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

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