinguishing between magmatic, meta-
morphic, and metasomatic andalusite;
(3) to evaluate the conditions and controls that
promote the formation of andalusite in naturally
occurring felsic igneous rocks.
If andalusite can have a primary magmatic origin, its
occurrence places important constraints on the T–P–X
conditionsofmagmacrystallization.
Petrological framework
Fig. 1. Relationship between the granite solidus and the andalusite–
The positions of the water-saturated granite solidus and sillimanitestabilityfieldboundary.(a)Thecombinationofthehaplo-
the andalusite–sillimanite stability field boundary in T– granite solidus (Tuttle & Bowen, 1958)and the And ¼ Sil boundary
P–Xspacearecriticaltotheoriginofandalusiteinfelsic ofHoldaway(1971;H71)permitsno overlapofthe stabilityfieldsof
andalusiteandsilicatemelt,andprecludesthestablecrystallizationof
igneous rocks. At one extreme, simple synthetic systems
primarymagmaticandalusite,whereasthecombinationofthehaplo-
involvingthewater-saturatedhaplogranite(Na2O–K2O– granite solidus and the And ¼ Sil boundary of Pattison (1992; P92)
Al O –SiO –H O) solidus (Tuttle & Bowen, 1958; permitstheformationofprimarymagmaticandalusite.(b)Thecom-
2 3 2 2
binationoftheperaluminousgranitesolidus(Johannes&Holtz,1996)
Holland&Powell,2001)andthealuminosilicatestability
andtheAnd¼SilboundaryofRichardsonetal.(1969;R69)expands
fields (Holdaway, 1971; Holdaway & Mukhopadhyay, the stability field for andalusite þ silicate melt (shaded area labelled
1993) show no overlap between the stability fields of ‘ANDMAX’).
silicate melt and andalusite, precluding a primary mag-
matic origin for andalusite (Fig. 1a). Accordingly, anda-
lusite in felsic igneous rocks must be xenocrystic, curve depending on their concentrations (Chorlton &
metasomatic, or the product of growth from a strongly Martin, 1978; London & Burt, 1982; Pichavant &
undercooledmelt.Attheotherextreme,simplesynthetic Manning, 1984). Natural Ca-bearing plagioclase raises
systems involving the water-saturated peraluminous thehaplogranitesoliduscurveby10–20(cid:8)C,dependingon
granite solidus (Abbott & Clarke, 1979; Holtz et al., theamountofCainthesystem( Johannes,1978).
1992; Joyce & Voigt, 1994) and the aluminosilicate sta- Thepositionoftheandalusite–sillimanitefieldbound-
bility fields of Richardson et al. (1969) show substantial ary in P–T space has been investigated many times, but
overlap, thereby permitting a primary magmatic origin itspreciselocationremainscontroversial(Kerrick,1990;
forandalusite(Fig.1b). Pattison, 1992, 2001; Holdaway & Mukhopadhyay,
The position of the water-saturated granite solidus 1993; Tinkham et al., 2001; Pattison et al., 2002; Cesare
curve is sensitive to the presence of other components. etal.,2003).UncertaintiesinthepositionoftheAnd¼Sil
In particular, excess Al O lowers the solidus curve by fieldboundaryarise,inpart,fromthestrongdependence
2 3
c.30(cid:8)C(Fig.1b),andcreatesamorefavourablecomposi- of the thermodynamic equilibrium conditions on the
tional environment in which to grow Al SiO poly- structural state of the material under investigation
2 5
morphs (Abbott & Clarke, 1979; Clemens & Wall, (Salje, 1986). Considerable discrepancy exists between
1981; Holtz et al., 1992; Joyce & Voigt, 1994). Fluorine, the experimental studies of Richardson et al. (1969),
lithium,andboronareothercomponentsthatmayhave who used fibrolitic sillimanite, and those of Holdaway
important roles in lowering the haplogranite solidus (1971) who used prismatic sillimanite. According to
3
JOURNALOFPETROLOGY
Salje(1986),a‘transitionfield’betweenthepolymorphsis request on the granite-research network for further
more appropriate than a ‘transition line’. Grambling & contributions to expand the coverage. The result is a
Williams(1985)andKerrick(1990)suggestedaneffectof database of 111 felsic igneous rock samples, 108 of
impurities (mainlyFe3þand Mn3þ) on the stabilityrela- them containing andalusite, contributed by the authors
tionsoftheAl SiO polymorphs.IncorporationofFeand of this paper. All authors have participated in the pro-
2 5
Mn enlarges the stability field of andalusite relative to ductionofthispaperthroughanexchangeoftext,tables,
that of sillimanite; however, Pattison (2001) argued that andfiguresontheInternet.
this effect is generally modest for natural Fe and Mn Mostofthesamplesweresubmittedashandspecimens
contents. and prepared as thin sections by Gordon Brown at
Owing to these difficulties in deciding between the Dalhousie University. Petrographic observations of all
different experimental calibrations, many investigators samplesweremadebyBarrieClarkeandMichaelDorais,
turned to natural parageneses to constrain the equilib- andverifiedbythepersonsubmittingthesamples.Inthis
rium (e.g. Greenwood, 1976; Vernon, 1982; Holland & way, we have applied a uniform nomenclature to all
Powell, 1985; Pattison, 1992; Pattison etal., 2002). Most samples. Bernardo Cesare examined all samples for
of these studies placed the And ¼ Sil equilibrium in melt inclusions. Dan Kontak examined all samples for
positions intermediate between the Holdaway (1971) fluidinclusions.Whereapplicable,mineralabbreviations
andRichardsonetal.(1969)curves.Ofparticularsignifi- usedinthispaperarethoseofKretz(1983).
cancetothisinvestigationisthatseveralstudiesofmeta-
peliticAnd¼Silphaseequilibriainlow-pressuresettings
(i.e. those most relevant to the issue of andalusite þ
PETROGRAPHIC OBSERVATIONS
silicate melt stability) rejected the Holdaway (1971)
And¼Silcurvebecauseitcreatedtoosmallanandalusite AND DISCUSSION
stability field to reconcile with a number of other phase Inafieldandpetrographicstudy,Hills(1938)notedthat
equilibriumconstraints(e.g.Vernon,1982;Vernonetal., ‘it is chiefly from those uncontaminated ... granites,
1990;Pattison&Tracy,1991;Pattison,1992;Johnson& pegmatites, and aplites...that what appears to be pri-
Vernon,1995).Pattison(1992)providedanevaluationof mary pyrogenetic andalusite has been recorded’. Hills’
theAnd¼Silequilibriumagainstanumberofkeyphase evidence included modal abundance, uniform distribu-
equilibrium constraints that supported his calculated tion, large size and euhedral habit of andalusite, lack of
position about midway between the Holdaway (1971) orientedcarbonaceousinclusions(chiastolite),absenceof
and Richardson et al. (1969) positions. This position metasedimentary xenoliths, association with topaz and
allows for an andalusite þ haplogranite melt stability tourmalineintwo-micagranites,and,forsome,apparent
field below (cid:9)3kbar, even without the need to invoke lack of opportunity for the magmas to assimilate pera-
F-, B-, Li- or excess Al-bearing components in the melt luminouswall-rock.Toestablishtheigneousoriginfora
(Fig.1a),andithasfoundsupportinanumberofrecent particularmineralrequires matchinganumberofthese,
papers (Spear et al., 1999; Tinkham et al., 2001; Cesare andother,inherentlyequivocaltexturalcriteria,detailed
etal.,2003;Johnsonetal.,2003;Larson&Sharp,2003). below. If andalusite in a felsic igneous rock satisfies at
Inaddition,thepresenceofmeltinclusionsinandalusite least some of these criteria, an igneous origin for that
from volcanic rocks (Cesare etal., 2003), the presence of andalusiteistenable.
euhedralcrystalsofandalusiteinsomeglassyfelsicvolca- Electronic Appendix Table A1 contains informa-
nic rocks (Pichavant et al., 1988), and the occurrence of tion about the samples, including source, location,
euhedralandalusitecrystalsingraniticrocksandanatec- environmentofcrystallization,andaliteraturereference
tic leucosomes (Clarke et al., 1976; Clemens & Wall, (if any); electronic appendices may be downloaded from
1981; Vernon et al., 1990; Pattison, 1992) suggest an the Journal of Petrology website at http://petrology.
overlap of the stability fields of andalusite and silicate oupjournals.org/.
meltandamagmaticoriginfortheandalusite.
Grain size
Methods Dimensional compatibility ofa mineral of unknown ori-
This project began as the result of an exchange of ideas gin with other magmatic rock-forming minerals in the
about andalusite in granites on the Granite-Research same sample could be used to argue a co-magmatic
Internet discussion group ([email protected], origin. The grain sizes of primary magmatic minerals in
now [email protected]). Subsequent to that an igneous rock can, however, vary by orders of magni-
discussion, Barrie Clarke and Michael Dorais tested tude; therefore, any grain-size test is not particularly
some ideas with their own andalusite-bearing and discriminating. Conversely, dimensional incompatibility
andalusite-free granitoid samples, and then put out a maysuggest,butdoesnotnecessarilydemandadifferent
4
CLARKEetal. ANDALUSITEINPERALUMINOUSFELSICIGNEOUSROCKS
aplites, e.g. WIL-01 (Fig. 3b), pass the grain-shape test
as potentially primary magmatic phases. The andalusite
inCLA-12isskeletal(Fig.3c),suggestingformationdur-
ing a temperature or pressure quench. Many subhedral
or anhedral andalusites in felsic igneous rocks have pink
cores that are euhedral to subhedral (VIS-01, Fig. 3d),
suggestingthatthosecores,atleast,mightbeigneous.
Anhedralandalusitegrainshapesmayreflectlate-stage
graininterferenceduringprimarymagmaticgrowth,the
result of a reaction relationship of an andalusite of any
originwiththesilicatemeltphase,anoriginallyanhedral
xenocrysticmorphology,oranoriginallyeuhedralxeno-
crystic morphology out of equilibrium with the melt.
Distinctly anhedral andalusite grains, apparently out of
equilibriumwiththefelsicmagma,includevolcanicsam-
ple BAR-01 (Fig. 3e) and plutonic sample ROT-05
(Fig.3f ).
State of aggregation
Andalusite in felsic igneous rocks may occur as single
grains(Figs3a,b,d–f;4a–d),isolatedfromotherandalu-
site grains by more common rock-forming minerals. It
may also occur as clusters of small grains. In some clus-
ters,theindividualandalusitegrainshaverandomorien-
tationsrelativetooneanother(Fig.5a–d).Whyshoulda
modally scarce mineral cluster? Either the individual
andalusites crystallized elsewhere and were brought to
thatlocationby some physical process such as synneusis
orsettling,or theyrepresent thesites ofadvanced diges-
tion of pelitic xenoliths, or they nucleated and grew at
that position in the sample. These common clusters of
Fig.2.Andalusitegrainsizes.Photomicrographsillustratingandalusite randomlyorientedgrainsofandalusitemayhavegenetic
grains that are significantly larger than the average grain size of the significance.
rock,suggestingthattheymaynothavethesameoriginastheother
Inotherclusters,theindividualandalusitegrainsarein
minerals in these felsic igneous rocks. (a) Sample BBR-01 (granite;
Oulad Ouslam Pluton, Morocco). (b) Sample CES-01 (dacite, optical and crystallographic continuity (e.g. Figs 3c and
Mazarr(cid:1)oon,Spain).Scalebarsrepresent1mm.A,andalusite. 4b).Ifincrystallographicalignment,theandalusitegrains
either grew as a spray of quench crystals (Figs 3c and
origin. Any andalusite grains that are significantly smal-
5b), or the clustering may only be apparent, as in the
ler, or significantly larger, than the main rock-forming
cases of many optically continuous andalusite grains
silicate minerals are potentially non-igneous. Figure 2
embedded in muscovite (Fig. 4b). In cases such as the
illustrates two samples (BBR-01 and CES-01) in which
latter,asinglegrainofandalusitewasirregularlyreplaced
andalusitefailsthegrain-sizetestbecausethecrystalsare
bymuscovite,yieldinganapparent‘cluster’ofanhedral,
much larger than the other minerals in the rock. Many
butcrystallographicallyaligned,andalusiteinmuscovite.
other samples contain andalusite grains that are consid-
erably smaller than the main rock-forming minerals;
although they also fail the grain-size test, they may still Textural relationship with muscovite
haveanigneousorigin. Manyandalusitegrainsinfelsicigneousrockshaveman-
tlesofmuscovite,andthesemuscoviterimsmayconsistof
Grain shape a single crystal or a polycrystalline aggregate. Figure 6
Euhedralandalusiteinafelsicigneousrockmayindicate combines the state of aggregation of andalusite grains
aformercotecticorperitecticrelationshipwithasilicate (above), and the common association of andalusite with
meltphase;however,euhedralandalusiteoccursbothin muscovite, to establish a six-fold textural classification
igneous and metamorphic rocks, and thus idiomorphic of andalusite. In some cases, more than one class of
grainshapesalonearenotdiagnostic.Someoftheanda- andalusitecanoccurinthesamerock(e.g.sampleGOT-
lusites in volcanic samples, e.g. LON-01 (Fig. 3a), or 02containsandalusitetexturaltypesS1,C1,andC2).
5
JOURNALOFPETROLOGY
Fig.3.Andalusitegrainshapes.(a)SampleLON-01(rhyoliteobsidianclast,Macusani,Peru;USNationalMuseumcatalogno.2143)showstwo
small euhedral to subhedral andalusite crystals in a predominantly glassy matrix. (b) Sample WIL-01 (aplitic granite; Velay Massif, France)
containseuhedralandalusite.(c)SampleCLA-12(aplite–pegmatite;SouthMountainBatholith,NovaScotia,Canada;sectionisslightlytoothick)
haselongate–skeletalandalusitegrainshapessuggestingcrystallizationbyquenching.(d)SampleVIS-01(granite;Makalunorthside,Tibet)has
andalusitewithanoverallanhedralgrainshape,butwithamoreeuhedralpinkcore.(e)SampleBAR-01(rhyolite;Lipari,Italy)isavolcanicrock
withanhedralandalusite.(f)SampleROT-05(granite;Telve,Cimad’Astapluton,southernAlps,Italy)containsanhedralandalusitethatexhibits
deformationtwinningincrossedpolars(notshown).Scalebarsrepresent1mm.A,andalusite.
In the Macusani rhyolites, muscovite and andalusite andalusite and muscovite are negatively correlated, sug-
coexist throughout the entire volcanic field (Pichavant gesting that, during the main crystallization stage of the
et al., 1988). No textural evidence exists for replacement Macusanimagmas,thereactionMsþQtz¼AndþSan
of one phase by the other, but the modal proportions of (in presence of melt) controls the modal proportions of
6
CLARKEetal. ANDALUSITEINPERALUMINOUSFELSICIGNEOUSROCKS
Fig.4.Singlegrainsofandalusite.(a)SampleCLR-01(migmatite;Mt.Stafford,AruntaBlock,Australia)withsubhedralandalusiteinamigmatite
leucosome(texturaltypeS1).(b)SampleTOS-05(pegmatite;VelascoBatholith,Argentina)showsasingleopticallycontinuousandalusitegrain
enclosedinsinglegrainofmuscovite(texturaltypeS2).(c)SampleROT-02(granite;Cotronei,SilaBatholith,Calabria,southernItaly)showinga
subhedral andalusite enclosed in a single crystal of muscovite (textural type S2). (d) Sample UGI-04 (granite; Plasencia granite, west Central
IberianMassif,Spain)showingananhedralandalusitewithapolycrystallinerimofmuscovite(texturaltypeS3).ClassificationoftexturaltypesS1,
S2,andS3isgiveninFig.6.Scalebarsrepresent1mm.A,andalusite.
andalusiteandmuscovite.ThisreactiondependsonP,T, (ROT-02,Fig.4c),andtherebycomplicateanydetermi-
and a , implying thatthe mineral assemblage charac- nationoftheoriginoftheandalusite.Becausemuscovite
HO
2
teristic of the main crystallization stage of the Macusani can haveprimary magmatic or secondary hydrothermal
magmas (Qtz, San, Plag, Ms, And, (cid:4) Bt) could have origins, with much the same texture (Miller et al., 1981;
crystallized over a range of P, T and f conditions. Zen, 1988), interpretation of this textural relationship
HO
2
However,theFcontentofmuscoviteisalsoanimportant betweenandalusiteandmuscoviteisdifficult.Onereason
controlling factor in this reaction. For a given a , for little or no bulk chemical compositional difference
HO
2
elevatedf woulddrivethereactiontotheleft(consum- between some andalusite-bearing two-mica granitoids
HF
ing andalusite, producing muscovite). Muscovite crystal- andandalusite-freetwo-micagranitoidsisjustaquestion
lization attheexpenseofandalusite doesnotnecessarily of how completely the andalusite is replaced (effectively
implyhigha (itcouldbelowerT,higherP,orhigher under magmatic conditions by primary muscovite, less
HO
2
f ).Theinversecorrelationbetweenthemodalpropor- effectivelyundersubsolidusconditionsbysecondarymus-
HF
tions of Ms and And in the Macusani volcanics also covite).Whetherandalusiteispreservedinplutonicrocks
occurs in peraluminous granites from the Bohemian dependsonitssurvivalunderconditionsofslowcooling,
Massif (samples ROT-03,04; D’Amico et al., 1982– allowingmagmaticperitecticrelationsofthetype
1983a,1982–1983b).
LþAndþOther Phases
Muscovite overgrowths on andalusite in plutonic
rocks may obscure a possible original euhedral shape ! LþMsþOther Phases
7
JOURNALOFPETROLOGY
Fig.5.Clustersofandalusitegrains.(a)SampleROT-04(granite;Rasnaquarry,Telc,southwesternMoravia,CzechRepublic)showingasmall
clusterofanhedralandalusitecrystalsinquartz(texturaltypeC1).(b)SampleELB-01(aplite;BeniBousera,Morocco)showingasub-parallel
cluster of andalusite grains in an aplite (textural type C1). (c) Sample ROB-02 (granite; South Bohemian Pluton, Austria) shows a cluster of
randomlyorientedandalusitesinasinglecrystalofmuscovite(texturaltypeC2).(d)SampleVIL-02(granite;Penn~a-HombrePluton,Spain)showsa
clusterofanhedralandalusitegrainsinapolycrystallineaggregateofmuscovite(texturaltypeC3).ClassificationoftexturaltypesC1,C2,andC3
isgiveninFig.6.Scalebarsrepresent1mm.
(whereLismelt),orsubsolidusreactionssuchas
And þ Kfs þ ðH OÞ ! Ms þ Qtz
2 v
to eliminate the early formed andalusite. Addition of
water to the left sides of these equations converts ‘dry’
andalusite-bearinggranitoidsto‘wet’muscovite-bearing,
and normally two-mica, granitoids; in other words, they
are compositional equivalents except for the amount of
water (Zen, 1989). Kinetically, a high-temperature,
Fig. 6. Textural classification of andalusite in felsic igneous rocks. melt þ fluid, condition may favour the formation of
Threetexturalparameters(theoccurrenceofandalusiteeitherassingle
coarse-grained single muscovite crystals, whereas a
grains or as clusters of grains, the occurrence of andalusite with or
subsolidus low-temperature, fluid-only, condition may
without muscovite, and if with muscovite, whether that muscovite
consists of a single grain or an aggregate of grains) produce the favourtheformationofsomefine-grainedpolycrystalline
followingsixtexturalcategories:S1,singleandalusitegrains,nomus- muscovite aggregates.
covite; S2, single andalusite grains, monocrystalline muscovite over-
Figure 7 illustrates four of the many types of
growth or reaction rim; S3, single andalusite grains, polycrystalline
muscoviteovergrowthorreactionrim;C1,clusteredandalusitegrains, textural relations between andalusite and muscovite.
nomuscovite;C2,clusteredandalusitegrains,monocrystallinemusco- Theoriginalandalusitemaybeasinglegrainoracluster,
viteovergrowthorreactionrim;C3,clusteredandalusitegrains,poly- themuscoviterimmaybemagmaticorsubsolidushydro-
crystalline muscovite overgrowth or reaction rim. Textural types S1
thermal,andtheAnd!Msreactionmaybeincomplete
and C1 can occur as discrete grains, or as inclusions in other grains
suchasplagioclaseorquartz. orcomplete.Inthelastcase,theandalusiteiscompletely
8
CLARKEetal. ANDALUSITEINPERALUMINOUSFELSICIGNEOUSROCKS
Fig.7.Developmentofseveralpossibletexturalrelationshipsbetweenandalusiteandmuscovite(arrowsrepresentthecrystallographicc-axisof
andalusite). Different processes can have similar end-points. (a) Single grain of magmatic muscovite overgrows a single grain of magmatic
andalusite.Suprasolidusorsubsolidusmuscovitecontinuestogrowtotheultimateeliminationofandalusite.Notexturalevidencefortheformer
existenceofandalusiteremains.(b)Subsolidusreplacementofasinglegrainofandalusitetoproduceapolycrystallinemuscovitepseudomorph.
(c)Quenchedskeletalandalusiteovergrownbymagmaticmuscoviteresultinginanapparentcluster,butthe‘grains’areinopticalcontinuity.
(d)Opticallydiscontinuousclusterovergrownbymagmaticmuscovite.
consumedinthereaction,leavinglittleornoevidenceof those inclusions may help to determine the origin of the
itsformerexistence. host andalusite. If an andalusite contains carbonaceous
material defining the chiastolite cross (e.g. BBR-01,
Fig. 2a), a metamorphic origin is probable. Some chias-
Inclusion relationships
tolite-likeandalusite may alsoform by peritectic melting
Mineral inclusions reactions in graphitic schists where inclusion of graphite
Ifandalusiteoccursasinclusionsinigneousmineralssuch particles may take place behind advancing crystal faces,
as feldspar and quartz (e.g. REN-03, UGI-06), little can butatthesametimetheandalusiteshouldalsotrapmelt
be deduced about its origin; however, andalusite rarely inclusions(Cesare&G(cid:1)oomez-Pugnaire,2001).Fewofthe
occursasinclusionsinanyphaseotherthanmuscovite.If andalusitesthatwebelieveareigneousonothergrounds
andalusite itself contains inclusions of magmatic miner- contain any mineral inclusions, and thus the mineral
als, the sizes, shapes, abundances, and compositions of inclusioncriterionisnotparticularlyuseful.
9
JOURNALOFPETROLOGY
inavolcanicrockisalmostcertainlymagmatic,whereasa
large anhedral andalusite with a chiastolite cross and a
reaction rim is probably xenocrystic. Also, we note that
thereisnoaprioritexturalreasonwhyafelsicigneousrock
cannotcontainmorethanonegenetictypeofandalusite
(e.g.magmaticandxenocrystic).
CHEMICAL COMPOSITION OF
ANDALUSITE IN FELSIC
IGNEOUS ROCKS
In this section,we examinethe chemical compositionof
andalusite, the nature of any chemical zoning, and the
chemicalcompositionsofcoexistingmicasandapatiteto
search for criteria that might provide information about
Fig.8.Meltinclusionsinandalusite.VolcanicsamplePIC-01(rhyolite;
theoriginofandalusiteinfelsicigneousrocks.Electronic
Macusani,Peru)showingconspicuousmeltinclusions.Alsotobenoted
Appendix Tables A2–A5 contain compositional data
is the sharp straight contact between the pleochroic core and the
colourlessrimoftheandalusite.Scalebarrepresents0(cid:2)1mm. for average biotite, muscovite, andalusite, and apatite,
respectively, in the samples we have studied. Not all
samples contain all four minerals, and even if they do,
Melt inclusions wedonotnecessarilyhaveanalysesforallfourphasesin
eachrock.
Melt inclusions in andalusite attest to its growth in the
presenceofmelt(Cesareetal.,2003).Glassinclusionsare
easytorecognizeinandalusitefromfelsicvolcanicrocks, Chemical composition
such as those from Lipari (BAR-01), Mazarr(cid:1)oon (CES- If a mineral exhibits a wide range of chemical substitu-
01,02) (Cesare & G(cid:1)oomez-Pugnaire, 2001; Cesare et al., tions that reflect its conditions of formation [e.g. Ti in
2002,2003),andMacusani(Pichavantetal.,1988;Fig.8). muscovite (Miller et al., 1981)], then the origin of that
In slowly cooled plutonic rocks or migmatites, any melt mineral may be determined from its chemical composi-
inclusions trapped in andalusite will have crystallized as tion alone. In stoichiometric andalusite (Al SiO ), half
2 5
polyphase aggregates of quartz, feldspars, and micas, a the Al cations reside in octahedral sites, and the other
useful criterion to infer an igneous origin for andalusite. halfresideinfive-coordinatedpolyhedra,whereasallthe
Polyphase inclusions in andalusite crystals of samples Sicationsoccupytetrahedralsites.Suchsimplechemistry
CLA-01,05,11,12,13, CLR-02, GOM-03, RIC-03, and and relatively simple structure provide limited opportu-
TOS-06provideadditionalsupportfortheircoexistence nityforchemicalsubstitution(Deeretal.,1982).Electro-
withafelsicsilicatemelt. nic Appendix Table A4 shows that the studied
andalusites from felsic igneous rocks have transition-
element compositions with the following ranges: FeO
Fluid inclusions T
(measured as Fe, reported as FeO) 0(cid:2)03–1(cid:2)70%, MnO
Examinationofallourandalusitesamplesforfluidinclu-
0(cid:2)00–0(cid:2)09%,andTiO 0(cid:2)00–0(cid:2)36%.Withoutacompar-
2
sionsyieldednegativeresults.Eithertherewasnofluidin
able database of andalusite compositions from meta-
equilibriumwiththeandalusiteasitgrew(unlikelyinthe
morphic rocks, little can be said about the existence of
cases ofpegmatites),or the surface properties ofandalu-
chemical discriminants to determine the origin of the
sitearesuchthatitisnotreadily‘wetted’byfluids.
andalusite.Traceelementsmightprovetobemoreuseful
thanmajorelements.
Summary of textural observations
Of the several possible textural tests for the origin of Chemical zoning
andalusite in felsic igneous rocks, no single criterion Optically zoned andalusite is common in metamorphic,
(grain size, grain shape, clustering, textural relations hydrothermal, and magmatic environments [e.g. review
withmuscovite,inclusionrelations)isnecessarilydiagnos- by Kerrick (1990)]. Andalusites from the studied felsic
tic of the origin of andalusite. The agreement of two or igneousrocksshowfourtypesofzoning,asfollows.
moreofthesetexturalandchemicalcriteriaconstitutesa (1) Concentric zoning. Concentric zoning consists of a
stronger collective case. For example, a euhedral, grain- sharp to gradational variation in the mole fraction of
sizecompatible,andalusitewithmeltinclusionsoccurring transition-element content (hereafter referred to
10