New age and geochemistry data from the Middle Allochthon ophiolitic units of the Morais Complex (Portugal).

The allochthonous complexes of the Galicia-Trás-os-Montes Zone (GTMZ) of the NW Iberian Massif consist of an ensemble of peri-Gondwanan terranes and ophiolitic units stacked during the Variscan orogeny. The Middle Allochthon also known as the ophiolitic complex represents the variscan suture of one or more peri-Gondwanan oceans, and includes Cambro-Ordovician to Lower Devonian units. In the allochthonous Morais Complex (Trás-os-Montes, Portugal), the ophiolitic complex comprises four structural units, which from bottom to top are Macedo de Cavaleiros, Pombais, Izeda-Remondes and Morais-Talhinhas.

The two first units are quite similar to each other and consist of greenschists and metapelites, with metabasites dominating in Pombais and metapelites in Macedo de Cavaleiros. No age data are available for these two units. Their structural position is comparable to that of the Cambro-Ordovician Vila de Cruces Unit in the Órdenes Complex, but also to that of the Lower Devonian Moeche Unit in the Cabo Ortegal Complex, both in Galicia.

The Izeda-Remondes and Morais-Talhinhas units mostly consist of fine grained amphibolites associated with deformed gabbros, mafic cumulates and serpentinized ultramafics. The Izeda-Remondes Unit is structurally the lower and the older of the two, dated by Pin et al. (2006) around 447 ± 24 Ma (Sm-Nd whole rock isochron). The upper ophiolitic Morais-Talhinhas Unit was also dated by Pin et al. (2006) giving U-Pb ages of 405 ± 1 Ma and 396 ± 1 Ma.

This contribution brings new geochronological and geochemical data from the Middle Allochthon providing new understanding of the history of the suture of the Morais allochthonous complex. Zircons have been collected for LA-MC-ICP-MS U–Pb analyses in two felsic intrusions in the Izeda-Remondes Unit giving concordant ages ranging from 422 ± 4 Ma to 432 ± 4 Ma. These ages together with new and previous whole rock geochemical data obtained from basic and felsic igneous samples from the ophiolitic complex are interpreted to date and reflect the formation of igneous protoliths in an oceanic ridge setting forming part of the Rheic oceanic realm, during Silurian to Devonian. The mantle source for the basic rocks of all four units is similar to that of N-MORB with some influence from a subduction zone.

REFERENCE:

Pin, C., Paquette, J. L., Ábalos, B., Santos, F. J., & Gil Ibarguchi, J. I. (2006). Composite origin of an early Variscan transported suture: Ophiolitic units of the Morais Nappe Complex (north Portugal). Tectonics, 25(5).

Acknowledgements: Spanish Ministry of Science and Innovation, project PID2020-117332GB-C21.