Study of nanogranites included in garnets from the Valpelline series (Austrlpine domain, Western Alps)

Since their discovery melt inclusions (MI) have been a very useful tool in igneous petrology and from the last decade they represent a novel approach for the study of metamorphic rocks. This work focuses on the investigation of high-grade metapelitic rocks (kinzigites) from the Valpelline Series and on the characterization of the melt inclusions hosted in peritectic garnet. These rocks belong to the Australpine domain of the western Alps and represent portions of the southern continental margin of the alpine Tethys. The partial melting in this rocks occurred at about 750-850 °C and 6-8 kbar during the Permian (300-250 Ma). One hundred and one samples have been collected from the Valpelline Series and seventy-seven thin sections have been prepared from them. Subsequently a preliminary work with the optical microscope has been done in order to identify the most promising samples for the study of MI. A detailed microstructural characterization has been performed on selected samples using electron scanning microscope (SEM-EDS). The inclusions are small in size (< 20 μm) and are of primary origin as testified by their distribution in the cores of peritectic garnet. They are formed by a polycrystalline assemblage of quartz + plagioclase + K-feldspar + biotite and they are call nanogranites or nanogranitoids; no glassy inclusions have been founded in any samples. An important goal of this work was the re-homogenization of MI in order to recover the bulk composition of the trapped melt. This has been done via remelting experiments under confining pressure in a piston cylinder apparatus. The inclusions did not re-homogenize after the first experiment conducted at 800 °C and 10 kbar for 24 hours and, hence, it was necessary to perform a second experiment at 850 °C and 10 kbar for 24 hours. In this case almost all nanogranite inclusions were partially re-melted but some of them displayed interaction with the host garnet, such as the recrystallization of garnet around the inclusions or the crystallization of new phases (e.g., orthopyroxene) at the inclusion wall. This occurs because the trapping temperature of MI was exceeded during the experimental run. One inclusion was almost totally re-homogenized, without clear evidence of overheating. Semi-quantitative analyses have been done using EDAX system. For the first time the probable composition of the melt produced by the partial melting of crustal rocks of the Valpelline Series was analysed (SiO2 ≈ 72%, Na2O ≈2%, CaO ≈0.7%, K2O ≈ 6%, FeO ≈ 4%, Al2O3 ≈ 13 %).This work also confirms the previous temperature estimates from the literature on peak conditions of the Variscan metamorphism.