68 / 352
by Arenas (1988) and Marcos
et al.
(2002). The
mélange was revisited by Arenas
et al.
(2009),
who provided a detailed mapping, compositional
data of igneous rocks and some U-Pb ages. This
unit, however, shows unique features at the scale
of the allochthonous complexes, the interpreta-
tion of its origin and meaning being a real chal-
lenge.
The Somozas Mélange is
c.
1800 m thick, dips
gently to the West and intersects the contact be-
tween the Basal and Ophiolitic units (Espasante
and Moeche units, respectively; Figs. 25 and 26).
The upper contact of the mélange is therefore
an out-of-sequence thrust that cuts part of the
allochthonous pile located above. The mélange
unit has two members with different lithologies
and meaning (Fig. 26). The upper member is a
c.
800 m thick serpentinite mélange, consisting of
a highly sheared serpentinite matrix that wraps
around tectonic blocks with variable thickness,
from metric to kilometric (Fig. 26). These blocks
are made up of gabbros, diabases, granitoids,
metabasalts, andesitic basalts, pillow breccias,
pillow lavas, hyaloclastites, marbles, phyllites,
sandstones and conglomerates, plus other high-T
metamorphic blocks of orthogneisses and am-
phibolites, some of which contain zoisite and
rutile. Many of these tectonic blocks are exotic,
as they are unknown in the rest of the allochtho-
nous complexes and Parautochthon. The low-
er member is a
c.
1000 m thick mélange with a
matrix formed by ocher-colored phyllites or blue
phyllonites, and tectonic blocks of the litholo-
gies involved in the upper serpentinite mélange
(Fig. 26). No evident high-P rocks have been de-
scribed so far in the Somozas Mélange, though
some of the high-T tectonic blocks reached at
least the higher pressure part of the amphibolite
facies (Arenas, 1988).
U-Pb zircon dating was performed in a to-
nalitic orthogneiss from a large high-T tectonic
block and two slightly deformed granitoids af-
fected by low grade recrystallization (Arenas
et
al.
, 2009). The high-T tonalitic orthogneiss has
an age of
c.
485 Ma (U-Pb in zircon, SHRIMP),
while the two granitoids yielded ages of
c.
499 Ma
(U-Pb in zircon, SHRIMP) and
c.
527 Ma (U-Pb
in zircon, LA-ICP-MS). The composition of the
igneous rocks involved in the mélange, either
volcanic, plutonic or dykes, shows clear affinity
with a volcanic arc, a setting also supported by
an ubiquitous negative Nb-anomaly (Fig. 12)
(Arenas
et al.
, 2009). On the other hand, U-Pb
ages of detrital zircons extracted from a tectonic
block of conglomerate indicate an Early-Middle
Ordovician maximum deposition age and deri-
vation from the West African Craton (Arenas
et
al.
, 2009).
The U-Pb ages along with the whole rock
geochemical data set favors the hypothesis that
the Somozas Mélange contains material derived
from a peri-Gondwanan magmatic arc. Although
the age of the mélange is unknown, the structural
position of this unit suggests a relationship with
the subduction and accretionary processes that
affected the Basal Units. The development of a
serpentinite mélange such as the upper member
of this unit implies the existence of a long-last-
ing subduction process, capable of generating
a low-viscosity channel in the overlying mantle
wedge (Gerya
et al.
, 2002; Hebert
et al.
, 2009).
The lower member was developed later, proba-
bly during the imbrication of the upper member
with the upper part of the Parautochthon (Fig.
26). Therefore, the age of the mélange should
be bracketed between the age of the subduction
of the Basal Units (
c.
370 Ma; Abati
et al.
, 2010)
and the age of its basal imbrication. The age of
the latter is unknown, but it should be similar or
slighty younger than the age of the basal thrust
that moved the Parautochthon and the rest of the
allochthonous complexes eastwards (
c.
343 Ma;
Dallmeyer
et al.
, 1997).
Provenance of the continental terranes
The provenance of the continental terranes in-
cluded in the allochthonous complexes has been
somewhat enigmatic until recent times. The Vari-
scan Belt was formed by the collision of two large
continental masses, Gondwana and Laurussia,
and this scenario leads to two different possibili-
ties for the provenance of the terranes located in
the most internal part of the belt. Moreover, it is
also believed that other suspect terranes located
in an intermediate paleogeographic position (
e.g.
microcontinents or volcanic arcs) could have
been involved in the orogen too, thus introduc-
ing further elements to be taken into account
when considering the origin of the allochthonous
52
3. GEOLOGICAL FRAMEWORK