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 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.

Age and petrogenesis of the Qassiarsuk carbonatite-alkaline silicate volcanic complex in the Gardar rift, South Greenland

The Qassiarsuk (formerly spelled Qagssiarssuk) complex is located in a roughly E–W trending graben structure between Qassiarsuk village and Tasiusaq settlement in the northern part of the Precambrian Gardar rift, South Greenland. The complex comprises a sequence of alkaline silicate tuffs and extrusive carbonatites interlayered with sandstones, and their subvolcanic equivalents, which represent possible feeders for the extrusive rocks. The Rb-Sr, Sm-Nd and Pb isotopic characteristics of 65 samples of extrusive carbonatite- and silicate tuffs and carbonatite diatremes have been determined by mass spectrometry. The Qassiarsuk complex can be dated to c. 1.2 Ga by Rb-Sr and Pb-Pb isochrons on whole-rocks and mineral separates, agreeing with previous isotopic ages for the volcanic rocks of the Eriksfjord formation in the Eriksfjord area of the Gardar rift, but not with previous, indirect age estimates of >1.31 Ga for assumed Eriksfjord equivalents in the Motzfeldt area further east. Recalculated isotopic compositions at 1.2 Ga indicate that the Qassiarsuk carbonatite- and alkaline-silicate magmas were comagmatic and derived from a depleted mantle source (εNd>4, εSr<−13, time-integrated, single- stage 238U/204Pb ≤ 7.4). The mantle-derived magmas were contaminated with crustal material, equivalent to the local, pre-Gardar granites and gneisses and sediments derived from these. The crustal component has a depleted mantle Nd model age of 2.1-2.6 Ga; at 1.2 Ga it was characterized by εSr = +76, εNd = −8.4, time-integrated, single- stage 238U/204Pb = 8.2−8.3. Strong decoupling of the Pb from the Sr and Nd isotopic systems suggests that the contamination happened only after carbonatitic and alkaline-silicate magmas had evolved from a common parent, by processes such as liquid immisicibility and/or fractional crystallization. Post-magmatic hydrothermal alteration (oxidation, hydration of mafic silicates, carbonatization of melilite) may have contributed further to the contamination of the carbonatite and alkaline silicate rocks of the Qassiarsuk complex.

Cathodoluminescence Petrography of Middle Proterozoic extrusive carbonatite from Qasiarsuk, South Greenland

The amygdaloidal carbonatite lavas at Qasiarsuk have a primary phenocryst assemblage of calcite, fluor-apatite and magnetite set in a groundmass of calcite, apatite and iron oxides, and minor dolomite. Cathodoluminescence reveals a complex history, both for the major minerals which show zonal growth, and for important Nb and REE accessory phases which include pyrochlore and perovskite. The REE reside in fluor/hydrous-carbonates included exclusively in apatite. These REE minerals are similar to synthetic phases from hydrothermal experiments, but probably crystallised in equilibrium with a late-stage volatile-rich carbonate melt. Apart from low-temperature alteration, the rocks have been little disturbed since their extrusion during the earliest phase of development of the Gardar Alkaline Igneous Province.