Geologia e caracterização química do magmatismo peralcalino ultrapotássico do enxame de diques Manaíra-Princesa Isabel, Província Borborema

Abstract

The Manaira-Princesa Isabel dike swarm forms one of the most expressive examples of Neoproterozoic (c. 600 Ma) peralkaline magmatism in the Borborema Province (NE Brazil). It consists of about a hundred NE-trending bodies intrusive in older, Neoproterozoic porphyritic granites (Princesa Isabel and Tavares plutons), and orthogneisses and low-grade metasediments of the Eo-neoproterozoic Riacho Gravata complex. The dike swarm includes mostly silica-saturated syenites, with potassic to ultrapotassic, peralkaline affinity, containing microcline and sodic amphibole ± pyroxene as the main mineral assemblage. Amphibole is dominantly Mg-riebeckite (Manaira, Princesa Isabel and Tavares sub-swarms), whereas pyroxene is mainly aegirine-augite (Manaira and Tavares sub-swarms). A minor set of dikes from this swarm is slightly metaluminous with Mg-biotite as the major mafic phase. Geochemical and isotopic signatures indicate strong enrichment in incompatible elements (Rb, Ba, K, Th, U), in association with a negative Nb anomaly, and strongly radiogenic 87Sr/86Sr and 143Nd/144Nd (= negative eNd values) initial ratios. These features suggest a common source, which was probably an enriched lithospheric mantle reservoir, chemically modified by an ancient subduction component inferred to be Paleoproterozoic from T DM model ages. Conversely, Pb isotopic ratios lower than average crustal values indicate the influence of a non-radiogenic component interacting with the enriched mantle source. A SHRIMP U-Pb age of c. 600 Ma obtained for the Manaira-Princesa Isabel dike swarm defines an important crustal exhumation period which was relatively synchronous with intensive tectonomagmatic activity related to the Brasiliano orogeny, which occurred in distinct structural domains within the Transverse Zone of the Borborema Province.