Origine of sapphirine-bearing rocks in contact with the lherzolite in the Etang de Lers area (Central Pyrénees) : cataclasis, metasomatism and reworking of peridotites and Late Triassic meta-ophite and evaporitic meta-sediments.

In order to decipher the mode of exhumation of the Pyrenean lherzolites and the mechanisms of extreme thinning of the continental crust, it is necessary to obtain consistent information from the direct geological environment of mantle bodies. Our field investigations conducted since ten years along the North Pyrenean Zone led us to the conclusion that, despite they are famous among Earth scientists, the lherzolite bodies of Etang de Lers and Freychinede forest still lack detailed geological mapping. Therefore, we achieved a study of the metasedimentary terranes in direct contact with the lherzolites of the inverted Aulus Basin of Albian- Cenomanian age, completed by the optical observation and microprobe analysis of more than 50 thin sections. This allows us providing robust constraints on the lithology of the units progressively exhumed during the successive stages of mantle unroofing. We show that various polymictic breccias, microbreccias and sandstones are exposed in the immediate vicinity of the mantle bodies and associated meta-ophites. Among these detrital sediments, the famous sapphirine-bearing sandstones (Monchoux 1970, 1972) are located on the northern edge of the Lers lherzolite. Here, we describe a progressive transition from the lherzolite body towards the surrounding massive limestones. This transition consists of breccias and sandstones, whose nature varies according to the distance to the lherzolite. The mm-sized clasts are mainly composed of Al and Mg bearing minerals, such as sapphirine, phlogopite and anthophyllite associated with kornerupine and (metamorphic) Al-spinels in a carbonate cement. By place, the polymineral clasts have been deposited along with the monomineral debris, often defining a clear grain-size sorting. In addition, minor isolated clasts originating from the disaggreggation of various rock types are observed in the sapphirine-bearing sandstones suite. We recognized : lherzolite, micaschists, quartzite, alkaline gabbro, meta-ophite, metaevaporite and marble debris. Sandstones composed of alterning beds of minerals deriving from these rock-types are also exposed close to the mantle body. All the sources of these detrital elements can be identified in the Aulus Basin, apart from the sapphirine-bearing rocks protolith which remains enigmatic. Microprobe analyses allow to estimate an average composition of this protolith. We found that the best candidate is a mix of evaporitic clays and dolomite, typical of the Late Trias sediments, that evolved under the HT-LP conditions of the Pyrenean metamorphism. The presence of evaporitic inclusions (Clapatite) in the sapphirine crystals, revealed by MEB analysis, strongly supports this hypothesis. Earlier observations of anhydrite enclosed in metamorphic enstatite also confirms these conclusions (Foucard, 1997). Finally, we propose that the sapphirine-bearing and associated sedimentary rocks originated from the transformation of sediments of Triassic and Lower Jurassic age during the Cretaceous metamorphism event, in relation with mantle exhumation. Metasomatism and cataclasis of the Mesozoic pre-rift cover occurred along the extensional detachment fault which was progressively exposing the lherzolites to the seafloor. The cataclastic debris were abandonned on the unroofed detachment surface and rapidly reworked through sedimentary processes. Foucard, F., 1997. Les inclusions d’anhydrite, d’apatite, de chlorite et de zircon dans l’enstatite : de nouvelles donnees pour les roches a sapphirine de Lherz. Unpubli. Master thesis Univ. Toulouse. Monchoux P., 1970. Les lherzolites pyreneennes : contribution a l'etude de leur mineralogie, de leur genese et de leurs transformations. Unpubl. thesis, Univ. Toulouse. Monchoux, P., 1972. Roches a sapphirine au contact des lherzolites pyrenennes. Contrib. Mineral. Petrol. , 37, 47-64.