Migmatites in the Ivrea Zone (NW Italy): Constraints on partial melting and melt loss in metasedimentary rocks from Val Strona di Omegna

The mid to lower crustal metamorphic field gradient through amphibolite and granulite facies rocks in the Ivrea Zone offers the potential to study partial melting and melt loss in the crust. Metapelitic rocks in Val Strona di Omegna show a progressive evolution in migmatite structures from metatexites with rare isolated leucosome veins in the amphibolite facies rocks to stromatic migmatites and diatexites in granulite facies rocks. Little field or petrographic evidence for melting can be seen on crossing the position of the modelled wet solidus, consistent with the small amounts of melt predicted to occur by H2O-saturated melting. The first field evidence for partial melting, in the form of narrow discontinuous leucosomes, coincides with the fluid-absent breakdown of muscovite and the prograde appearance of K-feldspar. The consumption of biotite, which is modelled to occur over a 50-100 °C wide field of coexisting garnet-sillimanite-biotite, led to more pronounced melting and the formation of abundant garnet-bearing leucosomes. At the highest grades, metagreywacke compositions contain leucosomes that are spatially focussed on orthopyroxene porphyroblasts. Calculated P-T pseudosections show that the metapelitic rocks could have produced up to 40 mol% melt and the metagreywackes up to 25% melt at peak metamorphic conditions of around 11 kbar, 900 °C. Modelling of granulite facies samples shows elevated solidi suggesting significant melt loss prior to cooling, consistent with depletion in SiO2, Na2O and K2O and enrichment in FeO, MgO and TiO2 relative to amphibolite facies samples. Zones of diatexite in the highest grade rocks indicate that, at least locally, melt loss was inefficient and/or accumulation of melt occurred. Zones of apparent accumulation of melt are common at the boundaries between metatexitic metagreywacke and diatexitic metapelite, and may indicate that the metagreywacke formed a low-permeability barrier that restricted melt flow.