Study of nanogranite inclusions in garnets from the Ulten zone (Austroalpine domain, Eastern Alps)

The study of melt inclusions (MI) in high-grade metamorphic rocks represents a novel tool to characterize anatectic melt and partial melting P-T conditions. In this work, high-grade migmatitic gneisses from the Ulten-Nonsberg Zone (UZ) have been investigated, in order to characterize MI hosted in peritectic garnet. UZ rocks belong to the Austroalpine domain of the Eastern Alps and represent a pre-Alpine basement unit that underwent a polymetamorphic evolution marked by high-pressure metamorphism and migmatization processes during the Variscan orogeny, related to continental collision between Laurussia and Gondwana in the late Carboniferous (about 330 Ma). Previous geothermobarometric studies suggest P-T conditions of the migmatization event of P<1.0 GPa and T<900°C and P-T conditions for the peak metamorphism of 1-2 GPa and 600-900°C (with the wide range depending upon the assumption of the H2O activity of the fluid phase during metamorphic evolution; Godard et al., 1996). Other studies by Braga et al. (2007) and Braga and Massonne (2008), based on thermobarometry on garnet-kyanite gneisses, suggest a P climax (w1.2 GPa) during progressive heating and a thermal peak between 700-750°C at about 1 GPa, during the first stages of the exhumation process. For this study, after the sampling activity, 67 thin sections have been analyzed. Optical microscope and SEM-EDS observations allowed to identify the best samples with MI-bearing garnets. MI appear as a polycrystalline aggregate of Qtz +- Bt +- Kfs +- Pl +- Chl, with a maimum size of about 20 μm, and are therefore dened nanogranites or nanogranitoids. The presence of MI in the core of peritectic garnets allowed to conrm their primary origin and thus to consider the EMP composition of garnet core in order to perform the phase equilibria modeling. Phase equilibria modeling was performed with the Perple X software (Connolly, 2009), in order to estimate probable P-T conditions of entrapment of MI in peritectic garnets. The mineral assemblage of sample N3L (the one selected for phase equilibria modeling) is Qtz + Kfs + Wm + Pl + Grt + Ky, and Rt as accessory mineral. Bulk rock composition of sample NB5, very similar to N3L, was used and the chemical system Na2O-CaO-MnO-K2O-Fe2O3-MgO-Al2O3-SiO2-H2O-TiO2 (NCMnKFMASHT) was selected. A phase diagram section was obtained for P=0.5-2.0 GPa and T=600-950°C and garnet isopleths (for almandine, grossular, pyrope and spessartine components) have been calculated within the same P-T space, in order to further constrain the stability field. Garnet core isopleths of sample N3L did not match the mineralogical association of the rock in the pseudosection; therefore, the core composition of garnet from a rock very similar to NB5 was used (sample NONS17, from Braga et al., 2007). These data allowed to constrain the stability field of rock NONS17 and rock NB5, corresponding to a soprasolidus field with P-T conditions ranging from 1-1.4 GPa and 800-845°C, characterized by the mineralogical association Grt + Bt + Wm + Pl + Kfs + Qtz + Rt + melt. Therefore, the investigated MI clearly represent droplets of the anatectic melt produced during the prograde phase of the host migmatites.