Detrital zircon and tectonostratigraphy of the Parautochthon under the Morais Complex (NE Portugal): implications for the Variscan accretionary history of the Iberian Massif

Zircon geochronology using laser ablation inductively coupled plasma mass spectrometry, combined with a new detailed geological map and a stratigraphic study of the Parautochthon at the eastern rim of the Morais Allochthonous Complex, has provided new insights into the evolution of the Iberian Variscan belt. The data confirmed that the Parautochthon is composed of two structural units. The higher and more deformed, called the Upper Parautochthon, consists of recumbently folded pre-Variscan low-grade metasediments deposited in the northern Gondwana passive margin during the opening of the Rheic Ocean. It represents the Variscan tectonic duplication of the NW Iberian autochthonous sedimentary sequence triggered by the advance of a stack of allochthonous units formed previously in an accretionary prism. Below this unit, the Lower Parautochthon is a less pervasively deformed tectonic slice including two lithostratigraphic units (Travanca and Vila Cha formations) composed of Culm-type synorogenic low-grade metasediments deposited in a tectonic trench located between the accretionary prism and a peripheral bulge, with detrital zircon age populations pointing to a Late Devonian-Early Carboniferous age. These sediments were subsequently detached from the Autochthon along mechanically weak Silurian carbonaceous slates. The provenance study on the detrital zircons implies that this basin was fed mainly from the active margin.