Isotope constraints on the age and origin of magmatism and metamorphism in the Malpica-Tuy allochthon, Galicia, NW Spain

Abstract

UPb analyses on zircon and plagioclase, and RbSr measurements on garnet, clinozoisite, phengite, plagioclase and biotite from a meta-granodiorite and an eclogitic felsic gneiss were undertaken to: (1) precisely date granodioritic magmatism; (2) decipher the magma origin; and (3) study the tectonometamorphic overprint in the Malpica-Tuy unit, which is part of the lower allochthonous terrains of the Palaeozoic orogenic belt in NW Spain. Zircons from the meta-granodiorite yield an emplacement age of 479.5 −1.7 +1.6 Ma (2a) Ma, and inherited components indicate an average age of ∼ 1812 Ma for continental magma source rocks. A largely anatectic origin of the intrusion, as substantiated by the inherited components in the zircons, is in agreement with its initial 87 Sr 86 Sr isotopic composition of 0.7081, as well as with the initial Pb isotopic composition of plagioclase. Major- and traceelement compositions indicate a calc-alkaline affinity of granodiorite and felsic gneiss in contrast to subsequent magmatism at Malpica-Tuy, which has a peralkaline character. From the meta-granodiorite, whole rock, plagioclase and two biotite fractions define a Rb-Sr isochron age of 352 ± 3 (2σ) Ma, indicating cooling below ∼ 320'C at 360-350 Ma. A RbSr age of 365 Ma was obtained for garnet-phengite from the associated eclogtte-facies felsic gneiss, whereas clinozoisite-phengite from the same sample yields a younger age of ∼ 335 Ma. This difference in age shows that mineral assemblages in the eclogitic gneiss are not in equilibrium; however, both ages confirm major cooling after ∼ 360 Ma following high-pressure metamorphism.