Geochimica Brasiliensis (May 2017)
The Anicuns-Itaberaí volcano-sedimentary sequence, Goiás Magmatic Arc: new geochemical and Nd-S isotopic data
Abstract
The Neoproterozoic Anicuns-Itaberaí volcano-sedimentary sequence exposed in the surroundings of Anicuns, western Goiás, comprises a complex association of igneous and supracrustal rocks, which has been divided into: (i) the Córrego da Boa Esperança Sequence including calc-alkaline meta-basalts, andesitic/dacitic metatuffs, metapelites, and iron formation, and (ii) the Anicuns-Itaberaí Sequence, in the east, including mafi c/ultramafi c rocks, metacherts, metarhytmites, and marble lenses. These sequences are interpreted to be part of the supracrustal associations of the Goiás Magmatic Arc, formed between ca. 890 and 630 Ma. Granitic rocks, as well as small mafi c and mafi c-ultramafi c bodies are intrusive into the supra-crustal sequences. The granitoid intrusions are tonalites, granodiorites, and granites with subordinate quartz syenite, monzonite, and monzodiorite. In this work we present and discuss geochemical and isotopic data from metaigneous and metasedimentary samples. Based on the variation patterns three different groups of metabasic rocks may be recognized: 1) very primitive N-MORB-like basalts; 2) moderate LILE-enriched rocks and 3) distinctively LILE-enriched rocks. Nd isotopic results mostly indicate TDMvalues of ca. 1.0 Ga and positive εNd(T), also similar to the known primitive nature of the meta-igneous rocks of the Neoproterozoic Goiás Magmatic Arc. Field observations, together with geochemical and isotopic data from this ontribution suggest that the supracrustal and associated magmatic sequences in the Anicuns area may represent an arc/fore-arc sequence, marking the tectonic boundary between the Goiás Magmatic Arc, to the west and, the westernmost exposures of the São Francisco Continental margin, to the east. This is also in agreement with regional geophysical data from the literature, which shows that the area of exposure of the Anicuns-Itaberaí sequence coincides with a steep gravimetric gradient. In general the geochemical and isotopic characteristics of the meta-igneous rocks are typical of island arc associations.
