Numerical modeling of post-collisional carbonated alkaline magmatism: Variscan style Orogeny (the Ivrea Zone as natural laboratory)

Abstract

The Ivrea Zone of northwest Italy exposes a section of the lower crust and lithospheric mantle deformed during the Variscan Orogeny, where a series of carbonated alkaline pipes was emplaced over a time span of ca. 40 Ma, following a crustal underplating event at ca. 288 Ma. We use a coupled 2D petrological-thermomechanical approach to geodynamically model the processes that led to the Variscan continental collision and subsequent emplacement of the alkaline pipes with application to craton margin metasomatic events. The following criteria are assessed to develop a generic model: 1) closure of Rheic Ocean through subduction of oceanic crust and subduction-related metasomatism of the lithospheric mantle, 2) high-temperature metamorphism of the lower crust associated with continental collision, preceding the 3) emplacement of a crustal underplating event, and 4) partial melting of metasomatized lithospheric mantle domains during gravitational collapse of the orogen to source the alkaline pipes. Our model investigates the collision process by varying thermal ages of the intervening oceanic crust (40, 60, and 80 Ma) and by changing the rheological strength of the lower crust. We employ two ideal end-member cases, wet quartzite and plagioclase An75, an intermediate dry quartz case to represent a migmatized lower crust, and two polymineralic cases of felsic granulite and mafic granulite as possible representations to the Ivrea Zone specifically. We discuss the results through three scenarios controlled by the age of the oceanic crust and rheology of the lower crust that can reproduce tectonic scale features that are observed in the Ivrea Zone. Scenario 1: the wet quartzite 40 Ma model features widespread H2O and CO2 fluxes within the continental lithospheric mantle. The model displays little melt productivity due to the presence of an overthickened crust and the continental lithospheric mantle decouples and peels off beneath the lower crust of the colliding terrane. Scenario 2: the plagioclase An75 60 Ma model shows high-temperature metamorphism in the lower crust and reproduces the metasomatism of the continental lithospheric mantle that is a precursor to the genesis of the alkaline pipes. The model is permissive for the emplacement of the crustal underplating event, but the relative timings are misplaced. Scenario 3: the mafic granulite 80 Ma model recreates the tectonic relationships between lower crustal high-temperature metamorphism followed by initiation of the magmatic underplating event, and lastly by localized melting of metasomatized domains of the lithospheric mantle. This model can potentially explain two of the prevailing hypotheses for the formation of the alkaline pipes. Whereas all models reproduced different stages of the tectono-magmatic evolution of continental collision, only scenario 3 could sufficiently recreate the relative sequence of events observed in the Ivrea Zone. Scenarios 1 and 2 may provide precious information about the nature and timing of metasomatic processes in the lithospheric mantle, with implications for the enhanced prospectivity for a wide range of ore systems in specific domains along the margins of lithospheric blocks.