serie NOVA TERRA nº 48

6. PROVENANCE OF THE HP-HT UPPER ALLOCHTHON

1.7–1.0 Ga ‘magmatic gap’, and therefore

Mesoproterozoic zircons are not expected to

be present in a WAC derived formation. In

the Banded Gneisses the Mesoproterozoic

zircon is scarce and scattered, constituting

2.8% of the total population and not defining a

clear maximum. Taking into account that this

population is isotopically depleted, it 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 (Avalonia and Ganderia), contain

isotopically depleted Mesoproterozoic zircon,

but not as depleted as the zircon from this study.

Dolerite dykes have been recently discovered in

the Anti-Atlas belt (WAC) with emplacement

ages of

1.65 Ga and

1.4 Ga. The

176

Hf/

177

age plot shows that the Banded Gneisses

Mesoproterozoic population has an isotopically

depleted source that undertook a similar Lu–

Hf isotopic evolution as the CHUR and the

DM. These observations seem to favour a WAC

juvenile DM-derived dyke provenance, rather

than an Amazonian or even a Laurentian one.

Therefore, the provenance of theMesoproterozoic

population remains enigmatic (because the

observations aforementioned are not conclusive),

but it does not seem necessary to assign far exotic

provenance sources to explain the presence of

this small population in the Upper Allochthon

(as it has been assigned in previous works).

The zircon detrital signature of the Banded

Gneisses has not only been compared with the

WAC, but also with cratons and terranes that

could potentially be the source areas for its

metasedimentary rocks. The high number of

zircon grains with mixing patterns at

1.5 and

1.0 Ga in the Avalonian terranes (Ganderia

and Avalonia) and the Laurentian craton are

not present in the HP–HT Upper Allochthon

spectrum. Therefore, the Upper Allochthon

detrital series are not derived from the mentioned

source areas.

The

Paleozoic–Neoproterozoic

zircon

population of the Banded Gneisses (with most of

its ages of

780–490 Ma and with a maximum

abundance at

522–512 Ma) coincides with the

reported ages for the Cadomian orogeny (

. 750–

540 Ma), but the

522–512 Ma Banded Gneisses

maximum is younger. Banded Gneisses zircon

crystals with ages between

780 and 590 Ma are

not abundant and are interpreted as to be formed

by crustal recycling due to the initial development

of the Cadomian arc system (‘proto–arc’ stage).

The

590–490Ma BandedGneisses zircon is very

abundant (‘arc’ stage) and their Lu–Hf isotopic

patterns are explained by the intrusion of DM-

derived magmas that triggered mixing processes

with an Eburnean and Archean crust (and with

a small proportion of reworked early Cadomian

crustal material), consistent with a peripheral arc

activity at the Northern WAC. If the magmatic

arc is the Cadomian arc, the magmatic activity

inferred by the Banded Gneisses lasted at least

until

510 Ma in the NW Iberian section of

the Gondwana margin, instead of

540 Ma (as

described in the Bohemian Massif). This could

imply a diachronous scissor-like continental arc

generation, due to oblique vector of subduction,

that propagated westwards (possibility already

proposed in the literature), but more likely, it

could imply that the

540–510 Ma arc activity

was not registered in the Bohemian Massif sector

of the margin of Gondwana, because magmatic

activity can strongly vary between segments of

the same arc system.

When comparisons are made with the unit

overlapping the HP–HT Banded Gneisses (the

IP Cariño Gneisses, studied in the previous

chapter) big similarities are observed. Their

maximum depositional ages (MDAs) are very

similar (Cariño Gneisses MDA = 510 Ma,

Banded Gneisses MDA = 521 Ma) and as no

big differences in their youngest U–Pb ages are

appreciated, it seems reasonable to conclude that

both units are temporally related. Sm–Nd isotopic

experiments also reveal the similarity between the

gneissic units as the

(t)

values for the Banded

Gneisses are in the range of those for the Cariño

Gneisses. The main difference between their U–

Pb age density distributions is that the Banded

Gneiss formation has an abundant

590–540

Ma Ediacaran population and the Cariño Gneiss

formation has not. Both formations represent the

same section of the Gondwanan margin. Their

bimodal detrital populations suggest that both

formations had the same geological setting,

i.e.

sedimentation in a back-arc type basin. Banded

Gneiss formation was profusely intruded by

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