S. MELI, A. MONTANINI, M. THÖNI & W. FRANK
ABSTRACT
In the Northern Apennines, the External Liguride ophiolite-bearing chaotic complexes of Upper Cretaceous age frequently include slide-blocks and breccias of granulite-facies continental rocks. These rocks record a complex tectono-metamorphic history which can be partially elucidated by geochronology. Sm/Nd and 40Ar/39Ar geochronology of a mafic granulite has provided three different ages: (1) a Sm/Nd mineral-WR isochron at 291+9 Ma represents the time of emplacement of the mafic igneous protolith in the lower crust followed by slow cooling and recrystallisation in granulite-facies conditions; (2) a 40Ar/39Ar amphibole age of 228+2 Ma is a cooling age reflecting the exhumation of the lower crust during the early phases of Tethyan rifting; (3) a 40Ar/39Ar plagioclase pseudoplateau of about 80 Ma could reflect a weak thermal overprint of the mafic granulite (and enclosing sedimentary mélanges) in the Late Cretaceous during the closure of the Ligure-Piemontese Basin.