U-Pb zircon age for the Lagoa augen gneiss, Morais complex, Portugal: tectonic implications

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Abstract
The Morais complex is a structural unit within a succession of allochthonous, exotic crystalline Variscan nappes exposed in northwestern Iberia. U-Pb analyses of tips broken from needle-like zircon in mylonitized Lagoa augen gneiss indicate a crystallization age of 496 +3 −2 Ma. Four other zircon fractions from the Lagoa augen gneiss suggest inherited zircon components with upper-intercept ages ranging between c. 2242 and 1941 Ma. The Cambrian to earliest Ordovician crystallization age is incompatible with previous interpretations of the Lagoa augen gneiss as Iberian Precambrian basement unconformably overlain by early Palaeozoic sedimentary rocks (Lagoa mica schist). The U-Pb results indicate that the Lagoa augen gneiss is coeval with, or perhaps younger than, the host sedimentary succession (the Lagoa mica schist). A 496 Ma crystallization age for the Lagoa augen gneiss is consistent with its derivation from an Avalonian-Cadomian source, at present situated west or northwest of the central Iberian autochthon.