Magnetic Fabric and Microstructures of Late Paleozoic granitoids from the North Patagonian Massif: Evidence of a collision between Patagonia and Gondwana?

LOPEZ DE LUCHI, M.G.; RAPALINI, A.E.; TOMEZZOLI, R.

TECTONOPHYSICS

ELSEVIER SCIENCE BV

Año: 2010 vol. 494 p. 118 - 137

0040-1951

Widespread Late Paleozoic magmatism in northern Patagonia is a target to test hypotheses on the origin of Patagonia which has long caught the attention of South American earth scientists. In recent years, a dispute over whether it is an accreted crustal block that collided with Gondwana in Paleozoic times or an autochthonous part of South America has taken place. As part of a multidisciplinary study, an integrated microstructural and magnetic fabric study was carried out on the Late Carboniferous Yaminué Complex (YC) and the Early Permian Navarrete Plutonic Complex (NPC), both exposed in the northeastern corner of the North Patagonian Massif (40.5°S, 67.0°W). Other investigated units are the Late Carboniferous Tardugno Granodiorite (TG), the newly defined Cabeza de Vaca Granite (CVG) and the Late Permian San Martin pluton (SM). Over 300 oriented cores from 60 sites were collected for anisotropy of magnetic susceptibility (AMS) measurements. A systematic analysis of around 100 petrographic thin sections was performed to characterize the microstructures of the different magmatic units. Microstructures in the YC are indicative of a transition from magmatic to solid state deformation. Orthogneiss of tonalitic composition suggest an early stage in the crystallization history of this complex. The CVG, intrusive in YC, is the most evolved unit and records less intense high-temperature solid state deformation which suggests that the stress field that controlled the emplacement of the YC outlasted it. According to petrologic and structural considerations, the NPC has been subdivided into three facies, i.e. Robaina, Guanacos and Aranda, respectively. Microstructures of the NPC are mostly magmatic to submagmatic, being the Robaina facies the only one that records a more developed high-temperature solid-state overprint when in contact with the YC. Combined analyses of AMS and microstructural data lead us to suggest that the YC, CVG and possibly TG were intruded during a major compressional event associated with top to the SSW thrusting. This event is most likely related to a frontal collision of the North Patagonian Massif and the southwestern Gondwana margin at around 300 Ma. The NPC and SM were emplaced after that tectonic event, which must have ended by 281 Ma. Previous magmatic, geochronological and paleomagnetic data that suggest close connection of the North Patagonian Massif with the South American Gondwana blocks during the Paleozoic, can be reconciled with a Late Paleozoic collision by a model of a para-autochthonous North Patagonian Massif that rifted away from Gondwana after the Ordovician and collided again in the Late Carboniferous – Early Permian.