Lamprophyres (Greek λαµπρός (lamprós) = "bright" and φύρω
(phýro) = to mix) are uncommon, small volume ultrapotassic igneous
rocks primarily occurring as dikes, lopoliths, laccoliths, stocks and
small intrusions. They are alkaline silica-undersaturated mafic or
ultramafic rocks with high magnesium oxide, >3% potassium oxide,
high sodium oxide and high nickel and chromium.
Lamprophyres occur throughout all geologic eras. Archaean examples are
commonly associated with lode gold deposits.
1 Petrology 2 Genesis 3 Petrography 4 Nomenclature
4.1 Vogesite 4.2 Minette
5 Distribution 6 References 7 External links
Petrology Modern science treats lamprophyres as a catch-all term for ultrapotassic mafic igneous rocks which have primary mineralogy consisting of amphibole or biotite, and with feldspar in the groundmass. Lamprophyres are not amenable to classification according to modal proportions, such as the system QAPF due to peculiar mineralogy, nor compositional discrimination diagrams, such as TAS because of their peculiar geochemistry. They are classified under the IUGS Nomenclature for Igneous Rocks (Le Maitre et al., 1989) separately; this is primarily because they are rare, have peculiar mineralogy and do not fit classical classification schemes. For example, the TAS scheme is inappropriate due to the control of mineralogy by potassium, not by calcium or sodium. Mitchell has suggested that rocks belonging to the "lamprophyre facies" are characterized by the presence of phenocrysts of mica and/or amphibole together with lesser clinopyroxene and/or melilite set in a groundmass which may consist (either singly or in various combinations) of plagioclase, alkali feldspar, feldspathoids, carbonate, monticellite, melilite, mica, amphibole, pyroxene, perovskite, Fe-Ti oxides and glass. Classification schemes which include genetic information, may be required to properly describe lamprophyres. Genesis Rock considered lamprophyres are part of a "clan" of rocks, with similar mineralogy, textures and genesis. Lamprophyres are similar to lamproites and kimberlites. While modern concepts see orangeites, lamproites and kimberlites as separate, a vast majority of lamprophyres have similar origins to these other rock types. Mitchell considered the lamprophyres as a "facies" of igneous rocks created by a set of conditions (generally; late, highly volatile differentiates of other rock types). Either scheme may apply to some, but not all, occurrences and variations of the broader group of rocks known as lamprophyres and melilitic rocks. Leaving aside complex petrogenetic arguments, it is fair to say that the essential components in lamprophyre genesis are;
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. Petrography
Microscope view (long dimension 2 mm) of a thin section of minette from the Colorado Plateau. Magnesium-rich biotite (P, phlogopite) and clinopyroxene (C) phenocrysts in a groundmass of alkali feldspar, pyroxene, and iron-titanium oxides.
Lamprophyres are a group of rocks containing phenocrysts, usually of
biotite and amphibole (with bright cleavage surfaces), and pyroxene,
but not of feldspar. They are thus distinguished from the porphyries
and porphyrites in which the feldspar has crystallized in two
generations. They are essentially dike rocks, occurring as dikes and
thin sills, and are also found as marginal facies of plutonic
intrusions. They are usually dark in color, owing to the abundance of
ferro-magnesian silicates, of high specific gravity and liable to
decomposition. For these reasons they have been defined as a
melanocrate series (rich in the dark minerals); and they are often
accompanied by a complementary leucocrate series (rich in the white
minerals feldspar and quartz) such as aplites, porphyries and
Minette contains biotite and orthoclase. Kersantite contains biotite and plagioclase. Vogesite contains hornblende and orthoclase. Spessartite contains hornblende and plagioclase. Each variety of lamprophyre may and often does contain all four minerals but is named according to the two which predominate.
These rocks contain also iron oxides (usually titaniferous), apatite,
sometimes sphene, augite, and olivine. The hornblende and biotite are
brown or greenish-brown, and as a rule their crystals even when small
are very perfect and give the thin section views an easily
recognizable character. Green hornblende occurs in some of these
normally occurring as porphyritic dikes containing matrix restricted feldspars and/or feldspathoids if present biotite or phlogopite is an essential mineral phase commonly extensively hydrothermally altered may contain primary calcite, zeolites and other more typically hydrothermal minerals higher than normal contents of K2O and/or Na2O, H2O, CO2, S, P2O5, and Ba
On a purely chemical basis, an extrusive lamprophyre (sp. minette)
might be classified as potassic trachybasalt, shoshonite, or latite
using the total alkali-silica diagram (see TAS classification), or as
absarokite, shoshonite, or banakite using a classification sometimes
applied to potassium-rich lavas. Such chemical classifications ignore
the distinctive textures and mineralogies of lamprophyres.
The naming and classification of lamprophyres has had several
revisions, and much argument within the geological community. Nicholas
Rock and colleagues devoted much time to a complicated descriptive
system of nomenclature which took after a series of nomenclature based
on regional examples of the very diverse mineralogical expression of
lamprophyres. This system was based on a somewhat provincial, rustic
system of naming after French villages nearby were found the first
described examples of various species of lamprophyre (
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, where it occurs…."
Examples include minettes in the Navajo Volcanic Field (e.g. dikes
^ a b c d e f Flett 1911, p. 135.
^ Johannsen, A., 1937, A Descriptive Petrography of the Igneous Rocks:
Volume III, The Intermediate Rocks. University of Chicago Press,
^ Le Maitre, R. W., editor, 2002, Igneous Rocks: A Classification and
Glossary of Terms. Recommendations of the International Union of
Geological Sciences, Subcommission of the Systematics of Igneous
Rocks. Cambridge University Press.
^ Mitchell, R.H., 1994b. Suggestions for revisions to the terminology
of kimberlites and lamprophyres from a genetic viewpoint. In Proc.
This article incorporates text from a publication now in the public domain: Flett, John Smith (1911). "Lamprophyres". In Chisholm, Hugh. Encyclopædia Britannica. 16 (11th ed.). Cambridge University Press. pp. 135–136.
IUGS addendum Igneous geology classification flowchart List of alkaline rocks in