[BPT05-P07] Occurrence and compositions of the oldest sedimentary rocks in the Nulliak supracrustal rocks, Labrador (>3.92 Ga): Implications for a chemical composition of the Eoarchean seawater
Keywords:Eoarchean, Early life, Supracrustal rocks
The Saglek block is located in the northeastern part of the Labrador Peninsula, northeast Canada, and is underlain by the Archean orthogneisses and supracrustal rocks. They underwent granulite to amphibolite facies metamorphism but escaped extensive carbonation and silicification. The supracrustal rocks contain ultramafic and mafic rocks, BIF, chert, carbonate rocks, pelitic rocks and conglomerates, and were intruded by the Mesoarchean mafic dikes (Saglek Dyke) and Eoarchean orthogneisses. Especially, in the St. John’s Harbour South area, the supracrustal rocks were intruded by the 3.92 Ga orthogneiss, indicating the supracrustal rocks have over 3.92 Ga age so that they are the oldest supracrustal rock in the world.
The carbonate rocks occur in the St. John’s Harbour South (SJHS), St. John’s Harbour East (SJHE), Big Island and Pangertok Inlet areas. We classified the carbonate rocks into three groups based on the field occurrence. The first group is characterized by the occurrence within mafic/ultramafic rock units and is composed of the carbonate rocks in the SJHS. The second group occurs between the BIFs and mafic volcanic rocks, and the type locality is the Pangertok Inlet. The third group comprises the carbonate rocks interlayered with or underlying the pelitic rocks in the SJHE and Big Island.
We analyzed major and trace element (Rb, Sr, Y, Zr, Nb, Cs, Ba, rare earth element (REE), Hf, Ta, Pb, Th, and U) contents of the carbonate rocks. Some carbonate rocks have high Zr, Ti and Al2O3 contents, possibly due to involvement of detrital and volcanic materials. We selected the carbonate rocks with low Zr, Ti and Al2O3 contents in order to remove the influence of contamination of the detrital and volcanic materials because they are not incorporated into the carbonate minerals. Their PAAS-normalized REE+Y patterns display flat to slightly LREE-depleted patterns with positive La, Eu and Y anomalies and without Ce anomaly. The positive Y anomalies indicate that the carbonate rocks were chemical sediments precipitated from seawater, and the lack of the Ce anomaly indicates that the Eoarchean seawater was anoxic as many previous works suggested. The presence of the Eu anomalies suggests that the carbonate rocks were deposited in marine environments strongly influenced by hydrothermal fluid.
The Eu anomalies are apparently correlated with lithostratigraphy and accompanied rocks; those in SJHS have larger positive Eu anomalies than those in the SJHE and Big Island, possibly due to the degree of the influence of hydrothermal influxes related to the distance from the volcanic sources. On the other hand, even the carbonate rocks accompanied with pelitic rocks have the distinct Eu anomalies, indicating that the seawater surrounding a continent had the Eu anomaly so that the seawater was dominated by hydrothermal influxes. In addition, there is a positive correlation between the Y and Eu anomalies.