Granitoid plutonism in the Iberian autochthon terrane

With a few possible exceptions, the Variscan plutonism in the Iberian autochthon is largely unrelated to the subduction of the oceanic crust dated at 380-390 Ma (Dallmeyer and Gil Ibarguchi, 1990; Peucat et al., 1990). The absence of linear belts resembling those of the South American Cordillera Belt (Atherton, 1984; 1993) and the scarcity of typical I-type granites certainly supports the above assumption.

The majority of the Variscan granitoids from the Iberian autochthon terrane were emplaced 340 to 270 million years ago (Ortega and Gil Ibarguchi, 1989). Regional structural constraints show that granite emplacement post-dates syn-collisional deformation (D1+D2) and is predominantly correlated with the last Variscan deformation phase (D3), which is marked by vertical folding and important intra-continental shearing (Noronha et al., 1981; Iglesias and Ribeiro, 1981; Díez Balda et al., 1990).

According to their relationships with the D3 deformation phase, the Iberian Variscan granitoids have been subdivided into two major groups: (a) syn-kinematic (340-320 Ma) and (b) late-post-kinematic granites (315-270 Ma) (Oen, 1970; Capdevila et al., 1973; Ferreira et al., 1987; Pinto et al., 1987) (Fig. 2). The range of emplacement ages for these granitoids is mainly based on Rb-Sr whole-rock radiometric data and points to a broadly continuous evolution in time. However, recent high precision U-Pb dating reveal that the syn- to post-kinematic Variscan granitoids may have been intruded during several, discrete, short-lived events, over a c.a. 20 Ma timespan (Valle Aguado et al., 2005).

Figure 2. Distribution of Variscan granitoids

Distribution of Variscan granitoids

Distribution of Variscan granitoids in the Central Iberian Zone (modified from Ribeiro et al., 1972).

From the petrological and geochemical point of view, the Iberian granitoids have been classified into two main suites: (a) strongly peraluminous leucogranites and two-mica granitoids, and (b) calc-alkaline granodiorites and biotite monzogranites, associated with minor intrusions of basic and intermediate rocks. These two granitoid series cannot be exclusively assigned to any particular group of ages. Synkinematic and late-post kinematic plutons of both granite types are found all over the Iberian Massif (Ferreira et al., 1987; Pinto et al., 1987; Ortega and Gil Ibarguchi, 1989).

Most petrogenetic models favour an origin by partial melting of pure metasedimentary crustal sources for the highly peraluminous two-mica granites and leucogranites (true S-type granites) (e.g. Reavy, 1989; Beetsma, 1995). However, Holtz and Johannes (1991) claim that the upper crustal metaigneous lithologies (orthogneisses) can also yield peraluminous melts and should therefore be regarded as potential source rocks for the group of peraluminous granitoids.

On the other hand, the I- and I-S transitional granite-types (granodiorite-monzogranite suites) have been alternatively interpreted as products of: (a) hybridization of felsic crustal melts with mantle-derived magmas, followed by further contamination and fractional crystallization (e.g. Castro et al., 1994; Dias and Leterrier, 1994, Beetsma, 1995; Azevedo and Nolan, 1998; Dias et al., 2002) or (b) partial melting of heterogeneous lower crustal metaigneous sources (e.g. Villaseca et al., 1998).