Ultramafic xenoliths in plio-pleistocene alkali basalts from the eastern transylvanian basin: Depleted mantle enriched by vein metasomatism

O. Vaselli, H. Downes, M. Thirlwall, G. Dobosi, N. Coradossi, I. Seghedi, A. Szakacs, R. Vannucci

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126 Citations (Scopus)


Ultramafic xenoliths from alkali basalts in the Perjani Mountains in the Eastern Transylvanian Basin (ETB) of Romania are mainly spinel Iherzolites, 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 Iherzolites 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. 87Sr/86Sr and 143Nd/l44Nd isotopic ratios of the xenoliths vary between 0·7018 and 0·7044, and 0·51355 and 0 51275, respectively. These value are more depleted than those obtained for xenoliths from the Pannonian Basin. The lower l43Nd/l44Nd and higher 87Sr/Sr86 values are found in anhydrous pyroxenites, metasomatic amphiboles in veins and amphibole pyroxenites, and in the only example of an equigranular spinel Iherzolite in the suite.The ETB xenoliths were brought to the surface in alkaline vokanism which post-dated a period of Miocene to Pliocene subduction-related cak-alkaline volcanism. However, the effects of the passage of either slab-derived fluids or cak-alkaline magmas through the ETB lithospheric mantle cannot be discerned in the chemistry of the xenoliths. The metasomatic amphibole has 87Sr/Sr86 and 143Sr/Sr144 ratios similar to the host alkali basalts, but the least evoked cak-alkaline magmas also have similar Sr and Nd isotope compositions. The REE patterns of the amphibole resembk those of amphiboles considered to have crystallized from alkaline melts. No preferential enrichment in elements typically associated with slab-derivedfluids (K, Rb and Sr) relative to elements typically depleted in cak-alkaline magmas (Ti, 2jr 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 close to the calc-alkaline volcanic arc 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 basal

Original languageEnglish
Pages (from-to)23-53
Number of pages31
JournalJournal of Petrology
Issue number1
Publication statusPublished - Feb 1995

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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