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Petrology and geochemistry of exhumed peridotites and gabbros at non-volcanic margins: ODP Leg 173 West Iberia ocean-continent transition zone

By
Réjean Hébert
Réjean Hébert
Département de Géologie et de Génie géologique, Pavillon Adrien-Pouliot, Université Laval, Ste-Foy, Qc., Canada, G1K 7P4 (e-mail: hebert@ggl.ulaval.ca)
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K. Gueddari
K. Gueddari
INRS-Géoressoures, Centre géoscientifique de Québec, 880 Chemin, Ste-Foy, Québec, Canada, G1V 4C7
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M. R. Laflèche
M. R. Laflèche
Géosciences Azur, Observatoire océanologique, B.P. 48, 06235 Villefranche-sur-Mer Cedex, France
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M.-O. Beslier
M.-O. Beslier
Géosciences Azur, Observatoire océanologique, B.P. 48, 06235 Villefranche-sur-Mer Cedex, France
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V. Gardien
V. Gardien
Université Claude Bernard Lyon I, 43 Boulevard du 11 Novembre 1918, Laboratoire de Pétrologie et Tectonique, UMR 5570, Bât. 402, 69622 Villeurbanne Cédex, France
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Published:
January 01, 2001

Abstract

Ultramafic and mafic rocks recovered at Holes 1068A and 1070A were drilled during Leg 173 of the Ocean Drilling Program (ODP) in the ocean-continent transition zone of the Iberia Abyssal Plain. Peridotites show contrasting petrographic characteristics. Hole 1068A peridotites are fine grained and show a well-defined high-temperature foliation marked by elongated pyroxene as well as aligned spinels. Hole 1068A peridotites are strongly serpentinized. Hole 1070A peridotites are coarse grained and show little evidence of high-temperature foliation. The degree of serpentinization is lower and relicts of silicate minerals are preserved. In both sets of recovered material, spinels show a wide range of composition and suggest a complex magmatic evolution. Gabbros dykes, which are found only in Hole 1070A, are very coarse grained and are locally sheared and/or crushed. Magmatic amphiboles are kaersutites and Ti-rich tschermakites that are partially replaced by hornblende and actinolite, and are associated with plagioclase of intermediate composition. Peridotites and pyroxenite have low TiO2, A12O3 and CaO contents in carbonate-free samples. Ni and Cr contents fall into the upper-mantle array. On the other hand, gabbros have relatively high TiO2 and V contents reflecting modal ilmenite, and suggesting that they are relatively differentiated. This paper presents the very first geochemical data on platinum group elements (PGE) of peridotites and gabbros from passive margins. Peridotites and gabbros show low PGE (25.83 ppb and 1.44 ppb), Pd (2.75 ppb and 0.15 ppb), Pd/Ir ratios (1.45 and 1.3) and mafic index. Pyroxenite has the highest PGE (27.97 ppb), Pd/Ir (19.87) and Pt/Ir (10.25). Interelemental correlation and observation of PGE-bearing sulphide phases suggest that the PGE are hosted by single sulphide phases. From a PGE point of view, extraction of magmas involved very PGE-depleted liquids similar to gabbroic veins cutting the peridotites at Hole 1070A. Partial melting is interpreted as occurring just before oceanic accretion. Geochemical attributes suggest that the peridotites belong to the Ronda and Beni Bousera peridotitic depleted end-member clan. Thus they are believed to be of subcontinental origin. Deformation and retrograde metamorphism of peridotites and gabbros are consistent with exhumation in a rift environment post-dating the 120 Ma magmatic stage.

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Contents

Geological Society, London, Special Publications

Non-Volcanic Rifting of Continental Margins: A Comparison of Evidence from Land and Sea

R. C. L. Wilson
R. C. L. Wilson
The Open University, UK
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R. B. Whitmarsh
R. B. Whitmarsh
Southampton Oceanography Centre, UK
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B. Taylor
B. Taylor
University of Hawaii, Hawaii
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N. Froitzheim
N. Froitzheim
University of Bonn, Germany
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Geological Society of London
Volume
187
ISBN electronic:
9781862394353
Publication date:
January 01, 2001

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