Shonkinite

Shonkinite is an intrusive igneous rock found in few places in the world. It is unique in having little silicates and large blocky crystals of black augite. It makes up much of the hard dark grey buttes of Montana that remain from the eroded rocks around the buttes.

Shonkinite from Mountain Pass in California

Composition

Shonkinite is an intrusive igneous rock. More specifically, it is a mafic foidal (feldspathoid bearing) syenite, a holocrystalline (completely crystalline) intrusive rock which, in the restricted sense, is composed of potassic feldspar in the form of sanidine, nepheline, augite, biotite, and olivine.[1] Shonkinite is also used for mafic nepheline syenite with aegerine-augite as the pyroxene, and with the addition of plagioclase (andesine to labradorite). Nepheline in shonkinite from the is largely altered to natrolite and stilbite.

The close view of the rocks in the Adel mountains show large glossy crystals of augite in a dark grey matrix made up of small crystals of augite and feldspar. This is unusual as augite is usually dull.[2]

Formation

In central Montana buttes of shonkinite are capped with white layers of syenite. There appear round globes of syenite at the boundary which suggest that the less dense syenite float up to the top of molten shonkinite as the mass cooled.[2]

Occurrence

Etymology

The rock gets its name from the type locality at Shonkin Sag in the Highwood Mountains of north-central Montana.[12]

References
  1. Weed, Walter H. and Pirsson, Louis V. Geology of the Little Belt mountains, Montana, With Note on the Mineral Deposits of the Neihart, Barker, Yogo, and Other Districts. Washington, D.C.: Government Printing Office, 1900, p. 319.
  2. Alt, David D.; Hyndman, Donald W. (1986). Roadside Geology of Montana. ISBN 9780878422029.
  3. Merrill-Maker, Andrea. Montana Almanac. 2d ed. Guilford, Conn.: Globe Pequot, 2005. ISBN 0-7627-3655-0
  4. Gunderson, Jay A.; Sheriff, Steven D. (1991). "A new Late Cretaceous paleomagnetic pole from the Adel Mountains, west central Montana". Journal of Geophysical Research. 96 (B1): 317. Bibcode:1991JGR....96..317G. doi:10.1029/90JB01963.
  5. Roberts, E. M.; Hendrix, M. S. (1 October 2000). "Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics". PALAIOS. 15 (5): 476–482. Bibcode:2000Palai..15..476R. doi:10.1669/0883-1351(2000)015<0476:TOAPFI>2.0.CO;2.
  6. Hyndman, D. W.; Alt, D. (November 1987). "Radial Dikes, Laccoliths, and Gelatin Models". The Journal of Geology. 95 (6): 763–774. Bibcode:1987JG.....95..763H. doi:10.1086/629176.
  7. Haxel, Gordon B. (2005). "Ultrapotassic mafic dikes and rare earth element- and barium-rich carbonatite at Mountain Pass, Mojave Desert, southern California: summary and field trip localities". Open-File Report. doi:10.3133/ofr20051219.
  8. He, Xiao-Fang; Santosh, M.; Zhang, Ze-Ming; Tsunogae, Toshiaki; Chetty, T.R.K.; Ram Mohan, M.; Anbazhagan, S. (2015). "Shonkinites from Salem, southern India: Implications for Cryogenian alkaline magmatism in rift-related setting". Journal of Asian Earth Sciences. 113: 812–825. doi:10.1016/j.jseaes.2015.07.002.
  9. Ghodke, Sachin S.; Rathna, K.; Kokandakar, Gaurav J.; Nagaraju, B.; More, Laxman B.; Bhosle, Munjaji V.; Vijaya Kumar, K. (December 2018). "Emplacement and growth of alkaline dikes: Insights from the shonkinite dikes (Elchuru alkaline complex, SE India)". Journal of Structural Geology. 117: 219–236. Bibcode:2018JSG...117..219G. doi:10.1016/j.jsg.2018.09.016.
  10. Cram, I (1932). "The Rest Island Granite of Minnesota and Ontario". The Journal of Geology. 40 (3): 270–278. Bibcode:1932JG.....40..270C. doi:10.1086/623946.
  11. Washington, Henry Stephens (1917). Chemical Analyses of Igneous Rocks: Pub. from 1884 to 1913, Inclusive, with a Critical Discussion of the Character and Use of Analyses, a Revision and Expansion of Professional Paper 14. U.S. Government Printing Office. p. 811.
  12. Iddings, Joseph Paxon. Igneous Rocks: Composition, Texture and Classification, Description and Occurrence. New York: J. Wiley & Sons, 1909, p. 402.
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