Lamprophyre

Any of a heterogeneous group of gray to black, mafic igneous rocks characterized by a distinctive panidiomorphic and porphyritic texture in which abundant euhedral, dark-colored ferromagnesian (femic) minerals (dark mica, amphibole, pyroxene, olivine) occur in two generations—both early as phenocrysts and later in the matrix or groundmass—while felsic minerals (potassium feldspar, plagioclase, analcime, melilite) are restricted to the groundmass (see illus.). Compared with the common igneous rocks, lamprophyres are chemically peculiar. They have low silicon, moderate aluminum, and variable calcium contents, but are rich in alkalies (potassium or sodium or both), magnesium, iron, and volatile constituents (H2O and CO2), and contain a wide variety of such minor and trace elements as titanium, phosphorus, barium, strontium, rubidium, zirconium, lanthanum, uranium, thorium, chromium, nickel, and cobalt. Many lamprophyres contain ocelli, small spheroidal bodies consisting of alkali-rich feldspar, analcime, or calcite with minor femic minerals.

Ali Fazeli = egeology.blogfa.com 

Specimens of lamprophyre. (a) Minette, found near Walsenburg, Colorado. The altered sanidine matrix also contains other minor minerals. (b) Camptonite, found in Campton Falls, New Hampshire. (After H. Williams and C. M. Gilbert, Petrography: An Introduction to the Study of Rocks in Thin Sections, W. H. Freeman, 1954)

 

 

 

Mineralogy and terminology

 Many varieties of lamprophyre are known; only the more abundant ones are listed in the table. Minettes, kersantites, vogesites, and spessartites are the most common and are sometimes collectively called calcalkaline lamprophyres. Camptonites and monchiquites are less common; alnoites are rare. These varieties, along with some others, are referred to as alkaline lamprophyres. Although lamprophyres are named on the basis of their dominant or diagnostic femic and felsic minerals, their terminology is not straightforward because there is much gradation in mineral assemblages, especially in number and abundance of femic minerals, and felsic minerals are often too altered to permit identification.

All lamprophyres contain essential hydrous minerals, such as dark mica and amphibole, frequently accompanied by significant pyroxene and less olivine. The mica is a magnesium-rich titanium-bearing biotite or phlogopite. Under the microscope the biotite in a thin section of a lamprophyre can be seen as pseudo hexagonal plates that are chemically zoned. The pale magnesium-rich centers of the crystals are surrounded by reddish-brown iron-rich rims; groundmass biotites are chemically similar to phenocryst rims. Amphibole is less common than biotite; it forms prisms or needles of a green or brown hornblende in calcalkaline lamprophyres, and a deep brown kaersutite in alkaline lamprophyres. Pyroxene occurs as stubby monoclinic crystals of diopsidic augite or titanaugite, frequently zoned. Orthorhombic pyroxene is absent. Magnesium-rich olivine is usually the least abundant femic mineral; unlike the biotite, amphibole, and pyroxene, it generally occurs only as phenocrysts and frequently is altered and pseudo­morphed by calcite and other secondary minerals such as chlorite and members of the montmorillonite group.

Although in hand specimen the groundmass appears dark, a large proportion of it is composed of colorless plates or laths of potassium feldspar (sanidine or orthoclase) or plagioclase feldspar (albite-andesine in calcalkaline lamprophyres, andesine-labradorite in alkaline). In the rare feldspar-free varieties, monchiquite and alnoite, the groundmass consists predominantly of an alkali-rich commonly altered glass ± analcime, or melilite and a calcium carbonate. Apatite, a magnetitelike spinel, and calcite (primary or secondary or both) are ubiquitous accessory minerals in all lamprophyres; other minerals that may occur include sphene, quartz, analcime, zeolites, nepheline, haüyne, monticellite, pyrite, perovskite, and ilmenite (many of these may be secondary).  

Occurrence

Lamprophyres are widespread but volumetrically minor rocks that apparently are restricted to the continents and are the last manifestation of igneous activity in a given area. They usually occur as subparallel or radial swarms of thin (∼1.6–160 ft or 0.5–50 m) dikes or, less commonly, sills, volcanic neck fillings, or diatremes, or, rarely, lava flows (lamprophyric lavas are sometimes called lamproites). A swarm may consist of one or more varieties of lamprophyre, but calcalkaline and alkaline lamprophyres usually do not occur together. Calcalkaline varieties frequently are found in the region of large granodiorite or granite bodies, while alkaline lamprophyres tend to be associated with alkaline basalts, nepheline syenites, carbonatites, or kimberlites; many exceptions to this generalization are known.

Alkaline lamprophyres and the rare minette may bear xenocrysts and xenoliths, foreign crystals and rock fragments representing the material that composes the Earth's mantle under the continents at depths greater than approximately 47 mi (75 km). Xenoliths of the continental crust can be found in all lamprophyres. See also: Phenocryst; Xenolith

 Origin

 Like all igneous rocks, a lamprophyre is the product of the crystallization of magma (molten rock). The ultimate origin of lamprophyre magmas is the upper portion of the Earth's subcontinental mantle where localized heating causes partial melting (anatexis) of mantle rock. The exact composition of the liquid or magma thus formed, which differs from that of the original unmelted mantle rock (source rock), depends on several factors, including the temperature and pressure at which anatexis occurs, the composition of the source rocks, and the degree of partial melting. Since magma is less dense than solid rock, it rises upward, invades the overlying continental crust, and eventually solidifies. During its ascent from the mantle to its site of final emplacement, a magma's composition may be changed by several processes, two of the most important being assimilation of foreign material, especially portions of the rocks making up the continental crust; and partial (fractional) crystallization of the magma, resulting in removal of early-formed crystals of femic minerals because of density differences between these crystals and the magma from which they were formed.

The controversy surrounding the origin of lamprophyres centers on the chemical composition of the mantle-derived magma and the extent to which it has been modified prior to its final solidification. Some geologists maintain that a normal basaltic magma (the most common product of anatexis of the mantle) is made lamprophyric by a combination of the magma composition-changing processes. But continuing studies of the major- and trace-element chemistry of lamprophyres is indicating increasingly that many minettes and alkaline lamprophyres result from the crystallization of nearly unmodified mantle-derived magmas. This implies that magmas of lamprophyric composition can be generated within the upper mantle. Although lamprophyric magmas are believed to form by small degrees of partial melting of mantle rock of unusual chemical and mineralogical composition, details of the origin of these magmas are unclear. By the operation of processes such as assimilation of foreign material and partial magma crystallization, lamprophyric magmas may themselves change or evolve during their ascent, and this probably accounts for the gradations between the varieties. The origin of kersantites, vogesites, and spessartites is still a matter of debate; they may represent modified minettelike or other lamprophyric magma. See also.

  • Ali Fazeli = egeology.blogfa.com
  • Ali Fazeli = egeology.blogfa.com
  • Ali Fazeli = egeology.blogfa.com