Transitional top normal MORB affinities in ophiolitic metabasites from the Zermatt-Saas, Combin and Antrona units, Western Alps: implications for the paleogeographic evolution of the Western Tethyan Basin

Abstract

The Zermatt-Saas, Combin and Antrona ophiolite units represent tectonic fragments of the oceanic to transitional lithosphere of the upper Jurassic-lower Cretaceous Piedmont basin, a section of the Western Alpine Tethyan basin. The investigated area is located around the Monte Rosa massif between the middle Aosta valley and the Ossola valley, Italian Northwestern Alps. The Piedmont ophiolite nappe, i.e. the couple of the Zermatt-Saas and Combin juxtaposed tectonic elements, is interposed between the overlying paleo-African continental crust (the Austroalpine tectonic system of the Dent Blanche and Sesia-Lanzo nappes) and the underlying paleo-European continental crust (the Pennine Monte Rosa and St. Bernhard nappes). On the contrary, the Antrona ophiolite unit occurs at a lower structural level. and is sandwiched between the overlying Monte Rosa nappe and the underlying Comughera-Moncucco units, the 'root zone' of the St. Bernhard nappe. Bulk rock analyses of 29 selected samples from these units demonstrate: 1- that the petrogenetic characteristics of the metabasalts and metagabbros from both the Zermatt-Saas and Combin units are strictly comparable to those of normal-MORB magmatism, and 2- that the features of the Antrona metabasalts indicate an oceanic nature for these metamorphic ophiolites with a distinct transitional-MORB affinity. This seems to reflect incipient oceanic rift conditions for the related segments of the Piedmont basin. The available petrochemical data from the Alps, Northern Apennines, Corsica and Calabria, indicate that ophiolites with transitional-MORB affinity represent an early ocean-type magmatism. This was later confined to the external sectors of the accreting Alpine-Apennine oceanic basin, while ophiolites with normal-MORB affinity appear to be related to a subsequent well-established mantle convection in a more developed ocean-ridge system.