ERUPTION STYLE AND PETROLOGY OF A NEW CARBONATITIC SUITE FROM THE MT-VULTURE SOUTHERN ITALY - THE MONTICCHIO LAKES FORMATION

The Monticchio Lakes Formation MLF is a newly identified carbonatite-m elilitite 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 smaller vo lume, but with similar characteristics, have been discovered recently. This volcanic event occurred in isolation after a 0.35 m.y. period of inactivity at Vulture. The eruption produced two maar-type vents and formed tuff aprons mainly composed of dune beds of lapilli. Deposition al features suggest that a dry surge mechanism, possibly triggered by CO2 expansion, was dominant during tuff emplacement. The MLF event inv olved a mixture of carbonatite and melilitite liquids which were physi cally separated before the eruption. Abundant mantle xenoliths are dir ect evidence of the deep-seated origin of the parental magma and its h igh velocity of propagation towards the surface. Often, these nodules form the core of lapilli composed of concentric shells of melilitite a ndror porphyritic carbonatite. Coarse-ash beds alternate with lapilli beds and consist of abundant lumps and spherulae of very fine-grained calcite immersed in a welded, highly compacted carbonatite matrix. Por phyritic carbonatite shells of the lapilli and fine-grained spherulae of calcite in the tuff matrix suggest incipient crystallisation of a c arbonatite liquid in subvolcanic conditions and eruption of carbonatit e-spray droplets. Dark coloured juvenile fragments mainly consist of m elilite, phlogopite, calcite, apatite, perovskite, and hauyne crystals in a carbonatite or melilitite matrix. The rocks have an extremely pr imitive, ultramafic composition with very high Mga) 85 and Cr and Ni c ontent 1500 ppm. The calcite contains high SrO, BaO and REE of up to 1 .5 wt.%. Similar compositions are typical of primary, magmatic carbona tes which are found in both intrusive and extrusive carbonatites. The high modal Sr-Ba-REE-rich calcite, the typical mineralogy, and the hig h amount of Sr-group elements identify the carbonate component as a ca rbonatite. The very high Mga, mantle debris and C, O, He isotope ratio s in the range of mantle values indicate a near-primary character for the carbonatite which is distinctive of a restricted group of extrusiv e carbonatites only found in continental rift areas. q 1997 Elsevier S cience B.V.