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), pre-Andean margin SW Gondwana

One major enigma about granitic magmas is the precise time span of their origin and emplacement. As is widely acknowledged thermal models demonstrate that crustal magma bodies should solidify rapidly upon emplacement. Small plutons can cool below the solidus in thousands of years, while even large plutons require hundreds of thousands of years, but not more than a million years. A common view is that the crystallization of different plutons forming a batholith may not exceed a million years (e.g., Pitcher, 1993). 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 data is necessary. The Devonian foreland magmatism of the Sierra de San Luis is made up of two distinctive suites, the Monzonite suite (< 65 wt.% SiO₂) and the Granite suite (> 65 wt.% SiO₂), both emplaced at ca. 3.7 kbar (López de Luchi et al., 2017 and references therein). Classification of the studied Devonian granitoids is debatable because they have a hybrid I- to A-type granite signature. Based on a robust geochronological U-Pb zircon dataset (n = 203) we corroborate the development of a protracted magmatic activity with three major crystallization events for this Devonian magmatism: 391 ± 1, 384 ± 1, and 379 ± 2 Ma. Considering these geochronological data, the traditional view of a parental granitic magma crystallizing mostly (or entirely) from a magmatic chamber hosted in the upper crust is questioned. Alternatively, we postulate the presence of a deep mush reservoir (ca. 8 kbar, the depth was indirectly established based on isotopic data from previous studies such as that of López de Luchi et al., 2017; Dahlquist et al., 2019); which defines a source for the parental magma in the lower crust or in the lithospheric mantle-crust boundary region), where an extended magmatic activity, permitted the prolonged crystallization (“zircon incubation”) of zircon antecrysts (ca. 395 – 384 Ma). 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 at shallow levels, where zircon autocrysts crystallized (ca. 379 Ma). Age spectra reported within individual samples support the idea of a massive migration of magma when conditions were favorable (e.g., thermally matured crust). Individual U-Pb crystallization ages (n = 20) recorded by monazites hosted in two-mica granites of the Sierra de San Luis, are comparable to those obtained from zircons, supporting the presence of a long-lasting hot source.

As indicated in previous works, one notable and general conclusion (consistent with this study) is that ascribing a distinct crystallization age to granitic formations through U-Pb zircon dating is hazardous due to the tendency of zircon to crystallize throughout the evolution of long-lasting magmatic systems. As a result, distinctions must be made between zircons entrained from the magmatic source and later transported to the emplacement level (i.e., antecrysts), and those zircons crystallized during the final emplacement of the magma (i.e., autocrysts).

References

Dahlquist, J.A., Macchioli Grande, M., Alasino, P.H., Basei, M.A.S., Galindo, C., Moreno, J.A., Morales Camera, M., 2019. New geochronological and isotope data for the Las Chacras–Potrerillos and Renca batholiths: a contribution to the middle-upper Devonian magmatism in the pre-Andean foreland (Sierras Pampeanas, Argentina). SW Gondwana. J. S. Am. Earth Sci. 93, 348–363.

López de Luchi, M.G., Siegesmund, S., Wemmer, K., Nolte, N., 2017. Petrogenesis of the postcollisional middle devonian monzonitic to granitic magmatism of the Sierra de San Luis, Argentina. Lithos 288-289, 191–213.

Pitcher, W.S., 1993. The Nature and Origin of Granite. Blackie Academic, London, p. 321.