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

摘要

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 (Fo_(90-92)), orthopyroxene (approx En_(89), Wo_(2.0)), clinopyroxene (Mg_(90-92), 3-6 percent Al_2O_3, 1-1.50 percent Cr_2O_3), and chrome-spinel (14-20 percent MgO, approx 30-40 percent Cr_2O_3). Many xenoliths contain partial melt glasses and accessory sulphide (pentlandite). Some contain primary mica (phlogopite with approx 4 percent FeO, 1.8 percent Cr_2O_3, 1.4-2.80 percent TiO_2) with slightly zoned rims (Fe-, Ti-, Al-enriched). One contains relics of garnet (pyrope; Mg_(84)). Secondary veins in several xenoliths contain carbonate with significant Sr levels (approx 0.5-1.0 percent 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 ming a range of geothermometers/barometers give values of 14-22 kbar and 1050-1150 percent 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.