serie NOVA TERRA nº 48

6. PROVENANCE OF THE HP-HT UPPER ALLOCHTHON

124

WAC sediments and

560

–

510 Ma arc-derived sedi-

ments.

Implications for the Devonian subduction recorded in the

Upper Allochthon

The metasedimentary sequences that constitute the

gneissic units of the Upper Allochthon, both in the

IP upper units (Cari

no Gneisses) and in the HP

–

upper units (Banded Gneisses) have the same detrital

sources and they seem to have been deposited in a

same back-arc basin. Therefore, these units were

probably the members of the same sedimentary suc-

cession. Between the time of deposition (

512

–

506 Ma) and high-grade metamorphism (

400

–

390 Ma), both formations constituted a part of the

margin of Gondwana. According to the sedimentary

and igneous processes recorded in this margin, it

changed from a volcanic arc to a passive margin set-

ting at around the Cambrian

–

Ordovician boundary,

after a short event of extension marked by the intru-

sion of alkaline

–

peralkaline igneous rocks (Rodrıguez

Aller, 2005; Abati

et al.

, 2010; Dıez Fernandez

et al.

2012a).

The different metamorphic conditions reached by

both units indicate that the HP

–

HT member of the

Upper Allochthon was affected by subduction at

400

–

390 Ma. This deep subduction affecting the mar-

gin of Gondwana is considered a clear evidence for a

first collision between this continent and the southern

margin of Laurussia (Arenas

et al.

, 2014a). Different

structures and metamorphic recrystallizations related

to this collision have also been described in the IP

upper units. In this case, the intensity of deformation

was much lower and only limited shear zones and

amphibolite facies Barrovian metamorphism were

generated (Gomez Barreiro

et al.

, 2006; Arenas

et al.

in press). The differentiated tectonothermal evolution

recorded in the Cari

no and Banded gneiss forma-

tions, could have been favoured by the previous

structure of the Gondwana margin, achieved during

the Cambrian

–

Ordovician extension. It has been pub-

lished (Dıez Fernandez

et al.

, 2014), that in the Cam-

brian

–

Ordovician transition, lithospheric extension

induced crustal necking and stretching, compartment-

ing the Gondwana lithosphere into several continen-

tal microblocks. During the Devonian collision the

thicker sections of the margin were affected by lim-

ited accretion and thickening, developing Barrovian

metamorphism. The adjacent thinned sections experi-

enced profound subduction directed to the North,

favoured by zones of inherited crustal weakness,

developing the HP

–

HT subduction-related metamor-

phic conditions of the Banded Gneiss formation and

its HP

–

HT equivalents along the Variscan belt

(Fig. 11b,c).

According to previously published structural data

of the Banded Gneiss formation, subduction affecting

the entire lower section of the Upper Allochthon was

probably oblique with a top-to-the-NNE sense of

shearing (present day coordinates; Abalos

et al.

2003, 2011). At first, this sense of movement is not

(a)

Long livedmagmatic arc

(c. 750-500Ma)

Ediacaran -Cambrian

peri-Gondwananseries

Neoproterozoicorogens

Cratonsandmeta-cratons

Active long lived subduction zone

Back-arc basin

c. 500 Ma

Avalonian -CadomianMagmaticArc

0.54-1.1

1.8-2.2

2.7-2.9

3.1-3.4

0.54-1.2

1.6-2.3

2.6-2.8

3.0-3.3

0.54-0.7

1.0-1.35

1.45-1.75

2.5-3.1

0.55-0.65

0.9-1.1

1.65-1.85

2.45-2.7

0.55-0.65

0.9-1.1

1.65-1.85

2.45-2.7

Zirconages from

the cratons

Upperunits inNW Iberia

Basalunits inNW Iberia

Autochthon inNW Iberia

Amazonian

Craton

West

African

Craton

Sahara

Meta-Craton

Congo

Craton

Kalahari

Craton

Arabian

Nubian

Shield

Indian

Shield

EastAntartic

Shield

Australian

Shield

Iapetus - Tornquist Ocean

South Pole

420 Ma

Contracting Rheic Ocean.

BALTICA

ASIATIC

GONDWANA

RHEICOCEAN

(b)

410–400 Ma

First collision: HP-UHP - event.

GONDWANA

BALTICA

ASIATIC

HP-UHPBELT

UPPERUNITS

(c)

Fig. 11.

(a) Sketch showing the proposed paleogeographic position of the upper units (Upper Allochthon) and other terranes from

the NW Iberian Massif at

500 Ma. Simplified paleogeography of Gondwana based on Fernandez-Suarez

et al.

(2013). Numbers

in squares are zircon ages in Ga from the cratons, from Linnemann

et al.

(2014) and references therein. New zircon ages have been

added to the West African Craton (Avigad

et al.

, 2012) and to the Arabian Nubian Shield (Morag

et al.

, 2012). (b & c) Sketch

showing the proposed paleogeographic position of the upper units (Upper Allochthon) during the Devonian collision (based on

Arenas

et al.

, 2014a).

2015 John Wiley & Sons Ltd

974

R. ALBERT

ET AL.