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5. PROVENANCE OF THE UPPER ALLOCHTHON
5.2. Partial conclusions
The protoliths of the metasedimentary rocks
of the Cariño Gneiss formation are formed by
siliciclastic series that represent the deposition
of greywackes by turbidity currents at
c.
510
Ma. The siliciclastic series were deposited in a
peripheral back-arc basin, somewhere between
an active volcanic arc and the Northern margin
of the West Africa Craton (WAC). In this
tectonic setting, detritus arrived from the active
part of the volcanic arc and from the mainland.
These metasedimentary rocks record Archean,
Eburnean, and late or post-Cadomian crust
formation events, with a 3.6% Mesoproterozoic
zircon input. In the source region of the gneiss
formation, the Archean crust formation event
is roughly dated at
c.
3.3–2.9 Ga, which is
represented by zircons with
c.
3.0–2.2 Ga
crystallization ages. These Archean materials
suffered important crustal reworking processes,
supporting an intracratonic or an active margin
setting. The recorded Eburnean events represent
the intrusion of juvenile Eburnean depleted-
mantle derived magmas into an Archean
basement, most probably the Archean basement
represented by the Cariño Gneiss Archean zircon
population. The most probable geodynamic
setting is a volcanic arc at the margin of a
pre-existing Archean craton. Archean and
Paleoproterozoic (Eburnean) zircon populations
are most probably derived from the WAC. The
Cambrian events recorded in the Cariño Gneisses
represent a magmatic arc at the periphery of
the North Gondwana margin, specifically the
North WAC. This arc records the intrusion of
c.
525 Ma juvenile DM-derived magmas into the
Eburnean and Archean basement of Gondwana.
The first stages of the arc were probably linked
to the Cadomian arc system, defined in other
parts of Europe as to have an activity bracketed
between 750 Ma and 540 Ma, and so it could
represent a long lasting volcanic-magmatic arc
system. Taking into account the Nd model ages
published from other metagreywacke series
included in the NW Iberian Upper Allochthon,
it can be concluded that this terrane contains
heterogeneous isotopic sources. These varied
sources indicate that different parts of the peri-
Gondwanan arc system may be preserved in the
Upper Allochthon.
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