Petrogênese das rochas máficas alcalinas do litoral entre São Sebastião (SP) e Parati (RJ)

Abstract

Alkaline lamprophyres and alkaline diabases of the Serra do Mar Dyke Swarm (SMDS) outcrop along the Southeastern Brazil. These dykes intrude rocks of the so-called Rio Negro Magmatic Arc within the Late Proterozoic-Ordovician Ribeira Orogen and are associated with the opening of South Atlantic Ocean during the Late Cretaceous. Typically, the intrusions are subvertical bearing a NE-SW preferential trend which follows the main lineaments in the area. The lamprophyres are classified as monchiquites and camptonites and show holocrystalline and hypocrystalline textures, respectively. They also have panidiomorphic texture and olivine and clinopyroxene phenocrysts within a matrix composed essentially by these same minerals, kaersutite and biotite. Camptonites still have plagioclase in the matrix. The alkaline diabases are holo- to hypocrystalline, equi- to inequigranular rocks with olivine and/or clinopyroxene and/or plagioclase phenocrysts in a matrix composed essentially by these minerals. These alkaline rocks have miaskitic compositions. Lamprophyres are sodic, potassic and ultrapotassic and alkaline diabases are predominantly sodic. Petrogenetic modeling indicates that lamprophyres are unlikely to be cogenetic by evolutionary processes involving either fractional crystallization, with or without concomitant assimilation, or hybridization. The same applies to the alkaline diabases. Modal-batch partial melting models using samples representing parental liquids indicate that these alkalines mafic rocks are related to lherzolitic sources and that partial melting occurred in the spinel stability field, pointing to a thin continental lithosphere in the region. Lithogeochemical and Sr-Nd isotopic data show that the petrogenesis of these alkaline rocks can be assigned to mixing of mantle sources. Binary mixing models and assimilation tests reveal that the petrogenesis of the lamprophyres and alkaline diabases involve a large amount of participation of a fertile component with small contribution of an enriched component. TDM model ages for the studied rocks suggest remobilization ages of the of the lithospheric mantle during the Neoproterozoic, possibly related to the subduction of the San Francisco plate prior to the collision of the Ribeira Orogen. The high CaO/Al2O3 ratios of less evolved lamprophyric melts, high Zr contents, Zr/Hf and Ti/Eu negative correlations and association with carbonatites point to high CO2/H2O conditions during metassomatism of the subcontinental lithospheric mantle. Geodynamic models based on the non-isothermal asthenospheric mantle seem to apply to the studied mafic alkaline magmatism on a local scale although the isothermal asthenospheric mantle model appears to conceal the petrogenesis of other coeval alkaline provinces in central and southeastern Brazil.