26 / 352
Upper Units). The Upper Units are considered
as a single and coherent terrane because of their
systematic position above the Ophiolitic Units.
In the first descriptions of the allochthonous
complexes these units were assigned to a common
tectonic setting and interpreted as sections of a
back-arc, a fore-arc, or the magmatic arc itself
(Arenas et al., 1986). This interpretation has not
changed to this day, as many subsequent works
have interpreted the origin of the Upper Units
in the context of a peri-Gondwanan magmatic
arc. The polymetamorphic nature of this terrane
has also been demonstrated, first it was affected
by a tectonothermal event related to the activity
of the magmatic arc, and then by a collisional
tectonothermal event present in the Upper Units,
but with different characteristics depending on
the intensity of deformation and metamorphism.
Intermediate Pressure Upper Units:
In the Cabo Ortegal Complex, the IP Upper
Units are represented by the Cariño Gneiss
Unit. It is formed by schists and paragneisses,
with compositions ranging from pelitic to
greywackic, intruded by small massifs of
gabbros and granitoids (Castiñeiras, 2005). The
metasedimentary rocks still preserve primary
sedimentary features, which indicate deposition
in a turbiditic system (Vogel, 1967). A maximum
depositional age of c. 510 Ma has been obtained
for the Cariño paragneisses using U-Pb dating
of detrital zircons, with a clear North African
provenance (Albert et al., 2015).
High Pressure and High Temperature Upper
Units:
The lower part of the Upper Units consists
of a HP-HT metamorphic belt. The protoliths
are similar to those described in the IP Upper
Units, but the intensity of deformation and
metamorphism hinders the preservation of
primary sedimentary or igneous features, which
in only a few cases can be recognised. The main
deformation and metamorphic events are also
comparable, regardless of their intensity, as
well as the isotopic geochronology obtained for
them. The second metamorphic event is the one
showing HP-HT characteristics. The tectonic
fabrics and mineral assemblages of the first
Cambrian, arc-related, metamorphic event were
in this case affected by strong reworking, and
they have been almost completely obliterated.
However, the initial existence of this first event
can still be detected, mainly due to the presence
of relict monazites in high-grade gneisses whose
age has been determined by U-Pb.
Equivalent HP-HT units are known to
be present in the Variscan belt outlining the
Variscan suture, both in the Armorican Massif,
French Massif Central and the Bohemian Massif
(Ballèvre et al., 2014). In some sectors the presence
of mineral assemblages including coesite suggests
that ultra-high-P conditions were reached at least
locally (Lardeaux et al., 2001). Peak pressures
calculated so far for the NW Iberian Massif are
close to ultra-high-P values, although evidence
of coesite is still missing.
The diversity and tectonothermal evolution
of the HP-HT rocks of the Cabo Ortegal
Complex were described in great detail in the
pioneering work of Vogel (1967), and later by
Engels (1972) and Gil Ibarguchi et al. (1990).
In this complex, the HP-HT Upper Units
include two main juxtaposed counterparts, the
Cedeira and Capelada units. Both units have a
similar lithological composition but different
record of high-P metamorphism. The Capelada
Unit occupies the upper structural level, and
developed eclogite facies metamorphism,
whereas the Cedeira Unit reached only granulite
facies conditions.
The eclogitic gneisses of the Capelada
Unit (Banded Gneisses) are mainly derived
from semipelitic and greywackic sediments,
interspersed with granitic and tonalitic
orthogneisses (Albert et al., 2012). These gneisses
are usually migmatised and show a mylonitic
foliation. Mafic rocks occur as numerous eclogite
pods or bands within the gneisses and also form
a thick layer of eclogite, separating this ensemble
from the mafic and ultramafic rocks underneath.
In some sectors of this layer, pre-metamorphic
mingling processes are still recognizable, as well
as some lenses of
augen
gneiss similar to those
observed in the IP Upper Units. The U-Pb dating
of detrital zircons from the paragneisses reported
similar age populations to those obtained in the
Cariño Unit (IP Upper Units). Therefore, it is
considered that the source area for this sequence
was also located in North Africa (Albert et al.,
2015).
10
3. GEOLOGICAL FRAMEWORK