COMPOSITE ORIGIN OF AN EARLY VARISCAN TRANSPORTED SUTURE: OPHIOLITIC UNITS OF THE MORAIS NAPPE COMPLEX (NORTH PORTUGAL)

Abstract

The two structural units that constitute the ophiolitic complex of the Morais allochthonous complex display different geochemical features. Amphibolites from the lower unit are nearly homogeneous, chemically similar to normal mid-ocean ridge basalts, with low Th/Nb and highly radiogenic Nd isotope signature (εNd = +8.7), and might derive from igneous protoliths formed in an oceanic ridge setting 447 +/- 24 Myr ago (Sm-Nd whole rock isochron). In contrast, the upper unit consists largely of metaperidotites together with gabbroic rocks and minor amphibolites and felsic veins. U-Pb zircon ages of the upper unit range from 405 +/- 1 Ma to 396 +/- 1 Ma. These rocks are geochemically more variable but share in common high Th/Nb ratios and high initial εNd (from +7.5 to +8.9) interpreted to reflect generation above an intraoceanic subduction zone. This implies that the Morais ophiolitic complex is composite and consists of two distinct, relatively thin tectonic slivers that were detached from contrasting oceanic domains before being finally imbricated. This occurred broadly coeval, at the regional scale, with eclogite-facies metamorphism documented in several separate tectonic units of NW Iberia. On the basis of this space and time association, we interpret the early stage of the Variscan (in the broad sense) evolution (circa >= 405-400 Ma) in terms of intraoceanic subduction, followed by an arc-continent collision, circa 395 Ma. This model may account for the long-recognized polycyclic evolution of the north Portuguese catazonal complexes without resorting to the existence of a high-grade Precambrian basement.