The Goli d'Aget member: Early Permian volcanoclastic and volcanic rocks within the Briançonnais Grand St-Bernard Nappe (Valais, Switzerland)

Abstract

The well-preserved sedimentary structures of the Goli d'Aget Member (Early Permian) allow to distinguish four successive environments of deposition: 1) An alluvial plain with important channeling and evidence of volcanic activity; 2) A shallow lacustrine environment with a low water level, deposition of massive carbonates and an increasing contribution of volcanic materials; 3) A fluviatile plain with the deposition of volcanic tuffs; 4) A deltaic to lacustrine environment occasionally covered by pyroclastic flows. An ignimbritic rhyolite from the upper part of the Goli d'Aget Member has an age of 267-282 Ma (U-Pb on zircon). Major and trace elements, as well as zircon typology point to a calc-alkaline and volcanic-arc affinity for that high-silica rhyolite. The Goli d'Aget Member may be the best-preserved unit among the Permian units from the Swiss Penninic Western Alps. This series provides an important point of comparison with other local Permian units (Dent de Nendaz and Cleuson Members) and with similar series in the Western Alps including the Melogno porphyroids in the Ligurian Alps and the Rochachille Series near Briançon (France).