Portal de Periódicos da CAPES

MULTIPLE SILLS OR A LAYERED INTRUSION? TIME TO DECIDE

2012; Geological Society of South Africa; Volume: 115; Issue: 3 Linguagem: Inglês

10.2113/gssajg.115.3.283

1996-8590

R. Grant Cawthorn,

Geological formations and processes

Research Article| September 01, 2012 MULTIPLE SILLS OR A LAYERED INTRUSION? TIME TO DECIDE R.G. CAWTHORN R.G. CAWTHORN School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa, e-mail: grant.cawthorn@wits.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information R.G. CAWTHORN School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa, e-mail: grant.cawthorn@wits.ac.za Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2012 Geological Society of South Africa South African Journal of Geology (2012) 115 (3): 283–290. https://doi.org/10.2113/gssajg.115.3.283 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation R.G. CAWTHORN; MULTIPLE SILLS OR A LAYERED INTRUSION? TIME TO DECIDE. South African Journal of Geology 2012;; 115 (3): 283–290. doi: https://doi.org/10.2113/gssajg.115.3.283 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Below many old continental flood basalt provinces typically there are contemporaneous sills, which are commonly quite thin. Some sills may show internal chilled margins that suggest repeated injection, but only once the previous sill had largely solidified and cooled. Much rarer are very thick intrusive bodies that have cooled sufficiently slowly to allow a layered complex to develop. It is generally accepted that they are the result of multiple intrusions that occurred prior to a significant degree of crystallization of the previous magma body. The Karoo Large Igneous Province and the Bushveld Complex are archetypal examples in South Africa of these processes. Both have volumes in excess of 106 km3. A consideration of the rate of cooling and crystallization of intrusions may provide an answer as to why the two different products result. Of 160 accurately measured sill thicknesses in the Karoo (from borecore information) relatively few exceed 50 m, and some may be composite. A 50 m thick basic sill will become 50% solidified in 10 years. The possibility of a second injection into the centre of that body in less than 10 years is generally unlikely. For a 100 m thick sill, the time period for the same degree of cooling is 37 years. Once the first sill has cooled by 100°C and become 50% crystalline, a second injection will chill, and form a distinct body within the first. It will not inflate the magma portion of the first sill. Average calculated emplacement rates for both the Karoo dolerites and the Bushveld Complex are far too slow for a layered complex to have developed. However, if there were three or perhaps four injections of magma about 100 m thick (or perhaps more than five injections of 50 m thickness) within 100 years into the same body, then the rate of cooling of that composite body could become sufficiently slow that it would act as a trap for any subsequent magma injection and a thick layered intrusion would result. Intensely pulsed volcanic episodicity in igneous provinces is beginning to be identified with increasing eruption intensity and decreasing time lapses, based on extremely precise measurements of palaeomagnetic directions. Hence, there need be no fundamentally different mechanism for the origin of the two different resultant igneous suites, merely a statistical peak in magma injection rate for a short period of time. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.