TECTONIC SETTING AND SOURCES OF MAGMATISM RELATED TO THE SOUTHERN BRAZILIAN SHEAR BELT

MARIA DE FÁTIMA BITENCOURT, LAURO VALENTIM STOLL NARDI

Resumo


The Neoproterozoic-Cambrian Brasiliano/Pan-African orogenic cycle is represented, in southern Brazil, by arc and collisional settings, with very expressive granitic magmatism. The post collisional stage, 650-580 Ma, is recorded by extensive magmatism along major transcurrent shear zones that compose the Southern Brazilian Shear Belt. The early post-collisional magmatism is high-K, calc alkaline, found along gently dipping shear zones; biotite-bearing granites and granodiorites are the most common rock types, often containing mafic microgranular enclaves. This is immediately followed by peraluminous two-mica leucogranites, emplaced either along flat lying or steeply dipping shear zones. The next magmatic manifestation has a shoshonitic affinity and is found either within the shear belt or far from deformation sites, piercing through cratonic areas. Shoshonitic rocks range from acid to basic, either as extrusive or plutonic terms. The latest magmatic event is a silica- saturated alkaline magmatism, occurring within or outside the shear belt. It is controlled by steeply dipping transcurrent structures, or discordantly intrusive in the previous types. Metaluminous biotite granites are widespread, with a coeval basic magmatism of mildly alkaline affinity. The shear belt acted as a conduit for magmas and also promoted decompression of shallower mantle levels, reactivating the sources of previous, calc-alkaline, pre-collisional magmatism. The role of previously modified mantle wedge in the generation of southern Brazilian postcollisional magmatism is here emphasized over the contribution of crustal melting processes.

Palavras-chave


Syntectonic magmatism; Magma sources; Neoproterozoic magmatism; Southern Brazilian Shear Belt.

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