Magmatismo básico mesoproterozoico associado às rochas metassedimentares da Formação Água Clara do Supergrupo Açungui: 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.RESUMO - As rochas ortoderivadas básicas e ultrabásicas da porção sul do Cinturão Ribeira, norte do Terreno Apiaí, no sudoeste do estado de São Paulo são definidas por anfibolitos, metabasitos, anfibólio xisto e localmente metaultrabasitos de idade meso- a neoproterozoica e encontram-se associadas às rochas da sequência metavulcanossedimentar da Formação Água Clara, Supergrupo Açungui. O metamorfismo regional progressivo é do tipo Barroviano e a paragênese mineral é constituída de pseudomorfos ígneos de piroxênios “augita” ± labradorita, associado às paragêneses metamórficas apresentando magnesiohornblenda ± oligoclásio/andesina, representando o ápice do metamorfismo regional progressivo em fácies anfibolito médio, de 579°C a 621°C de temperatura e 6,6 kbar a 7,3 kbar (±0,6). Uma paragênese mineral em fácies xisto verde baixo a médio é caracterizada pelo retrometamorfismo regional e milonítico, sendo constituída por actinolita/tremolita ± albita. Dados geoquímicos classificam as rochas como basaltos, sub alcalinos de baixo potássio a alcalinos, Fe-toleíticos a Mg-toleíticos, com leve tendência a basaltos komatiíticos. Já os dados geotectônicos, corroborados pelo comportamento dos elementos traços e dos ETRs, indicam três assinaturas principais para os grupos anfibolíticos: as rochas básicas toleíticas a komatiíticas associadas a cadeia mesoceânicas (MORB), com possível interação da crosta na fase de subducção; as rochas ultrabásicas alcalinas de ilha oceânica (OIA) e os basaltos toleíticos de arco de ilha. Portanto, os eventos magmáticos estão correlacionados ao estágio inicial de abertura de bacia ou ao início de fechamento à fase inicial de natureza toleítica de fundo oceânico, associado a basaltos alcalinos de ilha oceânica. Palavras-Chave: Metabásica; Água Clara; Supergrupo Açungui; Química Mineral; Litogeoquímica.   ABSTRACT - The basic and ultrabasic orthoderived 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. Keywords: Metabasic; Água Clara Formation; Açungui Supergroup; Mineral Quimic; Lithogeochemistr