geochemistry and petrogenesis of the tibbit hill metavolcanic suite of the appalachian fold belt ...

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[AMERICANJOURNALOFSCIENCE,VOL.299,MARCH,1999,P.210–237]
GEOCHEMISTRY AND PETROGENESIS OF THE TIBBIT
HILL METAVOLCANIC SUITE OF THE APPALACHIAN
FOLD BELT, QUEBEC-VERMONT: A PLUME-RELATED
AND FRACTIONATED ASSEMBLAGE
ABDEL-FATTAH M. ABDEL-RAHMAN* and P. STEPHEN KUMARAPELI**
ABSTRACT. The Late NeoProterozoic metavolcanic Tibbit Hill Formation
(THF)coversalargecrustalsegmentexposedalongabeltabout250kmlongin
the Appalachian fold belt (Quebec-Vermont). It is predominantly basaltic in
compositionbutcontainsaminorcomponentoffelsicandintermediatealkaline
volcaniclithologies.
Geochemically,theTibbitHillFormationformsacontinuumincomposition
and exhibits a wide range of SiO (44-76 wt. percent), covering the entire
2
spectrumfromalkalibasalttotrachyteandcomendite.Thismildlyalkalinesuite
is relatively enriched in incompatible elements and exhibits a wide range of Zr
(138-1493 ppm), Nb (15-139 ppm), and Y (18-185 ppm) concentrations, among
otherelements.TheconcentrationsoftheHFSEandtheREEgraduallyincrease
towardthemoreevolvedlithologies.ThechondritenormalizedREEpatternsare
fractionated (LREE-enriched over HREE), parallel to subparallel, and generally
uniform but with negative Eu-anomalies developed in the more felsic varieties.
Thesegeochemicalfeaturesunderlinethecomagmaticnatureoftheentiresuite
andareconsistentwithafractionatedbasalttocomenditesuites.Theincompat-
ibleelementprofilessuggestthatmostoftheseelementsincludingNb,Zr,Ti,Y,
andtheREEhavenotbeenaffectedbymetamorphism,astheyremainedlargely
intactwithintheTHFrocks.
Chemical features of the mafic rocks are typical of within-plate basalts and
suggest that their melts were derived from a fertile or plume-related mantle
source. Chemical features of the felsic and intermediate rocks are typical of
anorogenicA typesuites,relatedtohotspots,mantleplumes,orcontinentalrift
1
zones.Thisisconsistentwiththeregionalgeologicalcontextwiththevolcanism,
associatedwithanIapetanRRRtriplejunction,occurringshortlybeforetheonset
ofseafloorspreading.
Geochemical modelling shows that the THF basaltic magma was produced
by a very small degree of batch partial melting (F(cid:1)2.5 percent) of a garnet-
bearingprimitivemantlesource(garnetlherzolite).Afinalbasalticmeltsegrega-
tion depth is estimated at 80 to 100 km. Melting probably occurred within the
thermal anomaly of a rising mantle plume beneath the Sutton Mountains triple
junction(neartheQuebec-Maineborder).FractionationofTHFbasalticmagma
produced minor trachytic and comenditic magmas. The volcanic assemblage of
theAfarrift(forexample,Boinacentre)appearstorepresentamodernanalogue
totheTHFvolcanicsuite.
INTRODUCTION
Variablyaltered,metamorphosed,anddeformedvolcanicassemblages,thoughtto
be related to Late NeoProterozoic continental rifting (Bond, Nickeson, and Kominz,
1984),havebeenidentifiedatseverallocalitiesonthewesternflankoftheAppalachians
(Aleinikoff and others, 1995). Trans-Atlantic correlatives are probably represented by
volcanicpilessuchastheTayvallichvolcanicsintheScottishDalradians(Grahamand
Bradbury, 1981; Halliday and others, 1989). Of all these volcanic suites, the one that
makesuptheTibbitHillFormation(THF),exposedalongabeltabout250kmlongin
theAppalachianHumberZoneofsoutheasternQuebecandnorthernVermont(fig.1),is
insomewaysunique.First,itsageofabout555to560Ma(Kumarapeliandothers,1989)
*DepartmentofGeology,AmericanUniversityofBeirut,Beirut,Lebanon
**DepartmentofGeology,ConcordiaUniversity,7141SherbrookeStreetWest,Montreal,QC,Canada,
H4B1R6
210
211
A.-F.M.Abdel-RahmanandP.S.Kumarapeli
makesittheyoungestofthevolcanicsequences.Thishasledtothesuggestionthatthe
Tibbit Hill volcanism took place just before the onset of seafloor spreading and not
during rift initiation some 35 Ma earlier (Kamo, Krogh, and Kumarapeli, 1995).
Secondly,itistheclearestexampleoferuptionatanRRR(rift-rift-rift)triplejunction(the
Sutton Mountains triple junction) in a setting similar to that of the Afar triangle
(Kumarapeli,Goodacre,andThomas,1981).Theriftingeventtookplaceasapreludeto
theopeningoftheIapetusocean.Faill(1997)arguesthatthenameIapetusforthisocean
isinconsistentwithoriginaldefinitionsandthatitshouldbecalledTheia.However,we
usethenameIapetusinthispaperwiththeunderstandingthatitreferstothePaleozoic
oceanoffthe‘‘east’’marginofLaurentia.
Fig.1. MapshowingthesurfaceandsubsurfaceextentoftheTibbitHillFormation.Alsoshownare:(A)
probablelocationofandtectonic-magmaticfeatures(suchastheOttawaGraben,Grenville-,andAdirondack
dikeswarms)relatedtotheSuttonMountainstriplejunction,and(B)boundariesoftheAppalachianfoldbelt,
St.LawrencePlatform,andCanadianShieldinthemaparea.Inset:locationmap.Sources:Kumarapeli(1985),
Coishandothers(1985),Kumarapeli(1993),St.SeymourandKumarapeli(1995).
212 A.-F.M.Abdel-RahmanandP.S.Kumarapeli
The THF is predominantly composed of basalts. Previous geochemical studies of
thebasaltswererestrictedmainlytotwowidelyseparatedsegmentsofthevolcanicbelt
(fig.1):oneinVermont(Coishandothers,1985)andtheotherintheRichmondareaof
Quebec(Pintson,Kumarapeli,andMorency,1985;Pintson,1986).Thesestudiesshow
the effect of metamorphism and alteration on the incompatible elements was minimal.
ThepresenceofcomenditicrocksintheTHFoftheWaterlooarea(fig.1)suggestedthat
the Tibbit Hill volcanic suite is bimodal (Kumarapeli and others, 1989). However, our
studiesshowthattheTHFalsocontainsaminorcomponentofintermediatelithologies
(seebelow).
New chemical data have been obtained for 18 samples of the recently recognized
intermediatelithologiesoftheTHFandfor10basalticsamples,allfromtheRichmond
and Sutton areas in Quebec (fig. 1). Additional data on basalts from Vermont and
Richmond have been taken from Coish and others (1985) and Pintson (1986), respec-
tively.DataforthefelsicmembersoftheTHFaretakenfromtheonlyavailablestudyof
these rocks (10 analyses from the Waterloo area in Quebec, Kumarapeli and others,
1989). Samples for which the concentrations of major and trace elements are available
have been used in this investigation. Our aim is to (A) present the geochemistry of the
Tibbit Hill mafic to felsic volcanic assemblage based on a larger data set from various
partsoftheentirevolcanicshield;(B)proposeamodelfortheoriginoftheTibbitHill
magma,andinvestigateitspossibleconnectiontomantleplumes,(C)presentamecha-
nismfortheformationoftheobservedspectrumoflithologies,(D)furtherinvestigatethe
mobilityofelementsinresponsetothemetamorphicepisodethathasaffectedtheTHF
basalticrocks,(E)comparethecharacteristicsofvolcanicproductsattheabout560Ma
SuttonMountainstriplejunctionwiththoseatamorerecentone,namelytheAfartriple
junction,and(F)discusstherelationshipbetweentheTHFvolcanicrocks,theGrenville
dikeswarm,andtheAdirondackdikeswarm.
TheTibbitHillFormation
TheTHFoccursatorneartheexposedbaseoftheriftfaciesvolcanic-sedimentary
wedge that formed on the Grenvillian (approx. 1 Ga) rocks of the rifted margin of
Laurentia.InQuebec,theriftfaciesrockscomprisetheLowerOakHillGroup,andthe
THF, whose base is not exposed, is the lowest recognized formation. No fossils have
been found in the Lower Oak Hill Group, but Early Cambrian fossils have been
recordedfromtheoverlyingGilmanFormationoftheUpperOakHillGroup(Fritzand
Yochelson,1988).ComenditicrocksintheWaterlooareahavebeendated(U-Pbzircon)
at554(cid:1)4/(cid:2)2Ma(Kumarapeliandothers,1989),butthemaficrocksmayhavebegun
forming around 560 Ma. It should be noted that the only known rift-related igneous
eventofregionalsignificanceolderthantheTibbitHillvolcanismistheemplacementof
alkalic-carbonatitic complexes along the Ottawa and Saguenay grabens in eastern
Canadawithanageof565Ma(K-Arage,Gittins,MacIntyre,andYork,1967;Doigand
Barton,1968).
In Vermont siliciclastic metasediments, including conglomeratic units of the Pin-
nacle Formation (Camels Hump Group), make up the basal rocks of the rift facies
sequence;theTibbitHillmetavolcanicsuiteisamemberofthisformation.
Therelativelynarrow(lessthan10km)beltofTHFoutcropareasshowninfigure1
represents inliers of the THF exposed at structural culminations of a major anticlinal
axis. That these seemingly minor outcrop areas are merely the surface expression of a
largevolcanicmass,about250kmlong,upto45kmwideandupto8kmthick,hasbeen
demonstratedbyanalysisoftheassociatedgravityandmagneticanomalies(Kumarapeli,
Goodacre,andThomas,1981).Thetriangular-shapedareawhichrepresentsthesubsur-
face extension of the Tibbit Hill volcanic mass occurs in the apical area of the Sutton
GeochemistryandpetrogenesisoftheTibbitHillmetavolcanicsuite,Quebec-Vermont 213
Mountains salient (fig. 1) which is a prominent arcuate feature of the Appalachian fold
belt. Taconian (Early-Middle Ordovician) thrusting is thought to have transported the
volcanicmassandtheassociatedsedimentscratonwardfromtheirinitialsettingoverthe
triplejunction(fig.1;Kumarapeli,1993).
Thevolcanicstratigraphyisnotclearinanyoftheareasstudiedwhichisnotentirely
unexpectedbecauseofthecomplexdeformationandalterationoftherocks.However,
amygdulesarepresentandarelocallyabundant.Also,pillowstructures,althoughrare,
have been reported (Dennis, 1964; Cady, 1969). Thin, discontinuous, phyllitic layers
representsedimentintercalations.
TheTibbitHillfelsicrocksoccurmostlyinoneprincipalband(about15kmlong
andupto2.5kmwide)intheWaterlooarea(fig.1;Kumarapeliandothers,1989).These
felsicrocksareflankedonallsidesbybasalticrocks.Thetrachyticrocksareknownfrom
twosectorsoftheTHF:oneintheRichmondareaandtheotherintheSuttonarea.They
occur within the basaltic rocks as deformed and metamorphosed remnants of thin
(commonly (cid:3)1m thick) flows and dike-like igneous bodies. The largest trachytic body
occursabout5kmnorthofSutton(fig.1)andismappableasasinuousbandabout1km
long and 50 m wide. Field relations suggest that the trachytic bodies were initially
tabular.Thefactthatsomeofthemcontainlargeproportionsofamygdulessuggeststhat
theymayhavebeenlavaflowsandhenceapartofthevolcanicsequence.
PETROGRAPHIC DESCRIPTIONS
TheTibbitHillmaficandfelsicmetavolcanicrockshavebeendescribedindetailby
previousinvestigators(metabasalts:Booth,1950;Christman,1959;Osberg,1965;Clark
and Eakins, 1968; Pieratti, 1976; Coish and others, 1985; Pintson, Kumarapeli, and
Morency,1985;Pintson,1986;metacomendites:Kumarapeliandothers,1989).There-
fore,onlybriefgeneralizeddescriptionsoftheserocksaregiven.Sincetheintermediate
metavolcanicrockshavenotbeendescribedbyotherauthors,theyaretreatedingreater
detail.
Metabasalts.—MaficvolcanicrocksoftheTHFarerepresentedbygreenschists,with
atypicalmineralassemblageofalbite–chlorite–epidote.IntheRichmondarea,however,
the metabasalts contain abundant crossitic amphibole suggestive of blueschist–
greenschisttransitionfaciesmetamorphism(Trzcienski,1976;Pintson,1986).Coinciden-
tally,thisisalsotheareainwhichthevolcanicsequenceisthickest,and,accordingly,the
rocks may have been subjected to greater load pressure. Other minerals present are
Fe-Ti oxides, calcite, biotite, phengite, quartz, actinolite, titanite, and stilpnomelane.
Rare kaersutitic amphibole is preserved as cores within crossite (Trzcienski, 1976;
Pintson, 1986). Relict basaltic textures and sub-ophitic textures are occasionally pre-
served (Coish and others, 1985; Pintson, 1986). Knots of epidote occur in some of the
greenschists.Theproportionofsuchepidotisedrocks(whichisgenerallyminor)varies
alongthemetavolcanicbelt.
Metacomendites.—The volcanic protolith of metacomendites probably consisted of
lavaflowsaswellastuffs(Kumarapeliandothers,1989).Thevolcanicmineralsarenow
representedlargelybyaschistoseassemblageofmuscovite-quartz-albite,withupto10
percent(modal)opaquesandwithorwithoutcarbonatephases.However,relictpheno-
crysts of quartz (up to 20 percent modal) and feldspar (up to 5 percent modal) are
present. Textures of feldspars suggest they were originally potassic and have been
subsequentlyalbitized.
Metatrachytes.—Metatrachytic rocks from the Sutton area contain relict phenocrysts
offeldsparandpreservetrachyticflowtextures.Thephenocrysts(typicallylessthan25
214 A.-F.M.Abdel-RahmanandP.S.Kumarapeli
percentoftherock)aresetinaninequigranularmicrocrystallinegroundmasscontaining
feldsparlathsshowingdistinct,preferredorientationoftheirlongaxes.Thefeldsparlaths
inthematrixmakeupabout25percentoftherockandareonaverageabout0.3by0.03
mm. The phenocrysts are generally highly sodic alkali feldspars and occur as discrete
grains or clusters. The largest grain observed is 5 mm long. The microcrystalline
groundmass,comprisingnearly75percentoftherock,appearstobecomposedlargely
offeldspars(about60percent),minorepidote(5percent),andopaqueFe-Tioxides(10
percent).
Compared with the metatrachytic rocks from the Sutton area, those from the
Richmondareashowahigherdegreeofalteration.Maficmineralshavebeenalteredto
aggregatesofopaqueironoxidesandminorchlorite.Feldsparshaveundergonevarying
degrees of albitization but still preserve a preferred orientation of laths indicating their
originaltrachyticflowtexture.Somesamplesshowpervasivesilicification.Silicablebsin
these rocks are rimmed by specks of jasper which give the rocks their pinkish and
purplish colorations. Zircon is a common accessory mineral. Epidote is also present in
minoramountsasasecondaryphase.
ANALYTICAL TECHNIQUES
For data we used analytical techniques from previous studies on mafic and felsic
rocks from: (A) Vermont area, (B) Richmond area, and (C) comendites from the
Waterloo area, described in Coish and others (1985), Pintson (1986), and Kumarapeli
andothers(1989),respectively.Theanalyticaltechniquesforthenewdata(presentedin
tables1,2)aredescribedbelow.
Majorelements.—Concentrationsofthemajorelements(table1)weredeterminedon
fused lithium-metaborate discs by X-ray fluorescence spectrometry (Philips PW1400
SpectrometeratMcGillUniversity)usingaRhtubeoperatedat40kVand70mA.Loss
onignition(LOI)wasdeterminedbyheatingpowderedsamplesfor50minat1000°C.
Traceelements.—ConcentrationsofNi,Cr,Sc,V,Co,andBawerealsodeterminedon
fuseddiscsalongwiththemajorelementsasdescribedabove.ConcentrationsofRb,Sr,
Zr, Y, Nb, Ga, Pb, U, and Th (table 1) were determined on pressed pellets by X-ray
fluorescence (operating conditions: Rh radiation, 70 kV, 40 mA). The analytical preci-
sion,ascalculatedfrom20replicateanalysesofonesample,isbetterthan1percentfor
mostmajorelementsandbetterthan5percentformosttraceelements.
Rare earths, hafnium, and tantalum.—Concentrations of fourteen rare earth elements
(REE; La to Lu, all except Pm) as well as Hf and Ta (table 2) were determined by
ICP-MS.Theanalyticalprocedurewasasfollows:(1)sinteringofa0.2gsamplealiquot
withsodiumperoxide,(2)dissolutionofthesintercake,separationanddissolutionofthe
REE hydroxide-bearing precipitate, and (3) analysis by ICP-MS using the method of
internal standardization to correct for matrix and drift effects. The advantage of the
sintering technique is that it ensures complete digestion of resistant REE-bearing
accessory phases (for example, zircon, fluorite) which may not dissolve during an acid
digestion.FulldetailsoftheprocedurearegiveninLongerichandothers(1990).Apure
quartz reagent blank and several certified geological reference standards as well as
internal laboratory standards were analyzed with these samples. Detection limits and
reagentblanksaregenerallyabout10percentofchondritevalues.Thechondritevalues
usedfornormalizationarethoseofTaylorandMcLennan(1985),compiledfromAnders
andEbihara(1982)andEvensen,Hamilton,andO’Nions(1978).
For samples numbered W-01 to W-26 and RTH-3 (table 2), the concentrations of
theREE(La,Ce,Nd,Sm,Eu,Tb,Yb,andLu),alongwiththoseofHf,Sc,andCo,were
determined by instrumental neutron activation analysis (INAA) at the University of
Quebec at Montreal. Precision of trace element data obtained by INAA method is
GeochemistryandpetrogenesisoftheTibbitHillmetavolcanicsuite,Quebec-Vermont 215
generally better than 10 percent. Details of this INAA method are given in Pintson
(1986).
GEOCHEMISTRY
Major and trace element geochemistry.—The concentration of the major and minor
elements of the analyzed samples are given in table 1. The Tibbit Hill volcanic
assemblagespansawiderangeofSiO contents(44-76wtpercent),coveringtheentire
2
(mafictofelsic)compositionalspectrum.Thiswidecompositionalrangeisillustratedin
figure 2. The relatively high Na content (reaching up to 10 wt percent Na O) of some
2
intermediate and felsic rocks of the THF reflects the common presence of albite as
observedmicroscopically.Inthemorefelsicrocks,K Ovaluesarealsohighlyvariable
2
(Table 1). The wide variability of the contents of the alkalies in these rocks reflects the
mobility of the two elements occurring during post-magmatic alteration common in
volcanic systems and during metamorphism (see below). It should be noted that alkali
metasomatismisgenerallycommoninvolcanicrocksduetotheirstructure,texture,and
therelativelyhighporosityoftheintermediateandfelsicvolcanicrocks,asdocumented
for several igneous complexes such as the Deloro A-type complex, Madoc, Ontario
(Abdel-RahmanandMartin,1990a).
Despite the mobility of the alkalis, the silica versus alkalis diagram of Miyashiro
(1978; not shown) shows that most lithologies of the THF are alkaline. The relative
enrichmentofTiwithintheserocks(table1),comparedtocalc-alkalinevolcanicrocks,is
consistentwiththealkalinenatureoftheTHF.Thisisalsoreflectedbythepresenceof
kaersutiticamphiboleandFe-Tioxidesasobservedpetrographically.
IntheNb/Y—Zr/Tidiagram(fig.3)ofWinchesterandFloyd(1977)andFloydand
Winchester (1978), rocks of the THF occupy the entire spectrum of mafic to felsic
alkalinevolcanicrocks(alkalibasaltthroughcomendite/pantellerite).RocksoftheTHF
generally exhibit a wide range of Zr (138-1493 ppm), Nb (15-139 ppm), and Y (18-185
ppm) concentrations (table 1; Coish and others, 1985; Pintson, 1986; Kumarapeli and
others,1989).
In the Tibbit Hill rocks, the Zr/Hf ratio ranges from 32 to 50 in basalts with an
averageof41,whichfallswellwithinreportedratiosoftypicalOIB-typebasalts(37-43;
St.SeymourandKumarapeli,1995).TheZr/HfratiooftheTibbitHilltrachytesranges
from57to79,withanaverageof66.TherelativelywiderangeoftheZr/Hfratiosinthe
THFrocksmayhaveresultedfromminorredistributionofHfduringthemetamorphic
episode(seethesectiononthemobilityofelementsbelow).
TheNb/Taratiorangesfrom17.7to18.8intheTHFbasalticrocks,withanaverage
of18.3,whichisalsocomparabletoNb/Taratios(15-18)reportedforOIB-typebasaltic
rocks(St.SeymourandKumarapeli,1995).TheNb/TaratiointheTHFtrachytesranges
from17.7to19.8(19.1avg).Ingeneral,basalticrocksoftheTHFcontainrelativelyhigh
concentrations of the high field strength elements (HFSE), thus reflecting an enriched
source (see below). However, concentrations of U (0-5.8 ppm) and Th (0-9.4 ppm) are
relativelylowforsuchanalkalinesuite.
Mobilityofelements.—Resultsofthisstudyareusedtoillustratefurtherthemobilityof
elements in response to the metamorphic episode that affected the THF rocks. Our
results indicate that Ca, Na, and K exhibit wide compositional variations. The average
CaOcontentintheTHFbasalt(6.05wtpercent)iswellbelowaverageCaOconcentra-
tioninatypicalunalteredbasalt(9.66wtpercent)orinatholeiite(10.35wtpercent;Le
Maitre,1976).TheaverageNaOcontentintheTHFbasalt(4.35wtpercent)isrelatively
higherthanthatoftheaveragebasalt(2.97wtpercent),ortholeiite(2.44wtpercent;Le
Maitre,1976).ThedatareflectthemobilityofCaandNaduringmetamorphismofthe
THF. The wide variability in the concentration of K in the THF basalt (ranging from
216 A.-F.M.Abdel-RahmanandP.S.Kumarapeli
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GeochemistryandpetrogenesisoftheTibbitHillmetavolcanicsuite,Quebec-Vermont 217
218 A.-F.M.Abdel-RahmanandP.S.Kumarapeli
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GeochemistryandpetrogenesisoftheTibbitHillmetavolcanicsuite,Quebec-Vermont 219
Fig.2. SiO (inwtpercent)versusZr(inppm)variationdiagramforwholerocksamplesshowingthewide
2
(mafictofelsic)compositionalrangeoftheTibbitHillvolcanicassemblageanditsenrichmentinZr.Symbols:
triangles,felsicrocksofKumarapeliandothers(1989);circles,intermediaterocks;squares,maficrocks;(cid:1),
basalticrocksofCoishandothers(1985);andX,basalticrocksofPintson(1986).
0.01-1.2 wt percent K O; table 1) suggests also that K has been highly mobile during
2
metamorphism.
MajorelementssuchasSiO (rangingfrom44.2to50.5wtpercent;Table1),Al O
2 2 3
(15.23to16.98wtpercent),TiO (2.22-3.14wtpercent),andP O (0.32-0.59wtpercent)
2 2 5
exhibit much less variations and appear to have been much less affected by the
metamorphicevent.Theconcentrationofsilicaisstillrepresentativeoftheprotolithand
reflectsitsbasalticnature.
The incompatible trace elements (along with Ti and P) seem to have been least
affectedbymetamorphism.Chondrite-normalizedincompatibleelementpatternsofthe
THF basaltic rocks are shown in figure 4. The wide scatter observed for K and Sr
resultingfromthewiderangesintheirnormalizedabundanceswithstrongdepletionin
someoftheTHFbasalticrocks(fig.4)suggeststhatKandSrhavebeenhighlymobileas
aresultofmetamorphism.
Hfshowsaslightlywiderrangethantherestoftheincompatiblehighfieldstrength
elements. The normalized patterns of most of the other incompatible elements plotted
fallwithinaconsistentlynarrowrange,varysystematicallyfromonesampletoanother,
and are relatively smooth, parallel to subparallel patterns (fig. 4). These incompatible
element profiles suggest that, despite metamorphism, most of the HFSE (as Nb, Zr, Ti,
andY),alongwiththeREE,haveremainedlargelyintact.
It should be noted that relatively smooth patterns are commonly characteristic of
unalteredorfreshbasalts,wherenormalizedabundancesofZr,Ti,andHfcorrelatewith
the REE (Sun, Nesbitt, and Sharaskin, 1979; Sun, 1980). Thus, our results suggest that
mostoftheincompatibleelementscanbeusedinassessingthepetrologicalcharacterof
theprotolith.