The metasomatic history of the Finero Mafic Complex, Ivrea-Verbano Zone, Italy: melt migration, melt-rock interaction, and effects on geochronology
Unique in the modern solar system is the process of plate tectonics on Earth. Plate tectonics is driven by the migration of mass (melt) and heat (via conduction and advection) from the deep Earth to the surface. Understanding how mantle-derived melts migrate through the lower crust is critical in recognizing the microstructural and microchemical fingerprints they impart along migration pathways and their surficial expression upon eruption. The boundary between the mantle and lower crust is rarely exposed, limiting our understanding of melt flux at depth. Typically, these processes can only be understood from geophysical observations or via direct examination of lower-crustal xenoliths. The Ivrea-Verbano Zone, Southern Alps, northwest Italy, exposes a near-complete field gradient from the middle to lower continental crust. Starting in the Cretaceous, the Alpine orogeny tilted and uplifted this km-scale section allowing the in-situ study of deep melt-migration pathways. There is still considerable debate about the pre-Alpine history of the Ivrea-Verbano Zone due to gaps in knowledge on the roles of melt-migration and melt-rock interaction for crustal evolution, particularly in the lower crustal mafic and ultramafic rocks. This study focusses on the most metasomatized portion of the Ivrea-Verbano Zone, the Finero Mafic Complex since metasomatism is indicative of melt-rock interaction. Microstructural and microchemical analyses of gabbroic gneisses in the Finero Mafic Complex show that the garnet-amphibole-rich rocks are modified versions of a precursor diopside-plagioclase gabbroic gneiss. Reaction with melt drove hydration of diopside along grain boundaries to form new pargasite and introduced garnet into the assemblage. The trace element signature of all minerals shows hand-sample-scale equilibration at high temperatures allowing for the efficient redistribution of rare earth elements. The metasomatism and hydration are consistent with open system melt-mediated recrystallization as the primary process which modified the original gabbroic rocks. The documentation of this microstructural and microchemical evidence for diffuse-porous melt flow, i.e. open system melt migration of an externally derived melt along grain boundaries through the lower crust, serves as the context for the study of highly modified zircon grains in the gabbroic gneisses of the Finero Mafic Complex. These zircon grains have irregular grain shapes, internal microporosity, mineral inclusions, and display ghosted/blurry, overlapping cathodoluminescence textures. This physical evidence for modified zircon is supported by chemical data such as low concentrations of U and Pb, low Th/U ratios, and highly variable U-Pb isotopic disturbance leading to high concentrations of common Pb and highly discordant U-Pb age data. The relatively small proportion of concordant zircon age data gives a range of 206Pb/238U dates from 209 ± 10 Ma to 298 ± 14 Ma. Such a broad range of age data clouds the age determination and requires a significant interpretation that can be supported by trace element data and other isotopic systems. Hafnium isotopic analyses of zircon provide perspective as the broad range of εHf values (-1.68 – 14.44) indicates likely mixing of isotopic sources. The diffuse-porous melt-flow inferred at the mineral scale in the gabbroic rocks was likely responsible for the formation of the complex zircon age and Hf-isotope data. Melt migration possibly introduced antecrystic and xenocrystic zircon grains to the rocks, and melt-zircon interaction simultaneously caused melt-mediated coupled dissolution-precipitation recrystallization of zircon and modification of the geochemistry and internal microstructure of zircon grains. A second mechanism of melt migration may be inferred in the Finero Mafic Complex by field observation of cm-scale plagioclase-rich dikes, which are pervasive throughout the complex. The mineral assemblages and mineral chemistry indicate these dikes are cumulates of early crystallized minerals formed from highly evolved melts. The field relationships of the dikes serve as relative time markers for P-T estimation of the conditions of melt flow, as they cut a high-temperature foliation observed in peridotite rocks and are deformed in ultramylonite zones. The dikes also contain abundant zircon grains, which show similar physical and chemical modification patterns indicative of melt-mediated coupled dissolution-precipitation recrystallization. Centimeter-sized zircon grains from the dikes lack internal deformation and a U-Pb transect across the grain shows a ~40 Myr range. These observations suggest that sustained high temperatures and crystal-plastic deformation are unlikely to be the primary cause for the isotopic perturbations observed in the zircon grains, supporting the other microstructural evidence for coupled dissolution-precipitation. Concordant zircon 206Pb/238U ages from several analyzed dikes indicate four age populations, which range from 246 ± 12 Ma to 173 ± 6 Ma, extending the record of melt-migration via diking in the Finero Mafic Complex to the Middle Jurassic. Again, like zircon grains from the gabbroic rocks, those from the dikes have a broad range of Hf-isotope compositions (εHf = -0.21 to 8.55), indicating mixing of isotopic reservoirs occurred deep in the crust or upper mantle. The small degrees of partial melting represented by these dikes are consistent with melt generation by adiabatic heating of fertile components of the lithosphere during tectonic thinning as the Variscan mountain belt transitioned from a collisional setting in the Paleozoic to a rift setting in the Mesozoic.