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摘要
Here we present new field, petrographic and geochemical data from the similar to 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 meta-greywackes, 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 Al2O3 (up to 12 wt.%), TiO2, high field strength elements (HFSE) and transition metals. The chemically pure BIF is characterised by alternating high iron (similar to 68 wt%) and high silica (similar to 64 wt.%) 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 T-DM model ages of 3.23-2.85 Ga and initial epsilon(Nd) (epsilon(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 epsilon(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 epsilon(Nd) (i) values. The Nd-143/Nd-144 in the Itilliarsuk BIF, therefore, contrasts world BIFs by exhibiting radiogenic, positive epsilon(Nd)(i) values in shallow seawaters where the REY's were controlled by a local, depleted continental crust, whereas the negative epsilon(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 (SiO2/Al2O3 similar to 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 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.
- 出版日期2013-5
