Lamprophyre

Lamprophyres - from the Greek λαμπρός ( Lampros ) bright, shining ( with respect to the crystal faces of amphibole and biotite, which in freshly cracked condition bright light ) and φύρω ( Phyro ) = mix, introduced by Carl Wilhelm von Gümbel 1874 - are a rather unusual group of dark igneous rocks. They appear as little or as powerful intrusions courses and sills. Due to their chemical composition, they are among the alkaline rocks. Lamprophyres are mostly used as decorative stone and building materials.

Mineral composition and chemistry

It is fine to medium grained, sometimes strongly porphyritic rocks from biotite, amphibole and pyroxene, olivine and clinopyroxene partly also are mixed. The groundmass consists of feldspar or Foiden. Typically, their mean ( mesotype ) is darkening to large ( melanokrat ), but rarely are they ultramafisch. They are characterized by a high content lithophiler elements such as potassium, sodium, barium, cesium, rubidium and strontium and usually have high nickel and chromium contents. The silicon content is low.

Even if lamprophyres are generally ultrapotassisch, so they can still have variable K / Na ratios - Camptonite and Monchiquite example show sodium supremacy. Lamprophyres usually have very high contents of volatiles such as water, carbon dioxide, fluorine, chlorine and sulfur dioxide. LREE their values ​​are also increased, while the values ​​remain at HREE basaltic level.

Often the occurrence of secondary minerals such as calcite and zeolites, mostly tied to ocelli - leukokrate ball to lens-shaped inclusions, which are interpreted as immiscible interference or residual melts.

Formation

The origin of lamprophyres is not fully understood. According to some geologists its origin is related to Subduktionsvorgängen, others see them as residual melts of plutons on. Also a coupling at hotspot mantle plumes or deep -reaching tectonic faults can not be excluded.

The predominance of mafic minerals indicates a large melting depth of the magma from which the lamprophyres have emerged. The high content of potassium-rich minerals, makes it likely that the source rock was not partially melted ( partial melting ), as they would otherwise leave the melt. With few exceptions, there is little evidence of fractional crystallization, but for many for a mineral transport by gases and volatile residual solutions.

Because of its mineral holdings and its emergence as late phases of intrusions lamprophyres have often for signs of hydrothermal alteration.

Types of lamprophyres

The classification of lamprophyres by the usual methods of magmatic rocks ( QAPF or Streckeisen diagram TAS diagram) is difficult as its main components are not or only partly included in these charts. An official classification of lamprophyres has only existed since the late 1990s, although this continues to be identified as provisional. The subdivision of the lamprophyres is done primarily by their dark main components amphibole, pyroxene, biotite and olivine, plus the main light minerals such as feldspar and Foide come.

The various designations are mostly borrowed from the name of the sites from which they were first described. The following types are distinguished:

• Kersantit, biotite - hornblende - pyroxene - lamprophyre with a base material in which the plagioclase content predominates the orthoclase content
• Minette, biotite - hornblende - pyroxene - lamprophyre with a base material in which the orthoclase content predominates the plagioclase - share

• Spessartite, hornblende - pyroxene - lamprophyre, plagioclase is more abundant than orthoclase in the matrix
• Vogesit, hornblende - pyroxene - lamprophyre, orthoclase is more common than plagioclase in the matrix

• Sannait, amphibole - pyroxene - olivine - biotite - lamprophyre, orthoclase is more common than plagioclase in the matrix, Foide occur only subordinated to
• Camptonit, amphibole - pyroxene - olivine - biotite - lamprophyre, plagioclase is more abundant than orthoclase in the matrix, Foide occur only subordinated to
• Monchiquit, amphibole - pyroxene - olivine - biotite - lamprophyre, glassy matrix or exclusively Foide in the matrix
• Alnöit is ultramafisch - contains melilite.

According to the classification of the IUGS ​​Alnöit, a biotite / phlogopite rock with olivine, calcite and clinopyroxene, and the ultramafic Polzenit are no longer expected, with all its varieties Bergalit and Damkjernit to the lamprophyres, but assigned to the melilite rocks.

Presence of dark rock color and only predominantly OH-containing minerals such as biotite dark, phlogopite, amphibole and dark phenocrysts, which are often very large: For the determination of lamprophyres, which occur in corridors or low volume vents only, the following characteristics apply. Both porphyry and aporphyrischen structure must exist and be a so-called alteration, conversion of minerals into secondary minerals has occurred.

Groupings

In general, can be among the lamprophyres two major groups:

• Kalkalkalilamprophyre
• Alkalilamprophyre

The SiO2 - rich lamprophyres can be divided into shoshonitische and leucitische lamprophyres in turn.

The group of alkaline lamprophyres are also still the ultrabasic lamprophyres be added.

Shoshonitische lamprophyres:

• Minette
• Kersantit
• Vogesit
• Spessartite

Leucitische lamprophyres: extremely rare, ultrapotassische dyke rocks with glassy texture. Among the mafic rocks, they have the highest K2O content.

Alkaline lamprophyres:

• Camptonit
• Monchiquit
• Sannait

Ultrabasic lamprophyres: extremely SiO2 - poor rocks with very high Ca content, are close to carbonatites:

• Ouachitit

Occurrence

Lamprophyres are found worldwide, often in areas with granitic, granodioritic and dioritic intrusions. Important localities are:

• Germany: Spessart ( spessartite ) and resin, Emperor chair ( Monchiquit ), Lusatian Highlands, Odenwald
• France: Kersanton (Brittany - Kersantit ), Pyrenees ( Monchiquit ), Vosges ( Vogesit )
• Canada: British Columbia, Quebec ( Camptonit, Monchiquit )
• Norway: Telemark ( Sannait )
• Portugal: Monchique ( Monchiquit )
• South Africa:
• USA: Campton Falls (New Hampshire - Camptonit ), Hopi Buttes Volcanic Field ( Monchiquit ), Navajo Volcanic Field ( Minette, Monchiquit )

Types of natural stone

A selection of lamprophyres:

• Lamprophyre frontier (also called gesteinskundlich Spremberger or Lusatian syenite wrong), Valtengrund in Saxony
• Lamprophyre Snowflake ( also syenite or diabase gesteinskundlich called false), Ottendorf
• Lamprophyre Sora, Sora in Neukirch in Saxony
• Lamprophyre Friedersdorf ( Spessartite ) from Friedersdorf in Saxony
• Šluknov - Lipova and Šluknov - Rozany ( Spessartite ) in Šluknov in the Czech Republic