Geologia USP. Série Científica (Aug 2011)
Petrogenetic and Tectonic Evolution of the Cariris Velhos Event in the Afogados da Ingazeira Region (PE), Alto Pajeú Terrane, Borborema Province
Abstract
The Tonian Cariris Velhos event was studied in the area around Afogados da Ingazeira town, Pernambuco State,Northeast Brazil, which is situated within the Alto Pajeú terrane, a Tonian-Ediacaran composite domain of the BorboremaProvince. The exposed rocks belong to the São Caetano Complex, which is formed of biotite-muscovite paragneisses andquartz-feldspathic gneisses, derived from arkosic or felsic volcanic protoliths, with marble and quartzite intercalations. Thedeformation started with a D1/D2 thrusting episode, when metamorphism ranged from greenschist to amphibolite facies,and was accompanied by emplacement of orthogneiss sheets. The subsequent episode, the D3 phase, had an extensionalcharacter and was marked by intrusion of small metadiorite dikes, which were strongly transposed by the D4 phase, a newepisode of transcurrent deformation. The main structure of the D4 phase is the Afogados da Ingazeira shear zone (ZCAI), migmawhichEdiacaran age was well constrained by the associated Solidão granite, dating from 574 ± 54 Ma. A geochemical study of themetasedimentary rocks shows that the protoliths of the paragneisses located in the southeastern part of the ZCAI are greywackes,whereas those of the northwestern part are greywackes, lithic sandstones and arkoses, although the REE patterns of these samplesnormalized to NASC (North American Shale Composite) show minor differences between them. These metasedimentary rocks havegeochemical signatures and patterns of synorogenic sediments compatible with island and continental arc sources. The geochemicalpatterns of the orthogneisses are also consistent with a magmatic arc environment, the same conclusion reached by other authors withrespect to the metavolcanic rocks of the Alto Pajeú terrane. The occurrence of the pre-transcurrent D3 extensional episode suggests that the D1/D2 thrusting deformation and associated metamorphism may be related to orogenic processes, pointing to the existence of theCariris Velhos orogeny.
