REE and Sr/1bNd isotope geochemistry of the alkaline magmatism from the Cretaceous North Pyrenean Rift Zone (France-Spain)

Abstract

Cretaceous alkaline magmatism extends E-W within the north-Pyrenean Rift Zone from the Mediterranean Sea to the Spanish coast of the Bay of Biscay and consists of both effusive and intrusive rocks displaying a wide petrographic diversity: basalts, trachytes, lamprophyres, gabbros, teschenites, ultramafic cumulates and nepheline syenites. Sixty samples representing the main rock-types were analyzed for major and trace elements including the REE. From these, thirteen samples, mainly from mafic rocks, were selected for determination of Sr and Nd isotopic compositions. 87 Sr 86 Sr ratios range from 0.703 to 0.7064 and 143 Nd 144 Nd ratios from 0.5127 to 0.5129. ϵNd( t)-values of Pyrenean samples (+2 to + 6) are those commonly found for continental volcanism, particularly from extension and rift settings. High 87 Sr 86 Sr ratios in some samples coupled with high δ 18O-values suggest they suffered contamination by a Sr-rich, Nd-free fluid such as seawater. Regularities of REE distribution in basalts and associated teschenites substantiate that they formed by variable degrees of melting from LREE-enriched mantle sources which had similar (La/Yb) N close to 5 and probably were situated beneath the lithosphere. LREE enrichment took place 600–1000 Ma ago and is thus related neither to Mesozoic events in the Pyrénées nor to the breakup of Pangea. Finally, the isotopic data are used to discuss the genesis of the continental basalts which is interpreted in term of a three-component mixing: the absence of basalts with Nd as radiogenic as in MORB and the scarcity of mixed enriched types are shown to reflect unique Sr/Nd systematics among the components present in the mantle source of continental basalts.