Petrology and Genesis of Hercynian Alkaline Orthogneisses from Provence, France

Major and trace element variations in an Ordovician alkalineorthogneiss (ortholeptynites) series of the Maures massif, France,suggest that the pre-metamorphic rocks were alkaline and peralkalinerhyolites derived by fractional crystallization from trachy-rhyoliticmagma. Differentiation proceeded under low oxygen fugacity conditions,with the production of high-silica comendites. By comparisonwith unaltered alkaline and peralkaline rhyolites, it is possibleto prove that the parental magma was not similar to the metatholeiitesthat are associated with the orthogneisses in the field. Norwere the metarhyolites derived from the alkaline to transitionalmetabasalts of the same age which occur elsewhere in the Mauresmassif. The orthogneisses have geochemical features typicalof alkaline and peralkaline rhyolites occurring with basaltsin continental, anorogenic, bimodal associations. This hypothesisis also supported by zircon morphology and mineral chemistry.The Maures massif was a within-plate continental volcanic province,whose crust underwent doming and thinning, but not break up,during lower Palaeozoic times.