U-Pb zircon and monazite geochronology of Variscan magmatism related to syn-convergence extension in Central Northern Portugal

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 Viscu area: the syn-kinematic granitoids of Maceira (314 +/- 5 Ma), Casal Vasco (311 +/- 1 Ma) and Junqueira (307.8 +/- 0.7 Ma) and the late-kinematic biotite monzogranites of Cola (306 +/- 9 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. (c) 2005 Elsevier B.V. All rights reserved.