Sr, Nd a136 (1997) 99-122 Hercynian granodiorites and monzogranites, Massif Central, France

Variations in Sr, Nd and Pb initial isotope ratios of three large Hercynian granitoid massifs of the French Massif Central are consistent with formation of the granitoids by partial melting of the lower crust. The nature of the lower crust of the Massif Central is known from the granulite-facies lower-crustal xenoliths brought to the surface by Tertiary alkaline volcanics. Studies of the xenoliths have revealed that, during the Hercynian orogeny (360-290 Ma ago), the pre-existing metasedimentary and acid metaigneous lower crust was intruded by basic magmas, causing anatexis. The mantle-derived basic magmas mixed with melts from the pre-existing lower crust, forming the Hercynian granitoids. The peraluminous Gueret and Margeride granitoids, which are very similar in their major- and trace-element compositions, have Sr-87/Sr-86(i) (0.7089 to 0.7121) and epsilon Nd-i (-7.5 to -3.9) values which indicate that they have a high sedimentary component, considered to have been derived from lower-crustal metasedimentary rocks. However, the granitoids differ significantly in their initial Pb isotope compositions, with the Gueret granodiorites being less radiogenic, indicating the presence of a slightly different crustal source component for the Gueret massif. In contrast, monzogranites from the Millevaches massif are less peraluminous and have higher La/Yb-N and lower Sr-87/Sr-86(i) values (0.7064 to 0.7109). These features, together with the relatively radiogenic Pb isotope compositions of the Millevaches granitoids, suggest derivation from the acid metaigneous lower crust.