Geochemistry and zircon geochronology of the I-type high-K calc-alkaline and S-type granitoid ...

29 Pages·2007·2.93 MB·English

PrecambrianResearch155(2007)69–97
Geochemistry and zircon geochronology of the I-type high-K
calc-alkaline and S-type granitoid rocks from southeastern
Roraima, Brazil: Orosirian collisional magmatism evidence
(1.97–1.96Ga) in central portion of Guyana Shield
Marcelo E. Almeidaa,b,∗, Moacir J.B. Macambirab, Elma C. Oliveirab
aCPRM-GeologicalSurveyofBrazil,Av.Andre´Arau´jo2160,Aleixo,CEP69060-001,Manaus,Amazonas,Brazil
bIsotopeGeologyLaboratory,CenterofGeosciences,FederalUniversityofPara´,CP8608,CEP66075-110,Bele´m,Para´,Brazil
Received13August2006;receivedinrevisedform5January2007;accepted16January2007
Abstract
TheunderstandingofthegeologicalevolutionoftheUatuma˜-Anaua´DomaininsoutheasternRoraima,centralregionofGuyana
Shield, is of major significance in the study of the Amazonian craton. This region lies between some major Paleoproterozoic
geological–geochronologicalprovinces:Tapajo´s-ParimaorVentuari-Tapajos(dominant),Maroni-Itacaiu´nasorTransamazon(north-
west)andCentralAmazonianorCentralAmazon(southeastandeast).GeologicalmappingofthenorthernareaofUatuma˜-Anaua´
Domain, integrated with whole rock geochemistry data and previous and new Pb-evaporation and U–Pb zircon geochronology,
pointoutforaplutoniccollisionalmagmaticevent(1975–1968Ma)representedbyI-typehigh-Kcalc-alkaline(MartinsPereira)
andS-type(SerraDourada)granitoidrocks.Thismagmatismwasprobablygeneratedfromcrustalsourcesbypartialmeltingduring
amalgamationoftheTTG-likeAnaua´magmaticarc(2028Ma)withTransamazonian(2.2–2.0Ga)andCentralAmazonian(older
than2.3Ga)terranes.LocalcumulaticleucogranitesfillsplanarstructuresoftheMartinsPereiragranites.Theseleucogranitesshow
youngerages(1909Ma)andseveralinheritedzircons(2354,2134,1997and1959Ma),suggestingoriginfromcrustalsources.
©2007ElsevierB.V.Allrightsreserved.
Keywords: Paleoproterozoic;Granitoidrocks;GuyanaShield;Zircongeochronology;Geochemistry
1. Introduction and b). Despite representing an intricate component of
theAmazoniancraton,theGuyanaShieldhasseenonly
The Guyana Shield, with a surface area of nearly limitedattentionamongthescientificcommunity.Geo-
1.5millionkm2,representsthenorthernmostsectionof logicalmapsoftheregionarescarce(e.g.CPRM,1999,
the Amazonian craton. This shield was predominately 2000a; Delor et al., 2003), and only few petrographi-
formed during protracted periods of intense granitic cal,geochemicalandgeophysicalstudiesareavailable.
magmatism,bracketedbetween2.1and1.9Ga(Fig.1a ComparisonbetweenpreviousstudiesfromtheGuyana
Shieldhasbeenhamperedbyalackofdependableage
determinations, implying large errors (±50–100Ma),
whichprecludeanyattempttoestablishafinechronol-
∗ Correspondingauthor.Tel.:[phone];
ogyofthemagmaticandmetamorphicevents.
fax:[phone].
The study area is concentrates on the central part
E-mailaddress:[email protected]
(M.E.Almeida). of the Guyana Shield, in southeastern Roraima State
0301-9268/$–seefrontmatter©2007ElsevierB.V.Allrightsreserved.
doi:10.1016/j.precamres.2007.01.004
70 M.E.Almeidaetal./PrecambrianResearch155(2007)69–97
Fig.1. ThestudiedarealocationinRoraimaState,plottedonthesketchmapsoftheGeochronologicalProvincesoftheAmazoniancratonaccording
to(a)TassinariandMacambira(1999,2004)and(b)Santosetal.(2000,2006),andon(c)themapofthelithostructuraldomainsafterReisetal.
(2003)modifiedbyCPRM(2006).
(Brazil). Recent zircon geochronology data demon- al., 2002). It is hoped that such studies will contribute
strate that igneous rocks cropping out in this region to a better understanding of the lithostratigraphy, ori-
(c. 4970km2, Fig. 2) show ages of 2.03Ga (Faria et gin and geodynamic evolution of the northern part of
al., 2002) and 1.97–1.96Ga (Almeida et al., 1997; theUatuma˜-Anaua´Domain,inthecentralportionofthe
CPRM, 2003), similar to the Tapajo´s Domain in Cen- Guyana Shield (Fig. 1). These data will be integrated
tral Brazil Shield (Santos et al., 2000; Lamara˜o et al., with previous geochemical and geochronological data
2002). However, the characteristics and significance of for other magmatic associations, mainly calc-alkaline
theseeventsandagesarenotfullyunderstood(CPRM, granitoids older than 1.90Ga. Particular focus will be
2000a;AlmeidaandMacambira,2003).Anexampleof placedoncomparativegeochemicalandgeochronology
this can be seen in the 2.03–1.96Ga magmatic event datafrompreviousstudiesinthesameregionandfrom
which, unlike the 1.90Ga calc-alkaline magmatism, is thesouthernpartoftheAmazoniancraton.
uncommon in other regions of the world. In addition
the Tapajo´s Domain in Central Brazil Shield (south- 2. Geologicalsetting
ern Amazonian craton) and parts of the Sa˜o Francisco
Craton, Paleoproterozoic tectonic and magmatic activ- Regional geological maps (CPRM, 2000a; Almeida
ity between c. 2.0 and 1.9Ga is recorded in Western etal.,2002)andzircongeochronologicaldata(Almeida
Australia(GascoigneComplex,DalgaringaSupersuite, etal.,1997;Santosetal.,1997;Macambiraetal.,2002;
2005–1970Ma) by Sheppard et al. (2004) and south- CPRM,2003)haveshownthatPaleoproterozoicgrani-
ern Australia (Gawler Craton, Miltalie Gneiss, ca. toid and volcanic rocks (1.97–1.81Ga) are widespread
2000Ma)byDalyetal.(1998).TheTaltsonMagmatic in southeastern Roraima. These intrusive and extrusive
Zone of northern Canada (McDonough et al., 1993) rocks are emplaced within poorly exposed basement
and Kora-Karelian orogen of northern of Baltic Shield rocksthathavemaximumagesofaround2.03Ga(Faria
(Daly et al., 2001) also show 2.0–1.9Ga magmatic etal.,2002).
events. According to recent evolutionary models proposed
The aim of this paper is to provide new geochem- for the Amazonian craton, the study area can be
ical and geochronological constraints on the Martins divided into several provinces. According to the work
PereiraandSerraDouradagranitoidrocks(Almeidaet ofTassinariandMacambira(1999,2004),thestudyarea
M.E.Almeidaetal./PrecambrianResearch155(2007)69–97 71
Fig.2. SimplifiedgeologicalmapofSoutheasternRoraimaStatemodifiedfromAlmeidaetal.(2002)andCPRM(2000a,2005):(1)Plio-Pleistocene
sedimentarycovers;(2)Caracara´ıGabbro(1.52Ga?);(3)Foliatedgranitoids(1.72Ga),myloniticgranites(1.89Ga),granulites,augengneisses,
metagranitoidsandorthogneissesfromRioUrubuComplex(1.96–1.93Ga);(4)S-typeCuruxuim(garnet)Granite(1.97Ga?);(5)Metavolcano-
sedimentarysequence(CauaraneGroup>1.97Ga);(6)(a)A-typeModerna(1.81Ga)andMapueraGranites(1.87Ga),andminorenderbiteand
charno-enderbite(1.89Ga);(7)Igarape´AzulandCaroebeGranitesandminorvolcanicrocks(1.90–1.89Ga);(8)S-typeSerraDourada(cordierite)
Granite(1.96Ga);(9)High-Kcalc-alkalinegranitoidswith(a)normalto(b)highU,Th,Kcontents(MartinsPereiraGranite,1.97Ga);(10)
Metavolcano-sedimentarysequence(CauaraneGrouprelatedrocks;<2.03Ga?)andTTGcalc-alkalineassociation(Anaua´Complex,2.03Ga).
islargelyenclosedintheVentuari-Tapajo´sandCentral (e.g. Ventuari-Tapajo´s or Tapajo´s-Parima, and Maroni-
Amazonian provinces, with a subordinate northeastern Itacaiu´nas or Transamazon provinces) accreted to the
section falling within the Maroni-Itacaiu´nas province. Archeancratonwithtimeand/orrepresentasetofrocks
Santosetal.(2000,2006)arguethatallprovinceswere produced from melting of Archean crust (Fig. 1a and
collectivelyaffectedbytheK’MudkuShearBeltinthis b;Cordanietal.,1979;Teixeiraetal.,1989;Tassinari,
same region (Fig. 1a and b). These authors conclude 1996; Tassinari and Macambira, 1999, 2004; Santos et
that Paleoproterozoic orogenic belts or magmatic arcs al.,2000,2004,2006).
72 M.E.Almeidaetal./PrecambrianResearch155(2007)69–97
The Tapajo´s-Parima (or Ventuari-Tapajo´s) Province ment is composed of TTG-like metagranitoids to
is a Paleoproterozoic orogenic belt trends north– orthogneisses (Anaua´ Complex), enclosing meta-mafic
northwest and includes geological units which range tometa-ultramaficxenoliths,andisassociatedwithsome
from ∼2.10 to 1.87Ga in age (Santos et al., 2000, inliers of metavolcano–sedimentary rocks (Cauarane-
2006; Tassinari and Macambira, 1999, 2004). This like).ThebasementrocksareintrudedbyS-type(Serra
province was subdivided into four domains by Santos DouradaGranite)andhigh-K,I-typecalc-alkaline(Mar-
et al. (2000): Parima and Uaimiri, to the north, and tins Pereira) granite plutons of c. 1.97–1.96Ga (see
PeixotoAzevedoandTapajo´s,tothesouth.Southeastern zircongeochronologysection).
Roraima belongs to the Uaimiri Domain. The Central In the southern area of the Uatuma˜-Anaua´ Domain,
Amazonian Province of the study area (Fig. 1a and intrusive younger granites (with no regional deforma-
b) display granitoid and volcanic rocks (1.88–1.70Ga) tion and metamorphism) are very common. The most
lackingregionalmetamorphismandcompressionalfold- prominent magmatism is related to the calc-alkaline
ing (Santos et al., 2000; Tassinari and Macambira, Caroebe and Igarape´ Azul granitoids (A´gua Branca
1999), and its basement is exposed scarcely. The age Suite) with coeval Iricoume´ volcanic rocks. Locally
for this basement has been estimated via Nd-model igneous charnockitic (Igarape´ Tamandare´) and ender-
ages at around 2.3–2.5Ga (Tassinari and Macambira, bitic(SantaMaria)plutonswerealsorecorded.Several
1999, 2004). According to these authors, the recorded A-type granite bodies are widespread in the Uatuma˜-
ArcheanrocksintheAmazoniancratonareonlyexposed Anaua´ Domain (Fig. 2) represented by Moderna-
in Imataca (Venezuela), Caraja´s and southern Amapa´- A´gua Boa (1.81Ga) and Mapuera-Abonari (1.87Ga)
northwestern Para´ (Brazil). The Maroni-Itacaiu´nas (or granites.
Transamazon)Provinceisalsocharacterizedbyanoro-
genic belt with Rhyacian ages (2.25–2.00Ga, Fig. 1a 3. Analyticalprocedures
and b) and is correlated to the Birimian belt in West
Africa(TassinariandMacambira,1999,2004;Santoset 3.1. Whole-rockgeochemistryanalysis
al.,2000;Deloretal.,2003).TheK’MudkuShearBeltis
characterizedbylow-tomedium-grademyloniticzones Whole-rockchemicalanalysesof11samples(milled
with ca. 1.20Ga ages and cross-cuts the Rio Negro, under200mesh)weredoneattheAcmeAnalyticalLab-
Tapajo´s-ParimaandTransamazonprovinces,(Santoset oratoriesLtd.inVancouver,BritishColumbia,Canada.
al.,2000). The analytical package includes inductively coupled
Taking into account lithological associations and plasma-atomicemissionspectrometer(ICP-AES)anal-
geochronological data, Reis et al. (2003) and CPRM ysesafterLiO2fusionforallmajoroxides(SiO2,TiO2,
(2006), divided Roraima into four major domains— Al2O3,MnO,MgO,CaO,K2O,Na2O,P2O5)andLOI.
Surumu, Parima, Central Guyana and Uatuma˜-Anaua´ Total iron concentration is expressed as Fe2O3. The
(Fig.1c).Eachofthesedomainscontainsawiderangeof trace elements were analyzed by inductively coupled
rocktypesandstratigraphicunits.SoutheasternRoraima plasma-mass spectrometer (ICP-MS), with rare-earth
iscomposedoftheCentralGuyanaandUatuma˜-Anaua´ and incompatible elements determined from a LiBO2
lithostructuraldomains,thatcorrespondtotheK’Mudku fusionandpreciousandbasemetalsdeterminedfroman
Shear Belt and Uaimiri Domain, respectively (north- aquaregiadigestion.Additional16sampleswerecom-
ernTapajo´s-ParimaProvince)proposedbySantosetal. piled(andpartiallyreinterpreted)fromCPRM(2000a).
(2000). These last whole-rock chemical analyses were done at
The Central Guyana Domain (CGD) consists the Geosol Laboratories S.A., Belo Horizonte, Minas
primarilyofgranulites,orthogneiss,mylonitesandmeta- Gerais,Brazil.
granitoids (Rio Urubu Metamorphic Suite) associated
with low to high metamorphic grade metavolcanosed- 3.2. Zirconisotopeanalysis
imentary covers (Cauarane Group) and S-type granite
(CuruxuimGranite).Thelineamentstrendarestrongly Forisotopeanalysis,thezirconcrystalswereobtained
NE–SW trends (Figs. 1c and 2). The Uatuma˜-Anaua´ from samples with 2–20kg. After crushing (milled to
Domain(UAD)ischaracterizedbyE–WtoNE–SWlin- 60–80mesh)andsieving,heavymineralfractionswere
eaments and the northern part of the domain Northern obtainedbywater-mechanicalanddenseliquidconcen-
Uatuma˜-Anaua´ Domain shows an older metamorphic trations, and processed under hand magnet and Frantz
basement (Figs. 1 and 2) formed in a presumed Isodynamic Separator. In order to remove impurities,
◦
island arc environment (Faria et al., 2002). This base- zirconconcentrateswerewashedwithHNO3 at100 C
M.E.Almeidaetal./PrecambrianResearch155(2007)69–97 73
(10min), submitted to ultrasound cube (5min) and tory,oftenyieldmixedageswithnogeologicalmeaning
finally washed on bidistilled H O. The less magnetic (Dougherty-PageandBartlett,1999).Withtheseuncer-
2
zirconconcentrates(fromfivemagneticfractions)were tainties in mind, the age obtained for a single grain
preferredforhand-picking.Wheneverpossibleonlyzir- is considered as a minimum age. This being the case,
con grains free of alteration, metamictization features, as has been proposed in several studies (Kober, 1986;
inclusionsandfractureswereselectedforanalysis,how- AndsellandKyser,1991;MacambiraandScheller,1994;
everitwasnotalwayspossible(seezircondescriptions So¨derlund,1996),ifasetofmagmaticgrainsfromthe
below). All selected grains for analysis were photomi- samesampleyieldssimilarages,itispossibletosuggest
crographedviaaconventionalopticalmicroscope. thatsuchsimilaritiesindicatethetimeofthemagmatic
Single-zircon dating by Pb-evaporation and U–Pb crystallizationorepisodicPbloss.
isotopicdilutioninthermalionizationmassspectrometry The U–Pb ID-TIMS procedures undertaken in
(ID-TIMS)methodswasperformedattheIsotopeGeol- the Para´-Iso Laboratory followed those presented by
ogy Laboratory (Para´-Iso), Federal University of Para´ Krymsky(2002).Allzirconfractionsselectedforanal-
(UFPA),Brazil.TheUdecayconstantsarethoserecom- ysis were previously air abraded with pyrite crystals
mendedbySteigerandJager(1977),anderrorsaregiven (20mg and 0.1–0.5mm diameter) in 1.2–1.8psi pres-
atthe95%confidencelevel.Agesoffivesampleswere sure for 30–40min. After abrasion the zircon grains
◦
obtainedbythePb-evaporationtechniqueestablishedby werewashedwithHNO andHCl(100 C,30min)and
3
Kober(1986,1987),andonlyonesamplewasanalyzed H O-Millipore(3times).Thegrainswerethenwashed
2
byU–PbID-TIMS.Allisotopeanalyseswerecarriedout withmethanol,weighed,spikedwith235U–205Pbtracer
onaFinniganMAT262massspectrometerindynamic and dissolved with a mixture of HF and HCl in PTFE
modeusingtheioncountingdetector. Teflon® bombs.Uraniumandleadwereseparatedwith
InthePb-evaporationmethodonasinglezircon,the anion-exchangeresin(Dowex®1×8200–400mesh)in
selected grains were tied in Re-“evaporation”-filament HCl medium in 50–70(cid:2)l columns. Uranium and Lead
and introduced in the mass spectrometer. The Pb was wereloadedwithSi-gelandH PO (1N)ontothesame
3 4
◦
normallyextractedfromthecrystalsbyheatinginthree out-gassedRe-filamentandanalyzedat1400–1600 C.
evaporation steps at temperatures of 1450, 1500 and Forzirconanalyses,blanksare<30pgPband<1pgU.
◦
1550 C.TheevaporatedPbwasloadedonan“ioniza- The obtained data were processed in ISOPLOT/Excel
tion”filament,whichisheatedfortheisotopeanalyses. program version 2 (Ludwig, 1999). The commom Pb
In this technique, the data were dynamically acquired interference was corrected using also the Stacey and
usingtheioncountingsystemoftheinstrument.Pbsig- Kramers(1975)model.
nal was measured by peak hopping in the 206, 207,
208, 206, 207, 204 mass order along 10 scans, defin- 4. Whole-rockgeochemicalresults
ing one block of data with 18 207Pb/206Pb ratios. The
207Pb/206Pbratioaverageofeachstepwasbasedonfive 4.1. Majorandminoroxides,andtraceelements
blocksorless,tilltheintensitybeamwassufficientlyhigh geochemistry
forareliableanalysis.Usually,theaverage207Pb/206Pb
ratioobtainedinthehighesttemperaturestepwastaken Analytical results of representative samples from
foragecalculation,buttheotherstepsarealsoconsid- Martins Pereira, Serra Dourada and Anaua´ granitoid
ered. Outliers were eliminated using Dixon’s test. The rocksoftheNorthernUatuma˜-Anaua´Domain,including
207Pb/206Pb ratios were corrected for a mass discrimi- leucogranite blobs and lenses, are presented in Table 1
nation factor of 0.12%±0.03amu−1, and results with andplottedindiagramsinFigs.3–7.SerraDouradaand
204Pb/206Pbratioshigherthan0.0004were,ingeneral, Anaua´ results are extracted form CPRM (2000a). Cre-
discarded.Theageswerecalculatedwith2sigmaerror poriza˜o (CPRM, 2000b) and Old Sa˜o Jorge (Lamara˜o
andcommonPbcorrectionwasdoneusingappropriate et al., 2002) granitoid rocks data from Tapajo´s region
agevaluesderivedfromthetwo-stagemodelofStacey are also plotted in some diagrams for comparison with
andKramers(1975).Theobtaineddatawereprocessed MartinsPereiragranitoidrocks.
insharewareZirconprogram(Scheller,1998),DOSsys- The Martins Pereira Granite consists of a com-
temversion. positionally wide series of rocks with SiO contents
2
ThePb-evaporationonasinglezirconmethodyields between 49.3 and 74.6wt.% (Table 1). The associated
apparent 207Pb/206Pb ages and the degree of concor- leucogranite pods and lenses have high SiO contents
2
dance of the analytical points is not possible to assess. (72.9–73.9wt.%), but in the Harker diagrams show
Furthermore, zircon grains exhibiting a complex his- no correlation with the Martins Pereira Granite. For
7
4
Table1
ChemicalcompositionsofmainplutonicassociationsofNorthernUatuma˜-Anaua´Domain,suchasMartinsPereira(meta)granitoidrocks,lensesandblobsofleucogranites,SerraDouradagranite
andAnaua´Complex
M
.E
.
A
lm
e
id
a
e
t
a
l.
/
P
re
c
a
m
b
r
ia
n
R
e
s
e
a
rc
h
1
5
5
(2
0
0
7
)
6
9
–
9
7
Thegeochemicaldatawereobtainedfromthisstudy1 andCPRM(2000a)2.Abbreviations:a,amphibole;b,biotite;e,epidote;m,muscovite;D,diorite;Gb,gabbro;Gd,granodiorite;Mgr,
monzogranite;N,norite;Sgr,syenogranite;Tn,tonalite;Gn,gneiss;Lc,leuco;Mt,meta;P,porphyritic;–,notavailablevalues.
M.E.Almeidaetal./PrecambrianResearch155(2007)69–97 75
Fig.3. (a–h)SelectedHarkervariationdiagramsforMartinsPereiraandSerraDouradaGranites,leucogranitesandAnaua´Complex.Forreferences
seeTable1.FieldsrepresentingCreporiza˜o(CPRM,2000b)andOldSa˜oJorgegranites(Lamara˜oetal.,2002)oftheTapajo´sDomainareplotted
forcomparison.ForthelinearregressionareusedonlytheMartinsPereiraGranitesamples.
76 M.E.Almeidaetal./PrecambrianResearch155(2007)69–97
Fig.4. GeochemicaldiagramsshowingresultsfromMartinsPereiraGranite,leucogranitesandAnaua´Complexrocks(forreferencesseeTable1).
FieldsrepresentingCreporiza˜o(CPRM,2000b)andOldSa˜oJorgegranites(Lamara˜oetal.,2002)areplottedforcomparison:(a)K2Ovs.SiO2
contentsdisplayingtheshoshonite,high-K,medium-Kandlow-Kfields(fromPeccerilloandTaylor,1976;modifiedbyRickwood,1989).For
thelinearregressionareusedonlytheMartinsPereiraGranitesamples.(b)log[CaO/(Na2O+K2O)]vs.SiO2contents(fromBrownetal.,1984).
SymbolsasinFig.3.
instance, leucogranites with the same SiO contents higherNa OandMgO(Fig.3e–g)contents.TheSerra
2 2
as Martins Pereira granatoid rocks, have low Na O DouradaGraniteshowsremarkablelowNa O(Fig.3g)
2 2
(Fig. 3g), MnO (Fig. 3d), Fe O +FeO (Fig. 3c) and andK Ocontents(Table1).
2 3 2
veryhighK O(Fig.4a)values. ComparedtotheMartinsPereiragranitoidrocks,the
2
RocksamplesfromMartinsPereiraGranitearealso 1.99–1.96Ga Old Sa˜o Jorge and Creporiza˜o granites
characterizedbylineartrendsintheallHarkerdiagrams from the Tapajo´s Domain have lower TiO (Fig. 3a),
2
(Fig. 3a–h). In these diagrams, excluding Na O versus MnO (Fig. 3d) and P O (Fig. 3h), and higher Na O
2 2 5 2
SiO ,thestatisticparametersshownegativelinearcor- (Fig.3g)contents.ThecorrespondingHarkerdiagrams
2
relations with regression agreement between 99% and demonstratethattheCreporiza˜ograniteshaveashorter
99.9%.Lineartrendscanresultfromseveralpetrogenetic rangeandhighercontentsofSiO (65.2–73.4%,Table1),
2
processes, such as contamination, mixing, crystal frac- aswellaslowerMnO(Fig.3d),Al O (Fig.3b)andK O
2 3 2
tionationwithnocrystallizingphaseschangeandpartial (Fig.4a)contents.
melting(e.g.Coxetal.,1987;Wilson,1991).Thelack Inthelog[CaO/(Na O+K O)]versusSiO diagram
2 2 2
ofsignificantcompositionalgapsinHarkerdiagramsfor (Fig. 4b), all the studied granitoid rocks (excluding
MartinsPereirasamplessuggeststhatthemainpetroge- Serra Dourada Granite) indicate calc-alkaline affinity,
neticprocessislikelyrelatedtopartialmeltingorcrystal with the samples plotting in the normal calc-alkaline
fractionationwithnochangeinthemineralassemblage andesitic field. Only the Old Sa˜o Jorge Granite shows
beingfractionated.Howeverthislastonerequiresagreat exclusive transitional character between normal and
volume of mafic parental magma, not detected in the mature arc series. Most of the granitoid samples also
region.Thesameistrueforfractionalcrystallizationpro- plot in the high-K field (K O versus SiO diagram,
2 2
cesswithchangeinthecrystallizingmineralphases(e.g. Fig. 4a), however, the Martins Pereira and Old Sa˜o
hornblendeoutandbiotitein),butinthiscasetheresult Jorge granites have a transitional high-K to slightly
areusuallyresultsincurvelinearHarker-plottrends(e.g. shoshonitic character. It is also apparent from the K O
2
Coxetal.,1987;Wilson,1991)thatarenotobservedfor versusSiO diagramthatsamplesfromtheCreporiza˜o
2
MartinsPereirasamples. Granitedisplayatransitionalhigh-Ktolocallymedium-
IncontrasttotheMartinsPereirasamples,theAnaua´ Ktrend,whereasAnaua´typesaremedium-Ktoslightly
Complex and related rocks show lower SiO contents low-K.TheleucogranitesshowhighKcontents(>7%)
2
rangingfrom41.3%to59.8%(Table1),anddispersionin andahighlyfractionatedcharacter.
theHarkerdiagrams(e.g.TiO ,MnO,Al O ,Na Oand The Martins Pereira and Creporiza˜o granite com-
2 2 3 2
Fe O ). When compared to the Martins Pereira grani- positions are transitional between metaluminous to
2 3t
toidrocks,theAnaua´samplesarecharacterizedbylower peraluminous, while the Old Sa˜o Jorge granite is met-
K OandP O contents(Figs.4aand3h)whilehaving aluminous to slightly peraluminous (Fig. 5a). Almost
2 2 5
M.E.Almeidaetal./PrecambrianResearch155(2007)69–97 77
Fig.5. MartinsPereiraandSerraDouradagranites,leucogranitesandAnaua´Complexsamplesplottedinthediagrams(forreferencesseeTable1):
(a)MolecularAl2O3/(Na2O+K2O)vs.molecularAl2O3/(CaO+Na2O+K2O)(ManiarandPiccoli,1989;mod.Shand,1927)and(b)Na2Ovs.
K2OplotsshowingS-typeandI-typegranitescompositionaldatafromLachlanFoldedBelt(LFB);(c)Rbvs.(Y+Nb)and(d)(K2O+Na2O)/CaO
vs.(Zr+Nb+Ce+Y).FieldsrepresentingCreporiza˜o(CPRM,2000b)andOldSa˜oJorgegranites(Lamara˜oetal.,2002)areplottedforcomparison.
(c)VAG,VolcanicArcGranites;ORG,OceanRidgeGranites;syn-COLG,syn-collisionalGranites;WPG,Within-PlateGranites,fieldsarefrom
Pearceetal.(1984)andpost-COLG(post-collisionalgranites)fromPearce(1996).(d)OGT,Orogenicgranitetypes:unfractionedI-andS-type
granites;FG,FractionatedfelsicI-andS-typegranites,fieldsaretakenfromWhalenetal.(1987).CompositionalaverageofA-,M-,I-,S-and
I-typearealsorepresented.SymbolsasinFig.3.
allthesegranitesshowA/NKmolarratios<2.Onlythe generallyplotintheVAGfield,withcorrespondinglylow
Anaua´ Complexrocks(A/NKmolar>2)andthelenses Rbcontents.TheCreporiza˜oandOldSa˜oJorgegranites
ofleucogranitearemetaluminousandperaluminous.The samples dominantly plot in the VAG field, falling near
SerraDouradaGraniteisbroadlyperaluminous,plotting to the WPG boundary. Thus, excluding Anaua´ rocks,
in the S-type granite compositions. This granite body mostoftheanalyzedsamplesfallinthepostcollisional
showsA/CNKmolarratiosof1.1and1.3,whilesome fieldofPearce(1996),whichsuggestsanincreasingin
samplesoftheMartinsPereiraGranitealsoshowlocally arc maturity (Brown et al., 1984; see Fig. 4a) or the
highA/CNKvalues.IntheNa OversusK Odiagram, beginningofthetransitionfromcalc-alkalinetoalkaline
2 2
the Martins Pereira granitoid rocks plot on the I-type magmatic series in orogenic to post-orogenic tectonic
field, but several samples also plot near to the S-type settings(Bonin,1990;Barbarin,1999).
field boundary (Fig. 5b) showing, as well as the Serra Inthe(K O+Na O)/CaOversus(Zr+Nb+Ce+Y)
2 2
DouradaGranitesamples,thelowestNa O/K Oratios. diagram (Fig. 5d), the calc-alkaline Martins Pereira,
2 2
IntheRbversus(Y+Nb)tectonicdiscriminatordia- Creporiza˜oandOldSa˜oJorgegranitesplotontheunfrac-
grams (Fig. 5c), the Martins Pereira granite samples tionated S- and I-type granites to transitional A-type
show transitional VAG (granites to granodiorites) to fields. Concerning the Martins Pereira granitoid rocks,
WPG (biotite-rich tonalites) trends. Anaua´ rock-types the higher Zr, Y and Ce contents are probably related
78 M.E.Almeidaetal./PrecambrianResearch155(2007)69–97
Fig.6. Primitivemantle-normalizedspidergram(Wood,1979)for(a)MartinsPereiragranitoids(tonalitestomonzogranites);(b)Anaua´Complex;
(c)Podsandlensesofleucogranites,and(d)SerraDouradaGranite.Datafromgranitoidsofprimitive,normalandmaturearcs(Brownetal.,1984)
arealsoplottedforcomparison.SymbolsasinFig.3.
to the presence of accessory minerals such as epidote, The Anaua´ spidergram pattern (Fig. 6b) displays
allaniteandzircon,mainlyinbiotite-richtonalitetypes. coherent Rb, Ba and K contents with primitive arcs
However, their metaluminous to peraluminous nature (Brown et al., 1984), however, U, Ta, Zr, Sm, Ti and
(Fig. 5a) and VAG character (Fig. 5c) point to I-type Y are enriched in relation to the primitive arc pattern.
granite affinity. Samples from Anaua´ Complex plot in All other elements (Th, Nb, La, Ce, Sr and P) in the
theunfractionatedS-andI-typegranitesfieldwithlow Anaua´ rocksshowprimitivetomaturearcstransitional
(K O+Na O)/CaOratios.Onlytheblobsofleucogran- values. The spidergram pattern of the blobs and lenses
2 2
ites and few Creporiza˜o Granite samples plot on the ofleucogranite(Fig.6c)displaysthehigherNb,Ta,La,
fractionatedfelsicgranitesfield(Fig.5d). CeandHffractionatedvaluesandsteeplynegativePand
Intheprimordialmantle-normalizedspidergrams,the Tianomalies.Theleucogranitesalsoshowmoderateto
MartinsPereiraGranite(Fig.6a)exhibitsdepletedtrace highRb,Ba,ThandKcontentsinrelationtochondrite
elementpatternsforsomeHFSEsuchasTa,Nb,PandTi, values.TheSerraDouradaGranite(Fig.6d)hasRb,Ba,
anddoesnotshowsignificantLILEdepletion(e.g.Srand Sr,PandTi(locallyRb,NbandP)patternremarkably
Ba),asobservedforalkali-calcicgranitoidrocksofmore similar to the normal S-type average and La, Ce and
maturearcs(Brownetal.,1984).Thispatternissome- Y display more enriched contents than normal S-type
whatsimilartothoseofthecalc-alkalinegranitoidrocks granites(e.g.ChappellandWhite,1992).
fromnormalarcs(cf.Brownetal.,1984),thoughsome
samples of the Martins Pereira Granite show strong Ti 4.2. Rareearthelement(REE)geochemistry
andPdepletion,liketheaverageofmaturearcgranitoid
rocks.ThisgeochemicalbehaviorsuggeststhatMartins ThetotalREEcontentsofthestudiedgranitoidrocks
PereiraGranitebelongstotransitionalgraniticmagma- arerelativelysimilar(Table1),exceptintheAnaua´Com-
tism, from normal to mature continental arcs (see also plex (138–148ppm) and leucogranites (100–153ppm)
Fig.4a). samples.MartinsPereira(97–624ppm)andCreporiza˜o