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

We present petrographic and mineralogical data for 21 mantle xenoliths (12 Iherzolites, 8 wehrlites and 1 composite) selected from a suite of more than 70 samples collected from the Monticchio Formation, Mr. Vulture volcano, southern Italy. The xenoliths an rounded, coarse- to porphyroelastic-textured, and very fresh, with the following equilibrated mineral assemblages; olivine (Fo(90-92)), orthopyroxene (similar to En(89), Wo(2.0)), clinopyroxene (Mg(90)-(92), 3-6% Al(2)O(3), 1-1.5% Cr(2)O(3)), and chrome-spinel (14-20% MgO, similar to 30-40% Cr(2)O(3)). Many xenoliths contain partial melt glasses and accessory sulphide (pentlandite). Some contain primary mica (phlogopite with similar to 4% FeO, 1.8% Cr(2)O(3), 1.4-2.8% TiO(2)) with slightly zoned rims (Fe-, Ti-, Al-enriched). One contains relies of garnet (pyrope; Mg(84)). Secondary veins in several xenoliths contain carbonate with significant Sr levels (similar to 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 degrees 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.