Geología de las zonas Centro Ibérica y Galicia – Trás-Os-Montes en la parte oriental del Complejo de Morais, Portugal/España

Résumé

The Morais Allochthonous Complex (MAC) belonging to the Galicia ¿ Trás-os-Montes Zone, (GTMZ) is placed in the hinge zone of the Central Iberian Arc (CIA). Evidences of the pre- to syn-Variscan geological evolution of the NW Iberia are especially well exposed along the eastern rim of this first order structure, where this research work was achieved. Two major tectono-stratigraphic domains juxtaposed by a significant Variscan tectonic accident were redefined: the Schistose Domain of Galicia ¿ Trás-os-Montes (SDGTM) and Central Iberian Zone (CIZ). The first domain belongs to the lowermost tectonic slice of the different imbricated allochthonous units of the GTMZ. The SDGTM was settled over the CIZ during the Variscan C2 stage by the Main Trás-os-Montes Thrust (MTMT), and it is considered to be the least allochthonous unit (also called Parautochthonous or Peritransmontano Domain). Since the pre-variscan stratigraphic records of both domains are very similar, it is assumed that they must have been attached to the same continental margin but in different palaeogeographic locations, as their detrital zircon age populations point out. Both domains present a quite continuous and traceable Cambro-Silurian (?) stratigraphic sequence, although it underwent more Variscan deformation in the SDGTM than in its relative autochthonous (CIZ). In the CIZ the identification of two ichnofossil genera in the Desejosa Formation (Teichichnus rectus and Rosselia socialis) confirms that its upper part is at least uppermost-early Cambrian (Cambrian Stage 3), as previously proposed near the studied region with badly preserved trilobite remains (¿522Ma). A new stratigraphic unit (Montes Ermos formation, informal) was found concordantly overlaying the Desejosa Formation and lying below the Lower Ordovician unconformity (or Toledanic Unconformity) meaning that the uppermost record of the Slate and Greywacke Complex in Iberia could reach ages younger than Cambrian Stage 3. Over the Toledanic unconformity, a quite complete Lower-Middle Ordovician sequence was mapped, showing a laterally continuous and condensed Armorican Quartzite facies. At the base of the Upper Ordovician limestones of the Santo Adrião Formation another unconformity was identified, eroding the underlying units towards the Lower Ordovician formations, which locally eliminates the Middle Ordovician record (Moncorvo Formation). The youngest studied unit of the autochthonous domain recognized in this work is Silurian, but due to the strong C2 shearing that affects it no further assumptions on its age should be made. Although the pre-Variscan record of the SDGTM is intensely deformed under low metamorphic Barrovian conditions, it was possible to redefine its stratigraphy and the variscan structure based on field criteria and high resolution dating methods. With that purpose, two volcanic complexes were dated using U-Pb CA-ID-TIMS of magmatic zircon grains, proving the existence of two separated events in the same sedimentary sequence. The oldest, named Mora volcanics, is firstly described as a stratigraphic unit in this work. It presents a Furogian age (late Cambrian) and is characterized by a bimodal volcanic suite with sulphide mineralization. The youngest volcanic rocks dated in this thesis are Lower Ordovician. These are known as Saldanha Gneiss but the name Saldanha volcanics is preferably used. This unit is composed by porphyritic tuffs and rhyolites with calc-alkaline affinity. Since no important accident between these two volcanic complexes was identified, these ages add importance to the first order tectonic accident mapped at the base of the Upper Parautochthon, the MTMT, as Cambro-Ordovician rocks of the now proposed Mora-Saldanha Volcano-Sedimentary complex (MSVSc) are put over the Devono-Carboniferous synorogenic deposits of the Lower Parautochthon. The MSVSc is roofed by the white quartzite beds of the Algoso formation (informally proposed in this work) which is thought to be an equivalent to the Armorican Quartzite units of Iberia. Finally, the uppermost unit of the Upper Parautochthon studied in this work is composed by slates and basic to acid volcanic rocks with sulphide mineralization, known here as Peso formation (formerly as Vulcano- Siliceous Complex) and a possible correlation with the Middle Ordovician slaty units of Iberia is proposed. Along the boundary between the CIZ and the GTMZ, the two synorogenic deposits of the Lower Parautochthon ¿ Travanca and Vila Chã formations ¿ were detached over the autochthonous Silurian black slates and ampelites thus creating the Basal Lower Parautochthonous Detachment (BLPD) during the C2 Variscan phase. Because their zircon U-Pb age clusters are identical to the ones found in the pre-orogenic record of the SDGTM (and in the Basal Units) they were incorporated in the GTMZ. In this work it is proposed a new local tectono-stratigraphic sub-division of the SDGTM, where the synorogenic units define the Lower Parautochthon, whereas the pre-orogenic ones were included in the Upper Parautochthon. In this work it is also presented an integrated approach to the tectono-thermal evolution of the Variscan Orogeny in the eastern border of the MAC based on the regional study of macro and microscopic tectono-metamorphic fabrics. The first compressive event (C1) generated folding of both SDGTM and CIZ and marks the beginning of the prograde Barrovian evolution. Continuously the C2 event caused the thrusting of the Upper Parautochthon on top of the Lower creating the MTMT, and triggered the detachment of the synorogenic deposits over the CIZ along the BLPD. This event started the bending (CIA) of the previous C1 folds in both domains and it is thought to be related with the expansion of an extrusion wedge from the Variscan orogen axial zone. During this period the Barrovian metamorphism reached its peak. Following this initial compressional events, the instability of the thickened crust below the GTMZ, produced the synorogenic collapse and the formation of thermal domes (metamorphic core complexes) during the first extensional event (E1). This favoured the development of tectonic basins where the allochthonous complexes were preserved and the partial melt of the lower-middle crust while it was exhumed to upper crust conditions in the domes (following a Buchanian path). Immediately after the synorogenic extension a third compressive event (C3) generated disharmonic folding and lately fragile-ductile conjugate shear bands that led to the tightening of the CIA, and the formation of the Ibero-Armorican Arc. This event was accompanied by the intrusion of different granitic bodies. Finally, a late gravitational collapse (E2) followed by late-post Variscan compression (C4) produced gentle horizontal and vertical folding (respectively).