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1 Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK
2 Dipartimento di Scienze della Terra, Università di Perugia, Perugia, I-06100, Italy
3 Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
* E-mail: adrian.jones{at}ucl.ac.uk
We present petrographic and mineralogical data for 21 mantle xenoliths (12 lherzolites, 8 wehrlites and 1 composite) selected from a suite of more than 70 samples collected from the Monticchio Formation, Mt. Vulture volcano, southern Italy. The xenoliths are rounded, coarse- to porphyroclastic-textured, and very fresh, with the following equilibrated mineral assemblages; olivine (Fo90–92), orthopyroxene (~En89, Wo2.0), clinopyroxene (Mg90–92, 3–6% Al2O3, 1–1.5% Cr2O3), and chrome-spinel (14–20% MgO, ~30–40% Cr2O3). Many xenoliths contain partial melt glasses and accessory sulphide (pentlandite). Some contain primary mica (phlogopite with ~4% FeO, 1.8% Cr2O3, 1.4–2.8% TiO2) with slightly zoned rims (Fe-, Ti-, Al-enriched). One contains relics of garnet (pyrope; Mg84). Secondary veins in several xenoliths contain carbonate with significant Sr levels (~0.5–1.0% SrO), occasional apatite and scarce melanite, all typical of carbonatites and presumably related to the host magma (melilitite/carbonatite). Although amphibole is a common megacryst in the same volcanic units, no primary amphibole was found in the xenoliths themselves. Calculated pressures and temperatures using a range of geothermometers/barometers give values of 14–22 kbar and 1050–1150°C. In particular, the En-Sp and Di-Sp thermo/barometers (Mercier, 1980) show a good positive correlation between P and T. The Monticchio xenoliths lie on the high-T side of an ‘oceanic’ geotherm. The xenolith geotherm is hotter than general heat flow values in this region at the current day (50 mWm–2) but it compares well with the high-pressure end of a typical alkaline continental rift.
KEYWORDS: Italy, carbonatite, lherzolite, wehrlite, peridotite, mantle xenoliths, melilitite
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