Comparative geochemical study on Furongian (Toledanian) and Ordovician (Sardic) felsic magmatic events in south-western Europe

Abstract. A geochemical comparison of Early Palaeozoic felsic magmatic episodes throughout the south-western European margin of Gondwana is analysed. The comparison is made between (i) Furongian–Early Ordovician (Toledanian) activies recorded in the Central Iberian and Galicia-Tras-os-Montes Zones of the Iberian Massif, and (ii) Early–Late Ordovician (Sardic) activities in the eastern Pyrenees, Occitan Domain (Albigeois, Montagne Noire and Mouthoumet massifs) and Sardinia. Both phases are related to uplift and denudation of an inherited palaeorelief, and stratigraphically preserved as distinct angular discordances and paraconformities involving gaps of up to 30 m.y. The geochemical features of the Toledanian and Sardic, felsic-dominant activies point to a predominance of byproducts derived from the melting of metasedimentary rocks, rich in SiO2 and K2O and with peraluminous character. Zr / TiO2, Zr / Nb, Nb / Y and Zr vs. Ga / Al ratios, and REE and ƐNd values suggest the contemporaneity, for both phases, of two geochemical scenarios characterized by arc and extensional features evolving to distinct extensional and rifting conditions associated with the final outpouring of mafic tholeiitic-dominant lava flows. The Toledanian and Sardic phases are linked to neither metamorphism nor penetrative deformation; on the contrary, their unconformities are associated with foliation-free open folds subsequently affected by the Variscan deformation. The geochemical and structural framework precludes a subduction scenario reaching the crust in a magmatic arc to back-arc setting, but favours partial melting of sediments and/or granitoids in a continental lower crust triggered by the underplating of hot mafic magmas during extensional events related to the opening of the Rheic Ocean.