Evolution of the Palaeozoic basement of the North Patagonian Andes in the San Martín de los Andes area (Neuquén, Argentina): petrology, age and correlations

SERRA-VARELA, SAMANTA; GONZÁLEZ, PABLO D.; GIACOSA, RAÚL E.; HEREDIA, NEMESIO; PEDREIRA, DAVID; MARTÍN-GONZÁLEZ, FIDEL; SATO, ANA MARÍA

ANDEAN GEOLOGY

SERVICIO NACIONAL GEOLOGIA MINERVA

Año: 2019 vol. 46 p. 102 - 130

0718-7092

In San Martín de los Andes area (Argentinian Patagonia) the Palaeozoic basement consists of metamorphicand plutonic rocks. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemicaland petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes.Detrital zircon analysis (U-Pb Q-ICP-LA-MS) yielded a maximum depositional age of 501±14 Ma (Series 3 Cambrian)for this sedimentary protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of themetamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regionalfoliation is locally affected by open folds that develop an incipient crenulation cleavage (S3). The high-grade metamorphismincludes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut itin presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a U-PbConcordia age of 434.1±4.5 Ma (Llandovery-Wenlock, Silurian). The age of maximum sedimentation and the anatecticage constrain the metamorphic evolution of the basement into the lower Palaeozoic (between upper Cambrian and lowerSilurian). The igneous rocks of the basement are granodiorites, tonalities, and some gabbros that cut the metamorphicbasement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grademetamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age ofthese igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. 370and 400 Ma (Devonian). The maximum sedimentation age for the protolith and its metamorphic evolution seems to berelated to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. In contrast, the Devonianigneous rocks of San Martín de los Andes could represent a Devonian magmatic arc, related to subduction processesdeveloped at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.