Late mantle evolution of the Pyrenean sub-continental lithospheric mantle in the light of new 40Ar– 39Ar and Sm–Nd ages on pyroxenites and peridotites (Pyrenees, France)

Abstract

Spinel peridotite bodies from the North Pyrenean Zone (NPZ) contain ubiquitous anhydrous pyroxenite layers and, in the Lherz area exclusively, late dykes or veins of amphibole pyroxenites and hornblendites cross-cutting the layering. Anhydrous and amphibole-bearing pyroxenites represent the high-pressure segregates of successive tholeiitic and alkaline magmas under mantle conditions, respectively. In two localities (Caussou and Montaut) the peridotites themselves contain several percent of modal amphibole. 40Ar– 39Ar and Sm–Nd methods of dating have been used to decipher the late (Mesozoic) evolution of the ultramafic bodies from the French Pyrenees. 40Ar– 39Ar ages on amphiboles from Lherz and Caussou samples cluster in the 103–108 Ma range and Sm–Nd internal isochrons on garnet amphibole pyroxenites from Lherz yield ages of 104±5 Ma with initial ε Nd values between +5 and +7. Middle Cretaceous ages recorded by both dating methods argue for a rapid uplift of ultramafic slices into the crust. In agreement with previous studies, the ages and the range of initial ε 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 pyroxenites yield more dispersed Nd ages with from east to west, 153±3 Ma with ε 153 Ma Nd =+10.1 (Prades), 177±3 Ma with ε 177 Ma Nd =+10.8 (Moncaup–Arguenos) and 138±4 Ma with ε 138 Ma Nd =+6.6 (Moncaut–Hourat), respectively. The real significance of these ages remains uncertain. However, they are believed to correspond to incomplete Nd rehomogenizations during the fast ascent of the ultramafic bodies. Therefore, we conclude that the crustal emplacement 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 pyroxenites 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 the geological significance of the ages yielded by layered pyroxenites remains open to discussion, the Nd heterogeneities of the sub-continental lithospheric mantle below the Pyrenees can be explained by a long time integrated evolution of the Sm–Nd system recording the succession of Mesozoic, Phanerozoic and Proterozoic chemical fractionations under mantle conditions.