Electron microprobe monazite geochronology of magmatic events: Examples from Variscan migmatites and granitoids, Massif Central, France

Abstract

To cite this version:Eugène Abstract U-Th-Pb dating of monazite with the electron probe microanalyser (EPMA) is increasingly documented as a reliable geochronological method offering high spatial resolution. This method has been applied on monazite from the Cévennes migmatites and granitoids from the southeast of the French Massif Central. Measurements were performed on separated grains after systematic back-scattered electron (BSE) imaging. Monazites from migmatites record two main ages: (i) a protolith age of about 550-543 Ma obtained on inherited cores, and (ii) a migmatization event between 329 ± 5 and 323 ± 3 Ma recorded by monazite rims and all other monogenetic grains. Monazite from the peraluminous Rocles pluton yields a 318 ± 3 Ma age. Finally, three granite dykes are dated at 333 ± 6, 318 ± 5 and 311 ± 5 Ma; the older dyke is the most deformed of them and is interpreted as linked to the migmatization event; the two other dykes are geochronologically, petrologically and structurally coeval with the Rocles pluton. The data constrain the timing of crustal melting following Variscan thickening in the northern Cévennes area. Migmatization of Ordovician protoliths took place at 329-323 Ma and was shortly followed by intrusion of leucogranite at 318-311 Ma. The study shows that EPMA dating of monazite can be successfully used to resolve a close succession of regional melting events.

SHRIMP U–Pb zircon dating of anatexis in high-grade migmatite complexes of Central Spain: implications in the Hercynian evolution of Central Iberia

“…In the southeast of the French Massif Central, migmatization has been dated between 329 and 323 Ma (Be Mezeme et al 2006), closer in ages to those exposed in this work. Nevertheless, two successive high-T events have been described in the Velay anatectic dome (French Massif Central), the youngest (at -305 Ma) being responsible for most of the cordierite-bearing granites which characterize this dome (Ledru et al 2001).…”

A review of the pre-Permian geology of the Variscan French Massif Central

“…1), is dated between 333 Ma and 325Ma [1]. The tectonic setting of these migmatites is still disputed.…”

Precambrian protoliths and Early Paleozoic magmatism in the French Massif Central: U–Pb data and the North Gondwana connection in the west European Variscan belt

“…Two HT metamorphic events dated as Middle Devonian and Middle Carboniferous (late Visean) are known in the FMC ([Lafon, 1986], [Duthou et al, 1994], , [Be Mezeme et al, 2005a], [Be Mezeme et al, 2005b] and [Faure et al, 2008]). The Middle Devonian event is related to the nappe stacking event ( [Ledru et al, 1989] and and enclosed references).…”