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The Trans-amazonian cycle


Rio Sao Francisco
Rio Sao Francisco

Precambrian rocks constitute the oldest rocks of the continent and are preserved in the five core cratons. These rocks are represented by high- to low-grade metamorphosed assemblages along heavily deformed belts of plutonic (intrusive), metavolcanic (metamorphosed extrusive igneous rocks), and metasedimentary rocks. Rocks of Archean age (2.5 to 4 billion years old) are known in the Amazonia, Luis Alves, and São Francisco cratons, although precisely dated rock samples are scarce. Ages older than 3 billion years have been reported in the Imataca Complex of Venezuela and in the Xingu area of Brazil, both in the Amazonia craton. The oldest rocks found so far—with ages of some 3.4 billion years—are in the São Francisco craton in the Brazilian state of Bahia. In the other cratons (e.g., the Río de la Plata craton in Uruguay) the dating of Archean rocks has been inconclusive.

Greenstone belts, which are remnants of Archean oceanic crust emplaced in the suture zones (convergent plate boundaries), contain most of South America’s known large gold deposits, such as those located near Belo Horizonte, Braz. Two major cycles of crustal deformation occurred in the Precambrian, widely separated in time from each other. The first, called the Trans-Amazonian cycle, took place approximately 2.2 to 1.8 billion years ago; and the second, the Brazilian cycle, between about 900 and 570 million years ago.

Trans-Amazonian rocks can be subdivided into three distinct groups: orogenic belts, such as the Maroni-Itacaiúnas belt of the Amazonia craton or the Salvador-Juazeiro belt of the São Francisco; stable cover rocks, such as the Chapada Diamantina formation in Bahia or the Carajás and Roraima platform deposits; and large extensional dike swarms (groups of tabular intrusions of igneous rock into sedimentary strata).

The orogenic belts represent old mountain chains that had been formed either along the margins of the continent as geosynclines (downwarps of the Earth’s crust) and then uplifted, such as the Maroni-Itacaiúnas belt, or were the result of collisions between continental blocks, such as the Tandil belt in Buenos Aires, Arg.

Such collisions are believed to have formed a supercontinent (sometimes called the first Pangaea) some 1.8 billion years ago. The sedimentary cover of this supercontinent (preserved on the Amazonia craton), consisting of postcollision rhyolites and clastic shelf deposits, was deep and widespread and obliterated earlier suture boundaries. Extensive stratified iron and manganese deposits are found in these sequences, such as near Carajás, Braz. Early phases of continental-plate dispersal produced extensive dike swarms of mafic rock, including a zone some 60 miles wide in west-central Uruguay where hundreds of gabbro dikes are now emplaced along a 150-mile stretch. Britannica "South America" © Emmanuel BUCHOT, Encarta, Wikipedia

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