Les méta-péridotites de La Bessenoits (Massif central). Nature et signification géodynamique

Abstract

Located north of the Stephanian Decazeville basin (Aveyron) (figures 1 und 2), the La Bessenoits massif is composed of quartzo-feldspathic gneiss (both augen and ordinary), amphibolite and serpentinized peridotite (Raguin 1947, Monchoux 1961, Couturier 1985). According to Burg and Matte (1977), the massif is an isolated fragment (klippe) of the Upper Gneissic Unit (Ledru et al. 1989), lying southward of the main thrust upon the parautochtonous Lot valley micaschist. After finding relictual eclogites in the La Bessenoits massif, Monchoux and Couturier (1987) attributed it to the socalled Leptynitic-Amphibolitic Complexes, referring to the selected definition of Santallier et al. (1988). The serpentinite is either massive or schistose and generally located at the orthogneiss formation boundary. It crops out as bodies of various sizes, one of which, the so-called Puy de Voll (figure 3), is as much as 1 km. The Puy de Voll serpentinite exhihits orthopyroxene phenoclast-rich zones and, locally, a few centimeter-thick beds a alternating amphibolite and serpentinite. The original paragenesis qf the peridotite complex is generally completely altered. Nevertheless, relicts of olivine and in places, orthopyroxene are to be found in the massive serpentinite. Depending upon the sample, one notes two populations of olivine crystals: the one less than one millimeter wide, the other centimetric. The chemical composition of the olivines (Fo 88 to 90.8 %) is apparently not size dependent (table l), thus the two populations express a porphyroclastic texture. The orthopyroxene composition ranges from En 87 % to En 90 %, with a variable alumina content: Al2 03 varies in a single crystal from 5.1 % in the core to 0.9 % at the recrystallized rim (table 1). Clinopyroxene has not been observed. It is probably completely altered to amphibole. Spinel generally occurs as opaque and poikilitic chromite grains surrounded by chlorite nodules. Progressive retrograde metamorphism is reflected a strong negative correlation between Cr/Cr + Al and Mg/Mg + Fe2+ ratios (Evans and Frost 1975 and figure 5). In a trivalent cation diagram (figure 6), the observed correlation is in good agreement with the immiscibility domain boundary (Sack and Ghiorso 1994). Nevertheless, colourless spinels with relatively high Al2,03 contents have been observed in thin section (table 1). Most of the spinels plot in the metamorphic domain (figures 5 and 7). Chlorite in the nodules surrounding the chromite is in places, a Cr-rich clinochlore. It apparently crystallized before the amphibole, which constrains the beginning of the peridotite retrograde metamorphism at relatively high pressure (PH20 = PT > 10 Kbars for circa 800°C) (figure 12). Amphibole is widespread and mostly devoid of serpentinization. Chemically, it varies from Mg-hornblende to trentolite (figure 8), and the cation distribution between the various possible sites (figures 9 and 10) is good evidence for its metamorphic origin. Clinopyroxene has been completely altered by hydration. This is good evidence that the metamorphic fluid circulated at relatively high pressure (> 10 Kbars) with PH20 ? PFluid = PT and temperatures circa 700°C (fïgure 12 and Evans 1992). Chrysotile and lizardite mostly developed at the expense of olivine and orthopyroxene, and less commonly of amphibole and chlorite. They display pseudomorphic textures. Correlation diagrams between the elements were drawn up to investigate the protolith composition (figure 13). They show that the retrograde metamorphism was not isochemical. Nevertheless, remobilization was weak, except for Fe and Ca. The Fe evolution is probably due to a metasomatic episode prior to metamorphism. The Ca decrease reflects the high temperature retrograde metamorphism. The studied samples are scattered throughout the whole domain between fertile lherzolites and harzburgites, which provides a good argument for comparing the La Bessenoits peridotite to orogenic peridotite. The modal compositions, recalculated assuming the Ca0 contents to be ? 0.9 Al203 support a similar conclusion. The partial melting probably occurred in the spinel-lherzolite facies (figure 14). Consequently, the La Bessenoits serpentinite derived from a spinel-bearing lherzolite, which was aflected by partial melting and resulted in more depleted harzburgitic terms. Later, the harzburgitic paragenesis underwent a retrograde metamorphic evolution, as rejlected by the various intermediate stages indicated above. The fertile character of some of the rocks, as deduced from the mineralogical and chemical study, is good evidence that the original lherzolite was probably a subcontinental mantelic slice. Our conclusion is in good agreement with the most recent geochemical work devoted to the Massif central metabasites (Ouali 1993). It shows that the extensional process was limited in the eastern Massif central, where it never resulted in a complete oceanization. Consequently, the associated peridotite is of the sub-continental type. In such a diagram, the La Bessenoits klippe appears to be linked to the eastern Massif central zone.