38 / 352
been described so far in the NW Iberian Mas-
sif. The contact zone of the Cabo Ortegal Com-
plex with the Parautochthon is broad and shows
strong shearing, as evidenced by extensive devel-
opment of phyllonites in the southern part of the
complex. The basal contact of the Parautochthon
is strongly deformed too, and well exposed in the
coastal section near Loiba village. It is a thrust
that emplaced a thick series of metasedimenta-
ry and metavolcanic rocks, some of which are
Ordovician in age, on top of Silurian fossilifer-
ous ampelites typical of the Central Iberian Zone
(Arenas, 1988; Rodríguez
et al.
, 2004; Valverde
Vaquero
et al.
, 2005).
The Malpica-Tui Complex
The Malpica-Tui Complex occupies the core
of a narrow synform which extends
c.
150 km in
N-S direction (Figs. 2 and 5). This complex only
includes lithologies of the Basal Units, which
show here largest variety in terms of composition
and tectonic fabrics. The Upper Sequence crops
out in the northern sector, in the core of the syn-
form, forming the Ceán Unit (Fig. 2). The rest of
the complex is constituted by lithologies of the
Lower Sequence, also referred to as the Malpi-
ca-Tui Unit (Fig. 2).
The current basal contact of the Malpica-Tui
Unit is an extensional fault, the westward con-
tinuation of the Pico Sacro Detachment (Fig. 2).
This tectonic contact is reworked in strike-slip
shear zones that run parallel to the boundaries
of the complex and are responsible for its elon-
gated shape. The contact between the Upper and
Lower sequences is also an extensional fault that
reaches the Órdenes Comples to the East, the
Bembibre-Cean Detachment (Fig. 2). The Bem-
bibre-Cean Detachment is cut by the Pico Sacro
Detachment (Gómez Barreiro
et al.
, 2010a), and
has been dated at
c.
337 Ma (
40
Ar/
39
Ar on musco-
vite; López Carmona
et al.
, 2014).
Basal Units
They represent a coherent continental terrane
that extends from the Malpica-Tui Complex to
the eastern part of the Órdenes Complex. It can
be also followed discontinuously up to the east-
ern sector of the Cabo Ortegal Complex (Fig. 2).
Lithologies and chemical composition
The Basal Units are constituted by metasedi-
mentary rocks, orthogneisses and less abundant
mafic rocks, all of them grouped in two juxta-
posed lithological sequences, a Lower Sequence
and an Upper Sequence. The largest lithological
variety is found in the Malpica-Tui Complex,
where in addition deformation is heterogeneous,
thus enabling observation of primary protolith
features and reconstruction of the original litho-
stratigraphy (Díez Fernández
et al.
, 2010; Fig. 6).
In the Malpica-Tui Complex, the Lower Se-
quence was originally formed by geywackes ar-
ranged as siliciclastic turbiditic series together
with ampelitic shales, semipelites, and scarcer
limestones and cherts. Variscan deformation and
metamorphism transformed this rock ensem-
ble to a sequence of albite-bearing schists and
paragneisses, graphitic schists, calc-silicate rocks
and metacherts. The metasedimentary rocks are
intruded by a large variety of granitoids, later
transformed into orthogneisses, with composi-
tions ranging between granites, high-K granites,
quartz-syenites, granodiorites and tonalites. The
sequence also contains less abundant mafic rocks,
which now occur as amphibolites, blueschists
and variably retrogressed eclogites (Fig. 6a). The
granitic rocks define two compositional suites
(Floor, 1966; Rodríguez Aller, 2005; Montero
et
al.
, 2009), a dominant suite with calc-alkaline
composition and intruded by the mafic rocks (
c.
493 Ma; Abati
et al.
, 2010), and another younger
suite with alkaline-peralkaline affinity (
c.
475-
470 Ma; Díez Fernández
et al.
, 2012c). The alka-
line-peralkaline granitoids are not intruded by
the mafic rocks, which thence can be interpreted
as a dyke swarm emplaced in between the two
granitic series.
The Lower Sequence described in the Malpi-
ca-Tui Complex can be followed in the Santiago
and Lalín units of the Órdenes Complex (Arenas
et al.
, 1995). Both units are located in the nor-
mal limb of a large recumbent anticlinorium, the
Carrio Anticline (Fig. 2). This sequence contin-
ues into the Forcarei Unit to depict the reverse
limb of that fold (Martínez Catalán
et al.
, 1996).
Minor differences between these three units ex-
ist, for instance the mafic rocks are more abun-
dant in the Lalín Unit and the alkaline-peralka-
line orthogneisses are very rare or absent in the
22
3. GEOLOGICAL FRAMEWORK