METAKOMATIITIC AND META-ULTRAMAFIC ROCKS FROM THE RIO MANSO REGION, MINAS GERAIS: GEOLOGY TEXTURES AND METAMORPHISM

Abstract

The study area is located in the southwestern part of the "Quadrilátero Ferrífero", south of Serra do Curral, Minas Gerais (Brazil). Three major units occur in the area: Granite-Gneiss Complex, Nova Lima Group and intrusive bodies. The Nova Lima Group consists of metakomatiite and metakomatiitic basalt, meta-ultramafic rocks devoid of spinifex texture, rare iron formation, amphibolites and metasedimentary rocks. It comprises mainly meta-ultramafic rocks without primary igneous texture relicts and subordinate metakomatiitic rocks. Metakomatiites occur only in Morro da Onga, and display spinifex texture, especially of the random type. Thin layers interpreted as probable metacumulate parts of flows (B zone) occur locally. A structure that may correspond to a komatiite lava tube is also exposed, Metakomatiites are typically peridotitic komatiites and subordinately komatiitic metabasalts; the latter may exhibit pillow-lava structures. Progressive metamorphic evolution began with a Mg-chlorite + tremolite-actinolite paragenesis. In a second stage orthopyroxene and olivine porphyroblasts were developed in a Mg-hornblende groundmass. Progressive increase of Al in chlorite contributed to olivine and rare anthophyllite blastesis in the initial phases of the progressive metamorphism. Late metasomatism led to serpentinization and talcification. Amphibole plays an important role when the modal/textural relations of the highest-grade metamorphic parageneses are considered. Al enrichment of Ca-amphibole through tschermak and edenite replacements led to consumption of olivine to yield pyroxene and to spinel consumption through its incorporation as the aluminous component of amphibole. Thus the area consists of abundant meta-ultramafic rocks devoid of igneous texture relicts and subordinate komatiitic flows with thin cumulate zones. The former may partly correspond to an ultramafic layered sequence.