serie NOVA TERRA nº 48

6. PROVENANCE OF THE HP-HT UPPER ALLOCHTHON

109

Provenance of the HP

–

HT subducted margin in the Variscan

belt (Cabo Ortegal Complex, NW Iberian Massif)

R. ALBERT,

R. ARENAS,

A. GERDES,

2,3

S. SANCHEZ MARTINEZ

AND L. MARKO

Departamento de Petrologıa y Geoquımica & Instituto de Geociencias (UCM, CSIC), Universidad Complutense, 28040,

Madrid, Spain

(r.albert@geo.ucm.es

)

Institut f

€

ur Geowissenschaften, Goethe

–

University Frankfurt, 60438, Frankfurt, Germany

Department of Earth Sciences, Stellenbosch University, Private Bag X1, Matieland, South Africa

ABSTRACT

The Variscan Upper Allochthon is a continental-affinity terrane that recorded a Cambrian

–

Ediacaran

magmatic arc generation, a subsequent transition to a passive margin, and a collision-related high-

metamorphism during the Devonian

–

Carboniferous amalgamation of Pangea. The objective of this

article is to decipher which continental margin subducted in the Devonian high-

–

high-

(HP

–

HT)

event. To do so, a provenance study is presented using combined U

–

Pb (

613) and Lu

–

(

463) isotopic LA

–

ICP

–

MS zircon analyses and Sm

–

Nd whole

–

rock (

5) determinations.

These analyses have been performed on five samples of the Banded Gneisses (Cabo Ortegal Complex,

NW Iberia), which forms a part of the HP

–

HT bottom member of the Upper Allochthon. Palaeozoic

–

Neoproterozoic zircon ages (34.7%) have a maximum abundance at 522

–

512 Ma, peaks at 575, 561,

545 Ma and minor abundance peaks between 780 and 590 Ma, and show from their Lu

–

Hf composi-

tions a volcanic arc mixing pattern. This arc was probably related to the Cadomian arc system. The

Mesoproterozoic population is scarce and scattered (2.8%), and due to its Lu

–

Hf pattern, it is pro-

posed that this population is also West Africa Craton derived. The Paleoproterozoic population

(39.6%) is concentrated at 2.07 Ga and it is linked to the Eburnean Orogeny, where depleted mantle

derived magmas intruded an Archean craton margin. This craton is represented by the Archean pop-

ulation (22.8%), which is grouped at 3.0, 2.68-2.61 and 2.52-2.48 Ga, and shows long-term reworking

processes and at least two juvenile magma intrusions. These data show that the Variscan Upper

Allochthon has a West African provenance and therefore, it strongly suggests that the NW Iberian

allochthonous complexes and their correlative European terranes are also West Africa derived. These

results allow us to finally clarify that the first high-

event, recorded during the eo-Variscan amalga-

mation of Pangea, was attained by the subduction of the margin of Gondwana under Laurussia.

Key words:

Cabo Ortegal; LAICPMS; Lu

–

Hf; U

–

Pb; Variscan Allochthon.

INTRODUCTION

The Variscan belt is a long orogen developed during

the main stages of the Pangea assembly as a result of

the collision of two large continents, Gondwana to

the South and Laurussia to the North (Matte, 1991,

2001; Winchester

et al.

, 2002; Martınez Catalan

et al.

, 2009; Arenas

et al.

, 2014a). It is considered

that the final assemblage of Pangea took place

between Devonian and Early Permian, and that the

continental collision followed the closure of the main

Palaeozoic ocean, the Rheic Ocean. This ocean was

generated after the Late Cambrian

–

Early Ordovician,

after rifting and progressive north drift of Avalonia

and other peri-Gondwanan terranes (Stampfli &

Borel, 2002; Murphy

et al.

, 2006; Arenas

et al.

, 2007;

Murphy & Nance, 2008; Nance

et al.

, 2010). The

Variscan belt can be followed from Iberia, across

Brittany and the French Massif Central, to the Bohe-

mian Massif, extending towards the basement of the

Alps, Corsica and Sardinia (Fig. 1a). The orogen was

linear in origin, but it was affected by several late- to

post-orogenic oroclinal bends that hinder correlations

across the belt (Matte, 2001; Martınez Catalan, 2011;

Weil

et al.

, 2013). The Variscan orogen has continu-

ity in eastern North America in the Appalachian

–

Al-

leghanian Belt.

The Variscan orogen contains a long suture zone

outlined by several allochthonous terranes with ophi-

olites and high-

units (Fig. 1a). They preserve the

most significant information in relation to the oldest

orogenic events and the evolution of the oceanic

domains that preceded the collision. Therefore, a

clear understanding of the origin and tectonothermal

evolution of these terranes is a key issue to improve

the knowledge about the initial interaction between

Gondwana and Laurussia. The orogen is character-

ized by the development of two high-

events with

2015 John Wiley & Sons Ltd

959

J. metamorphic Geol.,

2015,

33,

959–979

doi:10.1111/jmg.12155