Using Nd isotopes to understand the provenance of sedimentary rocks from a continental margin to a foreland basin in the Neoproterozoic Paraguay Belt, Central Brazil

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• 作者：Elton Luiz Dantas ; Carlos Jose Souza de Alvarenga ; Roberto Ventura Santos ; Má ; rcio Martins Pimentel
• 关键词：Cap carbonate ; Nd isotopes ; Neoproterozoic ; Paraguay Belt
• 刊名：Precambrian Research
• 出版年：2009
• 出版时间：April 2009
• 年：2009
• 卷：170
• 期：1-2
• 页码：1-12
• 全文大小：2463 K

文摘

The Neoproterozoic Paraguay Belt (PB) in central South America includes a thick sequence of sedimentary rocks that were deposited and deformed along the southeastern margin of the Amazonian Craton. The stratigraphy is divided into four major successions: (1) a lower succession of fine-grained sedimentary rocks in the Cuiabá Group, (2) a glacial diamictite–turbidite succession in the Cuiabá Group and Puga Formation, (3) a carbonate succession in the Araras Group, and (4) an upper succession composed of sandstones, siltstones and arkoses in the Alto Paraguay Group. This sequence of sedimentary rocks represents a passive margin that was deformed during late Neoproterozoic-Cambrian epoch.

We investigated the Nd isotope signature of the sedimentary rocks, while considering the different lithostratigraphic sequences and their geographic extent within the PB. Nd isotopic ratios and T_DM model ages suggest a dominant continental source where are related to Paleoproterozoic to Mesoproterozoic rocks of the Amazonian Craton. The Nd isotopic compositions do not display a major contribution from the younger Neoproterozoic juvenile magmatic arc terrane of the Brasília Belt.

T_DM model ages plotted against the stratigraphy show old T_DM model ages of ca. 2.3–1.5 Ga and strongly negative _Nd (T) values of −11 to −6, revealing sedimentary changes in the source. The changing _Nd (T) values are interpreted to be function of the erosion/deposition ratio that occurred due to uplift during orogenic processes causing variations in the source area.

Compared to the lower glacial and carbonate successions, the upper siliciclastic succession shows lower Nd isotopic ratios that is consistent with a source from the Paraguay Belt (or another Neoproterozoic continental source?). This can be accommodated by the change from an extensional continental margin to a compressional foreland basin setting.

This provenance study of the Paraguay Belt sedimentary rocks has important implications for the West Gondwana collision history prior to its final amalgamation at the end of the Neoproterozoic and beginning of the Cambrian epoch.