LAUSR

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 SiO2, Al2O3, 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 SiO2, TiO2, Al2O3, 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 HREE‐enrichment 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 iron‐oxyhydroxide causing the Y anomaly was more significant near the hydrothermal fields than the continental margin, consistent with an alkaline hydrothermal model.