Petrology and geochemistry of the banded iron formation in the Eastern Sierras Pampeanas of San Luis (Argentina): Implications for the evolution of the Nogolí Metamorphic Complex

Abstract

The metamorphosed banded iron formation from the Nogolí Metamorphic Complex of western Sierra de San Luis, Eastern Sierras Pampeanas of Argentina (Nogolí area, 32°55’S–66°15’W) is classified as an oxide facies iron formation of Algoma Type, with a tectonic setting possibly associated with an island arc or back arc, on the basis of field mapping, mineral and textural arrangements and whole rock geochemical features. The origin of banded iron formation is mainly related to chemical precipitation of hydrogenous sediments from seawater in oceanic environments. The primary chemical precipitate is a result of solutions that represent mixtures of seawater and hydrothermal fluids, with significant dilution by mafic–ultramafic volcanic and siliciclastic materials. Multi-stage T DM model ages of 1670, 1854 and 1939 Ma and positive, mantle-like ξ Nd (1502) values of +3.8, +1.5 and +0.5 from the banded iron formation are around the range of those mafic to ultramafic meta-volcanic rocks of Nogolí Metamorphic Complex, which are between 1679 and 1765 Ma and +2.64 and +3.68, respectively. This Sm and Nd isotopic connection suggests a close genetic relationship between ferruginous and mafic–ultramafic meta-volcanic rocks, as part of the same island arc or back arc setting. A previous Sm–Nd whole rock isochron of ∼1.5 Ga performed on mafic–ultramafic meta-volcanic rocks led to the interpretation that chemical sedimentation as old as Mesoproterozoic is possible for the banded iron formation. A clockwise P–T path can be inferred for the regional metamorphic evolution of the banded iron formation, with three distinctive trajectories: (1) Relict prograde M 1–M 3 segment with gradual P and T increase from greenschist facies at M 1 to amphibolite facies at M 3. (2) Peak P–T conditions at high amphibolite-low granulite facies during M 4. (3) Retrograde counterpart of M 4, that returns from amphibolite facies and stabilizes at greenschist facies during M 5. Each trajectory may be regarded as produced by different tectonic events related to the Pampean? (1) and the Famatinian (2 and 3) orogenies, during the Early to Middle Paleozoic. The Nogolí Metamorphic Complex is interpreted as part of a greenstone belt within the large Meso- to Neoproterozoic Pampean Terrane of the Eastern Sierras Pampeanas of Argentina.