70 / 352
ing the Rheic Ocean in its wake. These events
would have coincided with the rift and drift of
the Avalonian microcontinent (Fig. 28). The Up-
per Units, however, would have had a different
identity and provenance since they were locat-
ed 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
high- to ultrahigh-pressure (HP-UHP) meta-
morphic event that affected the lower section
of the Upper Units at
c.
410-390 Ma would have
been generated during the subduction of this
terrane under the southern margin of Laurussia,
this process indicating the switch from a diver-
gent to a convergent setting in the evolution of
the Rheic Ocean (Fig. 28). This geodynamic evo-
lution would imply that the Variscan ophiolitic
units of NW Iberia 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 the Devonian ophiolites would
have been formed during the final stages of its
closure (Arenas
et al.
, 2007a). For this reason, it
had been proposed that the Devonian ophiolites
were formed in a northward dipping intra-Rhe-
ic Ocean supra-subduction zone located close to
the southern margin of Laurussia (Díaz García
et
al.
, 1999a; Sánchez Martínez
et al.
, 2007a). Such
intraoceanic subduction zone would have gen-
erated buoyant oceanic lithosphere that would
have been readily accreted beneath Laurussia
and eventually obducted over the external mar-
gin of Gondwana (Basal Units) at the beginning
of the Variscan cycle (
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
sutures.
However, the previous models that linked the
generation of the Devonian ophiolites to an open-
ocean setting were challenged by new isotopic
data that revealed interaction of the Devonian
gabbroic protoliths with old continental crust
(see above). Many of the zircons analyzed in maf-
ic rocks from the Purrido and Moeche ophiolites
show Lu-Hf isotopic compositions only compat-
ible with a continental origin. These zircons can
be only interpreted as inherited crystals incorpo-
rated into the mafic magmas (Sánchez Martínez
et al.
, 2011; Arenas
et al.
, 2014b). Consequently,
there is no conclusive evidence to link the gen-
eration of the Devonian ophiolites either to the
evolution of the Rheic Ocean or to an intraoc-
eanic subduction zone active in a mature ocean
basin. If the connection between the Variscan su-
ture of NW Iberia and the evolution of the Rheic
Ocean is called into question, so must be the in-
terpretation of the Upper Units as a peri-Gond-
wanan terrane that drifted away during the
opening of this Paleozoic ocean. Moreover, prob-
lems also exist in attributing the development of
HP–UHP metamorphism in the trailing edge of
a rather small terrane to its collision with Lau-
russia. In this regard, deep seating of continental
crust is usually associated with subduction of the
thinned margin of a large continent prior to its
collision with another large continent (Warren
et
al.
, 2008; Beaumont
et al.
, 2009).
The age of the HP-UHP metamorphic event in
the Upper Units (no younger than 400-390 Ma)
is close to that of the mafic rocks of the Upper
Ophiolitic Units (
c.
400-395 Ma). Yet, the U-Pb
geochronology provides the age of the HT zircon
growth, which probably occurred sometime af-
ter the continental margin sank into the mantle.
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 ex-
humation/decompression path after subduction
(Fernández-Suárez
et al.
, 2007). Consequently,
this continental subduction predates the gen-
eration of the Devonian mafic rocks (Sánchez
Martínez
et al.
, 2007a; Martínez Catalán
et al.
,
2009). Considering the new Lu-Hf isotope geo-
chemistry of the Devonian ophiolites (400-395
Ma) and the detailed U-Pb geochronology of the
two HP metamorphic events (>400 Ma and ~370
Ma), Arenas
et al.
(2014a) conceived a model of
two successive collision events between Gondwa-
na and Laurussia, each of them taking place in a
context of oblique convergence and separated by
the opening of a rather wide oceanic basin, prob-
ably of pull-apart type (Fig. 29).
According to the latter model, the Upper Units
became the most external part of the Gondwanan
margin, a rather wide continental shelf contain-
ing thick turbiditic series intruded by large mas-
sifs of gabbros and granitoids. That lithological
ensemble resulted from volcanic arc activity in
54
3. GEOLOGICAL FRAMEWORK