1,720,976 research outputs found
Vulcanismo carbonatitico medio-pleistocenico in Italia: attività eruttiva e depositi piroclastici.
No full textThe Italian carbonatites: field occurrence, petrology and regional significance.
No full textThe 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 carbonatitessensu 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 immiscibly 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
Extension of the melilite-carbonatite province in the Apennines of Italy: the kamafugite of Grotta del Cervo, Abruzzo.
No full textA 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, haüyne, 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
A multi-method analysis of Si-, S- and REE-rich apatite from a new find of kalsilite-bearing leucitite (Abruzzi, Italy).
No full textThe crystal chemistry characteristics of a hydroxyl-fluor apatite from a recently discovered kalsilite-bearing leucitite from Abruzzi, Italy, were investigated by electron microprobe, single crystal X-ray diffraction, IR, Raman and micro-Raman spectroscopy. The apatite has exceptionally high S and relatively high Si, Sr and LREE, whereas the HREE content is negligible. The 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/a.p.f.u.) and a systematic parallel increase in REE, S and Si indicate two substitution mechanisms, i.e. REE3+ + Si4+ = Ca2+ + P5+ and Si4+ + S6+ = 2 P5+. Site occupancy data and bond lengths, determined from structural refinements on selected samples, demonstrate that LREE and Sr show a
marked preference for the Ca2 site, even though in the LREE-rich samples a partial substitution of LREE 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 also confirmed by vibrational spectra. As (SiO4)4- and (SO4)2-
substitute for (PO4)3-, the relative intensity of n1 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 n3 modes disappear. Moreover, the (PO4)3- Raman peak broadening is linearly correlated with the Si and S concentrations. Apatite crystals are sometimes zoned with compositions varying from SiO2 = 1.15-2.07 wt.%, Σ(LREE2O3) = 0.56-1.08 wt.% and SrO = 0.58-1.02 wt.% 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 in Sr and LREE in the rim indicate that
the apatite suddenly became an acceptor of these elements in the late stages of crystallization
Carbonatite lapilli-bearing tuff and a dolomite carbonatite bomb from Murumuli crater, Katwe volcanic field, Uganda
No full textA group of carbonate-rich tuffs are described from the Murumuli crater, Katwe-Kikorongo volcanic field, 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
Leucite melilitolites in Italy: genetic aspects and relationships with associated alkaline rocks and carbonatites
No full text
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Comments on Melluso et al. (2003) reported data and interpretation of some wollastonite- and melilite-bearing rocks from the Central Apennines of Italy
No full textTwo 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)
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