Petrogenese do granodiorito Fazenda Gavião : registro de uma colisão arco-continente no greenstone belt do Rio Itapicuru, Craton do São Francisco, Bahia / Petrogenesis of the Fazenda Gavião granodiorite : a record of an arc-continent collision in the Rio Itapicuru greenstone belt, São Francisco Craton, Bahia

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

The Rio Itapicuru greenstone belt is a gold-bearing Palaeoproterozoic volcanic-sedimentary sequence intruded by several granitic plutons in the time interval 2160-2070 Ma. The Fazenda Gavião granodiorite (FGG) is one of a set of plutons emplaced along the western boundary of the greenstone belt with the Archaean basement complex. The FGG is largely composed of a homogeneous coarse-grained hornblende granodiorite, occasionally crosscut by appinitic mafic dykes. Minor mafic enclaves and amphibolite xenoliths are common close to the contact with the host metabasalts. The FGG is a metaluminous medium-K to high-K calc-alkaline body with relatively constant silica abundances (SiO2 ~ 63-66 wt %). It shows fractionated REE patterns (LaN/YbN ~22), high Sr (900-800 ppm), high Ba (1000- 1500 ppm), and high Sr/Y ratios. Comparing with the geochemistry of other plutons of the area, the trace elements signature of the FGG suggests a probable magmatic consanguinity between this pluton and the alkaline plutons of the area. Similar geochemical characteristics of the FGG are observed in the associated appinitic dykes, which show relatively higher abundances of Ba, Sr, MgO, Ni, Cr, K2O, and the light rare earth elements, suggesting they derived from partial melts of an enriched mantle source. The FGG shares several geochemical characteristics with adakites and TTG plutons, but unlike these series it shows relatively higher K2O abundance. It is suggested here that the FGG originated from partial melts of the mafic lower crust (with garnet and no plagioclase in the residue) possibly triggered by underplating of mantle-derived, enriched mafic magmas, of which the appinitic mafic dykes may be a representative remnant. The granitic melt mixed with minor amounts of the appinitic magma, as suggest by magma mingling. The 2106 + 6 Ma-old (UPb SHRIMP in zircons) FGG is younger in age than the early (2163-2127 Ma) TTG/calcalkaline arc plutons of the greenstone, and is close-related in time to the young potassicultrapotassic plutons (ca. 2110-2105 Ma) emplaced along the basement-greenstone boundary. We suggest that the K-rich plutons in the Rio Itapicuru greenstone belt are the records of island arc-continent collision at ~2110-2105 Ma, preceding major continental rework and collision at ~2080-2070 Ma.

ASSUNTO(S)

greenstone belt sedimentary rocks petrogenese quimica mineralogica chemical geology petrogenesis rio (ba) geoquimica - itapicuru

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