LATE MANTLE EVOLUTION OF THE PYRENEAN SUB-CONTINENTAL LITHOSPHERIC MANTLE IN THE LIGHT OF NEW AR-40-AR-39 AND SM-ND AGES ON PYROXENITES ANDPERIDOTITES (PYRENEES, FRANCE)

Spinel peridotite bodies from the North Pyrenean Zone (NPZ) contain ub iquitous anhydrous pyroxenite layers and, in the Lherz area exclusivel y, late dykes or veins of amphibole pyroxenites and hornblendites cros s-cutting the layering. Anhydrous and amphibole-bearing pyroxenites re present the high-pressure segregates of successive tholeiitic and alka line magmas under mantle conditions, respectively. In two localities ( Caussou and Montaut) the peridotites themselves contain several percen t of modal amphibole. Ar-40-Ar-39,nd Sm-Nd methods of dating have been used to decipher the late (Mesozoic) evolution of the ultramafic bodi es from the French Pyrenees. Ar-40-Ar-39 ages on amphiboles from Lherz and Caussou samples cluster in the 103-108 Ma range and Sm-Nd interna l isochrons on garnet amphibole pyroxenites from Lherz yield ages of 1 04 +/- 5 Ma with initial epsilon(Nd) values between +5 and +7. Middle Cretaceous ages recorded by both dating methods argue for a rapid upli ft of ultramafic slices into the crust. In agreement with previous stu dies, the ages and the range of initial epsilon(Nd) values confirm the genetic link, in this area, between the crystallization of amphibole under mantle conditions and the Cretaceous alkaline magmatism known in the NPZ since ca, 105 Ma. Sm-Nd linear arrays defined by whole rock, clinopyroxene and garnet analyses from layered anhydrous garnet pyroxe nites yield more dispersed Nd ages with from east to west, 153 +/- 3 M a with epsilon(Nd)(153 Ma) = +10.1 (Prades), 177 +/- 3 Ma with epsilon (Nd)(177 Ma) = +10.8 (Moncaup-Arguenos) and 138 +/- 4 Ma with epsilon( Nd)(138 Ma) = +6.6 (Moncaut-Hourat), respectively. The real significan ce of these ages remains uncertain. However, they are believed to corr espond to incomplete Nd rehomogenizations during the fast ascent of th e ultramafic bodies. Therefore, we conclude that the crustal emplaceme nt of the ultramafic bodies in the Pyrenees is an unique event linked to the rotation of the Iberian Peninsula relative to Europe at 110-105 Ma. Finally, whole-rock analyses on layered garnet-bearing pyroxenite s yield a Sm-Nd errorchron which is believed to approximate the age of an ancient fractionation in the mantle at ca. 450-500 Ma. Although th e geological significance of the ages yielded by layered pyroxenites r emains open to discussion, the Nd heterogeneities of the sub-continent al lithospheric mantle below the Pyrenees can be explained by a long t ime integrated evolution of the Sm-Nd system recording the succession of Mesozoic, Phanerozoic and Proterozoic chemical fractionations under mantle conditions. (C) 1998 Elsevier Science B.V. All rights reserved .