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Petrography and mineral chemistry of mantle xenoliths in a carbonate-rich melilititic tuff from Mt. Vulture volcano, southern Italy

Published online by Cambridge University Press:  05 July 2018

A. P. Jones ,
T. Kostoula ,
F. Stoppa  and
A. R. Woolley
A. P. Jones*
Affiliation:
Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK
T. Kostoula
Affiliation:
Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, UK
F. Stoppa
Affiliation:
Dipartimento di Scienze della Terra, Università di Perugia, Perugia, I-06100, Italy
A. R. Woolley
Affiliation:
Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK
*
*E-mail: adrian.jones@ucl.ac.uk
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Abstract

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

Italycarbonatitelherzolitewehrliteperidotitemantle xenolithsmelilitite
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 4 , August 2000 , pp. 593 - 613
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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