ZIRCON TYPOLOGY COMBINED WITH SM-ND WHOLE-ROCK ISOTOPE ANALYSIS TO STUDY BRIOVERIAN SEDIMENTS FROM THE ARMORICAN MASSIF

The identification of source materials contributing to mature terrigen ous elastic deposits is made difficult due to a lack of useful discrim inant criteria. Sm-Nd isotope analysis can provide some indication of the presence of mantle-derived constituents in elastic rocks. However, since this method is based on whole-rock samples, it is only possible to obtain the averaged composition of the different source materials involved. Moreover, Sm-Nd isotope systematics can be strongly influenc ed by the presence of heavy minerals rich in rare earth elements and/o r displaying isotopic ratios very different to the sedimentary host. I n this manner, 0.1% of zircon or 0.01% of monazite from an extraneous source is sufficient to modify significatively the Sm-Nd signature of the whole-rock. The typological study of zircon populations is an extr emely valuable tool, especially since this mineral is highly resistant and exhibits a morphology controlled by the physical and chemical con ditions under which it crystallized. Zircons that have been reworked i n a sedimentary deposit display typologies that make it possible to id entify the different igneous rock-types present in the source region. Moreover the typological study can be associated with a single-grain P b-Pb dating. This twofold approach was applied to Brioverian sedimenta ry rocks (Neoproterozoic III to Early Palaeozoic in age) from the Cent ral Brittany Domain (Armorican Massif, NW France). It is proposed that these deposits inherited a major component from juvenile crustal mate rials (epsilon(Nd(540)) = -1.4 to -6.3), reflecting the presence of ig neous precursors with a dominantly mantle-derived origin (zircon sub-t ypes S19, S20, S24, S25 etc.), to which was added another component of crustal anatectic affinity (zircon sub-types S2, S6, S7 etc.). Two ig neous source regions are identified on the basis of the present study: the anatectic granitic massifs of the Mancellian Batholith (540 Ma), along with a coeval ignimbritic suite of end-Proterozoic/Eocambrian ag e, and the syn- and post-tectonic dioritic massifs (590 to 530 Ma). Ap art from these main sources of elastic supply, it is very likely that sporadic contributions arose from basic to intermediate volcanic activ ity of mantle origin linked to the Cadomian orogeny and from the Icart ian basement.