Emplacement conditions and exhumation of the Varvarco Tonalite and associated plutons from the Cordillera del Viento, Southern Central Andes

During the Late Cretaceous Andean orogeny, the compressive deformation associated with the shallowing of the subducting slab caused the development of the arc-related igneous rocks known as the Naunauco Belt. This study presents petrographic, mineralogical and anisotropy of magnetic susceptibility data for the Varvarco Intrusives (the Varvarco Tonalite, Butalon Tonalite and Radales Aplite), which crop out in the Cordillera del Viento, Neuquen Province, Argentina. The assembly of plutons was formed by mafic magma episodic injection. Amphibole and biotite compositions suggest that the Varvarco Tonalite is related to calc-alkaline, I-type magmas, typical of subduction environments. Different geothermobarometers based on amphibole and plagioclase compositions for the Varvarco Tonalite suggest shallow emplacement conditions (similar to 2-3 kbar, equivalent to similar to 12 km depth). Apatite fission-track analyses give exhumation ages of 67.5 +/- 8 Ma for the Varvarco Tonalite and 50.3 +/- 5.9 Ma for the Butalon Tonalite. A calculated continuous fast exhumation rate of at least 330 degrees C Ma(-1) is consistent with the shallow emplacement conditions, textural data and geobarometric estimations. In agreement with the thermal profile, the magmatic system was exhumed by similar to 12 km within c. 2.1 Ma implying a geothermal gradient of similar to 62.5 degrees C km(-1). The last step of exhumation occurred between similar to 65.3 and 56.9 Ma. The magmatic fabrics observed in the studied plutons reflect mostly magma chamber processes. The Varvarco Intrusives represent satellite calc-alkaline plutons of the North Patagonian Batholith which were emplaced syn- to post-tectonically with respect to a major deformation stage of the Southern Central Andes.