K-Ar age of hornblende from a hornblendite vein in the Alpine-type ultramafic mass of the Étang de Lers (Ariège), French Pyrenees

The Alpine-type ultramafic mass of Étang de Lers in the French Pyrenees (the type locality of lherzolite) is transected by a number of hornblendite veins. These veins cut through the lherzolite-pyroxenite layering and obviously are the youngest ultramafic rocks present. Geological field evidence and petrofabric analysis indicate that the whole mass, including the hornblendite veins, was emplaced among Mesozoic sediments as a solid block in Upper Albian or Lower Cenomanian time, immediately before the main phase of Alpine orogenic movements. The rocks of the ultramafic mass are metamorphic tectonites affected by two Alpine sets of fracture cleavages. They do not show, however, any effects of the Alpine low-grade regional metamorphism that affected the country rocks. A detailed study of this ultramafic mass is given in the Ph. D. thesis by Avé Lallemant (1967). K-Ar age measurements were made on the hornblende from a hornblendite vein. The sample was collected at an altitude of 1365 m, about 175 m E. of the northern shore of l’Estagnon (the small pond S. W. of the Etang de Lers). Hornblende makes up about 75 % of the vein rock. Subordinate constituents are brownish augite and opaque ore minerals. The hornblende has a somewhat patchy appearance, with pleochroism from Z = dark yellowish brown (locally with slightly greenish tinge) to Y = brown to chesnut-brown and X = colourless, nz = 1.702 ± 0.002, nx \u2248 1.672, Z/c = 6° and 2VX = 80°. Part of the hornblende crystals are slightly bent.