Low P partial melting of metapelites during contact metamorphism in the Forcel Rosso-Mount Ignaga zone of the Adamello Massif (Northern Italy)

Partial melting of metapelitic lithologies in thermal aureoles around shallow felsic intrusions is localized to the first hundreds of meters from the magmatic body and its diagnostic features are often cryptic. The contact aureole of the post-collisional metaluminous tonalitic to granodioritic Adamello pluton (Eocene sup.) at Forcel Rosso-Mount Ignaga imprinted an unmetamorphosed continental to the marine sedimentary succession of Permian to Triassic age. Disorganized melt patches and aplitic-pegmatitic dykes and veinlets in the pelitic-psammitic sequence of the thermometamorphosed Permian Verrucano Lombardo formation indicate that diffuse partial melting occurred up to 500m from the contact. In the study area, metamorphic grade increases from phyllites and schists at sub-greenschist and greenschist facies on the top of mount Foppa and in the Fumo valley, to upper amphibolite facies migmatites near the Western Adamello Tonalite (WAT) intrusion (Adamè Valley). Petrological and geochemical variations together with phase equilibria modeling are used to investigate the metamorphic conditions that determined the formation of migmatites. Peak mineral assemblages are composed of Crd+Sil+Bt+Kfs+Pl+Qz+Tur+Mag indicating a pressure of ca. 300 MPa, obtained through phase equilibria modeling, and a peak metamorphic temperature of 700°C, on which phase equilibria modeling and the Na-in-cordierite geothermometer are concordant. Fluid fluxed, fluid assisted and fluid absent partial melting conditions were modeled using R crust for different unmetamorphosed and low-grade metamorphosed pelitic lithotypes. The obtained data suggest that the migmatites are the result of open-system processes in which fluid assisted melting played a major role as only the fluid present models are concordant with mass balance estimations while fluid absent models produce very low melt fractions in the expected P-T field. Geochemical evidence suggests that primary compositional variations characterizing the Verrucano Lombardo pelitic layers control both the amount of melt that was formed and its composition. Metatexites containing abundant deformed paleosome and cordierite have melt-depleted compositions (up to 20vol.% of produced and extracted melt). Diatexites are mainly composed of melanocratic residuum of high An86-96 plagioclase, high aspect ratio biotite schlieren, fibrolitic sillimanite, back-reacted cordierite, and euhedral schorl-dravitic tourmaline alternated to Kfs+Qz leucocratic levels. These rocks represent layered cumulate products with syn-anatectic flow structures that were formed after 26vol.% of melt was efficiently segregated from the protolith. The B and H2O-rich anatectic melts were locally lithium enriched and they collected in dilating fractures forming a limited pegmatitic field of similarly oriented barren to Li-rich pegmatites in the Verrucano Lombardo metapelites of the Forcel Rosso-Mount Ignaga zone.