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

Abstract

The La Hoya Pluton is a Late Jurassic shallow intrusive body emplaced in the North Patagonian Andes at the latitude of the city of Esquel in Argentina. The structural study of this pluton is interesting because its excellent exposures track the early stages of the building of the North Patagonian fold and thrust belt. This highly deformed area experienced many successive deformational events at different ages. This work presents a preliminary field, microstructural, and AMS study of this pluton. The La Hoya Pluton preserves magmatic and high- and low-temperature solid-state deformation structures of a subhorizontal attitude. The associated subhorizontal lineations have either E-W or N-S strike. These structures were probably formed during the cooling of the pluton, which, initially, could be envisaged as having a geometry compatible with coalescent laccoliths. The fact that the pluton cooled fast, as it was emplaced at shallow crustal levels, suggests that the subhorizontal foliation planes tracked by magmatic and high- to low-temperature solid-state microstructures started, at least, in Late Jurassic times, and were related to emplacement processes. NW-SE and NE-SW trending high-angle subvertical fractures and low-temperature mylonitic zones with steeply plunging lineations, which are compatible with an E-W shortening direction, cut the previously-formed subhorizontal structures. These low-temperature, high-angle compressive structures found in the La Hoya Pluton were probably developed during the Late Cretaceous main stage of tectonic deformation. The mylonitized areas of the La Hoya Pluton are cut by undeformed basaltic dikes of Eocene age ( 40 Ar- 39 Ar age in plagioclase of 42.15 ± 0.40 Ma). These dikes have geochemical features transitional between the alkaline-like signature of Pilcaniyeu Belt (Paleocene-Eocene) and the slab-derived magmas of El Maitén Belt (Late EoceneOligocene)