Fluid-assisted melting of amphibolites at low-medium pressure in the High-Grade Metamorphic Complex of the Sardinian Variscan Belt (Asinara Island, Italy)

Partial melting processes during orogenesis can involve a wide variety of protoliths and take place over a wide range of pressure. In particular, partial melting of mafic rocks at low-medium pressure/medium-high temperature is one of the least investigated case studies occurring in Nature. Amphibolites, amphibolitic gneisses and granulites from the High-Grade Metamorphic Complex (HGMC) of the Asinara Island (Sardinia, Italy), southern Variscan Belt, are a perfect target for studying these processes. In the Punta Scorno area, they occur as lenses and fragments, surrounded by granites and migmatitic metapelite. There are two varieties of amphibolites, which are either massive, composed mostly of hornblende, plagioclase and biotite with minor quartz, zoisite, K-feldspar and muscovite, or banded, containing plagioclase, hornblende, biotite, grunerite and garnet. Amphibolitic gneisses and granulites have the same mineral assemblage as the banded amphibolites but different modal proportions. Evidence of former melt, such as melt pseudomorphs and melt inclusions, is observed in banded amphibolites and amphibolitic gneisses. The mineral chemistry is, in general, similar throughout the different lithologies. Plagioclase composition is variable, with XAn = 0.29 – 0.96. Amphibole occurring as big crystals is classified as magnesio-hornblende with XFe = 0.46 – 0.68, whereas the one occurring on the grunerite rims is tschermakite with XFe = 0.62 – 0.73. Grunerite has XFe = 0.52 – 0.64. Biotite is also Fe-rich (XFe = 0.48 – 0.64). Si in muscovite reaches 3.12 apfu. Garnet in banded amphibolites is Alm69-72Grs11-14Prp7-9Sps8-10, whereas in amphibolitic gneisses the composition is Alm72-76Grs11-13Prp7-12Sps4-6. Pressure-temperature conditions of metamorphism have been preliminarily estimated with the Ti-in-amphibole thermometer (Liao et al., 2021) and thermodynamic modelling using the Perple_X program (Connolly, 2005) for ca. 0.6 GPa and 730ºC. Such a low temperature requires that partial melting took place in presence of a water-rich fluid which lowered the solidus of the system (Weinberg and Hasalová, 2015). The occurrence of partially melted amphibolitic rocks together with granites and metasediments suggests interactions between mafic and felsic crust. This topic will be the subject of further research, as this study is an introduction for investigations of the processes recorded in the rocks of the HGMC. References Liao, Y. et al (2021). American Mineralogist, 106(2), 180–191 Connolly J. A. D. (2005). Earth and Planetary Science Letters, 236, 524–541 Weinberg R. F. and Hasalová P. (2015). Lithos, 212–215, 158–188