FREI MARTINHO PLUG: CENOZOIC ALKALINE MAGMATISM IN THE NORTH OF THE STATE OF PARAIBA

Frei Martinho Plug: Cenozoic alkaline magmatism in the North of State of Paraíba

Abstract

The occurrence of small-scale volcanisms is common in many places around the world, being included in the most diverse tectonic contexts. In the central portion of the states of Rio Grande do Norte and Paraíba, there is a series of plugs and necks of Cenozoic age and predominantly alkali-basaltic composition. The Frei Martinho Plug (FMP) is composed of black and isotropic basanites, with an aphanitic to microporphyritic texture and presents an orientation related to the Macau-Queimadas alignment. Olivine and spinel occur as phenocrysts, while the matrix is composed of nepheline, diopside, opaque minerals, plagioclase, olivine, alkali feldspar, apatite and glass. Olivine shows compositional variation between Fo₈₀ to Fo₉₁ (chrysotile and fosterite), where the more magnesian terms represent xenocrystals. Diopside (Wo_41-50En_38-45Fs_09-17) is the predominant composition among clinopyroxene crystals. Plagioclase is sodium (albite and oligoclase) with an An < 22% molecule. Alkali feldspars have composition Or_33-46Ab_64-51 and correspond to anorthoclase and sanidine. Nepheline compositions vary in the range Ne_60-83Ks_19-08Q_27-00. Temperature estimates obtained from the compositions of feldspars and feldspathoids indicate temperatures between 1068°C and 500°C for the rocks that make up the FMP.