THE ORIGIN OF PYRENEAN HERCYNIAN VOLCANIC-ROCKS (FRANCE-SPAIN) - REE AND SM-ND ISOTOPE CONSTRAINTS

The Hercynian orogeny of the Pyrenees generated a suite of basalt to r hyolite calc-alkaline volcanic rocks that were erupted subaerially. Th ese melts may either have been generated above a subduction zone or by lower-crustal melting after input of mantle-derived magmas in an exte nsional environment. Sm-Nd garnet/whole-rock mineral isochrons date a rhyolitic ignimbrite from the Coll de Oli Ignimbrite Member at 313+/-1 4 Ma (2 sigma) (MSWD=0.7) and a rhyolitic lava of the Coll de Pi regio n at 320+/-2 Ma (2 sigma) (MSWD=0.2). These ages suggest that some of the volcanism was synchronous with high-temperature, low-pressure meta morphism and partial melting deep in the Hercynian crust as seen in ro cks now exposed in the axial and northern Pyrenean zones and dated by U-Pb on zircon at between 309+/-5 and 315+/-5 Ma. The silicic volcanic rocks are peraluminous and have highly fractionated REE patterns with low abundances of the HREE and (La/Yb)(n) approximate to 70. Mineral and whole-rock REE abundances suggest derivation of the silicic rocks by partial melting of a metasedimentary source in equilibrium with gar net and/or garnet fractionation in melts derived from peraluminous sou rces. Nd isotope compositions of the entire volcanic rock suite (mafic -silicic compositions) are within the same range as those of sediments metamorphosed during the Hercynian orogeny and Hercynian basement roc ks. They have mid-Proterozoic depleted-mantle model Sm-Nd ages and do not exhibit mixing trends with mantle sources. Even the most mafic of the volcanic rocks appear to have interacted extensively with the crus t. Analysis of the chemistry of the volcanic rocks does not unequivoca lly constrain the Hercynian tectonic setting of the Pyrenees.