Francesco Stoppa

Eruption style and petrology of a new carbonatitic suite from the Mt. Vulture ( Southern Italy) : The Monticchio Lakes Formation

Abstract The Monticchio Lakes Formation MLF is a newly identified carbonatite-melilitite tuff sequence which is exposed in the southwestern sector of the Vulture volcano. It is the youngest example ca. 0.13 m.y. of this type of volcanism in Italy, although other carbonatites of smaller volume, but with similar characteristics, have been discovered recently. This volcanic event occurred in isolation after a 0.35 m.y. period of inactivity at Vulture. The eruption produced two maar-type vents and formed tuff aprons mainly composed of dune beds of lapilli. Depositional features suggest that a dry surge mechanism, possibly triggered by CO 2 expansion, was dominant during tuff emplacement. The MLF event involved a mixture of carbonatite and melilitite liquids which were physically separated before the eruption. Abundant mantle xenoliths are direct evidence of the deep-seated origin of the parental magma and its high velocity of propagation towards the surface. Often, these nodules form the core of lapilli composed of concentric shells of melilitite andror porphyritic carbonatite. Coarse-ash beds alternate with lapilli beds and consist of abundant lumps and spherulae of very fine-grained calcite immersed in a welded, highly compacted carbonatite matrix. Porphyritic carbonatite shells of the lapilli and fine-grained spherulae of calcite in the tuff matrix suggest incipient crystallisation of a carbonatite liquid in subvolcanic conditions and eruption of carbonatite-spray droplets. Dark coloured juvenile fragments mainly consist of melilite, phlogopite, calcite, apatite, perovskite, and hauyne crystals in a carbonatite or melilitite matrix. The rocks have an extremely primitive, ultramafic composition with very high Mga) 85. and Cr and Ni content 1500 ppm-. The calcite contains high SrO, BaO and REE of up to 1.5 wt.%. Similar compositions are typical of primary, magmatic carbonates which are found in both intrusive and extrusive carbonatites. The high modal Sr-Ba-REE-rich calcite, the typical mineralogy, and the high amount of Sr-group elements identify the carbonate component as a carbonatite. The very high Mga, mantle debris and C, O, He isotope ratios in the range of mantle values indicate a near-primary character for the carbonatite which is distinctive of a restricted group of extrusive carbonatites only found in continental rift areas.

Is there a mantle plume below Italy

Some of the most diverse igneous rocks found on Earth occur along the length of Italy and in many of the islands in the southeastern Tyrrhenian Sea, all the result of Cenozoic magmatism. Magmas extremely rich in alkalis, particularly potassium, and many undersaturated with respect to silica, were erupted, as well as others of calc-alkalic affinity (see legend in Figure 1). Their origin has been the subject of heated debate, and there is still no general consensus about how they formed. Most attribute them to subduction-related processes (see Beccaluva et al. [2004] for a review); others consider them to be the result of within-plate magmatism [e.g., Vollmer, 1976; Lauecchia and Stoppa, 1996]. Still others consider magmatism the result of a deep, mantle upwelling within a slab window coupled with mixing between isotopically different reservoirs [Gasperini et al., 2002].

Late Pleistocene ultra-alkaline magmatic activity in the Umbria-Latium region (Italy): An overview

Abstract The “Umbria-Latium ultra-alkaline district” (ULUD), central Italy, consists of numerous, generally monogenetic igneous centres, all of which are Late Pleistocene in age. They show strong peculiarities in both volcanic behaviour and chemical characteristics. The igneous centres occur as cinder cones, lava flows, dykes, maars and diatremes along the Plio-Pleistocene graben faults and adjoining blocks. The existing subvolcanic rocks and lavas display rare mineral assemblages including melilite, leucite, kalsilite, monticellite, wollastonite, perovskite and BaSr-rich calcite. In addition, they contain very unusual Ti-rich garnet and ZrCaTi minerals, which are generally confined to carbonate-rich magmatic rocks. These rock types range from near-agpaitic melilitolite and melilitite to calcium-carbonatite. The pyroclastics commonly contain feldspars, mostly sanidine, and like the ULUD sub-volcanic rocks and lavas, commonly contain diopside, phlogopite, olivine and mantle micro-nodules. The pyroclastic rocks range from melilitite-carbonate tuffs to sanidine-bearing tuffs. In the ULUD, volcanic activity and rock types strongly resemble those generally observed in classical, continental-rift-related magmatic provinces. The affinity between the ULUD rocks and the Roman Campanian Province “High-Potassium Series” rocks helps in interpreting the magmatism developed at the Tyrrhenian eastern border as intra-continental and rift-related.

Petrography and mineral chemistry of mantle xenoliths in a carbonate-rich melilititic tuff from Mt. Vulture volcano, southern Italy

Abstract We present petrographic and mineralogical data for 21 mantle xenoliths (12 lherzolites, 8 wehrlites and 1 composite) selected from a suite of more than 70 samples collected from the Monticchio Formation, Mt. Vulture volcano, southern Italy. The xenoliths are rounded, coarse- to porphyroclastic-textured, and very fresh, with the following equilibrated mineral assemblages; olivine (Fo90−92), orthopyroxene (~En89, Wo2.0), clinopyroxene (Mg90−92, 3−6% Al2O3, 1−1.5% Cr2O3), and chrome-spinel (14−20% MgO, ~30−40% Cr2O3). Many xenoliths contain partial melt glasses and accessory sulphide (pentlandite). Some contain primary mica (phlogopite with ~4% FeO, 1.8% Cr2O3, 1.4−2.8% TiO2) with slightly zoned rims (Fe-, Ti-, Al-enriched). One contains relics of garnet (pyrope; Mg84). Secondary veins in several xenoliths contain carbonate with significant Sr levels (~0.5−1.0% SrO), occasional apatite and scarce melanite, all typical of carbonatites and presumably related to the host magma (melilitite/carbonatite). Although amphibole is a common megacryst in the same volcanic units, no primary amphibole was found in the xenoliths themselves. Calculated pressures and temperatures using a range of geothermometers/barometers give values of 14−22 kbar and 1050–1150°C. In particular, the En-Sp and Di-Sp thermo/barometers (Mercier, 1980) show a good positive correlation between P and T. The Monticchio xenoliths lie on the high-T side of an ‘oceanic’ geotherm. The xenolith geotherm is hotter than general heat flow values in this region at the current day (50 mWm-2) but it compares well with the high-pressure end of a typical alkaline continental rift.

The San Venanzo maar and tuff ring, Umbria, Italy: eruptive behaviour of a carbonatite-melilitite volcano.

The late Pleistocene San Venanzo maar and nearby Pian di Celle tuff ring in the San Venanzo area of Umbria, central Italy, appear to represent different aspects of an eruptive cycle accompanied by diatreme formation. Approximately 6x106 m3 of mostly lapillisized, juvenile ejecta with lesser amounts of lithics and 1x106 m3 of lava were erupted. The stratigraphy indicates intense explosive activity followed by lava flows and subvolcanic intrusions. The pyroclastic material includes lithic breccia derived from vent and diatreme wall erosion, roughly stratified lapilli tuff deposited by concentrated pyroclastic surge, chaotic scoriaceous pyroclastic flow and inverse graded grain-flow deposits. The key feature of the pyroclastics is the presence of concentric-shelled lapilli generated by accretion around the lithics during magma ascent in the diatreme conduits. The rock types range from kalsilite leucite olivine melilitite lavas and subvolcanic intrusions to carbonatite, phonolite and calcitic melilitite pyroclasts. Juvenile ejecta contain essential calcite whose composition and texture indicate a magmatic origin. Pyroclastic carbonatite activity is also indicated by the presence of carbonatite ash beds. The San Venanzo maar-forming event is believed to have been trigered by fluid-rich carbonatite-phonolite magma. The eruptive centre the moved to the Pian di Celle tuff ring, where the eruption of degassed olivine melilititic magma and late intrusions ended magmatic activity in the area. In both volcanoes the absence of phreatomagmatic features together with the presence of large amounts of primary calcite suggests carbonatite segregation and violent exsolution of CO2 which, flowing through the diatremes, produced the peculiar intrusive pyroclastic facies and triggered explosions.

An enriched mantle source for Italy’s melilitite-carbonatite association as inferred by its Nd-Sr isotope signature

Abstract New Sr-Nd isotope data were obtained from Late Pleistocene carbonatite-kamafugite associations from the Umbria-Latium Ultra-Alkaline District of Italy (ULUD) with the aim of constraining their origin and possible mantle source(s). This is relevant to the origin and evolution of ultrapotassic (K/Na ≫ 2) and associated rocks generally, notably the occurrences from Ugandan kamafugites,Western Australian lamproites and South African orangeites. The selected ULUD samples yielded 87Sr/86Sr and 143Nd/144Nd ranging from 0.7100 to 0.7112 and from 0.5119 to 0.5121 respectively, similar to cratonic potassic volcanic rocks with higher Rb/Sr and lower Sm/Nd ratios than Bulk Earth. Silicate and carbonate fractions separated from melilitite are in isotopic equilibrium, supporting the view that they are cogenetic. The ULUD carbonatites yielded the highest radiogenic Sr so far reported for carbonatites. In contrast, sedimentary limestones from ULUD basement formations are lower in radiogenic Sr, i.e. 87Sr/86Sr = 0.70745−0.70735. The variation trend of ULUD isotopic compositions is similar to that reported for Ugandan kamafugites and Western Australian lamproites and overlaps the values for South African orangeites in the ƐSr-ƐNd diagram. A poor correlation between Sr/Nd and 87Sr/86Sr ratios in ULUD rocks is inconsistent with a mantle source generated by subduction-driven processes, while the negligible Sr and LREE in sedimentary limestones from the ULUD region fail to account for a hypothetical limestone assimilation process. The Nd model ages of 1.5−1.9 Ga have been inferred for a possible metasomatic event, allowing further radiogenic evolution of the source, a process which may have occurred in isolation until eruption time. While the origin of this component remains speculative, the Sr-Nd isotope trend is consistent with a simple mixing process involving an OIB-type mantle and a component with low ƐNd and high ƐSr.

Erratum to “Eruption style and petrology of a new carbonatitic suite from the Mt. Vulture (Southern Italy): The Monticchio Lakes Formation” [Journal of Volcanology and Geothermal Research 78 (1997) 251–265]

Abstract The Monticchio Lakes Formation (MLF) is a newly identified carbonatite-melilitite tuff sequence which is exposed in the southwestern sector of the Vulture volcano. It is the youngest example (ca. 0.13 m.y.) of this type of volcanism in Italy, although other carbonatites of smaller volume, but with similar characteristics, have been discovered recently. This volcanic event occurred in isolation after a 0.35 m.y. period of inactivity at Vulture. The eruption produced two maar-type vents and formed tuff aprons mainly composed of dune beds of lapilli. Depositional features suggest that a dry surge mechanism, possibly triggered by CO2 expansion, was dominant during tuff emplacement. The MLF event involved a mixture of carbonatite and melilitite liquids which were physically separated before the eruption. Abundant mantle xenoliths are direct evidence of the deep-seated origin of the parental magma and its high velocity of propagation towards the surface. Often, these nodules form the core of lapilli composed of concentric shells of melilitite and/or porphyritic carbonatite. Coarse-ash beds alternate with lapilli beds and consist of abundant lumps and spherulae of very fine-grained calcite immersed in a welded, highly compacted carbonatite matrix. Porphyritic carbonatite shells of the lapilli and fine-grained spherulae of calcite in the tuff matrix suggest incipient crystallisation of a carbonatite liquid in subvolcanic conditions and eruption of carbonatite-spray droplets. Dark coloured juvenile fragments mainly consist of melilite, phlogopite, calcite, apatite, perovskite, and hauyne crystals in a carbonatite or melilitite matrix. The rocks have an extremely primitive, ultramafic composition with very high Mg# (> 85) and Cr and Ni content (1500 ppm). The calcite contains high SrO, BaO and REE of up to 1.5 wt.%. Similar compositions are typical of primary, magmatic carbonates which are found in both intrusive and extrusive carbonatites. The high modal Sr-Ba-REE-rich calcite, the typical mineralogy, and the high amount of Sr-group elements identify the carbonate component as a carbonatite. The very high Mg#, mantle debris and C, O, He isotope ratios in the range of mantle values indicate a near-primary character for the carbonatite which is distinctive of a restricted group of extrusive carbonatites only found in continental rift areas.

New mineral data from the kamafugitecarbonatite association: The melilitolite from Pian di Celle, Italy

SummaryA detailed mineralogical investigation of a Pian di Celle sill rock (San Venanzo, Italy), classified asmelilitolite and associated withvenanzite and carbonatitic pyroclasts, revealed new and rare mineral parageneses, considered as characteristic of thekamafugite-carbonatite association. These are formed by several accessory minerals, including minerals of the cuspidine family, götzenite, khibinskite, minerals of the rhodesite- delhayelite- macdonaldite family, pyrrhotite, bartonite and (Fe, Ni, Co) monoarsenide, mostly optically and chemically identified also in fluid inclusions. The chemical composition of these minerals and their probable crystallisation succession, deduced from textural relationships, demonstrates extensive atomic substitutions, notably for Ca, Ti, Mg and alkali, essentially reflecting high concentrations of REE, Sr, Ba, Nb and Zr, which significantly varied during crystallisation. Molecular alkali excess over Al and high Ca content in (H2O, F, CO2)-rich, Siundersaturated liquid(s) are considered the dominant factors in controlling the stability of disilicate-type minerals. Separation of the carbonatite liquid from the silicate magma, constrained by textural and fluid inclusion data, was fundamental in moving the residuum onto a strongly peralkaline trend which stabilised the sulphides under changed redox conditions.ZusammenfassungEine eingehende mineralogische Untersuchung eines Lagerganges von Pian di Celle, der als Melilitolit klassifiziert and mit Venanzit and karbonatitischen Pyroklasten assoziiert ist, ergab neue and seltene Mineral-Paragenesen, die als charakteristisch für die Kamafugit-Karbonatit-Assoziation gelten. Diese bestehen aus verschiedenen akzessorischen Mineralien, darunter Perovskit, Cuspidin, Götzenit, Khibinskit, Delhayelit, Macdonaldit, Bardonit and (Fe, Ni, Co) Monoarsenit; diese werden in Flüssigkeitseinschlüssen mit optischen and chemischen Methoden identifiziert. Die chemische Zusammensetzung dieser Minerale and ihre wahrscheinliche Kristallisationsabfolge, aus texturellen Beziehungen abgeleitet, zeigt extensive Substitutionen, vor allem für Ca, Ti, Mg and Alkelien, die im wesentlichen hohe Gehalte an SEE, Sr, Ba, Mb and Zr andeuten, die während der Kristallisation beträchtlichen Schwankungen unterlagen. Molekularer Alkali überschuß über Al in (H2O, F, CO2)-reichen Si-untersättigten Fluiden werden als wichtigste Faktoren für die Stabilität von Mineralen des DisilikatTyps gesehen. Trennung des Karbonatites vom Silikat, die durch texturelle und Flüssigkeitseinschluß-Daten genau fixiert werden konnte, war wichtig für die Verschiebung des Residuums auf einen deutlich peralkalinen Trend, welcher die Sulfide unter veränderten Redox-Bedingungen stabilisieren konnte.

Intrusive calcite-carbonatite occurrence from Mt. Vulture volcano, southern Italy

Abstract Intrusive calcite-carbonatite ejecta (sovite) in the lowermost tephra layers of the Mt. Vulture alkaline mafic-ultramafic volcano (Upper Pleistocene), is the first intrusive carbonatite sample from southern Europe. The sovite is of coarse granularity and shows some textural and mineralogical layering. It is mainly formed of calcite (up to 3.5 wt.% MgO, and 0.53 wt.% SrO), less dolomite (average 18.2 wt.% MgO, and up to 2.1 wt.% SrO), spinel (60 wt.% Al2O3, 26.5 wt.% MgO, 10.7 wt.% FeO) and olivine (Fo99). Perovskite and apatite have been found only as microlites. Intergranular vugs are scattered throughout the rocks and small composite inclusions occur in calcite. The mineral chemistry, high temperature melt inclusions, overall isotopic compositions, and the REE distribution are consistent with a primary igneous origin. Compared with world average sovite compositions, the Vulture sovite has lower LILE and HFSE but Rb, Sr, Th and U are high. The REE abundance is typical of carbonatites, having an LREE/HREE value of ~100. The δ13C ( 4.8% SMOW) is in the range for mantle-derived carbonatites. The 143Nd/144Nd (0.512648±15) and 87Sr/86Sr (0.705978±10) ratios show close similarity between the sovite and the Vulture alkaline mafic rocks. The sovite ejecta are interpreted as a crystallization product of carbonate ultramafic liquid. In common with many shallow-level carbonatites from other localities, the recrystallization of rather pure Mg-calcite, the presence of dissolution vugs and the depletion of some HFSE and the relatively high δ18O values, suggest that a secondary process, such as hydrothermal leaching, affected the rock.

The Tyrrhenian zone: a case of lithosphere extension control of intra-continental magmatism

Abstract By means of a multivariate analysis of the Tyrrhenian and peri-Tyrrhenian Late Miocene to Quaternary igneous rocks, performed through a statistical routine known as SIMCA (Soft Independent Modelling of Classes Analogy) and based on the Principal Components Analysis, it has been possible to distinguish, from an initially unstructured data set, seven minimum-variance subsets (classes). By analyzing the petrological meaning of these classes in the frame of the tectonic setting of the Tyrrhenian intra-continental eastward-migrating extensional lithospheric strain field, a tectonomagmatic model of the area is proposed which stresses the role played by the extending lithosphere in controlling the magmatism. This magmatism can be interpreted as being related to a limited number of parental magmas generated at well defined structural levels: continental crust, mantle lithosphere and metasomatized mantle asthenosphere. The large variety of rock types with different chemical compositions, which outcrop all over the studied area, can be considered to derive from these melts through a complex evolutionary history involving immiscibility, crystal fractionation and mixing among different bacsthes of magmas.