Lu-Hf zircon isotopes data from Devonian granitic rocks of Sierra de San Luis, Argentina: petrogenetic implications combining previous U-Pb and Hf zircon data

A robust U-Pb zircon dataset previously published in this journal revealed a magmatic system with protracted activity, characterized by three major crystallization events at 391 ± 1, 384 ± 1, and 379 ± 2 Ma. Based on these data, we proposed a conceptual model that suggests the existence of a deep mush reservoir, which enabled prolonged zircon antecrysts crystallization (ca. 395–384 Ma), followed by the crystallization of younger zircon (i.e. authocryst, 379 Ma) during emplacement. New Lu-Hf zircon data from the same U-Pb dated domains reveal that the parental magma was derived from a single heterogeneous source involving both subcontinental lithospheric mantle and Early Palaeozoic lower continental crust. Notably, Lu-Hf data from zircon indicate significant compositional variability in the magma, with a wide εHf_t ranges during the zircon crystallization events. This marked compositional diversity in εHf_t values are attributed to the crystallization of zircon antecrysts from isotopically distinct microdomains that formed melt pockets within the mush reservoir, accounting for the complex Hf isotopic signatures observed in magmatic zircon populations. These conditions persisted during magma ascent and shallow emplacement, where zircon autocrysts grew. Zircons that previously crystallized from compositionally distinct microdomains were juxtaposed within a single rock volume at the hand specimen scale, resulting in the presence of zircon crystals with different U – Pb ages but similar εHf_t values, or crystals with the same U – Pb ages but contrasting εHf_t values, all within the same rock sample. εHf_t zircon data, combined with geochronological and whole-rock chemistry evidence, support a model in which differentiation occurred mainly within the mush reservoir, with subsequent ascent and emplacement involving magmas with diverse geochemical compositions. Our findings indicate that whole-rock isotopic systems (e.g. Sm-Nd, Rb-Sr) reflect only the final integrated signal of the magmatic system, masking internal heterogeneity.