the rio apa craton in mato grosso do sul (brazil) and northern paraguay: geochronological ...

43 Pages·2011·1.25 MB·English

[AmericanJournalofScience,Vol.310,November,2010,P.981–1023,DOI10.2475/09.2010.09]
THERIOAPACRATONINMATOGROSSODOSUL(BRAZIL)AND
NORTHERNPARAGUAY:GEOCHRONOLOGICALEVOLUTION,
CORRELATIONSANDTECTONICIMPLICATIONSFORRODINIAAND
GONDWANA
UMBERTOG.CORDANI*,†,WILSONTEIXEIRA*,COLOMBOC.G.TASSINARI*,
JOSE´ M.V.COUTINHO*,andAMARILDOS.RUIZ**
ABSTRACT. TheRioApacratonicfragmentcropsoutinMatoGrossodoSulState
ofBrazilandinnortheasternParaguay.ItcomprisesPaleo-Mesoproterozoicmedium
grademetamorphicrocks,intrudedbygraniticrocks,andiscoveredbytheNeoprotero-
zoic deposits of the Corumba´ and Itapocumi Groups. Eastward it is bound by the
southernportionoftheParaguaybelt.Inthiswork,morethan100isotopicdetermina-
tions, including U-Pb SHRIMP zircon ages, Rb-Sr and Sm-Nd whole-rock determina-
tions, as well as K-Ar and Ar-Ar mineral ages, were reassessed in order to obtain a
completepictureofitsregionalgeologicalhistory.
ThetectonicevolutionoftheRioApaCratonstartswiththeformationofaseries
of magmatic arc complexes. The oldest U-Pb SHRIMP zircon age comes from a
banded gneiss collected in the northern part of the region, with an age of 1950 (cid:1) 23
Ma.ThelargegraniticintrusionoftheAlumiadorBatholithyieldedaU-Pbzirconage
of1839(cid:1)33Ma,andfromthesoutheasternpartoftheareatwoorthogneissesgave
zircon U-Pb ages of 1774 (cid:1) 26 Ma and 1721 (cid:1) 25 Ma. These may be coeval with the
Alto Terereˆ metamorphic rocks of the northeastern corner, intruded in their turn by
theBa´ıadasGarc¸asgraniticrocks,oneofthemyieldingazirconU-Pbageof1754(cid:1)49
Ma.Theoriginalmagmaticprotolithsoftheserocksinvolvedsomecrustalcomponent,
as indicated by the Sm-Nd T model ages, between 1.9 and 2.5 Ga. Regional Sr
DM
isotopic homogenization, associated with tectonic deformation and medium-grade
metamorphismoccurredatapproximately1670Ma,assuggestedbyRb-Srwholerock
reference isochrons. Finally, at 1300 Ma ago, the Ar work indicates that the Rio Apa
Craton was affected by widespread regional heating, when the temperature probably
exceeded350°C.
Geographic distribution, age and isotopic signature of the lithotectonic units
suggest the existence of a major suture separating two different tectonic domains,
juxtaposedatabout1670Ma.Fromthattimeon,theunifiedRioApacontinentalblock
behavedasonecoherentandstabletectonicunit.ItcorrelateswellwiththeSWcorner
of the Amazonian Craton, where the medium-grade rocks of the Juruena-Rio Negro
tectonicprovince,withagesbetween1600and1780Ma,werereworkedatabout1300
Ma.Lookingatthelargestscale,theRioApaCratonisprobablyattachedtothelarger
Amazonian Craton, and the actual configuration of southwestern South America is
possiblyduetoacomplexarrangementofallochthonousblockssuchastheArequipa,
AntofallaandPampia,withdifferentsizes,thatmayhaveoriginatedasdisruptedparts
of either Laurentia or Amazonia, and were trapped during later collisions of these
continentalmasses.
Keywords:RioApaCraton,Geochronology,SouthAmerica,Tectonicevolution,
Geotectoniccorrelations.
introduction
The Rio Apa cratonic fragment, which is located in the central part of South
America (fig. 1) and measures 220 km long (cid:1) 60 km wide, is poorly exposed, being
*InstituteofGeosciences,UniversityofSa˜oPaulo,RuadoLago562,05508-080,Sa˜oPaulo,SP,Brazil
**InstituteofGeosciences,FederalUniversityofMatoGrosso,AvFernandoCorreias/n,09923-900
Cuiaba´,MT,Brazil
†Correspondingauthor:[email protected]
981
982 U.G.Cordani&others—TheRioApaCratoninMatoGrossodoSul(Brazil)andnorthern
60º
CG
AMAZONIA
AT
SF
LTucavaca P
AA im
its ofA RA PR -20º
n
d
e
a
n
B
e
lt
Cratonic Masses and
Fragments
PA
SF = São Francisco
RP
PR = Paraná
RA = Rio Apa
RP = Rio de la Plata
PA = Pampia
?
AA = Arequipa - Antofalha
CG = Central Goiás
-40º
60º
1 2 3 4 5 6 7
8 9
Fig.1. GeotectonicsketchofCentralSouthAmerica,showingtheRioApaCratonasanallochthonous
tectonicfeature,attachedtotheAmazonianCratonintheprocessofagglutinationofGondwana(adapted
fromKro¨nerandCordani(2003).1—Cratonicareas,includingtheRondonian-SanIgna´cio(2)andSunsa´s
(3) belts (in Amazonia); 4—Central Goia´s Massif, including large mafic complexes; 5—Neoproterozoic
tectonicprovinces(forexample,P—ParaguayandAT—Araguaia-Tocantinsbelts);6—Tucavacaaulacogen;
7—Phanerozoicsedimentarycover;8—Pampeanmagmaticarc.9—Concealedcratonicareas.Seetextfor
details.
coveredbyextensivePhanerozoicsedimentarysequences.ItcropsoutattheBrazilian
borderwithBoliviaandParaguayandextendstothesouthintoParaguayanterritory.It
ispartofatectonicallystablecratonicdomainoftheParaguaybelt(whichwasfolded
andregionallymetamorphosedduringtheNeoproterozoicBrasilianoOrogeny),and
is overlain by the mainly carbonate platform covers of the Corumba´ and Itapocumi
Groups (Almeida, 1965 and 1967; Alvarenga and others, 2000; Boggiani and Alva-
renga,2004).
Almeida(1967)wasthefirsttosuggestthattheRioAparegionwasadirectlinkto
his “Guapore´ Craton,” which is the southern part of what is now named the “Amazo-
nian Craton.” Regarding the geotectonic setting of southern South America during
Neoproterozoic time, two main scenarios must be considered in relation to the Rio
Apa cratonic fragment. One scenario, proposed by several authors (Dell’Arco and
others,1982;AlvarengaandSaes,1992;Kro¨nerandCordani,2003)describedtheRio
Apa as an allochthonous feature, which, during the agglutination of Gondwana, was
Paraguay:Geochronologicalevolution,correlationsandtectonicimplicationsforRodinia 983
attached to the Amazonian Craton along the Neoproterozoic Tucavaca belt, which is
considered a suture. The other scenario, proposed by Ruiz and others (2005), and
followed by Cordani and others (2009), described the Rio Apa cratonic fragment, in
the Neoproterozoic, as a prolongation of the Amazonian Craton. In this case, the
Tucavaca belt would correspond to an aulacogenic feature (A´vila-Salinas, 1992)
developedovercontinentalcrustasareflectionofthecompressionaltectonicepisodes
oftheParaguay-Araguaia/Tocantinsorogen.Figure1,adaptedfromRuizandothers
(2005),illustratesthisidea.
ThepositionoftheRioApacratonicfragmentwithinthecontextoftheMeso-and
Neoproterozoicsupercontinents,andconsequentlyitscorrelationwithitsneighbour-
ingcontinentalmasses,isrelevantinordertoinvestigatethetectonicevolutionofthe
Grenvillian mobile belts related to the agglutination of Rodinia and Gondwana. For
the terminal Mesoproterozoic, attempts to establish a correlation should be made
taking into consideration the tectonic provinces of the Amazonian Craton, as well as
thedispersedGrenvillian-typebasementinlierswithintheyoungertectonicframework
of the Andean Cordillera. Therefore, the determination of its geological history is
crucial to put on a better basis its possible position within the context of Rodinia.
Moreover, it is also important to understand its role during the agglutination of
Gondwana.
A great deal of geochronological information about the Rio Apa cratonic frag-
menthasbeenavailablesincethefirstcomprehensivegeologicalmappingwascarried
out(Araujoandothers,1982;Godoiandothers,1999),inwhichseveralRb-SrandK-Ar
determinationswereobtainedatareconnaissancescale.Asaresult,thepolymetamor-
phic character of the region was clearly demonstrated. Later, a series of additional
Rb-Sr measurements, plus several Ar-Ar, U-Pb and Sm-Nd ages, were obtained, and
many of them were made known as preliminary notes (Cordani and others, 2005a;
Cordani and others, 2008a and 2008b). Some additional U-Pb SHRIMP ages and
Sm-Nd model ages were also included in the regional report of Lacerda-Filho and
others (2006), although in this case the analytical data for the U-Pb ages were not
reported. This important set of geochronological data makes it possible to compare
and evaluate the interpretative values of different dating methods employed on the
samerocksamples,whichwerecollectedwithinthesameareaandbelongtothesame
geological context. We recognize that the existing data falls into three categories:
(1)—alreadypublisheddataandinterpretations,suchasthosereportedbyAraujoand
others (1982) and Godoi and others (1999); (2)—data included only in internal
reportsorotherpublicationsnoteasilyaccessibleoutsideBrazil;and(3)—completely
newdataandideas,asthosereportedinthiswork.
The objective of this work is therefore to make a comprehensive report of the
geochronologicalstudiesconductedintheRioApaCratonandproduceaconsistent
interpretationofthetectonicevolutionofthisunit.Inthisrespect,ourobjectivesare:
(1) to integrate the interpretation of the geochronological data in order to
establishtherelativesequenceofregionaltectonicevents;
(2) totrytointerpretproperlythetectonicsignificanceoftheapparentagesand
isotopicconstraintsdeterminedbydifferentmethods;
(3) to try to correlate the Rio Apa cratonic fragment with the neighbouring
tectonicprovinceswithincentralSouthAmerica,inordertosuggestasuitable
relativepositionforitinRodiniaandGondwana;and
(4) toreportinthetablesandappendicesallthepertinentanalyticaldatarelated
to the U-Pb, Rb-Sr, Sm-Nd, K-Ar and Ar-Ar measurements, indicating the
sourceofeachagelisted.
984 U.G.Cordani&others—TheRioApaCratoninMatoGrossodoSul(Brazil)andnorthern
57º
Quaternary
GMR-27
Pão de Açúcar Alkaline Complex 1941 BODOQUENA
Paraná Basin
Paraquay Belt
Corumba Group (intracratonic)
Cuiabé Group (metamorphic)
Itapocumi Group
(intracratonic)
Amolar Group
Triunfo mafic intrusion
JV-14
Rio Apa Complex 21º
1791
Amoguijá Group
BONITO
Alumiador granite JV-31
Alto Tererê Association N 1794
40 km
CC-09
1867
PORTO
MURTINHO
+ CARACOL
22º ++ BELA 22º
+
VISTA
Brazi l
+
+
+
Paragu a y Apa River
+ SAN CARLOS
+
+
58º 57º
Fig.2. GeologicoutlineoftheRioApaCratoninBrazil.LocationofU-PbSHRIMPzirconagesfrom
LacerdaFilhoandothers(2006)isshownbyopentriangles,togetherwithageinMa.
geological setting
Figure 2 is a regional sketch map of the main area of exposure of the Rio Apa
Craton, which is bound to the east by the Paraguay belt, in SW Mato Grosso do Sul,
Brazil. This map was adapted from Lacerda-Filho and others (2006). These authors
consideredallinformationfromthegeologicalmapsproducedbyAraujoandothers
(1982),Godoi(1999)andGodoiandothers(1999),aswellasthedigitalgeologicmaps
(1:1 million scale) published by Delgado and others (2003). In their work, they
Paraguay:Geochronologicalevolution,correlationsandtectonicimplicationsforRodinia 985
presentednewstructural,geochemicalandgeochronologicaldataandre-interpreted
theregionaltectonicevolution.Anupdatedlithostratigraphiccolumnwassuggested,
andthegeodynamicenvironmentfortheformationofthemainPrecambriangeologi-
calunitswasproposed.Theirtectonicinterpretationwasconsideredinthisstudyasthe
latestcomprehensiveandupdatedsituation,priortothediscussioncarriedoutinthis
paper,whichisbasedonthenewisotopicdeterminations.Inthisfigure,theapproxi-
mate location of four samples dated by U-Pb SHRIMP in zircon at the Australian
National University at Canberra and reported by Lacerda-Filho and others (2006) is
shown. Regrettably, the complete analytical data for these dates, including the preci-
sionofeachagemeasurement,isnotavailable.
Largepartsoftheareashowninfigure2arecoveredbytherecentsedimentsof
the Pantanal Formation and by the Paleozoic sedimentary rocks of the Aquidauana
Formation. They will not be discussed here, as well as the Triassic intrusions of the
FechodosMorrosalkalineComplex.
ConsideringthegeotectonicsettingatthePrecambrian-Cambrianboundary,the
RioApaCratonstandsupastheforelanddomainfortheParaguaybelt.Thebasement
rocksareoverlainbytheintracratoniccoveroftheCorumba´GroupalongtheSerrade
Bodoquena in Brazil and the southernmost part of the region. In that stratigraphic
unit,carbonatesedimentspredominate,showingpeculiarmetazoanfossils(Cloudina,
Corumbella werneri), which indicate that their age is close to the Vendian/(cid:2)Early
Cambrianboundary(Boggianiandothers,1993).TheCorumba´ Groupshowsaclear
tectonic and metamorphic polarity increasing toward the easternmost part of the
region, where the low-grade metasedimentary rocks of the Paraguay belt occur as a
tectonic feature of the Neoproterozoic Brasiliano Orogeny. Figure 2 shows in its
north-eastern corner the low to medium-grade metamorphic rocks of the Cuiaba´
Group,whichiscomposedpredominantlyofmuscoviteschistsandquartzitesandhas
NWtrendingconspicuousstructures.Thesecorrespondtolargefaultzones,inwhich
the Cuiaba´ fold and thrust belt overrides the less metamorphosed sequences of the
Corumba´ Group. For the older Precambrian units making up the basement rocks in
figure 2, the interpretation of their geological history, and especially their structural
evolution,isverycomplicated.
Lacerda-Filho and others (2006) consider the Alto Terereˆ association, which is
composed of supracrustal rocks, to be the oldest unit in the region. It comprises a
sequence of low- to medium-grade metavolcano-sedimentary rocks, where muscovite-
biotite schists predominate, sometimes with garnet porphyroblasts. Muscovite-biotite
gneisses and quartzite intercalations are common. Metabasic rocks also occur in many
places.Thedifferentialerosionbetweentheschistsandthelargequartziteintercalations
enhancesthecomplicatedsinuousstructures,whichfurtherindicatethecomplexityofthe
structure of the Alto Terereˆ metamorphic terrain. The metabasic rocks are mainly
amphibolites with MORB-type chemistry, which were interpreted as remnants of an old
Paleoproterozoicoceaniccrust.Correˆaandothers(1976)namedthisunittheAltoTerereˆ
Group and this name was retained by Godoi and others (1999) and Lacerda-Filho and
others(2006),althoughwithsomewhatdifferentgeologicalmeanings.Inthenortheastern
partofthearea,closetothetownofBa´ıadasGarc¸asandveryclosetothecontactwiththe
overlying Corumba´ Group, the Alto Terereˆ schists are intruded by three small, slightly
foliatedgraniticmassifs,asshowninfigure2.
Araujo and others (1982) proposed the name “Rio Apa Complex” for a very
heterogeneous unit, comprising medium- to high-grade metamorphic rocks and
granitesandoccupyingalargeareainthecentralpartoftheregion.Thenameofthis
tectono-stratigraphic unit was retained by Lacerda-Filho and others (2006) with the
samemeaning.Inthenorthernpartofthearea,inthevicinityofthetownofMorraria
(fig.3),bandedgneissesandmigmatitespredominate,withfrequentintercalationsof
986 U.G.Cordani&others—TheRioApaCratoninMatoGrossodoSul(Brazil)andnorthern
58º 57º
0 20 km 4007 EG 50
1265 (M) RA-23
1950 (Z)
BODOQUENA
KRaedsiewrveeu 1283R A(-B22)R A-23 Morraria Chapena River
1272 (B) S
a
lo
58758 3E G(H 79) TAomldáeziaia 41033R76A 4E-3 G5(H A85) bro River MS-178
Aquidabã River MS-382 13R0A0- 3(2M ()Z) RRRAAA---833 78-A -1 -13 13430118 (0 B( B()B))
4036 EG 14 RA-40 - 1314 (B)
21º 1267 (H)
Baía das
Garças
Branco River 1300R A(M-33) 17R5A2- 4 0(Z) BONITO
MATO GROSSO
DO SUL 12R9A5- 5 (2M) Perdido RiveRrA-45 MS-178
1289 (M)
Amoguijá River
PORTO 12R9A5-6 2(FB) BR-267
MURTINHO
RA-76
1303 (B) RA-77
1314 (B)
RA-77
1839 (Z) RA-81 RA-83
1774 (Z) 1098 (B)
22º PVaulelert-omi MS-467 Perdido River CARACOL 17R2A1-8(4Z M)S-384 BVEISLTAA
Puerto CRioslsôonia CCaoclôhnoiAaePirAa RIVERSCaanrlos PABRRAAGZUILAY
La Victoria RA-88 Colônia
1292 (H) Felix
P Lopez
A RA-93A
RA excess Argon (H)
GU RA-95
AI R ~1400 (Z)
IV PARAGUAY
E
R
Pto. Pinasco RA-114
RA-111 1303 (H) Route 5
(Z) RA-112
1290 (B)
Fig. 3. Location of samples from the Rio Apa Craton dated by U-Pb, K-Ar and 40Ar/39Ar methods.
Greencircles(cid:3)40Ar/39Armethod;Redcircles(cid:3)K-Ar;B(cid:3)biotite;M(cid:3)muscovite;H(cid:3)hornblende;Z(cid:3)
zircon.Blacktriangles(cid:3)U-Pb.Numbersandagesrefertodatapresentedinthetables.
Paraguay:Geochronologicalevolution,correlationsandtectonicimplicationsforRodinia 987
amphibolite. These medium- to high-grade metamorphic rocks were attributed to a
seriesofPaleoproterozoiccalc-alkalinemagmaticarcs.TotheeastofthecityofPorto
Murtinho,bandedgneissicrocksalsooccur.TheserockswereconsideredbyLacerda-
Filho and others (2006) as correlative with the northern gneisses located near
Morraria.TheyarecoveredbytheSerradaBocainafelsicvolcanicsandintrudedbythe
granitoid rocks of the Alumiador batholith, which contains xenoliths of the banded
gneisses.
Inthecentralpartoftheregion,slightlyfoliatedhomogeneousorthogneissesare
widespreadfromaboutthelatitudeofthetownofBonitotothetownofCaracoltothe
south.TheserockswerealsoincludedintheRioApaComplexbyLacerda-Filhoand
others(2006),followingthepreviousworkofAraujoandothers(1982).However,they
arequitedifferentfromthebandedgneissesofthenorthernandwesternpartsofthe
region, especially in their paragenesis, which includes very small amounts of mafic
minerals.Theyareessentiallyorthogneisses,withaverysimplemineralogy,composed
of quartz, microcline and oligoclase as the main components. Later in this work, the
Rio Apa Complex will be divided into three separate litho-stratigraphic units: “the
MorrariaandPortoMurtinhobandedgneissesandtheCaracolleucocraticgneisses.”
According to Araujo and others (1982) and most subsequent authors, including
Lacerda-Filhoandothers(2006),theSerradaBocainavolcanicshavebeenconsidered
to be the extrusive equivalent to the Alumiador granites, as components of the
Amoguija´ Suite. The volcanic rocks include porphyritic rhyolites and dacites, associ-
atedwithminorpyroclasticrocksandvolcanicbreccias.TheAlumiadorbatholithtakes
the form of a large elongated intrusion in the central part of the region, showing
conspicuousNNEtrendinglineamentsalongtheSerradoAlumiadoranddeflectingto
a NW trend along the Serra da Alegria. It is formed essentially of fine- to medium-
grainedisotropicsyeno-tomonzogranites,alsoincludingsomegranophyricvarieties.
A second large portion of the Alumiador suite forms an extension to the north,
trending NW and including similar granitic rocks. In this region, the batholith is
surrounded and intruded by a gabbro-anorthositic suite, which was named Serra da
Alegria,asreportedbySilva(ms,1998),andthisnamewasretainedbyLacerda-Filho
and others (2006). It is a cumulative magmatic suite, in which anorthosites and
leuco-gabbros to mela-gabbros occur, some of them with igneous banding. The
gabbroicrocksoftheMorrodoTriunfoMaficIntrusive,indicatedinfigure2,maybe
coevalwiththeSerradaAlegriamagmaticrocks.Allintrusivegraniticbodiesoccurring
intheregionwereconsideredcorrelativewiththeAlumiadorgranitesmainlybecause
of the lack of geochronological control (Araujo and others, 1982; Godoi and others,
1999;Delgadoandothers,2003;Lacerda-Filhoandothers,2006).Thisisthecase,for
example, for the already mentioned granites intruding into the schists of the Alto
TerereˆGroupinthenortheastcorner,nearBa´ıadasGarc¸as,whichwillbeconsidered
asaseparateunitlaterinthispaper.
NeartheBrazil-Paraguayborder(fig.2),severaloutcropsoflow-tomedium-grade
metamorphic sequences were united by Lacerda-Filho and others (2006) under the
informalnameof“AmolarDomain.”Theyareconsideredascorrelativewiththe1.10
to 1.00 Ga Sunsa´s orogeny of the Amazonian Craton (see fig. 1) and, therefore, are
tentativelyattributedtothelateMesoproterozoic.Alargerareaoccupiedbythisunit
was identified near the Apa River, entering Paraguay and forming a large zig-zag
structure. The main lithologies include different types of supracrustal rocks, among
which quartzites and sericite-schists predominate, although meta-volcanic rocks are
alsopresent.TherocksoftheAmolardomainareintrudedbysmallgranitoidplutons.
No dating is available yet, either of the supracrustal rocks or the intrusive granites.
Therefore,anypossibilityofcorrelationisonlytentative.
988 U.G.Cordani&others—TheRioApaCratoninMatoGrossodoSul(Brazil)andnorthern
InthecaseoftheterritoryofParaguay,themaininformationwasobtainedfrom
the reconnaissance geological map produced by the Anchutz Corporation in the 80s
andlaterincorporatedbyF.WiensinhisPh.D.dissertation(Wiens,ms,1986).When
the samples used in this study were collected, some observations made by the senior
author,in2003,duringfieldworkinthearea,werealsoconsidered.
Inthefollowingcomments,wewilltrytocorrelatethelithologicandstratigraphic
units found in Paraguay with the ones already established in Brazil by Lacerda-Filho
andothers(2006).ItisobviousthatthesedimentaryrocksoftheParana´Basinarealso
presentattheeasternportionoftheParaguayanregion,anditispossibletoestablish
the correlation of a few small outcrops of limestone with the Corumba´ Group. The
Quaternary cover along the Paraguay River is the same in Brazil and in the western
portion of Paraguay. In addition, the metamorphic rocks of the Amolar domain are
also present in Paraguay, forming a coherent structure. The Itapocumi stromatolitic
limestone, containing minor intercalations of siliciclastic rocks and considered to be
correlative with the Corumba´ Group, occurs in this area covering the Amolar supra-
crustals.
Inthecentralpartofthearea,Wiens(ms,1986)namedas“PasoBravoProvince”a
complex and diversified region in which medium-grade metamorphic rocks are
predominantly exposed. The western part of this unit, formed by pink to gray,
medium- to coarse-grained, strongly foliated granitic gneisses, may easily be consid-
ered as the continuation in Paraguay of the Caracol leucocratic gneisses described
aboveandincludedbyLacerda-Filhoandothers(2006)intheirRioApaComplex.On
theotherhand,theeasternpartofWiens’(ms,1986)PasoBravoProvinceseemsnotto
haveacounterpartinBrazil.Inthatarea,thereisapredominanceofbandedgneisses,
in which, besides feldspars and quartz, a great deal of mafic minerals are recorded,
such as hornblende, biotite, garnet and pyroxene. Migmatites are also described, as
wellasafewgraniticintrusions,formedmainlybymassivetoweaklyfoliated,medium
to coarse-grained biotite granites, sometimes with muscovite, and locally exhibiting
porphyritictexture.
StructuralContext
Inordertorecognizethelarge-scaleregionalstructures,manyobservationsmade
ontheavailableoutcropsbydifferentauthors(forexampleAraujoandothers,1982;
Godoiandothers,1999;Ruizandothers,2005;Lacerda-Filhoandothers,2006;Godoy
and others, 2009), and by the present authors, as well as the available SLAR images
takeninthe70sandmorerecentsatelliteimageswereconsidered,andsomegeneral
ideasontheregionalstructuralevolutioncanbeproposedasfollows:
1—Asexpected,theareascoveredbytheQuaternaryformationsandthesedimen-
taryrocksoftheAquidauanaFormationandtheCorumba´ Grouparevirtually
structureless. This also is true for the peneplanized areas of the various
granitoid-gneissicterrainsinBrazilandParaguay.Somelowcrustal-levelfaults
producedbyrelativelyyoungPhanerozoictectonicsaffectedtheCorumba´and
Itapocumilimestones.Theyaremainlynormal,butsometimescanbecompres-
sional. Moreover, it is apparent that the Pantanal Formation, including
youngeralluviumdepositsformingswampyterrains,isnowsubsiding,charac-
terizingoneoftheinitialepisodesfortheformationofanewlargesedimen-
tarybasinincentralSouthAmerica.
2—Neoproterozoictectonics,relatedtotheactivityoftheParaguayfoldbelt,are
also at low crustal level. The tectonic polarity of the low-grade metamorphic
rocks of the Cuiaba´ Group towards the cratonic area is evident. Recumbent
foldsareobserved,andtherockshaveslatycleavageandaxialplaneschistosity.
Later deformational phases are also observed, producing crenulation and
myloniticfoliationalongtranspressivezones.Regardingtheplatformcoverof
Paraguay:Geochronologicalevolution,correlationsandtectonicimplicationsforRodinia 989
the Corumba´ Group, there is only gentle folding with eastward dips and
practically vertical axial planes. The same occurs for the Itapocumi Group in
Paraguay,butthereisonedifference;thegentledipsofthisunitarewestward
directed. Moreover, affecting the Puerto Valle-Mi outcrop of limestones and
shales of the Itapocumi Group, along the Paraguay River, Campanha and
others (2008) described a series of thrust faults associated with low-grade
metamorphism.RegardingtheRioApaCratonbasementrocks,evidencefor
Neoproterozoictectonicsisbarelyvisible.
3—Consideringthebasementrocks,aregionalfoliationcanbeobservedinallthe
lithological units, especially in the southern part of the area. In the gneisses
located near Caracol, it stands up as a penetrative schistosity, often with
variableattitudesandpossiblyrelatedtoapervasivemedium-grademetamor-
phic event. Along the BR 267 highway, these rocks show low dip angles
(around20°)totheSW.Incontrast,intheAlumiadorsuite,afewkilometers
to the west along the same highway, a similar structural deformation is
observed, showing similar trend but with high dip angles (70-80°) always
towards the SW. Farther west, the Serra da Bocaina volcanics have a slaty
cleavagewithmoderatedipstotheSW,whichmayalsoberelatedtothesame
regionaldeformation.
4—A strong deformational episode can also be observed in the central and
northern areas, where different gneissic rocks and the Alto Terereˆ supra-
crustalrocksoccur.Variablelithologieswithquitedifferentrheologicalprop-
ertiesandalsovariablestructuraltrendsarereported.Often,atleastoneolder
deformational phase is detected. Moreover, the strong penetrative and axial
planeschistosity,wherethiscanbeobserved,ispracticallyparalleltobedding,
indicatingtheexistenceofisoclinalfolding.
5—In a coherent structural picture, it is difficult to include in the Amolar
metamorphicdomainthesmallandsparseoutcropsoflow-gradesupracrustal
rocks present in the northwestern part of the region, as Lacerda-Filho and
others(2006)did.Thisisonlypossibleforthesouthernstructurethatcrosses
theRioApafromBraziltoParaguay.InBrazil,thisstructurehasaNEtrend,
makingupwhatseemstobeanantiformwithaninvertedflankandaxialplane
dippingtowardsthesouth-east,whosecoreisfilledwithsmallgraniticbodies
(fig. 2). In Paraguay, the same antiform bends sharply to a NW trend and
comesbacklatertoaNEtrend,butkeepingitsinternalgranites.Oneofthese,
a biotite granite, forms a nucleus of what Wiens (ms, 1986) characterized as
the “Centurion structural high.” This large zig-zag antiform indicates a west-
wardtectonictransport.
geochronological results
Dating of the Rio Apa Craton was carried out mainly at the Geochronology
ResearchCenteroftheUniversityofSa˜oPaulo(CPGeo-USP)(K-Ar,Ar-Ar,Rb-Srand
Sm-Nd), using the samples collected in 2003 by the senior author, firstly for the
RadamBrazilProject(Araujoandothers,1982)andmorerecentlyforthiswork.The
U-Pb ages were obtained at the Beijing SHRIMP Center (China) and some of the
Sm-Nd analyses were obtained at the Federal University of Brasilia (Brazil). Prelimi-
nary data were presented at a few scientific meetings (Cordani and others, 2005a,
2008a, 2008b), and the abstracts published in these events are mentioned in the
references. All available geochronological data for the region will be presented
accordingtothemethodologyemployedandexaminedandevaluatedintermsofthe
direct interpretative value of each method. The first four tables present the K-Ar,
40Ar-39Ar,Rb-SrandSm-Nddeterminations,respectively,andthecompleteanalytical
datafortheU-Pband40Ar-39AranalysesareshowninAppendices1and2,respectively.
990 U.G.Cordani&others—TheRioApaCratoninMatoGrossodoSul(Brazil)andnorthern
Alreadypublisheddata,suchastheK-ArandRb-SrmeasurementsreportedbyAraujo
andothers(1982),areindicatedinthepertinenttablesandappendices.However,the
data presented at scientific meetings, such as the above mentioned, as well as those
only included in internal reports or other publications not easily accessible outside
Brazil,willbeconsideredinasimilarwayasthenewdataproducedforthiswork.
U-PbSHRIMPDeterminations
U-Pbdatingwascarriedoutonsinglezirconcrystalsfromeightsamples,employ-
ingtheSHRIMPIIinstrumentinstalledattheChineseAcademyofGeologicalSciences
andoperatedfromSa˜oPaulousingtheSHRIMPRemoteOperationalSystem(SROS)
device.DetailsoftheanalyticalproceduresarepresentedbyWilliams(1998).Correc-
tion for common Pb was made based on the measured 204Pb, and the typical error
componentforthe206Pb/238Uratiosislessthan2percent.Uraniumabundanceand
U/Pb ratios were calibrated against the TEM standard. Zircons were extracted from
eight samples of granitoid rocks, and the location of these samples is indicated in
figure 3. The zircon typologies for the samples prepared at the CPGeo-USP are
describedbelow,andtheconcordiaplotsoftheanalyticalpointsareshowninfigure4.
Age calculations are based on Isoplot 3.0 Ludwig (2003). Appendix 1 presents the
apparentU-Pbagesandthecompleteanalyticaldata.
Sample RA 23 is a strongly foliated, medium-grained biotite-hornblende gneiss
fromthenorthernmostpartoftheareaandbelongstotheMorrariagneissicunit,in
whichplagioclase(45%)predominatesovermicrocline(15%).Italsoincludesquartz
(25%),biotite(10%)andhornblende(5%),plustitanite,apatiteandepidote.Zircons
aremainly200to300(cid:4)mlongsubhedralprismaticcrystalswithdarkoscillatory-zoned
coresandthinwhitelow-UrimsintheCLimages.AppendixIindicatesthissamplehas
U contents of 200 to 500 ppm, as well as quite low common 206Pb. In figure 4A, six
zirconsyieldanupperinterceptageof1950(cid:5)23Ma(MSWD(cid:3)1.06;model1)[95%
confidence]. The three most concordant zircons yielded a weighted mean age of
1935(cid:5)15Ma[0.76%](2(cid:6)internal).
SampleRA77isanunfoliatedpinkmonzogranitefromtheAlumiadorSuite.Its
mineralcompositionincludesplagioclase,microclineandquartzoccurringinsimilar
amounts, approximately 30 percent, plus biotite (less than 10%), titanite, zircon,
epidote, apatite and opaques. Its texture is equigranular, with millimetric grain size,
butincludingsomecentimeter-sizedK-feldsparcrystals.Inthissample,largeeuhedral
tosubhedralprismaticzirconcrystals,180to300(cid:4)mlong,arefound.CLimagesshow
oscillatory-zoned cores and dark (possibly magmatic) resorbed borders. The crystals
aredarkbrownandfractured,infilledwithhigh-Uzircon.Appendix1showsthattheU
contentisvariable,usuallybetween70to270ppm,butwithsomehigh-Ucrystals,upto
773 ppm. In figure 4B, nine analyses yield an upper intercept age of 1839 (cid:5) 33 Ma
(MSWD(cid:3)1.12;model1)[95%confidence].
SamplesRA35AandRA40,whichwerecollectednearBa´ıadasGarc¸asandhave
verysimilarmineralcompositionandtextures,areunfoliatedtoslightlyfoliated,pink
colored granitic rocks which intruded the Alto Terereˆ metamorphic rocks. They
contain plagioclase (40 to 50%), microcline (about 30%), quartz (20 to 30%) and
somebiotite.Zircon,allanite,apatite,chloriteandopaquemineralsarethecommon
accessories. These rocks are medium grained with magmatic textures. Sample RA 35
consistsofeuhedralzirconcrystals,usuallyshortprismswithpyramidalterminations,
50to150(cid:4)mlong.Asmallerpopulationoflargercrystals,upto300(cid:4)mlong,canalso
befoundinthissample.IntheCLimages,oscillatory-zonedcoresandrelativelylarge
darkhigh-Uzirconovergrowthsareobserved.TheUcontentismostlybetween250to
550 ppm (Appendix 1), but some zircons show higher contents, up to 2048 ppm. In
figure 4C, nearly concordant zircon yield a 207Pb-206Pb age close to 1730 Ma (see
Appendix 1), and the other more discordant zircons possibly indicate a multi-