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Mineralogy and Petrology

, Volume 49, Issue 3–4, pp 213–231 | Cite as

Mineralogy and petrology of the Polino Monticellite Calciocarbonatite (Central Italy)

  • F. Stoppa
  • L. Lupini
Article

Summary

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, schorlom ite, 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.

Keywords

Perovskit Apatite Ultramafic Rock Concentric Structure Alkali Basalt 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Mineralogie und Petrologie des Monticellit-Calciokarbonatites von Polino, Mittelitalien

Zusammenfassung

Zwei kleine, um 0.25 Millionen Jahre alte Diatreme durchschlugen liassische Kalkgesteine, ungefdhr 1 km NNE des Dorfes Polino (Long. 12°50'54″E-Lat. 42°35'34″N; Mittelitalien). Das Material, aus dem das größere Diatrem besteht, ist überwiegend Tuffisit mit häufigen Lapilli, die eine konzentrische Struktur zeigen. Sowohl nichtalterierte Nebengesteine, wie auch massive, hypabyssale Karbonatite treten im Tuffisit als eckige Klasten und Blöcke auf, mit einem Durchmesser von wenigen mm bis 1 m.

Das Polino-Gestein tritt in einem sehr Kalium-reichen Vulkan-Distrikt (der Umbria-Latium Ultraalkaline Distrikt) auf, der aus phonolitischen Pyroklastika und untergeordnet aus kamafugitischen Laven aufgebaut ist.

Massive Karbonatite bestehen im Durchschnitt aus 53% Sr-Ba-reichem Calcit, 23% Fe-Monticellit, 9% Th-Perovskit und Ti-Magnetit, 6% Cr-Phlogopit, 6% Fo-reichem Olivin, ungefdhr 2% Zr-Schorlomit und ca. 1 % Si-CO-OH-Apatit. Perovskit, Schorlo mit und Apatit bilden eine Mineralassoziation, während Olivin und Phlogopit, die häufig durch Monticellit verdrdngt sind, als Nodulen und Einzelkbrner auftreten. Letztere zeigen Zusammensetzungen und Deformationen wie sie für Mantel-Xenokristalle in Alkali-Basalten und ultramafischen Gesteinen typisch sind.

Der hohe Modalgehalt an Ca-Karbonat, hohe Sr, Ba und LREE-Gehalte im Calcit, das Vorhandensein von besonders für Karbonatite außergewöhnlichen Mineralen und häufiger Monticellit sprechen für eine Klassifizierung des Polino-Gesteins als Monticellit-Calciokarbonatit.

Das Polino-Gestein repräsentiert eine karbonatitische Schmelze, die von Mantelkristallen kontaminiert ist. Die außergewöhnliche Geochemie dieser Gesteine ist durch Spurenelemente, die eine engere Verwandtschaft zu den regional assoziierten Kamafugiten als zu üblichen Karbonatiten erkennen lassen, geprägt.

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Copyright information

© Springer-Verlag 1993

Authors and Affiliations

  • F. Stoppa
    • 1
  • L. Lupini
    • 1
  1. 1.Dipartimento di Scienze della TerraUniversità di Perugia, Piazza Università 1PerugiaItaly

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