An early Palaeozoic supra-subduction lithosphere in the Variscides: new evidence from the Maures massif |
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Authors: | Jean-Philippe Bellot Christine Laverne Georges Bronner |
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Institution: | 1. 4 Place de Brie, 13015, Marseille, France 2. Laboratoire de Pétrologie Magmatique, Faculté des Sciences et Techniques, Université Paul Cezanne Aix-Marseille III, Case 441, Avenue Escadrille Normandie-Niemen, 13397, Marseille Cedex 20, France
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Abstract: | Petrographic and geochemical studies of peridotites and melagabbros from the Maures massif (SE France) provide new constraints
on the Early Palaeozoic evolution of the continental lithosphere in Western Europe. Peridotites occur as lenses along a unit
rooted in the main Variscan suture zone. They are dominantly spinel peridotites and minor garnet–spinel peridotites. Spinel
peridotites represent both residual mantle and ultramafic cumulates. Mantle-related dunites and harzburgites display high
temperature textures, with olivine (Mg#0.90), orthopyroxene (Mg#0.90) and spinel (TiO2 < 0.2%; Cr#0.64–0.83) compositions typical of fore-arc upper mantle. Ultramafic cumulates are dunite adcumulates, harzburgite heteradcumulates
and mesocumulates, melagabbro heteradcumulates and amphibole peridotites, with olivine (Mg#0.85–0.89), orthopyroxene (Mg#0.86–0.89) and Cr-spinel (TiO2 = 0.5–3.3%; Cr#0.7–0.98) compositions typical of ultramafic cumulates. Cr-spinel compositions of both spinel peridotite types suggest their genesis
in a supra-subduction zone lithosphere. Core to rim zoning in spinel is related to the incomplete influence of regional metamorphism
and serpentinisation. The covariation of major and minor elements with Al2O3 for cumulates is consistent with igneous processes involving crystal accumulation. Both mantle and cumulate dunites and harzburgites
have U-shaped REE patterns and extremely low trace element contents, similar to peridotites from modern fore-arc peridotites
(South Atlantic) and from ophiolites related to supra-subduction zones (Semail, Cyclops, Pindos, Troodos). Melagabbros also
have U-shaped REE patterns similar to xenoliths from the Philippine island arc, but also similar to intrusive ultramafic cumulates
from the Semail nappe of Oman related to a proto-subduction setting. A wehrlite has a REE pattern similar to that of amphibole
peridotites reflecting metasomatism of clinopyroxene-bearing peridotites due to subduction-related fluids. The Maures spinel
peridotites and melagabbros are therefore interpreted as the lowermost parts of a crustal sequence and minor residual mantle
of lithosphere generated in a supra-subduction zone during Early Palaeozoic time. Garnet–spinel peridotites are chemically
close to melagabbros, but have recorded high pressure metamorphism before their retrogression similar to spinel peridotites
into amphibolites to greenschists facies metamorphism. They indicate burial to mantle depths of the margin of the supra-subduction
lithosphere during the Early Palaeozoic continental subduction. Both peridotite types were exhumed during the Upper Palaeozoic
continental collision. Comparable observations from other Variscan-related peridotites, in particular of the Speik complex
of the Autroalpine basement, and a common age for the subduction stage allow extension of these regional conclusions to a
broad area sharing the Cambrian suture zone, extending from the Ossa-Morena to the Bohemian massif. |
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