A conceptual model for the evolution of a magmatic system based on U-Pb zircon studies on the Devonian granites of Sierra de San Luis, Argentina

Abstract

It is widely acknowledged that thermal models clearly demonstrate that crustal magma bodiesshould solidify rapidly upon emplacement. Small plutons can cool below the solidus in thousandsof years, while even large plutons require hundreds of thousands of years, but not more thana million years. However, recent U-Pb zircon geochronological data contradict these results,strongly suggesting that magmatic systems are often characterized by protracted events.Therefore, a conceptual framework that reconciles thermal models and the geochronology datais necessary. The Devonian foreland magmatism of the Sierra de San Luis is made up of twodistinctive suites, the Monzonite suite (<65 wt.% SiO2) and the Granite suite (>65 wt.% SiO2), bothemplaced at ca. 3.7 kbar. Classification of the studied Devonian granitoids is debatable becausethey have a hybrid I- to A-type granite signature. Based on a robust geochronological U-Pb zircondataset we corroborate the development of a protracted magmatic activity with three majorcrystallization events for this Devonian magmatism: 391 ± 1, 384 ± 1, and 379 ± 2 Ma.Considering these geochronological data, we postulate the presence of a deep mush reservoir,where a prolonged magmatic activity, permitted the prolonged crystallization of antecrysts (ca.395–384 Ma). Migration of the parental magma from the mush reservoir zone occurred near thetime of emplacement and culminated in the formation of an ephemeral magma chamber locatedat shallow levels, where zircon autocrysts crystallized (ca. 379 Ma). Age spectra reported withinindividual samples support the idea of a massive migration of magma when conditions werefavourable (e.g. thermally matured crust). Individual crystallization ages recorded by monaziteshosted in two-mica granites are comparable to those obtained from zircons, supporting thepresence of a long-lasting hot source. Additional geochronological data indicate that a laterthermal event (ca. 353 Ma) could have partially affected some areas of the Devonian magmaticzircons promoting Pb loss, with subsequent partial resetting of the isotopic clock.