Sr-Nd isotope and trace-element geochemistry of late Variscan volcanism in the Pyrenees: Magmatism in post-orogenic extension?

Abstract

We obtained Sr-Nd isotope and trace-element data on late Variscan volcanic rocks in the Pyrenees: felsic lavas and andesites from the Ossau and the Serra de Cadi massifs and calc-alkaline andesites and alkali basalts from the La Rhune and the Anayet volcanic massifs. The calc-alkaline volcanic rocks have crustal trace-element and Sr-Nd isotopic characteristics. The andesites probably originated from melting of Pan-African paragneisses, with a limited contribution of mantle-derived sources having lithospheric characteristics. The andesitic lavas were more or less contaminated in magma chambers by Palaeozoic sedimentary rocks, which can be interpreted as a recycled Proterozoic crust. The felsic lavas are principally derived from anatexis of the Palaeozoic products, the rhyolites providing even peraluminous characteristics. The Anayet andesites show mantle-derived characteristics, and are interpreted as a mixture between a strongly undersaturated continental alkali basalt and a peraluminous end-member. However, Sr and Nd contents rale out any simple process. The late Anayet alkali basalts show typical asthenospheric Sr and Nd isotopic characteristics. The La Rhune basalts exhibit Sr and Nd sub-continental lithospheric isotopic characteristics, very similar to those of the Triassic Pyrenean continental basalts. It is thought that the granulite facies metamorphism dated at 310 Ma was caused by the ascent of the asthenosphere inducing a thermal anomaly. The mafic magmas ascending from the mantle are responsible for the crustal 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 either pull-apart basins opened by the reactivation of the major Variscan Pyrenean fault in a distensive setting or, better, in basins resulting from a post-orogenic collapse. Both these models involve a thinning of the subcontinental lithosphere, until its disappearance. Consequently, the inferred mantle-derived end-member of the Anayet lavas, which occurs in a post-Variscan distensive setting, clearly derives from the asthenosphere.