PB AND O ISOTOPE SYSTEMATICS IN GRANULITE FACIES XENOLITHS, FRENCH MASSIF-CENTRAL - IMPLICATIONS FOR CRUSTAL PROCESSES

Pb and O isotope data are represented for a suite of granulite facies xenoliths found within Tertiary alkaline volcanic rocks of the Massif Central, France. The suite consists of ultramafic and mafic cumulates, metagabbros which are considered to represent basis liquids, felsic meta-igneous lithologies (charnockites) and metasediments. Ranges of delta-O-18 values are + 6.9 to +9.8 parts per thousand for mafic xenoliths, +9.3 to +10.2 parts per thousand for felsic meta-igneous samples and +6.1 to +11.8 parts per thousand for the metasediments. By comparison, delta-O-18 values for mantle periodites from the same region range from +5.1 to +6.9 parts per thousand, whilst local Hercynian granitoids vary from +8.6 to +12.0 parts per thousand. The Pb-206/Pb-204 ratios of the granulite xenoliths are between 17.77 and 19.19, Pb-207/Pb-204 ratios vary from 15.51 to 15.69, and Pb-208/Pb-204 ratios range from 38.07 to 40.07. In general, metasedimentary granulites have the more radiogenic Pb isotope compositions, whereas mafic meta-igneous samples are less radiogenic. These isotopic characteristics can be explained as the result of the interaction of mafic magmas with the metasedimentary crust into which they intruded. The release of heat also provoked melting of the more fusible parts of the lower crust and led to the formation of late-orogenic Hercynian granitoids. However, an additional component which provides less radiogenic Pb is also needed in the source of the granitoids; this may be the felsic meta-igneous xenoliths or middle/upper crustal gneisses.