On the tectonic significance of retrograde P‐T‐t paths in eclogites of the French Massif Central

Abstract

The retrograde pressure‐temperature‐time (P‐T‐t) paths in eclogites of the French Massif Central constitute a record of evolutionary thermal structures (geotherms) of the internal Variscan zones during uplift. Crystallization of all eclogitic assemblages occurred in the same range of P, T conditions: 700°–750°C for a minimal pressure of 15 kbar. In contrast the retrograde P‐T‐t paths of eclogites in the western and eastern parts of the Massif Central are clearly different. The eclogitic assemblages are retromorphosed at low temperatures in the western Massif Central, under amphibolite facies conditions in the central domain (middle temperature), and under granulite facies conditions in the eastern Massif Central (high temperature). A simple analytical thermal model, well constrained by geological features, allows us to simulate the evolution of this temperature difference during uplift. This study shows that different initial conditions of burial (depth of burial), or differences in tectonic processes during uplift (number of units involved in nappe piling, occurrence of a delay before the emplacement of a last unit) can produce such contrasting P‐T‐t paths. These cannot be explained by variations in erosion rates. In the case of different histories during uplift the P‐T‐t paths can simply be related to an efficient screen effect of a third nappe.