Observations sur le mode de gisement et les conditions de mise en place des coulées volcaniques spilitisées de Peyrebrune (Région de Realmont) Tarn/France

In the district of Peyrebrune, several lava-flows are interstratified in a Cambrosilurian aged Schisto-Sandstone Series. The relationships between sediments and lavas, specified by a cut in detail, are the subject of a petrographic and sedimentologic study. According to this study each volcanic eruption is divided in two flows separated by a little amount of “mictites” (mixture of sediments which were rearranged during the eruption, and elements of volcanic origin). The upper flow is much than the lower one and is sometimes thicker cut in pillow-lavas. The mineralogical study shows that the center of the principal flow is made of albitic dolerite with preserved pyroxene and albite and that the boundaries up to one meter and also the lower flow have a spilitic composition. In one flow, taken for example, measurements of density show that the density is 2,99 in the center of the flow, and 2,81 in the boundaries. The chemical analyses indicate that the center has a hyperaluminous basaltic composition and that the more spilitified parts are considerably modified by:

• -Addition of Na (around 60 Kg by m³)
• -loss of Ca and Mg (corresponding to the possible formation of 350 Kg of magnesium calcarous by m³ of spilitified stony mass).