Shonkinite Explained

Shonkinite is an intrusive igneous rock found in few places in the world. It is unique in having low silica, feldspathoid minerals, and large blocky crystals of black augite. It makes up much of the hard dark grey rock comprising certain mountains and buttes in Montana that are remnants of laccoliths and stocks, such as the Highwood mountains.

Composition

Shonkinite is an intrusive igneous rock. More specifically, it is a mafic foidal (feldspathoid bearing) syenite, a holocrystalline (completely crystalline) intrusive rock which,, is composed of potassic feldspar (in the form of sanidine), with 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

• The Adel Mountains Volcanic Field in north-central Montana^{[3] [4] [5] [6]}
• Mountain Pass, California.^[7]
• India in the Salem Block in the Southern Granulite Terrane^[8] and the Elchuru alkaline complex in India^[9]
• Ontario, Canada^[10]
• Timor, Indonesia^[11]

Etymology

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

Notes and References

1. https://books.google.com/books?id=dLwNAAAAYAAJ&dq=shonkinite&pg=PA319 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
2. Book: 9780878422029. Roadside Geology of Montana. Alt. David D.. Hyndman. Donald W.. 1986.
3. Merrill-Maker, Andrea. Montana Almanac. 2d ed. Guilford, Conn.: Globe Pequot, 2005.
4. Gunderson . Jay A. . Sheriff . Steven D. . A new Late Cretaceous paleomagnetic pole from the Adel Mountains, west central Montana . Journal of Geophysical Research . 1991 . 96 . B1 . 317 . 10.1029/90JB01963 . 1991JGR....96..317G .
5. Roberts . E. M. . Hendrix . M. S. . 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 . 1 October 2000 . 15 . 5 . 476–482 . 10.1669/0883-1351(2000)015<0476:TOAPFI>2.0.CO;2 . 2000Palai..15..476R . 130166175 .
6. Hyndman . D. W. . Alt . D. . Radial Dikes, Laccoliths, and Gelatin Models . The Journal of Geology . November 1987 . 95 . 6 . 763–774 . 10.1086/629176 . 1987JG.....95..763H . 128562770 .
7. Haxel . Gordon B. . 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 . 2005 . 10.3133/ofr20051219 .
8. 10.1016/j.jseaes.2015.07.002 . Shonkinites from Salem, southern India: Implications for Cryogenian alkaline magmatism in rift-related setting . Journal of Asian Earth Sciences . 113 . 812–825 . 2015 . He . Xiao-Fang . Santosh . M. . Zhang . Ze-Ming . Tsunogae . Toshiaki . Chetty . T.R.K. . Ram Mohan . M. . Anbazhagan . S. .
9. Ghodke . Sachin S. . Rathna . K. . Kokandakar . Gaurav J. . Nagaraju . B. . More . Laxman B. . Bhosle . Munjaji V. . Vijaya Kumar . K. . Emplacement and growth of alkaline dikes: Insights from the shonkinite dikes (Elchuru alkaline complex, SE India) . Journal of Structural Geology . December 2018 . 117 . 219–236 . 10.1016/j.jsg.2018.09.016 . 2018JSG...117..219G . 135120633 .
10. Cram . I . The Rest Island Granite of Minnesota and Ontario . The Journal of Geology . 1932 . 40 . 3 . 270–278 . 10.1086/623946 . 1932JG.....40..270C . 129258198 .
11. Book: Washington . Henry Stephens . 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 . 1917 . U.S. Government Printing Office . 811 .
12. Iddings, Joseph Paxon. Igneous Rocks: Composition, Texture and Classification, Description and Occurrence. New York: J. Wiley & Sons, 1909, p. 402.