Pantelleritic Ignimbrite

The term "ignimbrite" (from the Latin igni- "fire" and imbri- "rain") was coined by the New Zealand geologist Peter Marshall in 1935. This term was originally used only to refer to welded tuffs. These are pyroclastic rocks that were so hot right after the deposition from the pyroclastic cloud that individual clasts adhered to each other. However, this restriction no longer applies. The term "ignimbrite" includes all pyroclastic flow deposits, no matter whether they are welded or not.

An ignimbrite is a welded pyroclastic rock that contains abundant flattened juvenile clasts often originally pumice. The flattened clasts within ignimbrites are termed fiamme and range from lapilli-sized (>2 mm) to block-sized (>64 mm). The layered texture produced by fiamme is termed a eutaxitic texture. The groundmass of ignimbrites is usually dominated flattened vitric shards, but can contain lithic and crystal fragments. The fine-grained groundmass of many ignimbrites has a reddish colour due to high temperature oxidation of iron, in particular in the upper parts of a pyroclastic flow deposit. Less welded flows tend to be white or grey, whilst intensely welded flows are often dark grey to black. Recrystallisation and alteration of glass within ignimbrite is common, in particular in ancient examples.

Ignimbrites form due to emplacement of high temperature pyroclastic flows that compact under their own weight. Exsolution of volatiles from pyroclasts after emplacement can cause alteration of the surrounding groundmass and generate vesicles. Rheomorphic flow of ignimbrites can occur after emplacement resulting in deformation of layering, clasts and vesicles. In thick ignimbrites columnar jointing may occur due to contraction during slow cooling.

The mineralogy of an ignimbrite is controlled primarily by the chemistry of the source magma. The typical range of phenocrysts in ignimbrites are biotite, quartz, sanidine or other alkali feldspar, occasionally hornblende, rarely pyroxene and in the case of phonolite tuffs, the feldspathoid minerals such as nepheline and leucite. Commonly in most felsic ignimbrites the quartz polymorphs cristobalite and tridymite are usually found within the welded tuffs and breccias. In the majority of cases, it appears that these high-temperature polymorphs of quartz occurred post-eruption as part of an autogenic post-eruptive alteration in some metastable form. Thus although tridymite and cristobalite are common minerals in ignimbrites, they may not be primary magmatic minerals.

Pantellerite is a peralkaline rhyolite. It has a higher iron and lower aluminium composition than comendite. It is named after Pantelleria, a volcanic island in the Strait of Sicily and the type location for this rock. On Pantelleria the rock is usually found as a vitrophyre containing phenocrysts of anorthoclase or sanidine. Quartz is found only in the most strongly peralkaline rocks. Mafic minerals may include aegirine, fayalite, aenigmatite, ilmenite, and sodic amphibole (often arfvedsonite or ferrorichterite).

Pantellerite melts, such as those from the Island of Pantelleria, Italy, are characterized by high alkali and halogen contents compared to metaluminous magmas of equivalent silica content (or "normal rhyolites"). These peculiarities in geochemistry affect the rheological properties of peralkaline magmas, resulting in viscosities that are several orders of magnitude lower than those of "normal” rhyolites".

The rheology of peralkaline magmas affects profoundly their vesiculation, outgassing, flow and eruption. Peralkaline volcanic systems commonly exhibit substantial differences in the physical nature of their eruptive products with respect to metaluminous rhyolites. In contrast with calcalkaline centres, which produce widespread plinian deposits and ignimbrites, the dynamics of peralkaline rhyolite volcanic systems are more varied and complex, and not yet fully understood. Peralkaline volcanic centres show a close association between fall units, ignimbrites, deposits originating from lava fountains and lava domes and flows, documenting a continuous transition from explosive to less explosive styles of eruption. These kinds of associations are unique to pantellerites.

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Green Pantelleritic Ignimbrite from Pantelleria (Italy)/



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Rheomorphic structure in a Pantellerite from Pantelleria (Italy)/




Bibliography



• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Eric A.K. (1985): Middlemost Magmas and Magmatic Rocks. Longman, London
• D’Amico C., Innocenti F. & Sassi F.P. (1987): Magmatismo e metamorfismo. UTET
• Innocenti F., Rocchi S. & Triglia R. (1999:) La classificazione delle rocce vulcaniche e subvulcaniche: schema operativo per il progetto CARG.
• Carmichael I.S.E., Turner F.J. & Verghoogen J. (1974): Igneous Petrology. McGraw-Hill.

Photo
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Sanidine, Aegirine and Aenigmatite (balck-red) in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Sanidine, Aegirine and Aenigmatite (balck-red) in a Pantellerite from Pantelleria (Italy). XPL image, 2x (Field of view = 7mm)
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Sanidine, Aegirine and Aenigmatite (balck-red) in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Sanidine in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Sanidine and Aegirine in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Sanidine and Aenigmatite (balck-red) in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Flattened pumice fragment in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Sanidine and Aegirine in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Sanidine and Aenigmatite (balck-red) in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Flattened pumice fragment, Aegirine and Aenigmatite (balck-red) in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Aenigmatite (balck-red) embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Aenigmatite (balck-red) embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Aenigmatite (balck-red) embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Pumice fragments in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Sanidine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Aenigmatite (balck-red) embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aenigmatite (balck-red) and Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aenigmatite (balck-red) embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Sanidine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Sanidine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aenigmatite (balck-red) embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aenigmatite (balck-red) embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Sanidine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aenigmatite (balck-red) and Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aenigmatite (balck-red) embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aenigmatite (balck-red) embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Flattened, deformed pumice fragment in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Flattened, deformed pumice fragment in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Flattened, deformed pumice fragment in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Sanidine, Aegirine and Aenigmatite (black-red) in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Flattened, deformed pumice fragment in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Flattened, deformed pumice fragments in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Aegirine and sanidine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Xenolith embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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sanidine, Pumice fragments, Aegirine and Aenigmatite embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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sanidine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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sanidine and Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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sanidine and Aenigmatite embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
igimbritepantelleritica0221(5).jpg

sanidine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
igimbritepantelleritica0221(6).jpg

sanidine and Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
igimbritepantelleritica0221(7).jpg

sanidine and Aenigmatite embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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sanidine, pumice fragment and Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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sanidine and AenigmatiteXenolith embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
igimbritepantelleritica0221(10).jpg

Aegirine and Aenigmatite embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 2x (Field of view = 7mm)
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Aegirine embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aegirine and Aenigmatite embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)
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Aenigmatite embedded in a Eutaxitic groundmass in a Pantellerite from Pantelleria (Italy). PPL image, 10x (Field of view = 2mm)