The Viseu area is located in the Central Iberian Zone of the Iberian Variscan Belt and hosts numerous post-thickening, collision-related granitoids intruded into upper and middle crustal levels. The present paper reports high precision U-Pb zircon and monazite ages for four plutons of the Viseu area: the syn-kinematic granitoids of Maceira (314F5 Ma), Casal Vasco (311F1 Ma) and Junqueira (307.8F0.7 Ma) and the late-kinematic biotite monzogranites of Cota (306F9 Ma). This points to a synchronous emplacement of the different syn-kinematic plutons shortly followed by the intrusion of the late-kinematic granites and shows that the Upper Carboniferous plutonism occurred within a short time span of ca. 10 million years.The ascent of granite magmas took place after an extensional tectonic event (D 2 ) and is coeval with dextral and sinistral crustal-scale transcurrent shearing (D 3 ). Field and petrographical evidence suggest a narrow time-span between peak T metamorphic conditions and the intrusion of granitic melts which implies very fast uplift rates accommodated through active tectonic exhumation. Magma compositions evolve through time, reflecting an increasing involvement of mid-crustal sources and the underplating effect of an upwelling asthenospheric mantle at the base of a thinning and stretching continental crust. D

Recent advances in geochemical studies of igneous rocks, isotopic age data for magmatism and metamorphism, quantitative pressure-temperature (P-T) estimates of metamorphic evolution, and structural geology in the northwestern Iberian Massif are integrated into a synthesis of the tectonic evolution that places the autochthonous and allochthonous terranes in the framework of Paleozoic plate tectonics. Because northwestern Iberia is free from strike-slip faults of continental scale, it is retrodeformable and preserves valuable information about the orthogonal component of convergence of Gondwana with Laurentia and/or Baltica, and the opening and closure of the Rheic Ocean.The evolution deduced for northwest Iberia is extended to the rest of the Variscan belt in an attempt to develop a three-dimensional interpretation that assigns great importance to the transcurrent components of convergence. Dominant Carboniferous dextral transpression following large Devonian and Early Carboniferous thrusting and recumbent folding is invoked to explain the complexity of the belt without requiring a large number of peri-Gondwanan terranes, and its ophiolites and highpressure allochthonous units are related to a single oceanic closure.Palinspastic reconstruction of the Variscan massifs and zones cannot be achieved without restoration of terrane transport along the colliding plate margins. A schematic reconstruction is proposed that involves postcollisional strike-slip displacement of ~3000 km between Laurussia and Gondwana during the Carboniferous.

Abstract:The Beiras batholith consists of four main Variscan granitoid suites intruded into metasediments of Proterozoic-Cambrian and Palaeozoic age in Central Northern Portugal: a) the early, syn-D 3 granodiorite-monzogranite suite (314-311 Ma); b) the highly peraluminous syn-D 3 two-mica / leucogranite suite (308 Ma); c) the late-post-D 3 granodioritemonzogranite suite (306 Ma) and (d) the late-post-D 3 , peraluminous, biotite-muscovite granite suite (300-295 Ma). Major, trace and isotopic data suggest that the S-type synkinematic two-mica granites result from moderate degrees of partial melting under vapour absent conditions of middle crustal metasedimentary sources comparable to the Proterozoic-Cambrian metapelite-metagraywacke units presently exposed in the studied area. A major contribution from metaigneous lower crust materials and/or interaction with mantle derived magmas appears to be required to produce the early, syn-D 3 granodiorite-monzogranite suite. The emplacement of large volumes of late-post-kinematic granites showing decoupled high-K calc-alkaline and peraluminous signatures documents the importance of combined fractional crystallization and mixing processes (AFC) in granite petrogenesis. In a scenario of post-collisional re-equilibration of a thickened lithosphere, asthenospheric mantle upwelling Origin and emplacement of syn-orogenic Variscan granitoids in Iberia the Beiras massifand underplating of abundant basaltic melts at base of the crust is thought to have lead to widespread dehydration melting of lower-to mid-crustal lithologies and consequent formation of peraluminous granite magmas (syn-D 3 two-mica granites). Mixing to various degrees of anatectic crustal melts with a juvenile asthenospheric mantle component is considered the major controlling process involved in the production of the late-post-D 3 , high-K calc-alkaline suite. Concomitant fractional crystallization can explain the geochemical signatures of the more evolved rocks, including those of the late-post-D 3 , peraluminous, biotite-muscovite granites.

No abstract

No abstract