Peralkaline rock

Chart illustrating the meaning of peralkaline, metaluminous, peraluminous and subaluminous

Peralkaline rocks include those igneous rocks which have a deficiency of aluminium such that sodium and potassium are in excess of that needed for feldspar. The presence of aegerine (sodium pyroxene) and riebeckite (sodium amphibole) are indicative of peralkaline conditions. Examples are the peralkaline rhyolites, comendite and pantellerite,[1] with comendite being the more felsic (silica-rich) rock.[2] Another example is the peralkaline granite that forms the islet of Rockall in the North Atlantic Ocean.[3]

Peralkaline rocks are indicative of continental rift-related volcanicity (e.g. the peralkaline rhyolites of the East African Rift in central Kenya) as well as continental and oceanic hotspot volcanicity (e.g. the peralkaline rhyolites of the Glass House Mountains in eastern Australia and the Canary Islands in the Atlantic Ocean).[4][5][6] Peralkaline rocks related to subduction zone volcanicity have also been reported (e.g. Sardinia in Italy).[5] Peralkaline magmas likely form when fractional crystallization removes a high proportion of plagioclase relative to mafic minerals.[7]

  1. ^ Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 143–146. ISBN 9780521880060.
  2. ^ McBirney, Alexander R. (1984). Igneous petrology. San Francisco, Calif.: Freeman, Cooper. p. 502. ISBN 0198578105.
  3. ^ Sutherland, D. S. (editor) (1982). Igneous Rocks of the British Isles. Chichester: John Wiley and Sons. p. 413. ISBN 978-0-471-27810-8. {{cite book}}: |author= has generic name (help)
  4. ^ Scaillet, Bruno; Macdonald, Ray (2003). "Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley". Journal of Petrology. 44 (10): 1867–1894. Bibcode:2003JPet...44.1867S. doi:10.1093/petrology/egg062.
  5. ^ a b Shao, Fengli; Niu, Yaoling; Regelous, Marcel; Zhu, Di-Cheng (2015). "Petrogenesis of peralkaline rhyolites in an intra-plate setting: Glass House Mountains, southeast Queensland, Australia". Lithos. 216. Elsevier: 196–210. Bibcode:2015Litho.216..196S. doi:10.1016/j.lithos.2014.12.015. ISSN 0024-4937.
  6. ^ Troll, Valentin R.; Schmincke, Hans-Ulrich (2002). "Magma Mixing and Crustal Recycling Recorded in Ternary Feldspar from Compositionally Zoned Peralkaline 'A', Gran Canaria, Canary Islands". Journal of Petrology. 43 (2). Oxford Academic Journals: 246. doi:10.1093/petrology/43.2.243. ISSN 0022-3530.
  7. ^ McBirney 1984, pp. 396–397.