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Lamprophyres are uncommon, small volume ultrapotassicigneous 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 nickeland chromium.
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.
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.
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