The Variscan basement in the western shoulder of the Lusitanian Basin (West Iberian Margin): insights from detrital-zircon geochronology of Jurassic strata

There is no consensus about the geological nature of the westernmost portion of the Iberian Massif. In the present research, the detrital zircon U-Pb signatures of Jurassic strata of the Lusitanian Basin, known to be west-sourced, are combined with published U-Pb data for the Precambrian-Palaeozoic basement and other Lusitanian Basin units to better understand this poorly exposed portion of the Iberian Massif. Cryogenian to Ediacaran ages prevail in a northern Upper Jurassic unit, while Lower and Upper Jurassic rocks in southern locations yield mostly Carboniferous to upper Permian zircons. These age results, coupled with their respective U/Th ratios, suggest that the basin covers two distinct terranes of the Iberian Massif. Another noteworthy feature of west-derived deposits is the abundance of < 310 Ma ages. It is proposed that a combination of crustal thinning in the West Iberian Margin with regional eastward basement tilt, favoured the enrichment of relatively young zircon in the western shoulder of the basin relative to its eastern margin. The detrital zircon age signatures also reveal a middle to late Permian thermal event in restricted areas, which is probably associated with the oldest stages of Alpine extension in West Iberia.