Importance of late-magmatic and hydrothermal fluids on the Sm–Nd isotope mineral systematics of hypersolvus granites

Abstract

Despite the significant impact of the Sm–Nd radiometric system in various fields of isotope geochemistry, very few attempts have been made to use it at the mineral scale in granite studies. This approach has been used on two granites from the anorogenic province of Corsica (SE France). Two statistically acceptable alignments were obtained in an isochron diagram for the hypersolvus fayalite-bearing granite of Mantelluccio. Alkali–feldspar, amphibole, inclusion-free zircon, fluorite and fergusonite give an age of 331±10 Ma, whereas allanite, whole-rock, amphibole and inclusion-free zircon yield a significantly younger age at 291±13 Ma. The latter is validated by a zircon U–Pb age of 283±1 Ma as the magmatic emplacement date of the granite. The other alignment, giving a 50-Ma older age, can be interpreted either as being inherited or, more likely, as the result of hydrothermal convection interacting with the country rocks at the end of granite crystallization. The Sm–Nd systematics of epidote records a second fluid–rock event, which probably occurred at least 200 Ma after the emplacement of the granite. Extremely low 143Nd/ 144Nd ratio indicates that the epidote crystallized from a fluid that carried lanthanides in the crust over a distance of at least 1 km. Zircons with inclusions have a lower initial 143Nd/ 144Nd ratio than inclusion-free zircons. This feature is attributed to apatite and/or zircon core inheritance, through contamination, during the ascent of the granitic melt in the crust. Two isochrons have been also obtained for the hypersolvus peralkaline granite of Evisa. Amphibole, fluorite, sphene and fergusonite yield an age of 259±6 Ma, whereas the whole-rock, alkali–feldspar and metamict zircon give an age at 209±14 Ma. In this case, the former value is roughly in agreement with a possibly slightly rejuvenated whole-rock Rb–Sr date of 249±3 Ma, suggesting that it documents the magmatic crystallization age of the granite. Sm–Nd isotope systematics of the alkali–feldspar, metamict zircon and possibly allanite, were re-equilibrated during an important hydrothermal event that affected Western Europe in Early Jurassic times. In both examples, however, it appears that the whole-rock Nd isotopic signatures were not strongly affected by the various fluid–rock events, indicating that they can still be used for petrogenetic investigations. In contrast, these results show that, like K–Ar and Rb–Sr, the Sm–Nd systematics of minerals with weak resistance to post-magmatic processes can be used to trace and date hydrothermal events.