40Ar 39Ar chronology of Variscan tectono-metamorphic events in an exhumed crustal nappe: the Monts du Lyonnais complex (Massif Central, France)

Abstract

The Monts du Lyonnais metamorphic complex is part of the high-grade nappes of the Massif Central, France. Retrogression of metamorphic conditions occurred synchronously with evolution of deformation from penetrative thrust structures at high- T conditions (stage P 1–2, 700±50°C) to strike-slip ductile shear at lower P- T conditions (stage P 3, T⩽600°C). This evolution distinguishes progressive exhumation and cooling of this previously buried Variscan segment. In this study, 40Ar 39Ar chronologic investigations are addressed to representative samples of each tectono-metamorphic stage associated with the exhumation of the nappe. The two mineral generations yield indistinguishable dates of 350-340 Ma which are interpreted as post-metamorphic cooling ages. However, the minerals which crystallized during the late intra-continental stage P 3 yield concordant plateau dates, while the minerals which grew during the P 1–2 stage and escaped later retrogressive deformation systematically display disturbed age spectra. These heterogeneous profiles could result from the presence of a minor excess Ar component, incorporated at the time of the P 3 event, when intense deformation took place under decreasing metamorphic temperature and synkinematic granite emplaced along ductile wrench faults. This tectono-metamorphic event could have been responsible for a local redistribution of Ar from the deformed rock volumes into the less or not deformed ones, or for a redistribution of Ar at the grain scale. In the Monts du Lyonnais complex, the effects of intense deformation and dynamic crystallization were probably associated with retrograde cooling to control the contrasting behaviour of the two mineral generations. 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, muscovites and biotites used in this study suggests relatively rapid cooling (at least 50°C Ma −1) of the complex after attainment of metamorphic conditions associated with the P 3 event. The younger values of 325-315 and 250 Ma produced by K-feldspar and plagioclase, respectively, may reflect that: (1) the metamorphic complex experienced slow cooling (at most 10°C Ma −1) between 325 and 315 Ma ago; and (2) a late, weak thermal event (<200°C) occurred 250 Ma ago.