C- and Sr-isotope systematics applied to Neoproterozoic marbles of the Seridó belt, northeastern Brazil

Abstract

The Seridó fold belt, situated in the northeastern part of the Borborema Province in northeastern Brazil, has been the target of several studies in the last two decades. Previous field and structural studies suggested a Paleoproterozoic age, but recent isotopic data raised the possibility that the original sediments were deposited and metamorphosed during the Neoproterozoic. Detailed C and Sr isotope studies in marbles of the Seridó Group clarify this issue. In the Jucurutu Formation, at the base of the Seridó Group, δ 13C fluctuations indicate three stratigraphic levels of carbonate sedimentation. Basal marbles, consisting of calcite, amphibole, phlogopite, quartz and opaques, display δ 13C between + 8.3 and + 10.8‰ V-PDB. Values of δ 13C < + 8.3‰ V-PDB are observed in marble samples close to the contact with metapelites, or when silicate minerals are more abundant. Intermediate marbles are composed of calcite, tremolite, phlogopite, quartz, opaques, and wollastonite and exhibit δ 13C values from + 6.7 to + 8.7‰ V-PDB. In wollastonite-bearing marbles, δ 13C is < + 6.7‰ V-PDB (sometimes negative). The youngest Jucurutu Formation marbles, consisting of calcite, dolomite, amphibole, mica, quartz, and opaque minerals in variable amounts, had δ 13C values from − 8.9 to + 3.8‰ V-PDB. δ 13C varies according to the abundance of silicate minerals present and with the calcite/dolomite ratio. Basal marbles of the Seridó Formation, composed of calcite, tremolite, phlogopite, quartz and opaques, exhibit δ 13C values from + 8.9 to + 10.7‰ V-PDB. A gradual declining δ 13C (+ 7.5 to + 4.4‰ V-PDB) coincident with an increase in silicate content, marks the basal contact with mica-schists. Negative δ 13C values mark the upper calcitic marble (± phlogopite) intercalations of the Seridó belt that exhibit values from − 4.6 to − 4.0‰ V-PDB. The C-isotope compositions observed in marbles of the Seridó belt vs. the C-isotope secular variation curve, suggest depositional age interval of 590–570 Ma. This is further corroborated by the Sr-isotope ratios (mostly between 0.7074 and 0.7077), typical of pre-Varangerian carbonates.