Journal of Petrology | Volume 39 | Number 8 | Pages 1453-1491 | 1998
© Oxford University Press 1998
The Geochemistry of Volcanic Rocks from Pantelleria Island, Sicily Channel: Petrogenesis and Characteristics of the Mantle Source Region
1 Osservatorio Vesuviano V. Manzoni 249, Napoli, I-80123, Italy
2 Dip. Geofisica E Vulcanologia, Università ‘Federico Ii’ Di Napoli L.GO S. Marcellino 10, Napoli, I-80138, Italy
3 Department of Geological Sciences, University of California Santa Barbara, CA 93106, USA
Received April 24, 1996; Revised typescript accepted February 27, 1998
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Abstract |
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Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for mafic and felsic volcanic rocks from the island of Pantelleria. The mafic rocks, mostly basalts, range from hy-normative transitional basalts, through alkali basalts, to basanites. Clinopyroxene in the mafic rocks varies in composition from Al, Ti-poor diopside to Al, Ti-rich augite. These two populations can be present simultaneously in the same sample and even in the same crystal, suggesting polybaric fractionation in the pressure range 0–4 kbar, or mixing between basaltic magmas with different degrees of alkalinity. On the basis of their major and trace element and Sr–Nd–Pb isotope composition and age of eruption, two groups of basalts are distinguished: a high TiO2–P2O5 group, erupted before 50 ka BP, and a low TiO2–P2O5 group, erupted after 50 ka BP, separated by a caldera collapse. The felsic volcanic rocks have compositions ranging from comenditic trachyte to comendite and pantelleritic trachyte to pantellerite, with progressively increasing peralkalinity. The Sr–Nd isotope compositions of most of the felsic volcanic rocks are similar to those of the mafic volcanic rocks, except for some very Sr-poor pantellerites, which show post-depositional exchange with seawater strontium. On the basis of their petrographic and geochemical characteristics, Sr–Nd–Pb isotope data, computer modelling and geological observations, it is suggested that the mafic volcanic rocks represent a number of different alkaline parental magmas from which the felsic volcanic rocks were derived via prolonged, closed-system fractional crystallization. The source region for the parental magmas was heterogeneous, and may have involved at least two distinct geochemical components: a mid-ocean ridge basalt (MORB) source, relatively depleted component, and a HIMU-like enriched component. A further enriched component, similar to the Enriched Mantle 1 (EM 1) component, could also have been involved. According to geophysical data, the lithosphere is thinned beneath the island, and the asthenospheric mantle rises to a depth of 60 km. Rare earth element data require residual garnet in the source and constrain the melting process to a depth of 70–80 km. The petrological and geochemical data suggest that the mafic magmas are generated within the asthenospheric mantle, from a deep plume bringing the HIMU–EM 1 isotopic and trace element signatures. Interaction of these OIB-like magmas with the shallower asthenospheric mantle, providing a depleted MORB signature, gives rise to magmas with the observed isotopic and geochemical characteristics.
KEY WORDS: Pantelleria; asthenosphere; isotope geochemistry; mantle components; HIMU
* Corresponding author. Present address: Dip. Geofisica e Vulcanologia, Università Federico II di Napoli, L.go S.Marcellino 10, Napoli, I-80138, Italy. Telephone: +39 81 5803110. Fax: +39 81 5527631. email: civetta{at}unina.it
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