Metabasite from the Variscan belt in NE Sardinia, Italy: within-plate OIB-like melts with very high Sr and low Nd isotope ratios

Abstract

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: SiO 2 2 O 3 2 O 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 Al 2 O 3 and Na 2 O 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 87 Sr/ 86 Sr (i) were obtained around 0.709 with e Sri = +69–71. The corresponding values of the 143 Nd/ 144 Nd (i) are about 0.51197 with e 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.