REEVALUATION OF LEAD ISOTOPIC DATA, SOUTHERN MASSIF-CENTRAL, FRANCE

Three independent Pb isotope homogenizing processes operating on large volumes of rock material during limited intervals in the Phanerozoic have been used to define a unique evolutionary curve for rock and ore lead isotopic compositions of the southern Massif Central, France. The model is x(t) = 18.641 - 9.56(e(L1t) - 1) y(t) = 15.678 - 0.06934(e( L2t) - 1) z(t) = 38.701 - 30.8(e(L3*t) - 1) where x(t), y(t) and z(t) are the calculated isotopic ratios (Pb-206/Pb-204, Pb-207/Pb-204 and Pb-208/Pb-204 respectively) at time t years ago and L1, L2 and L3 are the decay constants of U-238, U-235 and Th-232 respectively. The model gives the following ages for averages of the three sets of data used in its generation: age of mineralization for carbonate-hosted lead-zin c ore of Les Malines, ca. 150 Ma; emplacement of the Saint-Guiral-Liro n granite, ca. 290 Ma; and an estimated average age of a group of Camb rian syngenetic deposits, ca. 520 Ma. These ages are in close agreemen t with ages determined by independent methods.