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GEOREF RECORD

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

Gabriele Cruciani, Andrea Dini, Marcello Franceschelli, Mariano Puxeddu and Daniela Utzeri
Metabasite from the Variscan Belt in NE Sardinia, Italy; within-plate OIB-like melts with very high Sr and low Nd isotope ratios
European Journal of Mineralogy (August 2010) 22 (4): 509-523

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 (sub 2) <47.6 wt%, Al (sub 2) O (sub 3) <9.6 wt%, MgO> or =13 wt%, Fe (sub 2) O (sub 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 (sub 2) O (sub 3) and Na (sub 2) O and very high concentrations of MgO, Cr and Ni suggest a protolith of picritic nature. The steeply sloping REE patterns, with La (sub N) /Yb (sub 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 mantle-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 (super 87) Sr/ (super 86) Sr (sub (i)) were obtained around 0.709 with epsilon (sub Sri) =+69-71. The corresponding values of the (super 143) Nd/ (super 144) Nd (sub (i)) are about 0.51197 with epsilon (sub Ndi) approximately -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.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 22
Serial Issue: 4
Title: Metabasite from the Variscan Belt in NE Sardinia, Italy; within-plate OIB-like melts with very high Sr and low Nd isotope ratios
Affiliation: Universita di Cagliari, Dipartimento di Scienze della Terra, Cagliari, Italy
Pages: 509-523
Published: 201008
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 89
Accession Number: 2010-081052
Categories: Igneous and metamorphic petrologyGeneral geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables, geol. sketch map
N40°40'60" - N40°40'60", E09°43'60" - E09°43'60"
Secondary Affiliation: CNR, Istituto di Geoscienze e Georisorse, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201043
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