A vestige of an Ediacaran magmatic arc in southeast France and its significance for the northern Gondwana margin

The Maures–Tanneron Massif is a key to putting new constraints on the age, nature, and tectonic setting of pre-Variscan metaigneous rocks in the southern Variscan Belt. Whole-rock geochemistry was combined with U–Pb isotopic data on zircon to gain insight into the pre-Variscan evolution and improve the knowledge of the southern Variscan domain. The geochemical study shows that the protolith of the studied samples is a high-K calc-alkaline granodiorite emplaced in the upper plate of a subduction zone, most probably in a continental arc setting, as suggested by numerous microdioritic enclaves. Zircon cores record a spread of ages between 609 and 548 Ma and define an age peak at c. 590 Ma, interpreted as the most likely emplacement age of the granodioritic protolith. This Ediacaran population shows a consistent zircon Th/U ratio (~ 0.5) which likely indicates long-lived magmatic activity in a continental arc setting and corroborates our geochemical interpretations. The zircon overgrowth rims give ages from 505 to 460 Ma and might be of metamorphic origin (lower Th/U ratios), related to an important tectono-thermal event that developed during Lower to Middle Ordovician times. The occurrence of older zircon grains ( c. 1000 Ma and c. 1800–2500 Ma), either inherited from a crustal source or incorporated from country rocks during magma ascent, provides some constraints on the paleoposition of the magmatic arc, likely situated on the eastern shelf of the northern Gondwana margin.