KOMATIITE LAVAS FROM THE QUEBRA OSSO GROUP (RIO DAS VELHAS GREENSTONE BELT, SOUTHEAST BRAZIL): A FIELD GUIDE TO AN ARCHEAN FLOW FIELD

Abstract

The Quebra Osso Group encompasses one of the best preserved Archean komatiite flows in the Quadrilátero Ferrífero region, southern São Francisco craton (Brazil). It is commonly associated with the base of the Rio das Velhas greenstone belt within the Santa Bárbara tectonostratigraphic block. The Quebra Osso rocks are well exposed along the valley of the homonymous river, near the Catas Altas city (Minas Gerais state), and in inactive serpentine quarries. In this field guide, we provide the descriptions of the key outcrops in this area, including a variety of komatiite flows, sedimentary rocks and pyroclastic rocks. Despite the pervasive alteration of primary mineral assemblages due to greenschist to amphibolite facies metamorphism, the ultramafic rocks exhibit diverse igneous textures and structures. Volcanic textures are recognizable due to the morphology of the original igneous olivine and pyroxene, which underwent pseudomorphosis by hydrous minerals – serpentine and chlorite. These features include distinctive spinifex and cumulate textures that provide clues of the conditions operating during emplacement and crystallization of the ultrabasic liquids. The predominance of thick-packages of massive flows suggests channelized lava flows, with subordinate thin marginal layered (spinifex-textured), pillowed, brecciated and pyroclastic flows. Volcanic breccias, pillow lavas and chemical sedimentary rocks indicate that these rocks erupted underwater. These multiple volcanic products characterize a unique komatiite flow field and their remarkable preservation distinguishes the Quebra Osso Group from the other ultramafic associations within the São Francisco Craton.