U-Pb ages of zircons from meta-igneous and meta-sedimentary rocks of the Sierra de Guadarrama: implications for the Central Iberian crustal evolution

U-Pb data of zircons from various gneisses of the eastern Sierra de Guadarrama, Central Spain, make it possible to reconstruct the pre-Hercynian and Hercynian history of this area: (1) upper intercept ages (2400 Ma for meta-igneous and 2000 Ma for meta-sedimentary rocks) of discordias defined by data of anhedral zircon fractions document the existence of Early Proterozoic crust; (2) lower intercept minimum ages of anhedral zircon fractions show radiogenic lead loss at about 540 Ma due to a thermal event leading to volcanism and emplacement of granitoid rocks into Precambrian crust: growth of euhedral zircons is possibly related to this event; (3) lower intercept minimum ages of about 380 Ma defined by anhedral zircon fractions in meta-sedimentary rocks prove an Early Hercynian event. According to present knowledge on the metamorphism in the Sierra de Guadarrama this event could be explained in terms of a Barrow-type medium-pressure metamorphism. The inferred Cadomian igneous event relates the geological history of Central Spain with that of western Africa to the south and Brittany to the north. Furthermore, similarities of the crustal evolution in the area studied and other internal zones of the Hercynian belt (Moldanubian zone, French Central Massiv) are confirmed. The Early Hercynian event for the first time affected all the rocks of the area together. The pre-Hercynian evolution of the two complexes is different and the present association of the basement rocks may be explained by thrusting or in terms of Early-Hercynian nappe transport.