ORIGIN AND MULTIPLE CRYSTALLIZATION OF THE KAMAFUGITE-CARBONATITE ASSOCIATION - THE SAN-VENANZO PIAN-DI-CELLE OCCURRENCE (UMBRIA, ITALY)

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 co ntrol prompted a study of the kamafugite-carbonatite suite and related phase interactions to understand the nature of the distinct mineral a ssemblages of the pyroclasts, compared to that of the lavas, the forme r containing essential potassium feldspar and aluminous diopside cryst als, absent in the latter. The pyroclastic rocks represent a small amo unt of magma characterized by ubiquitous mantle xenocrysts and emplace d by early high-velocity eruptions. All the investigated specimens sho w 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 volc anic sequence. They yielded a (Na+K)/Al ratio of c. 1.1 and are larnit e-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 melili tolite yielded CIPW Or and Hy and is strongly peralkaline, i.e. (Na+K) /Al = 5-6. The lapilli typically exhibit concentrically zoned structur es which compound subliquidus venanzite phases, e.g. melilite, leucite , and kalsilite, with mantle xenolithic/xenocrystic debris and carbona tite phases. These lapilli represent a distinct variant of the venanzi te liquid, mechanically fractionated and quenched by the diatremic pro cess. Mantle-normalized HFSE for both lava and lapilli show typical ex trusive-carbonatite patterns. Carbonatitic beds intercalated with the pyroclastic suite are distinct and typically consist of carbonates hig h in Sr, Ba and REE. Primary carbonate yielded C isotope compositions ranging from -5.0 to -6.0 delta(13) C parts per thousand, falling with in the range of mantle compositions. Distinct differentiation trends o f the venanzite magma and its derivatives were recognized, hinging an the coexistence of the silicate and carbonatite fractions. Potential s anidine crystallization trends are suggested, distinct from the venanz ite --> melilitolite trend, reported for Oldoinyo Lengai assemblages. Unusual aspects of the San Venanzo rock association, relative to simil ar rock types elsewhere, include the combination of a rare mantle sour ce composition with a lithosphere about 80 km thick. A genetic model f or the origin of the San Venanzo kamafugite-carbonatite association an d related carbonate-silicate interactions is proposed and discussed. T his may be relevant to the petrogenesis of similar rocks elsewhere, pa rticularly in the light of the detailed data on the pyroclasts.