Calc-Alkaline Magmatism at the Archean^Proterozoic Transition

37 Pages·2007·1.69 MB·English

JOURNALOFPETROLOGY VOLUME48 NUMBER11 PAGES2149^2185 2007 doi:10.1093/petrology/egm055
Calc-Alkaline Magmatism at the
Archean^Proterozoic Transition: the Caico¤
Complex Basement (NE Brazil)
ZORANO SE¤ RGIO DE SOUZA1*, HERVE¤ MARTIN2, JEAN-JACQUES Do
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PEUCAT3, EMANUEL FERRAZJARDIM DE SA¤ 1 AND MARIA HELENA nlo
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DE FREITAS MACEDO1 ed
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1PO¤ S-GRADUAC(cid:1)A‹OEMGEODINA“MICAEGEOF|¤SICAANDDEPARTAMENTODEGEOLOGIA,CCET-UFRN,CAIXAPOSTAL h
1502, CEP 59078-970, NATAL/RN, BRAZIL ttps
2LABORATOIREMAGMASET VOLCANS, OPGC, CNRS, IRD, UNIVERSITE¤ BLAISEPASCAL,5, RUEKESSLER,63038, ://ac
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CLERMONT-FERRANDCEDEX, FRANCE de
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3GE¤OSCIENCESRENNES, CNRS, UNIVERSITE¤ DERENNES1,35042, RENNESCEDEX, FRANCE ic
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RECEIVEDJULY 26,2006; ACCEPTEDAUGUST15,2007
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ADVANCE ACCESS PUBLICATIONOCTOBER 9,2007 e
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The Paleoproterozoic metaplutonic rocks of the Caico¤ Complex KEY WORDS: calc-alkaline; magmatism; NE Brazil; Paleopro- rtic
Basement(Serido¤ region,NEBrazil)provideimportantandcru- terozoic;petrogenesis le
-a
cialinsightsintothepetrogeneticprocessesgoverningcrustalgrowth b
s
and may potentially be a proxy for understanding the Archean^ tra
Proterozoic transition.These rocks consist of high-K calc-alkaline INTRODUCTION ct/4
8
dioritetogranite,withRb^Sr,U^Pb,Pb^PbandSm^Ndagesof In Earth history, theArchean representsthe most impor- /1
1
c. 2(cid:1)25^2(cid:1)15Ga.They are metaluminous, with high YbN, K2O/ tantperiodofcontinentalcrustalgrowth.Itwascharacter- /21
Na OandRb/Sr,lowI ratios,andarelargeionlithophileelements izedbymuchhigherheatproductionthantodayand,asa 4
2 Sr 9
(LILE) enriched. Petrographic andgeochemicaldata demonstrate consequence, highergeothermalgradients, which resulted /1
5
6
thattheybelongtodifferentiatedseriesthatevolvedbylow-pressure inthegenesis of unique lithologies suchaskomatiites and 6
6
fractionation, thus resulting in granodioritic liquids.We propose a massive volumes of tonalite^trondhjemite^granodiorite 26
modelinwhichthepetrogenesisoftheCaico¤ Complexorthogneisses (TTG)magmas(Condie,1981;Taylor&McLennan,1985; by
beginswithpartialmeltingofametasomaticallyenrichedspinel-to Martin,1986,1987; Nisbet,1987).TTGshave strongly frac- gu
e
garnet-bearinglherzolite(withhigh-silicaadakitemeltasthemeta- tionated rare earth element (REE) patterns, with low st o
somaticagent),generatingabasicmagmathatsubsequentlyevolved heavy REE (HREE) contents (Yb (cid:2)8) and are devoid n
N 0
at depth through fractional crystallization of olivine, followed by of significant Eu anomalies.Their K O/Na O is low such 4
2 2 A
low-pressureintracrustalfractionation.Asubductionzonesettingis that, incontrastto classicalcalc-alkalinebasalt^andesite^ p
proposedforthismagmatism,toaccountforbothnegativeanomalies dacite^rhyolite(BADR)suites,theirdifferentiationresults ril 2
0
in high field strength elements (HFSE) and LILE enrichment. in a Na O enrichment defining trondhjemitic differentia- 1
2 9
Mantle-derived juvenile magmatism with the same age is also tiontrends.
knownintheSa‹oFranciscoandWestAfricacratons,aswellasin Basedonpetrological andexperimental studies, aswell
FrenchGuyana,andthustheArchean^Proterozoictransitionmarks asongeochemicalmodelling,thegenesisofArcheanTTG
averyimportantcontinentalaccretionevent.Italsorepresentsatran- hasbeenexplainedby partial meltingofan Archeantho-
sition from slab-dominated (in theArchean) to wedge-dominated leiitetransformedintogarnet-bearingamphiboliteoreclo-
post-Archeanmagmatism. gite (Barker & Arth,1976; Martin,1986,1987,1993,1994;
(cid:1) The Author 2007. Published by Oxford University Press. All
*Corresponding author. Telephone: [phone]. Fax: 55-84- rightsreserved.ForPermissions,pleasee-mail:journals.permissions@
32153831.E-mail:[email protected] oxfordjournals.org
JOURNALOFPETROLOGY VOLUME48 NUMBER11 NOVEMBER2007
Rappetal.,1991,2003;Rapp&Watson,1995;Martinetal., lowermost arc crust (Defant & Drummond, 1990;
1997, 2005; Foley et al., 2002; Martin & Moyen, 2002). Drummond & Defant,1990; Sen & Dunn,1994a,1994b;
Although there is consensus about the tholeiitic nature Rapp & Watson,1995; Schiano et al.,1995; Maury et al.,
of the source of Archean TTGs, the tectonic setting in 1996; Stern & Kilian, 1996; Sigmarsson et al., 1998;
which they were generated is still a subject of Martin,1999;Martinetal.,2005).
controversy. It hasbeen interpreted as either slab melting The Archean^Proterozoic boundary is marked by
in a subduction zone (Condie, 1981; Tarney et al., 1982; changes from a generalized high geothermal gradient
Martin, 1986, 1987; Rapp et al., 1991, 2003; Rapp & and subsequent production of about two-thirds to three-
Watson,1995; Foley et al., 2002; Martin & Moyen, 2002; quarters of the continental crust by accretion of juvenile
Martin et al., 2005) or hotspot-related melting of under- magmas in the Archean, to a regime of lower and more
plated basaltic crust (Atherton & Petford, 1993; Wolde diversified geothermal gradients, with predominance of Do
w
etal.,1996). crustal recycling during the Proterozoic (Taylor & n
lo
Most Archean K2O-rich granites with the isotopic sig- McLennan,1985; Martin,1986,1993,1994; Sylvester,1994). ad
nature of a mantle-derived source were emplaced at the Although at the world scale it is possible to findevidence ed
end of the Archean (2(cid:1)8^2(cid:1)5Ga), and intruded both for a progressive change inTTG composition throughout fro
m
greenstone belts andTTGs. They are also referred to as Archeantimes (Martin & Moyen,2002), the transitionat h
‘sanukitoids’ (Stern,1989; Stern & Hanson,1991; Smithies 2(cid:1)5Gawasnot sharp. Onthecontrary, it wasprogressive, ttp
s
& Champion, 1999) or ‘Closepet-type’ granites (Moyen such that someTTG are still known in Early Proterozoic ://a
etal.,2001,2003).However,theydisplaygeochemicalchar- terrains. In this context the orthogneisses of the 2(cid:1)2Ga ca
d
Caico¤ Complex in NEBrazilprovideanattractive oppor- e
acteristics intermediate between ArcheanTTG (strongly m
fractionated REE patterns and low YbN contents) and tunity to studycalc-alkaline magmatism at this period of ic.o
important petrogenetic changes. In addition, the Early u
modernjuvenile continental crust [K and more generally p
Proterozoic is also characterized by a very significant .c
large ion lithophile element (LILE) enrichment] and o
accretionevent,leadingtotheproductionofhugevolumes m
their petrogenesis is still under debate. Nevertheless, /p
of new juvenile continental crust; for example, in the e
tvhaeryiabgleeneexrtaelnlyts oapfpinetaerractotiohnasvbeetwbeeeenn mdearnivtleedpetrhirdooutgithe Sa‹o Francisco (Conceic(cid:1)a‹o,1997; Teixeira et al., 2000) and trolog
West Africa (Boher et al.,1992;Toteu et al., 2001) cratons, y
and TTG magmas (Jayananda et al., 1995; Moyen /a
et al., 1997; Smithies & Champion, 1999, 2000; Martin and in French Guyana (Gruau et al., 1985; Delor et al., rtic
2003). It also marks the formation of voluminous juvenile le
etal.,2005). crust after a period of (cid:3)300Myr (2(cid:1)5^2(cid:1)2Ga), character- -ab
When compared with TTGs, post-Archean granitoids s
izedbynegligiblemagmaticactivityandevenlackofmag- tra
are richer in K; their compositions range from granodio- c
arinteomtoaligesr.anAiten,uwmibtherhoigfhthYebmNw(4it1h0)traanced enleegmaetnivteaEnud mpuartpisomseionfstehviesrpaalpaereraiss:((M1)atrotidne,s1c9r9ib3)e.Ianlltmhiasgcmonatteicxtc,otmhe- t/48/11
ponents of thisjuveniletransitionalcrust from NE Brazil; /2
isotopic compositions of mantle-derived magmas are con- 1
(2) to constrain itspetrogenesis; (3) to discuss, inthelight 4
sidered as having been generated in a subduction zone of these data, theArchean^Proterozoic transition andthe 9/1
environment by partial melting of a fluid metasomatized subsequentPaleoproterozoicevolution. 566
mantle wedge.The dehydration of the subducted oceanic 6
2
6
crust produces LILE-enriched fluids that interact with b
theoverlyingmantlewedgeandinitiateitsmelting,result- GEOLOGICAL SETTING y g
u
ing in potassic calc-alkaline magmatism (e.g. Wyllie, Tectonic framework e
s
1983; Tatsumi, 1989; Hawkesworth et al., 1993; Keppler, Almeida et al. (1981) defined the Borborema Province t on
1996; Kogiso et al., 1997; Bureau & Keppler, 1999; in northeastern Brazil (Fig. 1), which consists of tectonic 04
Kesseletal.,2005). units stabilized during the Brasiliano orogeny Ap
In modern subduction zones, Archean TTG-like (0(cid:1)60(cid:4)0(cid:1)05Ga).Thisprovincedevelopedaftertheconver- ril 2
magmascanbegeneratedwhenhighgeothermalgradients gence of the West Africa^Sa‹o Lu|¤s and Sa‹o Francisco^ 01
9
are achievedalong the Benioff plane; for instance, during CongocratonsduringtheassemblyofWesternGondwana
subductionofanactivelyspreadingmid-oceanridge.These atc.600Ma.Inapre-driftreconstruction,itextendsfrom
magmas,referredtoasadakites,arericherinMg,Niand central and SE Brazil (Bras|¤lia^Ribeira mobile belt) to
CrthanPaleoarcheanandMesoarchean(43Ga)TTGbut West Africa through the Trans-Sahara belt composed of
are very similar to Neoarchean (53(cid:1)0Ga) TTG (Martin, theCameroon,NigeriaandHoggarshields(Caby,1989).
1999;Smithies,2000;Martinetal.,2005).Thesedifferences This area hasbeen studied for many years and several
areexplainedbyassumingthattheadakiticmagma,once contrasting geodynamic reconstructions have been pro-
generated by partial melting of the subducted oceanic posed (Almeida et al., 1981; Caby, 1989; Bertrand &
crust, interacts with the overlying mantle wedge and/or Jardim de Sa¤,1990; Caby et al.,1991; Jardim de Sa¤,1994;
2150
DESOUZAetal. PALEOPROTEROZOICCALC-ALKALINEMAGMATISM
In this context, the Serido¤ domain (Fig. 2), situated to
thenorthofthe Patos lineament, comprises: (1) theCaico¤
Complex Basement; (2) supracrustal sequences of indeter-
minate age belonging to the Serido¤ Group [late
Paleoproterozoic according to Jardim de Sa¤ (1994) and
Jardim de Sa¤ et al. (1995), or Neoproterozoic following
Hackspacher & Dantas (1997) and Van Schmus et al.
(2003)]; (3) granitoids of both late Paleoproterozoic (the
so-called G orthogneisses) and late Neoproterozoic ages
2
and interpreted as having been derived from melting of
D
an enriched lithospheric mantle or the lower continental o
w
crust, with variable crustal contamination and mixing n
lo
(Leterrier et al.,1990,1994; Jardim de Sa¤,1994; Hollanda ad
e
etal.,2003). d
fro
m
h
The Caico¤ Complex Basement ttp
s
Fieldrelationships ://a
c
Intheregionalliterature,theCaico¤ Complexcorresponds a
d
e
to the high-grade basement of the Serido¤ Group, which m
forms an area of (cid:3)60% ((cid:3)35000km2) of the exposed ic.o
Fig. 1. Pre-drift reconstruction forWest Africa and eastern South Precambrianunitsintheregionstudied(Fig.2).Itconsists up
America (afterJardim de Sa¤,1994). Rectangle outlinesapproximate .c
mainly of Paleoproterozoic meta-plutonic rocks, intruded o
area of Fig. 2. WAC, West Africa craton; AC, Amazonian craton; m
SFC,Sa‹oFranciscocraton;CC,Congocraton;BRMB,Bras|¤liaand and/or interlayered with older and subordinate meta- /p
e
RSPheiLbiee,lidPr;aerNnmBa,mobNbiuilgeceorbilaeinnltesBa;melBte;Pn,Ht;SBA,odHrLbo,ogArgedmaaramSahPoireuoladv;liinPncLeea;,mPCeantSot,.sClinaemaemroenotn; estupalr.a,1c9ru9s0t;adleroScokusz(aJaetrdali.m,19d9e3S).aT¤,1h9is84a,s1s9o9ci4a;tHionacokcscpuarcshienr trolog
y
boththeRioPiranhasandSa‹oJose¤ deCampestremassifs; /a
in the latter, Archean protoliths have also been identified rtic
le
Van Schmus et al., 1995, 2003). Briefly, the Borborema (Dantas et al., 2004). The present paper essentially deals -a
b
Province consists of several supracrustal sequences depos- with the Paleoproterozoic orthogneisses, which are here- s
ited over an Archean to Paleoproterozoic gneissic base- aftersimplyreferredtoastheCaico¤ Complex. trac
ment that has been intruded by large amounts of Theoldertectonicfabricintheseorthogneissesisahigh- t/48
iBtraassiblieainnog-amgaedgeraunpitoofidas.nuJamrdbiemrodfealSlao¤c(h1t9h9o4n)oiuntsetreprrreatiends gforladdseabnadnsdtrinongg(Dtr1a)naspssooscitiaiotend,fwoliltohwiesdocblyinaanlteoveinnttroaffotalina-l /11/21
4
thatamalgamatedjustbeforeand/orduringtheBrasiliano gentialkinematics(D2).D1andD2areusuallyinterpreted 9/1
orogeny, and Santos (1996) noted that tectonic collages astemporallydistinctevents(e.g.JardimdeSa¤,1984,1994); 56
6
occurredinboththeCaririsVelho^Kibaran(1(cid:1)1^0(cid:1)95Ga) thedepositionoftheSerido¤ GroupandintrusionoftheG 6
2 2
and Brasiliano^Pan-African orogenies in the so-called orthogneisses occurred between D1 and D2. The age of 6 b
TransversalZone. the D2 event is also controversial; the c.1(cid:1)8Ga age pro- y g
u
A notable feature of this province is the complex posed byJardim de Sa¤ (1994), Jardim de Sa¤ et al. (1995) e
s
system of crustal-scale high-temperature shear zones and others has been challenged by the younger t o
n
(Corsini et al., 1991; Jardim de Sa¤, 1994) that separate (Neoproterozoic) U^Pb detrital zircon and Sm^Nd 0
4
domains of variably strained massifs and supracrustal model dates of the Serido¤ belt supracrustal sequences A
p
sequences. These were developed (and/or activated) (Van Schmus etal.,2003). Recently, Hollanda etal. (2007) ril 2
during and after the collision between the West Africa, reported precise U^Pb sensitive high-resolution ion 0
1
Congo and Sa‹o Francisco cratons, and are closely asso- microprobe (SHRIMP) zircon ages of 2(cid:1)20(cid:4)0(cid:1)03Ga for 9
ciated with the emplacement of the Brasiliano granitoids G orthogneisses in the Serido¤ region, and thus con-
2
(Caby et al., 1981; Bertrand & Jardim de Sa¤, 1990; strained the timing of the D event. The last tectono-
2
Archanjo & Bouchez,1991; Corsini et al.,1991; Jardim de metamorphic event (D) is marked by transcurrent to
3
Sa¤, 1994). The Patos and Picu|¤^Joa‹o Ca“mara dextral oblique shear zones and emplacement of the late
shearzonesarebelievedtoaccommodatethedisplacement Neoproterozoic (Brasiliano) granitoids. The associated
of the Rio Piranhas massif toward the Sa‹o Jose¤ de metamorphism ranges from upper amphibolite to granu-
Campestre massif, which resulted in transpression of the lite facies near plutonic intrusions and crustal-scale shear
Serido¤ belt. zonestogreenschistfaciesinotherplaces.
2151
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dFeigC.a2m. GpeesotrloegMicaalssfirfa;mSBew,Soerrkidoof¤tbheeltS;ePrLid,oP¤ Datoomslainine,anmoerntht;oPfJCthSeZP,aPtoicsul|i¤^nJeoaam‹oeCnta“,mNaEraBSrahzeialr(Zmoondei;fiPedaSaZft,eProJartradliemgrdeeSShae¤,a1r9Z94o;nDe.antasetal.,2004).RPM,RioPiranhasMassif;SJCM,Sa‹oJose¤ ril 2
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DESOUZAetal. PALEOPROTEROZOICCALC-ALKALINEMAGMATISM
Geochronologyandgeochemistry 1404spectrometer,andsevenothersampleswereanalysed
Hackspacheretal.(1990)andVanSchmusetal.(1995)pub- for trace elements by inductively coupled plasma mass
lishedU^Pbdataonzirconsforgneissesandmetagabbros spectrometry (ICP-MS) at the Universite¤ de Lyon.
fromthe Sa‹oVicente^Flora“niaregion (Fig.2), whichgave Analyticalprecision for majorelements iswithin 2%, but
ages in the range 2(cid:1)16^2(cid:1)13Ga. For granodiorites of the may reach 10% for elements of low abundance (MnO,
Caico¤ area, Legrand et al. (1991) reported a whole-rock P2O5).Total iron isreportedas Fe2O3. Fortraceelements,
Rb^Sr isochronof 2(cid:1)12(cid:4)0(cid:1)08Ma anda U^Pbzirconage precisionisbetterthan5%,exceptforelementspresentat
of 2(cid:1)24(cid:4)0(cid:1)01Ma. Available Sm^Nddataformetagabbros concentrations 530ppm, where the uncertainties are
indicateT values of 2(cid:1)76^2(cid:1)62Ga (Hackspacher et al., within10%.TheREEcontentsofninesamplesweredeter-
DM
1990; Dantas,1992; Van Schmus et al.,1995). Whole-rock minedby ICP-MSattheUniversite¤ de Nancy (n¼3) and
D
Rb^Sr isochrons of granitic gneisses and porphyritic the Universite¤ Blaise Pascal (n¼6). Concentrations of o
w
granodioritic gneisses in both the Sa‹o Vicente^Flora“nia REE,Ta,U,Th,HfandScinninesamplesweremeasured n
lo
and Ac(cid:1)u areas give ages in the range 2(cid:1)2^2(cid:1)0Ga, and I byinstrumentalneutronactivationanalysis(INAA)atthe a
Sr d
of 0(cid:1)7041^0(cid:1)7028 (Macedo etal.,1984; Jardim de Sa¤ etal., PierreSuelaboratory(CEN,Saclay).Detailsoftheanalyt- ed
1987;Legrandetal.,1991;Dantas,1992). ical methods have been given elsewhere (Govindaraju fro
m
U^PbzircondatafromtheCaico¤ ComplexintheSanta et al., 1976; Martin, 1987). Chondrite normalization h
Cruz region, to the east of the Serido¤ belt, yield an age values used for the REE are from Sun & McDonough ttp
s
of 2(cid:1)18(cid:4)0(cid:1)02Ga (Dantas, 1996). In the Sa‹o Jose¤ de (1989). ://a
c
Campestre massif, Paleoproterozoic terrains surrounding Rb contents were measured by isotope dilution with a a
d
the Archean domains and correlated to the Caico¤ CamecaTHN-206 mass spectrometer atthe Universite¤ de em
Complex orthogneisses yield the following conventional Rennes I. A Finnigan Mat 262 multicollector mass spec- ic
.o
and SHRIMP U^Pb zircon and Nd model ages (Dantas, trometerwasusedtodetermineSrcontentaswellasisoto- u
p
1996; Dantas et al., 2004): (1) 3(cid:1)5Ga tonalitic gneiss with pic ratios.Totalblankswereas follows:0(cid:1)1ng for Rb,1ng .co
m
TDM of 4(cid:1)0^3(cid:1)8Ga; (2) 3(cid:1)3Ga grey monzogranitic gneiss for Sr, and measurements of NBS standard 987 gave an /p
withTDMof3(cid:1)7^3(cid:1)1Ga;(3)2(cid:1)7Gaalkalineclinopyroxene- 87Sr/86Sr value of 0(cid:1)71025(cid:4)0(cid:1)00001. Uncertainties of etro
bearing syenogranitic gneiss with TDM of 3(cid:1)5^3(cid:1)2Ga. 87Rb/86Sr are within 2%, and 87Sr/86Sr ratios are quoted log
However, no Archean terrain has been recognized to the at 2s. Sr and Nd isotopic compositions measured at y/a
westintheRioPiranhasmassif. Clermont-Ferrand were determined by mass spectrom- rtic
Geochemical studies of the Caico¤ Complex led to two etry with a Cameca THN-206 [analytical methods le-a
groups of genetic interpretation: (1) the orthogneisses have been described by Pin & Paquette (1997)]. bs
consist of Archean-like TTG suites formed by several 87Sr/86Sr ratios were normalized to 86Sr/88Sr¼0(cid:1)1194 tra
c
pulses of magmatism and associated processes of magma (Faure,1986), and 143Nd/144Nd ratios were normalized to t/4
8
mixing and mingling (Dantas,1992; Petta,1995), and sig- 146Nd/144Nd¼0(cid:1)7219. Single zircon analyses were per- /1
1
nificant contamination by crustal material accounts for formed at the Universite¤ de Rennes I using a Cameca /2
1
their negative eNd values (Dantas,1996); (2) the parental THN-206 mass spectrometer and steps at 2(cid:1)6, 2(cid:1)8 and 49
magmas were derived by partial melting of an enriched 3(cid:1)2A,followingtheprocedureofKo«ber(1986).Decaycon- /15
6
mantle; these melts then evolved by fractional crystal- stants and isotopic abundance ratios for all methods are 6
6
lization with little or no interaction with the continental those of Steiger & Ja«ger (1977). The ages, MSWD and 26
crust (Martin et al., 1990; de Souza, 1991; de Souza errors were calculated using the Excel-based version 3 of by
etal.,1993). Isoplot(Ludwig,2003).Allisotopicratiosandagecalcula- gu
e
s
tions in this paper, as well as previously published data, t o
ANALYTICAL PROCEDURES were(re)calculatedtoa2serror. n 0
4
Inthispaper,themodalcompositionhasbeenestablished Ap
from an average counting of 1300 points for each indi- STRATIGRAPHY AND ril 2
vidualthin section. Microprobe analyseswere carriedout 01
STRUCTURAL PATTERNS 9
attheUniversidadedeBras|¤liawithaCamecaSX50elec-
tron microprobe, operating at 15kVaccelerating voltage, The Caico¤ Complex is composed of two units, a meta-
25nA beam current, and 10s counting time, using syn- volcano-sedimentary unit andavolumetricallydominant,
thetic and natural minerals as standards. The analytical meta-plutonic one. In the region investigated, the supra-
errorsarewithin(cid:4)0(cid:1)5^2%forSiO,Al O,Fe O,MgO, crustal sequences represent 56% of outcropping area;
2 2 3 2 3
MnO,CaOandTiO,and4(cid:1)5^5(cid:1)6%forNa OandK O. theymainlyconsistofgarnet-bearingparagneissesandfine-
2 2 2
Concentrations of major and trace elements for grained amphibolites (meta-basalts and meta-andesites)
61 samples were determined by X-ray fluorescence togetherwithintermediatetofelsicgneisses(meta-rhyolites
(XRF) at the Universite¤ de Rennes I with a Philips PW and meta-greywackes). Subordinate amounts of banded
2153
JOURNALOFPETROLOGY VOLUME48 NUMBER11 NOVEMBER2007
iron formations (BIFs), quartzites, marbles and calc-sili- amphibole, biotite and feldspar are dynamically retro-
cate gneisses are also found.The meta-supracrustal rocks gressed into epidote, carbonate, chlorite, actinolite and
may form 20^150cm xenoliths included in the intrusive titanite.
meta-plutonic rocks. The meta-plutonic rocks consist
of(anestimationoftheexposedareaisindicatedasaper-
centage of the total area of basement rocks): (1) quartz PETROGRAPHY AND TEXTURES
dioritesandsubordinatemeta-gabbroandmeta-ultramafic General characteristics
(hornblendites, serpentinites, steatites) bodies ((cid:3)3%); Table1showstheaveragemodalcompositionsof128meta-
(2)fine-tomedium-grainedtonalitic((cid:3)28%)andgranitic plutonic rocks of the Caico¤ Complex from both the Rio
((cid:3)11%)gneisses;(3)medium-tocoarse-grainedporphyri- Piranhas and Sa‹oJose¤ de Campestre massifs. All samples
D
ticgranodioriticandgraniticgneisses((cid:3)52%). were plotted in the Q^A^P (quartz^alkali feldspar^ ow
Basic to intermediate rocks, which are volumetrically plagioclase) triangle (Fig. 3; Lameyre & Bowden, 1982). nlo
subordinate, may form 100^500m diameter stocks or, The modal compositions of basic to intermediate rocks ad
e
mthoeregracnomitomidosn,lya,ndocacsur1^a5sm10t^h2ic0k0csmheeetsncilnavtehsewmitehtian- aforlelowgababtrhoolaeniitdicqduiaffretrzendtiiaotriioten,trreesnpde.cTtiovnealyl,itiwchgincheissaelsl d from
supracrustal rocks. Quartz diorites, which are volu- plot along a low-K calc-alkaline (trondhjemitic) trend h
metrically more abundant than gabbros, diorites and akintothemostevolvedmembersofthePaleoproterozoic ttps
meta-ultramaficrocks,maycontainsmallellipticaldioritic low-K gabbro^diorite^tonalite^trondhjemite series of SW ://a
c
microgranular enclaves, and euhedral to rounded milli- Finland(Arthetal.,1978).Augengneissesvaryfromgrano- ad
e
metre-sized plagioclase phenocrysts. Field relationships dioritetosyenogranite,withafewsampleshavingmonzo- m
indicate that the tonalitic gneisses are intruded by augen dioritic andmonzonitic compositions. In fact, bothaugen ic.o
u
gneisses, which are in turn intruded by granitic gneisses. and granitic gneisses do not define real trends but rather p
.c
In low-strain regions, dioritic, quartz dioritic, granodio- plot on the medium-K to high-K calc-alkaline trends. o
m
ritic,graniticandtonaliticgneisseshavegradational,inter- Consequently, they are clearly different from typical /p
e
lobate, or wedge-shaped contacts, the first two lithotypes ArcheanTTG, which have low-K affinity (Martin,1987, tro
corresponding to the less differentiated petrographic 1994). On the other hand, they are very similar to log
y
faabcoievse. iAndllicfaetaetutrheastathnedminettrau-spilvuetorneilcatrioonckshsiopfstdheescCriabiecdo¤ NexeeompprolitfeierodzboyicrKoc2kOs-oefntrhiechAerdmcoarlicc-aanlkMaliansesifg(rGanriatvoiidous a&s /artic
le
Complex are coeval intrusions, spatially related to each Auvray,1985;Graviouetal.,1988). -a
b
otherandprobably withacommon, less evolved, basicto One outstanding feature of the Caico¤ meta-plutonic s
tra
intermediateparentalmagma. rocks is the abundance of ferromagnesian minerals, c
baTndhiengmthosattopveenreptrrianttisveeafraliberricma(Dgm2)atiiscfaabmricesta(mcoonrtpahcitcs mthaeminlyfrcolminot-hame pahmibpohleiboalned-poboiortittey,piwcahlichArdcihsteiannguTisTheGs t/48/11
between enclaves and more differentiated granitoid hosts; (Martin, 1987, 1994). In tonalitic, augen and granitic /21
4
alignment of feldspar and amphibole phenocrysts). The gneisses the less evolved facies are richer in amphibole 9/1
D2 fabric is also marked on G2 granitoid sheets intruded and poorer in biotite than the more differentiated mem- 566
intotheinterfacebetweenthe Caico¤ basement and supra- bers; this feature emphasizes the role played by the 6
2
crustal rocks of the Serido¤ Group. The metamorphism fractionation of these phases at the beginning of differen- 6 b
associated with D2 is generally in upper amphibolite tiation.Theregular variationof mafic andfelsic minerals y gu
faciesandoflowtomediumpressure,asindicatedbypara- together with preserved igneous textures (clinopyroxene, e
s
genesisincludingcordierite(cid:4)sillimanite(cid:4)kyanite(cid:4)rutile amphibole, feldspar, titanite and apatite phenocrysts), t o
n
in garnet-bearing paragneisses. Jardim de Sa¤ (1994) and absence of metasomatic replacementof K-feldsparby Na- 0
4
Jardim de Sa¤ et al. (1995) ascribed the D event to a late plagioclase (Drummond et al.,1986) and conservation of A
2 p
Paleoproterozoic stage, basedonthe assumptionof a syn- magmaticgeochemicaltrends (seebelow) all suggestthat ril 2
tectonic(syn-D)emplacementoftheG orthogneissesand themineralassemblage observedatpresentisthe sameas 0
2 2 1
meta-pegmatites, dated at1(cid:1)9^1(cid:1)8Ga according to U^Pb inthemagmaticprotoliths. 9
zircon and Rb^Sr isochron ages (Jardim de Sa¤ et al.,
1995); aU^Pbtitaniteage of 1(cid:1)97(cid:4)0(cid:1)02GafromaCaico¤ Basic to intermediate rocks (BIR)
Complexorthogneiss(Hackspacheretal.,1995)maybean According to their degree of recrystallizationthe basic to
indicationofbasementoverprintduringtheD thermotec- intermediate rocks of the Caico¤ Complex display grano-
2
tonic event.The D tangential fabrics are overprinted by blastic, nematoblastic, pokilitic and laminated textures.
2
NE^SW Brasiliano-age transcurrent (inthe Rio Piranhas Based on modal composition, three main petrographic
massif) and extensional (in the Sa‹o Jose¤ de Campestre facies can be distinguished: (1) hornblende4biotite,
massif)shear zones (D).Nearandinsidetheshear zones, the most widespread facies; (2) biotite4hornblende;
3
2154
DESOUZAetal. PALEOPROTEROZOICCALC-ALKALINEMAGMATISM
Table1: Averagemodalcompositionofmeta-plutonicrocksoftheCaico¤ Complex,BorboremaProvince,NEBrazil
Basictointermediaterocks(n¼29) Augengneisses(n¼55) Graniticgneisses(n¼21) Tonaliticgneisses(n¼23)
Facies: DioþHb Hb4Bio Bio4Hb Hb4Bi Bio4Hb Bio Hb4Bio Bio4Hb Bio Hb4Bio Bio4Hb Bio
n: 8 13 8 12 26 17 3 3 15 4 13 6
Qz(%) 2(cid:1)80 6(cid:1)7 10(cid:1)5 18(cid:1)4 24(cid:1)9 29(cid:1)3 27(cid:1)6 29(cid:1)4 32(cid:1)7 24(cid:1)7 31(cid:1)6 36(cid:1)8
AF 0(cid:1)01 0(cid:1)2 0(cid:1)0 14(cid:1)5 16(cid:1)8 27(cid:1)5 30(cid:1)7 19(cid:1)8 31(cid:1)9 3(cid:1)4 2(cid:1)6 5(cid:1)2
Pl 20(cid:1)1 37(cid:1)7 52(cid:1)1 38(cid:1)3 36(cid:1)6 32(cid:1)7 31(cid:1)1 35(cid:1)7 26(cid:1)9 39(cid:1)2 42(cid:1)7 44(cid:1)3 D
o
w
Bio 2(cid:1)8 14(cid:1)9 21(cid:1)7 10(cid:1)3 13(cid:1)5 7(cid:1)4 1(cid:1)4 6(cid:1)4 6(cid:1)1 6(cid:1)8 15(cid:1)4 11(cid:1)3 n
lo
Hb 56(cid:1)6 37(cid:1)1 10(cid:1)8 14(cid:1)5 4(cid:1)2 0(cid:1)0 6(cid:1)2 2(cid:1)1 0(cid:1)0 19(cid:1)0 3(cid:1)8 0(cid:1)1 a
d
Dio 13(cid:1)4 0(cid:1)0 0(cid:1)0 0(cid:1)0 0(cid:1)0 0(cid:1)0 0(cid:1)7 0(cid:1)0 0(cid:1)0 0(cid:1)0 0(cid:1)0 0(cid:1)0 ed
Tit 2(cid:1)0 1(cid:1)5 1(cid:1)6 1(cid:1)6 1(cid:1)4 1(cid:1)0 0(cid:1)8 1(cid:1)0 0(cid:1)3 1(cid:1)6 1(cid:1)1 0(cid:1)2 fro
m
Op 0(cid:1)4 0(cid:1)2 0(cid:1)2 0(cid:1)3 0(cid:1)5 0(cid:1)7 0(cid:1)2 1(cid:1)1 0(cid:1)6 0(cid:1)1 0(cid:1)5 0(cid:1)4 h
Ep 1(cid:1)3 0(cid:1)7 2(cid:1)6 0(cid:1)9 1(cid:1)4 0(cid:1)3 0(cid:1)8 3(cid:1)2 0(cid:1)3 4(cid:1)6 1(cid:1)5 0(cid:1)8 ttp
s
Apt 0(cid:1)1 0(cid:1)2 0(cid:1)2 0(cid:1)5 0(cid:1)5 0(cid:1)3 0(cid:1)4 0(cid:1)4 0(cid:1)2 0(cid:1)4 0(cid:1)6 0(cid:1)5 ://a
c
Zrn tr tr tr tr tr tr tr tr tr tr 0(cid:1)1 0(cid:1)1 a
d
e
Others 0(cid:1)4 0(cid:1)8 0(cid:1)3 0(cid:1)7 0(cid:1)2 0(cid:1)8 0(cid:1)1 0(cid:1)9 1(cid:1)0 0(cid:1)2 0(cid:1)1 0(cid:1)3 m
Total 100(cid:1)0 100(cid:1)0 100(cid:1)0 100(cid:1)0 100(cid:1)0 100(cid:1)0 100(cid:1)0 100(cid:1)0 100(cid:1)0 100(cid:1)0 100(cid:1)0 ic.o
u
(cid:1)M 75(cid:1)6 55(cid:1)3 35(cid:1)8 28(cid:1)1 21(cid:1)5 9(cid:1)7 10(cid:1)5 14(cid:1)3 7(cid:1)6 32(cid:1)4 23(cid:1)0 13(cid:1)3 p
.c
AF/Pl 0(cid:1)0 0(cid:1)0 0(cid:1)0 0(cid:1)4 0(cid:1)5 0(cid:1)8 1(cid:1)0 0(cid:1)6 1(cid:1)2 0(cid:1)1 0(cid:1)1 0(cid:1)1 o
m
/p
e
Qz, quartz; AF, alkali-feldspar; Pl, plagioclase; Bio, biotite; Hb, hornblende; Dio, diopside; Tit, titanite; Op, opaque tro
minerals; Ep, epidote; Apt, apatite; Zrn, zircon; Others, chlorite(cid:4)carbonate(cid:4)muscovite; (cid:1)M, total of mafic phases; lo
tr, trace. gy
/a
rtic
le
-a
thoseofcommongreenhornblende,butchemicalvariation b
s
fromMg-hornblendetoactinoliteandXMgof0(cid:1)7^0(cid:1)4were tra
c
reported by Petta (1995) in the Sa‹o Vicente^Flora“nia t/4
8
region. Plagioclase (An25^40) appears as millimetre-sized /1
1
phenocrystswithsharpcontactsorroundedmargins,com- /2
1
monlyformingrecrystallizedpolygonalmosaics. 4
9
Accessory minerals are: (1) grey to brown lozenge- /1
5
shapedtitanitephenocrysts (withquartz, amphibole, apa- 66
6
tite, andbiotiteinclusions),intergranularcrystalsorsmall 2
6
grainsalsofollowingthecleavage of biotite, amphibole or b
y
Fig. 3. Modal composition of orthogneisses of the Caico¤ Complex clinopyroxene; (2) small light yellow prismsandirregular gu
e
reportedintheQ^A^Ptriangle(Streckeisen,1976).To,tonalite;Gd, crystalsofepidote,withfrequentmetamicticallanitecore; s
granodiorite;Gr,granite;QM,quartzmonzonite;QMD,quartzmon- (3) opaque minerals that occur as lamellae andquadratic t on
z(Lodaimoreiyter.eT&heBoawrrdoewns,1c9o8r2r)e:sTp,onthdolteoiittiyc;piAca,laldkiaffleinreen.tCiaatilocn-altkraelnindes or poikilitic grains associated to biotite and titanite; (4) 04 A
trends: a, low-K; b, intermediate-K; c, high-K. BIR, basicto inter- apatite and zircon inclusions inclinopyroxene, amphibole p
mediaterock;TON,tonaliticgneiss;AG,augengneiss;GR,granitic and plagioclase. Plagioclase and biotite alteration occa- ril 2
gneiss. 0
sionally and locally gives rise to carbonate and chlorite, 1
9
respectively.
(3) clinopyroxeneþhornblende and rare biotite, subordi-
nateto(1)and(2). Augen gneisses (AG)
Clinopyroxene is a colourless or pale green diopside up The augen gneisses are derived fromporphyritic plutonic
to 2^5mm long that is sometimestransformed into green protoliths and have granoblastic to granonematoblastic
amphibole or brown biotite. Amphibole often occurs as textures.Themostimportantfeatureismillimetre-tocen-
euhedral to subhedral polygonal aggregates; it is strongly timetre-sized (0(cid:1)1^20mm) augens of perthitic K-feldspar
pleochroic (X pale yellow, Z deep green to blue) and its (microcline Or Ab,Table 2) and slightly zoned plagio-
93 7
length ranges from 0(cid:1)5 to 4mm. Its opticalproperties are clase (An ; Table 2). K-feldspar augens often contain
22^30
2155
JOURNALOFPETROLOGY VOLUME48 NUMBER11 NOVEMBER2007
inclusions of plagioclase, biotite, amphibole, titanite and or in reaction contact with amphibole, in this case asso-
zircon. Both types of augen can be deformed, recrystal- ciated with epidote and titanite. It has variable size
lized, and wrapped by quartz ribbons and new feldspar (0(cid:1)1^4mm), strong pleochroism (X light yellow, Z deep
grains. Myrmekite and replacement of plagioclase by yellow), with lowTi contents and X of 0(cid:1)5 (Table 2),
Mg
microcline is common between recrystallized aggregates andcanbeclassifiedasFe-biotite.
as well as in pressure shadows near feldspar augens.
Accessory minerals are: titanite (poikilitic or interstitial Granitic gneisses (GR)
grains); pistacite-rich epidote (P ¼37; Table 2) forming Granitic gneisses are mineralogically similar to augen
ss
anhedral rims around metamictic allanite core, anhedral gneisses, except that they contain smaller amounts of
grains in reactioncontactswithbiotiteandamphibole, or dark minerals (Table1).Texturally, they are equigranular
D
associated with saussuritization of plagioclase; oxides (1^2mm) or slightly inequigranular, and microporphyri- o
w
(usually bordered by epidote and titanite); and apatite tic. Plagioclase (An ) is slightly zoned or optically n
20^25 lo
andzirconincludedinothermineralphases. homogeneous. Biotite is brown and relatively rare, and ad
e
Amphiboleandbiotitemarkthemainplanarfabric(S). colourless clinopyroxene (diopside) has alsobeen scarcely d
Theformeroccursasanhedraltosubhedralprismatican2d observed in the less differentiated samples. Amphibole is fro
m
strongly pleochroic grains (x yellowish green, z deep green to blue with optical properties of common green h
green),0(cid:1)1^2mm in size. It consists of Ca-rich amphibole hornblende. Epidote, opaque minerals, apatite and zircon ttp
s
(Table 2) with (NaþK)A¼0(cid:1)7, (CaþNa)B¼1(cid:1)8, arefrequentaccessoryphases. ://ac
Ti¼0(cid:1)1p.f.u., Fe3 þ4AlVI and X ¼0(cid:1)4, and can be a
Mg d
classifiedas magnesian^hastingsitichornblende according Tonalitic gneiss (TON) em
totheclassificationof Leake (1978). Some crystals contain Tonaliticgneisses arecompositionallyandtexturallysimi- ic.o
u
inclusions of plagioclase, quartz, titanite, biotite and apa- lar to granitic and augen gneisses except that they have p
.c
tite. Biotiteappearsas isolatedflakes, locallyas inclusions little or no K-feldspar and are richer in mafic minerals. o
m
/p
e
Table2: MineralchemistryofselectedsamplesoftonaliticgneissandaugengneissfromtheAc(cid:1)uregion trolo
g
y
/a
Tonaliticgneiss(sampleAZ49C) Augengneiss(sampleAZ66C) rtic
le
-a
Amphibole Biotite Plagioclase Amphibole Biotite Plagioclase K-Feldspar Epidote bs
n: 15 3 4 6 7 5 1 1 tra
c
t/4
8
SiO2(wt%) 39(cid:1)51 35(cid:1)39 64(cid:1)83 41(cid:1)05 35(cid:1)46 63(cid:1)40 64(cid:1)79 37(cid:1)42 /11
/2
TiO2 0(cid:1)86 2(cid:1)23 – 0(cid:1)67 1(cid:1)84 – – 0(cid:1)07 14
Cr2O3 0(cid:1)02 0(cid:1)02 – 0(cid:1)05 0(cid:1)02 – – – 9/1
Al2O3 11(cid:1)64 14(cid:1)67 22(cid:1)16 11(cid:1)14 14(cid:1)84 23(cid:1)60 18(cid:1)66 22(cid:1)8 56
6
FeOt 23(cid:1)62 24(cid:1)14 0(cid:1)06 21(cid:1)38 20(cid:1)23 0(cid:1)08 0(cid:1)03 – 62
6
Fe2O3t – – – – – – – 14(cid:1)0 b
y
MnO 1(cid:1)51 1(cid:1)0 – 0(cid:1)4 0(cid:1)29 – – 0(cid:1)09 g
u
MgO 5(cid:1)62 8(cid:1)34 – 7(cid:1)75 10(cid:1)39 – – 0(cid:1)02 es
CaO 11(cid:1)03 0(cid:1)03 3(cid:1)22 11(cid:1)51 0(cid:1)06 4(cid:1)93 – 23(cid:1)31 t o
n
Na2O 1(cid:1)38 0(cid:1)06 9(cid:1)81 1(cid:1)36 0(cid:1)06 9(cid:1)17 0(cid:1)73 – 04
K2O 1(cid:1)50 9(cid:1)30 0(cid:1)21 1(cid:1)43 9(cid:1)28 0(cid:1)11 15(cid:1)63 – Ap
Total 96(cid:1)69 95(cid:1)18 100(cid:1)29 96(cid:1)74 92(cid:1)47 101(cid:1)29 99(cid:1)84 97(cid:1)71 ril 2
0
XMg¼0(cid:1)30 XMg¼0(cid:1)38 An¼15(cid:1)1 XMg¼0(cid:1)39 XMg¼0(cid:1)48 An¼22(cid:1)7 An¼0(cid:1)0 Pss¼37(cid:1)0 19
P17(cid:1)4 Ab¼83(cid:1)6 Ab¼76(cid:1)6 Ab¼6(cid:1)7
T2705 Or¼1(cid:1)1 Or¼0(cid:1)6 Or¼93(cid:1)3
1P ((cid:4)0(cid:1)6kbar) (Schmidt, 1992).
2T ((cid:4)758C) (Blundy & Holland, 1990).
X ¼Mg/(MgþFe2þ); P ¼Fe3þ/(Fe3þþAl); An¼Ca/(CaþNaþK); Ab¼Na/(CaþNaþK); Or¼K/(CaþNaþK).
Mg ss
Amphiboles were normalized to 23 O2(cid:5) atoms, 1 OH(cid:5) group; biotite to 22 O2(cid:5) atoms, 2 OH(cid:5) group; epidote to 12
O2(cid:5)atoms,1OH(cid:5)group;feldsparto8O2(cid:5)atoms.Fe3þofamphibolewerecalculatedassumingafixedratioFe3þ/Fe ¼0
t
(cid:1)27(Hammarstrom&Zen,1986),andtheremainingFewasassumedtobeFe2þ.Inepidote,FeO(wt%)wastransformed
into Fe O assuming an Fe O /FeO ratio of 1(cid:1)1114.
2 3 2 3
2156
DESOUZAetal. PALEOPROTEROZOICCALC-ALKALINEMAGMATISM
Plagioclase(An )isslightlylesscalcicthanintheaugen ((cid:4)0(cid:1)6kbar and (cid:4)758C), the calculated P^Tvalues are in
12^18
andgraniticgneisses(Table2).Amphiboleisstronglypleo- the range 7(cid:1)4^6(cid:1)8kbarand 732^7058C; theyarethe same
chroicrangingfrombrowntodeepgreen,withotheropti- for amphibole of both tonalitic and augen gneisses. This
calpropertiessimilartoamphibolesoftheaugengneisses. correspondstothetransitionbetweentheupperamphibo-
Chemically, they have (NaþK) ¼0(cid:1)7, (CaþNa) ¼1(cid:1)7, lite to granulite facies, in the field of partial melting of
A B
Ti¼0(cid:1)1p.f.u.,Fe3þ4AlVIandX ¼0(cid:1)3,theyareslightly water-saturated granitic systems.These values are consis-
Mg
Si-andMg-impoverishedwhencomparedwithamphibole tent withboth recrystallization of feldspar phenocrysts in
from the augen gneisses, and they can be classified as meta-plutonicrocksandmigmatizationofthemeta-pelitic
hastingsitic hornblende (Leake, 1978). Biotite is slightly components of the Caico¤ Complex. On the other hand,
Ti-richerandMg-poorerthanintheaugengneisses. as coexisting amphibole and biotite have different X ,
Mg
D
overall chemical equilibrium was not achieved (Vynhal o
w
P^Tconditions of both emplacement and etal.,1991).Itisproposedthatthesyntectonicemplacement n
lo
recrystallization and cooling of the meta-plutonic rocks occurred between a
d
TheCaico¤ Complexhasbeenvariablydeformedandrecrys- 7(cid:1)4 and 6(cid:1)8kbar and 732 and 7058C, which is consistent ed
tallized under amphibolite-facies conditions. Despite this, with all other field data, textural observations, and the fro
m
mineralshapesandinclusionrelationshipsallowustodistin- mineralogicalsequencedescribedabove.Thiscorresponds h
guishbetweenrelictsofigneoustexturesandmetamorphic toanaverage geothermalgradientof (cid:3)308C/km nearthe ttps
features. The former are represented by plagioclase and plutoncontacts. ://a
c
K-feldspar, as well as amphibole andtitanite phenocrysts. a
d
e
Generally,plagioclase,K-feldsparandamphibolearetextur- m
allystronglysimilartofeldsparandamphibolephenocrysts GEOCHRONOLOGY AND ic.o
described in quartz diorite and granodiorites from well- ISOTOPIC DATA up
.c
preserved calc-alkaline granitoids (Graviou & Auvray, FivesamplesoftonaliticgneissesfromCaico¤ andAc(cid:1)uwere o
m
1985;Graviouetal.,1988).Takingintoaccountthesepoints, analysed for Rb^Sr isotopic composition (Table 3).They /p
e
we selected the less deformed and/or metamorphically yielded an age of 2229(cid:4)64Ma with MSWD¼1(cid:1)9 and tro
recrystallizedsamplesformicroprobestudy. initial 87Sr/86Sr (I ) of 0(cid:1)7023(cid:4)0(cid:1)0005 (Fig. 4a). Single lo
Sr g
The Al-in amphibole geobarometer (Schmidt, 1992) zircon from sample EV10A gave a 207Pb/206Pb age of y/a
and the plagioclase^hornblende geothermometer (Blundy 2181(cid:4)10Ma (Table 4, Fig. 4a), which is within the error rtic
le
& Holland, 1990) were used to constrain the P^T limitsofthewhole-rock Rb^Srage.Thezircongrainsare -a
b
conditions of re-equilibration of amphibole (data from idiomorphic, dark (metamictic?) tolightbrown, andmay s
Table 2). Based onthe experimental errors of the method containminuteinclusionsofapatiteandfluid. trac
t/4
8
/1
1
/2
Table3: Rb^Srisotopedataformeta-plutonicrocksoftheCaico¤ ComplexbasementintheRioPiranhasmassif 1
4
9
/1
5
6
Lithology Facies Sample Rb(ppm) Sr(ppm) 87Rb/86Sr((cid:4)2%) 87Sr/86Srm((cid:4)0(cid:1)001%) 66
2
6
b
y
g
u
Tonaliticgneiss BioþHb EV10A 57 503 0(cid:1)32(cid:4)0(cid:1)01 0(cid:1)712612(cid:4)08 e
s
Tonaliticgneiss BioþHb EV10B 59 431 0(cid:1)40(cid:4)0(cid:1)01 0(cid:1)715147(cid:4)09 t o
n
Tonaliticgneiss BioþHb VC13C 108 487 0(cid:1)64(cid:4)0(cid:1)01 0(cid:1)722722(cid:4)13 0
4
Tonaliticgneiss BioþHb EV7B 113 493 0(cid:1)66(cid:4)0(cid:1)01 0(cid:1)723314(cid:4)09 A
p
Tonaliticgneiss Bio VC13D 98 417 0(cid:1)68(cid:4)0(cid:1)01 0(cid:1)724450(cid:4)13 ril 2
Augengneiss BioþHb EV12C 123 857 0(cid:1)42(cid:4)0(cid:1)02 0(cid:1)716119(cid:4)12 0
1
9
Augengneiss BioþHb EV13E 100 467 0(cid:1)61(cid:4)0(cid:1)01 0(cid:1)721942(cid:4)11
Augengneiss Bio EV12F 100 688 0(cid:1)42(cid:4)0(cid:1)01 0(cid:1)716226(cid:4)10
Augengneiss Bio EV12E 119 544 0(cid:1)63(cid:4)0(cid:1)01 0(cid:1)721890(cid:4)10
Augengneiss Bio EV13B 94 233 1(cid:1)16(cid:4)0(cid:1)02 0(cid:1)739604(cid:4)12
Augengneiss Bio EV13C 125 170 2(cid:1)13(cid:4)0(cid:1)04 0(cid:1)770845(cid:4)15
Augengneiss Bio EV13D 123 135 2(cid:1)63(cid:4)0(cid:1)05 0(cid:1)784864(cid:4)10
Graniticgneiss Bio EV7A 114 299 1(cid:1)10(cid:4)0(cid:1)02 0(cid:1)737658(cid:4)09
Bio, biotite; Hb, hornblende. The NBS 987 standard gave 87Sr/86Sr ratio of 0(cid:1)710248(cid:4)0(cid:1)000009. m, measured.
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JOURNALOFPETROLOGY VOLUME48 NUMBER11 NOVEMBER2007
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Fig.4. Rb^Srwhole-rockisochronandsinglezircon207Pb/206Pbagefortonaliticgneisses(a)andaugengneisses(b)oftheCaico¤ Complex rtic
fromtheRioPiranhasmassif. le
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Table4: SinglezirconPbisotopicdataforsamplesEV10AandEV12C t/4
8
/1
1
/2
Lithology Sample Current(A) 206Pb/204Pb 207Pb/206Pb Error(2s) 207Pb/206Pb Error(2s) 1
4
9
corrected age(Ma) /1
5
6
6
6
2
6
Tonaliticgneiss EV10A 2(cid:1)6 20000 0(cid:1)1363 0(cid:1)0008 2181 10 b
y
Augengneiss EV12C 2(cid:1)6 5806 0(cid:1)1356 0(cid:1)0002 2172 5 gu
e
2(cid:1)8 0(cid:1)1350 0(cid:1)0005 2164 5 s
t o
3(cid:1)2 0(cid:1)1362 0(cid:1)0030 2178 17 n
0
4
A
p
ril 2
0
1
9
SevensamplesofaugengneissesfromAc(cid:1)uwereanalysed In the Santa Cruz region (Fig. 2), in the Sa‹o Jose¤ de
for the Rb^Sr whole-rock composition (Table 3). They Campestre massif, seven samples of the Caico¤ Complex
define an isochronwith anage of 2195(cid:4)62Ma and I of were analysed for Sr and Nd isotopes (Table 5). For these
Sr
0(cid:1)7027(cid:4)0(cid:1)0009, with MSWD¼5(cid:1)1 (Fig. 4b). Three-step samples, an Rb^Sr isochron yielded an age of
heating of single zircon from sample EV12C gives similar 2144(cid:4)70Ma, with I of 0(cid:1)7025(cid:4)0(cid:1)0005 and MSWD of
Sr
results(Table4,Fig.4b),withanaverage207Pb/206Pbageof 24 (Fig. 5a). The whole-rock Sm^Nd isochron with all
2179(cid:4)17Ma,whichissimilartotheRb^Srage.Thezircon points resulted in an extremely elevatederror on age and
grains are idiomorphic, usually concentrically zoned, MSWD (2253(cid:4)450 Ma and 189, respectively). The best
colourlessorlightbrown,withmanyapatiteinclusions. fit is produced when samples ES56A, ES145 and ES196
2158