serie NOVA TERRA nº 48

Cambrian times, which was followed by signifi-

cant extension and rift-related magmatism in the

back-arc region. This shelf did not witness signif-

icant new igneous activity or deformation until

the onset of the HP–UHP metamorphic event,

and hence it shows the characteristics of a typical

passive margin for most of the Ordovician and Si-

lurian. In the geological record covering this time

interval there is no evidence suggesting any sig-

nificant separation of this continental shelf from

the Gondwanan mainland. Siluro-Devonian con-

vergence between Gondwana and Laurussia led

to a first continental collision before 400-390 Ma,

including the dextral, oblique subduction to the

North (Ábalos

et al.

, 2003) of the most external

and thinned part of the Gondwanan margin ac-

companied by the first HP–UHP metamorphism

(Albert

et al.

, submitted). The collision probably

affected the eastern part of Avalonia and the Bal-

tic margin (Fig. 29).

Further lateral motion between Gondwana

and Laurussia favored the rapid generation of a

rather wide pull-apart basin in Early Devonian

times, which we interpret as the tectonic setting

for the generation of the

395 Ma mafic rocks

forming the most typical ophiolites found in the

Variscan Belt (Fig. 29). The pull-apart basin cur-

rently being generated between the North Amer-

ican Plate and the Caribbean Plate can be consid-

ered a modern analogue for this setting, although

in this case the lateral component is sinistral. The

Gonâve microplate occupies the pull-apart basin

and comprises oceanic lithosphere with a rath-

er thin or completely absent sedimentary cover

(Brink

et al.

, 2002). This oceanic lithosphere is

being generated by the activity of the Mid-Cay-

man Spreading Centre. In this modern analogue,

as was probably the case at the beginning of

Pangea assembly, the pull-apart basin followed

a period of continental convergence responsible

for the high-P belts in northern Cuba and His-

paniola (García Casco

et al.

, 2008; Sommer

et al.

2011).

Subsequent convergence eventually closed

the pull-apart basin and forced the accretion of

buoyant oceanic lithosphere beneath the north-

ern continent starting at

380 Ma (Careón and

Purrido ophiolites; Dallmeyer

et al.

, 1997). The

accreted oceanic lithosphere is mostly meta-

morphosed to the amphibolite facies, but the

occasional presence of corundum-bearing met-

amorphic soles indicates the presence of local

higher thermal gradients (Díaz García

et al.

1999a). Later accretion of more Devonian mafic

slices took place under greenschist facies condi-

tions (Moeche Ophiolite), and was followed by

the accretion of mafic complexes rimming the

continental margin that were formed within the

Cambrian peri-Gondwanan volcanic arc (Vila

de Cruces Ophiolite). The final outcome was the

generation of a complex suture zone that records

protracted convergence and is characterized by

the presence of a double ophiolitic belt made of

lithological sequences with contrasting origin

and age: the Upper Ophiolitic Units of Devoni-

an age and the Lower Ophiolitic Units of Cam-

brian age. The occurrence of a thick serpentinite

mélange at the base of the allochthonous pile was

interpreted in the context of dextral convergence

(Somozas Mélange; Arenas

et al.

, 2009).

Further dextral convergence caused north-di-

rected subduction of the Gondwana margin

under Laurussia and the rest of the accreted

sections of Cambrian and Devonian ophiolitic

ensembles at

370 Ma (Díez Fernández

et al.

2012d, 2012e). This subduction affected a piece

of the Gondwanan margin with a more easterly

provenance (in Gondwanan margin coordinates;

Basal Units; Díez Fernández

et al.

, 2010; Fuen-

labrada

et al.

, 2012). This is the suggested setting

for the development of the second HP metamor-

phic event, formed under LIT conditions and

generating C-type eclogites, blueschists and HP

metapelites (Fig. 29). Continental subduction

evolved to a collisional scenario as more internal

sections of Gondwana were underthrust. Con-

vergence continued for about 70 m.y. (Dallmey-

et al.

, 1997) as intracontinental deformation

progressed southward, reaching inner sections

of Gondwana while building a foreland fold and

thrust belt in the external parts of the orogen

(Peréz-Estaún

et al.

, 1991).

Conclusions

NW Iberia includes a rather complete sec-

tion of the Variscan suture, where different ter-

ranes with continental or oceanic affinities ap-

pear with clear structural relationships between

them. Three groups of terranes, namely Upper,

3. GEOLOGICAL FRAMEWORK