Abstract
The present work is concerned with the geological description and interpretation of the evolutionary history of the Araxá Synform, in an area with approximately 2300 Km2, in the southern segment of the Brasilia Fold and Thrust Belt, Minas Gerais State, Brazil. Geological mapping, structural analysis, petrography, whole rock geochemistry, mineral chemistry and geochronology was the main methodological procedures.
The Araxá Synform is a regional fold, with WNW trending limbs, dipping NNE and SSW and with gently plunging hinges to WNW. The outcrops of Araxá, Ibiá and Canastra Groups take place at their limbs. The region represents the type locality of these geologic units. These Groups are ordered in three tectonic thrust sheets, delimitated by major gently deeping and subvertical shear zones: inferior, intermediate and superior.
The inferior thrust sheet is a psamo-pelitic metassedimentary sequence, metamorphosed at greenschist facies, chlorite and garnet zones, with Sm-Nd model age of 2,2 Ga. It represents a marine platform environment of sedimentation, and possibly belongs to an regressive marine cycle. It is formally designed Canastra Group.
Over these thrust sheet, occurs a pelitic metassedimentary sequence, arranged in fine rythmites, that is the lithologies of the intermediate thrust sheet. Their Sm-Nd model ages are between 1,1 and 1,3 Ga. and they were metamorphosed to chlorite zone at greenschist facies. Their origin is linked to the erosional processes of neoproterozoic magmatic arcs, and it was possibly deposited through distal turbiditic currents. It represents the Ibiá Group.
The superior thrust sheet comprise an igneous mafic sequence, with fine and coarse amphibolites, that is transitional to pelitic metassedimentary rocks with minor psamitic rocks.
All rocks were metamorphosed to amphibolite facies and were intruded by granitoid rocks. The amphibolites represent original basaltic and gabroic rocks, with minor ultramafics (serpentinites/ amphibole-talc schists). The basalts are high FeO toleiítes, with REE signatures that ressemble E-MORB and BABB. Therefore they could represent an oceanic crust, evolved from an asthenospheric and lithospheric magma mixing process. The metassedimentary rocks could be represent a marine deep water sedimentation. They have Sm-Nd model age of 1,9 Ga. The granitoid bodies are mainly two-mica leucogranites, with peraluminous affinity, and with a collisional geochemistry and mineralogic signature.
The deformational and metamorphic history of the Araxá Synform could be described through a sucession of events, which PTt path points to geological processes occurred from deep to high crustal levels. The main metamorphism, dated 630 M.a., is represented by the M1/D1 event, characteristically of barrovian type, and is showed off all thrust sheets. A coarse S1 schistosity, with obscure origin, was developed during this event. A retrometamorphic RM1 event, without deformational imprint, followed the first event, and was recorded only in the inferior and superior thrust sheets. The following event, D2/M2, was divided in an early stage, D2p, and in a later stage, D2t. Both developed in a collisional tectonic environment, with gently deeping shear zones that promoted the collage of the thrust sheets, in retrograde greenschist facies metamorphic conditions. On their initial stages, D2p was accompanied by the intrusion of granitic bodies. Kinematic criteria pointed out the thrust sheets were transported from southwest to northeast.
A D2t deformation began, apparently in continuity to D2p, however with a SE tectonic transport, accompanied by a SW-NE secundary compressional field. This deformation is founded in the majority of outcrops and represents the main deformational event of the region. It was the builder of the Araxa Synform.
The final event, M3/D3, was responsible by the development of wrench sinistral shear zones, in low greenschist facies metamorphic.conditions. Their stress field distribution is similar to that of D2t event, but their deformation occurred at higher crustal levels.
The final chapter is concerned with a synthesis about the evolution of Araxá Synform and employ the Terrane Tectonostratigraphic Analysis (Howell, 1993) to promove a background for the interpretation of the Brasilia Fold and Thrust Belt geological processes. The three thrust sheets of the Araxá region described like tectonostratigraphic terranes, with distinct stratigraphy that characterizes particular geological settings generated at different geographic positions.
The metamorphic and deformational history of the Araxá Synform reflect a regional tectonic framework based on the interaction of three major crustal segments: the Amazonas, São Francisco and Parana continents. The first interaction was represented by the collision between São Francisco and Parana continents. During this collisional process, another collision began between these continents and the Amazonas continent. All processes developed during 630 M.a. and 580 M.a., therefore during the Brasiliano Orogenic Cycle. This history is part of the final collage of Gondwana continent.