International Geologiical Congress - Oslo 2008

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AMS-07 Crustal evolution of the cratonic nuclei of South America

 

Relationships between geological units in the southern edge of the São Francisco Craton

 

Leticia Hirata Godoy, Universidade Estadual Paulista (UNESP) (Brazil)
Marcos Dias Alvim, Universidade Estadual Paulista (UNESP) (Brazil)
Carolina Del Roveri, Universidade Estadual Paulista (UNESP) (Brazil)
Norberto Morales, Universidade Estadual Paulista (UNESP) (Brazil)
Antenor Zanardo, Universidade Estadual Paulista (UNESP) (Brazil)
 

 

Field and laboratory activities carried out in the southern of the São Francisco Craton, SE Brazil, allowed the separation of petrographic associations of different ages, in addition to the recognition of a Neoproterozoic suture zone, strongly affected by late shear belt of the bulk same age. The oldest rocks (Archean) are petrographic associations of TTG and greenstone belt types, whose were reworked in varied degree in the Paleo- and Neoproterozoic, cut by intrusive rocks of basic to intermediary nature, between the Paleo- and Mesoproterozoic.

This set was overthrusted in Neoproterozoic from West to East by a metavolcanosedimentary tectonosequence, with inverted metamorphism (greenschist at the bottom and granulite of high pressure at the top), originated between Meso- and Neoproterozoic. Those rocks can be subdivided into three different units: a typical passive margin metamorphosed sequence consisting of metamorphosed psamites, pelites and carbonates, positioned at the base; calc-alkaline and aluminous gneisses intercalated with schists, quartzites, amphibolites and rare lens of retroeclogite, calc-silicated and ultramafic rocks and; the third is characterized by frequent lenticular ultramafic bodies interplanted with gneisses and schists identical to the previous sequence, defined as an ophiolitic sequence. With the subsidence of the underthrusted plate occurred the deposition of thin and carbonatic terrigen sediment layer, with intercalation of diamictites and conglomerates related to the elevation resulting of uplift movements in the allochthonous thrusted sequence.

With the progression this sequence also overthrusted the sediments promoting deformation and metamorphism of low grade (chlorite zone). In the extreme South, separated by a ductile shear zone, appears a granulitic belt with characteristics of lower crust of an overthrusted plate, with cinematic indicators associated to the main low angle foliation showing a sin-tectonic transport from SE to NW, overprinted by tectonic transport from W to E in final stages, therefore consistent with the tectonic move in the northern block only at the end of evolution. The entire set is cut by an anastomosed network of sinistral ductile to ductile-brittle shear zones striking WNW/ESE to E/W. NW shear zones show dominant transpressive behavior generating structural highs which allowed the exposure and exhumation of the Archean rocks by erosion.

Structural data show that the strike-slip movements were active before finalizing the tangential movements. The structural framework resulted from the collision of three cratonic blocks: São Francisco, Amaznico and Paraná, this last one cropping out in the studied area as the granulitic and associated rocks representing the lower crust.

 

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