DEGEO - Departamento de Geologia

URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8

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1998 - 199912010 - 20201

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Assunto: search.filters.subject.Gondwana assembly

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    The Ribeirão da Folha ophiolite-bearing accretionary wedge (Araçuaí orogen, SE Brazil) : new data for Cryogenian plagiogranite and metasedimentary rocks.
    (2020) Amaral, Leandro Silva dos Santos; Caxito, Fabrício de Andrade; Soares, Antônio Carlos Pedrosa; Queiroga, Gláucia Nascimento; Babinski, Marly; Trindade, Ricardo Ivan Ferreira da; Lana, Cristiano de Carvalho; Chemale Júnior, Farid
    The Araçuaí orogen and West Congo belt make up a singular confined orogenic system, embraced by the San Francisco – Congo craton: the AWCO. It includes a southern sector with ophiolite bodies and magmatic arc, and a northern sector free of them, suggesting the precursor basin was an embayment partially floored by oceanic crust. The northernmost ophiolitic rock-assemblage found in the Araçuaí orogen comprises metamafic and meta-ultramafic rocks with signatures of ocean-floor magmas, and associated pelagic to oceanic metasedimentary rocks of the Ribeirão da Folha Formation. Although tectonically dismembered and metamorphosed, those rocks resemble the classical ophiolite pseudostratigraphy. The Ribeirão da Folha Formation comprises rocks expected to be found in the upper units of an ophiolite edifice, like Al-rich micaschist (pelagic pelite), graphite-rich schist (black shale), sulfide-bearing metachert, diopsidite with massive sulfide, and banded iron formations (chemical-exhalative sediments), and sulfide-bearing fine-grained ortho-amphibolite with thin metachert intercalations (mafic volcanic unit). That formation hosts tectonic slices of banded ortho-amphibolite (dolerite) with plagiogranite veins, and coarse-grained massive ortho-amphibolite (gabbro), representing dismembered slivers from deeper mafic units, and slices of meta-ultramafic rocks from the deepest ophiolite units. Zircon crystals from a plagiogranite vein yielded the U-Pb SHRIMP age of 645 ± 10 Ma, providing a new time constraint for ocean-floor emplacement. Micaschist samples show chemical attributes typical of distal passive margin pelites. Among three progressive deformation phases, the main ductile phase (Dn) shows kinematic indicators related to top to SW mass transport, associated with intermediate P-T (St, Ky, Sil) metamorphic zoning. Although the few youngest grains of detrital zircon from three siliciclastic samples have distinct ages (around 599 Ma, 741 Ma, and 816 Ma), their wide-range age spectra and Hf signatures suggest similar sediment sources. The wide lithological variety and stratigraphic complexity along with the intricate tectonic framework of the Ribeirão da Folha region, comprising thrust slices of ophiolitic rocks tectonically interleaved with older rift-related rocks, characterize an accretionary wedge that was scrapped off the subducted slab and involved in collisional tectonics, marking the AWCO suture zone for some 250 km between the Guanhães basement block (lower plate) and Rio Doce magmatic arc (upper plate).
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    A structural transect across the Coastal Mobile belt in the Brazilian Highlands (Latitude 20°S) : the roots of a Precambrian transpressional orogen.
    (1998) Cunningham, Dickson; Alkmim, Fernando Flecha de; Marshak, Stephen
    We present results of a detailed structural analysis from a 250 km-long, east–west-trending transect crossing the Coastal Mobile Belt, a part of the Precambrian orogen which lies between the eastern edge of the São Francisco craton and the Atlantic coast. The region exposes amphibolite–granulite grade metamorphic rocks and migmatites which formed at mid-lower crustal depths during the Brasiliano orogeny (0.63–0.52 Ga). This event marked the closure of the northernmost Adamastor Ocean and collision between the São Francisco and Congo Cratons during West Gondwana assembly. Brasiliano deformation resulted in W-vergent structures including thrust faults which accommodated kilometer-scale transport of crystalline basement, overturned kilometer-scale folds, sheath folds and penetratively developed gneissosity and schistosity. Isolated relics of an older folded fabric occur locally and may represent Transamazonian (2.2–2.0 Ga) deformation. The orogen is kinematically partitioned with the eastern 175 km dominated by moderately- to steeply dipping north-trending dextral strike-slip and oblique-slip faults and associated flower structures, whereas the western 75 km is dominated by W-vergent shallowly- to moderately east-dipping thrust faults. The boundary between these two provinces may mark a Brasiliano suture. Throughout the transect, quartzite and metasedimentary belts form strongly deformed zones between massive crystalline basement thrust sheets. The granulite-cored Serra do Caparaù massif, the highest mountains in South America outside of the Andes and Guyana shield, occupies a restraining bend between two Brasiliano dextral shear zones. The W-vergent Coastal Mobile Belt formed contiguously with the E-vergent Pan-African West Congo orogen now exposed along the conjugate margin of Africa. Thus an important late Precambrian boundary between structurally linked but kinematically opposed structural provinces must lie hidden in the extended offshore continental margins of either continent. Cretaceous opening of the South Atlantic and separation of the West Congo belt from the Coastal Mobile Belt may have been structurally influenced by this boundary.