The sondalo gabbroic complex (central alps, Northern Italy): evidence for emplacement of mantle-derived melts into amphibolite-facies metapelites

The Sondalo gabbroic complex is related to the magmatic event that postdated the Variscan orogeny. The complex mainly consists of gabbronorites, associated with subordinate amounts of olivine-gabbros, and minor quartz-diorites. These rocks can be related to tholeiitic liquids that underwent a differentiation process controlled by fractional crystallisation and concomitant assimilation of crustal material. Basement country rocks are mostly made of two-mica amphibolite-facies metapelites that are locally sillimanite-bearing. Close to the contact with the gabbroic complex, the metapelites display migmatitic fabric. Melanosomes show the consumption of muscovite and the development of sillimanite and anhedral garnet. Leucosomes are quartz-feldspathic in composition and locally characterised by the growth of porphyroblastic garnet. Contact migmatites are in place characterised by the presence of peraluminous granitoid pods. Large xenolith blocks (up to hundreds of meters in size) showing granulite-facies assemblages, as well as peraluminous granitoid dykes, crop out within the gabbroic complex. The xenolith blocks are volumetrically dominated by cordierite- and sillimanite-bearing garnet-rich rocks. These rocks are characterised by low silica and alkalis, and high Al₂O₃, Fe₂O₃^tot, MgO, TiO₂ and MnO. These chemical features are interpreted to reflect a restitic origin, related to the extraction of “granitic” liquids from metapelitic sources. The garnet-rich xenolith blocks are discontinuosly wrapped by bodies of garnet-bearing mafic rocks. The Sondalo gabbroic complex resulted from the intrusion of mantle-derived liquids into intermediate levels of the continental crust. Such intrusion did not lead to regional granulite-facies metamorphism and triggered melting processes in contact metapelites. Extraction of anatectic liquids gave rise to dense restitic blocks that could sink into the magma chamber, where they possibly underwent further melting.