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The ophiolites of NW Iberia have received considerable attention in
thepastfewyearswithpapersdescribingindetailtheirlithologies,chem-
ical compositions and isotopic geochronology. It is now well-established
thattheirigneousprotolithsrangequitewidelyinage,therebyprecluding
their generation within a single oceanic domain. Two groups of Ophiolitic
Units have been distinguished (Fig. 2): an older group (Lower Ophiolitic
Units) containing metaigneous rocks of Late Cambrian age (c. 497–
495 Ma),andayoungergroup(UpperOphioliticUnits)includinggabbro-
ic rocks of Devonian age (Emsian–Eifelian; c. 395 Ma).
The Cambrian ophiolites comprise either thick sequences of
greenschists (with island-arc tholeiite compositions), with some alter-
nations of phyllites and rare tonalitic orthogneisses (Vila de Cruces
Ophiolite;Arenas et al., 2007a ; Fig. 2), or c. 4000 m of HT metagabbroic
amphibolites with N-MORB affinities and minor ultrama fic rocks (Bazar
Ophiolite; Sánchez Martínez et al., 2012; Fig. 2). The protoliths of the
Vila de Cruces Ophiolite were formed during the opening of a back-arc
basin at the periphery of Gondwana. In contrast, based on its chemical
composition and structural position, the Bazar Ophiolite probably rep-
resents a relic of the Cambrian peri-Gondwanan Iapetus-Tornquist
Ocean, accreted below a system of peri-Gondwanan volcanic arcs. This
accretion is dated at c. 475 Ma based on U– Pb zircon geochronology
for the timing of the HT metamorphism affecting the mafic protoliths.
The Lower Ophiolitic Units are interpreted to represent a series of
mafic complexes linked to the dynamics affecting the most external
margin of Gondwana in Cambrian–Early Ordovician times.
The Middle Devonian ophiolites are the most abundant group found
in the Variscan suture (Murphy et al., 2011). In addition to NW Iberia,
they occur in the Lizard Complex (Lizard Ophiolite; Clark et al., 1998;
Nutman et al., 2001), in the Armorican Massif (Drain Ophiolite;
Ballèvre et al., 2009, 2012), and in the Bohemian Massif (Ś lęża
Ophiolite; Dubińska et al., 2004; Kryza and Pin, 2010). In Galicia (NW
Spain), the Careón Ophiolite (395 ± 2 Ma, U–Pb zircon in metagabbro;
Díaz García et al., 1999; Pin et al., 2002; Fig. 2) has received special
attention and can be considered the type example of the group. It is
madeupofthree imbricate slices, thethickest(c. 1000 m)ofwhichcon-
tains a sequence of peridotites overlain by 500 m of isotropic gabbros.
Both the peridotites and thegabbros are intruded by stocks of pegmatit-
ic gabbros and numerous doleritic dykes. The ophiolite contains no
sheeteddykecomplexandnovolcanicor sedimentary rocks, atvariance
with common N-MORB oceanic lithosphere generated at a mid-ocean
ridge. The mafic rocks have compositions typical of island-arc tholeiites
(Sánchez Martínez et al., 2007). The U–Pb zircon age obtained for
this ophiolite is relatively close to the time calculated for its accretion
(c. 377 Ma, 40Ar/ 39Ar on hornblende concentrate; Dallmeyer et al.,
1997), therefore the unit represents a section of buoyant oceanic litho-
sphere that escaped subduction beneath Laurussia. New U–Pb zircon
data obtained from other upper ophiolites in NW Iberia yielded similar
crystallization ages for themafic protoliths(395 ± 3 Mafor thePurrido
Unit: Sánchez Martínez et al., 2011; 400 ± 3 Ma for the Moeche Unit:
Arenas et al., in press; Fig. 2). Moreover, a combined isotopic (Lu–Hf in
zircon and Sm–Nd in whole rock) study of these ophiolites shows that
a suite of Devonian gabbros with juvenile isotopic compositions and
mantle provenance (the mafic protoliths of the ophiolites) interacted
with old continental crust and were affected by limited mixing
(Sánchez Martínez et al., 2011; Arenas et al., in press). The involvement
of a continental component is revealed by Paleoproterozoic Hf model
ages in some of the zircons, which is inconsistent with the generation
of the igneous protoliths in an intra-oceanic setting far removed from
continents.
3. A two-stage collision model for the early history of Pangea
The Upper Units have been previously interpreted as a section of a
peri-Gondwanan volcanic arc that rifted off the continental margin in
Cambrian–Early Ordovician times and drifted northward, opening the
Rheic Ocean in its wake. This rifting would have coincided with the
rift and drift of the Avalonian microcontinent. The Upper Units, howev-
er, would have had a different identity and provenance since they were
located further to the east along the paleo-margin of Gondwana (Abati
et al., 2007; Gómez Barreiro et al., 2007; Díez Fernández et al., 2010).
The HP or UHP metamorphic event that affected the lower section of
the Upper Units at c. 400–390 Ma would have been generated during
the accretion of this terrane to the southern margin of Laurussia, this
process highlighting the switch from a divergentto a convergent setting
in the evolution of the Rheic Ocean. The geodynamic evolution hitherto
suggestedfor theUpper Unitsimplies thatthe ophiolites involvedin the
Variscan suture were developed in the realm of the Rheic Ocean. The
Cambrian ophiolites – with the exception of the Bazar Ophiolite –
would be related to early stages in the opening of this ocean, while
theDevonianophioliteswouldhave been formed duringthe final stages
of its closure (Arenas et al., 2007b). For this reason,it has been proposed
that the Devonianophiolites were formed in a northwarddipping intra-
Rheic Ocean supra-subduction zone located close to the southern
margin of Laurussia (Díaz García et al., 1999; Sánchez Martínez et al.,
2007). Such intraoceanic subduction zone would have generated buoy-
ant oceanic lithosphere that would have readily accreted beneath
Laurussia and eventually been obducted over the external margin of
Gondwana (Basal Units) at the beginning of the Variscan deformation
(c. 370 Ma). Furthermore, the activity of this intraoceanic subduction
zone would have consumed a significant tract of the Rheic Ocean, thus
explaining the general absence of typical N-MORB lithosphere in the
Variscan suture.
However, the aforementioned models that link the generation of the
Devonian ophiolites to an open-ocean setting are inconsistent with the
new isotopic data that clearly show the interaction of the gabbroic
protoliths with old continental crust. Many of the zircons analyzed
in mafic rocks from the Purrido and Moeche ophiolites show Lu–Hf
isotopic compositions only compatible with a continental origin. These
zircons can be only interpreted as inherited crystals incorporated into
the mafic magmas (Sánchez Martínez et al., 2011; Arenas et al., in
press). Consequently, there is no conclusive evidence to link the gener-
ation of the Devonian ophiolites either to the evolution of the Rheic
Ocean or to an intraoceanic subduction zone active in a mature ocean
basin. If the connection between the Variscan suture of NW Iberia and
the evolution of the Rheic Ocean is called into question, so must the
interpretation of the Upper Units as a peri-Gondwanan terrane that
drifted away during the opening of this Paleozoic ocean. Moreover,
problems also exist in attributing the development of HP–UHP meta-
morphism in the trailing edge of a rather small terrane to its collision
with Laurussia.The tectonothermalevolutionexpectedfor theaccretion
of a small, ribbon terrane to a large continent would be one more com-
patible with soft collision, without generation of important subduction
and, hence, lacking HP–UHP metamorphism. In contrast, the latter is
usually associated with deep subduction of the thinned margin of a
large continent during its collision with another large continent
(Warren et al., 2008; Beaumont et al., 2009).
It is alsonoteworthy that theage of the HP–UHP metamorphic event
in the Upper Units, which is constrained to be no younger than 400–
390 Ma (U–Pb zircon; Ordóñez Casado et al., 2001; Fernández-Suárez
et al., 2007), is similar to that of the mafic rocks of the Upper Ophiolitic
Units, repeatedly dated at c. 400–395 Ma (U–Pb zircon; Díaz García
et al., 1999; Pin et al., 2002, 2006; Sánchez Martínez et al., 2011;
Arenas et al., in press). In the case of the HP metamorphic event, the
U–Pb geochronology provides the age of the HT zircon growth, which
occurred sometime after the continental margin became involved in
the subduction system. Accordingly, the peak pressure of the HP event
must have been reached prior to 400–390 Ma since this age likely
marks a point along the exhumation/decompression path (Fernández-
Suárez et al., 2007). This subduction must also predate the generation
of the Devonian mafic rocks, as expressed in recent papers focused on
the origin of the allochthonous terranes of NW Iberia (Sánchez
Martínez et al., 2007; Martínez Catalán et al., 2009).
759 R. Arenas et al. / Gondwana Research 25 (2014) 756–763
8. TWO STAGE COLLISION
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