The Late Pleistocene kamafugite–carbonatite association at San Venanzo-Pian di Celle forms part of the Umbria-Latium Ultra-alkaline District (ULUD) of central Italy and, together with Toro-Ankole, SW Uganda and Mata de Corda, Brazil, represents one of three similar occurrences so far reported worldwide.
Excellent field exposure and stratigraphic control prompted a study of the kamafugite–carbonatite suite and related phase interactions to understand the nature of the distinct mineral assemblages of the pyroclasts, compared to that of the lavas, the former containing essential potassium feldspar and aluminous diopside crystals, absent in the latter.
The pyroclastic rocks represent a small amount of magma characterized by ubiquitous mantle xenocrysts and emplaced by early high-velocity eruptions. All the investigated specimens show a high Mg/(Mg+Fe2+) ratio (0.84–0.93) and high compatible elements (Ni+Cr>1000 ppm). Lavas (venanzite, i.e. leucite melilitite) and a sill (uncompahgrite, i.e. melilitolite) represent final events in the volcanic sequence. They yielded a (Na+K)/Al ratio of c. 1.1 and are larnite-bearing in the CIPW norm. Glass from the lapilli is peralkaline, i.e. (Na+K)/Al>2, and close to the lava in composition. Glass from melilitolite yielded CIPW Or and Hy and is strongly peralkaline, i.e. (Na+K)/Al = 5–6. The lapilli typically exhibit concentrically zoned structures which compound subliquidus venanzite phases, e.g. melilite, leucite, and kalsilite, with mantle xenolithic/xenocrystic debris and carbonatite phases. These lapilli represent a distinct variant of the venanzite liquid, mechanically fractionated and quenched by the diatremic process.
Mantle-normalized HFSE for both lava and lapilli show typical extrusive-carbonatite patterns. Carbonatitic beds intercalated with the pyroclastic suite are distinct and typically consist of carbonates high in Sr, Ba and REE. Primary carbonate yielded C isotope compositions ranging from –5.0 to –6.0 δ13C‰, falling within the range of mantle compositions. Distinct differentiation trends of the venanzite magma and its derivatives were recognized, hinging on the coexistence of the silicate and carbonatite fractions. Potential sanidine crystallization trends are suggested, distinct from the venanzite→melilitolite trend, reported for Oldoinyo Lengai assemblages.
Unusual aspects of the San Venanzo rock association, relative to similar rock types elsewhere, include the combination of a rare mantle source composition with a lithosphere about 80 km thick. A genetic model for the origin of the San Venanzo kamafugite–carbonatite association and related carbonate-silicate interactions is proposed and discussed. This may be relevant to the petrogenesis of similar rocks elsewhere, particularly in the light of the detailed data on the pyroclasts.