41 / 352
Altogether, the lithologies observed in these two
units seem to match those of the Lower Sequence.
These units reached the higher metamorphic
conditions in the Basal Units. They are currently
interpreted as a part of the Lower Sequence sub-
ducted to the greatest depths and subsequently
thrusted on top of this terrane.
The precise age of the sedimentary proto-
liths involved in the two lithological sequences
is unknown, even though in the Malpica-Tui
Complex they still preserve some palinological
record (Fombella Blanco, 1984). Using U-Pb
geochronology of detrital zircons, maximum
depositional ages of Late Neoproterozoic (
c.
560
Ma) and Late Cambrian to Ordovician (
c.
500-
480 Ma) were obtained for the Lower and Upper
sequences respectively (Díez Fernández
et al.
,
2010, 2013). Given that it is intruded by Cambri-
an granitoids (
c.
493 Ma), the depositional age of
the Lower Sequence can be further constrained
to 493-560 Ma.
According to geochemical discriminant di-
agrams for tectonic setting (Bathia and Crook,
1986), using immobile trace elements (Th-Co-
Zr-Sc-La), the metagreywackes of the Lower Se-
quence have chemical compositions which indi-
cate deposition in an active magmatic arc (Fig.
6c). This arc was formed in the periphery of a
continent, probably above thinned continental
crust. The calc-alkaline magmatism recorded in
the Basal Units would be also connected to the
activity of this arc system. However, the compo-
sitions of the younger pelitic-semipelitic schists
of the Upper Sequence are more typical of sed-
imentary rocks deposited in a passive margin
setting. In relation to PAAS (post-Archean Aus-
tralian Shale), the metasedimentary rocks of the
Basal Units show
c.
1 values in some significant
LILE elements (Rb, Th, Ce, K
2
O) and negative
anomalies in U, Sr, Hf and TiO
2
(Fuenlabrada
et
al.
, 2012). On the other hand, Nd model ages of
the metasedimentary rocks in both sequences are
very old in the context of the Iberian Massif, as
they range between 1743 and 2223 Ma (Fig. 6d).
These ages suggest that the original sedimenta-
ry basin was located near a cratonic area with
dominant Paleoproterozoic and Archean iso-
topic sources. Considering the age populations
obtained in detrital zircons from these metased-
imentary rocks (Díez Fernández
et al.
, 2010), the
whole dataset is compatible with a paleolocation
in the periphery of the West African Craton.
An idealized scheme for the Late Neoprotero-
zoic-Middle-Late Cambrian evolution of the
peri-Gondwanan magmatic arcs, including the
most probable location of the sedimentary series
of the Basal Units, is shown in Fig. 7. Cessation
of activity in this arc system occurred in a context
of extension affecting the margin of Gondwana,
coevally with the rifting and drifting of Avalonia
and possibly other smaller terranes of the Vari-
scan Belt. The intrusion of dyke swarms repre-
sented by the mafic rocks of the Basal Units, as
well as the later intrusion of alkaline to peralka-
line granitoids, would be probably related to this
event.
High pressure metamorphism
The Basal Units represent a long high-P belt
involved in the Variscan Orogen, in turn a typical
collisional belt. Considering the crustal affinity
and provenance of this terrane, the most proba-
ble setting for the origin of the high-P event en-
tails deep subduction of the Gondwanan margin
during the Variscan cycle.
Two different metamorphic groups can be dis-
tinguished in the Basal Units according to the
characteristics of the high-P metamorphism. A
Lower Metamorphic Group (LMG) constituted
by the Malpica-Tui, Santiago, Lalín, Forcarei,
Ceán, Lamas de Abade and Cercio units shows
hig-P and low to intermediate-T metamorphism
with variable intensity. However, an Upper Meta-
morphic Group (UMG) formed by the Agualada
and Espasante units was affected by high-P and
intermediate to high-Tmetamorphism(Martínez
Catalán
et al.
, 1996). The LMG contains abun-
dant high-P metapelites (garnet-phengite-chlo-
rite-chloritoid-quartz-albite-clinozoisite-ru-
tile±glaucophane), blueschists, lawsonite-bear-
ing blueschists, common eclogites, eclogites with
phengite and glaucophane and jadeite-bearing
orthogneisses (Van der Wegen, 1978; Gil Ibargu-
chi yOrtega Gironés, 1985; Arenas
et al.
, 1995; Gil
Ibarguchi, 1995, Rubio Pascual
et al.
, 2002; Ro-
dríguez Aller, 2005; López Carmona
et al.
, 2010,
2013, 2014) (Fig. 8). The UMG features a variety
of eclogites, including types with well developed
honeycomb pattern textures (Fig. 8), along with
high-P orthogneisses and paragneisses, frequent-
25
3. GEOLOGICAL FRAMEWORK