Michel Rossy

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 eNd 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 eNd 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 e 153 Ma Nd =+10.1 (Prades), 177±3 Ma with e 177 Ma Nd =+10.8 (Moncaup–Arguenos) and 138±4 Ma with e 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.

Synkinematic emplacement of the Pan-African Ngondo igneous complex (west Cameroon, central Africa)

Abstract The Ngondo Complex is one of the Pan-African plutons intruded in the West Cameroon Pan-African Orogenic Belt. The complex consists of three major groups of rocks: basic to intermediate rocks (diorites, granodiorites and minor gabbros), fined-grained granites and coarsed-grained granites successively emplaced in a metamorphic country rock of amphibolite-facies. Synkinematic emplacement of the complex, in relation with a ductile mega shear zone, is documented by a study of microstructures and foliation patterns which indicate a continuous transition from magmatic to high temperature solid-state deformation. The geometry of the internal foliation trajectories and the joint orientation in the complex suggest that the emplacement of the three groups of rocks was totally controlled by a N30° sinistral shear zone. Emplacement mechanisms, which are related in time and space to a continuum of deformation, may indicate a relative rheological change of the crust from ductile to brittle behaviour.

REE and Sr/1bNd isotope geochemistry of the alkaline magmatism from the Cretaceous North Pyrenean Rift Zone (France-Spain)

Abstract Cretaceous alkaline magmatism extends E-W within the north-Pyrenean Rift Zone from the Mediterranean Sea to the Spanish coast of the Bay of Biscay and consists of both effusive and intrusive rocks displaying a wide petrographic diversity: basalts, trachytes, lamprophyres, gabbros, teschenites, ultramafic cumulates and nepheline syenites. Sixty samples representing the main rock-types were analyzed for major and trace elements including the REE. From these, thirteen samples, mainly from mafic rocks, were selected for determination of Sr and Nd isotopic compositions. 87 Sr 86 Sr ratios range from 0.703 to 0.7064 and 143 Nd 144 Nd ratios from 0.5127 to 0.5129. ϵNd(t)-values of Pyrenean samples (+2 to + 6) are those commonly found for continental volcanism, particularly from extension and rift settings. High 87 Sr 86 Sr ratios in some samples coupled with high δ18O-values suggest they suffered contamination by a Sr-rich, Nd-free fluid such as seawater. Regularities of REE distribution in basalts and associated teschenites substantiate that they formed by variable degrees of melting from LREE-enriched mantle sources which had similar (La/Yb)N close to 5 and probably were situated beneath the lithosphere. LREE enrichment took place 600–1000 Ma ago and is thus related neither to Mesozoic events in the Pyrenees nor to the breakup of Pangea. Finally, the isotopic data are used to discuss the genesis of the continental basalts which is interpreted in term of a three-component mixing: the absence of basalts with Nd as radiogenic as in MORB and the scarcity of mixed enriched types are shown to reflect unique Sr/Nd systematics among the components present in the mantle source of continental basalts.

Stable isotope transfer in open and closed system across chemically contrasted boundaries: metacarbonate–granitoid contacts in the Quérigut magmatic complex (Eastern Pyrenees, France)

The processes of stable isotope transfer between chemically contrasted boundaries are examined for two metacarbonate–granitoid contacts in the Quérigut complex, Pyrénées, France. External contacts, between the sedimentary basement and granitic intrusions, behaved like a fluid-dominated open system whereas septa contacts, between carbonate septa and the host granitoids, behaved like a closed system with respect to external fluids. Along external contacts, skarns of several decimetres to several metres were developed. δ18O values of calcite, buffered at 13.5–14‰, suggest an advection of metamorphic aqueous fluid with a minor contribution of low δ13C CO2 during skarn formation. Internal contacts are characterized by a thin centimetre-sized skarn layer. δ13C and δ18O values of calcite follow an evolution explained by decarbonation processes alone. δ18O exchange profiles across the contact show a typical diffusion profile with their inflection points slightly displaced towards the metacarbonate side, interpreted as a limited influx of magmatic fluids. Moreover, the shape of septa profiles varies according to the thermal energy budget induced by the intrusive rock: more isotopic alteration appears where the intrusion size is larger.