The Agua Clara Formation, in the Ribeira Valley, State of São Paulo, consists of a volcanosedimentary sequence subjected to low- and medium-grade metamorphism in the Araçaiba region. This supracrustal sequence is composed predominantly of purê and impure marbles and calc-silicatic rocks with intercalations of amphibolites, schists and quartzites. Calc-silicatic rocks are always banded and are made up of diopside, tremoliteactinolite, feldspars, quartz and biotite/phlogopite in various combinations in the different bands. They may have an oriented fabric or may be massive. Marbles are calcitic and dolomitic, in variable proportions, with tremolite and phlogopite occurring in the impure terms. Metabasic rocks occur mainly as small bodies (ienses) associated with calc-silicatic rocks. Hornblende, plagioclase and epidote are the essential constituents. Clinopyroxene remains and relict ophitic-subophitic textures indicate an igneous origin, although most of these features have been obliterated by subsequent deformation. The marbles/calc-silicatic rocks have compositions indicative of carbonatic sediments and carbonate-pelite mixtures. The easternmost portion of the belt shows a predominance of marbles and pelite-rich calc-silicate rocks, while the westernmost pbrtion is dominated by carbonate-pelite mixtures. This spatial distribution suggests a facies variation with paleogeographical implications. The basic magmatism appears to have evolved in several tectonic settings. Early tholeiitic magmatism may represent transitional ocean floor basalts (possibly corresponding to the rifting stage) or, more probably, may be immature island are tholeiites generated in the initial stage of the basin closure. Shoshonitic basalts, possibly related to continental active margins, may have been associated with the increasing maturity and final events of this magmatic arc. Later, but still Precambrian events involved another basic magmatic episode with tholeiitic continental characteristies. This was repeated again in the Mesozoic with the intrusion of diabase dykes
