The age, origin, and tectonics of the Grão Pará Group and associated rocks, Serra dos Carajás, Brazil: Archean continental volcanism and rifting

Abstract

The Grão Pará Group, which is host to the large Serra dos Carajás Fe deposits, is located in the eastern part of the Central Brazil Shield, SSW of Belem, Brazil. It is composed of a lower sequence of metavolcanic rocks, a middle sequence of iron formation (Carajás Formation) and an upper sequence of mixed metavolcanic and metasedimentary rocks. The Grão Pará Group is overlain by an unnamed sedimentary (dominantly sandstone) unit, and unconformably overlies the older Xingu Complex. The lower metavolcanic sequence of the Grão Pará, in the region of the Serra Norte Fe deposits, is bimodal, with dominant metabasalts, metabasaltic andesites, and metatrachyandesites, and subordinate (10–15%) metarhyolitic tuffs and flows. Most of the metavolcanic rocks are at low to medium metamorphic grade, with more intense metamorphism and deformation along the northern contact with the Xingu Complex. The thickness of the metavolcanic section is not known, but may be ∼4–6 km. The mafic metavolcanic rocks are enriched in Rb, Ba and K with respect to oceanic basalts and basalts of greenstone belts, and are relatively depleted in Th, Sr, Ta, Ti and P. The mafic rocks also show an enrichment in LREEs with flat HREE patterns. The HREEs are correlated with Mg number. U-Pb zircon analyses of the metarhyolites from the Grão Pará Group yield an age of 2758±39 Ma. Rb-Sr wholerock analyses of the mafic rocks yield an isochron with an age of 2687±54 Ma, with an initial ratio of 0.7057±0.0010. The results indicate a late Archean age of volcanism in the Grão Pará Group, much older than previously thought. Rb-Sr results from the metarhyolites have been partially reset, yielding an age 2497±62 Ma. Sm-Nd analyses from the Grão Pará metavolcanic rocks were too scattered to yield an isochron, but ϵ Nd values relative to CHUR at 2758 Ma indicate values similar to depleted mantle at this time, although with some scatter. The trace element data, and the Sr and Nd isotopic results from the mafic rocks, are 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 Grão Pará Group originated on extending, older continental crust during the Archean. An age terrane boundary must exist between this Archean continental terrane and the apparently ensimatic early Proterozoic greenstone belts of the northern Amazonian craton. Similarities to other sequences (e.g., the Fortescue Group, Australia) suggest that similar tectonic settings were widespread at this time. Many major iron formations were deposited within these environments, indicating significant iron formation development on sialic crust in the late Archean. U-Ph zircon analyses from the Serra dos Carajás Granite, which intrudes the unnamed sedimentary unit overlying the Grão Pará Group, yield an age of 1820 ± 49 Ma, in agreement with previous age determinations. This age is similar to the ages for the Uatuma Supergroup volcanic and shallow plutonic rocks, which occur throughout the Amazonian Craton.