Geochemical characteristics of basalts related to incipient oceanization: The example from the Alpine‐Tethys OCTs

Abstract : Basalts exposed in the Platta and Tasna nappes (SE Switzerland) derive from the Alpine-Tethys Ocean-Continent Transitions (OCT) and overlie SubContinental Lithospheric Mantle (SCLM). We show that the trace element signatures of these basalts differ from Mid-Ocean Ridge Basalts (MORB). Two types of basalts occur in the OCT: a type-1 showing a "garnet signature" that can be modeled by the partial melting of the SCLM in the spinel stability field and a type-2 characterized by an enrichment in incompatible elements that can be explained by the mixing between garnet-pyroxenite-derived melts and the melting of either a depleted MORB mantle or a refertilized SCLM. Based on geological and geochemical observations we propose that the basalts from the Alpine-Tethys OCTs result from a poly-phase magmatic system that carry an inherited SCLM signature. These basalts should therefore be referred to as OCT-basalts rather than as MOR-basalts.