The role of the Rio Grande Rise on the continental Meso-Cenozoic alkaline magmatism in southwest Brazil

This contribution aims to present a proposal for the origin of the Cretaceous igneous alkaline rocks of SE Brazil. New studies with applications of petrographic, geochronological, structural, and geophysical tools establish a temporal and geodynamic correlation between the basaltic rocks of the Rio Grande Rise and alkaline rocks related to Mesozoic rifts of the SE Brazilian margin. Thus, a model of the generation of continental alkaline rocks is proposed as a result of the Rio Grande Rise formation, where the development of a meso-oceanic chain occurred as a result of mantle convection processes leading to spreading centers on the ocean floor at about 95–91 and 50 Ma, with subsequent volcanic activity until about 50 Ma. In this context, the extensional process consistent with the mantle geodynamics and the formation of meso-oceanic basalts resulted in the formation of mantle magmas whose intrusion through the continental crust resulted in alkaline magma (including crustal contamination) between 85 and 50 Ma. Interruption of the tentative opening of the oceanic crust in the region adjacent to the Rio Grande Rise was due to the obstruction of the South American Platform. This resulted in the abortion of the extensional process affecting the oceanic crust. In this sense, during the period of formation of oceanic basalts between 95 and 50 Ma, several continental intrusions are registered in the onshore region, a major magmatic event characterized by radiometric ages varying from 87 to 70 Ma. Between 70 and 60 Ma, the opening attempt continued with a new front of propagation and magmatic episodes resulting in the formation of an E–W chain of igneous rocks extending north of the Rio de Janeiro coastline, forming the Itatiaia-Cabo Frio lineament, with ages varying from 67 to 50 Ma. The consistency of 40Ar/39Ar ages of the continental alkaline rocks and the basaltic rocks of the Rio Grande Rise indicate that volcanism on the paleo-spreading center was coeval with an embryonic process of plate rupture and oceanic opening in regions adjacent to the continental margin. The process resulted from a mantle convection system that formed the oceanic crust at the Rio Grande Rise, simultaneous with the generation of continental alkaline magmas along igneous centers.