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 ∼2.9 Ga 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 ± 48 Ma. 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 Al 2 O 3 (up to 12 wt.%), TiO 2 , high field strength elements (HFSE) and transition metals. The chemically pure BIF is characterised by alternating high iron (∼68 wt.%) and high silica (∼64 wt.%) bands with low total rare earths and yttrium (REY), Al 2 O 3 , TiO 2 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 T DM model ages of 3.23–2.85 Ga and initial ɛ Nd ( ɛ Nd (i)) values in the range of +0.43 to +4.05, average of +1.35. In contrast, the chemically pure Fe-rich bands yield T DM model ages of 3.61–3.22 Ga and ɛ Nd (i) values from −2.87 to +0.09, average of −1.29. The associated supracrustal rocks in the study area have significantly higher, positive ɛ Nd (i) values. The 143 Nd/ 144 Nd in the Itilliarsuk BIF, therefore, contrasts world BIFs by exhibiting radiogenic, positive ɛ Nd (i) values in shallow seawaters where the REY's were controlled by a local, depleted continental crust, whereas the negative ɛ Nd (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 (SiO 2 /Al 2 O 3  ∼ 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 Fe 2 O 3 , 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 T DM 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.