Structural characterization of the south portion of the Guarinos Greenstone Belt, GO

Abstract

Geological mapping in the southern portion of the Guarinos Greenstone Belt in the state of Goias, Brazil, led to the characterization of structural features and recognition of the following lithostratigraphic units: Serra do Cotovelo (meta-ultramafic); Serra Azul (meta-mafic); Unit A (association of meta-ultramafic, meta-mafic, and psamo-pelitic rocks); Cabacal (carbonaceous shales with iron formations and gondites) subdivided into Ca-1, Ca-2, Ca-3 and Ca-4; Aimb, with lower (muscovite-rich banded iron formations) and upper (metapelites associated with iron formations) subunits; and Sao Patricinho (metapelites, subordinately gondites). Metasediments, referred to the Araxa Group in the local literature, unconformably overlie the greenstone sequence. During geological mapping, no structural feature resulting from ductile or brittle deformation (mylonite or breccia) was found at the contact between the so-called Araxa Group metasediments and the Guarinos greenstone belt lithotypes, which suggests that this contact may represent a stratigraphic unconformity and not a thrust fault, as described in the literature. On the basis of structural geometry and superposition criteria, five deformational events covering all units of the greenstone sequence have been characterized: Dn-2 and Dn-1, preceding the main phase (D-n), Dn+1 and Dn+2. The rocks were formed under greenschist facies conditions, from the biotite zone in the south to the garnet zone in the northern area. The minerals that define the metamorphic peak exhibit late to post-Dn growth for both the greenstone metasediments and the Araxa Group indicating that peak metamorphism was coeval in both sequences.