Summary
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.
Zusammenfassung
Diese Arbeit gibt einen Überblick über bereits veröffentlichte Ergebnisse und bringt neue Daten zu den vier Karbonatitvorkommen, die in Italien seit 1990 nachgewiesen wurden. Alle sind rezenten Alters. Drei davon sind extrusiv und umfassen Tuffe und Brekzien, während das vierte subvulkanisch ist und aus Tuffisiten besteht. Sie bilden Diatreme mit Tuff-Ringen, wobei drei von ihneneng mit melilithischen Tuffen und Laven assoziiert sind. Zwei der Vorkommen umfassen Karbonatite sensu strictu, i.e. Calcit > 50% Vol.%, während die anderen zwei “karbonatitisch” sind und nur 20–40% primären magmatischeu Calcit enthalten; dünne Aschenlagen bestehen aber aus echten Karbonatiten. Die Tuffe und Brekzien stellen Mischungen von karbonatitischen und melilithischen Lapilli und Bomben in einer Calcit-reichen Asche dar. Alle Vorkommen führen Reste von Mantelgesteinen in der Form kleiner Xenolithe, Xeno-Kristalle und als Kerne von kozentrischen Lapilli; hier liegt Olivin, Pyroxen und Phlogopit mit hohem Mg#, Ni, und Cr vor. In einem Vorkommen werden konzentrische Lapilli aus scharf begrenzten Lagen von Melilitit, Karbonat-führendem Melilitit und Calcit-phyrischem Karbonatit um Wehrlit-Kerne aufgebaut. Neue Gesamtgesteinsanalysen von allen vier Lokalitäten, sowie Mikrosondenanalysen von Calciten, lassen die Anwesenheit von beachtlichen Gehalten an Sr, Ba, und REE erkennen, was typisch für karbonatitische Calcite ist. Die Melilithite und Karbonatite sind gleichen Ursprungs und haben sich während raschen Transportes an die Oberfläche durch Magmenentmischung gebildet. Die frühere Verfestigung des Mililithites führte dann zur Bildung von karbonatitischen Schmelzen. Die Anwesenheit von Karbonatiten und Melilithiten in Zentralitalien wird als Hinweis darauf gesehen, daß diese magmatische Provinz nicht Subduktions-bezogen ist, die räumliche Verteilung dieser Gesteine in einer Zone östlich der römischen magmatischen Provinz wird vielmehr als Hinweis auf eine mächtigere Lithosphäre gesehen.
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Stoppa, F., Woolley, A.R. The Italian carbonatites: Field occurrence, petrology and regional significance. Mineralogy and Petrology 59, 43–67 (1997). https://doi.org/10.1007/BF01163061
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DOI: https://doi.org/10.1007/BF01163061