The evolution of the lithospheric mantle along the N. African Plate: geochemical and isotopic evidence from the tholeiitic and alkaline volcanic rocks of the Hyblean plateau, Italy |
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| Authors: | T Trua S Esperança R Mazzuoli |
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| Institution: | (1) Dipartimento di Scienze della Terra, Università degli Studi di Pisa, Via S. Maria 53, I-56100 PISA, Italy, IT;(2) Department of Geology, University of Maryland, College Park, MD 20742, USA, US |
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| Abstract: | The geochemical characteristics of primitive tholeiitic and alkaline volcanic rocks that erupted in the Plio-Pleistocene
along fissures on the Hyblean plateau in Sicily (Italy) were used to constrain the mantle sources involved in the volcanic
activity of this area of the Mediterranean. It is shown that some of the Hyblean plateau magmas with the most extreme isotopic
compositions have combined radiogenic Nd, Sr and Pb, a feature that is distinct from the mixing end-members of the oceanic
array. In addition, alkalinity in the basalts is found to be positively correlated with an increase in some HFSE ratios (e.g.
Nb/Ta, Zr/Hf) and negatively correlated with ratios between HFSE and MREE (e.g. Ti/Eu), a characteristic that is attributed
to a mantle source that has been modified by a carbonatitic metasomatic agent. This metasomatic enrichment had the effect
of increasing the U/Pb of portions of the lower lithosphere, possibly by adding phases such as sodic pyroxene and apatite
to the basalt-depleted lithosphere. It is suggested that rock types that formed by melting metasomatized portions of lithosphere-asthenosphere
boundary affected by this recent enrichment in U/Pb fall along a trend with a shallow slope in a plot of 206Pb/204Pb versus 207Pb/204Pb, and have Nd isotopic compositions that are as radiogenic as present day MORB. The isotopic compositions and trace-element
concentrations of the Hyblean plateau basalts are, therefore, mostly consistent with the interaction of a MORB-type mantle
source with a young lithosphere that was probably formed in the Phanerozoic and metasomatized by CO2-rich fluids, possibly during the Jurassic. The absence of a geochemical component indicative of involvement of older Proterozoic
lithosphere and continental crust in the evolution of these magmas distinguishes them from those erupted along the margins
of the Tyrrhenian sea, and supports the suggestion that at least portions of the lithosphere underneath Sicily have oceanic
affinity and may be genetically related to the adjacent Ionian abyssal plain.
Received 4 June 1997 / Accepted 25 November 1997 |
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