SR-ND ISOTOPE AND TRACE-ELEMENT GEOCHEMISTRY OF LATE VARISCAN VOLCANISM IN THE PYRENEES - MAGMATISM IN POSTOROGENIC EXTENSION

We obtained Sr-Nd isotope and trace-element data on late Variscan volc anic rocks in the Pyrenees: felsic lavas and andesites from the Ossau and the Serra de Cadi massifs and calc-alkaline andesites and alkali b asalts from the La Rhune and the Anayet volcanic massifs. The calc-alk aline volcanic rocks have crustal trace-element and Sr-Nd isotopic cha racteristics. The andesites probably originated from melting of Pan-Af rican paragneisses, with a limited contribution of mantle-derived sour ces having lithospheric characteristics. The andesitic lavas were more or less contaminated in magma chambers by Palaeozoic sedimentary rock s, which can be interpreted as a recycled Proterozoic crust. The felsi c lavas are principally derived from anatexis of the Palaeozoic produc ts, the rhyolites providing even peraluminous characteristics. The Ana yet andesites show mantle-derived characteristics, and are interpreted as a mixture between a strongly undersaturated continental alkali bas alt and a peraluminous end-member. However, Sr and Nd contents rule ou t any simple process. The late Anayet alkali basalts show typical asth enospheric Sr and Nd isotopic characteristics. The La Rhune basalts ex hibit Sr and Nd sub-continental lithospheric isotopic characteristics, very similar to those of the Triassic Pyrenean continental basalts. I t is thought that the granulite facies metamorphism dated at 310 Ma wa s caused by the ascent of the asthenosphere inducing a thermal anomaly . The mafic magmas ascending from the mantle are responsible for the c rustal anatexis. This results in the emplacement of both the Pyrenean composite plutons, such as the Querigut massif, and the Ossau volcanic rocks. The Pyrenean late Variscan volcanic rocks were emplaced in eit her pull-apart basins opened by the reactivation of the major Variscan Pyrenean fault in a distensive setting or, better, in basins resultin g from a post-orogenic collapse. Both these models involve a thinning of the subcontinental lithosphere, until its disappearance. Consequent ly, the inferred mantle-derived end-member of the Anayet lavas, which occurs in a post-Variscan distensive setting, clearly derives from the asthenosphere.