ULTRAMAFIC XENOLITHS IN PLIOPLEISTOCENE ALKALI BASALTS FROM THE EASTERN TRANSYLVANIAN BASIN - DEPLETED MANTLE ENRICHED BY VEIN METASOMATISM

Ultramafic xenoliths from alkali basalts in the Persani Mountains in the Eastern Transylvanian Basin (ETB) of Romania are mainly spinel lherzolites, although spinel harzburgites, websterites, clinopyroxenites and amphibole pyroxenites are also present. Amphibole veins cut some spinel peridotite samples. All are derived from the shallow lithospheric upper mantle. In general, textural variations are restricted to protogranular and porphyroclastic types, compared with the more varied textures found in mantle xenoliths from the alkali basalts of the neighbouring Pannonian Basin. Also, ETB peridotites are richer in amphibole. Thus, the mantle beneath the edge of the ETB is less deformed but more strongly metasomatized than the mantle closer to the centre of the Pannonian Basin. Mineralogical and bulk-rock geochemical variations resemble those of spinel lherzolites from other sub-continental shallow mantle xenolith suites. There is no apparent correlation between deformation and geochemistry, and much of the major and trace element variation is due to variable extraction of picritic melts The REE patterns of separated clinopyroxenes from the peridotite xenoliths are mostly LREE depleted, although clinopyroxenes from regions adjacent to amphibole veins have experienced an enrichment in La and Ce and a change in their Sr and Nd isotopic values towards those of the vein, while still retaining an overall LREE depletion. Clinopyroxenes from the websterites and clinopyroxenites are more variable. Amphibole in the hydrous pyroxenites and amphibole veins is strongly LREE enriched and is considered to be metasomatic in origin. Sr-87/Sr-86 and Nd-143/ Nd-144 isotopic ratios of the xenoliths vary between 0.7018 and 0.7044, and 0.51355 and 0.51275, respectively. These values are more depleted than those obtained for xenoliths from the Pannonian Basin. The lower Nd-143/Nd-144 and higher Sr-87/Sr-86 values are found in anhydrous pyroxenites, metasomatic amphiboles in veins and amphibole pyroxenites, and in the only example of an equigranular spinel lherzolite in the suite. The ETB xenoliths were brought to the surface in alkaline volcanism which post-dated a period of Miocene to Pliocene subduction-related calc-alkaline volcanism. However, the effects of the passage of either slab-derived fluids or calc-alkaline magmas through the ETB lithospheric mantle cannot be discerned in the chemistry of the xenoliths. The metasomatic amphibole has Sr-87/Sr-86 and Nd-143/Nd-144 ratios similar to the host alkali basalts, but the least evolved calc-alkaline magmas also have similar Sr and Nd isotope compositions. The REE patterns of the amphibole resemble those of amphiboles considered to have crystallized from alkaline melts. No preferential enrichment in elements typically associated with slab-derived fluids (K, Rb and Sr) relative to elements typically depleted in calc-alkaline magmas (Ti, Zr and Nb) has been observed in the vein amphiboles, although some interstitial amphibole is depleted in all incompatible trace elements, including LREE. Thus, despite its position dose to the calc-alkaline volcanic are of the Eastern Carpathians, we cannot readily detect any interaction between the lithospheric upper mantle beneath the ETB and subduction-related magmas or fluids. Metasomatism in the lithospheric mantle is instead largely related to the passage of a primitive alkaline magma similar to the host alkali basalts.