Mantle source heterogeneity beneath the Garrotxa Volcanic Field (NE Spain)

The Neogene volcanism in NE Spain, which is related to the rift-type extensional tectonics affecting the eastern margin of Iberia since late Oligocene, is well represented in the basalts and basanites from the Garrotxa Volcanic Field (GVF). Geochemistry of the GVF magmas is comparable to that of the Cenozoic Eastern Atlantic and Euro-Mediterranean eruptive centers related to the European Astenospheric Reservoir (EAR). The EAR is thought to have originated by the mixing between plume-like material (carrying a HIMU signature) and the shallower depleted mantle (DMM). The GVF mantle source and the EAR share comparable trace element distribution, enrichment levels and characteristic ranges of trace element ratios (e.g., Ba/La, K/La, Zr/Nb). However, isotope data from the GVF magmas call for the presence of a compositionally heterogeneous mantle source, which is considered to be the result of the mixing between two end-members. While a first end-member presents relatively high 143Nd/144Nd and 206Pb/204Pb values and shows geochemical features close to average EAR, the second end-member is characterized by relatively lower 143Nd/144Nd and 206Pb/204Pb and slightly higher 87Sr/86Sr ratios that are recurrent in EMI-type magmas. To constrain the isotope features of the GVF magmas, their Sr, Nd and 206Pb/204Pb isotopic compositions together with those of worldwide OIB, anorogenic European rocks, and lithosperic and crustal samples from the Pyrenean and Iberian domains were projected into a tethraedron with the four classical mantle end-members (DMM, EMI, EMII, HIMU) as vertices. The three-dimensional perspective indicates that the GVF mantle source isotope characteristics are consistent with a heterogeneous lithosphere, variously metasomatized by both the EAR and EMI components, and strongly related to the local tectonic structure, e.g., crustal thinning.