Variscan evolution of the Tanneron massif, SE France, examined through U–Pb monazite ages

Abstract

The Tanneron massif belongs to the southernmost segment of the Variscan belt in France and is composed of migmatitic orthogneisses and paragneisses cross-cut by a main tonalite–granite complex. Late Carboniferous detrital basins are bounded by north–south-trending ductile to brittle normal faults and delineate three distinct zones. Our new isotope dilution U–Pb monazite dating indicates a pre-Variscan history in the central part of the massif recorded by monazites from an orthogneiss yielding ages from 440 to 410 Ma, whereas monazites from a migmatitic paragneiss record a late Carboniferous high- T event at 317 ± 1 Ma. In the eastern part, a migmatization event is recorded by monazites from a synkinematic leucogranitic layer and a mylonitic orthogneiss yielding ages of 309 ± 5 and 310 ± 2 Ma, respectively. Later post-collisional magmatism related to lithospheric thinning is recorded by the intrusion of undeformed granitic bodies at 302 ± 4 and 297 ± 5 Ma, and marks the final stage of the Variscan evolution. The data indicate the presence of two crustal blocks, displaying different levels of exposure of the Variscan crust. The overall tectonomagmatic evolution of this massif suggests a strong affinity with the internal part of the Variscan belt.