Journal of Petrology, Volume 39, Issue 8: August 1 1998.

The Geochemistry of Volcanic Rocks from Pantelleria Island, Sicily Channel: Petrogenesis and Characteristics of the Mantle Source Region

LUCIA CIVETTA¹, MASSIMO D'ANTONIO², GIOVANNI ORSI² AND GEORGE R. TILTON³

¹OSSERVATORIO VESUVIANO, V. MANZONI 249, NAPOLI, I-80123, ITALY
²DIP. GEOFISICA E VULCANOLOGIA, UNIVERSITÀ `FEDERICO II' DI NAPOLI, L.GO S. MARCELLINO 10, NAPOLI, I-80138, ITALY
³DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CALIFORNIA, SANTA BARBARA, CA 93106, USA

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.

Keywords: Pantelleria;asthenosphere;isotope geochemistry; mantle components; HIMU

Pages 1453-1491

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