View source for Chemistry:Ultrapotassic igneous rocks

{{Short description|Class of rare ultramafic or mafic igneous rocks rich in potassium}}
[[File:Lamprophyre dike & hornfels & country rock (Archean; Route 17 roadcut southeast of Princess Lake & north of Wawa, Ontario, Canada) 2 (48278201871).jpg|thumb|346x346px|A [[Earth:Dike (geology)|dike]] composed of the ultrapotassic igneous rock, lamprophyre]]
'''Ultrapotassic igneous rocks''' are a class of rare, volumetrically minor, generally ultramafic or [[Earth:Mafic|mafic]] silica-depleted igneous rocks.

While there are debates on the exact classifications of ultrapotassic rocks, they are defined by using the chemical screens K<sub>2</sub>O/Na<sub>2</sub>O > 3 in much of the scientific literature.<ref>{{Cite journal |last=Foley |first=S.F. |last2=Venturelli |first2=G. |last3=Green |first3=D.H. |last4=Toscani |first4=L. |date=April 1987 |title=The ultrapotassic rocks: Characteristics, classification, and constraints for petrogenetic models |url=https://linkinghub.elsevier.com/retrieve/pii/0012825287900018 |journal=Earth-Science Reviews |language=en |volume=24 |issue=2 |pages=81–134 |doi=10.1016/0012-8252(87)90001-8}}</ref> 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 K<sub>2</sub>O/Na<sub>2</sub>O > 2 to indicate a rock is ultrapotassic.

== Conditions of formation ==
The [[Biology:Magma|magma]]s that produce ultrapotassic rocks are produced by a variety of mechanisms and from a variety of sources, but generally occur in a [[Chemistry:Homogeneity and heterogeneity|heterogenous]], anomalous, [[Chemistry:Phlogopite|phlogopite]]-bearing upper mantle.<ref>{{Cite journal |last=Peccerillo |first=Angelo |date=1992-12-01 |title=Potassic and ultrapotassic rocks : Compositional characteristics, petrogenesis, and geologic significance |url=http://www.episodes.org/journal/view.html?doi=10.18814/epiiugs/1992/v15i4/002 |journal=Episodes |language=en |volume=15 |issue=4 |pages=243–251 |doi=10.18814/epiiugs/1992/v15i4/002 |issn=0705-3797|doi-access=free }}</ref>

The following conditions are favorable for the formation of ultrapotassic magmas.<ref>{{Cite journal |last=Foley |first=Stephen |last2=Peccerillo |first2=Angelo |date=November 1992 |title=Potassic and ultrapotassic magmas and their origin |url=http://dx.doi.org/10.1016/0024-4937(92)90005-j |journal=Lithos |volume=28 |issue=3-6 |pages=181–185 |doi=10.1016/0024-4937(92)90005-j |issn=0024-4937}}</ref>

* [[Earth:Partial melting|partial melting]] at a great depth
* low degrees of partial melting
* lithophile element ([[Chemistry:Potassium|K]], [[Chemistry:Barium|Ba]], Cs, [[Chemistry:Rubidium|Rb]]) enrichment in sources
* enriched peridotite (variety harzburgite), especially in potassium
* [[Chemistry:Pyroxene|pyroxene]] and [[Chemistry:Phlogopite|phlogopite]]-rich volumes within the mantle, not from peridotite alone
* [[Chemistry:Carbon dioxide|carbon dioxide]] or water (each condition leading to a distinctive magma);
* reaction of melts with surrounding rock as they rise from their sources

[[Earth:Mantle (geology)|Mantle]] sources of ultrapotassic magmas may contain subducted sediments, or the sources may have been enriched in potassium by melts or fluids partly derived from subducted sediments. [[Chemistry:Phlogopite|Phlogopite]] and/or potassic [[Chemistry:Amphibole|amphibole]] are typical in the sources from which many such magmas have been derived. Ultrapotassic [[Chemistry:Granite|granite]]s are uncommon and may be produced by melting of the continental crust above upwelling mafic magma, such as at rift zones.

== Types of ultrapotassic rocks ==

* Lamprophyres and melilitic rocks
* [[Earth:Kimberlite|Kimberlite]]
* [[Earth:Lamproite|Lamproite]]
* Orangeite (see Group II [[Earth:Kimberlite|kimberlite]])
* [[Chemistry:Feldspathoid|Feldspathoid]]-bearing rocks such as [[Earth:Leucitite|leucitite]]s
* K-feldspar enriched leucogranites
* Vaugnerite and Durbachite

== Economic importance ==
The economic importance of ultrapotassic rocks is wide and varied. Because [[Earth:Kimberlite|kimberlite]]s, [[Earth:Lamproite|lamproite]]s and lamprophyres are all produced at depths of 120&nbsp;km or greater, they are known to be a major source of [[Chemistry:Diamond|diamond]] deposits and thus can bring diamonds to the surface as xenocrysts.<ref>{{Cite journal |last=Mitchell |first=Roger H. |date=2020-09-28 |title=Igneous Rock Associations 26. Lamproites, Exotic Potassic Alkaline Rocks: A Review of their Nomenclature, Characterization and Origins |url=https://journals.lib.unb.ca/index.php/GC/article/view/31174 |journal=Geoscience Canada |volume=47 |issue=3 |pages=119–142 |doi=10.12789/geocanj.2020.47.162 |issn=1911-4850|doi-access=free }}</ref> Additionally, ultrapotassic granites are a known host for granite-hosted [[Chemistry:Gold|gold]] mineralization and well as significant [[Earth:Porphyry (geology)|porphyry]]-style mineralization.<ref>{{Citation |title=Direct associations between potassic igneous rocks and gold-copper deposits |url=http://dx.doi.org/10.1007/bfb0017718 |work=Potassic Igneous Rocks and Associated Gold-Copper Mineralization |pages=85–134 |access-date=2023-11-28 |place=Berlin/Heidelberg |publisher=Springer-Verlag |isbn=3-540-62075-3}}</ref> Ultrapotassic A-type intracontinental granites may also be associated with [[Chemistry:Fluorite|fluorite]] and [[Chemistry:Columbite|columbite]] – [[Chemistry:Tantalite|tantalite]] mineralization.

==References==
{{Reflist}}

[[Category:Geochemistry]]



{{Sourceattribution|Ultrapotassic igneous rocks}}