Open system reaction between hydrous melt and gabbroic rock in the Finero Mafic Complex

Tracking melt migration in the lower crust is a foundational step in understanding the formation of Earth's crust. The Finero Mafic Complex of the Ivrea-Verbano Zone, Italy, exposes Mesozoic granulite to amphibolite grade gabbroic gneisses that are variably overprinted by brittle to semi-ductile fabrics inferred to have formed during the Late Cretaceous Alpine Orogeny. Whole-rock data compiled for gabbroic rocks of the Finero Mafic Complex supplemented with new data from representative samples analyzed in detail identify metasomatism involving iron and titanium (Fe–Ti) enrichment. As whole-rock Fe–Ti increases, the gabbroic rocks progress from diopside-plagioclase gabbroic gneisses to garnet-brown amphibole-rich gneisses. The modification involved hydration of diopside to brown amphibole along grain boundaries and the growth of poikiloblastic garnet grains. Brown amphibole and garnet microstructures indicative of the former presence of melt include small apparent dihedral angles and films or elongated fingers extending along grain boundaries. Mineral trace element and rare earth element analyses show a garnet-present (i.e., heavy rare earth element depleted) signature for all other minerals in garnet-bearing rocks, demonstrating high mobility of rare earth elements and a close approach to equilibrium at the hand sample scale during the modification event. The efficient redistribution of the rare earth elements supports the microstructural evidence for the former presence of melt. Thus, the observed whole-rock geochemical trend, textural progression, hydration, and recrystallization of gabbroic gneisses are interpreted to have developed during open system melt flow along grain boundaries of an externally derived hydrous mafic melt. Field relationships suggest melt flux through the gabbroic rocks was nearly pervasive at the unit scale and included sub-outcrop scale zones of concentrated flow marked by plagioclase-rich stringers with garnet reaction zones. This is the first interpretation that metasomatism by mantle-derived hydrous mafic melts modified the mantle and crustal components of the Finero Mafic Complex.