REE and Sr-Nd Isotopic Geochemistry of Eastern Pyrenean Peridotite Massifs: Sub-Continental Lithospheric Mantle Modified by Continental Magmatism

Abstract

Separated and leached minerals from >40 samples from Eastern Pyrenean ultramafic massifs have been analysed for rare earth elements (REEs), and Sr and Nd isotope ratios. The sub-continental lithospheric mantle in this region has interacted with several different episodes of niagmatic activity since at least the Palaeozoic. The range of isotopic composition in the massifs is extreme for mantle material (87Sr/86Sr =0·7020−0·7050; 143Nd/144Nd = 0·5139−0·5121), although the dominant lithologies, spinel lherzolite and spinel websterite, are isotopically much more homogeneous. Light rare earth element (LREE)-depleted patterns occur in clinopyroxenes from protogranular and porphyroclastic spinel lherzolites. These rocks have high ɛNd and lowɛSr values (mid-ocean ridge basalt—MORB-type) and represent lithospheric mantle unaffected or only slightly affected by subsequent magmatic events. With increasing refractory nature of the peridotites, heavy rare earth element (HREE) content and ɛNd of clinopyroxene decrease, and ɛSr and (Ce/Yb)N increase. Harzburgite bands may be formed by the passage of tholeitic melts unrelated to the host peridotites. Layered pyroxenites originated as crystal segregates from tholeiitic magmas. Clinopyroxenes from garnet clinopyroxenites have bell-shaped REE patterns, but websterites have reacted with host peridotites, acquiring similar REE patterns. Layered pyroxenites generally have higher ɛSr values than spinel peridotites, and show a wide range of ɛNd, resulting in part from time-integrated Sm/Nd fractionation. Some layered pyroxenites have extremely low ɛNdvalues (−5·6 to −10·2) resembling enriched mantle 1 (EMI); others have high ɛNd values (+ 25 to +28), although many have ɛNd values similar to those of their host peridotites. Cross-cutting amphibole pyroxenite and hornblendite veins were formed by 100-Ma-old alkali basaltic magmas which have reacted with pre-existing peridotite. The metasomatism of the Caussou massif is also related to this magmatism. REE patterns in clinopyroxene and amphibole from hydrous veins and metasomatized peridotites resemble those of megacrysts from alkali basalts. Magmatic minerals in hydrous veins have homogeneous #x025B;Nd values (+ 5·7 to +7·2), whereas metasomatic minerals have slightly higher values (+ 6·5 to +9·4). The highɛSr(+ 6·4 to +7·8) of some amphiboles from Caussou may be related to crustal fluids.