Age of the El Hornito pluton and thermobarometry of its thermal aureole: Insights into achalian (Devonian) magmatism in the Sierras Pampeanas of Argentina

Abstract

A comprehensive study of the El Hornito Pluton, located in the Sierra Grande de San Luis (Argentina) is presented in this work. The intrusive body is a composite pluton constituted by four main units: 1) Granitic Porphyritic Unit, 2) Granitic Red Unit, 3) Granitic External Unit and 4) Mafic-Hybrid Unit. The main intrusive (granitic) units are crosscut by syn-magmatic felsic dikes. A Devonian age of 385 ± 2 Ma was obtained by U–Pb SHRIMP analyses on zircons belonging to the Granitic Porphyritic Unit. This age is considered representative of the El Hornito Pluton crystallization since the Granitic Porphyritic Unit is the most voluminous one. The Granitic Red Unit crops out in the central zone of the El Hornito Pluton and hosts the largest proportion of the rocks that comprise the Mafic-Hybrid Unit, where mingling/mixing processes are evident. The host rocks located in the western sector of the El Hornito Pluton present an intense contact metamorphism. Electron microprobe analyses of plagioclase, garnet, biotite, and muscovite in a garnet-sillimanite hornfels were performed to calculate thermobarometry from multireaction procedures in these hornfels, yielding P-T values of 3.7 ± 1.1 kbar and 626 ± 32 °C. These conditions are considered representative of the emplacement level of the El Hornito Pluton. The results of this study show that the El Hornito Pluton was emplaced at a depth of 14.1 ± 4.2 km, evidencing deeper crustal levels of emplacement than the Achalian granites of the Sierra de Córdoba. The age obtained in this work, along with others reported for the Sierras de San Luis and Córdoba, indicate that the intrusions occurred first in the west and later in the east. Moreover, according to the emplacement levels at which intrusions took place, different exhumation rates are proposed to have occurred across the orogen (in an NE-SW direction) after the Devonian magmatism, being greater in the Sierra de San Luis (southwest) than in the Sierras de Córdoba (northeast).