Nd Isotopes in the Stratigraphic Record of the Montagne Noire (French Massif Central): No Significant Paleozoic Juvenile Inputs, and Pre‐Hercynian Paleogeography

Abstract

We present Sm‐Nd isotopic data on 46 shales, three sandstones, and four volcanogenic and volcanic rocks from the Montagne Noire in the southern French Massif Central. The sedimentary record that makes up one of the most complete Paleozoic sequences in Europe was sampled from Early Cambrian to Early Jurassic (560–180 Ma). This sequence is interrupted by two hiatuses. The first is situated in the Middle Ordovician (490–450 Ma) and the second during the Carboniferous (330–300 Ma). We analyzed volcanogenic materials and lavas in order to identify possible source materials. The Neodymium isotopic initial compositions, expressed in $$\varepsilon \mathrm{Nd}\,{}_{T}$$ or in $$T_{DM}$$ , are important tracers in shales because they represent an average of eroded source rocks. During the studied time span, most of the $$T_{DM}$$ 's are around 1.6 Ga. Some rapid $$\varepsilon \mathrm{Nd}\,{}_{T}$$ changes are observed toward less radiogenic values between 540 and 520 Ma and toward more radiogenic values at 367 and 300 Ma. These variations are related to different sedimentary settings and to a volcanic event (300 Ma) detected in a continental basin. Results indicate that, in any case, radiogenic $$\varepsilon \mathrm{Nd}\,{}_{T}$$ values do not reflect significant Paleozoic juvenile inputs related to orogenic processes in the south of the French Massif Central. Assuming that similar $$\varepsilon \mathrm{Nd}\,{}_{T}$$ values characterize a same tectonic domain, we compare our pre‐Hercynian shales data (before 330 Ma) with other published data from the Armorican domain and some Iberian domains (Central Iberian zone and Cantabrian zone). These constraints confirm that these three areas belonged to the same paleogeographic entity at least from the Early Ordovician to the Early Carboniferous (490–330 Ma). In addition, the Montagne Noire underwent a different tectonic evolution from Late Ordovician to Early Devonian time (450–400 Ma).