Tephritic leucitite

Leucitites: are a groups of fine-grained, often porphyritic extrusive or subvolcanic rocks composed essentially of Leucite and clinopyroxene (Titanoaugite, diopside or Aegirine). The term was used for the first time by Rosenbush in 1877.

Alkaline rocks

In petrology, the term alkaline is used to describe those rocks that contain essential amounts of foids (nepheline, sodalite, leucite) and/or alkali pyroxenes, and/or alkali amphiboles, and/or melilite. Alkaline rocks generally have more alkalis than can be accommodated by feldspars alone. The excess alkalis appear in feldspathoids, sodic pyroxenes-amphiboles, or other alkali-rich phases. In the most restricted sense, alkaline rocks are deficient in SiO₂ with respect to Na₂O, K₂O, and CaO to the extent that they become critically undersaturated in SiO₂, and Nepheline or Acmite appears in the norm. On the other hand, some rocks may be deficient in Al₂O₃ (and not necessarily in SiO₂) so that Al₂O₃ may not be able to accommodate the alkalis in normative feldspars. Such rocks are called peralkaline and may be either silica undersaturated or oversaturated.

The alkaline rocks can be grouped as:

1) Rocks having adequate or excess silica but deficient in alumina: These rocks having molecular proportion (Na₂O+K₂O)/Al₂O₃ > 1 are designated as peralkaline rocks. Typical Peralkaline silicic volcanics are represented by pantellerites (Na-rich rhyolite) and comendite (K-rich rhyolite).

2) Rocks in which alumina is adequate (to saturate feldspar composition) or in excess but silica is deficient: The rocks are then composed of feldspars and feldspathoids along with mica hornblende, corundum etc. Tephrites and phonolites belong to this group.

3) Rocks deficient in both silica and alumina relative to feldspar composition: The rocks contain besides alkali feldspars, both silica undersaturated minerals, feldspathoids and, also alkali rich mafic minerals. Foid-bearing-trachyte belong to this group.

Attempting to describe alkaline rocks is similar to opening Pandora’s box. They are petrologically fascinating but exhaustively diverse. They have attracted the attention of petrologists to an extent out of all proportion to their abundance. About half of the formal igneous rock names apply to alkaline rocks. When we consider that these rocks constitute less than 1% of the total volume of exposed igneous rocks, we might question the rationality of attempting so complete a description of such a bewildering array at this survey level. Several aspects of the classification of alkaline rocks are confusing and controversial, reflecting uncertainty about genetic relationships and the tectonic setting of many occurrences. Some investigators attempt to classify on the basis of genetic similarities (like the magmatic series), whereas others, recognizing the difficulty in assessing the genesis of such diverse rocks in complex continental settings, rely on more easily determinable features, such as chemical, mineralogical, and/or textural similarities.

Petrographic characteristics of alkaline rocks:

• Phonolites: Phonolites are extrusive rocks that are essentially composed of alkali feldspar (Anorthoclase and Sanidine), mafic minerals such as alkali amphiboles, alkali pyroxenes, Augite, Biotite and Olivine) and one or more foids. If nepheline is the only foid then the term phonolite may be used by itself but if, for example, leucite is the most abundant foid then the term leucite phonolite should be used etc. The name, coined by Cordier in 1816, comes from the greek meaning sounding stone because of the metallic sound it produces if an unfractured plate is hit; hence the English name clinkstone.

• Tephrites: Tephrites are extrusive rocks that are essentially composed of calcic plagioclase, clinopyroxene and foids. Foids normally constitute more than 10 % of the felsic minerals. Tephrites differ from basanites on that they do not contain essential olivine. Tephrites also contain minor amounts of alkali feldspar and they thus grade with increasing alkali feldspar contents into the phonolitic Tephrites and tephritic Phonolites. Both sodium rich and potassium rich Tephrites are found. Sodium rich varieties are known in Canary island and Thaiti while potassium rich varieties are known in the Roman magmatic provinces and Vesuvius. Tephrite is term of great antiquity usually attributed to Pliny, AD 77; from the Greek tephra = ashes.

• Basanites: Are a group of Tephritic rocks that essentially contain calcic plagioclase, clinopyroxene, foids (more than 10% of the felsic minerals) and olivine. Basanites can be Na ore K-rich. The term derives from the Latin basanites = hard rock.

Alkaline rocks that are essentially devoid of feldspar

• Foidites: A general term for volcanic rocks containing more than 60% foids in total light-coloured constituents.

• Nephelinites: are fine grained extrusive rocks composed by Nepheline and clinopyroxene; Nephelinites in which mafic minerals are more than foids are called melanephelinites. The term was used for the first time by Cordier in 1842.

• Leucitites: are a groups of fine-grained, often porphyritic extrusive or subvolcanic rocks composed essentially of Leucite and clinopyroxene (Titanoaugite, diopside or Aegirine). The term was used for the first time by Rosenbush in 1877.

Leucite-bearing rocks

Leucite-bearing rocks are a unique group of alkaline rocks that are found in a widely scattered localities all over the Earth like: Toro-Ankole procince in Uganda, Virunga province in Rwanda, Central massif in France, Roman magmatic province in Italy, Laacher see in Germany Mauricia and Almaria province in Spain, Leucite hills in USA, Kimberly region in South Africa, Java and Sulawesi in Indonesia.The Roman magmatic province, is probably the most and best studied ultrapotassic province. Is situated in central Italy and includes Vico volcano, Sabatini and Vulsini volcanoes, Alban Hills and Ernici-Roccamonfina volcanoes.

Leucite-bearing rocks, after the elimination of the lamproites and kamafugites, should be classified primarily on the basis of their chemistry and mineralogy, but on petrological stand point the mineralogical classification is more preferred here particularly in view of plethora of rock names given on the basis of localities from which these rocks have been described. Their nomenclature is therefore, not only confusing but also complex at times.

The most commonly occurring minerals found in these leucite-bearing rocks are leucite, clinopyroxene, plagioclase, potash feldspar, nepheline, and olivine. When plagioclase is present, melilite is absent. Phlogopite and potash richterite are fairly common. Accessory minerals include: biotite, perovskite, wadeite, priderite, apatite, magnetite, ilmenite, and spinel.

Leucite-bearing rocks Calssification

Petrogenesis of Alkaline rocks

Alkaline rocks occur in many different tectonic setting, petrological associations and have such diverse chemical compositions, so it is evident that no single parental magma is able to produce all the alkaline rocks. Their occurrence on oceanic islands and seamounts demonstrates that at least some of the parental magmas can be generated within the mantle and that these magmas can evolve in an environment far removed from any possible contamination by continental crust materials. Most petrologist now believe that the majority of alkaline rocks have evolved from parental magmas generated by partial melting within the mantle; and also that fractional crystallization, and other differentiation processes are usually only able to accentuate an alkaline tendency that has already been imparted to the parental magma.

According to Bailey (1983) alkaline magmatic activity is strictly controlled by the release of volatile-charged magmas from deep mantle source. Fractures through the continental lithosphere act as channel ways for these magmas, and volatiles and incompatible elements are drained through narrow fractures and rift zones. The latter process results in metasomatims; the composition of the ascending magma is to a large extent controlled by wall-rock reactions and polybaric fractional crystallization. Heat from the magma results in a gradual increase in the amount of partial melting and, depending on the composition of the materials being melted, a wide variety of different magmas may be generated. Has been proposed that undersaurated alkaline rocks are normally generated within the mantle at a depth of at least 80Km; and that the generation of these magmas tend to be triggered by the influx of low-viscosity fluids from degasing mantle.

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Pyroxene phenocrysts in a groundmass made by pyroxene, leucite and plagioclase. XPL image, 2x (Field of view = 7mm)	Pyroxene phenocrysts in a groundmass made by pyroxene, leucite and plagioclase. XPL image, 2x (Field of view = 7mm)	Pyroxene phenocrysts in a groundmass made by pyroxene, rare phlogopite and leucite and plagioclase. PPL image, 2x (Field of view = 7mm)
Pyroxene phenocrysts in a groundmass made by pyroxene, rare phlogopite and leucite and plagioclase. PPL image, 2x (Field of view = 7mm)	Groundmass made by pyroxene, leucite and plagioclase. XPL image, 10x (Field of view = 2mm)	Groundmass made by pyroxene, leucite and plagioclase. XPL image, 10x (Field of view = 2mm)
Groundmass made by pyroxene, leucite and plagioclase. XPL image, 10x (Field of view = 2mm)	Groundmass made by pyroxene, leucite and plagioclase. XPL image, 10x (Field of view = 2mm)