CRUSTAL EVOLUTION OF THE HERCYNIAN BELT OF WESTERN-EUROPE - EVIDENCE FROM LOWER-CRUSTAL GRANULITIC XENOLITHS (FRENCH MASSIF-CENTRAL)

Granulite-facies lower-crustal xenoliths scavenged by the Tertiary basaltic pipes of Bournac and Roche Pointue (Massif Central, France) have been analysed for major and trace elements and radiogenic (Sr, Nd) isotope ratios. They display wide variations in major elements, rare-earth elements and isotopes ( 87Sr 86Sr = 0.7041-0.7216; ε{lunate}Nd = -11.9 to +1.5), which tend to correlate with the three different rock types encountered: basic and acidic meta-igneous and metasediments. The basic liquids are of calc-alkaline affinity, and cumulates are abundant. There is a gap in silica content between the basic and acid meta-igneous xenoliths. The isotopic compositions suggest mixing of old pre-existing continental crust (represented by the metasediments) and mantle-derived basic melts which represent juvenile addition to the crust during the Hercynian orogeny. The basic meta-igneous rocks have been contaminated during their emplacement into the crust whereas the surrounding metasediments have suffered partial fusion and contamination by the same liquid, producing the Hercynian granitoids which have ε{lunate}Nd and ε{lunate}Sr initial ratios identical to those of the lower-crustal xenoliths. The residue from melting may be represented by the acid meta-igneous rocks, some of which have positive Eu anomalies. These results indicate that an important basaltic underplating event occurred during the evolution of Hercynian orogenic belt, although the exact timing of the underplating requires further constraint. © 1990.