-1Ci'1·14" InternationalConference on Basement Tectonics - Ouro Preto, MG - Brazil- 62-65

SOUTHERN BRASiLIA BELT (SE BRAZIL): THRUST-DISCONTINUITIES ANDEVOLUTION DURING THE NEOPROTEROZOIC BRASILIANO OROGENY

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VALERIANO'; C. M.; SIMOES2; L. S. A.; TEIXE IRA); W. & HEILBRON' , M.

lTEKTOS-lJniversidade do Estado do Rio de Janeiro-FGEL-DGRG (cmval@uerj .br);2Universidadc Estadual Paulista -IGCE-DPM;

3Universidade de Sao Paulo -IG-DGG

INTROD UCT IO N

The Brasilia belt, extending along 1500 km inN-S direction in central Brazil, is the geologicalrecord of convergence between the Amazonian andthe Sao Francisco cratons during the Neoproterozoicassembly of the Gondwana supercontinent (Neves &

Cordan i, 1991). The belt has its external (eastern)tectonic clements consistently thrusted eastwards ,towards the border of Sao Francisco craton (SFC).Southwards. it is limited by the MantiqueiraProvince, with overal l NE-SW trend along theAtlantic coast. This province includes the Ribeirabelt, which also evolved during the Neoproterozoic,but later, with two metamorphic peaks around 580and 530 Ma. (Machado et al, 1996; Hcilbron et al,this volume). Intense investigat ion of the southernsegment of the Brasilia Belt in the last decadebrought up significant data concerning its tectonicevolution. unique in many aspects with respect tonorthern segments and relevant to relationships withthe Mantiqucira Province. This paper presents anupdated overview of the structural organization of thesouthern segment of Brasilia belt. New K-Ar datahelp to constrain the timing and thermal conditions ofallochthony.

TECTONIC ORGANIZATION OF SOUTHERNBRASi LIA BELT

Important thrust surfaces of regionalexpression separate three major tectonic domains(Figure I) with contrast ing structural, metamorphicand lithologic characteristics, due to ear ly (pre-thruststacking) tectonic evolution in different crustallevels. From top to bottom, these arc the Interna l,External and Cra tonic domains, (Simoes &

Valeriano ( 1990); Fuck ( 1994). The InternalDomain- is represented by the the Passos nappe(PN), the uppermost structural unit, extending over at

least 150 km. To the west, the nappe is covered by .the Parana basin (Phanerozoic), so overlying tectonicdomains are not known. The PN displays a persistentmetasedimentary sucess ion (Araxa group) of twodistinct units (+metabasic intercalations): a LowerUnit (ca. 700 m thick) starts at the bottom with a

platform siliciclastic pelitic (with few carbonate

lenses) sequence with thin quartzite beds which,towards the top, become more frequent and thicker,suggesting a progradant shelf sequence. Atransitional contact with the Upper Unit (ca. 3800 m.thick) probably marks a reactivation of extensionalstresses, with eros ion of granite-gneiss ic basementsource , originating paragneisses and feldspathicschists with frequent intercalations of continentaltholleiitic metabasalts. To the top, the unit displayspredominant metapclites with subordinatedparagneiss, thin beds of calc-silicate rocks and Mn­garnet-quartz mctachcrt. Intercalations of metabasicrocks arc more frequent in thc upper portions of thcUpper Unit. The stratigraphy of PN, evolving fromplatform to deep marine facies, is interpreted as thcsed imentary record of Neoproterozoic lithosphericextension and construction of a passive margin. Thegeochemistry of intercalated metabasic rocks alsoindicates progressive lithospheric extension, startingwith the predominance of CFB-ty pe metabasalts,tend ing towards the top to a predominance of E­MORB type rnetabasalts, However, the persistence ofcontinental mctabasalts at thc top sequence suggeststhat lithospheric thining did not achieve oceanic cruststage in the preserved strat igraphy of the PN(Valeriano & Simoes, 1997). T he str ucturalorgani za tion of the Intcrnal Domain displays a solethrust always below the main quartzite level of theLower Unit. Lithologic contacts (enveloping surfacesof the relict bedding planes) at map scale aresubparallel to the basal thrust surface, regardless of \pervasive tight intrafolial recumbent folding. This }feature probably played an important role during thetectonic transport, in which the basa l metapelites,over lain by relatively competent thick quartzite beds,acted as a crustal detachment zone, preserving theoverall stratigraphy of the thrust sheet. Thc internaldefo rmation comprises early sin-metamorphic lowangle shear processes, which originated tight toisoclinal folds with gently W to WNW dipping axesand penetrative axial plane foliati ons related toprogressive refolding of early foliati on. Associatedstretching lineation is always subpara llel to fold axes.

International Basement Tectonics Association

14" International Conference on Basement Tectonics , Ouro Preto, MG' Brazil

Abundant kinematic shear sense indicators show

unequi vocal tectonic transport towards E to SEdirections. This main deformation is interpreted asthe initial stage of crusta l shortening related to aNeoprot erozoic subduction zone, followed by theformati on of the basal detachment zone and finalthrust-emplacement onto the External Domain(Simoes. 1995), The thrust-dri ven ascension of the

Passos nappe must have been relatively fast.indicated by the preservation of an inverted med ium

to high-pressure metamorphic gradient (S imoes et al..1988 : Simoes, 1995). ranging from greenschist toamphibolite facies. with metamorphic zonationnearly parallel to bedding and basal thrust. The post­metamorphi c peak deformations of the liN includetwo sets of gentle to open folds with steep axialplanes. one with NW trending axes and a later phasewith N-S axes. Subvertical faults. generallysubparallel to the axial surface of megascop ic fold s

are also frequent. Available K-Ar age determinationsin the PN (Correia et al., 1982; Machado Filho ct al..1983: Valeriano, 1(9 2) constrain the timing ofcooling related to the thrust-driven uplift. within the674-640 Ma. (hornblende) and 673-5M Ma (whitemica) time intervals. T hc Extcrn a l Domain is athrust-system marked by the complex imbrication ofnumerous thrust sheets that comprise lou r mainlithologic associations (Valeriano et al.. 1(95): i) a

low (chlo rit e zone) metamorphic grade siliciclas ticsuccess ion of undefined age, typified by quartziticrnctarenites, sericitic phyllites and quartz­metaconglomerates with subordinated iron formation .informall y referred to as the Serra da 80a EsperanceSequence by Valeriano (19 92) due to uncertaintiesregarding the correlation to the Canastra group byprevious authors on basis of lithology: ii) a granitc­grccnstonc basement associa tion, which displayspillow structured kornati it ic meta-lava with relictspini fcx textures, in association with basic to acidicvolcanic rocks and sediments (Fritzsons et al., 1980;

Schrank. 1982). Zircon from an anortho sitic gabbros ill within the volcanic sequence was dated by U-Pbmethod (Machado & Schrank, 1989). yielding anupper intercept at 3 116± I0 Ma. At the Piurnhi townarea. granitoid rocks display clear intrusiverelationship with the volcanic rocks, South of Piumhilocal ity. granitoid rocks predominate over thegreenstones. East of lIicinea locality, a deformedcalc-a lkaline granitoid suite predominates, displayingorthogneisses ranging from gabbroic diorite togranite in composition (Valeriano . 1992) ; iii) a thin­bedded turbidite sequence of low metamorphi cgrade: whose source area was probably the granite-

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greenstone association, indicated by the compositionof clasts in the metawackes: and iiii) thrust sliecs ofBa mbui gro up slates. Besid es the main continuousex posi tion under the Passes nappe. e leven smallklippen were mapped overlying the CratonicDomain . T he structural cvolution of thc Extcrn a lDomain contrasts with the other tectonic domain s.Most individual thrust sheets display an internaldeformational fabric originated by intense low-angleshear with N-verging tectonic transport marked by

streching lineation and associ ated shear senseindicators. This fabric was interpreted by Valerianoct al. (1995) as relicts of a significant prc-Brasilianocrustal shortening event of undetermin ed age. Thesepreviously deformed rock were involved in thcBrasiliano cast-vcrgc nt thrust stacking. at shallowcrustal levels. on the cratonic Bambui group. TheIlrasiIiano assoc iatcd delonn at ion produced normalchevron lillds 0\ erprinting thc old subhorizontalfol iation and N-S lineat ion. Usually. these folds arcassociated with brittle-ductile processes, such asinverse and vertical fault s, kink-band s, and intensejointin g along axia l surfaces, These folds have N-Strending a:-.es. overprinted by late and E-W to NW­SI·: tulds, resulting in dome-basin interferencepatterns. " -AI' dating on white mica from livesamples or Serra da Boa l:sperarH,a Sequencequartzites (Valeriano, 1l)9~ ) yield a range or 5XX :1: 15

to 5(171 17 Ma apparent ages, an indication or coo lingrelated to the Brasiliano tectonic imbrication . i\

significantly older age of 659 ± 8 Ma (white mica)from a basement granitoid-phyllonite thrust sheet. isinterpreted as a mixed age, due to the presence ofunrccrystalizcd coarse primal': mica crystals . Ahornblende separate fro m an orthogneiss sampicwithin the I'iumhi basement thrust slice. ncar Ilicincatown. dated 225 11 26 Ma. This value suggests thatmaximum metamorph ic temperatures in ExternalDomain were not high enough to reset old K-Arhornblend e systems. Thc Cratonic Domaincomprises lithological units correlative with.those ofthe southern portion of the SIT , in which basementrocks (Teixeira & Figueiredo, 1991) arc covered byanchimetarnorphic peIito-carbonate (Neoprot erozoic)platform deposits of the Bambui group (Dardenne,1978), the latter representing the footwall rocksunder the allochthonous front. The basement rocks,covered by the Bambu i group, extend continuouslyfrom the SW border of the SFC, along the southernlimb of the Passos nappe (see figure). This sectorcomprises an association of Archean toPaleoproterozoic migmatite -granitc complexes withgreenstone-belt remnant s (Te ixeira, 1978; Szabo,

14" International Conference on Basement Tectonics - Dura Preto, MG - Brazil

1989). Moderate to high angle ductile and brittleshear zones overprint and obliterate previousstructures, including a major NW-SE sinistral strike­slip crustal d iscontinuity, referred to as the Campo doMeio Shear Belt (Morales, 1993). A local Brasiliano

foreland basin is represented by the Sarnb uraform at ion, discordant on top of the Bambui group,underneath the external allochthons. The typicallithology is a polymictic metaconglomerate thatmakes up the basal facies of repetitive eros iveupward-finin g sedimentation cycles of metricthickness. some of them culminating with laminatedmetasiItite. Clasts show prcdorninant provenancefrom the External Domain and Bambui group rocks.

The Sarnbura formation is interpreted as sub-aqueousfan deposits related to the erosion of the advancingthrust front of the Brasilia belt (Va leriano. 1992:Castro & Dardcnne. 1( 96 ). E"o lution of theC ratonic Domain . The Bambui group displays adeformational gradient from undisturbedsubhorizontal beds in the craton, to highly shearedtectonites underneath the External Domain (Alkminet al., 1989). The steep slaty cleavage of the Bambuigroup rocks is commonly truncated by the basalthrust of the External Domain. This situation is bestexposed below several mapped klippen of theExternal Domain. As in the overlying domains, theoverprinting of t\\ 0 sets of late open folds and kinks,on the slat) cleavage. is also registered. The coo ling(K-Ar) pattern of the basement rocks from theCratonic Domain of the southern SFC area has beenstudied in detail, revealing a clear uplift and coolingevent at the end of the Transamazonian oroge ny,given by the widespread apparent ages around 1.8Ga. (Te ixeira. 1993). In the gneisses south of thePassos nappe, Teixe ira et al. ( 1989) detectedMesoproterozoic isotopic Rb-Sr rehomogenization( 1404±54 Ma.) and K-Ar hornblende dates related tothe Transamazonian event (2254±39 and 1988±29Ma), pointing to a complex thermal history for thearea. However, the Meso-Paleoproterozoic K-Ardatings indicate that, during the Brasiliano thrust­stacking, the Cratonic Domain basement rocks werenot heated enough to have their K-Ar systems reset,yielding pre-Brasiliano ages. Allochthony istherefore interpreted to have taken place onto a"co ld" Sao Francisco craton foreland, resulting in atypical thin-skinned nappe geometry.

CONCLUDING REMARKSThe Brasiliano tecton ic evolution, at the

southern Brasilia belt may be .outlined as: 1- crustalshortening deformational and metamorphic processes

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took place first at medium to lower crustal levels inthe Internal Domain, recorded as the maindeformation/metamorphism phase in the Passosnappe; 2- with development of thrust stacking andmountain build up, coarse clastic influx of molassic

character (Sarnbura formation) took place in theforeland zone, after the exposition of the ExternalDomain thrust sheets; 3- the timing of emplacementof the Passos nappe and of the External Domain, ontothe Cratonic Domain, is constrained by the K-Ar dataon white mica ranging 673-566 and 588-567,respectively; 4-the final thrust imbrication resulted inthe structural superposition of Internal and ExternalDomains over the the "cold" Cratonic Domain, as

presently displayed; 5- two sets of open folds andassociated vertical faults overprint the imbricatedthrust sheet pile.

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14" International Conference on Basement Tectonics - Duro Preto, MG - Brazil

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INTERNAL DOMAIN (pASS OS NAPPE)

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Geociencias. 25(4):22 1-234.VI\ LERII\NO, C.M. & SIMOES. L.SA 1997. Revis ta

Brasileira de Geociencias. 27( I):99-110.

TECTONIC OUTLINEOF THE SOUTHERN

BRASILIA BELT20Km

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