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Lęda Maria Fraga, CPRM - Geological Survey of Brazil (Brazil)
Nelson Joaquim Reis, CPRM - Geological Survey of Brazil (Brazil)
Roberto Dall`Agnol, UFPA - Federal University of Pará (Brazil)
Richard Armstrong, ANU - Australian National University (Australia)
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In the Guyana shield, northern part of the Amazonian craton, Archean terranes are locally exposed and Paleoproterozoic terranes are largely dominant. Systematic mapping and new geochemical and geochronological data have recently improved the geological knowledge of the central area of the Guyana shield. Allied to a review of the information made available by pioneering works on the shield, the new data led to the proposition of the Cauarane-Coeroene Belt (CCB), a sinuous NW-SE/NE-SW/NW-SE mega structure connecting exposures of amphibolite to granulite facies metamorphic supracrustal rocks. A granulitic belt, partly corresponding to the CCB has been proposed in the seventies. However, during the next decades it has been reinterpreted as corresponding to a straight NE-SW feature and the sinuous NW-SE/NE-SW/NW-SE disposition of metamorphosed supracrustal rocks have received little attention.
Recently few workers have mentioned this sinuous belt that is here redefined. The western part of the CCB, in Brazil, is represented by the NW-SE to NE-SW oriented Cauarane Group and Murupu gneisses, which prolong to the Kanuku Complex, disposed in a NE-SW to ENE-WSW direction in the central part of the shield, in Guyana. Towards the east, the Kanuku Complex in turn connects with the NW-SE oriented Coeroene Group in Surinam. In Brazil the belt can be traced farther northwest to the Parima region, where NW-SE discontinuous expositions of the Aracaraça paragneiss occur. The structure of the CCB largely fits to the main lineaments as observed in aeromagnetic and radar imagery. A recently obtained 1995±4 Ma U-Pb SHRIMP age on monazites of an S-type granitic body associated with Caurane migmatites is interpreted as reflecting the metamorphism in the CCB.
We consider that the CCB represents a key tectonic feature of the Guyana shield. It marks the approximate limit between two entirely distinct domains: (1) to the north, the preserved juvenile Rhyacian (2.22-2.08 Ga) granite-greenstone terrane and Archean and Rhyacia terranes, partially recycled at around 2.05 Ga; and (2) to the south, an intensely reworked domain in which the basement is only locally preserved and a 1.89-1.81 Ga old extensive volcano-plutonic magmatism is largely dominant. In the proximities of the CCB, fragments of a 2.03 Ga old crust (Anauá Complex and Trairão Suite), interpreted as remnants of magmatic arcs, have been identified. The recognition of the CCB led to the proposition of a new tectonic framework for the central Guyana shield and imposes a re-evaluation of the current geochronological province models for the Amazonian craton. The tectonic setting of the CCB is yet a speculative subject. It is tentatively proposed that the Cauarane-Coeroene basin evolved in response to transcurrent movements after the collision of the Anauá and Trairão arcs with Rhyacian and Archean continental blocks, and was then subsequently closed during the ongoing of this collisional process.
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