Age range of the latest alkaline magmatic activity of Uruguay

Abstract

Volcanic and intrusive activity corresponding to the Paraná Magmatic Province (PMP)[1], is found in the NW region of Uruguay. This magmatism is restricted to the locallyknown as Norte Basin, correspondent to the southern extreme of the Paraná Basin.Recent studies have shown the ocurrence of mafic alkaline subvolcanic rocks crosscutting the basaltic pile of the PMP, characterized as subvolcanic plugs, regarding totheir structural, textural and geomorphologic features. They were classified as Ne-tephrites, and also basalts and trachybasalts with nepheline and alkaline affinity [2].The first set of ages obtained for two plugs of Ne-tephrites ponted out the firstUruguayan record of a magmatic event in the Paleogene (63.7 +/- 2.5 Ma and 51.5 +/-1.7 Ma) [3]. The aim of this abstract is to present the results of K-Ar (whole rock)geochronology analyses perfomed at Actlabs Laboratories (Canada), obtained on nineplugs involving the different lithologies present in the area. The new results have enlarged the length time of this alkaline activity, establishing arange of ages between 82.4 +/- 1.3 Ma and 51.5 +/- 1.7 Ma. Previous data indicatedthat the youngest occurrences correspond to Ne-tephrites (65.5 +/- 4 Ma and 51.5 +/-1.7 Ma), while the oldest correspond to the occurrences of tracybasalts and alkalinebasalts. These new records of alkaline plugs allow to confirm the presence of a LateCretaceous to Eocene tecto-magmatic event with alkaline affinity in northwesternUruguay.