During Devonian, the Bas-Limousin series has been affected by two successive main deformation events associated with a plurifacial barrovian metamorphism grading from epizone up to Sillimanite + Muscovite zone (Santallier et al., 1978). The eclogites found in the series crystallized during the high pressure early phases of the Hercynian s.l. orogenic cycle, prior to Devonian (Santallier, 1981). They were amphibolitized during the first barrovian metamorphic phase, as a consequence of their tectonic emplacement in their present greywackous mesozonal country-rocks. They can be distributed into two geographically distinct groups (Figure 1). — The eclogites of the eastern area have a tholeiitic composition (Figure 3) either undifferenciate or enriched with plagioclase cumulates. In the first case, the eclogitic paragenesis comprised mostly garnet (Alm 39 to 57 and Py 16 to 39 : Figure 2) and omphacite (Jd + Ac 35 to 45). In the second case, kyanite and zoïsite were part of the eclogitic assemblage. After their tectonic emplacement in their present staurolite-, kyanite- or sillimanite-bearing country-rocks, the eastern eclogites have undergone a retrograde evolution with two separate episods. The first one predated the hydration process and developped an anhydrous paragenesis comprising mostly diopside (Figure 14) and oligoclase, which result from the omphacite breakdown (Figure 12). In the "bimineralic" eclogites (devoided of kyanite and/or zoïsite), the first so-called retromorphic assemblage is mostly composed of secondary diopside and oligoclase associated with undisturbed primary garnet. In the kyanite and/or zoïsite eclogites, the two mineral phases are retrogressed into anorthite with minor other constituents (spinel + corundum ± staurolite where primary kyanite was concerned). The connected breakdown of kyanite and omphacite induced the crystallization of a Mg-rich secondary garnet (Figure 10). The second assemblage is a hydrous one and is connected with amphibolitization. It comprises hornblende and andesine. — In the western area, the eclogites also present a tholeiitic composition. The primaiy, high pressure metamorphic mineral assemblage and its retrograde evolution represent a continual process occurring under regularly decreasing pressure conditions with the following characteristics : The jadeite content in the Cpx decreased progressively (Figures 13 and 14) from an hypothetical Jdx omphacite 1 (now disappeared and replaced by a Cpx + plagioclase intergrowth in the core of omphacite 2), through a massive Jd₃₅ to ₄₅ omphacite 2, a symplectitic Jd₂₃ omphacite 3, till a Jd₄ to ₁₂ Cpx in the reaction rims between quartz and garnet. Plagioclase was present in the western "eclogites" from the very beginning of the primary stage (garnets frequently have plagioclase inclusions), and its Ca content increases progressively as the jadeite content in the Cpx decreases (Figure 13) : Plagioclase 1 An₁₀ to ₁₅ is included in garnets ; plagioclase 2 An₂₀ is associated with massive and symplectitic omphacites 2 and 3 ; plagioclase 3 An₂₀ to ₃₀ appears in reaction rims with Cpx 4. Amphibolitization occurred a short time latterly : Cpx 2, 3 and 4 can be direcdy replaced by green hornblende. The P/T eclogitization conditions have been estimated (Santallier, 1981) at at least 15 kbar and 650 to 750 °C (Figure 19). These are just indicative values because of the scarcity of the generally poorly preserved available mineral relicts. But the results obtained in both western and eastern areas seem to be very comparable. The two eastern and western eclogites groups have a slightly different retromorphic history. But they all have the same chemical composition. They were tholeiites with alkaline affinities (Cabanis et al., in press) originally emplaced in a rift zone ; which suggests that they all belong to the same magmatic unit generated in a distensional environment. They all had very comparable primary eclogitic assemblages. They are all found as lense-shaped boudins in an homogeneous greywackous gneisses series. This gneisses series shows everywhere in Bas-Limousin the same number of deformation phases associated with the same number of metamorphic phases which occurred in the same P/T conditions. The eclogites boudins are everywhere moulded by the same F1 cleavage expressed both in the gneisses and in the external rim of the amphibolitized eclogite bodies. All these facts strongly suggest that all the Bas-Limousin eclogites were generated during the same high pressure event, that they were emplaced at the same time, during the first Devonian deformation phase, which induced two slightly different retrogressive processes occurring in the same barrovian P/T conditions.
Les éclogites du Bas-Limousin sont des témoins des phases précoces de haute pression du cycle Hercynien s.l. Elles ont été amphibolitisées au Dévonien à la suite de leur mise en place tectonique dans leur encaissant greywackeux actuel. Elles se répartissent en deux groupes. A l'Est, elles ont une composition de tholéiites soit indifférenciées (et sont alors essentiellement composées de grenat et d'omphacite), soit enrichies en plagioclases cumulatifs (le disthène et/ou la zoïsite font alors partie de la paragenèse éclogitique primaire). Les éclogites orientales ont subi une évolution rétrograde en deux stades distincts avec développement d'une première paragenèse anhydre, à diopside et oligoclase, dite de rétromorphose, puis d'une deuxième paragenèse hydratée qui correspond à l'amphibolitisation. A l'Ouest, la cristallisation de la paragenèse éclogitique primaire et sa déstabilisation au profit d'un assemblage rétrograde (à Cpx sodique et oligoclase) s'enchaînent en un processus continu à pression régulièrement décroissante avant l'amphibolitisation proprement dite.
