METAMORPHIC CORE COMPLEXES: CRUSTAL-SCALE PARAMETERS CONTROLLING FOOTWALL BEHAVIORS

In their initial definition, metamorphic core complexes involved a large metamorphic discontinuity between the hanging wall and footwall of extensional detachment zones. In some cases, the P-T conditions of footwall rocks have been used to infer the amount of displacement accommodated by the dipping detachment. While this is a reasonable assumption for some core complexes, if the footwall is dominated by migmatites – the rocks were once partially molten ­– then the trajectories of footwall rocks may not be controlled by the detachment geometry. Instead, the extension provided by the detachment system triggers upward flow of the partially molten crust, resulting in rapid exhumation of material to form migmatite domes. The question then becomes: where are the migmatites coming from, and more specifically, how deep were these rocks before they started their syn-detachment ascent? Because migmatites crystallize at relatively shallow depth, the pressures recorded in them are typically 0.5-0.8 GPa at temperatures around 700°C. These pressures are registered during the crystallization of migmatites at relatively shallow depths and do not, in general, represent the original depths of the material. Exceptionally, refractory rocks entrained in migmatite preserve significantly higher pressures, as is the case in the Variscan Montagne Noire dome in the French Massif Central. There, blocks of eclogite that crystallized at 1.5 GPa and 700°C yield zircon ages similar to the age of the felsic migmatite (~310-315 Ma). The presence of eclogite reveals the near-Moho depth of origin of the partially molten rocks (~50 km depth) before they were exhumed as a migmatite dome. Numerical modeling of low-viscosity deep crust predicts that exhumed partially molten material should originate around 40-50 km depth, follow very steep upward trajectories during exhumation, and fill migmatite domes in the core of metamorphic complexes.