AGE AND COMPOSITION OF THE GRÃO PARÁ GROUP VOLCANICS, SERRA DOS CARAJÁS

Abstract

Metavolcanic rocks underlying the Serra dos Carajas iron ore deposits yield Late Archean ages, and show isotopic and chemical evidence of assimilation of some of the continental crust through which they erupted. The lower metavolcanic sequence of the Grao Para Group North and East of the Serra Norte iron deposits is bimodal, with dominant basalts, basaltic andesites, and trachyandesites, and subordinate rhyolite tuffs and flows at several stratigraphic levels. Most volcanic rocks are of medium to low metamorphic grade, with more intense metamorphism and deformation along the northern contact with gneisses and granitoid rocks (Xingu Complex). Thickness of the volcanic section is not known, but may bc about 4·6 km. The basalts and basaltic andesites yleld a whole rock Rb-Sr isochron age of 2,687 ± 54 Ma. The initial Sr isotopic ratio of 0.7057 ± 0.0010 demonstrates that the basalts incorporated material that had history of elevated Rb/Sr ratios. Sm-Nd ratios in the mafic rocks are too tightly clustered to yield an isochron, and using the zircon age of 2,758 Ma they yield €Nd values varying from +4.6 to -7.0, averaging +0.6. The Nd and Sr isotopic data and the trace and rare-earth element compositions of these mafic rocks are ali consistent with contamination of mantle-derived melts by significant but variable amounts of diverse older continental crust. The stratigraphy, geochemical and isotopic compositions show that the group originated on extending, older continental crust. An age-terrane boundary must exist between this Arehean continental terrane and the apparently ensimatic Early Proterozoic greenstone belta of the northern Amazonian craton.