Paleoproterozoic subduction-related metasomatic signatures in the lithospheric mantle beneath NE Brazil: inferences from trace element and Sr–Nd–Pb isotopic compositions of Neoproterozoic high-K igneous rocks |
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| Authors: | Maria Helena Bezerra Maia de Hollanda Mrcio Martins Pimentel Emanuel Ferraz Jardim de S |
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| Institution: | a Institute of Geosciences/University of Brasília, Brasília, DF, CEP 70910-900, Brazil;b Department of Geology-CCET/Federal University of Rio Grande do Norte State, Natal, RGN, CEP 59072-970, Brazil |
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| Abstract: | Late Neoproterozoic (ca. 580 Ma), high-K, mafic-intermediate rocks represent voluminous bimodal magmatism in the Borborema Province, northeast Brazil. These rocks show the following chemical signatures that reflect derivation from a subduction-modified lithospheric mantle source: (1) enrichment in large ion lithophile elements (Rb, Ba, K, Th) and light rare-earth elements (REE) (La/YbCN=11–70), (2) pronounced negative Nb anomalies, and (3) radiogenic Sr (0.71202–0.7059) and unradiogenic Nd (Nd from −9.3–−20.1) isotopic compositions. TDM model ages suggest that modification of the lithospheric mantle source (metasomatised garnet lherzolite) may have occurred in the Paleoproterozoic during the Transamazonian/Eburnean tectonics that affected the region. Interaction with asthenospheric fluids is believed to have partially melted this enriched source in the Neoproterozoic, probably as a result of asthenosphere-derived fluid percolation in the Brasiliano/Pan-African shear zones that controlled the emplacement of these mafic-intermediate magmas. The involvement of this asthenospheric component is supported by the nonradiogenic Pb isotopic ratios (206Pb/204Pb=16–17.3, 207Pb/204Pb=15.1–15.6, 208Pb/204Pb=36–37.5), which contrast with the enriched Sr and Nd compositions and thereby suggest the decoupling of Rb–Sr, Sm–Nd, and U–Pb systems at the time of intrusion of the mafic-intermediate magmas in the crust. |
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| Keywords: | Mafic-intermediate rocks Subduction-modified lithospheric mantle source Transamazonian tectonics |
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