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es, with compositions ranging from pelitic to
greywackic, intruded by small massifs of gabbros
and granitoids (Castiñeiras, 2005). The metasedi-
mentary rocks still preserve primary sedimentary
features, which indicate deposition in a turbiditic
system. A maximum depositional age of
c.
510
Ma has been obtained for the Cariño paragneiss-
es using U-Pb dating of detrital zircons, with
clear North African provenance (Albert
et al.
,
2015). Both the turbiditic nature and the maxi-
mum depositional age suggest equivalence to the
greywackic series of the Betanzos Unit. Howev-
er, Nd model ages are much older in the Cariño
Unit, with an average of
c.
1730 Ma, which indi-
cates limited contribution of juvenile sources and
also a heterogeneous isotopic character of the IP
Upper Units (Albert
et al.
, 2015).
The lower sectors of the IPUpper Units are rep-
resented by two units with very different consti-
tution, the Monte Castelo and Corredoiras units
(Fig. 4). The Monte Castelo Unit includes a large
massif ofmetagabbros with scarce lens-shaped in-
clusions of migmatitic paragneisses (Díaz García,
1990). The Monte Castelo Gabbro Massif (
c.
150
km
2
) is constituted by fine to medium grained
metagabbronorites including types with olivine,
amphibole or biotite, with variable textures from
granular to intergranular and ophitic. Most of the
gabbro massif is not deformed and shows a rela-
tively homogeneous chemical composition char-
acteristic of island-arc tholeiites (Andonaegui
et
al.
, 2002). The Monte Castelo Gabbro has been
dated at 499 ± 2 Ma (U-Pb in zircon; Abati
et al.
,
1999). The Corredoiras Unit is formed by a large
massif of granitic orthogneisses that includes
two stocks of gabbro and some large inclusions
of migmatitic paragneisses (González Cuadra,
2007). The orthogneiss massif is mainly com-
posed of granodioritic orthogneisses, with small
bodies of tonalitic orthogneisses, scarce amphi-
bole-rich orthogneisses and metagabbronorites.
The granodioritic and tonalitic orthogneisses
have compositions characteristic of I-type granit-
oids. In some cases, the metagabbronorites show
chemical contamination by interaction with fel-
sic magma. All the igneous lithologies included
in the Corredoiras Massif have chemical compo-
sitions typical of magmas generated in magmatic
arcs (Andonaegui
et al.
, 2012) (Fig. 18a-f). On
the other hand, Nd model ages (T
DM
= 949-1578
Ma) fall within the range defined by the Betanzos
and Cariño units (Fig. 18g). The granodioritic or-
thogneisses of the Corredoiras Massif were dated
at 492 ± 3 Ma (U-Pb in zircon; Andonaegui
et
al.
, 2012).
Tectonothermal evolution: Arc–related event
and collisional reworking
The highest levels of the IP Upper Units, rep-
resented by the turbiditic series of the Betanzos
Unit, are affected by chlorite-zone low-grade
metamorphism. They contain a single foliation
(S
1
), axial planar to West vergent D
1
folds with re-
verse limbs less than 2 km in wavelength (Matte
and Capdevila, 1978; Díaz García
et al.
, 2010).
This regional foliation is intersected at high angle
by diabase dykes, one of which was dated at
c.
510
Ma (Díaz García
et al.
, 2010). Therefore, both D
1
and the greenschist facies metamorphism seem
to predate the Middle Cambrian. They are prob-
ably related to the dynamics of a peri-Gondwa-
nan magmatic arc, also the setting for the proto-
liths of the Upper Units (Díaz García
et al.
, 2010;
Fuenlabrada
et al.
, 2010).
A second regional foliation (S
2
) exists from the
base of the Betanzos Unit down, formed during
a second syn-metamorphic deformation phase
(D
2
). This schistosity is axial planar to minor
folds and contains a stretching lineation trending
N-S, and shear sense indicators pointing top-to-
the-North. S
2
increases down structure becom-
ing the dominant tectonic fabric until the bot-
tom of the IP Upper Units, with accompanying
progressive increase of P-T conditions. In the O
Pino Unit, metamorphism reached the amphi-
bolite facies, with regional development of Ky-
Sill-bearing mineral assemblages and widespread
migmatization over the lowest structural levels.
The schists and paragneisses SE of Santiago city
contain large idiomorphic crystals of andalusite,
also abundant within quartz veins, which ap-
pear invariably pseudomorphosed by light blue
kyanite, grown in a prograde counter-clockwise
P-T path (Castiñeiras, 2005). S
2
has been dated
at
c.
493-496 Ma, in migmatites from the base of
the O Pino Unit (U-Pb in monazites; Abati
et al.
,
1999; Abati, 2002).
The Monte Castelo Gabbro Massif contains
hectometric metapelitic xenoliths affected by
high-T recrystallization during their incorpora-
tion to the gabbro body at
c.
500-505 Ma (U-Pb
43
3. GEOLOGICAL FRAMEWORK