Abstract

Pb isotope data by Barrett on Upper Jurassic cherts and associated ophiolites in the northern Apennines form a mixing trend with only two end members. Oblique secondary trends in the data are most likely an effect of radiogenic growth since the Jurassic and not due to multiple end members as proposed by Thorpe in his criticism of Barrett. Implications of these data for the evolution of the lithosphere in this region have not been previously discussed: (1) the distinct Pb isotopic signature of the chert end member is shared by the Pb in the Cenozoic Tuscan anatectic magmas, which suggests the same crustal provenance for both leads, and (2) the Jurassic sea-floor basalts constrain the age of anomalous metasomatized mantle now underlying most of Italy to less than 140 Ma.

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