The Italian carbonatites: field occurrence, petrology and regional significance.

The paper reviews the published work, and presents new data, on the four occurrences of carbonatite that have been recognised in Italy since 1990. All are Recent in age. Three are extrusive and comprise tuffs and breccias while the fourth is high level and consists of tuffisites. They form diatremes with tuff rings, three of them being intimately associated with melilititic tuffs and lavas. Two of the occurrences include carbonatitessensu stricto i.e. calcite is > 50% by volume, while the other two are `‘carbonatitic'’, primary igneous calcite generally being 20-40%, but thin ash layers are true carbonatites. The tuffs and breccias are mixtures of carbonatitic and melilitic lapilli and bombs set in calcite-rich ash. All occurrences contain mantle debris in the form of small xenoliths, xenocrysts and cores to concentric lapilli, of olivine, pyroxene and phlogopite characterised by high Mg#, Ni and Cr. In one occurrence concentric lapilli are built of sharply bounded layers of melilitite, carbonate-bearing melilitite and calcite-phyric carbonatite around wehrlitic cores. New whole rock analyses for all four localities are given and electron microprobe analyses of calcite indicate the presence of significant Sr, Ba and REE, which is typical of carbonatitic calcite. The melilitites and carbonatites are taken to be consanguineous and to have separated immiscibly during rapid transport to the surface, earlier solidification of the melilitite producing final carbonatitic liquids. The presence of the carbonatites and melilitites in central Italy is taken as evidence that this igneous province is unlikely to be subduction related; instead the spatial distribution of these rocks in a zone east of the Roman Igneous Province is considered to reflect thicker underlying lithosphere.