Lamprophyre
Encyclopedia
Lamprophyres are uncommon, small volume ultrapotassic igneous rocks primarily occurring as dikes
, lopolith
s, laccolith
s, stocks and small intrusion
s. They are alkaline silica-undersaturated, 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. Cenozoic examples include magnesian rocks in Mexico
and South America
, and young ultramafic lamprophyres from Gympie
in Australia
with 18.5% MgO at ~250 Ma.
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
, feldspathoid
s, 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.
s and kimberlite
s. 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;
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.
s containing phenocryst
s, 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 crystal
lized in two generations. They are essentially dike
rocks, occurring as dikes and thin sill
s, 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 mineral
s); and they are often accompanied by a complementary leucocrate series (rich in the white minerals feldspar and quartz
) such as aplite
s, porphyries and felsite
s.
Biotite
, usually phlogopite
and amphibole
, usually pargasite
or other magnesian hornblende
are panidiomorphic; all are euhedral, well formed. Feldspar is restricted to the ground mass
. In many lamprophyres the pale quartz and felspathic ingredients tend to occur in rounded spots, or ocelli, 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 of analcite
probably represents an original miarolitic cavity
infilled at a later period.
The presence or absence of the four dominant minerals, orthoclase, plagioclase, biotite and hornblende, determines the species.
These rocks contain also iron oxide
s (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 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-like aggregates that are narrow in the middle and spread out towards both ends. As all lamprophyres are prone to alteration by weathering
a great abundance of secondary minerals is usually found in them; the principal are calcite
and other carbonate
s, limonite
, chlorite
, quartz and kaolin.
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, or xenocrysts, 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.
Lamprophyres (including minette) traditionally have been defined as:
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.
Modern nomenclature has been derived from an attempt to constrain some genetic parameters of lamprophyre 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's sake.
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 the Colorado Plateau
and in the Mexican Volcanic Belt.
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.
Lamprophyres are also known to be spatially and temporally associated with gold mineralisation. Rock (1991) considered them possible source rocks, but this view is not generally supported. The more reasonable explanation for the correlation is that lamprophyres, representing "wet" melts of the asthenosphere
and mantle
, correlate with a period of high fluid flow from the mantle through the crust, during subduction-related metamorphism, which drives gold mineralisation.
Non-melilitic lamprophyres are found in many districts where granites and diorites occur, such as the Scottish Highlands
and Southern Uplands, the Lake district, Ireland
; the Vosges
; Black Forest
; Harz
; Mascota, Mexico; Jamaica
and in certain locations of British Columbia
, Canada.
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
, lopolith
Lopolith
A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped feeder bodies below the body...
s, laccolith
Laccolith
A laccolith is a sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.Laccoliths tend to form at relatively...
s, stocks and small intrusion
Intrusion
An intrusion is liquid rock that forms under Earth's surface. Magma from under the surface is slowly pushed up from deep within the earth into any cracks or spaces it can find, sometimes pushing existing country rock out of the way, a process that can take millions of years. As the rock slowly...
s. They are alkaline silica-undersaturated, ultramafic rocks with high magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
oxide, >3% potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
oxide, high sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
oxide and high nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
and chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
.
Lamprophyres occur throughout all geologic eras. Archaean examples are commonly associated with lode gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
deposits. Cenozoic examples include magnesian rocks in Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
and South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
, and young ultramafic lamprophyres from Gympie
Gympie
Gympie may refer to:* Gympie, a city in Queensland, Australia** Gympie Airport** Electoral district of Gympie** Gympie Region, its local government authority* Gympie Gympie , a stinging plant...
in Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
with 18.5% MgO at ~250 Ma.
Petrology
Modern science treats lamprophyres as a grab-bag term for ultrapotassic maficMafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
igneous rocks which have primary mineralogy
Mineralogy
Mineralogy is the study of chemistry, crystal structure, and physical properties of minerals. Specific studies within mineralogy include the processes of mineral origin and formation, classification of minerals, their geographical distribution, as well as their utilization.-History:Early writing...
consisting of amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
or biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, and with feldspar in the groundmass.
Lamprophyres are not amenable to classification according to modal proportions, such as the system QAPF
QAPF diagram
A QAPF diagram is a double triangle diagram which is used to classify igneous rocks based on mineralogic composition. The acronym, QAPF, stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid ". These are the mineral groups used for classification in QAPF diagram...
due to peculiar mineralogy, nor compositional discrimination diagrams, such as TAS because of their peculiar geochemistry. They are classified under the IUGS
International Union of Geological Sciences
The International Union of Geological Sciences is an international non-governmental organization devoted to international cooperation in the field of geology.-About:...
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
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
and/or amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
together with lesser clinopyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
and/or melilite
Melilite
Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is 2[SiO7]. Discovered in 1793 near Rome, it has a yellowish, greenish brown color...
set in a groundmass which may consist (either singly or in various combinations) of plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
, alkali feldspar
Alkali feldspar
The alkali feldspar group are those feldspar minerals rich in the alkali elements like potassium. The alkali feldspars include: anorthoclase, microcline, orthoclase and sanidine....
, feldspathoid
Feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....
s, carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
, monticellite
Monticellite
Monticellite and kirschsteinite are gray silicate minerals of the olivine group with compositions CaMgSiO4 and CaFeSiO4, respectively. Most monticellites have the pure magnesium end-member composition but rare ferroan monticellites and magnesio-kirschsteinite are found with between 30 and 75 mol.%...
, melilite, mica, amphibole, pyroxene, perovskite
Perovskite
A perovskite structure is any material with the same type of crystal structure as calcium titanium oxide , known as the perovskite structure, or XIIA2+VIB4+X2−3 with the oxygen in the face centers. Perovskites take their name from this compound, which was first discovered in the Ural mountains of...
, 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 lamproiteLamproite
Lamproites are ultrapotassic mantle-derived volcanic and subvolcanic rocks. They have low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively high MgO content and extreme enrichment in incompatible elements....
s and kimberlite
Kimberlite
Kimberlite is a type of potassic volcanic rock best known for sometimes containing diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond in 1871 spawned a diamond rush, eventually creating the Big Hole....
s. 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
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
" 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 (phlogopitePhlogopitePhlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3O102. Iron substitutes for magnesium in variable amounts...
) and amphibole (pargasitePargasitePargasite is a complex inosilicate mineral of the amphibole group with formula NaCa24AlO222.It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality....
) 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
Lamprophyres are a group of rockRock (geology)
In geology, rock or stone is a naturally occurring solid aggregate of minerals and/or mineraloids.The Earth's outer solid layer, the lithosphere, is made of rock. In general rocks are of three types, namely, igneous, sedimentary, and metamorphic...
s containing phenocryst
Phenocryst
thumb|right|300px|[[Granite]]s often have large [[feldspar|feldspatic]] phenocrysts. This granite, from the [[Switzerland|Swiss]] side of the [[Mont Blanc]] massif, has large white [[plagioclase]] phenocrysts, [[triclinic]] [[mineral]]s that give [[trapezium|trapezoid]] shapes when cut through)...
s, usually of biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
and amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
(with bright cleavage surfaces), and pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, but not of feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. They are thus distinguished from the porphyries
Porphyry (geology)
Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are called phenocrysts...
and porphyrites in which the feldspar has crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
lized in two generations. They are essentially dike
Dike (geology)
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
rocks, occurring as dikes and thin sill
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
s, and are also found as marginal facies of plutonic intrusions.
They are usually dark in color, owing to the abundance of ferro-magnesian silicates
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
, of high specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
and liable to decomposition. For these reasons they have been defined as a melanocrate series (rich in the dark mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s); and they are often accompanied by a complementary leucocrate series (rich in the white minerals feldspar and quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
) such as aplite
Aplite
Aplite in petrology, the name given to intrusive rock in which quartz and feldspar are the dominant minerals. Aplites are usually very fine-grained, white, grey or pinkish, and their constituents are visible only with the help of a magnifying lens...
s, porphyries and felsite
Felsite
Felsite is a very fine grained volcanic rock that may or may not contain larger crystals. Felsite is a field term for a light colored rock that typically requires petrographic examination or chemical analysis for more precise definition...
s.
Biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, usually phlogopite
Phlogopite
Phlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3O102. Iron substitutes for magnesium in variable amounts...
and amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
, usually pargasite
Pargasite
Pargasite is a complex inosilicate mineral of the amphibole group with formula NaCa24AlO222.It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality....
or other magnesian hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
are panidiomorphic; all are euhedral, well formed. Feldspar is restricted to the ground mass
Matrix (geology)
The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
. In many lamprophyres the pale quartz and felspathic ingredients tend to occur in rounded spots, or ocelli, 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 of analcite
Analcite
Analcime or analcite is a white, grey, or colourless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is NaAlSi2O6·H2O. Minor amounts of potassium and calcium substitute for sodium...
probably represents an original miarolitic cavity
Miarolitic cavities
Miarolitic cavities are typically crystal lined irregular cavities or vugs most commonly found in granitic pegmatites, but also found in a variety of igneous rocks. The central portions of pegmatites are often miarolitic as the pegmatite dike crystallizes from the outside walls toward the center...
infilled at a later period.
The presence or absence of the four dominant minerals, orthoclase, plagioclase, biotite and hornblende, determines the species.
- 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 oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
s (usually titaniferous), apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
, sometimes sphene, augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
, and olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
. 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
Thin section
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
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-like aggregates that are narrow in the middle and spread out towards both ends. As all lamprophyres are prone to alteration by weathering
Weathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
a great abundance of secondary minerals is usually found in them; the principal are calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
and other carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
s, limonite
Limonite
Limonite is an ore consisting in a mixture of hydrated iron oxide-hydroxide of varying composition. The generic formula is frequently written as FeO·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.Together with hematite, it has...
, chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....
, quartz and kaolin.
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, or xenocrysts, 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.
Lamprophyres (including minette) traditionally have been defined as:
- normally occurring as porphyritic dikesDike (geology)A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts discordantly across* planar wall rock structures, such as bedding or foliation...
- containing matrixMatrix (geology)The matrix or groundmass of rock is the finer grained mass of material in which larger grains, crystals or clasts are embedded.The matrix of an igneous rock consists of finer grained, often microscopic, crystals in which larger crystals are embedded. This porphyritic texture is indicative of...
restricted feldsparFeldsparFeldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
s and/or feldspathoidFeldspathoidThe feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....
s if present - biotite or phlogopitePhlogopitePhlogopite is a yellow, greenish, or reddish-brown member of the mica family of phyllosilicates. It is also known as magnesium mica.Phlogopite is the magnesium endmember of the biotite solid solution series, with the chemical formula KMg3AlSi3O102. Iron substitutes for magnesium in variable amounts...
is an essential mineralMineralA mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
phase - commonly extensively hydrothermally altered
- may contain primary calciteCalciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
, zeoliteZeoliteZeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
s 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
Trachybasalt
Trachybasalt is a volcanic rock with a composition between trachyte and basalt. Minerals included in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and likely very small amounts of leucite or analcime. It contains high alkali content ....
, shoshonite
Shoshonite
Shoshonite is a basaltic rock, properly a potassic trachyandesite, composed of olivine, augite and plagioclase phenocrysts in a groundmass with calcic plagioclase and sanidine and some dark-colored volcanic glass. Shoshonite gives its name to the shoshonite series and grades into absarokite with...
, or latite
Latite
Latite is an igneous, volcanic rock, with aphanitic-aphyric to aphyric-porphyritic texture. Its mineral assemblage is usually alkali feldspar and plagioclase in approximately equal amounts. Quartz is less than five percent and is absent in a feldspathoid-bearing latite, and olivine is absent in a...
using the total alkali-silica diagram (see TAS classification
TAS classification
The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali content and the silica content. These chemical parameters are useful, because the relative proportions of alkalis and silica play an important role in...
), 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.
Nomenclature
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. 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's sake.
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 historical view of minette was provided by Johannsen (1937). He wrote that the name was " … used by the miners in the VosgesVosges
Vosges is a French department, named after the local mountain range. It contains the hometown of Joan of Arc, Domrémy.-History:The Vosges department is one of the original 83 departments of France, created on February 9, 1790 during the French Revolution. It was made of territories that had been...
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 the Colorado Plateau
Colorado Plateau
The Colorado Plateau, also called the Colorado Plateau Province, is a physiographic region of the Intermontane Plateaus, roughly centered on the Four Corners region of the southwestern United States. The province covers an area of 337,000 km2 within western Colorado, northwestern New Mexico,...
and in the Mexican Volcanic Belt.
Distribution
Lamprophyres are usually associated with voluminous granodioriteGranodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
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.
Lamprophyres are also known to be spatially and temporally associated with gold mineralisation. Rock (1991) considered them possible source rocks, but this view is not generally supported. The more reasonable explanation for the correlation is that lamprophyres, representing "wet" melts of the asthenosphere
Asthenosphere
The asthenosphere is the highly viscous, mechanically weak and ductilely-deforming region of the upper mantle of the Earth...
and mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
, correlate with a period of high fluid flow from the mantle through the crust, during subduction-related metamorphism, which drives gold mineralisation.
Non-melilitic lamprophyres are found in many districts where granites and diorites occur, such as the Scottish Highlands
Scottish Highlands
The Highlands is an historic region of Scotland. The area is sometimes referred to as the "Scottish Highlands". It was culturally distinguishable from the Lowlands from the later Middle Ages into the modern period, when Lowland Scots replaced Scottish Gaelic throughout most of the Lowlands...
and Southern Uplands, the Lake district, Ireland
Ireland
Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...
; the Vosges
Vosges
Vosges is a French department, named after the local mountain range. It contains the hometown of Joan of Arc, Domrémy.-History:The Vosges department is one of the original 83 departments of France, created on February 9, 1790 during the French Revolution. It was made of territories that had been...
; Black Forest
Black Forest
The Black Forest is a wooded mountain range in Baden-Württemberg, southwestern Germany. It is bordered by the Rhine valley to the west and south. The highest peak is the Feldberg with an elevation of 1,493 metres ....
; Harz
Harz
The Harz is the highest mountain range in northern Germany and its rugged terrain extends across parts of Lower Saxony, Saxony-Anhalt and Thuringia. The name Harz derives from the Middle High German word Hardt or Hart , latinized as Hercynia. The legendary Brocken is the highest summit in the Harz...
; Mascota, Mexico; Jamaica
Jamaica
Jamaica is an island nation of the Greater Antilles, in length, up to in width and 10,990 square kilometres in area. It is situated in the Caribbean Sea, about south of Cuba, and west of Hispaniola, the island harbouring the nation-states Haiti and the Dominican Republic...
and in certain locations of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, Canada.