La lherzolite à amphibole du gisement de Caussou (Ariège,France)

The Caussou outcrop consists mostly of a spinel-bearing lherzolite with irregular patches of amphibole lherzolite. The characteristic paragenesis of the latter is: forsterite + bronzite+Ti-rich K-bearing pargasite ± diopside, with 10 to 15% amphibole in the rock. Petrographic and chemical analysis of the two types of rocks and their constituent minerals lead to the conclusion that the spinel-lherzolite recrystallized locally as amphibole lherzolite in the presence of a gas phase containing water, and probably Ti and alkali elements as well, at approximately 7 to 8 Kb and 1100° C (for \(P_{{\text{H}}_2 {\text{O}}} = P_{{\text{total}}} \) ). Two hypothesis could account for this local recrystallization:

1. The amphibole lherzolite could represent a subsolidus recrystallization of the spinel lherzolite occuring in the stability field of plagioclase lherzolite at the time of the emplacement.
2. Or, in the same P-T conditions, the Ti-pargasite could precipitate from a liquid of nephelinite composition produced by limited partial melting of the spinel lherzolite.

In either case the original peridotite that produced the two existing types at Caussou could be considered as an undifferentiated fragment of the upper mantle.