Using garnet as a petrogenetic indicator: an example from Capo Vaticano intrusion (Calabria, Southern Italy)

Garnet is a common constituent of metamorphic rocks but may also be found in igneous lithologies. It records important information about the evolution of the crust, helping to reconstruct metamorphic and magmatic processes. This thesis focuses on investigating garnet contained in rocks from the Capo Vaticano intrusion with the specific aim at understanding its origin in these rocks. Samples were collected in Santa Maria beach (Calabria, Southern Italy), and, for comparison, garnet crystals from some metamorphic lithologies in the region, specifically metasedimentary migmatites and residual metapelites, were also examined. Polarized light microscopy was employed to identify the main minerals within the samples and select the most appropriate ones for further analyses, using SEM, EMPA and LA-ICP-MS. Major element analyses of garnet in intrusive rocks revealed a main composition Alm62-79 Prp11-29 Grs4-10 Sp1-8, while plagioclase is andesine - labradorite (An47-54). Major and trace element data collected in garnet from garnet-bearing tonalites revealed nearly homogeneous cores, interpreted as metamorphic xenocrystic cores, and rims enriched in Mn, Ca and HREE, interpreted as magmatic overgrowths. These results suggest that garnet crystals likely originated from high temperature metamorphism of the metapelitic basement. The subsequent arrival of mantle-derived magmas and their mixing with crustal anatectic melt produced a hybrid magma and the assimilation of garnet. The interaction with the surrounding hybrid magma likely caused garnet resorption, leading to an enrichment in Mn and HREE of the surrounding magma. Alternatively, this enrichment could be attributed to the arrival of anatectic melts with a high HREE content, originated in the metapelitic basement where high-temperature conditions promoted the dissolution of accessory minerals. Subsequent cooling and magma crystallization resulted in further garnet growth producing rims enriched in Mn, Ca and HREE and euhedral garnet shapes. Interestingly, the magmatic overgrowths appear to be optically separated from the core. The increase in H2O contents within the magma led to local resorption and the formation of biotite replacing garnet. The accessory phases found were zircon, monazite, xenotime, ilmenite and titanite, in tonalites and diorites, while in metamorphic rocks also rutile, tourmaline, magnetite and pyrite were found. The presence of monazite and xenotime in tonalites further supports the hypothesis that metasedimentary lithologies played a role in the formation of garnet-bearing tonalites.