Petrology and geochemistry of the ∼2.9 Ga Itilliarsuk banded iron formation and associated supracrustal rocks, West Greenland: Source characteristics and depositional environment

Abstract

Here we present new field, petrographic and geochemical data from the ∼2.9Ga Itilliarsuk banded iron formation (BIF) and associated lithologies within the Itilliarsuk supracrustal belt, south-eastern Nussuuaq, West Greenland. The supracrustals represent a volcanic–sedimentary sequence, which rests unconformably on a basement of tonalite–trondhjemite–granodiorite (TTG) lithologies. Felsic metagreywackes, meta-semipelites and thinly bedded ferruginous shales were identified intercalated with the Itilliarsuk BIF. Other associated rocks include metapelites, acidic metavolcanics and metagabbroic sills. The supracrustals have experienced amphibolite-facies metamorphism, which has resulted in complete resetting of the U–Pb system with an apparent age of 1895±48Ma. This tectono-metamorphic event corresponds well with the Paleoproterozoic Rinkian orogeny known from this region. The Itilliarsuk-(oxide-facies) BIF has been divided into two segments on the basis of major and trace elements chemistry: a shaley-BIF with a strong clastic component and a more chemically pure BIF. The shaley-BIF contains high terrigenous influx as reflected by elevated Al2O3 (up to 12wt.%), TiO2, high field strength elements (HFSE) and transition metals. The chemically pure BIF is characterised by alternating high iron (∼68wt.%) and high silica (∼64wt.%) bands with low total rare earths and yttrium (REY), Al2O3, TiO2 and HFSE contents, suggesting a low detrital component. The least altered bands of the BIF record diagnostic Archaean seawater features with Post-Archaean Average Shale (PAAS)-normalised positive La- and Eu-anomalies, enrichment in heavy rare earth elements (HREE) relative to light rare earth elements (LREE) [(Pr/Yb)PAAS<1], and absence of Ce-anomalies which suggests deposition from an anoxic water column. Sm–Nd isotopes of the chemically pure silica-rich bands record TDM model ages of 3.23–2.85Ga and initial ɛNd (ɛNd(i)) values in the range of +0.43 to +4.05, average of +1.35. In contrast, the chemically pure Fe-rich bands yield TDM model ages of 3.61–3.22Ga and ɛNd(i) values from −2.87 to +0.09, average of −1.29. The associated supracrustal rocks in the study area have significantly higher, positive ɛNd(i) values. The 143Nd/144Nd in the Itilliarsuk BIF, therefore, contrasts world BIFs by exhibiting radiogenic, positive ɛNd(i) values in shallow seawaters where the REY's were controlled by a local, depleted continental crust, whereas the negative ɛNd(i) values found in the iron-rich layers suggest that the submarine hydrothermal source was influenced by an enriched mantle, possibly an older subcontinental lithospheric segment. The felsic metagreywackes are immature, first-cycle (SiO2/Al2O3∼4.4, [La/Yb>>1]CHON) metasediments with affinities to TTG-suites, primarily extrusives, whereas the meta-semipelites and metapelites contain a larger mafic contribution with higher content of Fe2O3, MgO, Cr, Ni and HREEs. This suggests that the BIF was deposited in a highly unstable basin, presumably in a palaeo-continental slope or outer shelf environment, with frequent fluctuations of epiclastic and volcanogenic sediments derived from adjacent bimodal sources. The TDM model ages and the use of Th–Sc–Zr and La–Th–Sc tectonic discrimination plots indicate that the metasediments were sourced from a juvenile ocean island arc setting.