Carbonatite-melilitite association in the Italian collision zone and the Ugandan rifted craton: significant common factors

Abstract

AbstractItalian carbonatites form part of a suite with melilitites, normally an association characteristic of continental interiors; the perfect analogue of the Italian suite being the kamafugites (from the type area in SW Uganda, where the western branch of the East African Rift Zone cuts across the craton). The latter are commonly attributed to plume generation, whereas the Italian carbonatites, strung along the Appennine front, are usually linked to subduction. Evidently these two mechanisms are not essential, since neither can apply in both provinces. This conclusion is re-inforced by the related magmatism registered in both provinces in the Cretaceous. Phlogopite is ubiquitous in the mantle debris, and compositions from the two provinces overlap. Xenolithic phlogopites are distinct from cognate micas in the lavas, and from the carrier melt compositions, with similar distribution patterns in both suites. Kamafugitic magmas must be products of exceptional conditions, and added to the many near-identical magmatic features, the Italian and Ugandan volcanoes have sampled similar mantle conditions. Although the large scale geodynamic regimes are in total contrast, as are the deep mantle tomographic structures, the crucial common factor at the igneous province level is extensional tectonics. Extension, promoting release of volatiles (esp. CO2), is the vital trigger for this small volume, primary magmatism.