Résumé
La nappe alpine de Santa-Lucia se singularise par un socle antécrétacé comprenant des granodiorites et une association de granulites, de roches basiques stratifiées et de lherzolites, l’ensemble étant recouvert en discordance par des sédiments du Crétacé. Les diverses unités du socle sont affectées par une même foliation fortement pentée de direction N-S à NNW-SSE. Dans les kinzigites, les conditions initiales du métamorphisme sont estimées à environ 900 °C et 10 kbar. La déformation cisaillante principale a eu lieu lors de la décompression des granulites à température décroissante. Dans les roches mafiques stratifiées, la foliation de haute température acquise lors de l’écoulement des magmas dans des plans verticaux, résulte surtout d’une déformation à l’état solide commune avec celle des granulites. Dans les lherzolites, la déformation cisaillante à 900.
Mots-clés : Granodiorite, Granulite, Intrusion stratoïde, Composition mafique, Lherzolite, Linéation, Cisaillement, Rifting, Permien, Corse.
Abstract
The crystalline basement of the Santa-Lucia-di-Mercurio nappe is exceptional due to the occurrence, in addition to granodiorite, of kinzigite, layered diorite, gabbronorite, ferrogabbro and lherzolite (Libourel, 1985). The nappe overlies Eocene flysch of the lower Caporalino-Pédani unit with incipient slaty cleavage and low-grade Alpine metamorphism. A steep NNW-SSE-trending high-temperature foliation is recorded in all the basement units. The kinzigite, with a high-temperature mylonitic fabric, displays a primary mineral assemblage of pyrope-rich garnet, sillimanite, K feldspar, biotite, rutile and Zn-rich (9% ZnO) spinel, for which peak TP metamorphic conditions are calculated at around 900 °C and 10 kbar. Pervasive shearing took place under decreasing pressure and temperature, as evidenced by the cordierite-plagioclase-biotite mineral assemblages. The layered mafic rocks display a high-temperature foliation inherited from magmatic flow within the subvertical planes and reflecting solid-state shearing concomittant with that of the enclosed granulite. The foliation in the lherzolite relates to mantle flow along vertical planes during the ascent of deep mantle at the transition between spinel and plagioclase facies. The mineral and stretching lineations in all the units are roughly parallel and plunge steeply south. Shear-direction indicators, such as winged porphyroclasts and sigmoidal features, indicate a constant sinistral shearing. This tectono-magmatic event occurred during the Permian, according to U-Pb zircon dating at 286 and 281 Ma (Paquette et al., 1996). Stocks of undeformed gabbro, and dolerite dykes cutting the high-temperature foliation of the mafic rocks, indicate that these mantle-derived magmas were emplaced at shallow depths after cooling and uplift of the magmatic country rocks. The subvertical geometry of the rock units around the mantle/crust boundary suggests major thinning and tilting of the whole continental crust before deposition of the Tomboni conglomerate that contains large clasts of proximal origin, and other deep-sea Cretaceous sediments. These characteristics allow us to reconstruct the western side of a narrow Red Sea-type rift, which may have opened in a transtensional setting as a consequence of the diapiric emplacement of slices of spinel lherzolite that may represent deep lithospheric mantle. This setting compares well with the Zabargad island model (Boudier et al., 1988) and with continental rifts in general (Nicolas et al., 1995). Rifting here did not lead to oceanization, which only occurred in the adjacent Liguro-Piemont oceanic domain. This situation is thus inconsistent with the low-angle listric fault geometry linked with unroofing of the cold lithospheric mantle and leading to ocean spreading, as reported from the Galicia margin (Boillot et al., 1995) and also reconstructed for the Permian-Jurassic period in part of the Central Alps (Froitzheim and Rubatto, 1998). The high and frontal position of the Santa-Lucia-di-Mercurio nappe in Alpine Corsica, the Permian age of the gabbro, the presence of granulites and the post-Permian evolution of this basement slice suggest a comparison with the Ivrea zone in the Western Alps. However, preliminary structural data on the Alpine kinematics of the nappe and its Eocene substratum indicate early northeastward tectonic transport. These results suggest that the Santa-Lucia-di-Mercurio basement rocks represent a very small crustal fragment from the roots of an aborted Permian-Mesozoic rift that may have opened at the southeastern margin of the western European paleocontintent.
Key words: Granodiorite, Granulite, Layered intrusion, Mafic composition, Lherzolite, Lineation, Shear, Rifting, Permian, Corsica.
Dernière mise à jour le 02.07.2015