Thermobarometry, Sm/Nd ages and geophysical evidence for the location of the suture zone between Cuyania and the western proto-Andean margin of Gondwana

Abstract

Basement rocks comprising ortho- and paragneisses and schists whose tectono-metamorphic evolution is poorly known, are exposed in the Sierras de Umango, Maz-Espinal and Las Ramaditas, in the northwest of the La Rioja Province, Argentina. These units were included in the Maz, El Taco, El Zaino complexes, as well as the Tambillos Metamorphics that would be part of the northern end of the Cuyania terrane, a microcontinent derived from Laurentia that collided with Western Gondwana during the early Paleozoic, or belong to the active margin of the continent. To recognize rocks belonging to each of these tectonic units and to understand the history and physical conditions of accretion were some of the main goals of the multidisciplinary investigation whose preliminary results are presented here. Geochemical studies, trace and REE elements and Sm-Nd model ages allowed the recognition of several episodes of crustal accretion in these rocks. The oldest one occurred at ca. 2.2 Ga in an arc/back-arc environment along the eastern segment of the Sierra de Maz, and was possibly coeval with development of a early Proterozoic continental crust that acted as source to sediments of Maz Complex. The following episode of crustal accretion that formed rocks in this region was at ca. 1.4 Ga and is registered by tonalites emplaced in an extensional environment cropping out in the western flank of the Sierra del Espinal. In the Sierra de Umango, an arc/back-arc sequence registered an episode of crustal accretion during the Grenvillian Cycle (ca. 1.3 Ga). The last episode of crustal accretion detected in this area (800 Ma) is represented by an old alkaline volcanism in the Sierra de Umango. This episode could be representing the first stage of break-up of the Rodinia supercontinent during the Neoproterozoic. The metamorphic grade reached by these rocks is mostly represented by fabrics with mineral assemblages of intermediate to high pressures and high temperatures, typical of collisional environments. The oldest rock-forming fabrics tectono-metamorphic episode recognized is of middle Proterozoic age (ca. 1.04 to 0.969 Ga, garnet-whole-rock Sm/Nd age) being registered by metapelites from Maz Complex that attained temperatures of 650°C-6.3 kbar. A younger metamorphic event (463 Ma, garnet - whole-rock Sm/Nd age) is verified in metatonalites intrusive in these metapelites. Another metamorphic event at ca. 301 Ma (garnet-WR Sm/Nd age) was recognized in metasediments from El Taco Complex. Peak metamorphic conditions of this event, probably registering the last major tectonic episode that affected rocks of this area was established in 868°C-9.8 kbar. It is impossible to distinguish fabrics belonging to totally different tectonic episodes based on structural or metamorphic data. Therefore, distinction between major tectono-thermal events of totally different ages such as the high-T middle Proterozoic deformation and with N-NWestwards tectonic transport direction registered in the Sierras de Maz-Espinal and Umango from the youngest one (ca. 301 Ma) that attained the highest-P/T conditions, recognized in the Sierra de Las Ramaditas, had to be done on the basis of Sm/Nd ages. Geophysical evidence indicates the presence of extensive WNW-oriented lineaments that separate basements blocks of different magnetic and gravimetric signatures that are thought to represent ancient Grenvillian age suture zones. On the other hand, the northern segment of the Valle Fétil lineament that runs between Sierras de Umango and Maz-Espinal is at present interpreted as marking the eastern boundary of the Cuyania terrane. This is supported by isotopic data as well as the contrasting history of tectono-metamorphic events as determined for both of these segments of the NW Sierras Pampeanas.