913
Thc CanodinnM ineralogist
Vol. 33,pp. 913-915(1995)
A REPORT
ON THEDERVAilONAND PROPER
USE
OFTHETERMANORTHOSITE*
TOMAS FEININGER
Centreg&oscientifiquede Qulbec, CasePostale7500,Sainte-Foy,Qulbec GIV 4C7
ABsrRAcr
The term anorthosite,coinedby T. Sterry Hunt in 1862,appliesonly to a specificrock type, The origin ofthe term and a
review of the definition of anorthositeare given, as well as six recommendations
on relevantpetrogmphicnomenclature.The
recornmendationscall for adherenceto tle IUGS definition of anorthosite,use of modifiers where appropriate(andesine
anorthosite,augite anorthosite,and so on), abandonmentof suchmeaninglessexpressionsas anor&ositic gabbroand gabbroic
anorthosite,and use of the term meta-anorthositewhere evidenceof igneousparentagehas been erasedby deformationand
metamorphism.
Keywords: anorthosite,petrography,terminology.
SoLnaans
[€ lerme anorthosite,introduit par T. Sterry Hunt en 1862,se limite A un seul type de roche. L'origine du Grme et un
historique de la d6finition d'anorthositesont pr6sent6s,ainsi que six recommandationsconcernantI'utilisation de sa terminologiep6trographique:I'adh6sionstrictei la d6finition de I'anorthositede la UISG, I'utilisation desqualificatifs tels anorthosite
d andesine,anorthositei augite, etc., I'abandondes termes erron6s "gabbro anortlositique" et "anorthosite gabbrolQue",
et I'utilisation du terme mdta-anorthositepour les anorthositesdont l'origine ignle a 6tA effac6epar la d6formation et le
m6tamorphisme.
Mots-cl6s: anorthosite,p6trographie,terminologie.
INrnopucrroN
Recently, I have had the opportunity to map the
otype" anorthosite
@eininger 1993). That work has
servedto makeme consciousof the uniquenessof this
venerablerock name.Nevertheless.abuseand misuse
of the term anorthositeby geologistsare widespread
and seemunabatedby the passageof time.
ffus rim of the presentnote is pragmatic:to insure
the usefulnessof an especiallyspecificrock name.The
text is divided into three sections.The fust discusses
the origin of the tern anorthosite.The secondsection
reviews the definition of anorthosite. The closing
section offers six recommendations on relevant
petrographicnomenclatue.
ooRecherches
sur I'origine des roches"l before the
Soci6tdG6ologiquede Francein 1858. In discussing
exffusive(= ootrue
igneous")rocks,Delessesaid(1858,
p. 751): "Comme leur feldspathdominantest tant6t
l'orthoseet tantOtI'anorthose"il conviendrad'examiner
successivementces derx cas"2. To clarify, Delesse
added:"Ir mot anorthosed6signeici, d'une manidre
g6n6rale, tous les feldspaths qui appartiennentau
sixibmesystemecristallin'.3
The 1863compendinmGeologyof CanadaimFlies
that it was Delesse who introduced the term
o'anorthosite".In the compendium,a footnote reads:
"Since all thesevarying triclinic feldsparsare anorthic
in cryst4llization,and approachmore or less to anorthite in their compositioD,Delessethus proposedto
Onrcnt or rHETBRM
The French geologist Achille-Ernest-Oscar-Joseph rlnvestigationson tle origin of rocks.
Delesse (1817-1881) read a landmark paper 2Astheir dominanlfeldsparis in somecasesorthoclaseandin
* GeologicalSurveyof Canadacontributionnumber42992.
others"anorthose".it will be suitableto look at eachcasein
turn.
3The word anorthasehere refers, in a generalsense,to all
feldsparbelongingto the sixth (= triclinic) crystal system.
914
THE CANADIAN
designatethem by the commonnameof anorthose,as
distinguishedfrom orthoseor orthoclase,andthe rocks
characterizedby their presence as anorthosite. In
accordancewith this we haveadoptedthe genericname
of anorthositefor these rocks" (Logan et al. 1863,
p. 22). However,the term anorthositeappearsnowhere
in Delesse'spaperor, to my knowledge,in any other of
his early writings. On the other hand, tradition has it
that T. Sterry Hunt coinedthe term. It may havebeen
asearlyas 1862(ruGS 1989,p. 47), althoughthe first
useof the term in a readily availablepublicationwasin
the above-citedcompendium,wherein the body of the
text one reads: '0...destituteof quartz and composed
chiefly of a lime-sodafeldspar,varying in composition
from andesine to anorthite. and associatedwith
pyroxene or hypersthene. This rock we shall
distinguishby the nameanorthosite",and "The hyperstheneis howeveroftenreplacedby ordinarypyroxene,
or is wholly wanting in the Laurentiansystem,giving
rise to a purely feldspathicrock'o(Loganet aI.1863,
p.22,27).
The picture was cloudedwhen, sometimebetween
1883and 1888.the term anorthosein Francebecame
synonymouswith anorthoclase.This synonymy was
fust registeredin the North Americanliteratureby J.D.
Dana in the 6th edition of his Systemof Mineralogy,
publishedn L892.Not long into the presentcentury,
however. use of the term anorthose had been
abandoned,at least in English, and it was droppedby
E.S. Dana in the 3rd edition of hts Textbook of
Mineralogy, publishedn L922.
Drpnvmowor ANonrgosrrs
MINERALOGIST
townite. or anorthite.In an earlierhistorical discussion,
Johannsen(1937, p. L96-205)had restrictedthe modal
abundanceof mafic accessoryminerals yet more
tightly, settingthe color index (CI) of anorthositeat no
more than five.
Clearly, all definitions aim toward a generalconsensus,but at the sametime they allow someleeway.
The threecategoriesin which leewayis evidentareorigin, texture,andmodal composition.
Origin
Anorthosites are conveniently divided into the
anorthositesof layeredmafic intrusionsandthe massif
anorthosites.The igneousparentageof anorthositesin
layered intrusions is generally obviousoparticularly
where such intrusions are pristine or have been only
feebly modified by deformation or metamorphism.
Many massifanorthosites,on the otherhand,areextensively recrystallized,and their ultimate origin cannot
be read in the field or under the microscope.
Furtlermore, they may carry assemblagesof metamorphic minerals, and mineralogical disequilibrium
may be reflectedin coronatextures.In short,probably
all anorthositesbegan as igneous rocks. Many were
subsequenflydeformed and recrystallized to a point
where they have the characteristicsof metamorphic
rocks.
Texture
Anorthosites of layered infusions typically are
plagioclasecumulates.They arecomposedof crowded
euhedralto subhedralcumulusplagioclase,and small
amountsof intercumulusmafic minerals.Grain size is
mostly medium to coarse.Massif anorthosites,on the
otherhand,exhibit a much wider spectrumof textures,
commonly within individual outcrops. They range
from massiveto intenselyfoliated and lineatedrocks.
Someexhibit layeredstructures.Many anorthositesare
extremely coarse,among the coarsestrocks known,
composedof jammed-togethergiant crystalsof plagioclaseup to a meter long. Recrystallizationis common
in massif anorthosites,and a medium- to fi:re-grained
granoblastic matrix supporting bent relics of large
crystals of plagioclase is a particularly widespread
texture.In places,massif anorthositeshavebeencomminutedto the point wheresuperficialtythey re$emble
aplite.
In summaryothe texturesof anortlositesvaxyenormously,ran$ng from igneousto wholly metamorphic.
Grain sizeextendsfrom pegmatiticto aphanitic.Fabric
may be massive,intensely foliated and lineated, or
anyvvherebetweentheseexfremes
Anorthosite is a plutonic rock in the Dicrtonary
of Mining, Mineral, andRelatedTenzs(U.S.Bureauof
Mines), as well as in the first edition of the AGI
Glossary of Geology. By the second edition of the
AGI Glossary, anorthosite had become a plutonic
igneous rock, as indeed it is in R.S. Mitchell's
Dictionary of Rocks and the British Penguin
Dictionary of Geology,which alsoaddsthat it is coarse
grained.Anorthositeis a memberof the gabbrogroup,
according to both Russians(Geologiches6ySlovar,
Moscow) and Spaniards(Diccionario de Geologtay
CienciasAfines, Barcelona),whereasin Tomkeieff's
Dictionary of Petrologyit is, texturally at least,merely
a "granularrocK'.
Quantitative definitions of anorthositepivot upon
the modal compositionof the rock; that it is composed
ooalmost
wholly", "essentially",or "nearly exclusivelyo'
of plagioclase which, in addition, commonly is
describedas o'calcic"or oobasic".
The definitive IUGS
classificationstates:Q < 5; P(P + A) > 90; M < 10.
Aside from this unpoetic definition, the Penguin
Dictionary of Geology offers the only other current Modal composition
definition to setnumericallimits, calling on anorthosite
to contain more than 90Vo modal labradorite" bvAnorthositeis a plagioclaserock. Accessorymafic
915
REPORTON T}IE TERM'ANORTIIOSITE"
minerals are chiefly pyroxene, olivine, and oxides;
hornblendeandbiotite arerarer. Somemetamorphosed
anorthositescarry accessorygarnet.The highestmodal
contentof darkmineralsallowedis mostly unspecified,
althougha few authorsventureto cite 5 or llVa. One
form or another of a scale of rock types bridging
anortlosite with gabbro and spannedby such vague
terms as gabbroic anorthositeand anorthositicgabbro
is commonplace.Nevefiheless,it must be pointed out
that suchusagedoesnot conformto the IUGS scheme,
under which the color index of gabbro sweepsacross
the wide gap from anorthosite(CI < 10), to ultramafic
rocks(CI 2 90).
Rncommxpenoxs
In a personal effort to arrest what appearsto be
unendingdisorder in the use of the term anorthosite,
I offer the following six recommendations:
1. Anorthosite is a specific rock type, and the name
shouldat no time be dilutedby applicationto rocksthat
are not anorthosite.For this purpose,the IUGS definition (Q 3 5; P/(P+ A) > 90; M < 10)shouldbe adhered
to strictly.
2. The plagioclaseof anorthositeis commonlycited as
"calcic" or oobasic",with at least the allusion that it
must not be more sodic than labradorite.This is misleading becausethe plagioclaseof many anorthosites
(inclurling the type anorthosite:Feininger 1993) is
andesine.By the IUGS definition, the plagioclaseof
anorthosite may fall anywhere between Ano, and
An,*. To clariff, "andesineanorthosite",'labradorite
anorthosite",and so on are useful terms and may be
usedwhereappropriate.
3. A sampleof anorthositecharacterizedby a specific
mafic accessorymineral may advantageouslyhave its
name modified. Thus, olivine anorthosite,augite
anorthosite, and ilmenite anorthosite are legitimate
terms.For each,ofcourse,the color index ofthe rock
mustbe lessthan ten.
4. Anorthosite is a noun and must not be used as an
adjective to modify other rock names."Anorthositic
gabbro" is meaninglessand should be replacedby a
term suchas leucogabbro.
5. In orderto preservethe singularityandusefulnessof
the term anorthosite, such terms as 'ogabbroic
anorthosite" and "anorthositic gabbro" must be
abandoned,to be replaced by "leucogabbro" or yet
more specific terms as warranted by individual
situations.Suchusagecanbe preciseandsuccinct.One
examplesufnces.The gradationof an anorthositewith
accessory orthopyroxene (modal plagioclase, 97Vo;
enstatite,3Vo)nto a more mafic rock (modal plagioclase,887o,enstatiteol2%o)shouldbe describedas a
transition from anorfhositeto leuconorite.A few bare
words carry a detailedmessage.
6. Whereasmost anorthositesultimately have some
sort of igneousparentage,diversemassif anorthosites
havehad their primary textureserasedby deformation
and metamorphicrecrystallization.Suchrocks arebest
referredto as meta-anorthosite.
AcIc.{owHDGElvm.{Ts
Encouragementby severalof my colleaguesin the
GeologicalSurvey is much appreciated.I am particularly honoredby the commentsof Prof. F. Fitz Osborne
whosepersonalfield experiencewith anorthositenow
datesback 65 years,beginninglong beforeI wasborn.
REFERENCES
DELEssE,
A.-E.-O.-J.(1858): Recherchessur I'origine des
roches.BulI. Soc.G6ol. France,sdr. II,15,728-782.
Fnnvnonn,T. (1993):Geologyandgeophysicsofthe "type"
anorthosite,ChAteau-RicheqQu6bec.Can. Mineral. 31,
849-859.
hcrEpNerroNer-UMoN oF Grolocrcar, Scnncrs (IUGS)
(1989):A Ckusificationof lgneow Rocl<s
and Glnssaryof
Terms.Blacl<ttell Scientific Publ., Oxford U.K.
Jonerr'nsrn, A. (1937): A Descriptive Petrography of the
Igneow Rocla.3. TheIntermcdiateRocl<s.The University
of Chicagohess, Chicago,Tllinois.
LocaN, Sn W.E., Munnav, A., Htn'r, T.S. & Bnmlcs, E.
(1,863):Geol.ogicalSuney of Cannl4 Reporton Progress
to 1863. Dawson Brothers,
f'rom its Commencement
Montr6al, Quebec.
ReceivedOctober31, 1994.