CONICET | Buscador de Institutos y Recursos Humanos

The compositional variability of crustal-derived granites has been attributed to a multitude of processes. Magma mixing and fractional crystallisation have previously been given a great deal of attention, and used to account for the wide variation in granite compositions. We focus instead on restite unmixing as the dominant process influencing chemical variability in granitic rocks. We describe the relationshipbetween granulite facies migmatites and granites in the Sierra de Quilmes. The north-south trending mountains that define the region, known as the Sierras Pampeanas, are characterised by variably metamorphosed turbidites of the Neoproterozoic?Cambrian Puncoviscana Formation. Early Ordoviciansubduction on the western margin of Gondwana produced widespread high-T, low-P metamorphism of pelites and greywackes during the 470 Ma Famatinian Orogeny, resulting in extensive anatexis and granite plutonism. The Sierra de Quilmes is a tilted metamorphic complex in the northern Sierras Pampeanas, and provides exposure of the near-complete metamorphic sequence from granulite-faciesmagma source rocks in the southwest to granite emplacement levels in amphibolite and greenschist facies rocks in the northeast. Anatexis is associated with the fluid-absent incongruent breakdown of biotite, evidenced by the presence of peritectic Grt±Crd±Opx in restitic assemblages. In the southwest Sierra de Quilmes, the Ovejeria Complex is dominated by gradational contacts between diatexite migmatites and granites. This corresponds with greater heterogeneity in the form of schollen and Bt-schlieren within granites. In contrast, in the northeast Sierra de Quilmes, the Laguna Complex has common melt segregation structures in metatexites from which cleaner and more leucocratic andhomogeneous granites are extracted feeding the leucocratic Cafayate pluton. Bulk rock geochemistryof migmatites and granites reveals two contrasting trends between the two complexes. The diatexites and granites from the Ovejeria Complex are compositionally similar to the Puncoviscana Formation, suggesting mobilisation of residuum to produce the heterogeneous granites. In contrast, such rocks inthe Laguna Complex define geochemical trends indicative of melt-residuum separation, with residual compositions enriched in FeOT, MgO and TiO2, and a complementary fractionated assemblage with elevated SiO2 and K2O related to the homogeneous leucocratic plutons. The difference in geochemical trends between the two complexes, combined with distinct field relationships, indicates how the process of magma segregation and extraction from the source impacts on their geochemistry. Here we have two complexes side by side, one that records restite unmixing trends and the other that is dominated by segregation of melt from the source and subsequent fractionation.