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Lamprophyre

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Lamprophyre is ultrapotassic igneous rock that is occurring as dikes, lopoliths, loccoliths, stocks and small intrussion. It is alkaline silica-undersaturated mafic or ultramafic rocks with high magnesium oxide, >3% potassium oxide, high sodium oxide and high nickel and chromium. Four minerals dominate these rocks: orthoclase, plagioclase, biotite, and hornblende. Amphibole and biotite tend to occur in a matrix of various combinations of plagioclase and other sodium- and potassium-rich feldspars, pyroxene, and feldspathoids.

In general, they form at great depth and are enriched in sodium, cesium, rubidium, nickel, and chromium, as well as potassium, iron, and magnesium. Some are also source rocks for diamonds. The exact origin of lamprophyres is still debated. They form along the margins of some granites and are often associated with large bodies of intrusive granodiorite.

Name origin: Lamprophyres (Greek λαµπρός (lamprós) = “bright” and φύρω (phýro) = to mix)

Color: Dark brown to black

Group: Extrusive igneous rock

Minerals: Amphibole and biotite tend to occur in a matrix of various combinations of plagioclase and other sodium- and potassium-rich feldspars, pyroxene, and feldspathoids

Dominant Minerals: Orthoclase, plagioclase, biotite, and hornblende

Lamprophyre Classification

Classification of lamprophyres has had several revisions and so much argument within the geology. Modern naming has been derived from an attempt some genetic parameters of lamprphyre genesis.This has, by and large, dispensed with the previous provincial names of lamprophyre species, in favor of a mineralogical name. The old names are still used for convenience.

Streckeisen recognized three main type of lamprophyres:

  • Calc-alkaline lamprophyres
  • Melilitic lamprophyres
  • Alkaline lamprophyres

Calc-Alkaline Lamprophyres

The calc-alkaline lamprophyres are also known as ordinary lamprophyres and they consist of Minettes, Vosegites, Kersantites and Spessartites. This lamprophyres are, in term of average chemical composition, virtually indistinguishable.

Vogesite: Vogesite was first described from the Vosges mountains, France, where rocks of this type (actually, minette) were described in the early 20th century.

Minette: A dike of minette near Shiprock, Navajo Volcanic Field. A historical view of minette was provided by Johannsen (1937). He wrote that the name was ” … used by the miners in the Vosges apparently for oolitic or granular iron ore, and possibly derived from the valley of Minkette.

Spessartites: From Spessart mountains east of Aschaffenburg in Germany. A spessartite is a porphyritic alkaline igneous rock dominated by essential amphibole, usually hornblende, and plagioclase feldspar, often with augite present as an accessory. Plagioclase occurs in the groundmass and potassic feldspar is absent or present in low abundance.

Kersantites: From Kersanton, a village in France, are Plagioclase, Honrblende, Augite lamprophyres.

Alkaline Lamprophyres And Melilitic Lamprophyres

The Alkaline and melilitic lamprophyres will be considered together, because both groups contain alkaline rocks and are usually associated with alkaline complexes and the rocks of the Carbonatite-nepheline-ijolite association. The common alkaline lamprophyres are comptonites, Sannaites and monchiquites and they are chemically akin to the alkaline basalt, basanites and Nephelinites.

Comptonites: From Campton in the New Hampshire (USA). A camptonite is a porphyritic alkaline igneous rock dominated by essential plagioclase and brown amphibole, usually hornblende, often with titanaugite. Plagioclase occurs in the groundmass.

Sannaites: From Sannavand, Fen complex, Sweden. Sannaites are broadly to Comptonites, except that they contain alkali feldspar in place of plagioclase.

Monchiquites: From Sierra de Monchique in Southern Portugal. A monchiquite is a porphyritic alkaline igneous rock dominated by essential olivine, titanaugite and brown hornblende.

Alnöite: From Alno island, Sweden. A alnöite is a porphyritic alkaline igneous rock dominated by essential olivine, biotite and pyroxene, in a groundmass containing melilite. It can contain monticellite.

Polenzite: From Polzen area of the Bohemian massif, Czechoslovakia. Is a melilitic lamprophyre that usually contain between 10-30% of feldspathoids (Nepheline and Hauyne) and it normally contains the same minerals as occur in alnöite.

Lamprophyre Composition

Lamprophres is all term for ultrapotassic mafic igneous rocks which have primary mineralogy consisting of amphibole or biotite, and with feldspar in the groundmass. Four minerals dominate these rocks: orthoclase, plagioclase, biotite, and hornblende. Amphibole and biotite tend to occur in a matrix of various combinations of plagioclase and other sodium- and potassium-rich feldspars, pyroxene, and feldspathoids. Because of their relative rarity and varied composition, lamprophyres do not fit into standard geological classifications. In general, they form at great depth and are enriched in sodium, cesium, rubidium, nickel, and chromium, as well as potassium, iron, and magnesium. Some are also source rocks for diamonds.

Lamprophyre Formation

In all geological periods, lamprophy occurs. Archaic examples are usually associated with gold reserves. Among the Cenozoic examples, the magnesian rocks in Mexico and South America and the young ultramafic lamp lamps from Gympie in Australia are 18.5% MgO at ~ 250 Ma.

Rock lamps thought to be rock are part of a “clan” of rocks with similar mineralogy, textures and formations. Lamprofiller, lamproite and kimberlites. While modern concepts see orange, lampogens and kimberlites separately, the vast majority of the lamprophytes have similar origins to these other rock species.

Mitchell considers the lamprophytes as a “facies” of magmatic rocks created by a number of conditions (usually; late, high volatile differences of other rock species). Both schemes can be applied to all and some of the large rock group known as lamprophyres and melilitic rocks.

Leaving aside the complex petrogenetic arguments, the basic components in the formation of lamprophyre;

  • high depth of melting, which yields more mafic magmas;
  • low degrees of partial melting, which yields magmas rich in the alkalis (particularly potassium);
  • lithophile element (K, Ba, Cs, Rb) enrichment, high Ni and Cr,
  • high potassium and sodium concentrations (silica undersaturation is common)
  • some form of volatile enrichment, to provide the biotite (phlogopite) and amphibole (pargasite) mineralogy
  • lack of fractional crystallisation (generally; there are exceptions)
  • high Mg# ( MgO/(FeO + MgO) )
  • Individual examples thus may have a wide variety of mineralogy and mechanisms for formation. Rock considered lamprophyres to be derived from deep, volatile-driven melting in a subduction zone setting. Others such as Mitchell consider them to be late offshoots of plutons, etc., though this can be difficult to reconcile with their primitive melt chemistry and mineralogy.

Where is the Lamprophyre Rock

Lamprophyres are usually associated with voluminous granodiorite intrusive episodes. They occur as marginal facies to some granites, though usually as dikes and sills marginal to and crosscutting the granites and diorites. In other districts where granites are abundant no rocks of this class are known. It is rare to find only one member of the group present, but minettes, vogesites, kersantites, etc., all appear and there are usually transitional forms.

Non-melilitic lamprophyres are found in many districts where granites and diorites occur, such as the Scottish Highlands and Southern Uplands of Scotland; the Lake District of northwest England; Ireland; the Vosges Mountains of France; the Black Forest and Harz mountain regions of Germany; Mascota, Mexico; Jamaica[8] and in certain locations of British Columbia, Canada.

Economic Importance

The economic importance of ultrapotassic rocks is wide and varied. Kimberlites, lamproites and perhaps even lamprophyres are known to contain diamond. These rocks are all produced at depths in excess of 120 km and thus can bring diamond to the surface as xenocrysts. Ultrapotassic granites are a known hos for much granite-hosted gold mineralisation. Significant porphyry-style mineralisation is won from highly potassic to ultrapotassic granites. Ultrapotassic A-type intracontinental granites may be associated with fluorite and columbite-tantalite mineralization.

Conclusion

  • Lamprophyres are melanocratic, porphyritic, hypabyssal rocks
  • The lamprophyres commonly consists of alkali rich calc-alkali to ultramafic minerals
  • The economic importance of ultrapotassic rocks are wide and varied.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Wikipedia contributors. (2019, March 14). Lamprophyre. In Wikipedia, The Free Encyclopedia. Retrieved 19:30, May 11, 2019, from https://en.wikipedia.org/w/index.php?title=Lamprophyre&oldid=887734669
  • Vale, L. (2019). ALEX STREKEISEN-Sannaite-. Alexstrekeisen.it. Available at: http://www.alexstrekeisen.it/english/vulc/sannaite.php [Accessed 11 May 2019].
Cite this article as: Geology Science. (2019). Lamprophyre. [online] Available at: http://geologyscience.com/rocks/lamprophyre/ [9th December 2019 ]
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