Lamprophyre

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Lamprophyre

The Henneberg granite is intersected by numerous lamprophyre veins. They were formed after the granite solidified from basaltic melts that used extensional fractures for ascent. The original composition of olivine, pyroxene and potassium feldspar was strongly altered by the influx of fluids (water and carbon dioxide), so that the rock is now dominated by biotite, potassium feldspar and carbonate predominate in the rock.

Kersantite from the Henneberg granite, Upper Carboniferous,
Heberndorf, Thuringian Slate Mountains, ca. 320 Ma

Lamprophyre

Location: Heberndorf, Thuringian Slate Mountains

Age: ca. 320 million years

Lamprophyre is an unusual group of dark, igneous rocks, which due to their chemical composition belong to the alkali rocks. The name "lamprophyre" comes from the Greek and means "bright" or "shiny", referring to the crystal faces of amphibole and biotite, which shine brightly when freshly struck. Carl Wilhelm von Gümbel introduced the term in 1874. Lamprophyres occur either as little thick intrusions or as veins and stock veins. They are often used as decorative stones and building materials [1External link].

Lamprophyres consist of fine- to medium-grained rock components, often with a strongly porphyritic texture. Major minerals include biotite, amphibole, and pyroxene, with clinopyroxene and olivine sometimes present. The groundmass of the lamprophyre is either feldspar or foids. Characteristic of lamprophyres is their medium to strong dark coloration, although they are rarely ultramafic. They contain high concentrations of lithophilic elements such as potassium, sodium, barium, cesium, rubidium, and strontium, as well as high nickel and chromium contents. Silicon content, on the other hand, is low. Lamprophyres are known for their high variability in K/Na ratios. Some lamprophyres exhibit a sodium predominance, although they are generally ultrapotassic [1External link].

Lamprophyres often contain secondary minerals such as calcite and zeolites, which are commonly associated with ocelli. Ocelli are leucocrate spherical to lenticular inclusions interpreted as immiscible foreign or residual melts. The origin of the lamprophyre is not yet fully understood. Some geologists suspect a connection with subduction processes, while others consider them as residual melts of plutons. A connection to hotspot mantle plumes or deep tectonic faults is also possible [1External link].

Lamprophyres show evidence of a great depth of melting of the magma from which they were formed, as they show a predominance of mafic minerals. The high content of potassium-rich minerals suggests that the source rock was not partially molten. There is little evidence of fractional crystallization, but much evidence of mineral transport and mixing processes during formation.