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(2005) Origin of trondhjemitic and granitic leucosomes in the pelitic migmatite from the Variscan chain of NE Sardinia, Italy: a working hypothesis.
No full textMetabasite from the Variscan belt in NE Sardinia, Italy: within-plate OIB-like melts with very high Sr and low Nd isotope ratios
No full textA 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 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
Metamorphic evolution of the amphibolites from the Posada Valley, NE Sardinia, Italy: first results
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Pre-Variscan metagabbro from NW Sardinia, Italy: evidence of an enriched asthenospheric mantle source for continental alkali basalts
No full textSmall metagabbro bodies are enclosed in the metasedimentary sequence of NW Sardinia. The metagabbro represents the last magmatic episode before the continent-continent collision which built up the Variscan chain of north Sardinia.
The metagabbro are composed up of variable proportions of plagioclase and pyroxene igneous relics and metamorphic minerals. Major and trace element data, i.e high TiO2, P2O5 and low K, Rb content, as well as LREE, Nb, Ta enrichment, suggest an alkaline affinity for the gabbro and emplacement in a within-plate tectonic setting. The gabbro was derived from an island alkali basalt-like asthenospheric mantle source enriched with incompatible elements and uncontaminated by crustal or subducted materials. Non-modal modelling indicated a 5-7% partial melting of the asthenospheric mantle
Segnalazione di una lente di metabasite alcalina nel basamento metamorfico della Sardegna nord-orientale
No full textEclogitized ultramafic cumulate from a WPB suite, Variscan NE Sardinia: OIB-like asthenospheric source with memory of kimberlite II- lamproite-type subcontinental lithospheric mantle
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