MESOPROTEROZOIC BASIC MAGMATISM ASSOCIATED WITH METASEDIMENTARY ROCKS OF THE ÁGUA CLARA FORMATION, AÇUNGUI SUPERGROUP

Abstract

The basic and ultrabasic orthoderivative rocks in the southern portion of the Ribeira Belt, north of the Apiaí Terrain in southwestern São Paulo state, are primarily composed of amphibolites, metabasites, amphibole schists, and locally, metaultrabasalts. These rocks, dated to the Meso- to Neoproterozoic, are associated with the metavolcanic-sedimentary sequence of the Água Clara Formation within the Açungui Supergroup. The regional metamorphism is of the Barrovian type, with mineral paragenesis consisting of igneous pseudomorphs of pyroxenes ("augite") ± labradorite, associated with a metamorphic assemblage dominated by magnesiohornblende ± oligoclase/andesine. This represents the peak of progressive regional metamorphism in the medium amphibolite facies, with temperatures ranging from 579°C to 621°C and pressures between 6.6 kbar and 7.3 kbar (±0.6). Additionally, a mineral paragenesis in the low- to medium-grade greenschist facies is characterized by regional and mylonitic retrometamorphism, consisting of actinolite/tremolite ± albite. Geochemical data classify these rocks as basalts, ranging from low-potassium to alkaline sub-alkaline, with compositions transitioning from Fe-tholeiitic to Mg-tholeiitic and a slight tendency toward komatiitic basalts. Geotectonic data, supported by the behavior of trace elements and rare earth elements (REEs), indicate three main geochemical signatures among the amphibolitic groups: basic tholeiitic to komatiitic rocks associated with mid-ocean ridge basalts (MORB), possibly influenced by crustal interaction during subduction; ultrabasic alkaline rocks characteristic of oceanic islands (OIA); and tholeiitic basalts associated with island arcs. These magmatic events are therefore correlated with the initial stage of basin opening or the early phase of oceanic closure, specifically the tholeiitic nature of the ocean floor, and are associated with the alkaline basalts of an oceanic island.