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3. GEOLOGICAL FRAMEWORK
Fig. 27.
Cartoon showing the paleoposition of the continental terranes exposed in the NW of the Iberian Massif.
terranes.
The provenance of the Upper Units was ini-
tially ascribed to a section of the Gondwanan
margin surrounding the West Africa Craton
(WAC), based on U-Pb data of detrital zircons
from the uppermost greywackic series of the
Betanzos Unit, in the Órdenes Complex (Fernán-
dez-Suárez
et al.
, 2003). Additional U-Pb and Hf
isotopes in zircon from the metagreywackes of
the Cariño Unit (Cabo Ortegal Complex) have
provided firmer grounds to this interpretation.
They contain zircon populations with Archean,
Eburnean and Cadomian ages, as well as scarce
and scattered Mesoproterozoic grains unlikely to
represent a major crust generation pulse in the
source area of the siliciclastic unit (Albert
et al.
,
2015). Moreover, considering the Sm-Nd iso-
topic signature and abundant input of juvenile
material (Fuenlabrada
et al.
, 2010), it can be con-
cluded that the source area for the Upper Units
was also located in the proximity of a volcanic arc
(Fig. 27).
In the Basal Units, the U-Pb data of detri-
tal zircons from the metagreywackes are also
consistent with source areas in the periphery of
the WAC. However, their higher content in Ste-
nian-Tonian zircons was considered indicative of
a more easterly provenance,
i.e.
closer to the Sa-
hara Craton (Díez Fernández
et al.
, 2010). On the
other hand, Nd model ages obtained in the grey-
wackes of the Basal Units fall in the range 1743-
2223 Ma (Fuenlabrada
et al.
, 2012), suggesting
dominant WAC isotopic sources, but located far
away from any source of juvenile material such as
peri-Gondwanan arc system (Fig. 27).
The source area for the Parautochthon and
autochthonous domains of the Iberian Massif
was probably located further to the East, as they
are characterized by higher Stenian-Tonian zir-
con populations (Fig. 27) (Díez Fernández
et al.
,
2012a; Fernández-Suárez
et al.
, 2014; Shaw
et al.
,
2014).
Paleogeography and Variscan evolution
Previous models dealing with the evolution of
the Upper Units considered them as a section of
a peri-Gondwanan volcanic arc rifted from the
continental margin in Cambrian–Early Ordovi-
cian times, and then drifted northward, open-