Les lherzolites à spinelle et les pyroxénites à grenat associées de Bestiac (Ariège, France)

Abstract

Compared to the other ultramafic associations from the French Pyrénées, the small layered spinel-lherzolite bodies scattered around Bestiac (Ariège) all display some unusual mineralogical characters : 1) Kelyphitic assemblages (Opx + Sp ± Cpx) appear locally as coronas around the interstitial spinel in lherzolites as well as in interbedded olivine-websterites. This suggests that the kelyphitic assemblages resulted from the destabilisation of a garnet which crystallized after the spinel in equilibrium with the olivine. Thus the hypothesis that the parageneses, now observed, are due to a tectonic melange (Kornprobst, 1966) must be here ruled out ; 2) Also present in the same horizons, are some sharply-defined layers (3 to 20 cm-thick), composed of websterites with abundant coarse-grained garnets, containing numerous tiny exsolutions of rutile, and the highest mg ratios (80 to 83) ever found in pyroxenites associated with spinel-lherzolites. In fact, the coarse-grained garnets are relicts of a primary paragenesis : Al-rich pyroxenes (Opx + Cpx) + garnet ± spinel. However, despite its unique character, the layered formations composed of kelyphitic lherzolites and garnet pyroxenites are intimately associated, without discontinuity, to massive spinel-lherzolites, more or less rich in olivine-websterite layers, free of any kelyphitic assemblage. All the features suggest conditions of crystallization and re-crystallization next to the transition between the spinel- to the gamet-lherzolite stability fields, the growth of garnet in some olivine-bearing parageneses being controlled by the bulk chemistry. From a detailed chemical study and by applying different geothermobarometers, it may be concluded that : 1) The estimations deduced from the coarse-grained garnets and pyroxenes in websterite show pressures between 16 and 19 kbar, and temperatures of about 1 000 °C. Several evidences support the hypothesis that a pressure of about 20 kbar represents the greatest value at which crystallized the primary garnet-bearing assemblages, the corresponding temperatures being at least between 1 200 °C and 1 300 °C ; 2) In an ultramafic association which exhibits a relative chemical heterogeneity, the local growth of garnet after spinel in some olivine-bearing parageneses resulted from a near-isobaric cooling, at pressure of approximately 18 kbar, through the spinel-lherzolite stability field into the transition zone ; 3) Recrystallization that occurred during the ascent of the ultramafic body, first resulted in the destabilisation of the garnet coexisting with olivine, at pressures of 14-16 kbar, and continued during the main episode of plastic deformation which ended between 12-15 kbar and approximately 900 °C, i.e. in the same conditions as in all the ultramafic complexes of Ariège ; 4) Lastly, the ultramafïc association partially recrystallized at about 700 °C and 6-8 kbar, in the deep crust. The unique character of the ultramafïc association at Bestiac may thus result essentially from the differentiation of a crystalline mush which occurred at somewhat greater depth (~ 70 km) than usual for the spinel-lherzolite associations in the North-Pyrenean zone (50-60 km). Subsequently, it has suffered the same complex geotectonic history, including partial recrystallization in the deep crust, until its emplacement within Cretaceous limestones.