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6. PROVENANCE OF THE HP-HT UPPER ALLOCHTHON
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grade metamorphic events (see the horizontal trend
of this population in Fig. 8). There is also a small
data set with crystallization ages of c. 3.0 Ga (eHf( t)
from c. +2.2 to 4.8) pointing to a similar geologi-
cal framework, an intrusion at c. 3.0 Ga and a
reworking process (data with average TDM of
3.5 Ga). In the Northern WAC, Archean igneous
rocks have mainly been reported in the Western
Reguibat Rise. The different terranes in this rise
contain zircon crystallization ages of c. 3.50–3.45,
3.04–2.92, 2.75–2.70 and 2.50–2.46 Ga (Potrel et al.,
1996, 1998; Key et al., 2008; Schofield et al., 2012;
Bea et al., 2013; Montero et al., 2014). These studies
suggest that the Western part of the Reguibat Rise
is a viable candidate for the provenance of the
Banded Gneisses Archean zircon (despite the c.
3.5 Ga population, which is only found in the
Banded Gneisses as TDM in some zircon).
The Banded Gneisses Paleoproterozoic fraction
makes up 39.6% of the total population (Fig. 5).
Maximum abundance clusters at c. 2.14–1.88 Ga
with a maximum peak at 2.07 Ga (Fig. 4). This Pale-
oproterozoic population falls within the time span of
the Eburnean orogeny (2.2–2.0 Ga according to Egal
et al., 2002; or 2.2– 1.8 Ga according to Ennih &
Liegeois, 2008), so the Paleoproterozoic materials of
the Banded Gneisses are possibly derived from rocks
generated or reworked during the Eburnean orogeny.
Eburnean ages between c. 2.1 and 2.04 Ga in igneous
and sedimentary rocks have been reported in the
Eastern Reguibat Rise (Peucat et al., 2005; Schofield
et al., 2006) and in the Anti-Atlas belt (Thomas
et al., 2002; Abati et al., 2012; Avigad et al., 2012),
supporting a WAC provenance for the Banded
Gneisses Paleoproterozoic zircon. The Banded
Gneisses Lu –Hf data (Fig. 7a) for zircon of
Eburnean age (c. 2.20– 1.88 Ga, n = 135) are
arranged as a cluster with positive eHf( t) values
(n = 96/135) representing juvenile rocks (average
TDM of c. 2.2 Ga) and few negative e Hf( t) values
(n = 39/135), suggesting a mixing process, i.e. Ebur-
nean DM derived magmas intruding an older crust
triggering mixing processes (Fig. 8). As the most neg-
ative eHf(t ) value for Eburnean zircon is 18, and the
Archean linear arrangement intersects at c. 2 Ga at
~eHf(t ) = 17, this old crust could well be the one
represented by the Banded Gneisses Archean zircon.
All these observations are in agreement with the geo-
dynamic setting proposed by Egal et al. (2002), where
the Eburnean is an active margin orogen formed by
oceanic subduction along the edge of the pre-existing
Archean craton.
In the Mesoproterozoic Era, the WAC became a
stable craton (Ennih & Liegeois, 2008) which
resulted in a characteristic c. 1.7–1.0 Ga ‘magmatic
gap’ (Linnemann et al., 2008 and references
therein). Nevertheless, in some peripheral WAC
derived samples, Mesoproterozoic zircon is relatively
common in Ediacaran –Ordovician (and younger)
siliciclastic samples (Anti-Atlas, Avigad et al., 2012;
Autochthon, Fernandez-Suarez et al., 2013; Parau-
tochthon, Dıez Fernandez et al., 2012c; Basal
allochthonous units, Dıez Fernandez et al., 2010).
In the Banded Gneisses the Mesoproterozoic zircon
is scarce and scattered, constituting 2.8% of the
total population and not defining a clear maximum
(Fig. 4). Taking into account its juvenile character
(eHf(t) from +6.5 to 5.5), this population could
have been derived from the Amazonia craton or
from Mesoproterozoic dykes intruding the WAC.
Terranes that clearly derive from the Amazonian
craton and have similar Neoproterozoic–Cambrian
arc developments as the Upper Allochthon (Avalo-
nia and Ganderia), contain juvenile Mesoproterozoic
zircon (Willner et al., 2013, 2014; Fig. 10), but not
as juvenile as zircon from this study (Figs 7a & 10).
Dolerite dykes have been recently discovered in the
Anti-Atlas belt with emplacement ages of c.
1.65 Ga (Kouyate et al. , 2013) and c. 1.4 Ga (El
Bahat et al. , 2013; Michard & Gasquet, 2013;
S€oderlund et al., 2013). The 176Hf/177Hf v. age plot
(Fig. 8) shows that the HP–HT Upper Allochthon
Mesoproterozoic population seems to have a source
that undertook a similar Lu/Hf isotopic evolution
as the CHUR-depleted mantle (with a similar
176Lu/177Hf ratio of ~0.033–0.038). These observa-
tions seem to favour a WAC juvenile dyke prove-
nance rather than an Amazonian or even a
Laurentian source. Therefore, the provenance of the
Mesoproterozoic population remains enigmatic (be-
cause the observations aforementioned are not con-
clusive), but it does not seem necessary to assign
far exotic provenance sources to explain the pres-
ence of this small population in the Upper Alloch-
thon.
The proximal (<1 Ga) detrital zircon spectrum
The Palaeozoic– Neoproterozoic fraction constitutes
34.7% of the Banded Gneisses zircon, most with ages
of c. 780 –490 Ma and with a maximum abundance at
c. 522–512 Ma. These ages coincide with the reported
ages for the Cadomian orogeny ( c. 750 –540 Ma; Lin-
nemann et al., 2014), but the c. 522 –512 Ma Banded
Gneisses maximum is younger, suggesting a different
metacratonic WAC activity or, more likely, a late
development of the Cadomian orogeny. Palaeozoic–
Neoproterozoic populations in the WAC are very
common, and range from c. 770 to 530 Ma (Anti-At-
las belt, Thomas et al., 2002; Abati et al. , 2012; Avi-
gad et al., 2012). The Ganderia and Avalonia
terranes are characterized by a c. 700–500 Ma devel-
opment of magmatic arcs that were built on the
Amazonian craton sector of the Gondwanan margin
(Willner et al., 2013, 2014; Fig. 10). Banded Gneisses
zircon with ages between c. 780 and 590 Ma are not
abundant and fall in the Lu–Hf diagram (Fig. 7a,b)
around the CHUR evolution trend (eHf(t) from +4 to
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972 R. ALBERT ET AL.