Source and evolution of the clinopyroxenes in the Loire and Seine basins (France) based on grain morphology and color

Abstract

Variation in the characteristics (color, morphology) of clinopyroxene grains (CPX) in alluvial deposits and other surficial formations in the Loire valley, the Gâtinais and the Beauce show that many are pristine and come directly from recent volcanic eruptions, whereas weathered grains, clearly reworked, come from erosion of Cenozoic and Pleistocene volcanic rocks of the French Massif Central. After deposition, the CPX have been increasingly altered by longer exposures. Weathering of brown CPX yields paler minerals which are greenish-brown, colourless or two-coloured. Similarly, the CPX found in the Seine basin are from recent volcanic ash or old Sologne deposits, and not from Loire alluvial deposits, so that a Pleistocene palaeo-Loire-Seine river is improbable.