Combined use of Sm–Nd isotopes and lithogeochemistry in the sedimentary provenance of the southern Ediacaran-Cambrian Bambuí foreland basin system, Brazil.
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2021
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Abstract
Sedimentary provenance analysis based on lithogeochemistry and isotope data has been extensively applied in the
investigation of ancient sedimentary successions. This approach contributes to understand the tectonic setting in
which these strata were deposited and to recognize major provenance patterns in time and space. The Ediacaran to
Cambrian Bambuí Group is exposed in the Sao ̃ Francisco craton (SE Brazil) and represents the sedimentary record of a
foreland basin system developed during the assembly of the West Gondwana. Different lines of evidence indicate that
this basin system evolved into a confined setting, due to the overloads imposed on the western and eastern margins of
the Sao ̃ Francisco paleoplate by the diachronic Brasília and Araçuaí orogenic systems, respectively. New whole-rock
lithogeochemistry and Sm–Nd isotopic data were applied to evaluate the overall sedimentary provenance patterns of
the Bambuí strata exposed in the southern portion of the Sao ̃ Francisco craton (i.e., within the Sete Lagoas basement
high) and to identify its possible sources. Coupled (La/Yb)n and Th/Sc ratios with 147Sm/144Nd and εNd isotopes
allowed the recognition of three contrasting source patterns associated with different 2nd order sequences. The
lower sequence comprises carbonate ramp deposits and shows highly heterogeneous and more negative εNd ranging
from − 10.5 to − 4.5, as well as Th/Sc and (La/Yb)n ratios between 0.6 and 0.7 and 0.54 and 0.76, respectively. The
provenance patterns point toward intermediate-dominated and isotopically evolved crustal sources, with mean TDM
model ages between 1.5 and 2.0 Ga. The overlying 2nd-order sequence is composed of fine-to medium-grained
siliciclastics grading upward into platformal oolitic calcarenites and calcilutites. This succession exhibits homo-
geneous and less negative εNd(t) values between − 6.8 and − 5.2, with low Th/Sc (0.6–0.8) and (La/Yb)n (0.60–0.84)
ratios. The uppermost Bambuí 2nd-order sequence, marked by greenish siltstones and arkoses grading upward into
storm-bedded arkoses and lithic sandstones, shows homogeneous εNd(t) ranging from − 8.5 to − 6.1, slightly more
negative than the underlying successions, and relatively higher Th/Sc and (La/Yb)n ratios ranging from 0.7 to 1.3
and 0.85 to 1.28, respectively. The Sm–Nd isotopic signature of the upper deposits indicates felsic-dominated and
more isotopically juvenile crustal sources with TDM model ages of 1.5–1.7 Ga, characteristics compatible with
Ediacaran arc-related rocks and post-tectonic granites exposed in the Araçuaí orogen. Our data indicate that the
Proterozoic cratonic covers, their correlatives in the surrounding orogens, and magmatic rocks from the Araçuaí
orogen acted as major sedimentary sources during the first evolutionary stages of the Bambuí basin-cycle in the Sete
Lagoas basement high, which behaved as a forebulge depocenter. During the deposition of the overlying sequences,
the Araçuaí orogen rocks became increasingly important in sourcing the Ediacaran- Cambrian foreland system in this
depocenter. Besides revealing the major provenance patterns through the evolution of an ancient and partially
confined foreland basin system, our data also demonstrates that even minor changes in the Sm–Nd system might be
useful as reliable elements on the provenance analysis of sedimentary systems elsewhere.
Description
Keywords
Sm–Nd systematics, West – Gondwana, Bambuí group
Citation
PARAVIDINI, G. et al. Combined use of Sm–Nd isotopes and lithogeochemistry in the sedimentary provenance of the southern Ediacaran-Cambrian Bambuí foreland basin system, Brazil. Journal of South American Earth Sciences, v. 111, 2021. Disponível em: <https://www.sciencedirect.com/science/article/pii/S0895981121002765>. Acesso em: 29 abr. 2022.