Elemental and stable isotopes geochemistry of Paleoproterozoic dolomites from Fecho do Funil Formation, Quadrilatero Ferrifero - Brazil

摘要

Geochemical and stable isotopic characteristics of thirteen samples taken from Paleoproterozoic Fecho do Funil Formation, Quadrilatero Ferrifero, Brazil have been analyzed to investigate the depositional conditions and source of rare earth elements (Sigma REEs) in Cumbi quarry dolomites. The major oxides such as CaO and MgO show variable compositions ranging from 20 to 29 wt % and 14-21 wt % respectively in most of the samples. The contents of loss on ignition (LOI) are lower (26-42 wt %) than that of the pure dolomite (48 wt %). These mass discrepancies are compensated by other oxides including AI(2)O(3) (1:9-18.7 wt %), Fe2O3 (1.2-6.1 wt %), and K2O (0.5-6.8 wt %) that had been incorporated into the dolomite samples by the contamination of terrigenous input. Further, Sigma REE contents (20-101 ppm) display significant variation that also corroborate with contamination of studied samples by detrital materials. The regression lines of Sigma REE against Al2O3 (R-2 = 0.96), Fe2O3 (R-2 = 0.65), ,Ni (R-2 = 0.94), Cr (R-2 = 0.95), Th (R-2 = 0.98), and Sc (R-2 = 0.98) show positive correlation which is probably associated with the input of terrigenous materials during the deposition of Cumbi quarry dolomites. On the other hand, a negative correlation between Sigma REE and CaO combined with a large variation in Y/Ho (27-50) is also interpreted as the sea-water like Sigma REE patterns have been masked by the contribution of variable amounts of terrigenous materials in Cumbi quarry dolomites. All the dolomite samples analyzed from the Fecho do Funil Formation exhibit a subtle negative cerium anomaly (Ce/Ce* = 0.85-0.95). The dolomites from Cumbi quarry - Fecho do Funil Formation show narrow variations in Eu anomalies (Eu/Eu* = 1.02 to 1.25). The positive correlation between Eu with Zr, Th and Y supports the non diagenetic influence on this element (R-2 = 0.94, 0.98, 0.84 respectively). Eu contents, in this study show significant positive correlation with AI(2)O(3) (R-2 = 0.96 ), suggesting the detrital origin.& para;& para;The delta C-13(VPDB) (+6.0 to +7.2 parts per thousand) and delta O-18(VPDB) (-10.9 to -10.4 parts per thousand) values in our samples display a narrow range which are identical to those shown by successions, characterized by positive carbon excursions, deposited during the Lomagundi event. We infer that the elevated carbon isotope values of the Fecho do Funil dolomites likely reflect primary carbon isotope compositions. (C) 2017 Elsevier Ltd. All rights reserved.