Morphological and mineralogical characterization of weathering zones in tropical climates: A basis for understanding the weathering process on granitic rocks in southeastern Brazil

Abstract

Weathering zones of granitic rock masses are of particular interest in tropical climate regions because of their morphologies and mineralogical properties. To address the lack of studies on weathering profiles of granitic rocks in Brazil, this research aimed to perform a complete morphological and mineralogical characterization of weathering profiles of a syenogranite developed in tropical southeastern Brazil. Fieldwork was performed, resulting in the identification and investigation of five weathering profiles. The morphological description included the spatial distribution of the different layers, the weathering features and the mechanisms responsible for the transformation of rock masses. At the rock matrix scale, the mineralogical characterization was based on petrographic thin sections made from samples of each of the different degrees of weathering. Petrographic indices such as the micropetrographic index (Imp) and microfracturing index (Imf) were determined and compared with data obtained in the literature. The results indicated that the weathering in the profiles studied was facilitated by a network of joints and fractures, which allowed water access, causing chemical decomposition at a certain depth and then resulted in relatively thick weathered profiles characterized by the presence of corestones. On a microscopic scale, changes in unweathered rock are controlled by the opening of pre-existing microcracks and the development of new cracks in the early stages of weathering, as well as chemical decomposition through the transformation of primary minerals into secondary minerals from medium-weathered rock. It is clear that physical weathering controls the initial changes and chemical weathering controls the later stages of syenogranite alterations. The increase in the microfracturing index from 3.2 to 7.7 had a larger influence on the control of physical weathering in the early stage. The micropetrographic index decreases to 2.1 at the most advanced stage of weathering, demonstrating the dominance of chemical weathering of the matrix in the final stages of rock weathering.