Abstract
Abstract A complementary series of 40 K 40 Ar dates of some selected lavas (flow and intrusive) from southern France and associated clays indicate that the magmatic event took place in the Massif Central during Jurassic period around 170 – 165 ± 8 Ma ago. Dating of magma emplacement and of later geochemical effects is based on: (1) K exchange linked to alteration immediately after emplacement; and (2) a strong remobilization that affected the base of the volcanic pile 30 Ma later. Paleomagnetic studies of the dated lavas give positions of the geomagnetic pole in accordance with the previous polar wandering curve for this period. The stratigraphic position of volcanism between Upper Bajocian and Bathonian sediments allows us to propose an age between 170 and 165 ± 8 Ma for the Bajocian-Bathonian boundary.
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