Deformation, age, and provenance of the Nogolí Metamorphic Complex protoliths, Sierra de San Luis, Argentina: evidence of a non-collisional history for the Cambrian in the western Gondwana margin

Situated in the western Sierras Pampeanas of San Luis, the Nogoli Metamorphic Complex has become a key geological locality for unraveling the construction of the western Gondwana margin on the South American tectonic plate. This study presents a comprehensive analysis, encompassing new petrological and geochronological data, along with a meticulous structural examination of two distinct regions within the Nogoli Metamorphic Complex. Our primary aim is to assess the age and provenance of the protoliths, thereby shedding light on their geodynamic evolution. Detrital zircon U-Pb ages, obtained from a quartz-feldspathic-schist paleosome within a metatexite exposure in the La Barranquita area, disclosed a polymodal distribution. The predominant age population, centered around 616 +/- 11 Ma, suggests a late Neoproterozoic source (Brasiliano-Pan African orogen). A secondary, yet significant, age peak at 542 +/- 12 Ma points to a late Ediacaran to Early Cambrian origin (Pampean orogen). Additionally, a peak ranging from 930 to 1144 Ma indicates contributions from an early Neoproterozoic source (Grenvillian-Sunsas orogen). The maximum depositional age was determined to be 533 +/- 14 Ma, based on the calculation of the youngest grain cluster at 2 sigma uncertainty. This polymodal zircon-age spectrum indicates that the sedimentary protolith of the Nogoli Metamorphic Complex derived from a Cambrian-age crystalline basement situated along the western margin of Gondwana. Contrary to models invoking continent-continent collision in western Gondwana during the early Cambrian, our findings better support a ridge-trench collision as the most plausible hypothesis for the origin of the Pampean orogeny.

Automated summary

The Nogoli Metamorphic Complex, located in the western Sierras Pampeanas of San Luis, is an important geological area for understanding the construction of the western Gondwana margin in South America. This study presents new petrological and geochronological data to assess the age and origin of the protoliths in order to shed light on their geodynamic evolution. The findings support a ridge-trench collision as the most plausible explanation for the origin of the Pampean orogeny, contrary to previous models invoking continent-continent collision in western Gondwana during the early Cambrian.