Geochemistry and geochronology of three plutons from central Uruguay: tectonic implications for The Transamazonian orogeny

Abstract

The Arroyo Grande Belt is the smallest of the three metamorphic belts recognized in south-central Uruguay and is located along the northern margin of the Piedra Aha Terrane. The belt is host to a number of granitic intrusions, three of which (the Marincho Complex, the Arroyo Grande Pluton and the South Granite) are discussed in this report. The three intrusions vary in composition from homblendite to leucogranite and have themselves been intruded by a number of dykes, also of variable composition. The Arroyo Grande Pluton formed in an extensional environment whereas the main portion of the Marincho Complex formed in a compressional tectonic regime. The main unit of the Marincho Complex has been dated at 2,291 ± 65 Ma whereas a leucogranite member of the mass has been dated at 2,067 ± 24 Ma, thereby demonstrating that the Marincho Complex is a composite mass. A leucogranitic dyke, intrusive into the Marincho Complex, yields an age of 1,969 ± 25 Ma. The leucogranitic South Granite has been dated at 2,180 ± 50 Ma. The Arroyo Grande Pluton has not been dated. The available data indicate three periods of emplacement of granitic plutons in southern and central Uruguay: 1. Archean, involving greenschist facies metamorphism; 2.2,290 Ma - 2,180 Ma.- the time of emplacement of massive granitoids; 3. 2,040 Ma -1,894 Ma. - emplacement of predominantly small granitic dykes.