Petrology and Structure of the Aluminé Igneous-Metamorphic Complex, Neuquén province, Argentina.

Abstract

In the present contribution, the partial results of a detailed study of the magmatic, metamorphic and tectonic events developed in the Aluminé Igneous-Metamorphic Complex (AIMC) are given. We define the AIMC as the set of pre-andean and andean intrusive igneous and metamorphic rocks, outcropping in the area located between the Aluminé, Moquehue, Ñorquinco and Pulmarí lakes in the Neuquén province, Argentina. Based on field and petrographic observations and geochemical characteristics, the granitic rocks forming part of the AIMC have been classified as: 1. quartz diorites, 2. tonalites-granodiorites, 3. granodiorites and 4. granites. The geochemical parameters indicate that the defined lithologies derive from calcic to weakly calcalkaline magmas of metaluminous composition, typical of Type I Cordilleran batholiths associated with active continental margins. Intercalated between the magmatic rocks, some outcrops of metamorphic basement composed mainly of schists, gneisses, amphibolites and scarce quartzose sandstones. Andesites, tuffs and basalts cover in part the metamorphites and granitoids are exposed. The structure of the sector is characterized by the presence of three main sets of regional lineaments, with NE-SW, NW-SE and N-S orientations. These directions coincide with the orientations of the three groups of penetrative foliations recognized in the magmatic rocks of the sector. On the basis of the petrographic-microstructural analysis and deformation mechanisms in minerals, at least two ductile deformation events were identified in the magmatic rocks of the area. Field mapping and petrological-structural studies carried out so far, allow to differentiate two sectors with clearly contrasting characteristics. The north sector is characterized by the predominance of granitoids with andean ages (Cretaceous or younger), whereas in the southern sector dominates the pre-andean granitoids of permotriassic-jurassic ages emplaced in a Paleozoic basement. Although the geochemical diagrams allow to clearly distinguish the different lithologies defined petrographically and their evolutive trends could indicate fraccional crystallization processes, its areal distribution, their petrographic-microstructural characteristics and the geochronology indicate that they correspond to different magmatic pulse-events separated in time, although developed in a similar tectonic framework.