amphibole-rich polycrystalline clots in calc-alkaline granitic rocks and their enclaves

20 Pages·2006·3.74 MB·English

1093
Canadian Mineralogist
Vol. 30, pp. lW3-1112 (1992)
AMPHIBOLE-RICPHO LYCRYSTALLINCELO TS
IN CALC-ALKALINGE RANITICR OCKS
AND THEIRE NCLAVES
ANTONIOCASTRO
Departamento de Geologia y Mineria, Universidad de Sevilla, E-21819 La Rdbida, Huelva, Spain
W. EDRYD STEP}MNS
Departmenotf Geographayn dG eologyU, niversiroyf St.A ndrewsS, t.A ndrewsF, ifeK Y169 STS, cotland
ABS'IRAC|
The presenceo f amphibole-ricpho lycrystallinea ggregate(sc lots)o f millimetrics izei s a widespreafde atureo f calc-alkaline
graniticr ocksa ndr elatedm icrogranularenclaveAs p. etrologicasl tudyo fthesec los, includingfi eldr elationshiptse, xturesm, ineral
chemistrya, ndc ompositionazlo ningh asb eenc arriedo ut to constrainp ossibleo riginsa ndp etrogenetisci gnificanceT. he two
plutonsc hosen,t he Quintanap luton in the Hercynianb elt of Iberia andt he Strontianp luton in the Caledonideso f Scotland,d iffer
in agea ndt ectonics ettingI.n both,a mphibole-ricchl otsa rep resenitn microgranulaern clavesa ndi n theh ostg raniticr ocks.T he
clotsa reg enerallyz onedT. he margini s rich in amphiboleb, ut with significanpt roportionso fbiotite andp lagioclasen,o rmally
intergrownw ith otherm ineralso f the groundmassT.h e interiori s dominantlyc omposedo f multiplec rystalso f amphibolein
mutualc ontacst howinga typicalg ranoblastitce xture.I ndividualc rystalsth eres howc ompositionazlo ning.I n generatle rms,t he
corei s actinolitea, ndt he rim, magnesiahno mblendeT. his "reverse"z oningc anb e explainedb y a complexr eactionw ith early
pyroxeneT. hisr eactionp robablyto okp lacei n two stageso, nep roducingm agnesiahno mblendein equilibriumw ith them elt,a nd
theo therproducinagc tinolitein thes olids tatef,r om reactiono fpyroxenea rmoredb y eadyh omblendeT. heo rigino fthe pyroxene
precursonto thesea mphibolec lotsi s notw ell constrainebdy thiss tudy,b utt hep rismaticfo rm of thesec lotsi n thee nclaveds oes
suggestth at thosep reservedin enclavesw ereo riginallyi diomorphicm agmaticp henocrystsw, hereasth osei n the hostg ranite
coulda lsoh avea restitico rigin.
Keywordsg: ranitec, lot,a mphibolec, alc-alkalinem, agmam ixing,r estite,Q uintanap luton,S pain,S trontianp luton,S cotland.
SolaNaeRs
I-a pr6senced 'agrdgaa millim6triquesp olycristallinsr iches en amphibolee st r6pandued ans les rochesg ranitiques
calco-alcalineest lese nclavesm icrogrenueqsu 'ellesc ontiennenNt. ousa vons6 tudi6c esa grdgatsle, ursr elationsd et errain,le un
texfures,e t la compositione t la zonation de leurs min6raux,a fin de placer des limites sur leur origine et leur signification
p'6trog6n6tiqukes. deuxs uitesp lutoniquecsh oisiesQ, uintana(c eintureh ercyniennieb 6riquee) tS trontian(C al6donideds'E cosse)
difFdrentd ansl eur 6ge et leur contextet ectonique.A ux deux endroits,l es agr6gatsr iches en amphiboles ont prdsentsd ansl es
enclavesm icrogrenuese t dans les rochesg ranitiquesh 6tes. Ces agrdgatss ont zon6s,e n gdndral.l ,a bordure est enrichie en
amphibole,m ais contientu ne proportioni mportanted e biotite et de plagioclase,n ormalemenet n intercroissancea vecl es autres
min€raux de la pdte.L 'int6rieure sts urtoutc ompos6d 'une multituded e cristauxd 'amphiboled ontl es contactsd 6finissenut ne
textureg ranoblastiquet. es cristauxi ndividuels sont zon6s.E n g6n6ral,l e coeure st fait d'actinote,e t la bordure,d e homblende
magndsiennCe.e ttez onation"i nvers6ep" euts 'expliquear um oyend 'uner dactionc omplexea vecu np yroxbnep rdcoceL. ar6action
aurait eu lieu en deux stades,le premier donnantu ne hornblendem agndsiennee n 6quilibre avec le magma,I 'autre produisant
I'actinotei 1'6tat solide,p arr6actiond upyroxbner ecouvertd eh ornblendep r6coceL. 'origine du pyroxbnep r6curseudr cesa grdgats
richese n amphibolen 'est pasb ien d6finiep ar nosr 6sultatsm, aisl a formep rismatiqued e cesa gr6gatsd ansl es enclavesfa it penser
qu'il s'agissaita u d6part,d ansc esc asd u moins,d 'un ph6nocristaml agmatiqueid iomorphe.C euxd u graniteh 6tep ourraienta ussi
6tre un mat6riau" restitique"'
oraduit par ra R6daction)
Mots-clis: granite, agr6gats,a mphibole,c alco-alcalin,m 6langed e magmas,r estite, pluton de Quintana,E spagne,p luton de
Strondan.E cosse.
r094 THE CANADIAN MINERALOGIST
IN'rRoDUcroN thep resenceo f solid material( restitef raction)c aniedu p
from depthb y the graniticm agma( Chappelle ta l. 1987).
2. Crystalc umulate:a n accumulationo f melt-precipi-
MaJic clots (millimetric aggtegateso f mafic miner-
tated mafic phasesb ecomese ntraineda nd dispersed
als) are remarkablyc ommon featureso f calc-alkaline
throught he magmab ody.
tonalites and granodioriteso f widely varying age and
3. Mafic remnantso f mingledm agmasr:e rnnantsfr om
tectonics etting( e.9.,h esnall & Bateman1 973,V ernon
the quenchinga nd dynamicd isruptiono f mafic magma
1983,C antagreel t al. 1984,Z nrpi et a/. 1989"C astro
in a lower-temperafurefe lsic magmad uring magmatic
1990,C astroe ra l. l99la).InI-type graniticr ocks,t hese
flow.
clots are most commonly amphibole-rich.T heir origin
4. Disaggregatede nclaves:a clustero fattachedm afic
hasb eenv ariouslya ttributedt o restite,a ccidentaxl eno-
mineralsf orms by the chemicalo r mechanicadl isaggre-
liths, cognatex enoliths,r emnantso f mingled magmas,
gation of a mafic enclave( either cognateo r accidental
mineral cumulates,p olycrystalline pseudomorphso f
xenolith).
phenocrystsa, ndc rystala gglomerationsw, ith effectso f
5. Pseudomorphs:p henocrysts recrystallize into a
recrystallizationw herea ppropriateA. knowledgeo f the
clustero f secondarmy inerals( Vernon1 983,M azzone
origin of suchc lots is most desirable,a s their presence
et al. 1987).
is generallyt akena s a surei ndicationo f one or other of
In severalo fthe abovec asesi,t mayb e appropriateto
thesep etrogeneticp rocessesa, nd their normally homo-
considert hat thereh as beena precursorp hase,s ucha s
geneousd isribution over a pluton or plutonicu nit is
pyroxene,t hatr ecrystallizesto an amphibole-dominated
taken as evidence of the widespreade xtent of that
assemblageT.h is precursoris requiredf or the formation
processI.t is very difficult, howevegt o designf ield or
of clots by the replacemento f phenocrystsb, ut could
petrographict ests to choosea mong these models; in-
also occur in the other modelsw heret he early-formed
deed, if recrystallization of clots has occuned, the
mafic phasew as pyroxenet hat later reactedw ith melt,
ultimateo rigin may be moreo bscure.I n volcanicr ocks,
or fluid, to form a new assemblaged ominated by
clots of similar form but rather different composition
amphibole.
also may occur,a nd havee venb eeni nferredt o control
the compositiono f calc-alkalines eries( Scarfe& Fujii
Gsor-ocrcArS- elI'INc
1987).I nterpretatioonf texturesin plutonicr ocksi s less
well constraineodw ingt o thep ossiblem odificationsb y
The two plutons chosen for this study are t}te
recrystallization during slow cooling. Thus, for this
study we have adopteda n approacht hat places most Strontian pluton in the Scottish Caledonidesa nd the
emphasiso n mineral compositionsa nd their patternso f Quintanap luton of theS panishH ercynian( Fig. I ). Their
zonation. common featuresi nclude: (a) mineralogicala nd bulk
In comparingw ithin-clota ndb etween-clocto mposi- chemicalc ompositions( metaluminousf)y pical of l-type
plutons,( b) thed ominanceo fgranodiorite(,c ) abundant
tional and textural variations.w e find the combination
ofback-scatterede lectroni magery (BEI) and electron- microgranulare nclaveso f diorite andt onalitec omposi-
microprobea nalysist o be particularly useful. Whereas tion, and (d) the presenceo f abundanta mphibole-rich
somei mportantt gxtural featuresa re visible using nor- clots.
mal polarized-lightp etrographicte chniqueso, therst hat
are controlled solely by bulk composition are only The Caledonianp luton of Strontian
revealedb y BEI (a Z-contrastte chniqueL: loyd 1987).
Thus BEI can guide the points for analysisp articularly The Strontianp luton( Fig. 1B) intrudedM oine ho-
efficiently and discloset extures and compositional terozoicm etasedimenrty andm eta-igneourso ck at 425
characteristictsh at would otherwiseo nly becomea ppar- Ma (Rogers& Dunning 1991)d uring the Caledonian
ent only with a high densityo f spote lecFon-microprobe orogeny in northern Britain. The metasedimentws ere
analyses. regionally metamorphosetdo the sillimanite zone of a
kyanite-sillimanite sequencef lilinchester 1974), and
thesew ereo verprintedbya na symmetricaml etamorphic
MODEIS FORTHE ORIGINoF CLoTs aureolet hatr eachesc ordierite- K-feldspara ssemblages
at the contact.T he asymmetryis attributedt o a gradient
, The literature on amphibole-rich clots in granitic in the regional metamorphici sograds at the time of
rocks is quite extensive.H ere,w e attemptt o summarize emplacemen(tA shworth & Tyler 1973).E mplacement
the models for their origin in terms of end-member of the pluton wasp assive,in part controlledb y contem-
processesc,o mbinationso f which may be possible. poraneousm ovementso n the GreatG len Fault (Hutton
I . Restite:a patcho f mafic phasess urvivest hem elting 1988ab ).
processa nd is transporteda longw ith the magma,e ither The plutoni s internallyz oned( Fig. lB), with an outer
unmodifiedo r afterrecrystallizationin equilibriumw ith nonporphyriticg ranodiorite( ONPG),i nsidew hich is the
theh ostm agmah. esnall & Bateman(1 973)i nterpreted outerp orphyritic granodiorite( OPG),a nd an inner core
clots in the Sierra Nevadab atholith to be indicatorso f of granite (IG). These plutonic membersa re easily
AMPHIBOLE.RICH CLOTS IN CALC.ALKALINE GRANMC ROCKS 1095
Los Pedrochesb atholith
tanaP luton
$
tl-
ffi Cordierirc monzogranitcs- S n6r domdns
/CIkm
@f Granodiorircs ,/ = E ouar domains
l-l
Frc. lA, B. Geologicals ketcha nd regional seningo f the two granitic plutonsc hosenf or study.A . The Quintanap luton (Spain).
B. TheS trontianp luton( Scotland).
mappedi n the field. The namesu sedh ere are thoseo f Detailed petrographicd escriptionsm ay be found in
theI UGS classification( Streckeisen1 976)a ppliedt o the thesep apers.T he boundaryb etweent he ONPGa ndt he
"tonalite", "porphyritic granodiorite",a nd" biotite gran- OPG is a transition over about l0 metres,w hereast he
ite", respectively,o f the earlier investigators( MacGre- OPG-IG boundaryi s quite sharp.T he outerm emberso f
gor & Kennedy 1932, Sabine 1963, Munro 1973). the pluton are host to somel arge bodieso f "appinite",
1096 THE CANADIAN MINERALOGIST
hornblendite- diorite seriesr ocks that showl obatea nd basiso f whole-rockg eochemicaslt udies(i ncludingc lots
crenulatel iquid-liquid contactsin dicatingc ontempora- butn ote nclavesS; tephenes/ a/.,i n prep.),t hed ifference
neous intrusion. Enclaves of microdiorite aspect are betweent he two facies is more textural than composi-
abundanti n the ONPG and OPG, locally occurring as tional.T he boundaryb etweenth e ONPGa ndt he OPG
sw.umso f apparentlyd isaggregatesdy nplutonicd ykes. is transitionala long the Loch Sunarts ectiono f the
The ONPG and OPG are characterizedb y abundant northerlyp arto f thep luton( Fig. 1B).T herei s ani nverse
amphibole-richc lots aligned within the plane of folia- correlation between the abundanceso f idiomolphic
tion: the idiomorphic crystalso f amphiboleg rew ran- amphibolea nd mafic enclaves,e xceptw heret here are
domly. The Sr isotopei nitial ratiosf or the pluton are swarmso f enclaves.
about0 .7053-O.706f0o r the ONPGa ndO PG.0 .7068- At Quintana,t here is no preferentiald isribution of
0.7072f or theI G, 0.7051-{.7066fo r the enclavesa. nd the clot-rich facies,w hich appeari n domainsr icher
0.7056-7060f or the appinites( Halliday et al. 1979, in amphiboleM. ost of the amphiboleo ccursi n clots,
Hamiltone t al.'|,980"H oldene t al. 1987\. with only a minor proportiono f isolatedi diomorphic
crystalsI.n generalt,h e Quintanag ranodioriteis more
TheH ercynianp luton of Quintana biotite-rich in comparisonw ith the Strontianr ocks,
in which amphiboled ominateso ver biotite. In both
The Quintanap luton (Fig. lA) is part of the Los plutons,t here is an appreciablec orrelationb etween
Pedrochebs atholith,o neo fthe largestg raniticb atholiths thea bundanceosf enclavesa ndc lots.T hatt hee nclave-
of the Iberian massif. It was emplaceda s an elongate rich faciesa re also richer in clots perhapsin dicatesa
body parallelt o the NW-SE trendo f regionals tructures, common genetic relationshipb etweene nclavesa nd
at the southem branch of the Central Iberian Zone clots.T he samec orrelationh asb eeno bservedin other
(Julivert et al. 1974).T he pluton was emplaceda t high calc-alkalineg raniticb odiess ucha st he CentraSl ystem
levels into Upper Paleozoicm etasedimentas,n d em- batholitho f Spain( Castroe t al. 1991b)a, ndt he Sierra
placementt ook place after the first phaseo f Hercynian Nevadab atholitho f California( hesnall & Bateman
deformation, and probably during the second phase, 1973't.
which was dominatedi n this sectoro f the chain by EnclavesI.n generala, ndc ertainlyin thet wo plutons
intracontinentaslh earm ovement(sC astro1 985,1 986). consideredh ere,n ot all mafic,m icrogranulaer nclaves
Full descriptionsa nd a petrogeneticin terpretationw ere contain amphibole-richc lots. Clots are particularly
recentlyp resentebdy Castro( 1990). associatewdi th thes o-calledtr ansitionael nclave(sT RE)
The pluton is a medium-grainebdi otite hornblende of the Quintanap luton (Castro 1990), where clots
granodiorite.S everalf aciesa rei dentifiedo n the basiso f represenmt oret han lOVob y volumeo f the enclaveA.
their amphibole content (normally 0-57o) and color similart ypeo f enclaveis alsoc ommonin Strontiana nd
index.H owever,t hesef aciesd o not showa ny regular other Caledonianp lutons( e.g.,t he Criffell pluton, as
distributionw ithin the pluton,a ndr hep lutoni s consid- describedb y Holden et al. 1987). Characteristicso f
eredt o be essentiallyu nzonedo n a broads cale.M afic thesee nclavesa re: (1) a fine-grainedm atrix, (2) the
microgranulaern clavesa rea bundan(tl -57a) andm ore- presenceo f plagioclasem egacrysths avinga resorbed
or-lesse venlyd istributedo ver the pluton.T hreet ypes core,a nd (3) the presenceo famphibolei n the form of
of enclavew ere classifiedb y Castro (1990) as: (a) clots varying in abundanceb etween5 Voa nd 307ob y
porphyritict onalitee nclaves(P TE),( b) hornblende-pla- volume. The fine-grainedm afix of these enclaves
gioclased iorite enclaves( HPDE),a nd (c) transitional typically has a serial texture and comprisesb iotite,
enclaves( TRE) that have features of both PTE and plagioclasela ths, quartz,a mphibole,a ciculara patite
HPDE.P etrographidce scriptionosf thesety pesa rea lso and other accessorym ineralsi ncluding titanite, zircon
giveni n Castro( 1990). and opaquem inerals.P oikilitic quartz is a feature of
many tonalitic enclaves( Castro et al. 1,991c)a nd
Clot-bearingr ocks in the Strontiana nd synplutonicd ikesi n zoneso f magmain teraction(C astro
Quintanaplutons et al. l99la), suggestinga complexh istoryo f crystal-
lization.T he two othert ypeso f enclaves( porphyritic
In both plutons, amphibole-rich clots tend to be tonalites,P TE and hornblende-plagioclasdeio rites,
preferentially concentratedin the more mafic granitic HPDE) arep ooreri n amphibolec lots.A mphibole-rich
faciesa nda lsoi n certaint ypeso fmicrogranulare nclave. enclave(sH PDEtype)w, ith modala mphibolea pproach-
Granitic rocl<sA. t Sfontian, amphibole-bearincgl ots ing 907o,a re presenti n both plutons,b ut are more
are abundanti n the ONPG. They are also present, abundanitn the Strontianp luton; clots arel essa bundant
however,w ith somewhatle ssera bundancei.n both the in this type. The PTE have few if any clots; these
OPG and ONPG facieso f the pluton. It is importantt o enclaves are particularly abundant in the Quintana
note that the ONPG, which is richest in clots, has few granodioriteb ut are very scarcea t Strontian,g enerally
crystals of idiomorphic amphibole, whereas in the supporting the correlation noted between amphibole
porphyritic granodioritest,h e modalp roportiono fidio- content of the host granite and the abundanceo f
morphica mphiboleto clotsi s approximatel9y: l . Ont he amphibole-ricehn claves.
AMPHIBOLE.zuCH CLOTS IN CALC.ALKALINE GRANITIC ROCKS 1097
Tnxrunal- FsafllnEs oF AMprilBoLEC lors ranges from 2 to 8 mm (Figs. 2, 3); we note a
correspondencebetweesinz eo fthe clotsa ndt hea verage
Amphibole-bearingc lots, as describedf rom batho- grain-sizeo fthe rock. In flne-grainede nclavesc, lots are
liths from a wide variety of orogenics ettings,a ppeart o typically smallert han thoseo f the coarser-grainedh ost
ber emarkablys imilar in their texturala ndc ompositional granite,b ut in both casesu, nusuallyl argec los (> l0
characteristicsW. e have studiedt hesef eaturesi n both mm) may be present.
the Strontiana nd Quintanap lutons.
Shape
Size
In undeformedg ranitic rocks, amphibole-richc lots
Amphibolec lots vary from lesst han I mm up to a few typically have a round shape,w hereasi n deformed
cm, but with an even grain-size for the individual rocks,i ncluding the marginalf aciesa nd enclaveso fthe
crystals of amphibole. However, the dominant size Strontianp luton (Hutton 1988b),a mphibole-richcl ots
FIc. 2. Aspecto f a clot-bearingt onalite of the Strontianp luton.N ote the homogeneoussi ze
of the clots (darkers pots).
1098 TTIE CANADIAN MINERALOGIST
Ftc. 3. Comparisonsb etweens hapea nd size of amphibolec lots in different rock types of the Strontiana nd Quintanap lutons.A
and C aret he Quintanag ranodioritea ndr elatedt onalitic enclavesr, espectivelyB. andD aret he Strontiant onalitea nd related
enclavesr, espectively.N ote the compositionalz oning in the clots of B; the lighter core is richer in actinolite. Scaleb ar = 6
mm.
have ellipsoidal shapesw, ith the principal axis aligned (quafiz and K-feldspar) suggestst hat clots were de-
parallel to other deformation-relateds tructuresi n the formed when the rock was in a magmatic state.T he
rock. In the case of deformed rocks, the absenceo f absenceo f any kind of intracrystallined eformationi n
intracrystallined eformationi n the latestm ineralp hases the amphiboleo f theseo rientedc lots may indicaret hat
AMPHIBOLE-RICH CLOTS IN CALC.ALKALINE GRANMC ROCKS 1099
somer ecrystallizationm ay haveo ccurrede itherd uring, ous growth in the solid state.M ost biotite occursn ear
or after, magmaticf low. The fact that the textureso f the rim, in part encircling the clot and intergrownw ith
thesec lots do not correspondto thoseg enerallya ttrib- the outermosta mphibolec rystals,a nd overlapsi n the
utedt o magmaticp rocesse(sa si llustratedb elow) makes crystallization sequencew ith the groundmassp lagro-
it difficult to interprett he history of clots. Specifically, clasea nd quafiz. Quartza nd opaquem ineralsa ref ound
it is not possiblet o demonstrateth at they formed from asi nclusionsin amphibolew, ithquartzoccurrinags v ery
the crystallization of a silicate melt during laminar small vermicular-likeg rains typical of symplectitic
magmaticf low. intergrowths.O paquem ineralsa re principally sulfides
Occasionally,a mphibolec lots show approximately (pyrite and chalcopyrite),e xcepti n a few casesw here
prismatics hapest;h is featurei s mostc ommonlys eeni n probablec hrome-richs pinelw asq ualitativelyid entified
clots within mafic enclaves,w hereast he clots of host with the electronm icroprobe.A patite and intergranular
granites tend to be more inegular, and generally titanite are common phasesi n amphibole-rich clots.
roundedT. hep rismatics hapeis conspicuouisn theT RE Apatitet lpically has an acicularh abit and appearsa s
of the Quintanag ranodiorite (Fig. 3C) and in similar inclusionsin amphibolew ithin the outerp artso f clots.
enclaveso f the Strontianp luton.A comparisonb etween
clot shapei n enclavesa nd in related granitic rocks is
showni n Figure3 . This featurem ay be indicativeo f
pseudomorphisomf earlyf erromagnesiasnil icates( py- TABLE 1. REPFESENTATIVEF LECTRON-MICROPROBED ATA
roxene,o livine), as describedb y Vernon (1983) and ON AMPHIBOLEF ROMT HE QUINTANAG RANODIORTTE
AND RELATEDE NCLAVES
Tnrpi et al. (1989).
Texture Rml( typ€ PT snct. PT eml. PT eml. TR €nd. TR mcl. GsEd. G6od
sample G2 c2 O2 c3 B3 D-l D-l
Analysb 10 12 37 2, 4t g.ga 19
A major difficulty with the interpretationo f rextures Desipdon Oor clot oot Llatix clot Oot Matix
of amphibole-richc lots is that they do not exhibit the
Sio2wto/o 50.66 51.16 50.94 50.61 48.68 47.13 49.91
texture typical of serial crystallization in magmatic Ttrc2 0.5r 0.17 0.49 0.62 O.7e 0.95 0./l5
rocks.M ost clotsa rec haracterizebdy a granoblastic-like At203 3.55 S.49 3.57 4.17 5.34 5.92 4.12
CY2O3 0.04 0.04 0.00 0.2 0.14 0.6
texturei n which individual grains display near-120o
FeO 10.34 7.57 8.51 11 11.75 12 10.7
triplejunctions(F ig.4).T hist extureis widespreaidn the Fo2@' 2.79 5.87 5.73 2.35 29 2.24 e69
core of t}rec lots,i n which amphibolea lonei s present. F€ot 12.85 12.€5 19.66 19.12 14.36 14.01 13.12
lt.lr|o 0.47 0.63 0.55 o.17 0.48 0.48 0.42
Toward t}le rim, plagioclaseo r biotite or both may be MO 0.03 0.04 0.05 0.02
present together with amphibole. In these marginal t go 14.41 14.61 14.66 14.01 1S.A 12.U 13.@
domainst,h et extureis serial,a ndp lagioclasaen db iotite CaO 11.68 10.86 11.48 11.8 11.89 11.1 11.34
lla2o 0.7S 0.64 0.65 0.62 0.95 o.Ss 0.&
fill intersticesb etweeni diomorphic crystalso f amphi Kn 0.3 0.9 0.28 0.4 0.52 0.6 0.31
bole (seeb elow,F ig. 9A). This sequentiala, pparently
magmatic,t exture is best developedi n the outermost TOTAL 95.52 95.66 96.98 95.24 96.@ 91r.35 94.42
partso f the clots,i n which amphibolei s intergrownw ith Str|rtel iffmulae (O-3)
other minerals of the groundmass,n amely biotite, q 7.41 7.495 7.412 7.443 7.N 7.216 7.85
plagioclasea ndq uartz.G enerally,t his concentricz oning Axr(0 rv)) 0.5089 880.505 0.5s8 0.55s/ 8808.795 0.7U 0.537
of texture is reflected in a weak optical zoning of
amphibole,w ith the marginala mphiboleh aving deeper Ar( vr) 0.113 0.@ 0.025 0.167 0.137 0.285 0.t91
pleochroicc olorst han amphibolei n the core. ri 0.057 0.051 0.05s 0.069 0.088 0.11 0.05
Cr0+ 0.@4 0.@4 0.01 0.023 0.015 0.67
FBS +' 0.311 0.651 0.6s 0.26 0.s24 0.268 0.303
Rehrtonshipsw ith other minerals Fo2+ 1.41 0.9tts 1.041 1.355 1.456 1.59S 1.U2
Mn 0.06s 0.071 0.067 0.059 0.081 0.06t] 0.053
M 0.003 0.@4 0.@5 0.@2
Biotite, plagioclaseq, uafrz,t itanite and opaquem in- MS 3.17t 9.191 3.17S 3.O7 2,919 2.718 3.056
eralsa rec ommonlyp resentin modalq uantitiese achl ess x(c) 5.003 5.01 5.(X}9 6.0q1 5.0G 5.@1 5.@2
than 5Voi n amphibole-richc lots. Plagioclaset ypically
Fl2+ 0.003 0.01 o.@9 0.@1 0.@ 0.001 0.@2
showsa n intergranularte xture,i ndicatingt hat it formed 't.851 1.705 1.79 1.959 1.885 1.921 1.&.
as a late-crystallizingp hase.T he compositiono f this Ba
plagioclaseis similart o that ofplagioclasei n the host >l,la( B(M)4) 0.14t 7 01.2.s8o5 i.09.92 0.1242 202.112 0.178 0.178
granite. These crystals are normally zoned, with the
morea lbitic, extemalb andsc ontouringa mphibolec rys- t'la (A) 0.062 0.037 0.161 0.084 0.059
K 0.056 0.055 0.059 0.078 0.097 0.118 0.059
tals. However, lobate amphibole-plagioclasec ontacts r(A) 0.118 0.055 0.053 0.112 0.259 0.42 0.117
also arep resent,s uggestings imultaneousg rowth. Simi-
lar intergranulart exturesa re shown by biotite crystals Toblcdm 15.118 14.952 15.0s5 15.112 f5.259 15.n2 1F.117
%An in Plsg. 26.65 30.06 38.4t 21.* n.41
closet o therim of thec lots.H owever,b iotite maye xhibit
triple junctions with amphibole,s uggestings imultane- ') Caloiabd (s t€td)
I 100 THE CANADIAN MINERALOGIST
MINERALC gstvrrsrnv Amph ibol e compo s i tions
Silicatep hasesin clots were analyzedb y electron Representativree sultso f analysesa rel isted in Tables
microprobe( JEOL JCXA733 Superprobe)a t the Uni- 1.a nd2.S tructuralf ormulaew erec alculatedo n theb asis
versity of St. Andrews, with more than 500 point of 23 oxygen atoms, with recalculationo f ferric iron
analysesp erformed.A nalysesw ere carriedo ut using a basedo n the maximump er formula unit (pfu) to satisry
beamc urrento f 20 nA and an accelerationp otential of stoichiometr(yR obinsone ra /. 1982)w, ith thei dealt otal
l5 kV. References tandardsw erea combinationo f pure of cations set at 13 (exclusiveo f Ca Na and K).
metals and silicate minerals; the ZAF procedurew as Amphibole compositions,i ncluding those from clots
usedt o conect apparentc oncentrationsB. ack-scattered andi diomorphicc rystalsin bothh ostrocksa nde nclaves,
electroni maging (BEI) was used to reveal featureso f displaya wider angeo f Si pfu, from 6.8t o 7.8 ass hown
compositionazl oning in amphibole,a nd texturala nd in theI MA classificauon(F ig. 5; trake 1978).[ n most
reactionr elationshipbse tweena mphibolea ndc ontigu- cases, the amphibole is magnesianh ornblende and
ousp hases. actinolitic homblende;p argasitich ornblendeo nly ap-
l10l
tryryrffi{
Rc. 4. Texturalr elationshipisn severaal mphibole-ricchl otsf rom microgranulaern claves
of the Quintanag ranodiorite.A . The mineralsa t the edgea re intergrownw ith minerals
ofthe fine-grainemd atrix.B . Polygonapl ackingw, ith abundanttriplejunctionbse tween
differentamphibolcer ystalsa ndbetweeanm phibolea ndb iotite( arrow)C. .Intergranular
texture is displayed by plagioclase (arrow) near the edge of the clot. Scale bar
correspondtso 3 mm in A, and I mm in B andC .
pearsin theS trontianp lutoni n thec oreo f idiomorphic the host granodioritea nd within enclaveso f the Quin-
crystals. tanap luton (Fig. 7B). However,a nalogousd ifferences
Complexc oupleds ubstitutionbse tweenv ariousc at- are not found in the amphibolec rystals from the
ions in differenta mphiboles itesa re common.T heir Strontianp luton (Fig. 68). It mustb e notedt hat therei s
identification is possible using simple schemes a smallerd ifferencein bulk compositionb etweene n-
(Czamansk&e Wones1 973,B lundy & Holland1 990) claves and host in the Strontianp luton (both are
in which any amphibolec ompositionis relatedt o the tonalitesc) omparedw ith theQ uintanap luton.
tremolite (Tr) end-memberC. hargec ompensationd ue The linear correlationb etweenI vAl and other tetra-
to the introduction of Al in tetrahedrals ites may be valenta nd trivalentc ations( Figs.6 ,4,,7 A) is betteri n
balancedi n various ways, the most cornmoni n natural amphibolefr om Strontiant han in that from Quintana,
and synthetica mphibolesb eing incorporationo f alkalis but in both casesc orrelationsa rel esss ignificantt hanf or
in theAs ite( Blundy& Holland1 990)T. hisi st hee denite the Ed-types ubstitutionT. his indicatesth at otherc ou-
(Ed)-types ubstitution,f ormulateda s: pleds ubstitutionisn volvingh ornblende(H bl),p argasite
(Prg) and tschermakite( Ts) are rather less important
Si +no = lvAl + Naa than the Ed-types ubstitutioni n determininga mphibole
compositionin thesetw o plutonsC. ombinationosf these
Anotherc ommonw ay of achievingc hargeb alanceis
by incorporation of octahedrally coordinated Al in substitutionsa re shown [Fig. 8, basedo n Blundy &
Holland( 1990)1T. hed ominanct ompositionavle ctorfor
substitutiono f divalentc ationsin M1-M3 sites.T his is
both plutons startsf rom the Tr end-membear nd moves
the tschermakite( Ts)-types ubstitution,f ormulateda s:
towarda positioni ntermediatbee tweenE d andh g but
Si + Rff,r,_r.,- IvAl + vrAl rather closer to Ed. The dominant substitutionsa re
similar for the amphibolec ornpositionos f clots from
Figures6 and 7 show cation plots usedt o identify enclavesa ndh ostg ranitei n both plutons.G raniticr ocks
these coupled substitutionsE. d-type substitutioni s andr elatede nclavesa re similar in overall bulk compo-
dominantin the amphibolesa, s evidencedb y the good sition,b ut the PT conditions( mainlyP ) duringc rystal-
conelation between tetrahedrallyc oordinatedA l and lization were different. The Quintanap luton was em-
alkalis in the A site, for amphibole from both the placeda t shallowerl evelst han the Strontianp luton.
Strontiana nd Quintanap lutons.T his plot also shows Total Al content is strongly dependento n T, as
compositionald ifferencesb etween amphibole within demonstratebdy Blundy& Holland( 1990),b ut balanc-
tta2 THEC ANADIANM INERALOGIST
TABLE2 , REPRESENTATTEVLEE CTRON.MICROPRODBAET AO IiIA MPHIBOLEFR OM
THES TRONTIAN.GRANMRCO CKSA NDR ELATEDE NCI.AVES
Rek type Tmallb Ends Etrlm Tmalhe Tonallb EndaF Erdaw &dm EndaE Emlaw llybtd Hybrid
sample sRz sR2 sR2 sR4 SR4 SRo gRB sR6 sR6 SRB sFo gRE
Anat6ls 8 15 & U 41 a(@n) 5(rtn) l9(@D)28(dm) 36 2S(coro)24(ttft)
Doscdpdon ldbm. ldom. ml.Plag ldom. Fumclot Clot Clor Oor Cbt Rimclot Oot Clot
Sfo2wto, 8,m O.78 47.48 48.00 48.46 62.s /a.49 55.d2 50.94 /15.34 55.30 51.84
1io2 0.30 |u 0.96 1.06 0.96 0.2, o.75 o.to 0.46 1.21 0.10 0.50
4f203 4.53 6.dr 8.05 6.68 6.16 L72 5.80 1.38 4.42 7.8 1.52 3.73
G203 - 0.01 0.06 0.06 0.03 0.05 - 0.10 0.01 0.03 0.07
F6O S.&f 9.At 10.11 10.70 9.97 6.88 9.19 5./tt 8.01 1r.18 5.18 7.79
Fe2O3' 4.63 5.08 4.U 4.42 4.3s 4.61 4.52 3.80 4.73 4.5't 9.03 3.e
Mno O.a7 0.29 0.35 0./O' 0.31 0.35 0.36 0.38 0.28 0.n 0.20 0.&
Nlo O.O2 0.04 0.03 0.m 0.14 0.07 0.05 0.@ 0.08
MSO 15.33 13.60 13.74 13.31 13.V 16.4 14.27 17.@ t5.50 1a&4 18.68 16.64
CeO 11.96 ll.te 11.7s 11.71 11.69 11.73 11.67 11.85 11.76 11./A 12.4 12.24
Na2O 0.91 '1.44 1.11 1.19 1.03 0.56 't.21 0.35 0.82 1.51 0.32 0,74
l<n 0.36 0.78 0.62 0.68 0.57 0.4 0.58. 0.@ 0.31 0.80 0.@ 0.34
TOTAL 57,62 57,31 98.3r! 98.22 97.34 96.92
Sn|shmlbmulae (O-23)
7.@9 7.M 7.U4 6.7n 7.7U 7.44
At0v) 0.681 o.s6 0.s59 1.@1 0.914 0.404 0.917 0.200 0.656 1.m. 0.216 0.s74
tcD 8.0@ 8.o0 8.000 8.@0 8.0e 8.0o 8.@ 8.0@ 8.0@ 8.00 8.@ 8.@
Al (VD 0.090 0.147 0.OSg 0.145 0.1,4 0.056 0.@8 0.030 0.@5 0.168 o.@ 0.66
11 0.0311 0.114 0.107 0.116 0.105 0.024 0.082 0.010 0.050 0.139 0.010 0.054
cr& - 0.ol 0.@8 0.007 o.@4 0.06 - 0.011 0.@2 0.@4 0.o8
Fegr' 0.505 0.662 0./162 0.486 0.484 0.5@ o.afii 0./O7 0.515 0.5@ 0.321 0.s59
Fe2+ 1.0115 1.174 1.257 13@ 1.8 O.8A 1.127 0.7/2 0.9@ 'l.rl02 0.728 0.S5
Mn 0.045 0.@6 0.0/t4 0.049 0.09 0.043 0.044 0.0i16 0.035 0.0i8 0.024 0.039
i0 - - o.@3 0.@4 0.0@ 0.o2 0.015 o.@ 0.006 0.010 0.m9
Mg 9.N 2.972 9.038 2.892 3.@2 3.541 3.108 g.ru 3.3it1 2.750 3.913 3.552
>(o) 6.006 6.@7 5.@7 5.010 6.008 5.@5 5.@ 5.019 5.014 5.012 5.012 5.012
R2+ 0.006 0.O7 0.007 0.010 0.008 0.005 0.@9 0.019 0.014 0.012 0.012 0.012
Cs 1.849 1.753 1.872 1.82s 1.831 1.802 't.s28 1.800 1.817 1.8ri1 1.873 l.S7r
l,,la(M4) 0.146 0.2/O 0.'121 0.161 0.161 0.166 0.155 0.@5 0.170 0.15'/ 0.@7 0.t@
>(B) e@ a@ 2.000 2.000 2.@ 1.S 2(rl0 1.914 2.000 2.@ 1.972 /@
r\b(A) 0.1€ 0.1@ 0.108 0.174 0.132 - 0.176 - 0.061 0.282 - O.@7
K 0.067 0.142 0.118 0.127 0.16 0.040 0.108 0.016 0.058 0.t6tr 0.015 0.002
>(A) 0.171 0.311 0.316 0.301 0.23s 0.040 0.284 0.016 0.117 0.415 0.015 0.159
ToiElcadm: 15.182 15.318 15.323 15.311 15.2,181 5.@ 15.2S9 14.949 15.131 15.447 14.9S 15.171
%&t in Pla( 16.17 21.50 20.80
') Calculalgd( s ren)
ing ofchargesb y coupleds ubstitutiondse pendms oreo n differencesa re very similar, and the boundaryb etween
componenat ctivitiest hanP T conditionsE. d-types ub- adjacenzt onesis sharp.
stitution is dominanti n amphibolec rystallizedi n equi Zoning in the core is displayedb y the amphibole
librium with plagioclaset,h e norm in calc-alkaline grains displaying a granoblastict exture, in the central
magmas. part of the clots (Figs. 9A, B). Coresh avea n actinolitic
compositionri cheri n Mg and Si andp ooreri n Al, Fe,
Compositionazl oning in the clot amphiboles and alkalis. than the rim" which is hornblende in
compositionT. hes hapeo f actinoliticc orei s moreo r less
A significant feature of the clots is the presenceo f inegular, but generallyf ollows the externals hapeo fthe
discontinuousr,e gular and sectorz oning in the amphi- zonedc rystalsF. igurel 0A is a compositionaplr ofileo f
boles. There exists a compositional gap of abott 57o a zoned crystal in a clot within the Strontian NPG.
SiO2a nd 2VoM gO (seeT able 2). Geometricallyt,h e Zoning, althoughr evealeda sb eing sharpb y BEI @igs.
compositionalz oning appearsi n three different situ- 9A, B), does not appeart o be sharp in terms of the
ations:( 1) core zoning,( 2) oscillatoryz oning,a nd (3) Mg(Mg+Fe) ratio, indicating either that this parameter
patchy zoning. In all three cases,t he compositional was not important in the primary zonation or that