Ultrapotassic igneous rocks
Ultrapotassic igneous rocks are a class of rare, volumetrically minor, generally ultramafic or mafic silica-depleted igneous rocks.
While there are debates on the exact classifications of ultrapotassic rocks, they are defined by using the chemical screens K2O/Na2O > 3 in much of the scientific literature. However caution is indicated in interpreting the use of the term "ultrapotassic", and the nomenclature of these rocks continues to be debated, with some classifications using K2O/Na2O > 2 to indicate a rock is ultrapotassic.
Conditions of formation
The magmas that produce ultrapotassic rocks are produced by a variety of mechanisms and from a variety of sources, but generally occur in a heterogenous, anomalous, phlogopite-bearing upper mantle.The following conditions are favorable for the formation of ultrapotassic magmas.
- partial melting at a great depth
- low degrees of partial melting
- lithophile element enrichment in sources
- enriched peridotite, especially in potassium
- pyroxene and phlogopite-rich volumes within the mantle, not from peridotite alone
- carbon dioxide or water ;
- reaction of melts with surrounding rock as they rise from their sources
Types of ultrapotassic rocks
- Lamprophyres and melilitic rocks
- Kimberlite
- Lamproite
- Orangeite
- Feldspathoid-bearing rocks such as leucitites
- K-feldspar enriched leucogranites
- Vaugnerite and Durbachite
Economic importance