Geocronologia 207Pb/206Pb en cristal único de circón y Rb-Sr del plutón de la Cabrera (Sierra del Guadarrama)

A Pb-Pb crystallization age of 302 ±3 Ma was found for the La Cabrera granite by the single zircon Pbevaporation technique. Moreover the whole rock Rb-Sr isotope systematics provided an errochrone of 303 ± 24 Ma (I.R. = 0.709275; MSWD = 23.4). This age is Upper Carboniferous and is close to the end of a stage of regional N-S extension subsequent to the climax of Variscan regional metamorphism. The high MSWD of the Rb-Sr systematics might be attirbuted to either an ubiquitous weak pervasive hydrothermal alteration or to the absence of initial isotopic equilibrium throughout the pluton. The second possibility is favored from Nd isotope composition considerations.