RB-SR AND SM-ND GEOCHRONOLOGY AND ISOTOPE GEOCHEMISTRY OF CENTRAL IBERIAN METASEDIMENTARY ROCKS (PORTUGAL)

Rb-Sr and Sm-Nd isotopic data are reported for 27 samples of fine-grai ned metasediments from five different localities of the Slate Greywack e Complex of the Central Iberian Zone, Portugal. Over most of the area , Rb-Sr whole-rock isotope systematics yields a 440 to 400 Ma time int erval, which is considered to correspond to an important metamorphic e pisode. Initial Sr-87/Sr-86 ratios of most Variscan granitoids from no rthern Portugal together with the Sr isotopic evolution curves for all studied metapelites, show that the latter are not suitable main sourc es for these granitoids. Sm-Nd depleted-mantle model ages and whole-ro ck Sm-Nd isochron ages provide values of 1.35 to 1.25 Ga, which may co rrespond to the average age of the mantle-extraction of the sources of the sediments. These data provide evidence that metamorphism of green schist facies does not induce Nd isotopic rehomogenization in fine-gra ined sediments. The Sr and Nd isotopic signatures of the analysed meta sediments (Sr-i = 0.7090 to 0.7170; and epsilon(Nd) (430) = -2.6 to -4 .18) suggest derivation from young continental crust, and deposition p robably in a tectonically passive setting.