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Hur bildades Bushveldkomplexet?

Ivarsson, Filip LU (2015) In Examensarbeten i geologi vid Lunds universitet GEOL01 20151
Department of Geology
Abstract
The Bushveld complex is the world’s largest mafic intrusion and located on the Archean Kaapvaal craton, southern Africa. The complex’s host rock has anomalous quantities of platinum-group elements (PGE), chromite and other valuable metals of high demand. The origin of the complex remains to this day discussed, despite a plethora of geochemical and petrological studies. There are in general four proposed models for Bushveld’s formation. One hypothesis propose that the magmatism was triggered by an impact event (from asteroids and/or comets) and are supported by the circular form, PDFs and impact melt breccia. Another model suggests a formation from a back-arc as structural features, flow direction of magma from the feeder dyke TML and high... (More)
The Bushveld complex is the world’s largest mafic intrusion and located on the Archean Kaapvaal craton, southern Africa. The complex’s host rock has anomalous quantities of platinum-group elements (PGE), chromite and other valuable metals of high demand. The origin of the complex remains to this day discussed, despite a plethora of geochemical and petrological studies. There are in general four proposed models for Bushveld’s formation. One hypothesis propose that the magmatism was triggered by an impact event (from asteroids and/or comets) and are supported by the circular form, PDFs and impact melt breccia. Another model suggests a formation from a back-arc as structural features, flow direction of magma from the feeder dyke TML and high δ37Cl and Cl/F ratio support this. A third model suggests a mantle plume, this is supported by the geomorphological structure, the geochemistry and the contamination of the magma by the crust. A later model is a combination between the mantle plume and a delamination of the mantle. Arguments for this model are the spatial relationship between the Transvaal Supergroup and the Bushveld Complex, seismic analysis and dykes that converge towards the complex. Lack of representative macro and microscopic indicators, along with paleomagnetism that suggests that the strata was horizontal, makes the meteorite impact model unlikely. Same goes for the mantle plume theory, due to the absence of radiation dyke swarms from 2.06 Ga BP. The delamination hypothesis explains the formation of the Bushveld complex well, however the age difference between the swarms is a counter-argument. Other counter-arguments are the presence of dykes south of the complex with a differing orientation and diamonds on shallow depths. There are arguments against the subduction hypothesis as well. The Bushveld complex is the only intrusion found in a sedimentary environment in which no calc-alkaline bedrock has been found. High δ18O could be explained by influx of sediment. No other model can explain the high chlorine ratios which leads to the conclusion that the Bushveld complex is related to some type of subduction processes. (Less)
Abstract (Swedish)
Bushveldkomplexet är beläget på den arkeiska kratonen Kaapvaal och är världens största mafiska intrusion. Den innehåller stora mängder platinagruppmetaller (PGE), kromit och andra eftertraktade metaller. Bildningen av komplexet är än idag omdiskuterat trots många tidigare studier. Generellt finns fyra förslagna modeller för komplexets bildning. En hypotes är att magmatismen har skett som ett resultat av nedslag från asteorider och/eller kometer, där den cirkulära formen, PDF (planar deformation features) och impaktbreccia är de främsta argumenten. Vulkanisk aktivitet bakom en aktiv kontinetal öbåge är en annan förslagen hypotes som baseras på observationer så som flödesriktningar utifrån TML (Thabazimbi–Murchison lineament) och höga δ37Cl... (More)
Bushveldkomplexet är beläget på den arkeiska kratonen Kaapvaal och är världens största mafiska intrusion. Den innehåller stora mängder platinagruppmetaller (PGE), kromit och andra eftertraktade metaller. Bildningen av komplexet är än idag omdiskuterat trots många tidigare studier. Generellt finns fyra förslagna modeller för komplexets bildning. En hypotes är att magmatismen har skett som ett resultat av nedslag från asteorider och/eller kometer, där den cirkulära formen, PDF (planar deformation features) och impaktbreccia är de främsta argumenten. Vulkanisk aktivitet bakom en aktiv kontinetal öbåge är en annan förslagen hypotes som baseras på observationer så som flödesriktningar utifrån TML (Thabazimbi–Murchison lineament) och höga δ37Cl och Cl/F värden. En tredje modell innebär att komplexet bildades av en mantelplym. Detta styrks av den geomorfologiska uppbyggnaden av komplexet samt geokemiska bevis. En senare modell bygger på en kombination av mantelplymimpakt och delaminering av manteln. Modellen styrks bl.a. av den spatiala relationen mellan Bushveldkomplexet och Transvaal bassängen, samt radierande gångsvärmar i öster vilka konvergerar in mot komplexet. Bristen av representativa makroskopiska och mikroskopiska indikatorer tillsammans med paleomagnetiska bevis gör meteoritmodellen till den minst troliga. Avsaknad av associerande gångsvärmar som är likåldriga, dvs. ca 2.06 Ga gör också mantelplym teorin mindre trolig. Delamineringshypotesen är en trolig bildningsprocess för Bushveldkomplexets. Det finns även andra gångsvärmar söder om komplexet med annan riktning tillsammans med diamant på litet djup. Argument mot subduktionshypotesen är att intrusioner med snarlika åldrar saknas i norra Kaapvaal. Modellen är dock förenlig med geokemisk data som indikerar kontaminering av magman. Då ingen annan modell kan förklara de höga klorvärdena verkar den rimligaste hypotesen för Bushveldkomplexets bildning vara subduktion. (Less)
Please use this url to cite or link to this publication:
author
Ivarsson, Filip LU
supervisor
organization
course
GEOL01 20151
year
type
M2 - Bachelor Degree
subject
keywords
chromite, intrusion, platinum, LIP, Bushveld Complex
publication/series
Examensarbeten i geologi vid Lunds universitet
report number
452
language
Swedish
id
7753123
date added to LUP
2015-08-13 11:17:01
date last changed
2015-08-13 11:17:01
@misc{7753123,
  abstract     = {The Bushveld complex is the world’s largest mafic intrusion and located on the Archean Kaapvaal craton, southern Africa. The complex’s host rock has anomalous quantities of platinum-group elements (PGE), chromite and other valuable metals of high demand. The origin of the complex remains to this day discussed, despite a plethora of geochemical and petrological studies. There are in general four proposed models for Bushveld’s formation. One hypothesis propose that the magmatism was triggered by an impact event (from asteroids and/or comets) and are supported by the circular form, PDFs and impact melt breccia. Another model suggests a formation from a back-arc as structural features, flow direction of magma from the feeder dyke TML and high δ37Cl and Cl/F ratio support this. A third model suggests a mantle plume, this is supported by the geomorphological structure, the geochemistry and the contamination of the magma by the crust. A later model is a combination between the mantle plume and a delamination of the mantle. Arguments for this model are the spatial relationship between the Transvaal Supergroup and the Bushveld Complex, seismic analysis and dykes that converge towards the complex. Lack of representative macro and microscopic indicators, along with paleomagnetism that suggests that the strata was horizontal, makes the meteorite impact model unlikely. Same goes for the mantle plume theory, due to the absence of radiation dyke swarms from 2.06 Ga BP. The delamination hypothesis explains the formation of the Bushveld complex well, however the age difference between the swarms is a counter-argument. Other counter-arguments are the presence of dykes south of the complex with a differing orientation and diamonds on shallow depths. There are arguments against the subduction hypothesis as well. The Bushveld complex is the only intrusion found in a sedimentary environment in which no calc-alkaline bedrock has been found. High δ18O could be explained by influx of sediment. No other model can explain the high chlorine ratios which leads to the conclusion that the Bushveld complex is related to some type of subduction processes.},
  author       = {Ivarsson, Filip},
  keyword      = {chromite,intrusion,platinum,LIP,Bushveld Complex},
  language     = {swe},
  note         = {Student Paper},
  series       = {Examensarbeten i geologi vid Lunds universitet},
  title        = {Hur bildades Bushveldkomplexet?},
  year         = {2015},
}