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

Abstract

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

Zircon U-Pb and Hf isotopic constraints on the genesis of a post-kinematic S-type Variscan tin granite: the Logrosán cupola (Central Iberian Zone)

“…The S2 schistosity is occasionally recognized in the studied area. The D3 phase took place between 320 and 312 Ma (López-Moro et al, 2012;Valle Aguado et al, 2005;Dias et al, 2002) and constitutes the last ductile regional deformation phase. It is characterized by the generation of open to tight vertical folds with subhorizontal axes and subvertical shear zones with sinistral (NW-SE) or dextral (NE-SW) wrench movement (Dias et al, 1998).…”

LP-HT anatectic processes and lithological heterogeneity in the Mindelo Migmatite Complex (NW Portugal)

“…Continuous to D 1 a second deformation phase (D 2 ) manifests mainly through movements along the base of subhorizontal major thrusts (Noronha et al, 1979;Noronha et al, 2000). These events were followed by a predominantly extensional phase, named D 2 by some authors (Valle Aguado et al, 2005) that allowed anatexis in the middle crust and segregation of magmas. These anatectic magmas rise at the beginning of D 3 leading to the emplacement of large volumes of peraluminous granites (Noronha et al, 1979;Casquet et al, 1988;Escuder Viruete, et al, 1994;Díez Balda et al, 1995;Ábalos et al, 2002;Azevedo & Valle Aguado, 2013).…”

“…Thus, the magmatic rocks mainly represent orogenic granitoids, which are defined according to different age groups. In recent years both geochronological and structural data point to the definition of several intrusion times for iberian granitic rocks, which can be divided in three major groups: pre-orogenic granites, orogenic granites and post-orogenic granites (Ferreira et al, 1987;Díez Balda et al, 1995;Valle Aguado et al, 2005;Martins et al, 2011;Azevedo & Valle Aguado, 2013) ( Fig. 1B).…”

“…The widespread occurrence of Crdbearing granites has led some petrologists to suggest an intermediate series or family of "granites of mixed features" (Capdevila et al, 1973;García-Moreno et al, 2007). Most of leucogranites are related to the principal metamorphic episodes and their emplacement is associated with the last deformation phase (D 3 ) and the development of ductile extensional shear zones (Ferreira et al, 1987;Escuder Viruete et al, 1994;Díez Balda et al, 1995;Valle Aguado et al, 2005;Azevedo & Valle Aguado, 2013). They are spatially related to low-pressure, high-temperature anatectic domains.…”

“…In the Variscan Belt the metamorphism occurred mostly under low pressure and high temperature (Martínez et al, 2001;Valle Aguado et al, 2005;Ribeiro et al, 2008;Catalán et al, 2014). In CIZ the high-grade rocks, belonging mainly to the Schist-Greywacke Complex (SGC), are associated with peraluminous granitic plutons and anatectic complexes, materializing thermal domes developed in a late stage of the Variscan Orogeny (Escuder Viruete et al, 1994;Martínez et al, 2001;Bea et al, 2003;Valle Aguado et al, 2005;Ribeiro et al, 2008;Catalán et al, 2014). The association of amphibolite facies metamorphic rocks with migmatites grading into granitic rocks has been described in different sectors of the Iberian Variscan Belt (Pereira & Bea, 1994;Barbero & Villaseca, 1995;Valle Aguado et al, 2010).…”