PROPRIEDADES GEOQUÍMICAS E PROVENIÊNCIA DE ROCHAS METASSEDIMENTARES DETRÍTICAS ARQUEANAS DOS GREENSTONE BELTS DE CRIXÁS E GUARINOS, GOIÁS

Abstract

Metasedimentary rocks derived from detrital clay-rich protoliths occur only in the upper stratigraphic sections of the Crixas and Guarinos greenstone belts, Central Brazil. They consist of carbonaceous schist (+ or -100 m thick), overlain by 300 m thick distal metaturbidites (fine-grained clayey metarenite, metasiltite and metashale). In 85% of the samples, the SiO (sub 2) /(Al (sub 2) O (sub 3) +CaO+Na (sub 2) O+K (sub 2) O) ratio is similar to post-Archaean detrital sedimentary rocks suggesting that their source-area underwent moderate weathering. The average abundances of major, minor and trace elements of carbonaceous schist and the metarenite/metashale turbidite pair, suggest that no major compositional changes of their respective source-areas took place during sedimentation. Petrographic and geochemical data show that the metarenites and metasiltites derived from clay-rich protoliths and in the average are geochemically consistent with the genetically related metashales. Major and minor oxides as well as trace elements vary consistently with an empirical detrital sedimentary maturity index (SiO (sub 2) /SiO (sub 2) +Al (sub 2) O (sub 3) )X100, and with the white mica/chlorite + or - biotite ratios and probably also respond to the composition of white mica and chlorite. The majority of samples have a Sigma REE compatible with other Archaean equivalent rocks and Phanerozoic quartz-poor to quartz-rich graywackes. Normalized to NASC, the carbonaceous schist, metashales and metarenites (+metasiltites) derive two populations distinguished by their REE abundance. In contrast to post-Archaean equivalent rocks, the studied lithotypes are poorer in LREE, have positive EuN/Eu (super *) N anomalies and lack HREE fractionation. In the average, the composition of the studied rocks are richer in Fe (sub 2) O (sub 3) (super *) , CaO, MgO, MnO, Cr, Ni, Co, Sc, V, and Ba as compared to port-archean rocks. The La/Sc Sc/La, Th/La and Cr/Ti ratios are higher and the Zr/Y ratios are lower than post-Archaean rocks and lead to a bimodal provenance model. Modeling of the source-area shows that a geochemically anomalous horizon of Guarinos may be explained by a source-area with 1:1, while other samples by a 9:1 basalt:komatiite ratio, both with similar contribution of a tonalite/trondhjemite felsic component.