ERUPTION STYLE AND PETROLOGY OF A NEW CARBONATITIC SUITE FROM THE MT.VULTURE (SOUTHERN ITALY) - THE MONTICCHIO LAKES FORMATION (VOL 78, PG251, 1997)

The Monticchio Lakes Formation (MLF) is a newly identified carbonatite -melilitite tuff sequence which is exposed in the southwestern sector of the Vulture volcano. It is the youngest example (ca. 0.13 m.y.) of this type of volcanism in Italy, although other carbonatites of smalle r volume, but with similar characteristics, have been discovered recen tly. This volcanic event occurred in isolation after a 0.35 m.y. perio d of inactivity at Vulture. The eruption produced two maar-type vents and formed tuff aprons mainly composed of dune beds of lapilli. Deposi tional features suggest that a dry surge mechanism, possibly triggered by CO, expansion, was dominant during tuff emplacement. The MLF event involved a mixture of carbonatite and melilitite liquids which were p hysically separated before the eruption. Abundant mantle xenoliths are direct evidence of the deep-seated origin of the parental magma and i ts high velocity of propagation towards the surface. Often, these nodu les form the core of lapilli composed of concentric shells of meliliti te and/or porphyritic carbonatite. Coarse-ash beds alternate with lapi lli beds and consist of abundant lumps and spherulae of very fine-grai ned calcite immersed in a welded, highly compacted carbonatite matrix. Porphyritic carbonatite shells of the lapilli and fine-grained spheru lae of calcite in the tuff matrix suggest incipient crystallisation of a carbonatite Liquid in subvolcanic conditions and eruption of carbon atite-spray droplets. Dark coloured juvenile fragments mainly consist of melilite, phlogopite, calcite, apatite, perovskite, and hauyne crys tals in a carbonatite or melilitite matrix. The rocks have an extremel y primitive, ultramafic composition with very high Mg# (> 85) and Cr a nd Ni content (1500 ppm). The calcite contains high SrO, BaO and REE o f up to 1.5 wt.%. Similar compositions are typical of primary, magmati c carbonates which are found in both intrusive and extrusive carbonati tes. The high modal Sr-Ba-REE-rich calcite, the typical mineralogy, an d the high amount of Sr-goup elements identify the carbonate component as a carbonatite. The very high Mg#, mantle debris and C, O, He isoto pe ratios in the range of mantle values indicate a near-primary charac ter for the carbonatite which is distinctive of a restricted group of extrusive carbonatites only found in continental rift areas. (C) 1997 Elsevier Science B.V.