H. Downes et al., SR, ND A136 (1997) 99-122 HERCYNIAN GRANODIORITES AND MONZOGRANITES, MASSIF-CENTRAL, FRANCE, Chemical geology, 136(1-2), 1997, pp. 99-122
Variations in Sr, Nd and Pb initial isotope ratios of three large Herc
ynian granitoid massifs of the French Massif Central are consistent wi
th 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 Ter
tiary alkaline volcanics. Studies of the xenoliths have revealed that,
during the Hercynian orogeny (360-290 Ma ago), the pre-existing metas
edimentary and acid metaigneous lower crust was intruded by basic magm
as, 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 simil
ar 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 indic
ate that they have a high sedimentary component, considered to have be
en derived from lower-crustal metasedimentary rocks. However, the gran
itoids differ significantly in their initial Pb isotope compositions,
with the Gueret granodiorites being less radiogenic, indicating the pr
esence of a slightly different crustal source component for the Gueret
massif. In contrast, monzogranites from the Millevaches massif are le
ss peraluminous and have higher La/Yb-N and lower Sr-87/Sr-86(i) value
s (0.7064 to 0.7109). These features, together with the relatively rad
iogenic Pb isotope compositions of the Millevaches granitoids, suggest
derivation from the acid metaigneous lower crust.