Sources of Hercynian granitoids from the French Massif Central: Inferences from Nd isotopes and consequences for crustal evolution

Abstract

The French Massif Central is characterized by a great abundance and variety of granitoids emplaced during Hercynian times (360-280 Ma). Distinct ϵ Nd(i) and ( 87 Sr 86 Sr) i are not associated with particular petrographic types; however, lower ϵ Nd(i)-values (−8.5) occur among the younger leucogranites from the western part of the massif. Intermediate ϵ Nd( i) (−6) correlate with aluminous monzogranites and higher values (−6 to −3) with calc-alkaline granodiorites. In the eastern Massif Central ϵ Nd(i) are homogeneous at the regional scale. In the western Massif Central, the leucogranites and the monzogranite-granodiorites display different trends of ϵ Nd( i) with emplacement ages. From these results, extreme models involving a depleted mantle, or an old (Archean or Early Proterozoic) source may be ruled out. The trend of increasing ϵ Nd( i) with decreasing age observed in the monzogranite-granodiorite group suggests either an increasing contribution of a mantle component, or the progressive melting of more mafic, more refractory crustal sources, or both. The reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile. The large amount of granites may be the result of favourable processes during the collision tectonics, combined with the unusual abundance of fertile materials, enriched in water and heat-producing elements, in the pre-Carboniferous crust.