Estructura y fabricación Magnética del Batolito Pilcaniyeu (Jurásico Inferior), Precordillera Patagónica, Río Negro.

Abstract

The Pilcaniyeu Batholith is a Jurassic plutonic body emplaced in the Patagonian Precordillera (Río Negro). The intrusion comprises three main facies: (1) a granodiorite facies intruded by monzodioritic dikes; (2) a syenogranite facies intruded by aplite–muscovite dikes; and (3) late rhyolitic dikes that cut all previously mentioned units. The country rock is attributed to schists and metatexites of the Cushamen Formation. Among the numerous uncertainties surrounding this intrusive body, questions remain regarding its age and particularly its emplacement conditions. In this regard, Giacosa et al. (2001) proposed that the batholith would have been emplaced during the Early to Middle Jurassic under an extensional or transtensional tectonic regime within dilatational zones associated with the bending of the Río Chico and Pichileufú sinistral faults. However, until now, no structural data were available to support this model. This study demonstrates the potential of integrating igneous petrology, U-Pb zircon geochronology, and structural techniques such as AMS to precisely characterize the internal structure, age, and tectonic emplacement context of igneous suites. Mineral fabrics (magmatic and solid-state) measured in the field are always parallel or subparallel to magnetic fabrics. In this sense, the granodiorite facies exhibits predominantly subhorizontal mineral fabrics, characterized by a subvertical minimum magnetic axis (K3) and a subhorizontal maximum axis (K1) striking NE–SW. The syenogranite facies displays predominantly subvertical fabrics striking NW–SE, with a subhorizontal maximum axis (K1) oriented NE–SW. The LA-ICP-MS U-Pb zircon dating from the syenogranite facies yielded an age of 183.34 ± 0.84 Ma, corresponding to the Early Jurassic (Toarcian). This event is temporally concomitant with the Subcordilleran Batholith and with Stage V1 of the Chon Aike Silicic Large Igneous Province.The integration of structural, microstructural, and magnetic data allowed us to validate the emplacement model of the Pilcaniyeu Batholith as being associated with a transtensional tectonic regime, involving the development of pull-apart-type dilatational zones with an NE-SW–oriented extension direction. This emplacement style is linked to the evolution of a releasing bend between the Río Chico and Pichileufú sinistral faults.