The Pb-Sr-Nd isotope geochemistry of some recent circum-Mediterranean granites

Pb, Sr and Nd isotopic compositions have been analyzed in recent granites from Northern Africa, Northern Italy and Greece. Lead isotope compositions of K-feldspars are rather homogeneous, and cluster close to the modern lead of Stacey and Kramers (1975) but with slightly higher²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ratios. The Cyclades samples, however, have higher²⁰⁶Pb/²⁰⁴Pb ratios. Addition of mantle-derived lead was probably very limited, which supports a quasi-closed system evolution of this element in the continental crust. The Sr, Nd data fall in the enriched part of the¹⁴³Nd/¹⁴⁴Nd vs.⁸⁷Sr/⁸⁶Sr diagram and define a smooth hyperbolic mixing curve. Over a wide area, straddling different orogens, most granites may be accounted for by a binary mixture between a recycled crustal component and a depleted mantle-like component. No correlation is observed between either Pb and Sr or Nd isotopic ratios, or any isotopic ratio and major element contents. Quantitative modelling suggests that two cases fit the Sr and Nd characteristics of these granites: they both require anatexis of the crust on a scale large enough to average the isotopic properties of heterogeneous terranes. In the first case, the mantle-derived component may be represented by differentiated Island Arc-type magmas, and the granites result from mixing these magmas with anatectic melts. In the second case, mantle-derived igneous rocks, such as obducted ophiolites, are part of the crustal source and their variable involvement in the anatectic process causes isotopic variations.CRPG Contribution n 630.