Water-fluxed melting of an A-type granite associated with shear zones: Insights from the São Pedro da União Migmatite Unit, Brazil

Understanding the role of H₂O-fluxed melting in Fe-rich granites is crucial for evaluating their contribution to crustal differentiation. In this study, we examine the São Pedro da União Migmatite Unit (SE Brazil), where a Paleoproterozoic A-type granite was reworked during the Neoproterozoic, as part of the assembly of western Gondwana. New U-Pb and Lu-Hf zircon data indicate that the protolith crystallized at ca. 1.72 Ga, was derived from a quartz-feldspathic continental crustal source, and that magma generation was linked to Statherian continental rifting.

Phase equilibrium modelling suggests subsequent Ediacaran partial melting at ca. 600 Ma, which required an influx of externally derived H₂O at 670–720 °C and 1.0 GPa. Despite the extensive anatexis, the migmatite preserves geochemical characteristics of the protolith, including high X_Fe in peritectic hornblende (hastingsite). The increase in leucosome proportion towards the Jacuí Shear Zone—from stromatic metatexite at the top to homogeneous diatexite at the base—suggests progressively greater H₂O availability in a zone of syn-anatectic deformation.

These results reveal that, although A-type granites are typically considered hot and anhydrous, they can undergo significant reworking and generate substantial melt volumes when infiltrated by external H₂O. In this scenario, shear zones likely acted as localized pathways for fluid ingress.