Partial melting of metabasites in the crustal environment undergoing metamorphism produces various peritectic minerals including garnet, orthopyroxene, sillimanite and amphibole [1]. Although the latter is a common peritectic phase [1], the presence of grunerite (i.e., the Fe-dominated monoclinic magnesium-iron-manganese amphibole) in migmatites has been rarely reported. Its unusually great abundance has been recently pointed out as a striking characteristic of metabasites from the Punta Scorno area, in the northern part of the Asinara Island (NW Sardinia, Italy), belonging to the high-grade Metamorphic Complex of the Sardinian Variscan Chain. In the studied area, a large (200 x 50 m) integral body composed of massive amphibolites, banded amphibolites with more felsic portions of grunerite-bearing granulites and garnet granulites crops out. Except for the massive amphibolites, all the rocks contain evidence for partial melting, such as crystallised melt inclusions and melt pseudomorphs. Grunerite exclusively occurs in the melt-bearing rocks and forms (sub-)millimetric, subhedral, pale-yellow crystals often intergrown with garnet and subsequently overgrown by blueish-green tschermakite. Among the investigated rock types, grunerite-bearing granulites contain the highest modal proportion of grunerite. We interpret this rock type to represent the leucocratic portion produced by partial melting of the banded amphibolite. In the grunerite-bearing granulites, grunerite is characterised by a more variable XFe (XFe = 0.51 – 0.64) than in the banded amphibolites (XFe = 0.55 - 0.59). Pressure-temperature (P-T) conditions were estimated with Ti-in-amphibole [2,3] and Ti-in-quartz [4,5] thermometers coupled with phase equilibria modelling using Perple_X [6]. The prograde P-T conditions of grunerite-bearing granulites show isobaric heating at 0.5 GPa from ca 650°C, corresponding to grunerite crystallisation and melting, up to ca 730°C, when grunerite was possibly replaced by garnet. This stage was followed by late tschermakite overgrowth on grunerite during the retrograde P-T path at ca 600°C. Given the mafic protolith and the relatively low peak P-T conditions, the partial melting process must have been driven by a water-rich fluid influx, which lowered the solidus temperature. The growth of grunerite may be potentially triggered by this fluid and favoured by temperatures too low for the crystallisation of more common peritectic phases such as pyroxenes. This work benefited from Fondazione di Sardegna Progetto RAWEX and the European Union’s Horizon Europe research and innovation programme under grant agreement No 101131765 (EXCITE2).
Prograde grunerite in partially molten amphibolites from the High-Grade Metamorphic Complex of the Variscan Chain (Asinara Island, Italy).
Olga Turek;Silvio Ferrero;Alfredo Idini;Lorenzo Dulcetta;Gabriele Cruciani;
2026-01-01
Abstract
Partial melting of metabasites in the crustal environment undergoing metamorphism produces various peritectic minerals including garnet, orthopyroxene, sillimanite and amphibole [1]. Although the latter is a common peritectic phase [1], the presence of grunerite (i.e., the Fe-dominated monoclinic magnesium-iron-manganese amphibole) in migmatites has been rarely reported. Its unusually great abundance has been recently pointed out as a striking characteristic of metabasites from the Punta Scorno area, in the northern part of the Asinara Island (NW Sardinia, Italy), belonging to the high-grade Metamorphic Complex of the Sardinian Variscan Chain. In the studied area, a large (200 x 50 m) integral body composed of massive amphibolites, banded amphibolites with more felsic portions of grunerite-bearing granulites and garnet granulites crops out. Except for the massive amphibolites, all the rocks contain evidence for partial melting, such as crystallised melt inclusions and melt pseudomorphs. Grunerite exclusively occurs in the melt-bearing rocks and forms (sub-)millimetric, subhedral, pale-yellow crystals often intergrown with garnet and subsequently overgrown by blueish-green tschermakite. Among the investigated rock types, grunerite-bearing granulites contain the highest modal proportion of grunerite. We interpret this rock type to represent the leucocratic portion produced by partial melting of the banded amphibolite. In the grunerite-bearing granulites, grunerite is characterised by a more variable XFe (XFe = 0.51 – 0.64) than in the banded amphibolites (XFe = 0.55 - 0.59). Pressure-temperature (P-T) conditions were estimated with Ti-in-amphibole [2,3] and Ti-in-quartz [4,5] thermometers coupled with phase equilibria modelling using Perple_X [6]. The prograde P-T conditions of grunerite-bearing granulites show isobaric heating at 0.5 GPa from ca 650°C, corresponding to grunerite crystallisation and melting, up to ca 730°C, when grunerite was possibly replaced by garnet. This stage was followed by late tschermakite overgrowth on grunerite during the retrograde P-T path at ca 600°C. Given the mafic protolith and the relatively low peak P-T conditions, the partial melting process must have been driven by a water-rich fluid influx, which lowered the solidus temperature. The growth of grunerite may be potentially triggered by this fluid and favoured by temperatures too low for the crystallisation of more common peritectic phases such as pyroxenes. This work benefited from Fondazione di Sardegna Progetto RAWEX and the European Union’s Horizon Europe research and innovation programme under grant agreement No 101131765 (EXCITE2).
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