Coticules of the Belgian type area (Stavelot-Venn Massif): Limy turbidites within the nascent Rheic oceanic basin

Coticule is the name originating in Belgium for a fine-grained metasedimentary yellowish rock mainly composed of quartz, spessartine and mica, which is repeatedly interstratified in the hematite-rich purple slates of the Lower Ordovician of the Ardenne region. It was described for the first time in the nineteenth century in the Stavelot-Venn Massif (Ardenne), which is accordingly the type-area for this peculiar lithology. Since then, numerous occurrences were described all over the world in metamorphic rocks of various grades and ages. The exact nature of the Belgian coticule protolith remained hypothetical, although intensively discussed for a long time in the literature. After an extensive review of all the stratigraphical, sedimentological and geochemical results and new field, geochemical (major and trace elements) and Nd isotopic investigations, we propose a significantly improved and sustained genetic model: the thin coticule layers were deposited offshore in a deep oceanic basin as limy mud turbidites. During the early Floian (c. 477 Ma, Early Ordovician), density currents came from the north, from the shelf which bordered the emergent Brabant Massif at that time and flowed down the slope to the deep basin-plain. The purple shales enclosing the coticule layers represent the normal pelagic sedimentation in the basin. These shales are exceptionally rich in Fe and Mn, because of the hydrothermal activity of the nearby and young Rheic Ocean ridge. Indeed, the Rheic Ocean opened in the Early Ordovician (c. 482 Ma). During diagenesis, the strongly oxidizing depositional environment favoured the mobility of Mn2 +, as opposed to Fe3 + that remained insoluble and immobile. This allowed for the migration of divalent manganese from the pelagic shales and for replacing calcium in the turbiditic carbonate fraction, to form rhodochrosite. Later, epizonal metamorphism transformed the clay-quartz-rhodochrosite paragenesis of the protolith into the muscovite-quartz-spessartine paragenesis of the coticule. Hence, in the type-area, coticule genesis needed a peculiar environment including a continental shelf with limy mud deposits, a continental slope generating periodical turbidites and a nearby oceanic ridge, here the nascent Rheic Ocean, delivering hydrothermal iron and manganese.