Geochemistry and C and O isotope composition of carbonate rocks from Bemil and Lagoa Seca quarries, Gandarela Formation, Quadrilátero Ferrífero - Brazil.

Abstract

Geochemical and isotopic (carbon and oxygen) compositions of Paleoproterozoic carbonate rocks of the Bemil and Lagoa Seca quarries – Gandarela Formation – Quadrilátero Ferrífero – Brazil, were determined. Few samples (12 out of 69) show relatively higher REE contents that are associated with strongly positive correlation with the immobile elements including Al, Ni, Th, Cr, Sc and Y coupled with a negative correlation between ΣREE and CaO, suggesting that the observed variations in the ΣREE values in these samples were controlled mainly by the input of terrigenous materials. The Eu and Ce anomalies (normalized by the PAAS) of the analyzed samples vary from 1.1 to 2.13 and 0.79 to 1.03, respectively for Bemil quarry samples and 0.74 to 2.7 and 0.32 to 0.99, respectively, for samples from the Lagoa Seca quarry. In the present study, Al2O3 content do not show significant correlation with Eu/Eu* (correlation coefficient r = 0.23). Hence the positive europium anomalies in these samples seem to have not been influenced by the amount of terrigenous materials in them. Y/Ho ratios range from 27 to 93 for Bemil quarry samples and 24 to 132 for samples from the Lagoa Seca quarry. Some samples have Y/Ho ratio more or less similar to chondritic values (∼28). High REE contents, non-seawater-like REE patterns, the large abundance of certain trace elements such as Sc, Th and Hf, high (La/Yb)sn ratios and low Y/ Ho ratios collectively suggest that few samples were mainly influenced by the incorporation of terrigenous materials. Most of the studied samples (38 out of 69) of the Bemil and Lagoa Seca quarries have δ18O (VPDB) values lower than −12‰, corresponding to carbonates that underwent changes by post-depositional processes. The δ13C values (0.69‰ to −4.46‰ and 0.07‰ to −6.18‰) of the majority of the analyzed samples, both of the Bemil quarry and the Lagoa Seca quarry, are similar to those marine carbonates of the same age (∼2.42 Ga). Samples that have δ13C values around −4‰ are likely to result from decarbonation reactions. Two samples (LS11 and LS12) may record glacial periods.