66 / 352
in the pyroxenite rocks yielded an age of
c.
500
Ma, interpreted as the chronology of the proto-
liths (Santos
et al.
, 2002). These data confirm the
similar age of this ultramafic section relative to
the rest of the formations included in the Cape-
lada Unit.
The Cedeira Unit has a similar thickness to the
Capelada Unit and it is constituted by migmatit-
ic gneisses (Chimparra Gneisses) and metabasic
rocks, which include high-P granulites, high-T
amphibolites and coronitic metagabbros (Can-
dieira Formation) (Fig. 3). The gneissic rocks
have comparable composition to the upper
gneisses of the Capelada Unit, but very scarce
mafic inclusions exist that in this case have min-
eral assemblages typical of high-P granulites (Vo-
gel, 1967). Coronitic gabbros similar to those of
the Sobrado Unit of the Órdenes Complex are
locally abundant. The basal part of this unit is
a complex tectonic boundary, locally known as
the Carreiro Shear Zone, which separates the
HP-HT Upper Units from the Purrido Ophiolite
(Vogel, 1967) (Fig. 3). This shear zone consists
of an imbrication of paragneisses and ultramaf-
ic rocks (Azcárraga, 2000). The latter are mylo-
nitic garnet-bearing harzburgites and Ti-clino-
humite-bearing orthopyroxenites, with mineral
assemblages equilibrated at high to ultra-high-P
(Gil Ibarguchi
et al.
, 1999).
The most relevant structural feature in the HP-
HT Upper Units is the presence of a widespread
mylonitic foliation (Figs. 21b and 24) (Marcos
et al.
, 1984). This mylonitic foliation S
2
reworks
and usually obliterates previous tectonic fabrics
and mineral assemblages generated during the
high-P event (D
1
). Any previous tectonothermal
record associated with the Cambrian history of
these units has been almost entirely overprint-
ed. During the development of S
2
, the Bacariza
Formation and its equivalents in the Sobrado
Unit, as well as all the high-P gneisses, were af-
fected by intense hydration and migmatization.
P-T paths contemporary to S
2
depict a drastic
isothermal exhumation. D
1
mineral assemblages
with Grt+Cpx+Zo+Rt were replaced by more hy-
drated phases, such as Grt+Hbl+Czo+Spn (Vo-
gel, 1967; Gil Ibarguchi
et al.
, 1990; Arenas, 1991;
Mendia, 1996). The stretching lineation associ-
ated with S
2
trends NNE-SSW and D
2
kinematic
indicators consistently indicate top-to-the-North
sense of shearing (Ábalos
et al.
, 1994, 1996). Yet,
S
1
prevails in some eclogites and granulites of the
Cabo Ortegal Complex and presents, relative to
S
2
, similar kinematics and stretching lineation
trend (Ábalos, 1997; Ábalos
et al.
, 2003). In the
same complex, S
2
is affected by a train of large
recumbent folds vergent to the East (Fig. 25). The
largest of these folds is the recumbent synform
of the Capelada Unit, first described by Mar-
cos
et al.
(1984) (Fig. 3). There are other similar
folds related to the same generation, as the large
recumbent antiform which inverted the posi-
tion of the Cariño gneisses below the Capelada
Unit (Fig. 25). These large folds do not show in
general penetrative axial planar foliation, which
suggests limited contribution of compressional
forces during its nucleation and amplification.
An exception exists in the surrounding areas of
the hinge zone of the large recumbent synform
of the Capelada Unit, where the development
of a slightly pervasive axial planar S
2
can be ob-
served in the gneisses of the Cariño Unit. This
train of recumbent folds developed during the
exhumation of a high-ultra-high-P continental
subduction complex, most probably as a result of
thrusting over the underlying units (Albert
et al.
,
2012) (Fig. 25).
Chronology of the metamorphic events
The gneissic rocks of the HP-HT Upper Units
contain monazite dated at
c.
500-480 Ma, both in
the Cabo Ortegal Complex (Chimparra Gneiss-
es) and the Órdenes Complex (Belmil and Sob-
rado gneisses) (Fernández-Suárez
et al.
, 2002).
No other evidence for such Cambrian tectono-
thermal activity has been found so far, as the HP-
HT event seems to have reworked any previous
tectonic fabrics and mineral assemblages. The
age of this event coincides with the first tectono-
thermal event described for the IP Upper Units,
so they both can be integrated in the dynamics
of a peri-Gondwanan magmatic arc during the
Cambrian.
The second HP-HT event has been dated in a
variety of rocks. A first age of
c.
390 Ma obtained
in the large eclogite layer of the Cabo Ortegal
Complex was interpreted as dating the high-P
metamorphism (SHRIMP, U-Pb in zircon; Or-
dóñez Casado
et al.
, 2001). The same method-
ology was used later to constrain the age of the
high-P event in a mafic granulite of the Sobrado
50
3. GEOLOGICAL FRAMEWORK