The Niquelândia Mafic-Ultramafic Complex, Goias, Brazil: a contribution to the ophiolite X stratiform controversy based on new geological and structural data

Abstract

The results of a geological/structural mapping in a slice of the Niquelândia layered Mafic-Ultramafic Complex are presented. The complex stratigraphy is made up of a possible basal zone - the East Mafic Unit - (mainly gabbronorites with interlayered pyroxenites), followed by the Ultramafic Unit (mainly interlayered dunites and pyroxenites), followed in turn by the Central Mafic Unit (mainly gabbronorites with minor interlayered pyroxenites), and the Serra dos Borges Unit (mainly gabbros, leuco-troctolites, and anorthosites) at the upper part. The most important tectonic feature all across the complex is the pervasive presence of westward dipping NS-N10E ductile shear zones with well developed tectonic foliation parallel with the primary bedding. The metamorphic paragenesis within the deformed zones progressively change from hypersthene-granulite facies in the basal and central units, to hornblende-granulite facies, and amphibolite facies in the upper unit. Some micro-tectonic features described in the basal units are typical of alpine-type peridotites, and demonstrate that the basal cumulitic rocks underwent deformation and recrystallization under conditions generally considered to take place at the lower crust/upper mantle at an estimate pressure of 8–10 kbar. The tectonic features post-date the primary late-magmatic features (like unmixing in pyroxenes and reaction coronas) what indicate that the tectonic event is not closely related with the emplacement of the Niquelândia Complex. The Niquelândia Complex is now considered as a single, major stratiform, mafic-ultramafic complex which subsequently underwent tectonism with associated high-grade metamorphism.