Age of amphibolitic metamorphism in the ophiolitic unit of the Morais allochthon (Portugal): implications for early Hercynian orogenesis in the Iberian Massif

摘要: Structurally lower portions of the allochthonous Morais complex are comprised an ophiolitic unit exposed in two contrasting structural units separated by a ductile thrust fault. The is dominated variably foliated amphibolite whereas upper largely metaperidotite and metagabbro. Most has recrystallized synkinematically under facies metamorphic conditions ( c. 500°C 5 kbar). Amphiboles formed during this metamorphism typically magnesio-hornblende. An amphibole concentrate from records 39 Ar/ 40 Ar plateau age 397.3 ± 4 Ma. displays internally discordant spectrum corresponding to total gas 431.1 27.3 Gas fractions evolved each at intermediate high experimental temperatures characterized similar apparent K/Ca ratios yield well-defined 36 v. isotope correlations which result ages 384.2 5.2 392.4 7.3 These considered be more reliable than interpreted as dating last cooling through those required for intracrystalline retention argon within constituent grains. associated tectonothermal event represented first stage Hercynian orogenesis Iberian massif. It was probably related oceanic closure obduction granulitic/eclogitic massifs present structurally overlie unit.