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5. PROVENANCE OF THE UPPER ALLOCHTHON
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2010). Both studies focussed on the siliciclastic series located at the up-
permost structuralposition of theÓrdenes Complex whichis situated to
theWestofthe Cabo OrtegalComplex (Fig. 1b–c), where the mentioned
series are not represented. According to the data in the works
mentioned above, this terrane has been considered as WAC derived,
but the young Nd model ages obtained from the metagreywackes
(c. 1 Ga) are inconsistent with the old isotopic sources expected from
the WAC. This observation requires additional explanations. Also, new
and integrated U–Pb/Lu –Hf (zircon) and Sm–Nd (whole rock) data
are needed to present a complete picture for the provenance of the
upper units, as the previously studied greywacke series are only
representative for the uppermost stratigraphic levels of this thick
terrane (c. 10,000–12,000 m).
This contribution presents a provenance study of the upper units
combining U–Pb and Lu–Hf in zircon (LA-SF-ICPMS) and Sm–Nd
whole rock analyses. The rocks studied belong to the Cariño Gneiss
formation, which is the top member of the Upper Allochthon of the
Cabo Ortegal Complex, different to and occupying a lower position
than the metagreywackes previously studied in the Órdenes Complex.
This will allow a straightforward comparison and a better understand-
ing of the origin of this terrane.
2. Geological framework of the Upper Allochthon in NW Iberia
The NW sector of the Iberian Massif includes an autochthonous do-
main, represented by the Cantabrian (CZ), West-Asturian-Leonese
(WALZ) and Central Iberian (CIZ, Fig. 1a) zones. This domain is overlain
by an allochthonous domain which is represented by the Galicia-Trás-
os-Montes Zone (GTOMZ). In the GTOMZ a lower assemblage is consid-
ered as the Lower Allochthon or Parautochthon. Above it, a group of five
allochthonous complexes include the remnants of the Variscan suture
which can be followed throughout Europe, from the Iberian to the
Bohemian Massif (Fig. 1a; Martínez Catalán et al., 2009). These alloch-
thonous complexes have an intricate internal structure, but they appear
clearly constituted by three terranes with contrasting origin, structure
and tectonothermal evolution. The complexes are nappe piles
structured as late Variscan synforms, and they are divided upwards as
basal, ophiolitic and upper units.
The basal units represent a crustal terrane formed by the subduction
of the Gondwanan continental margin below Laurrusia at c. 370 Ma (Gil
Ibarguchiand Ortega Gironés,1985;Arenas et al., 1995; Rodríguez etal.,
2003; Abati et al., 2010b; Díez Fernández et al., 2011; López-Carmona
et al., 2014). This terrane contains metasedimentary rock series with
Ediacaran to Early Ordovician maximum depositional ages (Díez
Fernández et al., 2010; 2012b), intruded by abundant calc-alkaline
(c. 493 Ma; Abati et al., 2010b) to alkaline–peralkaline (c. 475–
470 Ma; Díez Fernández et al., 2012a ) granitic bodies.
The ophiolitic units are formed by mafic–ultramafic rock series that
have been the object of recent works. They represent a varied
assemblage with units formed in different stages in the evolution of
the Paleozoic oceans. Part of these ophiolites have been interpreted as
Cambrian remnants of the peri-Gondwanan oceans, including se-
quences accreted at the base of a fore-arc (Sánchez Martínez et al.,
2012) and back-arc sequences (Arenas et al., 2007). However the
most extended ophiolitic unitsin NW Iberia,and alsoalongthe Variscan
suture in the rest of Europe, are c. 395 Ma mafic–ultramafic rock series.
The interpretation of these units has changed over time. They were first
considered supra-subduction zone ophiolites formed during the last
stages of the closure of the Rheic Ocean (Díaz García et al., 1999;
Sánchez Martínez et al., 2007). However these ophiolites, according to
new U–Pb/Lu–Hf zircon data, have been recently considered to be relat-
ed to the opening of a long pull-apart basin, which show an interaction
of the mafic rocks with an old continental basement ( Sánchez Martínez
et al., 2011; Arenas et al., 2013, 2014).
The upper units or Upper Allochthon constitutes a terrane with
continental-crust affinity which can be divided into high pressure–
high temperature (HP–HT) units below and intermediate pressure
(IP) units above. The HP–HT base of this terrane is mainly composed
of ultramafic rocks, basic, intermediate and acid granulites, eclogites,
orthogneisses and paragneisses (Vogel, 1967; Gil Ibarguchi et al.,
1990; Peucat et al., 1990; Ábalos et al., 2003). The protolith ages for
the igneous rocks are clustered around c. 490 Ma and the age of the
HP–HT metamorphism is c. 400–390 Ma (Ordóñez Casado et al., 2001;
Fernández-Suárez et al., 2007). The uppermost section of the Upper
Allochthon, the IP upper units, is composed of a thick siliciclastic series
with greywacke and pelitic members and minor conglomerates (Díaz
García, 1990; Díaz García et al., 2010). In the Órdenes Complex the
greywackes have chemical compositions characteristic of volcanic-arc
settings (Fuenlabrada et al., 2010) and maximum depositional ages in
the range of 530–500 Ma (Fernández-Suárez et al., 2003; Fuenlabrada
et al., 2010). This series is intruded by large massifs of gabbros with
tholeitic arc affinity (Andonaegui et al., 2002) and granitoids with
calc-alkaline compositions (Andonaegui et al., 2012 ). U–Pb dating of
these mafic and felsic metaigneous rocks yielded protolith ages at
c. 500 Ma (Abati et al., 1999; Andonaegui et al., 2012). A doleritic dyke
swarm dated at c. 510 Ma also intrudes into the top greywacke series
(Díaz García et al., 2010). The IP upper units metamorphic grade de-
creases upwards from granulite to greenschistfacies,with characteristic
counter-clockwise P–T paths interpreted as associated to magmatic un-
derplating in a volcanic arc setting (Abati et al., 2003). This arc-related
metamorphic event is dated at c. 496–482 Ma (Abati et al., 1999,
2007). Therefore multiple evidence support the original volcanic-arc
settingfortheupperunitsoftheallochthonouscomplexes.Thisvolcanic
arc was part of the Early Paleozoic peri-Gondwanan arc system. The up-
permost partof the preserved section of this arc seemsto be only slight-
ly affected by the Variscan and eo-Variscan activity, whereas the lower
part was strongly reworked during the Variscan collision developing a
high-P and high-T recrystallization in the Devonian.
Most of the terranes and minor members described in the
allochthonous complexes of NW Iberia are present in the Cabo
Ortegal Complex, a small but well exposed complex on the Northern
Galician coast (Fig. 1a). The upper units are mostly represented by
the HP–HT units (Fig. 1b and c). The IP upper units are only repre-
sented by the Cariño Gneiss formation, which is the uppermost
structural unit of the Cabo Ortegal Complex. It is formed by
metasedimentary rocks in amphibolite facies with peak metamor-
phic conditions at 700 °C and 12 kbar (Castiñeiras, 2005). It is a
homogeneous metaturbiditic sequence of centimetre-scale alterna-
tions of fine-grained pelitic bands with lepidoblastic texture and
medium-grained psamitic bands with granoblastic texture, where
the amount of medium-grained bands seems to decrease upwards
(Castiñeiras, 2005). On the base of their mineralogical composition
the psamitic types are described as quartz- and normal greywackes
and the pelitic bands as normal and quartz-poor greywackes (Vogel,
1967). This compositional banding is an inherited sedimentary
feature representing the deposition of greywackes by turbidity
currents. The limits between sedimentary bands also coincide with
the foliation, which is defined by preferential orientation of the
rock forming minerals. This foliation strikes N-S to c. N45° and
dips up to 75° westwards at the western part of the formation
and eastwards at the eastern part defining a wide and open
antiform (Fig. 2). The dominant regional lineation plunges c. 5° to
30° with a N20°–N55° trend. The formation is folded in a large re-
cumbent synform with subordinated folds with an East vergence.
Its axial plane strikes c. N-S and dips to the West in the western
part of the formation. Associated with this synform an axial plane
foliation developed locally in the fine-grained bands. This large
synform does not only affect the Cariño Gneiss formation but also
all of the upper units of the Cabo Ortegal Complex. The synform
was earlier mapped by Vogel (1967) and Marcos et al. (1984), and
a recent interpretative section has been presented by Arenas et al.
(2013).
1435 R. Albert et al. / Gondwana Research 28 (2015) 1434–1448