Potassic magmatism within Continental plates

  • Marjorie Wilson


Most petrogenetic models assume that primary basic and ultrabasic magmas are generated by varying degrees of partial melting of fertile Iherzolite within the upper mantle (Ch. 3). The degree of partial melting and the depth of segregation of the magmas are considered to be the main variables in controlling the composition of the melt. Additionally, variations in the volatile content and mineralogy of the source mantle, and the extent of subsequent fractional crystallization and crustal contamination are invoked to explain the wide range of terrestrial basaltic magma compositions (Chs. 3 & 4). In the majority of tectonic settings which we have discussed thus far, an important characteristic of the primary basaltic magmas is that they contain significantly greater concentrations of Na2O than K2O on a weight per cent basis. Exceptions to this are the more potassic members of the subduction-related magmatic series (Chs. 6 & 7) and certain potassic oceanic-island suites (e.g. Tristan da Cunha; Ch. 9). However, even within these relatively potassic suites Na2O is still greater than K2O in the basaltic end-members. Rather more rarely, and almost totally restricted to within-continental plate-tectonic settings, basic and ultra-basic magmas are generated in which the content of K2O exceeds that of Na2O, often significantly so. Kimberlites are included within this category in addition to a range of highly potassic igneous rocks with exotic names and frequently equally exotic mineralogies, generated in a wide variety of tectonic settings.


Partial Melting Mantle Source Incompatible Element Continental Plate Crustal Contamination 
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Further reading

  1. Bergman, S.C. 1987. Lamproites and other potassium-rich igneous rocks: a review of their occurrence, mineralogy and geochemistry. In Alkaline igneous rocks, J.G. Fitton B.G.J. Upton), 103–89. Geol Soc. Spc. Publ. 30.Google Scholar
  2. Foley, S.F., G. Venturelli, D.H. Green L. Toscani 1987. The ultrapotassic rocks: characteristics, classification and constraints for petrogenetic models. Earth-Sci. Rev. 24, 81–134.CrossRefGoogle Scholar
  3. Hawkesworth, C.J., K.J. Fraser & N.W. Rogers 1985. Kimberlites and lamproites: extreme products of mantle enrichment processes. Trans Geol Soc. S. Afr. 88, 439–47.Google Scholar
  4. Mitchell, R.H. 1986. Kimberlites. New York: Plenum Press, 442 pp.Google Scholar
  5. Nelson, D.R., M.T. McCulloch, S.-S. Sun 1986. The origins of ultrapotassic rocks as inferred from Sr, Nd and Pb isotopes. Geochim. Cosmochim. Acta 50, 231–45.CrossRefGoogle Scholar
  6. Rock, N.M.S. 1987. The nature and origin of lamprophyres: an overview. In Alkaline igneous rocks In J.G. Fitton, B.G.J. Upton (eds), 191–226. Geol Soc. Spc. Publ. 30.Google Scholar

Copyright information

© Springer 2007

Authors and Affiliations

  • Marjorie Wilson
    • 1
  1. 1.Department of Earth SciencesUniversity of LeedsLeedsUK

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