Rio Perdido Intrusive Suite: intraplate magmatism in southern Amazonian Craton – Rio Apa Block

Abstract

The dyke swarms of the Rio Perdido Intrusive Suite are located on the southern Amazonian Craton, Rio Apa Block, and they stand for an important fissural magmatic event that took place before the agglutination of the Rodinia Supercontinent. The dykes are 1 to 30 m-thick, with tabular to lenticular shape, and follow two preferential trends: N70º-90ºE and N70º-90ºW. They occur parallel to each other showing abrupt and discordant contacts relative to the regional NS trend. These rocks are compositionally homogeneous being composed of very fine- to fine-grained diabases, as well as fine- to medium-grained microgabbros that show no evidence of ductile deformation or metamorphism. Under microscope, these rocks are holocrystalline, and display several types of textures such as ophitic to subophitic, intergranular, porphyritic in places, and locally, quench texture. The essential constituents are plagioclase, pyroxene, and olivine. Harker-type diagrams show a tholeiitic trend. They mostly plot as basalts and andesitic basalts. Diagrams suggest geological settings similar to that of Phanerozoic intraplate settings for the emplacement of these rocks. The geochemical signature of rare earth elements shows strong fractionation of heavy REE relative to light REE, with La/Yb ratios ranging from 2.8 to 6.2. Taking into account a crystallization age around 1100 Ma for the rocks from the Rio Perdido and Huanchaca Intrusive Suites, and from the Rincón del Tigre Complex, here we suggest the existence of a Stenian Large Igneous Province (LIP) which may be associated with an attempted continental breakup related to the evolution of the Aguapeí Aulacogen.