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

Abstract

A robust U-Pb zircon dataset previously published in this journal revealed a magmatic system withprotracted 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 ofa 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 wasderived from a single heterogeneous source involving both subcontinental lithospheric mantleand Early Palaeozoic lower continental crust. Notably, Lu-Hf data from zircon indicate significantcompositional variability in the magma, with a wide εHft ranges during the zircon crystallizationevents. This marked compositional diversity in εHft values are attributed to the crystallization ofzircon antecrysts from isotopically distinct microdomains that formed melt pockets within themush reservoir, accounting for the complex Hf isotopic signatures observed in magmatic zirconpopulations. These conditions persisted during magma ascent and shallow emplacement, wherezircon 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 εHft values, or crystals with the same U – Pb ages but contrasting εHft values, all within the same rock sample. εHft 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 wholerock isotopic systems (e.g. Sm-Nd, Rb-Sr) reflect only the final integrated signal of the magmatic system, masking internal heterogeneity.