AR-40-AR-39 CHRONOLOGY OF VARISCAN TECTONO-METAMORPHIC EVENTS IN AN EXHUMED CRUSTAL NAPPE - THE MONTS-DU-LYONNAIS COMPLEX (MASSIF-CENTRAL, FRANCE)

The Monts du Lyonnais metamorphic complex is part of the high-grade na ppes of the Massif Central, France. Retrogression of metamorphic condi tions Occurred synchronously with evolution of deformation from penetr ative thrust structures at high-T conditions (stage P-1-2, 700 +/- 50- degrees-C) to strike-slip ductile shear at lower P-T conditions (stage P3, T less-than-or-equal-to 600-degrees-C). This evolution distinguis hes progressive exhumation and cooling of this previously buried Varis can segment. In this study, Ar-40-Ar-39 chronologic investigations are addressed to representative samples of each tectono-metamorphic stage associated with the exhumation of the nappe. The two mineral generati ons yield indistinguishable dates of 350-340 Ma which are interpreted as post-metamorphic cooling ages. However, the minerals which crystall ized during the late intra-continental stage P3 yield concordant plate au dates, while the minerals which grew during the P1-2 stage and esca ped later retrogressive deformation systematically display disturbed a ge spectra. These heterogeneous profiles could result from the presenc e of a minor excess Ar component, incorporated at the time of the P3 e vent, when intense deformation took place under decreasing metamorphic temperature and synkinematic granite emplaced along ductile wrench fa ults. This tectono-metamorphic event could have been responsible for a local redistribution of Ar from the deformed rock volumes into the le ss or not deformed ones, or for a redistribution of Ar at the grain sc ale. In the Monts du Lyonnais complex, the effects of intense deformat ion and dynamic crystallization were probably associated with retrogra de cooling to control the contrasting behaviour of the two mineral gen erations. As a regional consequence, these radiometric data imply that complete retrogression of the Monts du Lyonnais complex was achieved 340 Ma ago. The sharp cluster of ages obtained for the amphiboles, mus covites and biotites used in this study suggests relatively rapid cool ing (at least 50-degrees-C Ma-1) of the complex after attainment of me tamorphic conditions associated with the P3 event. The younger values of 325-315 and 250 Ma produced by K-feldspar and plagioclase, respecti vely, may reflect that: (1) the metamorphic complex experienced slow c ooling (at most 10-degrees-C Ma-1) between 325 and 315 Ma ago; and (2) a late, weak thermal event (< 200-degrees-C) occurred 250 Ma ago.