The tectonic significance of juvenile intra-oceanic magmatic arcs: The Case of South America

CORDANI, U.G.; TEIXEIRA, W.; RAPELA, C.W.; HERVE, F.

Florencia, Italia

Congreso; 32nd International Geological Congress; 2004

IUGS

Classical models of orogens involve either Andean-type accretionary belts, where subduction beneath the front of a continental block give rise to voluminous calc-alkaline magmatism, or a Wilson cycle of ocean opening and closing, where the final event is a continent-continent collision. In both cases, a pre-existent basement is involved in the petrogenetic processes related to the production of granitoid rocks, whose isotopic signatures indicate reworked crustal components in the parental magmas. A somewhat different model of accretionary orogens is related to intra-oceanic magmatic arcs, formed within large oceanic domains, in areas of long-lived plate convergence and B-subduction, when both convergent plates consist of oceanic lithosphere. They comprise calc-alkaline igneous rocks, with typical juvenile isotopic signature, indicating mantle derived parental magmas. Juvenile rocks produced in such processes of "soft accretion" are formed by lateral accretion of successive magmatic arcs, and are widespread over very large areas. Typical examples are the Eoproterozoic Svecofennian belt in the Scandinavian Shield, and the Arabian-Nubian Shield in northeastern Africa.Granitoid rocks are essentially juvenile in the Archean, when continental masses were much smaller, whereas in the Phanerozoic reworked material is largely more common. In the Proterozoic, a clear distinction of juvenile vs. reworked material can be made, and we suggest: (1) - large oceans existed where major juvenile magmatic arcs are reported and (2) - small ocean basins or continental margins were present where reworked material is predominant. In South America, the largest region with juvenile granitoid rocks comprises the Ventuari-Tapajós and Rio Negro-Juruena tectonic provinces of the Amazonian Craton. The rocks are Mesoproterozoic, indicating the existence of a very large ocean before the formation of Rodinia. Another very important tectonic province is the Neoproterozoic Goiás Magmatic Arc, revealing a large oceanic domain separating the Amazonian and São Francisco cratons before the agglutination of West Gondwana. In contrast, within the Neoproterozoic belts of the Borborema, Mantiqueira and Pampean provinces only reworked material is indicated for their granitoid belts, suggesting the presence of restricted oceanic basins. Similar mixed isotopic signature is noted for the Andean granitoid belts formed during the Phanerozoic at the long-lived Pacific continental margin of the continent.