Igwisi Hills

The Igwisi Hills are a volcanic field in Tanzania. Three tuff cones are found there, one of which is associated with a lava flow. They are one of the few locations of possibly kimberlitic lava flows on Earth.

Igwisi Hills
Highest point
Elevation1,145 m (3,757 ft) [1]
Listing
Coordinates4°53′13.18″S 31°56′4.46″E [2]
Geography
LocationTanzania
Geology
Age of rockPleistocene?
Mountain typePyroclastic Cone
Last eruptionunknown

The volcanoes are located in the middle of the Tanzania craton, away from other Tanzanian volcanoes. There have been prior episodes of kimberlitic volcanism in the craton, however.

The age of the Igwisi Hills is poorly known but may be early Holocene-late Pleistocene in age. Some rainfall-induced chemical modification is found, and the hills have a unique vegetation profile.

Geology

The Igwisi hills are formed by three tuff cones formed in the middle of the Tanzania craton. They are 70 metres (230 ft) above the landscape with a karst morphology and craters covered with grass, on a low ridge that may be the product of early eruptive stages. The northeastern hill has two craters, one with a breach from which a 500 metres (1,600 ft) long lava flow originates, probably formed when a lava lake in the crater escaped through a breach. The central volcano has a lava coulee and a tephra cone in its crater.[3][4][5]:72, 73[6] Craters have diameters of 200–400 metres (660–1,310 ft).[7] The total volume of these cones is less than 0.001 cubic kilometres (0.00024 cu mi).[6] Weak pyroclastic activity probably accompanied the eruptive activity.[3] Presumably, low intensity explosive activity built the cones, starting from the northeast cone and ending with the southwest cone. Afterwards, lava flows were generated.[8]

The Igwisi Hills are the only places in the world where possible kimberlite lava flows have been found,[9] in form of calcite-olivine lavas.[10] Kimberlite tuffs are also found, a rare species which is very susceptible to erosion.[11] True kimberlites are usually very old eruptive rocks, consequently any subsurface volcanic structure has long since been eroded away.[8] These kimberlites were erupted in a fairly nonexplosive fashion.[12] Not all researchers agree that these lavas are kimberlites, however,[13] with the low alkali content being cited as a difference although the Benfontein "kimberlites" share this property with the Igwisi hills ones.[14] If the Igwisi Hills aren't true kimberlites, the next youngest would be the 32.3 ± 2.2 Ma Kundelungu plateau pipes in the Democratic Republic of Congo.[8]

These kimberlites are also the youngest kimberlites in the world by over thirty million years, cosmogenic helium-3 dates of olivine indicates they were erupted in the late Pleistocene-Holocene,[15] some indicated ages being 11,200 ± 7,800 ± and 12,400 ± 4,800.[3] A poorly constrained U-Pb date is 0 ± 29 million years. The cones display a young morphology.[5]:72, 73 The basement terrain belongs to the 2,500 ± 100 million year old Dodoman sequence.[3] The hills are remote from all other Tanzania volcanoes.[1] Prior kimberlite activity in the Tanzania craton is recorded 1,150, 189 and 53 million years ago.[8]

The tuffs are highly calcitic, vesicular and contain numerous microxenoliths. The petrologically similar lavas show evidence of a differentiation by flow and gravity and have trachytic textures.[16][5]:72, 73[7] Lavas have a carbonatitic composition.[4] Olivines with diopside cores are found at the Igwisi Hills. Garnet and orthopyroxene is associated with the diopside.[17] Olivines are surrounded by chromite.[7] Olivines characterize the texture of the Igwisi rocks, where they form spherical inclusions. The olivines are primarily forsteritic in composition.[18] Inclusions in the kimberlite include skeletal apatite, stellate aragonite and calcite.[19] High concentrations of CO
2 are found in the rock,[12] which may have resulted in the depolymerization of the melt, increasing its fluidity and resulting in effusive activity.[20] Peridotite xenoliths originate from 180 kilometres (110 mi) of depth.[5]:15 Spinels in the groundmass suggest that crustal contamination was extensive.[20] The geochemistry suggests an origin at high pressures (depths of 150–200 kilometres (93–124 mi)) and equilibrium temperatures of 1,000 °C (1,830 °F).[18] Rainfall has subsequently modified the pyroclastics and formed secondary calcite, while the less permeable lava flows were less modified.[8]

The hills have a unique vegetation, with aquatic plants found in the middle of the craters and distinct vegetation on inner crater slopes from the extra-crateric territory.[21] The hills have a rare occurrence of Asclepias pseudoamabilis.[22]

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References
  1. "Igwisi Hills". Global Volcanism Program. Smithsonian Institution.
  2. Brown, R.J.; Sparks, R.S.J. "Mapping the Igwisi Hills kimberlite volcanoes, Tanzania: understanding how deep-sourced mantle magmas behave at the Earth's surface" (PDF). GEF Scientific Reports. Natural Environment Research Council. Retrieved 20 March 2016.
  3. Brown, Richard J.; Manya, S.; Buisman, I.; Fontana, G.; Field, M.; Niocaill, C. Mac; Sparks, R. S. J.; Stuart, F. M. (13 June 2012). "Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)" (PDF). Bulletin of Volcanology. 74 (7): 1621–1643. doi:10.1007/s00445-012-0619-8.
  4. Dawson, J. Barry (1980). "Distribution and Tectonic Setting of Kimberlites". Kimberlites and Their Xenoliths. Minerals and Rocks. 15. Berlin, Heidelberg: Springer Berlin Heidelberg. p. 9. doi:10.1007/978-3-642-67742-7_2. ISBN 978-3-642-67742-7.
  5. Dawson, John Barry (2008). The Gregory Rift Valley and Neogene-recent Volcanoes of Northern Tanzania. Geological Society of London. ISBN 9781862392670. Retrieved 28 February 2016.
  6. Brown, R. J.; Valentine, G. A. (7 June 2013). "Physical characteristics of kimberlite and basaltic intraplate volcanism and implications of a biased kimberlite record" (PDF). Geological Society of America Bulletin. 125 (7–8): 1226. doi:10.1130/B30749.1.
  7. Mitchell, Roger H. (2013-06-29). Kimberlites: Mineralogy, Geochemistry, and Petrology. Springer Science & Business Media. pp. 31–32. ISBN 9781489905680. Retrieved 28 February 2016.
  8. Willcox, A.; Buisman, I.; Sparks, R.S.J.; Brown, R.J.; Manya, S.; Schumacher, J.C.; Tuffen, H. (June 2015). "Petrology, geochemistry and low-temperature alteration of lavas and pyroclastic rocks of the kimberlitic Igwisi Hills volcanoes, Tanzania". Chemical Geology. 405: 82–101. doi:10.1016/j.chemgeo.2015.04.012.
  9. Mitchell, Roger H. (1995). Kimberlites, Orangeites, and Related Rocks. Springer Science & Business Media. p. 37. ISBN 978-1-4613-5822-0. Retrieved 28 February 2016.
  10. Sparks, R.S.J.; Baker, L.; Brown, R.J.; Field, M.; Schumacher, J.; Stripp, G.; Walters, A (July 2006). "Dynamical constraints on kimberlite volcanism". Journal of Volcanology and Geothermal Research. 155 (1–2): 18–48. doi:10.1016/j.jvolgeores.2006.02.010.
  11. Chalapathi Rao, N. V.; Anand, M.; Dongre, A.; Osborne, I. (10 September 2009). "Carbonate xenoliths hosted by the Mesoproterozoic Siddanpalli Kimberlite Cluster (Eastern Dharwar craton): implications for the geodynamic evolution of southern India and its diamond and uranium metallogenesis". International Journal of Earth Sciences. 99 (8): 1791–1804. doi:10.1007/s00531-009-0484-7.
  12. Brooker, Richard A.; Sparks, R. Stephen J.; Kavanagh, Janine L.; Field, Matthew (8 September 2011). "The volatile content of hypabyssal kimberlite magmas: some constraints from experiments on natural rock compositions". Bulletin of Volcanology. 73 (8): 959–981. doi:10.1007/s00445-011-0523-7.
  13. White, J.D.L.; Ross, P.-S. (April 2011). "Maar-diatreme volcanoes: A review" (PDF). Journal of Volcanology and Geothermal Research. 201 (1–4): 1–29. doi:10.1016/j.jvolgeores.2011.01.010.
  14. BATUMIKE, J; GRIFFIN, W; BELOUSOVA, E; PEARSON, N; OREILLY, S; SHEE, S (30 March 2008). "LAM-ICPMS U–Pb dating of kimberlitic perovskite: Eocene–Oligocene kimberlites from the Kundelungu Plateau, D.R. Congo". Earth and Planetary Science Letters. 267 (3–4): 609–619. doi:10.1016/j.epsl.2007.12.013.
  15. D. Graham Pearson; et al., eds. (2013). "How Structure and Stress Influence Kimberlite Emplacement". Proceedings of 10th International Kimberlite Conference. New Delhi: Springer. p. 56. doi:10.1007/978-81-322-1173-0_4. ISBN 978-81-322-1173-0.
  16. Dawson, J. Barry (1980). "Geology of Kimberlite Intrusions". Kimberlites and Their Xenoliths. Minerals and Rocks. 15. Berlin, Heidelberg: Springer Berlin Heidelberg. p. 37. doi:10.1007/978-3-642-67742-7_2. ISBN 978-3-642-67742-7.
  17. Sazonova, L. V.; Nosova, A. A.; Kargin, A. V.; Borisovskiy, S. E.; Tretyachenko, V. V.; Abazova, Z. M.; Griban, Yu. G. (12 May 2015). "Olivine from the Pionerskaya and V. Grib kimberlite pipes, Arkhangelsk diamond province, Russia: Types, composition, and origin". Petrology. 23 (3): 252. doi:10.1134/S0869591115030054.
  18. Reid, Arch M.; Donaldson, C.H.; Dawson, J.B.; Brown, R.W.; Ridley, W.I. (January 1975). "The Igwisi Hills extrusive "kimberlites"". Physics and Chemistry of the Earth. 9: 199–218. doi:10.1016/0079-1946(75)90017-8.
  19. Dawson, J. Barry (1980). Kimberlites and Their Xenoliths. Berlin, Heidelberg: Springer Berlin Heidelberg. pp. 88–92. ISBN 978-3-642-67742-7.
  20. van Straaten, Bram I.; Kopylova, M. G.; Russell, J. K.; Scott Smith, B. H. (25 June 2011). "A rare occurrence of a crater-filling clastogenic extrusive coherent kimberlite, Victor Northwest (Ontario, Canada)". Bulletin of Volcanology. 73 (8): 1057. doi:10.1007/s00445-011-0507-7.
  21. Cantrill, David J.; Bamford, Marion K.; Wagstaff, Barbara E.; Sauquet, Hervé (September 2013). "Early Eocene fossil plants from the Mwadui kimberlite pipe, Tanzania". Review of Palaeobotany and Palynology. 196: 19–35. doi:10.1016/j.revpalbo.2013.04.002.
  22. Goyder, D. J. (18 October 2009). "A synopsis of Asclepias (Apocynaceae: Asclepiadoideae) in tropical Africa". Kew Bulletin. 64 (3): 380. doi:10.1007/s12225-009-9133-3.
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