Nd isotopes in the stratigraphic record of the montagne noire (French Massif Central): No significant Paleozoic juvenile inputs, and pre-Hercynian paleogeography

We present Sm-Nd isotopic data on 46 shales, three sandstones, and four vol canogenic and volcanic rocks from the Montagne Noire in the southern French Massif Central. The sedimentary record that makes up one of the most compl ete Paleozoic sequences in Europe was sampled from Early Cambrian to Early Jurassic (560-180 Ma). This sequence is interrupted by two hiatuses. The fi rst is situated in the Middle Ordovician (490-450 Ma) and the second during the Carboniferous (330-300 Ma). We analyzed volcanogenic materials and lav as in order to identify possible source materials. The Neodymium isotopic i nitial compositions, expressed in epsilon Nd-T or in T-DM, are important tr acers in shales because they represent an average of eroded source rocks. D uring the studied time span, most of the T-DM's are around 1.6 Ga. Some rap id epsilon Nd-T changes are observed toward less radiogenic values between 540 and 520 Ma and toward more radiogenic values at 367 and 300 Ma. These v ariations are related to different sedimentary settings and to a volcanic e vent (300 Ma) detected in a continental basin. Results indicate that, in an y case, radiogenic epsilon Nd-T values do not reflect significant Paleozoic juvenile inputs related to orogenic processes in the south of the French M assif Central. Assuming that similar epsilon Nd-T values characterize a sam e tectonic domain, we compare our pre-Hercynian shales data (before 330 Ma) with other published data from the Armorican domain and some Iberian domai ns (Central Iberian zone and Cantabrian zone). These constraints confirm th at these three areas belonged to the same paleogeographic entity at least f rom the Early Ordovician to the Early Carboniferous (490-330 Ma). In additi on, the Montagne Noire underwent a different tectonic evolution from Late O rdovician to Early Devonian time (450-400 Ma).