Isotopic age constraints and geochemical results of disseminated ophiolitic assemblage from Neoproterozoic mélange, central Brazil

Abstract

The Neoproterozoic Brasília belt, central Brazil, formed during the amalgamation of West Gondwana, and contains a magmatic arc and a large metasedimentary pile. An ophiolitic mélange is contained within one of the metasedimentary units from the pile, and includes bodies of metamafic and ultramafic assemblages in a metasedimentary rock matrix. One of these metamafic bodies encompasses a serpentinite body, which displays a well-developed metasomatic “black-wall” alteration zone at its contact. Samples of metamafic, serpentinite and black-wall rocks were collected, as well as two garnet-mica schists in contact with the metamafic assemblage. The metamafic assemblage is composed of epidote-amphibole schist, chemical analysis of which indicates that its protolith was a tholeiitic basalt with a MORB signature that may have been formed in a back-arc environment. U-Pb analyses indicate an age of crystallization of c. 800 Ma for the protolith of the epidote-amphibole schist. The black-wall is mostly composed of chlorite schist, with the exception of an albite-hornblende fels at the contact with the epidote-amphibole schist. The black-wall has a range of ages from c. 800 to 760 Ma. ƐHf values of selected zircon grains of the epidote-amphibole schist sample range from −21.67 to + 13.66, with TDM ages of 0.80–4.22 Ga, while the black-wall samples have highly depleted ƐHf values ranging from + 9.16 to + 23.98, with TDM model ages of 0.34–0.96 Ga. The studied garnet-mica schists have zircon population peaks of c. 790 to 890 Ma. ƐHf ranges from −14.87 to + 6.46 and TDM ages from 1.47 to 1.91 Ga. The epidote-amphibole schist, serpentinite and black-wall assemblages all have positive ƐNd values from + 4.95 to + 7.89, indicating a mantle signature. ƐNd values for the mica schists are −8.31 and −8.16, and their TDM values are 1.83 and 2.36 Ga, which indicate continentally derived components. The results suggest that the ophiolitic component of the Araxá mélange formed in a back-arc basin environment, which formed at c. 800 Ma and lasted till c. 760 Ma, while the studied schists probably represent sediments deposited in the newly formed back-arc basin.