Lamprophyres
Despite over 100 years of study lamprophyres remain to most petrologists one of the most obscure and least understood rock groups. Commonly, lamprophyres are treated, if at all, as an afterthought in most petrology texts by authors who merely reiterate old shibboleths and otiose terminology. Descriptions of petrological provinces or alkaline complexes tend to ignore or downplay any lamprophyric components and/or dismiss them as unimportant.The lamprophyres are a complex group of rocks that have mineralogical similarities to some kimberlites and lamproites. Lamprophyres are difficult to classify unambiguously using existing criteria. They are not amenable to classification according to modal proportions, such as the system QAPF, nor compositional discrimination diagrams, such as TAS. It seems unlikely that a simple taxonomic system will be found unless appropriate genetic criteria are applied, that is, unless the classification takes into account the genesis of the rocks. The term "lamprophyre", from "lampros" and "porphyros" (glistening porphyry), was introduced by von Gumbel in 1874 for a group of dark rocks that form minor intrusions, contain phenocrystal brown mica and hornblende, but lack feldspar phenocrysts. Following its introduction, the term was broadened to encompass a wide variety of hypabyssal rocks containing ferromagnesian phenocrysts (Rosenbusch, 1897). As more rocks that were difficult to classify were found in the late 19th and early 20th centuries, these were added to the lamprophyre group. Eventually the group became a repository for any difficult to characterize mafic phenocryst-rich rock. Unfortunately, the practice of type locality nomenclature led to the introduction of a "legion of obscure rock types named after equally obscure European villages" (Rock, 1990, p. 1). This archaic, commonly imprecise, nomenclature has been a particular hinderance in lamprophyre petrogenetic studies as rocks of diverse parentage became grouped under one petrographic banner. This grouping has been interpreted to imply genetic relationships where none actually exist.
During the past decade interest in lamprophyres has been rekindled because of their importance as windows into the deep mantle and supposed association with diamonds and gold deposits. Rock (1986, 1990) has even linked lamprophyres, kimberlites and lamproites into a super-group of rocks termed the "lamprophyre clan". Rock's (1990) use of the term "clan". He used it to refer to a group of rocks that superficially look the same, are commonly associated in the field and have a number of petrological characteristics in common, e.g. richness in volatiles, porphyritic texture, occurrence as minor intrusions. A lamprophyric character commonly implies the presence of mica, amphibole or pyroxene phenocrysts set in glassy or felsic matrix. Lamprophyres are a diverse group of polygenetic rocks united by the common trait of crystallization under volatile-rich conditions. Because of the diverse mineral assemblages which may crystallize from these magmas, it is not possible to devise a succinct and short definition of the term. An extended definition that highlights the principal characteristics of the group has been proposed by Mitchell (1992). This states that:
"Lamprophyres are rocks which are characterized by the presence of euhedral- to-subhedral phenocrysts of mica and/or amphibole together with lesser clino- pyroxene and/or melilite set in a groundmass which may consist (either singly or in various combinations) of plagioclase, alkali feldspar, feldspathoids, carbonate, mon- ticellite, melilite, mica, amphibole, pyroxene perovskite, Fe-Ti oxides and glass".
The great variety of lamprophyric rocks can be simplified into four well-defined subgroups:
• Calc-alkaline (shoshonitic) lamprophyres.
• Leucite lamprophyres.
• Alkaline lamprophyres.
• Ultramafic lamprophyres.
Calc-alkaline (shoshonitic) lamprophyres
These are near-saturated, mildly potassic (Na < K) lamprophyres of intermediate SiO2 content (ca. 53%), which many accompany either post-orogenic granites or mildly potassic (shoshonitic) alkaline rocks. Their chemistry, mineralogy, field relations, xenolith content and possibly Sr isotopic ratio have been taken to suggest they are typically hybrids between basic magma and granitic residua or crustal sediments. The calc-alkaline lamprophyres are also known as ordinary lamprophyres and they consist of minettes, vosegites, kersantites and spessartites. All four varieties are chemically similar on average, although exact inter- relationships are unclear.Vosegites: From Vosges in Northern France. A vogesite is a porphyritic alkaline igneous rock dominated by essential amphibole, usually hornblende, and potassic feldspar, often with augite and plagioclase present as accessories within the groundmass.
Minettes: From an old term used by miners in the Vosges. A minette is a porphyritic alkaline igneous rock dominated by essential biotite and potassic feldspar, often with augite and plagioclase present as accessories within the groundmass.
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, hornblende, augite lamprophyres.
Leucite lamprophyres
These are extremely rare, ultra-potassic, glassy extrusives which have often been described as "lamprophyric" because of their porphyritic character and large biotite or magnophorite phenocrysts (Sahama, 1974). Some are near-saturated, and not dissimilar to minettes, falling within the broad category of potassic alkaline rocks. Their true affinities how- ever are uncertain and their petrogenesis is probably quite distinct from the other lamprophyre groups.Cascadite: From Cascade Creek, Highwood Mts, Montana, USA. Cascadite is a sodic melanocratic lamprophyre composed of phenocrysts of biotite, olivine, and augite in a matrix essentially of alkali feldspar with patches of what could have been leucite.
Fitzroydite: From Fitzroy River Basin, Australia. Fitzroydite is leucite-phlogopite lamprophyre composed essentially of leucite and phlogopite in a chlorite-rich groundmass.
Orendite: From Orenda Butte, Leucite Hills, Wyoming, USA. Orendite is leucite lamprophyre essentially composed of leucite and alkali feldspar with subordinate clinopyroxene, mica and amphibole.
Jumillite: From Jumilla, Murcia, Spain. Jumilite is leucite lamprophyre composed essentially of leucite and diopside, and with subordinate olivine, alkali feldspar and phlogopite.
Alkaline lamprophyres
These lamprophyres are effectively hydrous basanites, all associate with alkali basalts or with nepheline syenite/gabbro plutons; monchiquites and sannaites also occur in carbonatite complexes. The alkaline lamprophyres are generally interpreted as being derived from hydrous basanitic, or tephritic magmas that evolved in areas of crustal doming, and they may be emplaced during the early stages of continental rifting. They include: camptonite, monchiquite, sannaite, fourchite and other rare rocks.Camptonites: 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 titan-augite. Plagioclase occurs in the groundmass.
Monchiquites: From Sierra de Monchique in Southern Portugal. A monchiquite is a porphyritic alkaline igneous rock dominated by essential olivine, titan-augite and brown hornblende.
Sannaites: From Sannavand, Fen complex, Sweden. Sannaites are broadly to camptonites, except that they contain alkali feldspar in place of plagioclase.
Fourchite: From Fourche Mts, Arkansas, USA. Fourchite is a melanocratic analcime lamprophyre containing abundant augite but devoid of olivine and feldspar.
Other, very rare, members of this subgroup include:
Mondhaldeite: From Mondhalde, Kaiserstuhl, Baden, Germany. A variety of sannaite composed of equal proportions of plagioclase and alkali feldspar, with subordinate clinopyroxene, amphibole and leucite, in a glassy matrix.
Espichellite: From Cape Espichel, Lisbon, Portugal. A variety of camptonite containing analcime.
Eustratite: From Haghios Eustratios, (now Ayios Evstra´tios Island), Greece. A glassy sannaite with rare phenocrysts of olivine, corroded hornblende and occasional augite in a groundmass of augite, titanomagnetite, feldspar and glass.
Heptorite: An obscure local name from the Greek hepta = seven. A haüyne-monchiquite consisting of phenocrysts of titanian augite, barkevikite, olivine, and haüyne in a groundmass of glass and labradorite laths.
Heumite: From Heum, Oslo Province, Norway. A sodalite-sannaite composed of biotite, hornblende, sanidine and foids.
Giumarrite: From Giumarra, Sicily, Italy. A variety of hornblende monchiquite.
Ultramafic lamprophyres
These are biotitic, ultramafic and ultrabasic (SiO2 < 35%) lamprophyres associated almost exclusively with carbonatites. Melilite, perovskite and calcite are characteristic phases. Alnoites are closely related to kimberlites, which might be included within the subgroup. Aillikites are compositionally closer to carbonatites than alnoites. Ultramafic lamprophyres form localized dyke-swarms or diatreme-clusters, mainly related to continental rifting, and may represent parent magmas for coeval carbonatite complexes. Their additional occurrence in an oceanic setting, their mantle xenolith content, and their high mg, Cr and Ni, together suggest that many of them are primary, mantle-derived magmas, generated at depths between those of melilitites and kimberlites (c. 100-150 km), but at higher CO2 pressures than melilitites. Other ultramafic lamprophyres, however, have been extensively modified from primary compositions by fractionation, accumulation, or interaction with alkali + volatile-rich fluid. They include: aillikite, alnoite, bergalite, damkjernite, ouachitite, polzenite.Aillikite: From Aillik Bay, Canada. An ultramafic carbonate-rich lamprophyre consisting of various phenocrysts including olivine, diopsidic pyroxene, amphiboles and phlogopite in a matrix of similar minerals with at least partly primary carbonate and minor perovskite but no melilite.
Alnöite: From Alnö island, Sweden. An ultramafic lamprophyre with phenocrysts of phlogopite-biotite, olivine and augite in a groundmass of melilite (often altered to calcite), augite and/or biotite with minor perovskite, garnet and calcite.
Bergalite: An obscure local name coined by Soellner (1913) from Oberbergen, Kaiserstuhl, Baden, Germany. An ultramafic lamprophyre similar to alnöite except that the matrix contains essential feldspathoids as well as melilite.
Damkjernite: From Damtjern, Fen area, Norway. A melanocratic variety of nepheline lamprophyre containing phenocrysts of biotite and titanian augite in a groundmass of the same minerals with some nepheline, calcite, and orthoclase.
Ouachitite: From Ouachitas Mtns., Arkansas, USA. A rare variety of ultramafic lamprophyre consisting of combinations of forsteritic olivine, diopsidic pyroxenes, various amphiboles and phlogopite in a matrix of similar minerals with abundant primary carbonates and feldspathoids, often with minor perovskite, but little or no melilite.
Polenzite: From Polzen area of the Bohemian massif, Czechoslovakia. A melilitic lamprophyre that usually contain between 10-30% of feldspathoids (nepheline and haüyne).
Lamprophyres, like other alkaline rocks, are most abundant in continental rifts, failed arms of triple junctions and on some oceanic islands. However, they also occur widely in orogenic zones and their peripheries (Himalayas; Alps; Pyrenees; Caledonides; Hercynides), in island arcs (Japan; Solomons), in passive to destructive continental margins, in anomalous uplifted fragments of oceanic crust and near major transcurrent faults. Collectively, therefore, lamprophyres are associated not only with intra-plate magmatism, but also with divergent, convergent and even passive plate-margin magmatism. Owing to their abundant phenocrysts, lamprophyres were assumed by Bowen (1928) never to correspond to fully liquid magmas. Although lamprophyre phenocrysts need not in fact represent liquidus phases but merely the crystals which grew fastest in such a volatile-rich medium, evidence such as a lack of holohyaline lamprophyres can be used to support Bowen's conclusion (Rock 1984).
Lamprophyres might then be attributed to crystal-laden, volatile-charged fluids with only minimal silicate-melt component. These might be generated by gas-phase metasomatism rather than by actual fusion of mantle material and emplaced by fluidization rather than by normal intrusion (Bailey 1984, 1987). However, the lamprophyre combination of groundmass + phenocrysts + abundant micas, carbonates etc. could also be taken as a “frozen” sample representing melt + suspended crystals + volatile phase, in fact a “complete” magma system. Lamprophyres would then be closer to intratelluric magma compositions than aphanitic mafic extrusives, the more fashionable yardstick of parental magmas (Hughes 1982), because they alone retain some representative of the volatile phase. Thus, if it is assumed a priori that primary magmas are fully liquid at the time of generation, natural lamprophyres cannot represent primary magmas. If intratelluric magmas are allowed to be crystal laden and volatile rich, then lamprophyres could uniquely approximate such magmas and many glassy lavas would have to be regarded as fractionated.
Two so far largely unaddressed questions are:
1. Are lamprophyre mineral assemblages primary? Luhr & Carmichael (1981) and Allan & Carmachael (1984) have presented evidence that recent lamprophyres may be erupted as relatively volatile-poor mag- mas, and that their volatile-rich mineralogy is subsequently acquired by interaction with an aqueous (deuteric, connate) phase during cooling. The bulk compositions of older lamprophyres may not therefore represent magmas but be determined by syn- or post-magmatic exchanges.
2. Are lamprophyre mineral assemblages in equilibrium? Commonly coexisting mineral pairs such as analcime with K-feldspar and forsterite or diopside with quartz or alkali feldspars, together with resorbed phenocrysts (especially phlogopite, Mitchell 1985), may point to disequilibrium. Indeed, many are coming to regard lamprophyre assemblages as 'frozen' mixtures of products and reactants from incompleted reactions (Yoder 1979; Rock 1979, 1986).
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Lamprophyre dyke swarm, Aiguablava, Catalunya, Spain. From Jordi Carreras Planells
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Lamprophyre dyke, La Grève au Lanchon, Jersey. From Jersey Geology Trail
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Lamprophyre dyke in a granodiorite, Oberlausitz, Germany. From Wikipedia.
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Gold-brown phlogopite crystals in a Alnoöite from Näset, Alnoö island. From Hildegard Wilske
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Minette (aphanitic) from New Mexico, USA. From James St. John.
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Spessartite dyke with altered mantle xenoliths (light colored objects), Wawa, northern Ontario, Canada. From James St. John.
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Monchiquite dyke with altered mantle xenoliths (with brownish-weathering), Wawa, northern Ontario, Canada. From James St. John.
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Diamontiferous minette, Gibson Lake area, Northwest Territories, northern Canada. From James St. John.
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Camptonite, Campton Falls, Grafton County, USA. From Geology Science.
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Lamprophyre from Torpa, Sweden. From Hildegard Wilske.
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Lamprophyre with amphibole (black) and altered olivine (brown) from Scania, Sweden. From Hildegard Wilske.
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Lamprophyre with amphibole (black) and altered olivine (brown) from Scania, Sweden. From Hildegard Wilske.
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Mondhaldeite with amphibole (black) and feldspar (white) from Kaiserstuhl, Baden, Germany. From lgrb-wissen.
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Mondhaldeite (aphanitic) from Kaiserstuhl, Baden, Germany. From Universität Hamburg.
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Camptonite with amphibole crystals (dark). Dike south of Hoover Dam, Arizona. From Stan Celestian.
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Camptonite with amphibole crystals (dark). Dike south of Hoover Dam, Arizona. From Stan Celestian.
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Kersantite with plagioclase (white) and hornblende (dark). Kersanton, Brittany, France. From Wikipedia.
Bibliography
• Mitchell, R. H. (1994). The lamprophyre facies. Mineralogy and Petrology, 51(2), 137-146.
• Rock, N. S. (1977). The nature and origin of lamprophyres: some definitions, distinctions, and derivations. Earth-Science Reviews, 13(2), 123-169.
• Rock, N. M. S. (1986). The nature and origin of ultramafic lamprophyres: alnöites and allied rocks. Journal of Petrology, 27(1), 155-196.
• Rock, N. M. (1987). The nature and origin of lamprophyres: an overview. Geological Society, London, Special Publications, 30(1), 191-226.