The geodynamic significance of alkaline magmatism in the western Mediterranean compared with West Africa

Abstract

Alkaline magmatic provinces occur globally in regions subjected to cratonization. Therefore, they may appear just after the end of an orogenic episode, when collision between two continental plates caused rapid uplift and unroofing of orogenic batholiths by isostatic adjustment under intense stress fields. A reversal in the sense of movement in a pre-existing fault system provoked alkaline magmatism. As reversals in the sense of movement are signs of global tectonics, it can be shown that at each orogeny an alkaline magmatism developed, essentially when relative plate motions became stationary. The Corsican alkaline province is a part of a larger western Mediterranean province, emplaced at the end of the Hercynian orogeny. Lithospheric faults along which melts from the mantle have ascended through the continental crust may subsequently act as preferential lines of weakness for oceanic basin development. In West Africa, alkaline provinces are essentially emplaced in Pan-African domains. A number of provinces are Cambrian in age, forming just after the termination of the Pan-African orogeny and located near major suture zones. Unlike the western Mediterranean province, the lithospheric faults, responsible for the ascent of the alkaline magmas, have not been reactivated in West Africa and no subsequent oceanic opening has occurred. In addition, Pan-African domains have been the sites of later alkaline provinces, during various time periods since the end of the Pan-African orogeny. Subsolvus granites predominate as early as 10 Ma after the last compressive tectonic phase, then hypersolvus granites supersede them during a 500 Ma period. Lastly, mixed and undersaturated complexes developed with increased proportions of carbonatites.