Layered intrusion

Chromitite and anorthosite layered igneous rocks in Critical Zone UG1 of the Bushveld Igneous Complex at the Mononono River outcrop, near Steelpoort, South Africa

A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around 100 km2 (39 sq mi) to over 50,000 km2 (19,000 sq mi) and several hundred metres to over one kilometre (3,300 ft) in thickness.[1] While most layered intrusions are Archean to Proterozoic in age (for example, the Paleoproterozoic Bushveld complex), they may be any age such as the Cenozoic Skaergaard intrusion of east Greenland or the Rum layered intrusion in Scotland.[1][2] Although most are ultramafic to mafic in composition, the Ilimaussaq intrusive complex of Greenland is an alkalic intrusion.[3]

Layered intrusions are typically found in ancient cratons and are rare but worldwide in distribution. The intrusive complexes exhibit evidence of fractional crystallization and crystal segregation by settling or floating of minerals from a melt.

Ideally, the stratigraphic sequence of an ultramafic-mafic intrusive complex consists of ultramafic peridotites and pyroxenites with associated chromitite layers toward the base with more mafic norites, gabbros and anorthosites in the upper layers.[4] Some include diorite, and granophyre near the top of the bodies. Orebodies of Nickel-Copper-Platinum group elements (Ni-Cu-PGE), chromite, magnetite, and ilmenite are often associated with base metal Sulfide mineral assemblages within these rare intrusions.[5][6][7] Often overlooked is that economically significant Ni-Cu-PGE deposits can occur in the country rock spatially associated with the layered intrusion.[8][9]

  1. ^ a b Blatt, Harvey and Tracy, Robert J. (1996) Petrology: Igneous, Sedimentary and Metamorphic, 2nd ed., pp. 123–132 & 194–197, Freeman, ISBN 0-7167-2438-3
  2. ^ Hamilton MA, Pearson DG, Thompson RN, Kelly SP, Emeleus CH (1998). "Rapid eruption of Skye lavas inferred from precise U-Pb and Ar–Ar dating of the Rum and Cuillin plutonic complexes". Nature. 394 (6690): 260–263. Bibcode:1998Natur.394..260H. doi:10.1038/28361. S2CID 262556547.
  3. ^ Sørensen, H. (2001), Brief introduction to the geology of the Ilímaussaq alkaline complex, South Greenland, and its exploration history (PDF), Geology of Greenland Survey Bulletin, vol. 190, archived from the original (PDF) on 2017-08-10
  4. ^ Emeleus, C. H.; Troll, V. R. (August 2014). "The Rum Igneous Centre, Scotland". Mineralogical Magazine. 78 (4): 805–839. Bibcode:2014MinM...78..805E. doi:10.1180/minmag.2014.078.4.04. ISSN 0026-461X.
  5. ^ Cite error: The named reference Zuccarelli-2018 was invoked but never defined (see the help page).
  6. ^ Cite error: The named reference Mungall-2010 was invoked but never defined (see the help page).
  7. ^ Cite error: The named reference Howarth-2013 was invoked but never defined (see the help page).
  8. ^ Cite error: The named reference Smith-2022 was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference Hall-2020 was invoked but never defined (see the help page).