Using petrochronology to re-investigate the age of the HP metamorphism in the French Massif Central

The P–T–t evolution of eclogite samples from a locality of the French Massif Central where a Silurian age for the high-pressure metamorphism is commonly accepted is reinvestigated. Petrology combined with LA-ICP-MS U-Pb dating and trace-element analysis in zircon and apatite discard the Silurian age and rather reveal an Ordovician (c. 490 Ma) rifting, a Devonian (c. 370 to 360 Ma) subduction and a Carboniferous (c. 350 Ma) exhumation in this part of the French Massif Central.

The petrological study using pseudosection document a prograde evolution in the eclogite facies marked by an increase of pressure above 20 kbar associated with a strong temperature increase from 650 to 850 °C. Peak-temperature and incipient decompression to the high-pressure granulite facies (19-20 kbar and 875°C) were accompanied by partial melting of the eclogite. Further decompression resulted in partial equilibration in the high-temperature amphibolite facies (<9 kbar, 750-850°C). Local fractures filled by analcite and thomsonite testify to late interaction with alkaline fluids. Metamorphic zircon with eclogitic REE patterns (no Eu anomaly, flat HREE) and inclusions (garnet, rutile and probably omphacite) shows concordant apparent ages that spread from c. 370 down to c. 310 Ma. A c. 350 Ma age of apatite attributed to cooling following decompression from the eclogite facies indicates that zircons younger than 350 Ma, were rejuvenated but preserved an apparent eclogitic signature. It is suggested that interaction with alkaline fluids at low temperatures would lead to the recrystallisation of zircon while leaving apatite unaffected.

Comparison with available P–T–t data from eclogites in Western Europe shows that Devono-Carboniferous high-temperature eclogites are also recognized in the Saxo-Thuringian and Moldanubian zones of the Bohemian Massif suggesting they belonged to the same subducting bloc. Devono-Carboniferous trench/arc and arc/back-arc relationships recognized in the Bohemian Massif and the French Massif Central respectively point to a southward subduction in both areas. This comparison challenges the historical interpretation of a northward subduction in France and brings an overall more coherent picture of the Variscan belt.