Late to post-hercynian dyke activity in the Sardinia-Corsica domain: A transition from orogenic calc-alkaline to anorogenic alkaline magmatism.

The paper describes the Corsica and Sardinia Late Hercynian dyke magmatism, which developed in two partially overlapping main phases of calc-alkaline and basaltic rock-suites. The two episodes were accompanied by rhyolitic dykes. Dyke activity began in Corsica around 320 Ma ago and ended in Sardinia about 230 Ma ago. Field data and geochronology suggest that the onset of hypabyssal magmatism occurred soon after the emplacement of the various Hercynian granitoid plutons, but almost always after complete solidification of the host rock. Petrography and mineral chemistry show significant differences between calc-alkaline, basaltic and rhyolitic rocks. The behaviour of trace elements with different degrees of incompatibility show intermediate characters for calc-alkaline magmatism, between those of intraplate basalt and volcanic-arc products. The nature of the source of the calc-alkaline rocks, tholeiitic and transitional basalt, suggests derivation from a subcontinental enriched lithospheric mantle. Crustal contamination during magma ascent appears to have played a minor role. Alkaline rocks (basanite, alkali basalt and their fractionated types: hawaiite and mugearite) show incompatible element distribution and initial Sr and Nd isotopic ratios typical of mantle-derived astenospheric uncontaminated magmas. Three rock-groups of felsic dykes are present: peraluminous, metaluminous and peralkaline rhyolite. The first, similar to S-type granite, have been related to partial melting of a metapelitic crustal source. The metaluminous rhyolite shows remarkable similarities with the calc-alkaline granitoid of the Sardinia-Corsica Batholith, and is linked to melting processes of a basic-intermediate meta-igneous source. The peralkaline rhyolite shows anorogenic features, a strong affinity with A-type granite, and a mantle signature with possible crustal involvement. The dyke magmatism reflects the evolution of mantle sources from orogenic up to anorogenic types, predating the Tethys opening and the beginning of the Alpine cycle.