Field geology, petrology and geochronology of pluton–dyke systems: emplacement conditions of the Trapecio and Ushuaia peninsula dyke swarms (Fuegian Andes) and their relations with tectonics

Abstract

We report the field geology, petrography, and geochemical and geochronological data for two dyke swarms in the Fuegian Andes, with the aim of correlating them with known suites and improving our knowledge of the magmatism of the Late Cretaceous rear arc and its relations with ductile deformation. We provide keys for correlation based on the K 2 O content of amphiboles. The Trapecio dyke swarm is mildly alkaline and ferriferous, with a zircon U–Pb age of 75 Ma, and therefore belongs to the Fuegian potassic magmatism (78–68 Ma). The Ushuaia peninsula dyke swarm, also known as the Ushuaia Peninsula Andesites, is shown to be high-K, calc-alkaline and magnesian, with new radiometric ages of 87–86 Ma. Although the Ushuaia peninsula dyke swarm exhibits foliation associated with ductile deformation, the Trapecio dyke swarm is post-kinematic and reveals emplacement controlled by the slaty cleavage in the metapelite host. Field and geophysical evidence suggest both swarms overlie small upper crustal plutons that are mostly buried. The variable composition of the dykes suggests a protracted history of dyke injection, mostly fed from deeper reservoirs. Contact metamorphism around both pluton–dyke systems is very weak. By comparison with adjacent plutons, we argue that the main variables controlling the development of the aureole are the small magma volumes and low rates of injection. Supplementary material: Full analytical procedures (Online Resource 1); detailed petrography of the investigated Trapecio dyke swarm samples (Table S1 and Fig. S1) with geochemical data (Table S3 and Fig. S2) (Online Resource 2); extended petrography and photomicrographs (Fig. S3) of the Ushuaia Peninsula Andesite dyke swarm (Online Resource 3); mineral chemistry data (Table S2) and the results of zircon U–Pb and hornblende Ar–Ar age measurements and ratios (Tables S4–S8) (Online Resource 4), and thermobarometric results (Table S9) (Online Resource 5) are available at https://doi.org/10.6084/m9.figshare.c.7097733