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

ZAFFARANA, CLAUDIA; BOLTSHAUSER, BÁRBARA; GONZÁLEZ, VÍCTOR RUIZ; ORTS, DARÍO; SERRA-VARELA, SAMANTA; PUIGDOMENECH, CARLA

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2020

0895-9811

The magmatic record of the North Patagonian Batholith starts in Middle Jurassic times andrapidly 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 experiencedits first contractional deformation pulse. The La Hoya Pluton is an Upper Jurassic shallowintrusive body emplaced in the North Patagonian Andes in the proximities of the city ofEsquel in Argentina. Its excellent exposures, which bear magmatic and high- and lowtemperaturesolid-state deformation structures, are studied in this work through field,microstructural, and AMS studies. The initial shape of the pluton could be envisaged ashaving a geometry compatible with coalescent laccoliths, given by the presence ofsubhorizontal magmatic to solid-state foliations with associated E-W or N-S trendingmagmatic to solid-state lineations. These magmatic structures, which bear a mild solid-stateoverprint, were probably formed during progressive pluton cooling. Moreover, the nature ofthese structures suggest that the pluton cooled fast at shallow crustal levels, indicating thatthey are probably related to its emplacement processes that started, at least, in Late Jurassictimes. Contrastingly, NW-SE and NE-SW trending high-angle subvertical fractures andlow-temperature mylonitic zones with steeply plunging lineations, which are compatiblewith an E-W shortening direction, cut the previously formed subhorizontal magmaticstructures. These compressive structures found in the La Hoya Pluton were probablydeveloped during the Late Cretaceous main stage of contractional deformation. Thecontractional deformation is constrained to have happened before the intrusion ofundeformed basaltic dikes crosscutting the mylonitized areas of the La Hoya Pluton. Thesedikes have an 40Ar-39Ar age in plagioclase of 42.15 ± 0.40 Ma and geochemical featurestypical of volcanic arcs above a subduction zone. Their geochemical signature is sensitiveto the process of arc resumption after the Paleogene waning of arc activity, as it istransitional 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; LateEocene-Early Miocene). These dikes probably formed part of the early stages of the ElMaitén Belt.