Pb and O isotope systematics in granulite facies xenoliths, French Massif Central: implications for crustal processes

Details Pb and O isotope systematics in granulite facies xenoliths, French Massif Central: implications for crustal processes Pb and O isotope systematics in granulite facies xenoliths, French Massif Central: implications for crustal processes

Mar 1991

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991e%26psl.102..342d&link_type=abstract

Earth and Planetary Science Letters, Volume 102, Issue 3-4, p. 342-357.

Computer Science

11

Scientific paper

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 basic liquids, felsic meta-igneous lithologies (charnockites) and metasediments. Ranges of δ18O values are +6.9 to +9.8‰ for mafic xenoliths, +9.3 to +10.2‰ for felsic meta-igneous samples and +6.1 to +11.8‰ for the metasediments. By comparison, δ18O values for mantle peridotites from the same region range from +5.1 to +6.9‰, whilst local Hercynian granitoids vary from +8.6 to +12.0‰. The 206Pb/204Pb ratios of the granulite xenoliths are between 17.77 and 19.19, 207Pb/204Pb ratios vary from 15.51 to 15.69, and 208Pb/204Pb 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.

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