Post-collisional, Late Neoproterozoic, high-Ba-Sr granitic magmatism from the Dom Feliciano Belt and its cratonic foreland, Uruguay: Petrography, geochemistry, geochronology, and tectonic implications

Abstract

Post-collisional, granitic magmatism with high-barium-strontium (HiBaSr) content and shoshonitic affinity has been recently recognized both in the southern section of the Dom Feliciano Belt of Uruguay and its cratonic foreland. This group of granitic rocks has a distinctive age, mineralogy, chemistry and field characteristics. New zircon U–Pb LA-ICP-MS data for three of the plutons confirm their Late Neoproterozoic age; 634±7.3 (Sierra de los Caracoles), 604±3.0Ma (Cortez Blanco) and 597±3.6Ma (Guayabo). Similar ages were published previously for the Solis de Mataojo Granitic Complex (584±13Ma) as well as Las Flores (586±2.7Ma) and Sobresaliente plutons (585±2.5Ma). Mineral assemblages of the studied quartz-monzonites, granodiorites and monzogranites comprise quartz, orthoclase and microcline, plagioclase (Ab10–30), hornblende, green biotite, apatite, titanite and allanite. They plot predominantly in the high-K calc-alkaline field with the exception of a few that plot in the shoshonitic field; characteristically they are relatively high in Na2O (normally >4.5%) in acid varieties (SiO2>65%) decreasing to between 3 and 4% for more basic types; K2O normally exceeds 3.5% but can be as low as <2% in the basic varieties. The Uruguayan HiBaSr granitoids show high abundances of Ba (>700ppm), Sr (>500ppm) and light REEs alongside low Nb, Ta and heavy REEs. The Eu anomalies are negligible to slightly positive. Intermediate initial 87Sr/86Sr values (0.7077 to 0.7090) and very low initial epsilon Nd values (−15.8 to −19.3) at 600Ma with high Nd TDM (2.2–2.8Ga) suggest a recycling of ancient Paleoproterozoic to Late Archaean sources. Late Neoproterozoic granitoids of Uruguay have been emplaced within a post-collisional tectonic setting controlled by major shear zones and strike-slip fault zones. Current field and analytical data suggest that these granitoids could have been mostly generated through the partial melting of an intermediate to lower mafic continental crust and/or “enriched” crustal sources (in the presence of garnet±amphibole as stable residual mineral assemblage) with a possible minor mantle component.