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 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 felsites. 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: Minette contains biotite and orthoclase. Kersantite contains biotite and plagioclase. Vogesite contains hornblende and orthoclase. Spessartite contains hornblende and plagioclase. Monchiquite contains no feldspar, has a glassy or feldspathoid-bearing groundmass, and contains amphibole phenocrysts. Each variety of lamprophyre may and often does contain all four minerals but is named according to the two