Evolution of Devonian and Carboniferous pre-Andean arc magmatism in the Frontal Cordillera (27°-35°S), Argentina: Insights from U–Pb zircon and isotopic studies

Abstract

We propose a conceptual model for the magmatic system that gave rise to the Devonian and Carboniferous subduction-related magmatism in the Frontal Cordillera of Argentina, (representing the pre-Andean margin of SW Gondwana), integrating previous geochronological studies with new geochronological data from granitoids and subvolcanic dikes, which include reported petrological, geochemical, and isotopic data. This conceptual magmatic system postulates an extended magmatic activity, similar to that reported in a previous study for the Devonian foreland magmatism located in the present-day Sierras Pampeanas of Argentina. The geochronologicaldata play a central role in the model, and lead us to postulate the presence of a deep mush reservoir, where a prolonged magmatic activity, permitted the extended crystallization of Devonian (ca. 400 ± 3 and 414 ± 3 Ma) and Carboniferous antecrysts (334 ± 2 Ma, 341 ± 2, and 348 ± 2, where the age of 348 Ma maybe considered as derived from the source, i.e. xenocrysts). Migration of the parental magma from the mush reservoir zone occurred near the time of emplacement and culminated in the formation of an ephemeral magma chamber located in shallow levels, where Devonian and Carboniferous zircon autocrysts crystallized (382 ± 5 Ma and 325 ± 2 Ma, respectively). Age spectra reported within individual sample, favors the idea of a massive migration of magma when conditions were favorable (e.g., thermally matured crust). Notably, the Carboniferous geochronological data is strongly consistent with previous geochronological data recently reported for the granitic rocks of the Tabaquito batholith. Although this model supports a protracted magmatic system for the Devonian and Carboniferous subduction-related magmatism, previous and new isotopic data reveal a significant difference in the petrogenetic processes. While the Carboniferous arc magmatism shows significant juvenile material contribution, this is not the case for the Devonian magmatic arc, where isotopic data suggest an older continental lithosphere as the dominant source. These different sources are attributed to two contrasting geodynamic settings, with advancing and retreating oceanic slabs, respectively.