Trace element and isotope geochemistry of depleted peridotites from an N-MORB type ophiolite (Internal Liguride, N. Italy)

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doi: 10.1007/s004100050143
Authors:Rampone, E.; Hofmann, A. W.; Piccardo, G. B.; Vannucci, R.; Bottazzi, P.; Ottolini, L.
Author Affiliations:Primary:
Universita di Genova, Genova, Italy
Max Planck-Institut für Chemie, Federal Republic of Germany
Universita di Pavia, Italy
CNR, Italy
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 123(1), p.61-76. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1996
Note:In English. 68 refs.; 5 tables
Summary:Mantle peridotites of the Internal Liguride (IL) units (N Apennines) constitute a rare example of the depleted lithosphere of the Jurassic Ligurian Tethys. Detailed chemical and isotopic investigation were performed with the major aim of constraining the timing and mechanism of their evolution, and to furnish new data for the geodynamic interpretation. The samples consist of clinopyroxene-poor spinel lherzolites, showing incipient re-equilibration in the plagioclase-facies stability field. The spinel-facies assemblage records high (asthenospheric) equilibration T (1150-1250°C). Whole rocks, and constituent clinopyroxenes, show a de-coupling between severe depletion in highly incompatible elements (LREE, Sr, Zr, Na, Ti) and less pronounced depletion in moderate incompatible elements (Ca, Al, Sc, V). Bulk rocks also display a relatively strong MREE/HREE fractionation. These compositional features indicate low-degree (<<<[dollar]I> 10%) fractional melting, which presumably started in the garnet stability field. In this respect, the IL ultramafics show strong similarity to abyssal peridotites. The Sr and Nd isotopic compositions, determined on hand-picked clinopyroxene separates, indicate an extremely depleted signature. The Sm/Nd model ages suggest that the IL peridotites melted most likely in Permian times. They could record, therefore, the early upwelling and melting of MORB type asthenosphere, in response to the onset of extensional mechanisms which led to the opening of the W Tethys. They subsequently cooled and experienced a composite subsolidus evolution testified by multiple episodes of gabbroic intrusion and high-T-low P retrograde metamorphic re-equilibration prior to their emplacement on the sea-floor. The most important trace element feature concerns the occurrence of Sr and Zr negative anomalies (relative to adjacent REE in both clinopyroxene and bulk rocks). It is suggested that such anomalies reflect changes in the relative magnitude of Sr, Zr and REE partition coefficients, depending on the specific melting conditions. [P.Br.]
Subjects:Alkaline earth metals; Basalts; Chemical composition; Depletion; Geochemistry; Geodynamics; Igneous rocks; Isotopes; Major elements; Mantle; Metals; Mid-ocean ridge basalts; Nd-144/Nd-143; Neodymium; Partial melting; Peridotites; Plutonic rocks; Rare earths; Sr-87/Sr-86; Stable isotopes; Strontium; Trace elements; Ultramafics; Volcanic rocks; Apennines; Europe; Italy; Liguria Italy; Ligurian Apennines; Southern Europe; Northern Italy
Abstract Numbers:96M/3166
Record ID:1996066699
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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