Lamprophyres (from Ancient Greek λαμπρός (lamprós) 'bright' and φύρω (phúrō) 'to mix') are uncommon, small-volumeultrapotassic igneous rocks primarily occurring asdikes,lopoliths,laccoliths,stocks, and smallintrusions. They arealkalinesilica-undersaturatedmafic orultramafic rocks with highmagnesium oxide, >3%potassium oxide, highsodium oxide, and highnickel andchromium.
Lamprophyres occur throughout all geologiceras.Archaean examples are commonly associated with lodegold deposits.Cenozoic examples include magnesian rocks inMexico andSouth America, and young ultramafic lamprophyres fromGympie inAustralia with 18.5% MgO at ~250 Ma.
Modern science treats lamprophyres as a catch-all term for ultrapotassicmaficigneous rocks which have primarymineralogy consisting ofamphibole orbiotite, and with feldspar in the groundmass.
Lamprophyres are not amenable to classification according to modal proportions, such as the systemQAPF, because of their peculiar mineralogy, nor compositional discrimination diagrams, such asTAS, because of their peculiar geochemistry. They are classified under theIUGS 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[1] has suggested that rocks belonging to the "lamprophyre facies" are characterized by the presence of phenocrysts ofmica and/oramphibole together with lesser clinopyroxene and/ormelilite set in a groundmass which may consist (either singly or in various combinations) ofplagioclase,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 (Tappe et al., 2005).
Rock[2] considered lamprophyres to be part of a "clan" of rocks, with similar mineralogy, textures and genesis. Lamprophyres are similar tolamproites andkimberlites. While modern concepts see orangeites, lamproites and kimberlites as separate, a vast majority of lamprophyres have similar origins to these other rock types (Tappe et al., 2005).
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, the essential components in lamprophyre genesis are:
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.
Lamprophyres are a group ofrocks containingphenocrysts, usually ofbiotite andamphibole (with bright cleavage surfaces), andpyroxene, but not offeldspar. They are thus distinguished from theporphyries and porphyrites in which the feldspar hascrystallized in two generations. They are essentiallydike rocks, occurring as dikes and thinsills, and are also found as marginal facies of plutonic intrusions. They are usually dark in color, owing to the abundance of ferro-magnesiansilicates, of highspecific gravity and liable to decomposition. For these reasons they have been defined as a melanocrate series (rich in the darkminerals); and they are often accompanied by a complementary leucocrate series (rich in the white minerals feldspar andquartz) such asaplites, porphyries andfelsites.[1]
Biotite (usuallyphlogopite) and amphibole (usuallypargasite or other magnesianhornblende) are panidiomorphic; all are euhedral, well formed. Feldspar is restricted to theground mass. In many lamprophyres the pale quartz and felspathic ingredients tend to occur in rounded spots, orocelli, in which there has been progressive crystallization from the margins towards the center. These spots may consist of radiate or brush-like feldspars (with some phlogopite and hornblende) or of quartz and feldspar. A central area of quartz or ofanalcite probably represents an originalmiarolitic cavity infilled at a later period.[1]
The presence or absence of the four dominant minerals, orthoclase, plagioclase, biotite and hornblende, determines the species:[2]
Each variety of lamprophyre may and often does contain all four minerals but is named according to the two which predominate.[1]
These rocks contain alsoiron oxides (usually titaniferous),apatite, sometimessphene,augite, andolivine. The hornblende and biotite are brown or greenish-brown, and as a rule their crystals even when small are very perfect and give thethin section views an easily recognizable character. Green hornblende occurs in some of these rocks. Augite exists as euhedral crystals of pale green color, often zonal and readily weathering. Olivine in the fresh state is rare; it forms rounded, corroded grains; in many cases it is decomposed to green or colorless hornblende in radiating nests (pilite). The plagioclase occurs as small rectangular crystals; orthoclase may have similar shapes or may be fibrous and grouped in sheaf-likeaggregates that are narrow in the middle and spread out towards both ends. As all lamprophyres are prone to alteration byweathering a great abundance of secondary minerals is usually found in them; the principal arecalcite and othercarbonates,limonite,chlorite, quartz andkaolin.[1]
Ocellar structure is common; the ocelli consist mainly of orthoclase and quartz, and may be up to one quarter of an inch in diameter. Another feature of these rocks is the presence of large foreign crystals, orxenocrysts, of feldspar and of quartz. Their forms are rounded, indicating partial resorption and the quartz may be surrounded by corrosion borders of minerals such as augite and hornblende produced where the magma is attacking the crystal.[1]
Lamprophyres (including minette) traditionally have been defined as:[4]
On a purely chemical basis, an extrusive lamprophyre (sp.minette) might be classified as potassictrachybasalt,shoshonite, orlatite using the total alkali-silica diagram (seeTAS 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 (Vosges being the prime example).
Modern nomenclature has been derived from an attempt to constrain some genetic parameters of lamprophyre genesis.[5] 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.
Vogesite was first described from theVosges mountains, France, where rocks of this type (actually, minette) were described in the early 20th century.
A historical view of minette was provided by Johannsen (1937). He wrote that the name was " ... used by the miners in theVosges 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 near Shiprock and Mitten Rock, NM) of theColorado Plateau[6] and in the Mexican Volcanic Belt.[7]
Kersantite is named after the village of Kersanton,Brittany, France, where the rock was first identified. An obsolete name for kersantite is kersanton.[8]
Lamprophyres are usually associated with voluminousgranodiorite intrusive episodes.[9] They occur as marginal facies to some granites, though usually as dikes and sills marginal to and crosscutting the granites and diorites.[10] 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.[1]
Lamprophyres are also known to be spatially and temporally associated withgoldmineralisation, for exampleorogenic gold deposits.[11] Rock (1991) considered lamprophyres to be possible source rocks for the gold,[9] but this view is not generally supported. The more reasonable explanation for the correlation is that lamprophyres, representing "wet" melts of theasthenosphere andmantle, correlate with a period of high fluid flow from the mantle through the crust, during subduction-related metamorphism, which drives gold mineralisation.[12]
Non-melilitic lamprophyres are found in many districts where granites and diorites occur, such as theScottish Highlands andSouthern Uplands of Scotland;[13][14] theLake District of northwest England;Ireland; theVosges Mountains of France; theBlack Forest andHarz mountain regions of Germany;Mascota, Mexico;Jamaica[10] and in certain locations ofBritish Columbia, Canada.[15]
This article incorporates text from a publication now in thepublic domain: Flett, John Smith (1911). "Lamprophyres". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 16 (11th ed.). Cambridge University Press. pp. 135–136.