Cone sheet

A cone sheet is a type of high-level igneous intrusion of subvolcanic rock, found in partly eroded central volcanic complexes. Cone sheets are relatively thin inclined sheets, generally just a few metres thick, with the geometry of a downward-pointing cone. Viewed from above, their outcrop is typically circular to elliptical. They were originally described from the Ardnamurchan, Mull and other central complexes of the British Tertiary Volcanic Province (now recognised as part of the North Atlantic Igneous Province).

A cone sheet at Mingary, Ardnamurchan, Scotland
Closer view of a cone sheet at Mingary, Ardnamurchan

Occurrence

Cone sheets are widely distributed at the lower levels of volcanic complexes.

Examples of cone sheet complexes
NameLocationAgeDominant rock typeReference
Ardnamurchan Scotland Paleogene dolerite [1][2]
Tejeda Gran Canaria Miocene trachyte, phonolite [3]
Vallehermoso La Gomera Miocene trachyte, phonolite [4]
Jabal Arknu Libya Tertiary [5]
Otoge Japan Miocene alkali basalt, trachyandesite [6]
Zarza Mexico Cretaceous gabbro [7]
Houshihushan China Cretaceous granite porphyry [8]
Boa Vista Cape Verde Miocene phonolite [9]
Ruri Hills Kenya Miocene carbonatite [10]
Bagstowe Queensland late Paleozoic rhyolite [11]
Thverartindur Iceland [12]
Tehilla Sudan Cambrian–Ordovician granite, monzonite [13]

Formation

Soon after cone sheets were first described, their formation was explained in terms of intrusion along conical fractures extending from the top of an intrusive body into the overlying rocks, caused by high magmatic pressure.[14][15]

References
  1. Geldmacher, Jörg; Haase, Karsten M.; Devey, Colin W.; Garbe-Schönberg, C. Dieter (1998). "The petrogenesis of Tertiary cone-sheets in Ardnamurchan, NW Scotland: petrological and geochemical constraints on crustal contamination and partial melting". Contributions to Mineralogy and Petrology. 131 (2–3): 196–209. doi:10.1007/s004100050388.
  2. Geldmacher, J.; Troll, V. R.; Emeleus, C. H.; Donaldson, C. H. (May 2002). "Pb-isotope evidence for contrasting crustal contamination of primitive to evolved magmas from Ardnamurchan and Rum: implications for the structure of the underlying crust". Scottish Journal of Geology. 38 (1): 55–61. doi:10.1144/sjg38010055. ISSN 0036-9276.
  3. Donoghue, Eleanor; Troll, Valentin R.; Harris, Chris (2010). "Fluid–Rock Interaction in the Miocene, Post-Caldera, Tejeda Intrusive Complex, Gran Canaria (Canary Islands): Insights from Mineralogy, and O- and H-Isotope Geochemistry". Journal of Petrology. 51 (10): 2149–2176. doi:10.1093/petrology/egq052.
  4. Rodriguez-Losada, J.A.; Martinez-Frias, J. (2004). "The felsic complex of the Vallehermoso Caldera: interior of an ancient volcanic system (La Gomera, Canary Islands)". Journal of Volcanology and Geothermal Research. 137 (4): 261–284. doi:10.1016/j.jvolgeores.2004.05.021.
  5. Tawadros, E. Edward (2011). Geology of North Africa. Boca Raton: CRC Press. p. 79. ISBN 9780415874205.
  6. Geshi, N. (2005). "Structural development of dike swarms controlled by the change of magma supply rate: the cone sheets and parallel dike swarms of the Miocene Otoge igneous complex, Central Japan". Journal of Volcanology and Geothermal Research. 141 (3–4): 267–281. doi:10.1016/j.jvolgeores.2004.11.002.
  7. Johnson, S. E.; Paterson, S. R.; Tate, M. C. (1999). "Structure and emplacement history of a multiple-center, cone-sheet–bearing ring complex: The Zarza Intrusive Complex, Baja California, Mexico". Geological Society of America Bulletin. 111 (4): 607–619. doi:10.1130/0016-7606(1999)111<0607:SAEHOA>2.3.CO;2.
  8. Wen, X.; Ma, C.; Mason, R.; Sang, L.; Zhao, J. (2015). "Subterranean origin of accreted lapilli in cone-sheets of the houshihushan sub-volcanic ring complex, Shanhaiguan, China". Journal of Earth Science. 26 (5): 661–668. doi:10.1007/s12583-015-0581-4.
  9. Ancochea, Eumenio; Huertas, María José; Hernán, Francisco; Brändle, José Luis (2014). "A new felsic cone-sheet swarm in the Central Atlantic Islands: The cone-sheet swarm of Boa Vista (Cape Verde)". Journal of Volcanology and Geothermal Research. 274: 1–15. doi:10.1016/j.jvolgeores.2014.01.010.
  10. King, B. C.; Le Bas, M. J.; Sutherland, D. S. (1972). "The history of the alkaline volcanoes and intrusive complexes of eastern Uganda and western Kenya". Journal of the Geological Society. 128 (2): 173–205. doi:10.1144/gsjgs.128.2.0173.
  11. Branch, C.D. (1959). Progress Report on Upper Palaeozoic Intrusions Controlled by Ring Fractures near Kidston, North Queensland (PDF). Bureau of Mineral Resources Geology and Geophysics, Department of National Development, Commonwealth of Australia.
  12. Klausen, M.B. (2004). "Geometry and mode of emplacement of the Thverartindur cone sheet swarm, SE Iceland". Journal of Volcanology and Geothermal Research. 138 (3–4): 185–204. doi:10.1016/j.jvolgeores.2004.05.022.
  13. Ahmed, F. (1977). "Petrology and Evolution of the Tehilla Igneous Complex, Sudan". Journal of Geology. 85 (3): 331–343. doi:10.1086/628303.
  14. Walter, Thomas R.; Troll, Valentin R. (2001-06-01). "Formation of caldera periphery faults: an experimental study". Bulletin of Volcanology. 63 (2): 191. doi:10.1007/s004450100135. ISSN 1432-0819.
  15. Magee C.; Stevenson C.; O'Driscoll B.; Schofield N.; McDermott K. (2012). "An alternative emplacement model for the classic Ardnamurchan cone sheet swarm, NW Scotland, involving lateral magma supply via regional dykes" (PDF). Journal of Structural Geology. 44: 73–91. doi:10.1016/j.jsg.2012.08.004.
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