Ultrapotassic Magmatism
Ultrapotassic igneous magmatism produces quite rare and insignificant igneous rocks, predominantly of a volcanic or sub-volcanic nature. Based on their chemical composition, they can be classified as undersaturated in silicon , ultramafic to mafic rocks. Nevertheless, precisely because of their versatility, they are of great importance for petrology . Its importance lies primarily in the area of magma formation .
definition
The definition of ultrapotassic igneous rocks, ie alkaline rocks with a predominance of potassium , is usually based on the molar ratio [K 2 O] / [Na 2 O] in the scientific literature . Until 2005, the limit for this ratio was> 2, but it is now generally set at> 3.
Petrogenesis
Ultrapotassic magmas can be generated by a variety of processes and, moreover, are likely to originate from very different source regions. The following points must be taken into account:
- Their partial melting takes place at great depth.
- The partial melting rate is low.
- Their source region shows high contents of lithophilic elements such as potassium , barium , cesium and rubidium .
- The mantle peridotite ( Harzburgite , Lherzolite ) is enriched, especially in potassium.
- In addition to the peridotic mantle rock, there are also volumes rich in pyroxene and phlogopite .
- The source region is penetrated by water and carbon dioxide . Depending on the liquid present, very different magmas are created.
- During the ascent, the melts react with the surrounding host rocks.
The source regions of the ultrapotassic magmatism in the upper mantle may also contain subducted sediments or may have been enriched in potassium by means of melts or metasomatic fluids originating from these sediments . Typical of this enrichment process are the minerals phlogopite and potassium amphiboles such as high-potassium Richterite .
Ultrapotassic granitoids are very rare; they may arise in rift zones through anatectic melting of the continental crust, with the heat required for this being added by billowing mafic magmas.
Ultrapotassic rock types
The following rocks are counted among the ultrapotassic magmatites:
- Lamprophyre
- Melilithites and Melilithic rocks
- Kimberlite
- Lamproite
- Group II orangite or kimberlite
- Feldspar representatives - bearing rocks ( e.g. leucitite )
- Alkali calcareous Leukogranite
- Vaugnerite and Durbachite
Economical meaning
The economic importance of ultrapotassic magmatism is very complex and far-reaching. Kimberlites, lamproites and possibly also lamprophyres are host rocks for diamonds . Their magmas arise at depths of up to 120 kilometers and, during their sometimes rapid ascent, bring diamonds as xenocrystals to the surface of the earth. The extremely rare ultrapotassic granites are known for their gold potential . Potassian to ultrapotassian granites also contain porphyry mineralizates such as copper . Ultrapotassic granites of the A-type located within the continents can be associated with fluorite deposits as well as with columbite - tantalite mineralizations.
Individual evidence
- ↑ Stephen Foley and Angelo Peccerillo: Potassic and ultrapotassic magmas and their origin . In: Lithos . v. 28, 1992, pp. 181-185 .