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Tectonostratigraphic Terrane Analysis on Neoproterozoic Times

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Tectonostratigraphic Terrane Analysis on Neoproterozoic Times Revista Brasileira de Geociências 30(1):078-081, março de 2000 TECTONOSTRATIGRAPHIC TERRANE ANALYSIS ON NEOPROTEROZOIC TIMES: THE CASE STUDY OF ARAXÁ SYNFORM, MINAS GERAIS STATE, BRAZIL: IMPLICATIONS TO THE FINAL COLLAGE OF THE GONDWANALAND. HILDOR JOSÉ SEER1 AND MARCEL AUGUSTE DARDENNE2 ABSTRACT The Araxá Synform is a regional fold with gently WNW plunging. The outcrops of the Araxá, Ibiá and Canastra Groups occur at their limbs. The region is the type locality of these units. These Groups belong to the Brasilia Fold Belt, a Neoproterozoic tectonic unit evolved at the western margin of the São Francisco-Congo Craton. Geological mapping, structural analysis, whole rock geochemistry, mineral chemistry, petrography and geochronology are the main tools to understand the tectonic evolution of these geological units. In this paper we apply tectonostratigraphic terrane analysis to the solution of stratigraphic problems in the Brasília Belt and Gondwanaland collage. Keywords: Brasília Belt, Neoproterozoic, Brasiliano orogeny, Tectonic Evolution, Structural Geology INTRODUCTION All geological information is necessary to The upper thrust sheet (Araxá Group) comprises a dominantly understand the history of an orogenic belt, basically it comprises the metamafic sequence (fine and coarse amphibolites, with rare ultramafic integration of stratigraphic data with structural geology. The rocks) which is transitional to pelitic metasedimentary rocks, both stratigraphic data provide information about paleogeography and ages metamorphosed under amphibolite facies conditions and intruded by of the geological units of an orogenic belt. The structural data describe granitoid rocks. The amphibolites represent gabbroic and basaltic the configuration of these units. To Howell (1993) an orogenic belt is protoliths. The basalts are high FeO tholeiites with REE signatures that essentially a puzzle, composed by a collection of crustal pieces. To resemble E-MORB (Figs. 4 and 5). Therefore, they could represent an reconstitute the sequence of events that give origin to an orogenic belt, oceanic crust evolved from a mixture of asthenospheric and we must identify the fundamental pieces that compose the puzzle. lithospheric sources. The metasedimentary rocks could represent marine deep-water sedimentation associated with chert layers (fine These pieces are defined as tectonostratigraphic terranes: “a fault- ε orthoquartzites). They have Sm-Nd model age of 1,9 Ga., and NdT = bounded package of rock with a distinct stratigraphy that characterizes -10,21. Thus, their crustal residence time was longer than that of the a particular geologic setting” (Howell 1993). As a consequence, a Ibiá Group and shorther than the Canastra Group. In other places of tectonostratigraphic terrane has no genetic relationships with its Brasilia Belt, the metasediments of Araxá Group yield bimodal Sm-Nd neighboring terranes. In the Araxá Synform, southern Brasilia Fold model ages, with values of 1,9 and 1,3 Ga. (Pimentel et al. 1999b). Belt, Minas Gerais State, Brazil (Fig. 1), we can apply the The granitic bodies are mainly two-mica leucogranites, tectonostratigraphic terrane methodology to pre-Cambrian rocks (Seer peraluminous, with geochemistry and mineralogical signatures (Fig. 6) 1999). This approach is different from that of earlier authors ( Barbosa compatible with their origin in a collisional environment, and they et al. 1970, Paulsen et al. 1974, Ferrari 1989) who believed that contain xenoliths of the surrounding rocks. Their emplacement distinct stratigraphic units may represent transitions or facies from one occurred through subhorizontal shear zones (thrust faults). All these another, a fixist tectonic conception. A high mobility of the different features imply that their genesis is similar to that of two-mica geologic units without apparent genetic links, and originated by peraluminous granitoids from another collisional belts (Barbarin allocthonous processes from distinct geographic regions, is assumed in 1996). this paper. Geological mapping, structural analysis, petrography, whole Therefore, the three thrust sheets of Araxá Synform may be rock geochemistry, mineral chemistry and geochronology were the described as tectonostratigraphic terranes, with distinct characteristics, main methodological procedures used to support such conclusions. representing specific geologic environments, originated at different geographic positions: a fragment of passive continental margin (Canas- TECTONOSTRATIGRAPHY The Araxá Synform (Simões and tra Terrane), a fragment of the sedimentary portion of a magmatic arc Navarro 1996) is a regional fold, with WNW trending limbs, dipping (Ibiá Terrane), and a fragment of ocean basin intruded by collisional NNE and SSW and with gently plunging hinges to WNW (Fig. 1). The granites (Araxá Terrane). outcrops of Araxá, Ibiá and Canastra Groups (Barbosa et al. 1970) occur at their limbs. The region represents the type locality for these TECTONIC EVOLUTION The deformational and metamorphic geologic units. history of the Araxá Synform can be described through a succession of Araxá, Ibiá and Canastra Groups are ordered in three tectonic thrust events, whose PTt path points to a geological process varying from sheets, delimited by gently dipping and subvertical shear zones (Fig. deep to upper crustal levels (Fig. 7). The main metamorphism (M1), 2). dated 630 Ma (Sm-Nd mineral isochrons), characteristically of the The lower thrust sheet (Canastra Group) is a psamo-pelitic Barrovian type, is recorded in all thrust sheets. During this event, a D1 metasedimentary sequence, metamorphosed at greenschist facies deformational phase was developed associated with coarse S (chlorite and garnet zones). It represents sedimentation on a marine 1 schistosity of uncertain origin. A retrometamorphic RM1 event, platform environment and apparently constitutes a regressive marine without deformational imprint, followed the first event, and was ε cycle with predominant quartzites at the stratigraphic top. Their NdT recorded only in the lower and upper thrust sheets. The following = -12,77 (Brod 1999) and Sm-Nd model age of 2,2 Ga. indicate high event, M /D , can be divided into an early stage (D p), and a later crustal residence time and provenance from ancient source regions. 2 2 2 stage (D2t). Both developed in a collisional tectonic environment, with This geologic unit was overthrusted the Bambuí Group, an gently dipping shear zones that promote the collage of the thrust autochthonous sedimentary cover deposited on the São Francisco sheets, in retrograde greenschist-facies metamorphic conditions. Craton (Fig. 1). During its initial stages, D2p was accompanied by granitic intrusions. The intermediate thrust sheet (Ibiá Group) is a pelitic Kinematic criteria indicate that these thrust sheets were transported metasedimentary sequence arranged as fine grain rhythmites. Their from southwest to northeast. A D t deformation began, apparently as Sm-Nd model ages are between 1,1 and 1,3 Ga. and they are 2 a continuity of D2p, though with a SE tectonic transport, accompanied metamorphosed to the chlorite zone of greenschist facies. Thus, they by a SW-NE secondary compressional field. This deformation is found are short time crustal residence sediments, which is strengthened by in the most outcrops and represents the main deformational event in feldspar abundance, especially plagioclase. Geochemically, these the region. It was responsible for the construction of the Araxá sediments are similar to graywackes from Oceanic Island Arcs (OIA) Synform. The final event, M3/D3, was responsible for the development (Fig. 3). Their origin is linked to the erosional processes of of wrench sinistral shear zones at lower greenschist-facies neoproterozoic magmatic arcs (western Goiás State magmatic arcs) metamorphic conditions. Their stress field distribution is similar to that and they were possibly deposited through distal turbiditic currents. of D2t event, but their deformation took place at higher crustal levels. 1 - CEFET-UNED - Araxá - C.P. 208 – 38183-070 – Araxá – MG – Brazil 2 - Instituto de Geociências, Universidade de Brasília – 70910-900 – Brasilia – DF - Brazil Revista Brasileira de Geociências, Volume 30, 2000 79 Figure 1 - Geologic map of the Araxá Synform showing the distribution of Araxá, Ibiá and Canastra Groups and the main tectonic structures (Seer 1999). 80 Revista Brasileira de Geociências, Volume 30, 2000 1,4 OIA % c i basalt - Serra Geral Formation o 2 z o s aeolic arenite - Botucatu Formation O 0,7 e i M nonconformity T MIA micaceous quartzite/micaschist/chlorite schist v v amphibolite/granite v v ACM t p e PM e am phibolite/micaschist/chlorite schist u h o s r am phibole-talc schist t s G u r á h t x r a e r p 0 p A u 0 3 6 9 12 15 c amphibolite/quartzite/chlorite schist/granite i o Fe O +MgO% z thrust fault t 2 3 t e e o p 0,4 h quartzite/rhythmites r u s t o e s r phy ll ite t u r G h o t á r e i l rhythm ites d b p d I i o m thrust fault e quartzite/micaceous quartzite/phyllite 2 N t OIA p e phyllite/ micaceous quartzite u O e h o carbonaceous phyllite s r i t G s MIA carbonaceous phyllite/phyllite S 0,2 u a r r h t micaceous quartzite t / s r a carbonaceous phyllite/metamarl/chlorite schist e 3 ACM n w a o l C chloritoid-graphyte schist/ micaceous quartzite O 2 l PM chloritoid-garnet-graphyte schist/ micaceous quartzite A pegmatite (quartz+alkali-feldspar+m uscovite) 0 Figure 2 - Tectonostratigraphy of the Araxá Synform (Seer 1999). 0 3 6 91215 Fe2O3t +MgO% Figure
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