Anisotropy of magnetic susceptibility (AMS) data (180 sites) from Hercynian granitoids from Southern Corsica (France) and Northern Sardinia (Italy) provide a high-density net of structural information for this part of the European Hercynian belt. Two main groups of granitoids, defined according to petrography and relative chronology, provide contrasting AMS results. For both groups, shallowly dipping magnetic foliations and lineations suggest that these granitoids were emplaced as large-scale shallowly dipping sheets or open funnel-shaped structures at a high structural level. In non-leucocratic granitoids (305–300 Ma), foliation and lineation trajectory maps reveal antiforms and synforms with a N130 axial trace. Intersection of the foliation planes indicates a N120 low-plunging ‘fold’ axis, revealing a N120 maximum stretching direction. Slightly younger leucocratic granitoids (295–285 Ma), bounded by late Hercynian faults, display N50 lineations parallel to the elongated shapes of these intrusions. This structural pattern is undisturbed by the faults, and suggests an intrusion in decakilometre-scale tensional fractures indicating a N140 instantaneous stretching direction compatible with the stretching direction evidenced in non-leucocratic granitoids. A younger
c
. N20-oriented Permian dyke swarm confirms the permanence of this stretching direction over at least 35 Ma and indicates a coaxial strain regime. To integrate these new data within a broader context in the Hercynian belt, Corsica has to be back-rotated clockwise by 70° to its Permian position, which indicates a north to N30 stretching direction for the 305–285 Ma interval, compatible with the post-collisional strain and stress fields of the French Massif Central at about the same period.
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 (TiO 2 \ 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 (TiO 2 = 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 Al 2 O 3 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 Variscanrelated 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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