46 / 352
al.
2004; Kryza and Pin 2010). These ophiolites
have been considered as related to the evolution
of the Rheic Ocean, being originated during stag-
es shortly before its closure (Díaz García
et al.
,
1999a; Sánchez Martínez
et al.
, 2007a). The Low-
er Ophiolitic Units include mafic-ultramafic se-
quences with Cambrian age and were developed
in different geodynamic contexts, either during
the opening of the Rheic Ocean (Arenas
et al.
,
2007b), or in previous stages to the opening of
this large Paleozoic ocean (Sánchez Martínez
et al.
, 2012). Five ophiolitic units have been de-
scribed in the NW of the Iberian Massif, Gali-
cia. The Upper Ophiolitic Units are represented
by the Careón, Purrido and Moeche ophiolites,
while the Lower Ophiolitic Units are formed by
the Vila de Cruces and Bazar ophiolites. Other
additional ophiolitic units, with similar chronol-
ogy and meaning, appear in the nearby region
of Trás-os-Montes, Portugal (Pin
et al.
, 2006). In
the following, the Galician ophiolites will be de-
scribed separately, as they differ from each other
in lithological composition and tectonothermal
evolution.
Upper Ophiolitic Units
Careón Ophiolite
This is the ophiolitic unit with the best pre-
served metaigneous succession in the NW of
the Iberian Massif. It is located in the East of
the Órdenes Complex (Fig. 4) and is made up
of three imbricated slices that repeat an oceanic
crust-mantle transition zone, with a total thick-
ness of
c.
1500 m (Fig. 11). The thicker slice is
the Careón Sheet, constituted by
c.
1000 m of
serpentinized ultramafic rocks and isotropic
metagabbros, with abundant stocks of pegmatoid
gabbros and late pegmatoid gabbros emplaced at
all levels of the succession, and scarce wehrlite
sills. Diabase and late diabase dykes also appear
in all the exposed levels, from the deepest mantle
sections to the shallower crustal levels (Fig. 11).
The Careón Ophiolite shows a different litholog-
ical constitution to the HOT and LOT ophiolitic
types described by Nicolas (1989, 1995) (Fig.
11). Therefore, it was interpreted as an ophiolite
generated in a supra-subduction zone setting by
Díaz García
et al.
(1999a). The intrusion of abun-
dant diabase dykes in all the levels of the Careón
Sheet is an additional argument for the genera-
tion of this ophiolite in a context of pronounced
extension and thinning, above a subduction
zone. Using immobile trace element (Hf, Th, Ta)
and the diagram of Wood (1980) (Fig. 12a), the
gabbros show compositions of typical island-arc
tholeiitites, but the younger diabase dykes have
transitional compositions to N-MORB. All the
studied lithologies show a negative Nb-anomaly
in relation to N-MORB, which is consistent with
the generation of this ophiolite in an active sub-
duction zone setting (Sánchez Martínez
et al.
,
2007a) (Fig. 12a).
The Careón Ophiolite was the first ophiolite
dated in the NW of the Iberian Massif. A proto-
lith age of
c.
395 Ma was obtained from a sample
of pegmatoid gabbro of the Careón Sheet (U-
Pb zircon; Díaz García
et al.
, 1999a). The same
U-Pb protolith age was reported later for a sim-
ilar gabbro of the same slice by Pin
et al.
(2002).
The three tectonic slices of the Careón Ophiolite
show a pervasive foliation, which developed at
higher temperature towards the upper part of
each slice. In the contact zone with the overlying
ultramafic rocks, the gabbros are transformed in
garnet-bearing amphibolites. High temperature,
corundum-bearing metamorphic soles with a
thickness of
c.
2 m can be locally developed (Fig.
11). P-T conditions reached during imbrication
of the ophiolitic slices were estimated at c 11.5
kbar and 650 °C in the garnet amphibolites from
the uppermost part of the Careón Sheet (Fig.
11). The stretching lineation of the amphibolites
shows a consistent E-W trending, and shear sense
indicators in the main fabric point to top-to-the
East kinematics in present coordinates (Gómez
Barreiro
et al.
, 2010b). The imbrication of the
ophiolitic slices occurred at
c.
377 Ma, according
to the
40
Ar/
39
Ar dating of hornblende from the
amphibolitic foliation (Dallmeyer
et al.
, 1997).
The Careón Ophiolite was originally interpret-
ed as a supra-subduction zone ophiolite gener-
ated in an intra-Rheic Ocean subduction zone
(Díaz García
et al.
, 1999a; Sánchez Martínez
et
al.
, 2007a). (Fig. 13). This subduction zone would
have consumed almost all the old and dense lith-
osphere of the Rheic Ocean, whose common ba-
satic types with N-MORB composition are yet to
be found in the Variscan Belt. The youngest oce-
anic lithosphere generated in this intra-oceanic
subduction zone at
c.
395 Ma was accreted below
30
3. GEOLOGICAL FRAMEWORK