FARAONE D. & STOPPA F. (1990). Petrology and regional implications of Early Cretaceous Alkaline-Lamprophyres in the Ligure Maremmano Group (Southern Tuscany, Italy): an outline. Ofioliti, 15 (1), 45-59
FARAONE D. & STOPPA F. (1990).
Petrology and regional implications
of Early Cretaceous Alkaline-Lamprophyres in the Ligure Maremmano Group
(Southern Tuscany, Italy): an outline.
Ofioliti, 15 (1), 45-59
Major geological and petrological features of igneous rocks within the Late Jurassic-Early Cretaceous Ligure Maremmano Group (Southern Tuscany) are examined and related to an extensional geodynamic environment. Mineral assemblage, texture, characteristics of occurrence and whole rock chemistry suggest that the rocks belong to the lamprophyre clan and point to an alkaline lamprophyre affinity. These rocks have been interpreted as a product of a within-plate magmatism, possibly related to extension during a late stage of the Liguria Ocean development. An age of about 100 Ma (Albian) has been found. This age represents a lower limit for the beginning of the compressional Alpine deformations in this area.
LAVECCHIA G. & STOPPA F. (1990). The Tyrrhenian zone: a case of litosphere extension control of intra-continental magmatism. Earth Planatary Science Letters, 99, 336-350.
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 analysing the petrologic meaning of these classes in the framework of the tectonic setting of the Tyrrhenian intra-continental eastward-migrating extensional lithosphenc 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 lilhosphere and metasomatized mantle asthenosphere. The large variety of rock types with different chemical compositions, which outcrop all over the investigated area, can be considered as derive from these melts through a complex evolutionary history involving immiscibility, crystal fractionation and mixing of different batches of magmas.
STOPPA F. & LAVECCHIA G. (1992). Late Pleistocene Ultra-alkaline magmatic activity in the Umbria-Latium region (Italy): an overview. Journal of Volcanological and Geothermal Research, 52, 277-293.
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.
STOPPA F. & LUPINI L. (1993). Mineralogy and Petrology of the Polino monticellite-calciocarbonatite (Central Italy). Mineralogy and Petrology, 49 (3-4), 213-232.
Two small diatremes, about 0.25 my old, cut through Liassic limestones about 1 km NNE of the village of Polino (Long. 12°50'54"E-Lat. 42°35'34"N; Central Italy). The material filling the larger diatreme is mainly composed of a tuffisite with abundant lapilli showing concentric structure. Both unaltered country-rocks and massive hypabyssal carbonatite occur in the tuffisite as angular clasts and blocks, from a few mm up to more than 1 m in diameter. The Polino rock occurs in a strongly-potassic igneous district (Umbria Latium Ultra-alkaline District) which comprises phonolitic pyroclastic rocks and very rare kamafugitic lavas. Massive carbonatite blocks have an average mode of 53% Sr-Ba-rich calcite, 23% Fe-monticellite, 9% Th-perovskite plus Ti-magnetite, 6% Cr-phlogopite, 6% forsteritic olivine, about 2% Zr-schorlomite and ca.1% Si-CO-OH apatite. Perovskite, schorlomite, and apatite form cognate phases, whereas olivine and phlogopite, often replaced by monticellite, occur as nodules and as discrete grains with compositions and deformation features typical of mantle xenocrysts found in alkali basalts and ultramafic rocks. High modal content of Ca-carbonate, high Sr, Ba and LREE contents of calcite, the presence of rare minerals peculiar to carbonatitic rocks and an essential amount of monticellite indicate classification of the Polino rock as a monticellite calciocarbonatite. The Polino rock represents a carbonatitic melt strongly contaminated by mantle crystal debris. It displays unusual geochemical features having trace elements closer to those of the regional-associated kamafugitic rocks rather than to those of common carbonatites.
STOPPA F. & LIU YU (1995). Chemical composition and petrogenetic implications of apatites from some ultra-alkaline Italian rocks. European Journal of Mineralogy, 7, 391-402.
The compositional characteristics of apatite group minerals from some Italian late Pleistocene ultra-alkaline rocks were studied and compared with apatites from other ultra-alkaline suites. The new data, together with data from the literature, were investigated using Principal Component Analysis (PCA). Three genetic groups of apatites were identified in terms of their compositional differences by means of the PC diagram and a Tetrahedra Substitution Index (TSI = 100*(Si + S + C)/P atom/apfu). Group I is made up of apatite samples from the ijolite-sovite association, with high contents of P, Sr, REE and considerable F (TSI = 1.6-6.8); Group II samples are apatites from fine-grained extrusive calcite carbonatites with high C, Si, OH, low S and a dominant substitution of C and Si for P (TSI = 7.9~8.5); Group III corresponds to the apatites from alkaline rocks with high S, F, and low C and OH contents and a dominant substitution of S and Si for P (TSI = 2.9-14.0). The apatite group geochemistry suggests that an ijolite-sovite magma existed as a single parent system over a relatively large range of P-X-T conditions. Major geochemical differences between the apatites from extrusive calcite carbonatites and alkaline rocks could well reflect separation of the ijolite-sovite magma into silicate and carbonate liquids under sub-volcanic or extrusive conditions.
STOPPA F. & CUNDARI A. (1995). A new Italian carbonatitic occurrence at Cupaello (Rieti) and its genetic significance. Contribution to Mineralogy and Petrology, 122, 275-288.
Field, mineralogical and petrological data are presented on a newly found carbonatite occurrence associated with kamafugite lava at Cupaello, central Italy. This carbonatite occurrence is part of the Late Pleistocene Umbria-Latium ultra-alkaline district (ULUD) which extends southwards within the Apennines to Mount Vulture, delineating an important magmatic province along the most peripheral belt of the Tyrrhenian extensional tectonic system. This province is distinct, but probably related genetically with the more abundant and common leucite-bearing assemblages of the Roman Comagmatic Region and represents the first reported occurrence of carbonatite assemblages in the Mediterranean Basin. The Cupaello suite indicates that primary or near-primary mantle silicate melts of "kamafugitic" composition are transitional with Ca-carbonatite liquid and provides direct evidence of immiscibility of carbonatite from "kamafugite" magma. It is inferred that a primary mantle origin of Ca-carbonatites is conditional upon a potential silicate magma that may be coupled with the carbonatite, but may not have reached the surface. The data indicate a strong genetic link between ULUD Ca-carbonatites and some African analogues, supporting the view that their genesis depends on similar source and associated tectonic conditions.
STOPPA F. (1996). The San Venanzo maar and tuff ring, Umbria, Italy: eruptive behaviour of a carbonatite-melilitite volcano. Bollettin of Volcanology, 57, 563-577.
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 6 x 106 m3 of mostly lapilli sized, juvenile ejecta with lesser amounts of lithics and 1 x 106 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 triggered by fluid-rich carbonatite-phonolite magma. The eruptive centre then 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.
SHARYGIN V.V., STOPPA F. & KOLESOV B.A. (1996). Zr-Ti disilicates from the Pian di Celle volcano, Umbria, Italy. European Journal of Mineralogy, 8, 1199-1212.
Phases of the cuspidine and götzenite groups, as well as khibinskite have been observed in melilitolite of the Pian di Celle volcano both as groundmass minerals and as trapped/daughter phases of melt inclusions hosted by melilite and olivine. Their approximate crystallisation sequence deduced from relationships of these minerals in melilitolite might be as follows: Zr,Ti-rich cuspidine Zr-rich cuspidine götzenite, khibinskite. Zr-rich cuspidine shows notable variations in CaO (58.2-40.4 wt.%), ZrO2 (0.2-11.2 wt.%), Na2O (0.5-4.5 wt.%) and F (10.1-8.1 wt.%). These variations may suggest partial solid solution between monoclinic cuspidine, Ca4[Si2O7]F2, and pseudomonoclinic hiortdahlite-2, NaCa2 Zr[Si2O7]OF, due to possible combined substitution 2Ca2+ + F- Na+ + Zr4+ + O2-. The X-ray patterns of the Pian di Celle cuspidines, with a different abundance of ZrO2, fall between those of pure cuspidine and hiortdahlite. Raman spectroscopy supports the possibility of the above solid solution and shows that the Pian di Celle cuspidine reserves monoclinic symmetry with a maximum content of Zr+Ti (up to 0.5 a.f.u.). Götzenite, (Na,Ca)3.5Ti0.5[Si2O7](F,O)2, generally varies in CaO, REE2O3, TiO2, ZrO2 and Na2O, which may indicate two main cation substitutions: 2Ca2+ REE3+ + Na+ and Ti4+ Zr4+, while REE and Zr contents do not exceed 0.14 a.f.u Both diffraction and spectroscopic data of Pian di Celle götzenite show negligible differences from götzenite elsewhere. The Zr-Ti-cuspidine composition ranges from Na0.7Ca2.8(Ti,Zr)0.5[Si2O7]F1.7O0.3 to Na0.8Ca2.4(Zr,Ti)0.8[Si2O7]F1.2O0.8. The extreme Zr,Ti-rich composition approaches that of hiortdahlite (possibly, hiortdahlite-1) on chemical, diffraction and Raman data. Khibinskite differs from the ideal composition K4Zr2[Si4O14] in high Na (up to 0.25 a.f.u) and Fe (up to 0.12 a.f.u).
LAVECCHIA G. & STOPPA F. (1996). The tectonic significance of Italian magmatism: an alternative view to the popular interpretation. Terra Nova, 8, 435-446
The Tyrrhenian rift zone has been the site of widespread magmatism since late Tortonian times. A pronounced asymmetrical distribution, reflecting the tectonic structure, characterizes Italian magmatism. Sodic basalts occur on the western Tyrrhenian flank and transitional-MORB basalts occur in the Tyrhenian Sea. The eastern flank, however, is characterized by K-alkaline and HK- to ultra-alkaline (e.g. carbonatites and melilitites) rocks. Major trace elements and isotopic compositions allow two major magmatic lineages to be identified: one relating to a depleted, non-radiogenic basaltic end-member and the other to a mantle end-member enriched in Ca, with high LILE/HFSE ratio and high Sr isotopic ratios. Their mantle sources are located within the lithosphere thermal boundary layer (TBL) and the metasomatized phlogopite-carbonate asthenosphere at the base of the TBL, respectively. The composition and spatial distribution of volcanism and relative mantle sources tend to map the geometry of the lithospheric mantle and to define a pronounced increase in depth from less than 60 km to about 100 km across the boundary between the thinned Tyrrhenian lithosphere and the Adriatic lithosphere. A mechanism of intra-continental passive rifting, which drives mantle upwelling, is sufficient to satisfy the petrological and geochemical constraints and the observed tectonic environment without requiring a subduction plane.
STOPPA F. & WOOLLEY A.R. (1997). The Italian carbonatites: field occurrence, petrology and regional significance. Mineralogy and Petrology, 59, 43-67
The paper reviews the published work, and presents new data, on the four occurrences of carbonatite that have been recognised in Italy since 1990. All are Recent in age. Three are extrusive and comprise tuffs and breccias while the fourth is high level and consists of tuffisites. They form diatremes with tuff rings, three of them being intimately associated with melilititic tuffs and lavas. Two of the occurrences include carbonatites sensu stricto i.e. calcite is >50% by volume, while the other two are 'carbonatitic', primary igneous calcite generally being 20-40%, but thin ash layers are true carbonatites. The tuffs and breccias are mixtures of carbonatitic and melilitic lapilli and bombs set in calcite-rich ash. All occurrences contain mantle debris in the form of small xenoliths, xenocrysts and cores to concentric lapilli, of olivine, pyroxene and phlogopite characterised by high Mg#, Ni and Cr. In one occurrence concentric lapilli are built of sharply bounded layers of melilitite, carbonate-bearing melilitite and calcite-phyric carbonatite around wehrlitic cores. New whole rock analyses for all four localities are given and electron microprobe analyses of calcite indicate the presence of significant Sr, Ba and REE, which is typical of carbonatitic calcite.The melilitites and carbonatites are taken to be consanguineous and to have separated immiscibility during rapid transport to the surface, earlier solidification of the melilitite producing final carbonatitic liquids. The presence of the carbonatites and melilitites in central Italy is taken as evidence that this igneous province is unlikely to be subduction related, instead the spatial distribution of these rocks in a zone east of the Roman Igneous Province is considered to reflect thicker underlying lithosphere.
STOPPA F. & PRINCIPE C. (1997). High energy eruption of carbonatitic magma at Mt. Vulture Southern Italy): the Monticchio Lakes Formation. J. Volcanol. Getherm. Res. 78, 251-265.
The Monticchio Lakes Formation (MLF) is a newly identified carbonatite - melilitite tuff sequence which is exposed in the SE sector of the Vulture volcano. It is the youngest example (c. 0.13 my) of this type of volcanism in Italy, although other carbonatites of smaller volume, but with similar characteristics, have been discovered there recently. This volcanic event occurred in isolation after a 0.35 M.a. interruption in the volcano's activity. A high energy 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 Monticchio Lakes Formation is a volcanic mixture of carbonatite and melilitite liquids which were physically separated before the eruption. Abundant mantle xenoliths are 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. The juvenile fragments mainly consist of melilite, phlogopite, calcite, apatite, perovskite, and häuyne crystals in a carbonatite or melilitite matrix. The rocks have an extremely primitive, ultramafic composition with very high Mg# (>85) and Cr+Ni content (1500 ppm). The calcite contains SrO, BaO and REE of up to 1.5 wt%. Similar compositions are typical of primary, magmatic carbonates which are found in intrusive and extrusive carbonatites. The high modal Sr-Ba-REE- rich calcite, the typical mineralogy, and the high amount of Sr-group elements identify this rock as a carbonatite. Very high Mg#, mantle debris and C, O, He isotope ratios in the range of mantle values indicate a near-primary character which is distinctive of a restricted group of extrusive carbonatites only found in continental rift areas.
F. STOPPA, V.V. SHARIGIN & A. CUNDARI (1997). New mineral data from the kamafugite-carbonatite association: the melilitolite from Pian di Celle, Italy. Mineralogy and Petrology, 61, 27-45.
A detailed mineralogical investigation of a Pian di Celle sill rock, classified as melilitolite and associated to venanzite and carbonatitic pyroclasts, revealed new and rare mineral parageneses, considered as characteristic of the kamafugite-carbonatite association. These are formed by several accessory minerals, including perovskite, cuspidine, götzenite, khibinskite, delhayelite, macdonaldite, bartonite and (Fe,Ni,Co) monoarsenide, 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 in (H2O,F,CO2)-rich, Si-undersaturated liquid(s) are considered the dominant factors in controlling the stability of disilicate-type minerals. Separation of the carbonatite from the silicate, 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.
F.STOPPA & A.CUNDARI (1998). Origin and multiple crystallization of the kamafugite-carbonatite association at S.Venanzo- Pian di Celle Umbria, Italy). Mineralogical Magazine, 62, 273-289
The Late Pleistocene kamafugite-carbonatite association at S.Venanzo-Pian di Celle forms part of the Umbria-Latium Ultra-alkaline District (ULUD) of central Italy and represents one of three similar occurrences so far reported worldwide, which include Toro Ankole, SW Uganda and Mata da Corda, Minas Gerais, Brazil. Excellent field exposure and good stratigraphic control prompted a study of the kamafugite-carbonatite interaction 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 Al-Cr diopside crystals, absent in the latter. The pyroclastic rocks represent a small amount of magma, emplaced by high velocity eruptions and characterized by ubiquitous mantle xenocrysts. Lapilli yielded a high Mg/(Mg+Fe2+) ratio (0.84-0.93) and high content of compatible elements (Ni+Cr(1000). Lavas (venanzite) and a sill (melilitolite) 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 is closely related to the above compositions forming a kamafugite trend. Glass from melilitolite yielded CIPW Or and Hy and is strongly peralkaline, i.e. (Na+K)/Al=5-6. The lapilli represent a distinct variant of the venanzite liquid, mechanically fractionated and quenched by the diatremic process, which compounded subliquidus venanzite phases, e.g. melilite, leucite and kalsilite, with mantle xenolithic/xenocrystic debris and carbonatite phases. Mantle-normalized HFSE for both lava and lapilli show typical carbonatite patterns, except for a negative spike for the Nb. Carbonatitic beds intercalated with the pyroclastic suite are distinct and tipically consist of carbonates high in Sr, Ba and REE. Primary carbonate yielded C isotope compositions varying from -5.0 to -6.0 d 13C °/°°, falling within the range for mantle compositions. Essential aspects in the genesis of the S.Venanzo rock association include primarily the combination of a mantle source of unusual composition with a high velocity ascent through the lithosphere. A common mantle source for the S.Venanzo kamafugite-carbonatite association and related carbonate-silicate interactions during emplacement is proposed and discussed.
COMODI P, LIU YU, STOPPA F & WOOLLEY (1998) A multi-method analysis of Si-, S- and REE-rich apatite from a new find of kalsilite-bearing leucitite (Abruzzi, Italy). Mineralogical Magazine, 63, 661-672
The crystal chemistry characteristics of a hydroxyl-fluor apatite from a recently discovered kalsilite-bearing foiditolite from the Abruzzi region, Italy, were investigated by electron microprobe, single crystal X-ray diffraction, IR, Raman and micro-Raman spectroscopy. The apatite has an exceptionally high contents of S and a relatively high content of Si, REE and Sr. IR spectra confirm the presence of OH calculated from formula difference. A high positive correlation between Ca-site Substitution Index (CSI = 100 (10-Ca)/Ca) and Tetrahedral Substitution Index (TSI = 100 (Si+C+S)/P atom/apfu) and a systemic parallel increase in REE, S and Si indicate two substitution mechanisms: REE3+ + (SiO4)4- = Ca2+ + (PO4)3- and (SiO4)4- + (SO4)2- = 2(PO4)3-. Site occupancy data and bond distances determined from structural refinements on selected samples demonstrate that REE and Sr show a marked preference for the Ca2 site, even though in the REE-rich samples a partial substitution of REE for Ca in the Ca1 site was observed. Tetrahedral distances (from 1.535 to 1.541 Å) reflect the substitution of Si4+ and S6+ for P5+, which is confirmed also by vibrational spectra. As (SiO4)4- and (SO4)2- substitute for (PO4)3-, the relative intensity of *1 Raman bands of (SO4)2- (at 1007 cm-1) and (SiO4)4- (at 865 cm-1) increase systematically, while that of phosphate decreases and the five components of phosphate *3 modes disappear. Moreover, the (PO4)3- Raman peak broadening is linearly correlated with the Si and S content of samples. The apatite is sometimes zoned with compositions varying from 1.15-2.07 wt% SiO2, 0.56-1.08 wt% REE2O3 and 0.58-1.02 wt% SrO in the core to 3.98-5.03, 4.14-6.73 and 1.97-2.17 respectively in the rim. A sharp, strong enrichment of Sr and LREE and an increase of the La/Nd ratio in the rim indicate that the apatite suddenly became the best acceptor of these elements in the late stages of crystallization.
E.LLOYD, A.R. WOOLLEY, F. STOPPA & N.EBY (1999) Rift valley magmatis: is there evidence for laterally variable alkali clinopyroxenite mantle? Golines, 9, 76-83
Alkaly clinopyroxenite xenoliths from three volcanic fields in Uganda are largely composed of clinopyroxenite (cpx) and phlogopite-biotite (together>70% of mode). Inter-field compositional variation in these minerals, shown 749 cpx analyses and 237 mica analyses from 34 xenoliths, indicate bulck-chemical lateral variation in the xenoliths source. the ubiquitous presence of alkali clinopyroxenite xenoliths in all the fields suggests this litology is widespread beneath Uganda's Western Rift. Nd-Sr and Pb isotopes systematics indicate that the xenoliths are not cumulate from their host kamafugites, while P-T experiments indicate that the kamafugites were in equilibrium with clinopyroxewnite at >60Km depth. It is argued therefore that the xenoliths are fragments of a laterally variable clinopyroxenite layer in Uganda's Westren Rift deep crust-mantle.
F. CASTORINA, F. STOPPA, A. CUNDARI & M. BARBIERI (2000). An enriched mantle source for Italy's melilitite-carbonatite association as inferred by its eNd-Sr isotope signature Mineralogical Magazine, 64(4), 15-169
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 143 Nd/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 eSr-eNd 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 eNd and high eSr. The potential role of the lower mantle in generating an enriched component, relative to Bulk Earth, emerges as an attractive alternative to a lithospheric mantle source.
G. ROSATELLI, F. STOPPA AND A. P. JONES (2000) Intrusive calcite-carbonatite occurrence from Mt. Vulture volcano, southern Italy Mineralogical Magazine, 64(4), 155-164
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.% Al2 O3 , 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 d13C (-4.8 % SMOW) is in the range for mantle-derived carbonatites. The 143Nd/144 Nd (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 d18O values, suggest that a secondary process, such as hydrothermal leaching, affected the rock.
STOPPA F., WOOLLEY A.R., LLOYD F., EBY N. (2000) Carbonatite lapilli-bearing tuff and a dolomite carbonatite bomb from Murumuli crater, Katwe volcanic field, Uganda Mineralogical Magazine, 64(4), 155-164.
A group of carbonate-rich tuffs are described from the Murumuli crater, Katwe-Kikorongo volcanicfield, SW Uganda which contain abundant carbonatite pelletal lapilli, together with melilitite lapilli and a range of xenocrysts and lithic fragments including clinopyroxenites considered to be of mantle origin. The carbonatite lapilli consist essentially of Sr-bearing calcite and Mg-calcite which form quench-textured laths. The lapilli contain microphenocrysts of Ti-magnetite, perovskite, apatite, clinopyroxene, sanidine and altered prisms of melilite. A 7 cm long dolomite carbonatite bomb is described which displays a form typically assumed by lava clots erupted in a molten state. Chemical analyses of a tuff, the bomb and a range of minerals are presented. Carbonatite clearly played an important role in the Katwe-Kikorongo magmatism and it is suggested that carbonatite magma evolved from carbonate-bearing melilitite.
JONES A. P., KOSTOULA T., STOPPA F. & WOLLEY A.R. (2000). Petrography and mineral chemistry of mantle xenoliths in a carbonate-rich melilititic tuff from Mt. Vulture volcano, southern Italy Mineralogical Magazine, 64(4), 341-361.
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), clinopyroxene (Mg20 92, 36% 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% Cr2 O3 , 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.
BARBIERI M ., BARBIERI M jr, D'OREFICE M., GRACIOTTI R., STOPPA F (2001). Il vulcanismo monogenico medio-pleistocenico della conca di carsoli (L’Aquila). Geologica Romana, 36, 13-31.
The volcanic field is comprised of several Upper Pleistocene small tuff cones, tuff rings and maars (531 ka), aligned along a NNW-SSE normal fault of regional meaning. Most of the deposits are directly related to vent structures and preserve signs of primary proximal origin consisting of high-energy structures, welded lapilli and ballistic impacts. Characteristic sequences of vent coring eterolithic breccias, dry lapilli-ash surges of high-temperature and wet surges of ash-lapilli tuff are exposed and found in well coring. Juvenile fragments consist of plastically moulded lapilli, essentially composed of diopside, phlogopite, leucite, K-feldspar, apatite and immersed in a turbid micro-crypto crystalline matrix of Ca-carbonate. Lapilli shape indicates that they agglutinated and quenched when still hot plastic. Silicate glass shards are present and have typical cuspate wedges produced by bubbles expansion and disruption during magmatic activity sustained by juvenile gases. A late phreato-strombolian phase builted several tuff-rings and cones. At Oricola - Carsoli - Rocca di Botte - Camerata Nuova volcanic field, juvenile fragments and tuffs range from phonolitic-foidite to foiditic-carbonatitic to carbonatite s.s. The latter forms small pyroclastic flows and surge deposits. Geological context, age, mineralogy and petrology are germane with the near Grotta del Cervo occurrence and are consistent with the carbonatitic-kamafugitic suite of Italy (Stoppa and Wolley, 1996). This new carbonatitic outcrop, that joins the increasing number of Italian carbonatites, puts Italy in a relevant place for what concerns carbon-rich mantle magmatism occurrences. Actually, the six extrusive carbonatites and the intrusive one so far reported, represent one of the largest concentrations of such a kind of rocks all over the world.
LLOYD F., WOOLLEY A.R., STOPPA F., AND EBY N. (2001) -Phlogopite-biotite parageneses from the K-mafic-carbonatite effusive magmatic association of Katwe-Kykorongo, SW Uganda. Mineralogy and Petrology, Edgar Alan volume, 1-24.
Ti-bearing phlogopite-biotite is dominant in Ugandan kamafugite-carbonatite effusives and their entrained alkali clinopyroxenite xenoliths. It occurs as xeno/phenocrysts, microphenocrysts and groundmass minerals and also as a major xenolith mineral. Xenocrystic micas in kamafugites and carbonatites are aluminous (> 12 wt% Al2O3 typically contain significant levels of Cr (up to 1.1 wt% Cr2O3 and are Ba-poor. Microphenocryst and groundmass micas in feldspathoidal rocks extend to Al-poor compositions, are depleted in Cr, and are generally enriched in Ba. In general, xenocrystic micas occupy the Al2O3 and TiO2 compositional field of the xenolith mica, and on the basis of Mg#’, and high P,T experimental evidence they probably crystallised at mantle pressures. Mica xenocryst Cr contents range from those in Cr-poor megacryst and MARID phlogopite to higher values found in primary and metasomatic phlogopites in kimberlite- hosted peridotite xenoliths. Such Cr contents in Ugandan mica xenocrysts are considered consistent with derivation from carbonate-bearing phlogopite wehrlite and phlogopite- clinopyroxenite mantle. Olivine melilitite xenocryst micas are distinguished by higher Mg# and Cr content than mica in clinopyroxenite xenoliths and mica in Katwe-Kikorongo mixed melilitite-carbonatite tephra. Higher Al2O3 distinguishes Fort Portal carbonatite xenocrysts and some contain high Cr. It is suggested that the genesis of Katwe-Kikorongo olivine melilitite and Fort Portal carbonatite involves a carbonate-bearing phlogopite wehrlite source while the source of the mixed carbonatite-melilitite rocks may be carbonate-bearing phlogopite clinopyroxenite.
STOPPA F., WOOLLEY A.R., CUNDARI A. (2002). Extension of the melilitite-carbonatite province in the Appennines of Italy: The kamafugite of Grotta del Cervo. Abruzzo. Min. Mag., 66 (4), 555-574.
A new occurrence of a rare kamafugite near L'Aquila, Abruzzo, is described in detail to characterize its paragenesis and to establish possible genetic links with similar alkaline mafic igneous rocks from the Oricola-Camerata Nuova (OC) volcanic field, ~20 km to the west. Both occurrences belong to the Umbria-Latium-Ultralkaline-District (ULUD), an igneous district represented by rare kamafugites and carbonatites and distinct from the much more voluminous Roman Region (RR) rocks. The new kamafugite was found in a cave known as Grotta del Cervo (GC), associated with epiclastic and pyroclastic rocks. In the latter, lapilli ash tuff, welded lapilli, ultramafic xenoliths, cognate lithics and pelletal lapilli have been identified. The mineralogy of the welded lapilli comprises, in order of decreasing abundance, diopside, leucite, hauÈ yne, Mg-mica, andraditic garnet, apatite, magnetite, kalsilite and olivine. The rock is carbonate-free. Based on bulk rock chemistry it is classified as a kamafugite, closely approaching the composition of ULUD kamafugites, according to Sahama's (1974) criteria. Separate lapilli ash tuff, characterized by the same silicate mineralogy as that of the welded lapilli, plus modal carbonate exceeding 10 wt.%, is classified as a carbonatitic kamafugite. Bulk-rock and trace-element compositions confirm that the Grotta del Cervo rocks closely approach the ULUD analogues. The Grotta del Cervo occurrence partially fills the geographical and compositional gap between ULUD rocks and the rocks from the Vulture Complex, also a carbonatite and melilitite locality ~200 km south of GC, and adds considerably to the bulk of kamafugitic and related rocks lying along the Italian Apennines. The petrogenesis of these kamafugites rocks is discussed and possible mineralogical similarities with the Roman Region rocks are highlighted.
Eby, G. N., Lloyd, F. E., Woolley, A. R., Stoppa, F., and Weaver, S. D. (2003). Geochemistry and Mantle Source(s) for Carbonatitic and Potassic Lavas, Western Branch of the East-African Rift System, SW Uganda. Geolines, 15, 15-19.
Samples from the various volcanic fields in the Uganda portion of the western branch of the East African rift system were analyzed for major and trace elements. The northernmost Fort Portal field consists of extrusive carbonatite tuffs and lavas. All these samples are mixtures of carbonatite, basement rock fragments and peridotite xenoliths. The central fields, Katwe-Kikorongo and Bunyaraguru, and Kasenyi Crater, are ultrapotassic, but detailed sampling indicates that the degree of K enrichment, with respect to Na, varies geographically with Bunyaraguru and Kasenyi Crater showing the greatest enrichment. The southern field, Bufumbira, while also potassic shows a much lower degree of K enrichment. In terms of a variety of trace elements, and trace element ratios, various mantle domains can be identified that gave rise to the magmas in each of the volcanic areas. The data indicate that the subcontinental mantle under this portion of Uganda has undergone variable degrees of metasomatism. A complete description of the character of the primary magmas requires consideration of both the degree of metasomatism and the degree of melting of the garnet-bearing source.
PANINA L.I., STOPPA F. and USOL’TSEVA L.M. (2003) Genesis of Melilitite Rocks of Pian di Celle Volcano, Umbrian Kamafugite Province, Italy: Evidence from Melt Inclusions in Minerals". Petrology, 2003, Vol.11, 4, 365-382.
We have found and studied syngenetic inclusions of silicate and carbonatitic melts in olivine and melilite of fine-grained melilitite lavas, bombs and lapilli tuffs of Pian Di Celli volcano (Central Italy). Silicate melts conserved in the inclusions have different chemical compositions. The rarest and highest-temperature (>1360oC) type of melts, corresponding to the composition of alkaline pyroxenites, was found in inclusions from the fragments of the highest-magnesian phenocryst Fo97, which is likely a mantle xenocryst. We think that the composition of these inclusions reflects either a composition of primitive melts or the composition of conserved relics of depleted mantle. The most common inclusions in Fo92-91 and melilite are melilitite melts and their differentiates, and Fo91-98 also contain inclusions of trachybasalt-trachyte melts. Their evolution trends have different directions. The melilite trend occurs toward hyperagpaitic melts with enrichment in alkalies and depletion in aluminum. Trachybasalt melts during evolution are enriched in Si and Al, i. e., toward miaskitic alumina-rich residual melts. Inclusions of melilitite melts are higher-temperature (/1360oC) and trachybasalt-trachyte inclusions homogenize at 1100-1120oC. The presence of trachybasalt inclusions suggests that the magma chamber along with predominant kamafugitic melt also contains trachybasalt melts. The latter can be genetically related to alkali-basaltoid magma of the Rome comagmatic region. Among carbonate inclusions there are silicate-carbonate and essentially carbonatitic varieties. Their presence is undoubtedly related to the phenomena of carbonate-silicate immiscibility which, most likely, occurred under deep-level conditions at temperatures higher than 1300oC. Salt melts evolved from silicate-bearing alkaline to essentially calcite liquids with their enrichment in Ba, Sr, F, and Cl. The results obtained and analysis of literature data bring us to a conclusion that in natural larnite-normative systems melilite most likely crystallizes only under the conditions of carbonate-silicate liquid immiscibility.
F. STOPPA, A. CUNDARI, G. ROSTELLI, A.R. WOOLLEY (2003) Leucite melilitolites in Italy: genetic aspects and relationship with associated alkaline rocks and carbonatites Periodico di Mineralogia, Special Issue Eurocarb 72, 223-251.
New bulk-rock and mineral data on leucite melilitolite from Italy are presented, compared and discussed in terms of their parageneses, petrological significance and petrogenesis. Melilitolite is an intrusive assemblage with more than 10% modal melilite. Leucite-bearing melilitolite (Italite?) is so far only known from Italy, contains about 30vol.% melilite and up to 25vol.% leucite. Other felsic constituents are kalsilite, nepheline and haüyne. It occurs as dykes, sills and a plug in the kamafugite –carbonatite suite forming the Pleistocene Ultra-alkaline Intra-mountain Province (IUP). In addition, ejecta of melilite-bearing, leucite and/or kalsilite clinopyroxenite as well as foid-free ultramelilitolite occur in alkaline, high-K volcanics from the Roman Comagmatic Region (RR). Essential mineral chemistry shows that the ubiquitous clinopyroxene signals crystallization from peralkaline liquids in its T site configuration but also, notably in RR ejecta, crystallization from metaluminous liquids. Melilite is characterised by a high gehlenite composition, similar to the melilite from a Ugandan (Fort Portal) calcite carbonatite lava. All IUP leucite melilitolites yielded lower Mg# and Cr+Ni, relative to the associated melilitites and their parental liquid is residual with respect to the initial melilititic melt. The melilitolite liquid was highly enriched in CaO and alkali and depleted in Al2O3 (agpaitic index > 0.9). High CaO and association with carbonatites have been proved to be unrelated to sedimentary limestones, but are linked to CaCO3 decoupling and reaction with the silicate fraction to form melilitolites and/or, by early CaCO3 immiscibility at high temperature, to form carbonatites. The occurrence of carbonate in globules, ocelli, and patches in melilitolite groundmass, is interpreted to have resulted from limited, late-stage immiscibility at relatively low temperature (670-800oC) and low pressure (<1 kb), favored by residual fluids concentration. Based on stratigraphic and structural observations, IUP melilitolites represent a final event in the related volcanic activity, inferred to have occurred as a slow, sub-volcanic intrusion which mechanically deformed the pre-existing rocks (brecciation, dragging and warping). IUP melilitolites and RR ejecta yielded a distinct mineral chemistry and modal abundance which reflect their initial peralkaline and metaluminous nature, respectively. This distinction is sharp for IUP melilitolites, but is blurred for RR ejecta. This may be due to the absence at the surface of a carbonatite component, non-essential modal melilite and essential feldspars in the RR assemblages. It is inferred that kamafugites may have originated from a deeper source, under a thicker lithosphere and lower heat flow, reflecting their close association with carbonatite, in contrast with conditions that prevailed for the generation of the much more abundant RR plagio-leucitite melts. and mixing of different batches of magmas.
F. STOPPA, F. E. LLOYD, G. ROSATELLI1 (2003). CO2 as the propellant of carbonatite-kamafugite cognate pairs and the eruption of diatremic tuffisite. Periodico di Mineralogia, Special Issue Eurocarb 72, 205-222.
Tuffisite pipes are characteristic of ultramafic rocks among which the rare association of kamafugite and carbonatite is prominent. Four main areas are known at present, where this association is found: Katwe-Kikorongo and Bunyaruguru in Uganda, IUP localities in Central Italy, Mata da Corda in Brazil and Qinling in Gansu, China. In all areas dominant volcanic forms are pipes (diatremes) with maar-tuff rings. Tuffisite lapilli-bearing breccia fills the diatremes and tuffisite lapilli tuff surges form pyroclastic aprons around the vents. Tuffisite lapilli are so named to reflect their specific structure, which has probably been generated during a deep magmatic tuffisitisation. Mantle xenoliths are typical of these rocks and the only realistic agent of acceleration to the surface is a deep-seated concentration of volatiles with a high proportion of CO2. The genesis of the kamafugite-carbonatite association can be assessed both in terms of field occurrence and geochemistry. In three cases out of the four the kamafugite-carbonatite association occurs in intracratonic grabens, only Italy being controversial. Specific magma composition is germane to a specific magma propagation mechanism producing a distinctive style of intrusive and extrusive pyroclastics that argues cogently for a fundamentally similar tectonic environment.
F. STOPPA, G. ROSATELLI, F. WALL, M. J. LE BAS (2003). Texture and mineralogy of tuffs and tuffisites at Ruri Volcano in western Kenya: a carbonatite, melilitite, mantle-debris trio. Periodico di Mineralogia, Special Issue Eurocarb 72, 181-204.
Ruri is located in the Kavirondo rift, near Homa in western Kenya It is a twin volcano with an ijolite-sövite core in its northern half, coupled with a carbonatite-melilitite pyroclastic centre in the southern half. The petrography and mineral chemistry of pyroclastic rocks from the collection of Ruri material at The Natural History Museum, London, have been studied in order to learn more of the nature of the volcanic activity and the erupted magmas Four types of tuffs have been recognised at South Ruri, on the basis of rock structure and texture, the nature of fragmental materials and matrix relationships: 1- heterolithic tuff: a mixture of accidental fragments with no discrete physical juvenile component; 2- pelletal tuff: heterolithic tuff with a carbonatitic matrix, with a minor juvenile component of rounded melilititic lapilli cored by mafic HP xenocrysts; 3-lapilli tuff: formed mainly of juvenile lapilli, with concentric melilitite layers around a kernel of clinopyroxene or Cr-spinel, immersed in a carbonatite ash-matrix; 4-lapilli-ash tuff: mostly microporphyritic carbonatite lapilli in a micritic carbonatite matrix (ie. extrusive carbonatite). Types 1 to 3 are interpreted as conduit and/or vent facies and type 4 is a surge deposit. The juvenile components of the deposits are of melilititic-carbonatitic type. Their proportion increases with eruptive sequence, indicating a progressively shallower level of magma fragmentation in the conduit. The juvenile silicate material is a feldspar-free association of melilite (now altered) and olivine, with foids, although these are also highly altered. Clinopyroxene, phlogopite and amphibole are slightly fresher. Some Cr-rich clinopyroxene, phlogopite, olivine and chromite have strain features and compositions indicating they are from disaggregated mantle xenoliths The melilititic melts were co-eruptive with igneous carbonate at Ruri They may represent a small volume, near-primary magma that erupted directly from the mantle, whereas the subvolcanic rocks at Ruri are more differentiated, as are the carbonatites and nephelinites of the nearby Kisingiri stratovolcano. Ruri adds another example to the general pattern that the most primitive magma compositions are erupted at small centres adjacent to the large alkaline/carbonatitic volcanoes. Other examples are Deeti, close to Kerimasi and the Monticchio Lakes at Vulture. At the liquidus temperature and low pressure of melilitite stability, carbonatite is not a miscible phase and may have erupted as mechanically separated, fine sprays of droplets and ash fragments that now form the matrix of the tuffs, as well as discrete porphyritic carbonatite lapilli.
F.STOPPA (2003). Consensus and open questions about Italian CO2 –driven magma from the mantle. Periodico di Mineralogia, Special Issue Eurocarb, 72, 1-8.
The significance of the heavy Carbon moves the frontier of interpretation for the Italian carbonatite magmatism and carbonatites worldwide toward progressively deeper mantle sources. Italian kamafugites and carbonatites are co-magmatic and erupt explosively in continental grabens. This system has some peculiarities which allow a direct link with the mantle. Spinel lherzolite xenoliths and other mantle debris are carried to the surface by carbonatites and kamafugites in the Italian Province. The extreme chemical nature of these rocks challenges conventional petrogenetic thinking. The gurus of the Italian magmatism cannot convincingly persist in preaching a geodynamic model based on Recent subduction.
K.BELL, CASTORINA F., LAVECCHIA G., ROSATELLI G., STOPPA F. (2004) Is there a mantle plume below Italy?. EOS, 85-50, 541-546.
Some of the most diverse and complex igneous rocks found on Earth are the result of Cenozoic magmatism that extended along the length of Italy and in many of the islands in the southeastern Tyrrhenian Sea. Large volumes of extremely rich in alkalies, particularly K2O, mostly silica undersaturated rocks were erupted whereas a few others are calc-alkaline in composition (see Fig. 1). Their origin has been the subject of heated debate and there is still no general consent about how they were formed. Many attribute them to subduction-related processes (Lustrino 2002 for a review), a few others consider them to be the result of within-plate magmatism (Cundari, 1994; Lavecchia & Stoppa, 1996; Bell et al., 2003) or considers the formation of slab windows with mixing between subslab and supraslab magmatic reservoirs (Gasperini et al., 2002). We have re-evaluated some of the chemical (isotopic) data, along with some new, and propose that magmatism is related to a large-scale, asymmetric plume underlying the western Mediterranean region that has been in existence for at least 30 Ma.
F. Stoppa, G. Rosatelli, F. Wall, T. Jeffries (2005).
geochemistry of carbonatite - silicate pairs in nature: a case history from Central Italy.
Lithos, Carbonatites Plus, Volume 85, 26-47
At Oricola (Aquila-Abruzzo, Italy) carbonatite is associated with phonolitic foidite tuff. The Oricola carbonatite contains fresh silicate glass of kamafugitic foidite composition which, compared with carbonate, shows similar trace element patterns but lower concentrations. As a whole, the mineralogy of the Oricola rocks matches that of the neighbouring Grotta del Cervo kamafugitic foidite and carbonatitic foidite and is in the range of the Intramountain Ultralkaline Province (IUP) of melilitites and carbonatites of Italy.
The IUP carbonatites and kamafugitic foidites definitely form intra-outcrop conjugate pairs. All these co-eruptive rocks have parallel trace element patterns, namely REE, which implies a dilution–concentration relationship among them but always with higher contents in primary calcite. Based on current textural and compositional criteria, as well as comparable experimental data, we attribute this feature to liquid immiscibility dominant over crystal fractionation at crustal pressure. This relatively late immiscibility phenomenon is superimposed on co-magmatic features shown by inter-outcrop conjugate rock couples. In fact if we consider San Venanzo kamafugite and Polino Ca-carbonatite, or Grotta del Gervo kamafugite and Oricola Ca-carbonatite, we note couple by couple that they are chemical heteromorphs erupted in isolation in different place. The REE distribution is another distinctive feature of these couples and shows a marked crossover at MREE level. A fact we interpret as produced by near mantle-solidus immiscibility. After this early phenomenon the two members of the couple can erupt in a near primary state carrying mantle xenoliths or undergo some evolution including settling out mantle xenoliths and crystals and fractionation and finally exsolve a carbonatitic residuum by immiscibility.
G. VICHI, F. STOPPA, F. WALL (2005). The carbonate fraction in carbonatitic Italian lamprophyres. Lithos, Special Issue Eurocarb, 85, 154-170.
Alkaline and ultramafic lamprophyres represent the bulk of pre-Pleistocene alkaline mafic magmatic activity in Italy and have been described from several localities. The age of magmatism ranges from Triassic to Lower Oligocene. Some contain appreciable amounts of carbonate. The primary carbonate of the Italian carbonatitic lamprophyres is mainly Sr- or Mn-rich calcite that occurs mostly as immiscible ocelli or as groundmass. Its textural occurrence, composition, and relationship with co-precipitating silicate phases is taken as evidence of an igneous origin. Low BaO and REE contents in the carbonate are explained by early crystallization of essential mica and subordinate apatite. Whole rock analyses and isotopic data (work in progress, Rukhlov A.S. et al.) suggest a mantle origin for these rocks and rule out contamination in either high or low pressure regimes. The bulk compositions of the carbonatitic lamprophyres have high HFSE/LILE and LREE/HREE ratios and although the abundances of these elements are generally lower than for carbonatites s.s., they are comparable with the abundances in other ‘carbonate-free’ Italian lamprophyres and Italian carbonatites, suggesting similar mantle sources. Moreover, the age of the Italian lamprophyres, ranging from Middle Triassic to Lower Oligocene, is much greater than the Pleistocene age of Italian carbonatites and indicates that the source remained similar over a long time span.
K. Bailey, F. Lloyd, S. Kearns, F. Stoppa, N. Eby and A. Woolley (2005). Melilitite at Fort Portal, Uganda, another dimension to the carbonate volcanism. Lithos, Special Issue Eurocarb, 85, 15-25.
Because the calciocarbonatite lavas at Fort Portal were the first ever described they have received great attention, with the pyroclastic rocks being relatively neglected. Volumetrically the lavas are minute, and the major deposit is a 2 m thick blanket of 'flaggy' tuffs, long regarded as carbonatite tuff with crustal debris. Fresh examination shows these tuffs to contain melilitite previously unreported from Fort Portal. The rock is a mix of crust and mantle debris with near-isotropic lapilli, set in a matrix composed predominantly of carbonate. The low birefringence parts of the lapilli are devitrified melilitite glass. Compound lapilli are abundant, containing aggregates of globules, together with xenolithic/crystic fragments. In some, there are concentric zones of more carbonate rich material alternating with melilitite: tangential phlogopite flakes mark the outer zones, in marked contrast to their planar distribution through the enclosing rock matrix. Euhedral titano-magnetite (10-15%) is the most obvious cognate mineral. Devitrified melilitite contains abundant small crystals and microlites of melilite, apatite, magnetite, and carbonates, mostly formed during disequilibrium quench crystallisation. Because of this, and widespread fine grained accidental debris, a precise bulk melt composition is hard to obtain, but the average is close to melilitite with high P2O5. Mantle debris is largely disaggregated magnetite-phlogopite clinopyroxenite, which could give a bulk composition close to the melt. Low Mg and high Mg calcite are present in the melilitite lapilli, and in the enclosing carbonate rich matrix. Previously, high Mg calcite was reported only as cement in lapilli tuffs, while the lavas contain only low Mg calcite in the assemblage calcite-periclase (consistent with low pressure carbonate melt crystallisation). Carbonatite-melilitite magma left the mantle carrying restite debris. Melt fragmentation took place in the deep crust, with rapidly quenched droplets enclosing crust debris. Chemical covariations within the flaggy tuffs are uniform and explicable as carbonatite-melilitite plus a thoroughly mixed combination of crust and mantle debris. New links are indicated with the alkaline ultramafic-carbonate volcanism to the south, in Uganda, and parallels with that in Italy.
I.P. Solovova, A.V. Girnis, L.N. Kogarko, N.N. Kononkova, F. Stoppa and G. Rosatelli
(2005). Compositions of magmas and carbonatevsilicate liquid immiscibility in the Vulture alkaline igneous complex, Italy. Lithos, Special Issue Eurocarb, 85, 113-128.
This paper presents a study of melt and fluid inclusions in minerals of an olivine-leucite phonolitic nephelinite bomb from the Monticchio Lake Formation, Vulture. The rock contains 50 vol.% clinopyroxene, 12% leucite, 10% alkali feldspars, 8% hauyne/sodalite, 7.5% nepheline, 4.5% apatite, 3.2% olivine, 2% opaques, 2.6% plagioclase, and b1% amphibole. We distinguished three generations of clinopyroxene differing in composition and morphology. All the phenocrysts bear primary and secondary melt and fluid inclusions, which recorded successive stages of melt evolution. The most primitive melts were found in the most magnesian olivine and the earliest clinopyroxene phenocrysts. The melts are near primary mantle liquids and are rich in Ca, Mg and incompatible and volatile elements. Thermometric experiments with the melt inclusions suggested that melt crystallization began at temperatures of about 1200 8C. Because of the partial leakage of all primary fluid inclusions, the pressure of crystallization is constrained only to minimum of 3.5 kbar. Combined silicate–carbonate melt inclusions were found in apatite phenocrysts. They are indicative of carbonate–silicate liquid immiscibility, which occurred during magma evolution. Large hydrous secondary melt inclusions were found in olivine and clinopyroxene. The inclusions in the phenocrysts recorded an open-system magma evolution during its rise towards the surface including crystallization, degassing, oxidation, and liquid immiscibility processes.
KEITH BELL, GIUSY LAVECCHIA & FRANCESCO STOPPA (2005). Reasoning and beliefs about Italian geodynamics. Boll. Soc. Geol. It., Volume Speciale n. 5, 119-127.
We propose that starting in the late Eocene-early Oligocene, the progressive eastward growth of an asymmetric plume head trapped within the transition zone (410 to 670 km depth) has played an active role in the fragmentation of the European and African continents and in the progressive opening of the Mediterranean Sea. Such an extensional geodynamic environment, as opposed to a subduction-related setting, is supported by the petrological, geochemical and isotopic features of the peri-Tyrrhenian magmatism, and in particular by the abundance of leucite-bearing rocks in peninsular Italy and by the occurrence of the rare carbonatite-kamafugite suite within the intra-Apennine graben system. The plume is isotopically associated with three end-members. Two are similar to FOZO and EM1 defined on the basis of data from ocean island basalts while the third one, ITEM (ITalian Enriched Mantle), is highly radiogenic and has Pb, Sr and Nd isotopic signatures similar to continental crust and to within-plate, deep-mantle, diamond-bearing rocks such as lamproites and micaceous kimberlites. The FOZO-like component, along with EM1, establishes a firm link between Italian magmatism and the deep mantle, and the simplest way of upward migration of mantle rock volumes and volatiles/fluids is by plume activity. ITEM is an unusual end-member, and coupled with its presence in ultramafic ultra-alkaline rocks requires an unusual mantle source. Whether ITEM reflects some fundamental character of the deep mantle, or crustal material associated with subduction is a question that remains to be answered.
F.Stoppa, G. Rosatelli, A. Cundari, F. Castorina and A. R. Woolley (2005). Comments on Melluso et al. (2003) and their reported data and interpretation of some wollastonite- and melilite-bearing rocks from the Central Apennines of Italy. American Mineralogist, 90, 1919-1925.
Two distinct occurrences of wollastonite- and melilite-bearing rocks from Ricetto and Colle Fabbri, which are located in the central Italy Apennine Range, are referred to as paralavas generated by melting and recrystallization of marly sediments likely due to coal fires by Melluso et al. (2003). We submit data demonstrating that these conclusions are incorrect. Ricetto is clearly pyrometamorphic, as described by Capitanio et al. (2001, 2004). Colle Fabbri is clearly an igneous outcrop as described in Stoppa (1988). It is quite different in scale, field relationships, and chemistry, and thus Ricetto is irrelevant to its petrogenesis. The sediment mixing model of Melluso et al. (2003) does not relate to the Colle Fabbri field data. Furthermore, the Ricetto Miocene flysch country-rock and the Colle Fabbri enclosing Pliocene clays were not analyzed by Melluso et al. (2003). A limestone-shale mixing calculation using Sr and Nd isotopic data in Melluso et al. (2003, Tables 3.4) and a 143Nd/144Nd ratio of 0.51213 for Apennine shales (op.cit. p. 1297) fails to account for the Colle Fabbri samples, which remain well clear of the hypothetical mixing line. Rocks of mantle origin, including micaceous kimberlites and Western Australia lamproites, plot in the variation field of Colle Fabbri and regionally associated igneous rocks. Colle Fabbri is but one of a series of similar igneous melilite-bearing and carbonatite occurrences that constitute the Intramontane Ultra-alkaline Province of central Italy (IUP), although Colle Fabbri does have unique features, as do other of these occurrences (Stoppa et al. 2003).
F.STOPPA (2005) The Sirente crater, Italy: impact versus mud volcano origins. Meteoritics and Planetary Sciences, vol. 41, pp. 467-477
Sirente crater has a rim with a circumference of about 30m. The rim-deposit is consistently an inverse-graded, matrix-supported breccia. Sedimentological features of the rim-deposit suggest that the crater is not related to an explosion (i.e. gas expansion) or violent mechanical displacement. Structure and texture of the deposit exhibit a primary sedimentary feature. The rim deposits do not contain artefacts and do not show evidence of reworking. A multistage formation is reconstructed for the rim growth and associated deposits. The geometry and sedimentology of the deposits indicate that they were produced by emission and accumulation of mudflow deposits. The dominant ejection mechanism was low mud fountains and the transport medium was water. Petrography and geochemistry do not indicate any physical or cryptic trace of an extraterrestrial body.The most realistic agent which explains the observed effects is a rapid local emission of mud and/or water. Geological processes capable of producing the observed features include piping sinkholes or, better, ‘caldera’-type mud volcanoes and may result from underground water-table perturbation and/or decompression of deep CO2/hydrocarbons gas–reservoirs due to tectonic deformation or faulting activity during a seismic event. In both cases, the name ‘crater’ for this geological form may be maintained but there is no evidence for a meteorite or bolide. In this paper the scientific literature on the Sirente Crater is re-considered in the light of new morphological, sedimentological, geochemical and archaeological data. A new mechanism is proposed involving mud-fountaining
WOOLLEY R.A., BAILEY D.K., CASTORINA F., ROSATELLI G., STOPPA F., WALL F. (2005). Reply to: “Carbonate-rich pyroclastic rocks from central Apennines: carbonatites or carbonated rocks? A commentary”. Periodico di Mineralogia, 74, pp. 183-194.
In a recent paper Peccerillo (2004) quoted chemical, including isotopic, mineralogical and field data which he claimed “..cast serious doubt on the hypothesis that the carbonate-rich pyroclastics from central Italy represent carbonatitic magmas.”. He concluded that the carbonate is derived from sedimentary limestone. He also noted that he had enunciated these ideas earlier (Peccerillo, 1998) but that he had not been answered and further that “the Editor of this journal [Periodico di Mineralogia] has repeatedly asked me to set down my case again and I have, reluctantly, agreed to do so.”. We, some of the principal advocates of the carbonatitic interpretation for the igneous carbonate-rich rocks of central Italy, have, therefore, taken this opportunity to answer, in detail, the various arguments advanced by Peccerillo. We remain firmly convinced that the evidence demonstrates that the carbonate-rich rocks at the five localities of San Venanzo, Cuppaello, Polino, Oricola and Vulture are carbonatitic, the carbonate deriving from deep within the mantle, and that sedimentary limestones played no immediate part in their genesis.
G. Lavecchia, F. Stoppa and N. Creati (2006) Carbonatites and kamafugites in Italy: mantle-derived rocks that challenge subduction. Annals of Geophysics, 49-1, 389-402
The carbonatite and kamafugite (i.e. potassic melilitite) rock-types of the Intramontane Ultra-alkaline Province (IUP, Italy) form a geologically rare, albeit not unique, ultramafic association. Their composition is in the range of other kamafugites and carbonatites, especially extrusive carbonatites, in the world (Katwe-Kikorongo and Bunyaruguru in Uganda, Oldoinyo Lengai in Tanzania and Quinling in Gansu, Central China). The kamafugitecarbonatite rock association requires specific tectonic-magmatic prerequisites: 1) metasomatised mantle source (enriched carbonate/phlogopite-bearing peridotite); 2) magma rapid rise to the surface to preserve a near primary state, to carry large mantle nodules and to limit the interaction with the crustal rocks and/or fluids; 3) geometry of the lithosphere suitable to favour the intersection of the local geotherm with the metasomatised peridotitesolidus and to allow only very small-degrees of partial melting. The Italian kamafugite carbonatite geochemistry encompasses high LILE (Cs, Rb, K, Ba), high Sr-group (Sr plus REE), high LREE/HREE, high incompatible elements (Cr-Ni), but negative Eu anomaly, low light-HFSE4+ (Zr-Ti) and intermediate heavy-HFSE (Hf-Ta). This distribution requires a high CO2 fugacity in the source able to fractionate the HFSE and to produce the Eu anomaly. Based on geochemical criteria used for basaltic rocks, the IUP rocks were attributed by some authors to a subduction geodynamic setting, a fact which produced a catastrophic misunderstanding of these rocks. Worldwide kamafugites and carbonatites occur in intraplate and continental rift settings, the magmatogenetic conditions required for their genesis being unsuitable in a subduction environment, which is instead the dominating model for Italian volcanism. This consideration stimulated this review of the IUP field relationships, mineralogy, petrology and geochemistry. The IUP is sited about 50 km east of the large volcanoes of the Roman Comagmatic Region (RCR). Plagio-leucitites which are typical of the RCR, are also geologically rare and, when regionally associated with carbonatites and kamafugites, form a regional triad very typical of continental rifts. This triad has never been observed in subduction related environments, that are dominanted by large volumes of andesites and dacites. It may be better explained in the frame of a plume model that links the metasomatic process of the IUP mantle source with hydroxyl-CO2 rich radiogenic fluids and volatiles released within the Mediterranean asthenosphere from a plume head trapped in the transition zone (between the 410 and 670 km discontinuities).
K. Bell, F. Castorina, G. Rosatelli, F. Stoppa (2006) Plume activity, magmatism, and the geodynamic evolution of the eastern Mediterranean. Annals of Geophysics, SI 49-1, 357-372.
On the basis of isotopic and geochemical data we propose that most of the volcanic activity in Italy is plume rather than subduction related. We propose that a large plume underlies the Tyrrhenian Sea, extending westwards under Sardinia and Corsica, northwards towards the Western Alps and eastwards under the Italian mainland. The plume is isotopically defined in terms of three end-members, all quite different to any of those found in subduction-related environments. Two of the end members are similar to FOZO and EMI defined on the basis of data from OIBs, while a third, here called ITEM (ITalian Enriched Mantle), is characterized by a high 87Sr/86Sr ratio (>0.7200) and quite different to any well-defined component found in oceanic environments. Two distinct trends in isotope ratio diagrams reflect mixing between a common end member (FOZO) and the two others. Variation in the isotopic compositions of the Italian magmas is attributed to partial melting of an isotopically heterogeneous, plume head containing both source and entrained material. Widespread extensional tectonics, lithospheric thinning, and deep-seated CO2 emissions add further support to mantle plume activity in Italy.
F. Stoppa, C. Principe and Giannandrea P., (2007). Comments on: Carbonatites in a subduction system: the Pleistocene alvikites from Mt. Vulture (southern Italy) by d’Orazio et al., (2007). Lithos, in press.
D’Orazio et al., (2007) describe a new finding of alvikite Ca-carbonatite at Vulture. They stress its importance as being the first carbonatite to be discovered in a subduction environment. They suggest that this rock is different from the other Italian carbonatites, considered as ‘rocks sharing a carbonatitic affinity’, which are radiogenic and chemically diluted by addition of sedimentary limestone. They note that Vulture ‘alvikite’ is not diluted and is very unradiogenic with respect to other Italian carbonatites. However, they maintain that Vulture ‘alvikite’ carbonate is derived from subducted limestones. We present an account of the field relationships relating to the above-mentioned rocks, setting the geological and petrographic records straight and describing pyroclastic rocks. We did not find that these rocks are formed from alvikite dykes or lava, but instead recognised them to be a continuous blanket of ‘flaggy’, welded tuff. We found that the rocks consist of physically separated melilitite and carbonatite juvenile lapilli settled into a carbonatite ash matrix form the rock. We disagree with the geochemical interpretation of the rock by d’Orazio et al., (2007), and are particularly concerned by their conclusion of its carbonate origin. We remark on the rock’s geodynamic assignment in the frame of an extensional tectonic setting, also referring to the other Italian carbonatite occurrences. We reject any ad hoc modified subduction as a direct source of Vulture and Italian carbonatites.
Stoppa F. , (2007). CO2 magmatism in Italy: from deep carbon to carbonatite volcanism. In Alkaline Magmatism, its sources and plumes. N.V. Vladykin ed.. Irkutsk, IGC. 109-126. ISBN 5-94797-095-3
The importance of CO2 in the Italian mantle and magmatism is supported by the presence of Middle-Upper Pleistocene carbonatites along the Italian Apennine graben-systems. Carbonatites are co-eruptive and chemically conjugate to kamafugites (kalsilite melilitite or foidite). Immiscibility phenomena largely explain the genetic relationship between the two rock-types. Their peculiar geochemistry reflects the differing solubility of the high field strength elements in rocks which have different peralkalinities. Dolomite and calcite dominated inclusions occur in mantle nodules and plutonic rocks such as soviets and melilitolites, which have been brought up by extrusive carbonatites. Nyerereite was found in melilite and apatite inclusions although the Italian extrusive carbonatites are always calcitic. Sequestration by abundant leucite, kalsilite, haüyne and nepheline, and dispersion of alkalis in subvolcanic aqueous fluids, may explain the low alkaline character of Italian carbonatites. The high Ca, CO2, F, S, Cl content of primitive melt inclusions in high pressure crystals suggests that these elements were not assimilated by the magma from the crust during its ascension towards the surface. In addition, the high radiogenic isotopic composition and compatible element content are regarded as primary magmatic characteristics. Veined mantle nodules show pervasive reaction with carbonatitic melts as illustrated by the presence of amphibole, fassaitic cpx and phlogopite phases. Metasomatising carbonatitic melts are thought to be released from a deep plume. The geodynamic and geochemical setting of Italian carbonatites implies a very deep source for heavy Carbon and it has even been speculated that this Carbon could be originated from the Earth’s core.
E. Schingaro, F. Scordari, S. Matarrese, E. Mesto, F. Stoppa, G. Rosatelli and G. Pedrazzi (2007) Phlogopite from the Ventaruolo subsynthem volcanics (Mt Vulture, Italy): a multi-method study. Mineralogical Magazine, 71(5),519–537
Volcanic activity at Mt Vulture lasted throughout the Middle Pleistocene and produced SiO2-undersaturated volcanics. Deposits from the Monte Vulture stratovolcano have been classified into four subsynthems and clustered into the Barile Synthem. In the present investigation, trioctahedral micas from the uppermost units (the Ventaruolo Subsynthem) of the Barile Synthem are considered. The samples are labelled VUT187. The phlogopitic micas were separated from the host rock (an olivinefoidite) and underwent chemical (electron microprobe analysis - EMPA and C-H-N), structural (singlecrystal X-ray diffraction) and spectroscopic (Mo¨ssbauer) investigations. The EMPA yielded: MgO (17.62 21.89 wt.%), FeOtot (5.98 9.78 wt.%), TiO2 (1.81 3.92 wt.%) and Al2O3 (14.47 17.98 wt.%), with H2O contents = 2.86 (E0.42) wt.% determined by C-H-N analyses. Mo¨ssbauer investigation provided [VI]Fe2+ = 12.6%, [VI]Fe3+ = 87.4%. The chemical and structural data are consistent with the occurrence of Ti-oxy, [VI]M2+ + 2(OH) > [VI]Ti4+ + 2O2 + H2, and M3+ oxy substitutions, [VI]M2+ + (OH) > [VI]M3+ + O2 + HH2, with M3+ = Fe3+, Al3+. In particular, Fe3+-oxy substitution has affected the Fe2+/Fe3+ ratioin the studied sample. This is probably due to the fact that interaction with underground water or a hydrothermal system may have altered the oxygen fugacity and raised the Fe3+ content of VUT187 phlogopite with respect to magmatic values.
STOPPA F., (2008). Alkaline and ultramafic lamprophyres in Italy: Distribution, mineral phases, and bulk rock data. In Alkaline Magmatism, its sources and plumes. N.V. Vladykin ed.. Irkutsk. Several lamprophyre outcrops occur in Italy paralleling the European cycles. Lamprophyre occurrences from the lower Cretaceous to the Oligocene are comprised primarily of dykes and rarely lavas, generally emplaced in isolation. They are ultramafic or alkaline lamprophyres located in eight different places ranging from the eastern Alps to the areas of Tuscany, Sardinia, Abruzzi, and Puglia and exhibit similar geochemistry, suggesting that they originated from the same mantle source, despite different tectonic conditions. Lamprophyres relate to partial melting of a mantle source rich in large-ion lithophile-elements (LILEs) and C-O-H. Distribution of high-field-strength elements (HFSEs) and their ratio to LILEs depends upon the presence of specific scavenger phases, which makes the geochemistry of these and similar rocks very different with respect to that of basalts. Italian Lamprophyres preserve different specific isotopic features that can be described in term of mixing of two mantle end-members, one of which is highly radiogenic. Lamprophyre magma emplacement should occur during structural extension after major compression episodes. However, this tectonic model seems much too simple for the Italian lamprophyres and is inconsistent with the Mediterranean Tethys geodynamic evolution. On the other hand, Italian lamprophyres evolved into lamproites after the Lower Oligocene era and finally into leucitites and kamafugites (plus carbonatites), ultrapotassic rocks which could be considered anhydrous, petrologic equivalents of lamprophyres. Certainly this requires very specific source conditions, possible related to a mature stage of mantle metasomatisms triggered by repeated episodes of (alkaline) carbonatite invasion, melt extraction, and upper mantle decompression. Italian lamprophyres demonstrate, with their long-term constant geochemistry and isotopic features, that the metasomatic agent is unrelated in time and space to the relatively shorter subduction phases of the Mediterranean Sea area and better fits with the presence of a large, long-lived, pulsing deep plume.