Does the metavolcanic-sedimentary Rio do Coco Group, Araguaia Belt, Brazil, represent a continuity of the Quatipuru ophiolitic complex? – Constraints from U-Pb and Sm-Nd isotope data

Abstract

Serpentinized peridotites and basaltic volcanic rocks from the Quatipuru, Serra do Tapa, and Morro do Agostinho complexes are part of dismembered ophiolite bodies hosted by metasedimentary units of the Neoproterozoic Araguaia Belt, Tocantins Province, central Brazil. Approximately 10 lenticular ultramafic bodies, which contain primitive (serpentinite) and more evolved (actinolite schist, actinolite-talc schist, actinolite-serpentinite schist, talc schist, talc-serpentine schist, actinolitite, cloritite) rocks, were identified in the Rio do Coco Group (RCG), a metavolcanic-sedimentary sequence in the southern portion of the belt. These bodies are intercalated with metasedimentary rocks. All lithologies are metamorphosed to upper greenschist and lower amphibolite facies. The sequence was initially characterized as a classic Archean greenstone belt, but the present study suggests that it should be considered an ophiolite suite. On the basis of the ƐNd(t) values the RCG rocks can be subdivided into two groups: Group 1, with positive ƐNd(t) values, neodymium model ages (TDM) of ca. 1.0 to 1.1 Ga, with ocean-island-basalt (OIB) affinity, and being less contaminated with metasedimentary rocks of the Baixo Araguaia Supergroup; and Group 2, with negative ƐNd(t) values, TDM ranging from 1.2 to 1.7 Ga, with a variable ocean island basalt (OIB), plume or enriched mid-ocean-ridge basalt (P- and E-MORB) affinity, and being more contaminated with metasedimentary rocks. Two samples were analyzed by the U-Pb method on zircon (LA-MC-ICPMS). The weighted average age from a serpentinite (715 ± 26 Ma) and an actinolite schist (702 ± 24) indicates a maximum emplacement age for this sequence at 708 ± 35 Ma, which is similar to the previously published age for the Quatipuru Complex (Sm-Nd isochron age of 757 ± 49 Ma). This may suggest that the metavolcanic-sedimentary sequence of the Rio do Coco Group and the Quatipuru ophiolite system could be part of the same dismembered oceanic lithosphere.