Provenance of the c. 390 Ma HP–HT subducted continental margin (Variscan belt, Cabo Ortegal Complex, NW Iberian Massif)

Abstract

The Variscan Upper Allochthon is a continental-affinity terrane that recorded a Cambrian–Ediacaran magmatic arc generation, a subsequent transition to a passive margin, and a collision-related high-P metamorphism during the Devonian–Carboniferous amalgamation of Pangea. The bottom member of the Upper Allochthon recorded this HP-HT Devonian metamorphism, which was attained by a subduction process. To know which continental margin subducted a provenance study was carried out on the metasedimentary rocks of the Banded Gneisses (5 samples), which form part of the Upper Allochthon subducted terrane. The provenance of this terrane has been established using combined U–Pb (n = 613) and Lu–Hf (n = 463) isotopic LA–ICP–MS zircon analyses. These data show that the Variscan Upper Allochthon has a West African provenance and therefore, it strongly suggests that the NW Iberian allochthonous complexes and their correlative European terranes are also West Africa derived. These results allow us to finally clarify that the first high-P event, recorded during the eo-Variscan amalgamation of Pangea, was attained by the subduction of the margin of Gondwana under the colliding retro-continent, presumably Laurussia.