Sill (geology)
In geology, asillis a tabularsheet intrusionthat hasintrudedbetween olderlayersofsedimentary rock,beds of volcaniclavaortuff,or along the direction offoliationinmetamorphic rock.A sill is aconcordant intrusive sheet,meaning that it does not cut across preexisting rock beds. Stacking of sills builds a sill complex[1]and a largemagma chamberat highmagma flux.[2]In contrast, adikeis a discordant intrusive sheet, which does cut across older rocks.
Formation[edit]
Sills are fed by dikes,[3]except in unusual locations where they form in nearly vertical beds attached directly to a magma source. The rocks must be brittle and fracture to create the planes along which the magma intrudes the parent rock bodies, whether this occurs along preexisting planes between sedimentary or volcanic beds or weakened planes related tofoliationin metamorphic rock. These planes or weakened areas allow the intrusion of a thin sheet-like body of magma paralleling the existing bedding planes, concordant fracture zone, or foliations. Sills run parallel to beds (layers) and foliations in the surroundingcountry rock.They can be originally emplaced in a horizontal orientation, although tectonic processes may cause subsequent rotation of horizontal sills up to near vertical orientations.
Sills can be confused with solidified lava flows; however, there are several differences between them. Intruded sills will show partial melting and incorporation of the surrounding country rock. On both contact surfaces of the country rock into which the sill has intruded, evidence of heating will be observed (contact metamorphism). Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows will typically show evidence ofvesicles(bubbles) where gases escaped into the atmosphere. Because sills form below the surface, even though generally at shallow depths (up to a few kilometers),[4]the pressure of overlying rock means few if any vesicles can form in a sill. Lava flows will also typically show evidence ofweatheringon their upper surface, whereas sills, if still covered by country rock, typically do not.
Associated ore deposits[edit]
Certainlayered intrusionsare a variety of sill that often contain importantoredeposits.Precambrianexamples include theBushveld,Insizwaand theGreat Dykecomplexes of southern Africa; and theDuluth intrusive complexalongLake Superior,and theStillwater igneous complexof the United States.Phanerozoicexamples are usually smaller and include theRùmperidotitecomplex[5]ofScotlandand theSkaergaard igneous complexof eastGreenland.These intrusions often contain concentrations ofgold,platinum,chromiumand other rare elements.
Transgressive sills[edit]
Despite their concordant nature, many large sills change stratigraphic level within the intruded sequence, with each concordant part of the intrusion linked by relatively short dike-like segments. Such sills are known astransgressive.The geometry of large sill complexes in sedimentary basins has become clearer with the availability of 3Dseismic reflectiondata.[6]Such data has shown that many sills have an overall saucer shape and that many others are at least in part transgressive.[7]
Examples include theWhin Silland sills within theKaroo basin.[8][9]
See also[edit]
References[edit]
- ^Leuthold J.; Müntener O.; Baumgartner L.; Putlitz B. (2014)."Petrological constraints on the recycling of mafic crystal mushes and intrusion of braided sills in the Torres del Paine Mafic Complex (Patagonia)".Journal of Petrology.55(5): 917–949.Bibcode:2014JPet...55..917L.doi:10.1093/petrology/egu011.hdl:20.500.11850/103136..
- ^Annen C.; Blundy J.D.; Leuthold J.; Sparks R.S.J. (2015). "Construction and evolution of igneous bodies: Towards an integrated perspective of crustal magmatism".Lithos.230:206–221.Bibcode:2015Litho.230..206A.doi:10.1016/j.lithos.2015.05.008.
- ^Meade, F. C.; Chew, D. M.; Troll, V. R.; Ellam, R. M.; Page, L. M. (2009-12-01)."Magma Ascent along a Major Terrane Boundary: Crustal Contamination and Magma Mi xing at the Drumadoon Intrusive Complex, Isle of Arran, Scotland".Journal of Petrology.50(12): 2345–2374.doi:10.1093/petrology/egp081.ISSN0022-3530.
- ^Bell, B.; Butcher, H. (2002). "On the emplacement of sill complexes: evidence from the Faroe-Shetland Basin".Geological Society, London, Special Publications.197(1): 307–329.Bibcode:2002GSLSP.197..307B.doi:10.1144/GSL.SP.2002.197.01.12.S2CID128669544.
- ^Emeleus, C. H.; Troll, V. R. (2014-08-01)."The Rum Igneous Centre, Scotland".Mineralogical Magazine.78(4): 805–839.Bibcode:2014MinM...78..805E.doi:10.1180/minmag.2014.078.4.04.ISSN0026-461X.
- ^Thomson K.; Hutton D. (2004). "Geometry and growth of sill complexes: insights using 3D seismic from the North Rockall Trough".Bulletin of Volcanology.66(4): 364–375.Bibcode:2004BVol...66..364T.doi:10.1007/s00445-003-0320-z.S2CID128713713.
- ^Planke S.; Rasmussen T.; Rey S.S.; Myklebust R. (2005). Doré A.G.; Vining B.A. (eds.).Seismic characteristics and distribution of volcanic intrusions and hydrothermal vent complexes in the Vøring and Møre basin.Petroleum geology: north-west Europe and global perspectives: Proceedings of the 6th petroleum geology conference held at the Queen Elizabeth II Conference Centre, London 6–9 October 2003.ISBN9781862391642.
- ^Hamilton M.A.; Pearson D.G. (2011). Srivastava R. (ed.).Precise U-Pb Age for the Great Whin Dolerite Complex, N.E. England and its Significance.Dyke Swarms: Keys for Geodynamic Interpretation. Springer Science & Business Media.ISBN9783642124969.
- ^Polteau S.; Mazzini A.; Galland O.; Planke S.; Malthe-Sørenssen A. (2008)."Saucer-shaped intrusions: Occurrences, emplacement and implications".Earth and Planetary Science Letters.266(1–2): 195–204.Bibcode:2008E&PSL.266..195P.doi:10.1016/j.epsl.2007.11.015.