O mapeamento faciológico em rochas ígneas plutônicas

Abstract

The mapping of plutonic igneous rocks poses special problems, since the usual stratigraphic laws are not applicable to such cases. Most igneous massifs are frequently the result of complex sequences of magma emplacement, to be unraveled by mapping procedures. Special care is thus needed with the definition of the several mapping units, and the mapping activity itself becomes a foremost tool for the interpretation of igneous processes and data. Initially, the igneous rock fades is recognized as the core mapping unit, each defined entirely by macroscopic-mesoscopic features (mineral content, texture, color and color index, structure). Most cannot be represented on a map, on the usual standard scales such as 1:10,000 or 1:25,000, so that the next step is to identify the mappable unit (or association), usually various that can clearly be grouped together, on account of similarities in composition, structure, and relative ages, as shown by crosscutting relationships. Mapping of these units outlines the igneous massif, its structure, geometry and emplacement behaviour. The term igneous body is here proposed as a synonym for this rheologic or emplacement unit, and the neutral name seems fit to identify any medium- or large-sized igneous occurrence. Facies and facies associations, or mappable units, should be named informally (e.g., A, B; of red granite; etc.), while a geographic addition identifies the entire massif (.e.g., Atibaia massif, etc.). All cited mapping categories (facies, association, body, massif) are informal and should therefore be clearly separated from lhe formal names defined in the official Stratigraphic Codes. Several geologically-geotectonically related massifs, grouped together, identify a still informal category of higher hierarchy, the magmatic or genetic unit, informally akin to the suites and supersuites of other authors, to be defined only after comprehensive regional studies.