Occurrence and chemical composition of the Eoarchean carbonate rocks of the Nulliak supracrustal rocks in the Saglek Block of northeastern Labrador, Canada

Abstract

The Eoarchean Nulliak supracrustal rocks in the Saglek Block of northeastern Labrador, Canada, contain some of the world's oldest carbonate rocks. This work attempted to reveal the origin of the carbonate rocks and estimate the surface environmental conditions of the early Earth based on their occurrence and geochemistry.They occur together with mafic and ultramafic rocks in Pangertok Inlet and St. John's Harbour South, whereas they are interlayered with pelitic rock layers with quartzofeldspathic mineral assemblages in St. John's Harbour East and Big Island. The geological occurrence suggests that the formers were formed around hydrothermal fields, whereas the latters were deposited near a continental margin. Some carbonate rocks have high SiO 2 , Al 2 O 3 , and Zr contents, indicating that the silicification and involvement of detrital materials influenced their composition; thus, pure carbonate rocks were selected using a combined filter of the SiO 2 , TiO 2, Al 2 O 3 , Zr, and Ba contents. The selected carbonate rocks have positive La, Eu, Gd, Y, U, Pb, and Sr anomalies, negative Nb, Zr, and Hf anomalies, and relatively small enrichment in heavy rare earth elements (HREEs). The La and Y anomalies suggest that they originated from chemical sediments precipitated from seawater. On the other hand, the small HREEenrichment suggests that REEs were mainly dissolved as REE-carbonate complexes in seawater or that the riverine influxes were dominated by the detritus of Eoarchean continental crusts, presumably composed of HREE-depleted TTG. The U anomaly suggests that uranium was more dissolved than Th as U-bearing carbonate complexes in seawater. The Nulliak carbonate rocks also show a positive correlation between Y and Eu anomaly values, suggesting that the precipitation of ironoxyhydroxide causing the Y anomaly was more significant near the hydrothermal fields than the continental margin, consistent with an alkaline hydrothermal model.

The δ13C–δ18O variations in marble in the Hida Belt, Japan

“…Sedimentary carbonate-rich rocks are often found in a continental platform, and the metamorphosed equivalents occur as lenses and discontinuous layers of marble in various orogenic belts since the early Archean (e.g., Yoshida et al, 2021). Tectonometamorphic and metasomatic processes in orogens change not only textures, mineral assemblages and chemical compositions, but also the isotope compositions of marble (e.g., Castelli, 1991;Kato et al, 1997;Liu et al, 2015;Ogasawara et al, 2000;Omori et al, 1998;Proyer et al, 2008;Schumacher et al, 2008;Tang et al, 2006;Wan et al, 2020).…”