Magmatic and tectonic fabrics in the Upper Jurassic La Hoya Pluton, North Patagonian Batholith (∼43ºS) as a record of the early stages of the Andean deformation

Abstract

The magmatic record of the North Patagonian Batholith starts in Middle Jurassic times and rapidly grew in Early Cretaceous times, where it experienced its main building phase. Slightly after, in the Late Cretaceous, the North Patagonian fold and thrust belt experienced its first contractional deformation pulse. The La Hoya Pluton is an Upper Jurassic shallow intrusive body emplaced in the North Patagonian Andes in the proximities of the city of Esquel in Argentina. Its excellent exposures, which bear magmatic and high- and low-temperature solid-state deformation structures, are studied in this work through field, microstructural, and AMS studies. The initial shape of the pluton could be envisaged as having a geometry compatible with coalescent laccoliths, given by the presence of subhorizontal magmatic to solid-state foliations with associated E-W or N–S trending magmatic to solid-state lineations. These magmatic structures, which bear a mild solid-state overprint, were probably formed during progressive pluton cooling. Moreover, the nature of these structures suggests that the pluton cooled fast at shallow crustal levels, indicating that they are probably related to its emplacement processes that started, at least, in Late Jurassic times. Contrastingly, NW-SE and NE-SW trending high-angle subvertical fractures and lowtemperature mylonitic zones with steeply plunging lineations, which are compatible with an E-W shortening direction, cut the previously formed subhorizontal magmatic structures. These compressive structures found in the La Hoya Pluton were probably developed during the Late Cretaceous main stage of contractional deformation. The contractional deformation is constrained to have happened before the intrusion of undeformed basaltic dikes crosscutting the mylonitized areas of the La Hoya Pluton. These dikes have an 40Ar-39Ar age in plagioclase of 42.15 ± 0.40 Ma and geochemical features typical of volcanic arcs above a subduction zone. Their geochemical signature is sensitive to the process of arc resumption after the Paleogene waning of arc activity, as it is transitional between the alkaline-like signature of Pilcaniyeu Belt (~57.8–42; Paleocene-Eocene) and the predominantly slab-derived magmas of El Maitén Belt (~37–19; Late Eocene-Early Miocene). These dikes probably formed part of the early stages of the El Maitén Belt.