Rb-Sr and Sm-Nd geochronology and isotope geochemistry of Central Iberian metasedimentary rocks (Portugal)

Abstract

Rb-Sr and Sm-Nd isotopic data are reported for 27 samples of fine-grained metasediments from five different localities of the Slate Greywacke 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 interval, which is considered to correspond to an important metamorphic episode. Initial 87Sr/86Sr ratios of most Variscan granitoids from northern Portugal together with the Sr isotopic evolution curves for all studied metapelites, show that the latter are not suitable main sources for these granitoids. Sm-Nd depleted-mantle model ages and whole-rock Sm-Nd isochron ages provide values of 1.35 to 1.25 Ga, which may correspond to the average age of the mantle-extraction of the sources of the sediments. These data provide evidence that metamorphism of greenschist facies does not induce Nd isotopic rehomogenization in fine-grained sediments. The Sr and Nd isotopic signatures of the analysed metasediments (Sri = 0.7090 to 0.7170; and εNd (430) = -2.6 to -4.18) suggest derivation from young continental crust, and deposition probably in a tectonically passive setting.