The basement of the Deseado Massif at Bahı́a Laura, Patagonia, Argentina: a proposal for its evolution

The Rı́o Deseado Complex, the basement of the eastern Deseado Massif geological province of Argentina, is composed of five dispersed, small outcrops of upper Proterozoic–lower Paleozoic metamorphic and igneous rocks. We propose a possible evolution for these rocks on the basis of a detailed study of the easternmost outcrop (Bahı́a Laura). The Bahı́a Laura metamorphic rocks are metasedimentary and igneous-derived rocks formed prior to the Pampean metamorphic event. Metasedimentary schists, garnet-rich and micaceous gneisses, and the migmatiteʹs paleosome are interpreted as marine sediments (pelites, sandstones, and limestones). The igneous-derived rocks are metaluminous and peraluminous granodiorites (hornblendic and granodioritic gneisses) and tholeiitic basalts (amphibolites) that were produced in a magmatic arc and a primitive island arc, respectively. These rocks underwent regional metamorphism during the Pampean cycle (Neoproterozoic–Paleozoic boundary), then a plutonic intrusion (Bahı́a Laura peraluminous monzogranite) and several granitoid injections,both of which are correlated with the Famatinian (Ordovician–Silurian) magmatic arc proposed for this region. Finally, all rocks were slightly deformed by a post-Famatinian tectonic event that may have produced the basement exhumation. According to the proposed evolution of the Rı́o Deseado Complex in Bahı́a Laura, the basement of the Deseado Massif could be part of a Late Proterozoic-Mid Paleozoic orogenic belt.