U-Pb, Hf isotope and REE constraints on high-pressure acid migmatites from the Cabo Ortegal Complex (NW Spain): New evidence of short-duration metamorphism in a Variscan subduction channel

Abstract

Paired LA-ICP-MS UPb and LuHf isotopic data and trace element data are reported on zircon in thin section from poorly known acid migmatites exposed in the Cabo Ortegal Complex, Western Variscides, NW Spain. The migmatites are part of the high-pressure granulite unit, tectonically placed between the eclogite and ultramafic units. Large oscillatory zoned zircon cores date the felsic igneous protolith at 459 ± 7 Ma as a minimum age reflecting a protracted basic magma differentiation in a Cambro-Ordovician arc setting. Lower Th/U, weakly- to un-zoned overgrowth rims are attributed to partial melting at 392 ± 5 Ma during high-pressure metamorphism. The distinctly lower epsilon Hf (+4.0) of the rim relative to the core (+8.2) implies an open-system behavior metamorphism and mass transfer between different units in deep parts of a subduction channel during the Variscan Orogeny. Tiny xenomorphic zircon crystals devoid of internal microstructures dated at 355 ± 4 Ma and with nearly flat REE patterns are interpreted as product of metamorphic overprint driven by hot fluids. The latter is envisaged as a subduction channel short-duration metamorphic phenomenon.