Manganese in pelagic carbonates:: indication of major Tectonic events during the geodynamic evolution of a passive continental margin (the Jurassic European margin of the Tethys-Ligurian Sea)

Mid-oceanic hydrothermal activity is the main source of manganese in the pelagic realm. Consequently, long-term fluctuations of manganese in bulk carbonates could represent the variations in the hydrothermal flow intensity through time depending on global tectonic activity. Within this scope, we analysed three Jurassic sedimentary series in the Southeastern Basin (France), thus providing valuable information for further palaeogeographic and geodynamic reconstructions of the European Tethyan passive margin. The long-term variations of the manganese curve can be correlated with second-order sea-level changes. The main transgressive phases (Early Toarcian, Late Aalenian to Bajocian and Late Bathonian to Callovian) are marked by a manganese content increase whereas the regressive phases (Late Pliensbachian, Late Toarcian to Middle Aalenian, Early Bathonian to Middle Bathonian, Oxfordian) are characterized by decreasing trends. Furthermore, some manganese positive anomalies can be superimposed on the global geochemical record of sea-level changes. These coincide with periods of increasing subsidence rate (Early Toarcian, Late Bajocian, Middle and Late Oxfordian) and also with phases of extension (Early Aalenian) of the European margin of the Ligurian Ocean. The major event is the very sharp increase of manganese content localised in the Athleta Zone (Late Callovian) and corresponds to the first occurrence of oceanic crust. Moreover, the manganese enrichments recorded in carbonates deposited before Callovian times. may have resulted from hydrothermal emanations during the first stages of rifting. (C) 2000 Elsevier Science B.V. All rights reserved.