Remelting of Ordovician Migmatites and Granites During Variscan High-Grade Metamorphism (aar Massif, Central Alps, Switzerland)

The remelting of felsic gneisses, migmatites and granite during a later orogenic cycle can produce rocks of different age with comparable field appearance, microstructure, mineralogical and chemical composition. Here, we compare two gneiss units in the Aar Massif (an External Crystalline Massif of the Central European Alps, Switzerland), which include migmatites and rocks with meta-igneous textures: the Erstfeld Gneiss Complex in the northern part and the Strem Granite unit in the south-eastern part of the Aar Massif. Both units show a range in zircon U-Pb dates of several hundred million years, which seemingly is caused by the presence of two melting events of Ordovician and Variscan age. A distinction between these two melting events is not possible based on field observations, but it becomes evident via in-situ U-Pb geochronology of distinct zircon growth domains identified through cathodoluminescence imaging. An Ordovician age group (ca. 455 Ma) is well preserved in zircon of both units. However, only in the southern unit is a Variscan overprint (ca. 310–315 Ma) recorded in zircon rims and by euhedral anatectic allanite with a U-Pb age of 315.1 ± 3.7 Ma. Geochronology and intrusive field relationships suggest that, during the late stage of the Variscan orogeny, Ordovician migmatites in the Strem Granite unit were remelted, forming a generation of anatectic granites that had not been previously identified. The remelting of Ordovician migmatites during Variscan high-grade metamorphism likely occurred at the minimum melt temperatures of the quartz-feldspar system through fluid-assisted melting. Due to the absence of any signs of dehydration melting, we propose that external water was advected through numerous trans-crustal shear zones that were active during the late stage of the Variscan orogeny. This melting episode is bracketed by pulses of late-orogenic magmatism between ca. 335 and 309 Ma. The initial Hf isotope composition of zircon provides evidence for multiple recycling of the late Proterozoic to early Ordovician chemical inheritance, restricting the addition of juvenile components to the latest stages of Variscan magmatism.