• journal_title：European Journal of Mineralogy
• Contributor：Gabriele Cruciani ; Andrea Dini ; Marcello Franceschelli ; Mariano Puxeddu ; Daniela Utzeri
• Publisher：E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.1127/0935-1221/2010/0022-2042
• journal_abbrev：Eur J Mineral
• issn：0935-1221
• volume：22
• issue：4
• firstpage：509
• section：Articles

A metabasite lens with eclogite-facies relics discovered within high-grade gneisses, at Punta Orvili (Posada) in NE Sardinia, has proven to be the first example of within-plate basalt among the eclogite-derived, tholeiitic metabasites from the Variscan belt in Sardinia. The composition of the Punta Orvili metabasite is unusual: SiO₂ < 47.6 wt%, Al₂O₃ < 9.6 wt%, MgO ≥ 13 wt%, Fe₂O_3tot >13 wt%, Cr > 550 ppm, Ni > 270 ppm, Mg # 68–70. Detailed investigation of the major and trace element content rules out any significant geochemical modification of the protolith by alteration and/or metamorphic processes. Normative contents of mafic minerals up to 55–79 wt%, very small amounts of Al₂O₃ and Na₂O and very high concentrations of MgO, Cr and Ni suggest a protolith of picritic nature. The steeply sloping REE patterns, with La_N/Yb_N = 13.0–17.3, suggest low-degree partial melting and the presence of garnet, as retention phase of HREE in the mantle source. Enrichment factors are 60–180 times higher for LREE, 30–60 times for MREE, 10–20 times for HREE. The primitive manntle-normalized trace element abundance patterns of the Punta Orvili metabasite show enrichment in Ba, Th, Nb, La, Ce, compared to eclogite-derived metabasites. Values of La/Nb, La/Ta, Ce/Nb, Th/Nb, Sm/Yb, Ce/Sm, Th/Y, Nb/Y, Th/Yb and Ta/Yb differ significantly from those of average crust, upper, lower and mid-crust, indicating an OIB-like source. Two values of ⁸⁷Sr/⁸⁶Sr_(i) were obtained around 0.709 with ε_Sri = +69–71. The corresponding values of the ¹⁴³Nd/¹⁴⁴Nd_(i) are about 0.51197 with ε_Ndi ~ −1.50 assuming an age of 460 Ma, i.e. the mean age of Sardinian eclogite. Isotope and geochemical data suggest that the Punta Orvili protolith was generated by interaction of OIB-like melts with an enriched sub-continental lithospheric mantle, similar in composition to Group II kimberlites and/or to their garnet peridotite xenoliths.