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%, Al 2O3 < 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 LaN/YbN = 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 εSri = +69-71. The corresponding values of the 143Nd/144Nd(i) are about 0.51197 with εNdi ∼-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.

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

CRUCIANI, GABRIELE;FRANCESCHELLI, MARCELLO;
2010

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: SiO2 < 47.6 wt%, Al 2O3 < 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 LaN/YbN = 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 εSri = +69-71. The corresponding values of the 143Nd/144Nd(i) are about 0.51197 with εNdi ∼-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.
Ocean island basalt; Within- plate basalt; Sr, Nd isotopic composition; Variscan orogeny; Sardinia; Sub-continental lithospheric mantle; Geochemistry
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11584/19796
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