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Variscan intra-orogenic extensional tectonics in the Ossa–Morena Zone (Évora–Aracena–Lora del Rı́o metamorphic belt, SW Iberian Massif): SHRIMP zircon U–Th–Pb geochronology

By
M. Francisco Pereira
M. Francisco Pereira
1
Departamento de Geociências, Centro de Geofı́sica de Évora
,
Universidade de Évora
,
Apt. 94, 7001-554 Évora
,
Portugal
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Martim Chichorro
Martim Chichorro
2
Centro de Investigação em Ciência e Engenharia Geológica
,
Universidade Nova de Lisboa
,
Portugal
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Ian S. Williams
Ian S. Williams
3
Research School of Earth Sciences
,
The Australian National University
,
Canberra, ACT, 0200
,
Australia
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José B. Silva
José B. Silva
4
Departamento de Geologia, Faculdade de Ciências
,
Universidade de Lisboa
,
Edifı́cio C3, Campo Grande, Lisboa
,
Portugal
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Carlos Fernández
Carlos Fernández
5
Departamento de Geodinámica y Paleontologı́a, Facultad Ciencias Experimentales
,
Universidad de Huelva
,
Campus Carmen, 21071 Huelva
,
Spain
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Manuel Dı́az-azpı́roz
Manuel Dı́az-azpı́roz
6
Departamento Ciencias Ambientales
,
Universidad Pablo Olavide
,
41013 Sevilla
,
Spain
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Arturo Apraiz
Arturo Apraiz
7
Geodinamika Saila, Zientzia eta Teknologia Fak.
,
Euskal Herriko Unibertsitatea, Apt. 644, 48080 Bilbo
,
Spain
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Antonio Castro
Antonio Castro
8
Departamento de Geologı́a
,
Universidad de Huelva
,
E21819 La Rábida, Huelva
,
Spain
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Published:
January 01, 2009

Abstract

Following a Middle–Late Devonian (c. 390–360 Ma) phase of crustal shortening and mountain building, continental extension and onset of high-medium-grade metamorphic terrains occurred in the SW Iberian Massif during the Visean (c. 345–326 Ma). The Évora–Aracena–Lora del Rı́o metamorphic belt extends along the Ossa–Morena Zone southern margin from south Portugal through the south of Spain, a distance of 250 km. This major structural domain is characterized by local development of high-temperature–low-pressure metamorphism (c. 345–335 Ma) that reached high amphibolite to granulite facies. These high-medium-grade metamorphic terrains consist of strongly sheared Ediacaran and Cambrian–early Ordovician (c. 600–480 Ma) protoliths. The dominant structure is a widespread steeply-dipping foliation with a gently-plunging stretching lineation generally oriented parallel to the fold axes. Despite of the wrench nature of this collisional orogen, kinematic indicators of left-lateral shearing are locally compatible with an oblique component of extension. These extensional transcurrent movements associated with pervasive mylonitic foliation (c. 345–335 Ma) explain the exhumation of scarce occurrences of eclogites (c. 370 Ma). Mafic-intermediate plutonic and hypabyssal rocks (c. 355–320 Ma), mainly I-type high-K calc-alkaline diorites, tonalites, granodiorites, gabbros and peraluminous biotite granites, are associated with these metamorphic terrains. Volcanic rocks of the same chemical composition and age are preserved in Tournaisian–Visean (c. 350–335 Ma) marine basins dominated by detrital sequences with local development of syn-sedimentary gravitational collapse structures. This study, supported by new U–Pb zircon dating, demonstrates the importance of intra-orogenic transtension in the Gondwana margin during the Early Carboniferous when the Rheic ocean between Laurussia and Gondwana closed, forming the Appalachian and Variscan mountains.

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Contents

Geological Society, London, Special Publications

Ancient Orogens and Modern Analogues

J. B. Murphy
J. B. Murphy
St Francis Xavier University, Canada
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J. D. Keppie
J. D. Keppie
Universidad Nacional Autonoma de Mexico, Mexico
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A. J. Hynes
A. J. Hynes
McGill University, Canada
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Geological Society of London
Volume
327
ISBN electronic:
9781862395756
Publication date:
January 01, 2009

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