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Geochemistry of two associated ophiolites from the Cabo Ortegal Complex (Variscan belt of NW Spain)

By
Sonia Sánchez Martínez
Sonia Sánchez Martínez
1
Departamento de Petrología y Geoquímica, Universidad Complutense, 28040 Madrid, Spain
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Ricardo Arenas
Ricardo Arenas
1
Departamento de Petrología y Geoquímica, Universidad Complutense, 28040 Madrid, Spain
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Pilar Andonaegui
Pilar Andonaegui
1
Departamento de Petrología y Geoquímica, Universidad Complutense, 28040 Madrid, Spain
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José R. Martínez Catalán
José R. Martínez Catalán
2
Departamento de Geología, Universidad de Salamanca, 37008 Salamanca, Spain
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Julian A. Pearce
Julian A. Pearce
3
School of Earth, Ocean and Planetary Sciences, Cardiff University, CF10 3YE Cardiff, UK
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Published:
January 01, 2007

Two Paleozoic ophiolites of different age mark the Variscan suture in the Cabo Ortegal Complex (northwestern Iberian Massif). The Moeche and Purrido allochthonous ophiolites are structurally located between an exotic terrane of continental affinity (basal units), below, and another terrane with arc affinity (upper units), above. The mafic rocks of the Moeche unit are greenschist with a composition typical of tholeiitic basalt. Their rare earth element (REE) concentrations range from 12 to 31 times the chondritic abundances, with almost flat chondrite-normalized REE patterns ([LaN/YbN]avg = 0.83), and they have no significant Eu anomalies. According to their contents in some immobile trace elements, the geochemical composition of these metabasalts seems to be transitional between normal mid-ocean-ridge basalt (N-MORB) and island-arc tholeiites. Their abundances in the most immobile trace elements with the highest discriminating power (Th, Nb, Ce, Zr, Ti, and Y), normalized to an average tholeiitic N-MORB, depict quite flat patterns close to the average N-MORB concentrations, although they are slightly enriched. A small negative Nb anomaly points to an origin linked to a subduction zone.

The Purrido unit consists of amphibolites with compositions also equivalent to tholeiitic basalt. REE compositions range from 6 to 22 times chondritic abundance, with essentially flat chondrite-normalized REE patterns ([LaN/YbN]avg = 0.99) and very small positive Eu anomalies ([Eu/Eu*]avg = 1.08). The abundance of some trace elements in these metabasites shows an affinity with island-arc tholeiites, but, unlike the greenschist of the Moeche unit, their N-MORB–normalized trace-element patterns are slightly depleted in Nb, Zr, Ti, and Y, with a somewhat steeper negative Nb anomaly. These features are found in basalts generated in some back-arc basins.

Both ophiolites seem to represent oceanic lithosphere that developed in a supra-subduction zone context. The Moeche unit was probably formed during the opening of a back-arc basin at the Cambrian-Ordovician boundary, whereas the Purrido unit was generated later, during the Early Devonian, as a result of the intraoceanic subduction of the older oceanic lithosphere represented by the Moeche unit. The two ophiolites are representative of the Variscan suture in northwest Iberia, and they preserve a significant part of the history of the oceanic realms that closed during the final amalgamation of Pangea.

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GSA Memoirs

4-D Framework of Continental Crust

Robert D. Hatcher, Jr.
Robert D. Hatcher, Jr.
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Marvin P. Carlson
Marvin P. Carlson
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John H. McBride
John H. McBride
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José R. Martínez Catalán
José R. Martínez Catalán
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Geological Society of America
Volume
200
ISBN print:
9780813712000
Publication date:
January 01, 2007

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