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Petrology of Birimian granitoids in southern Ghana : petrography and petrogenesis

Grenholm, Mikael (2011) In Dissertations in Geology at Lund University
Department of Geology
Abstract (Swedish)
Den Paleoproterozoiska Birimiska terrinen består av vulkaniska och sedimentära bergarter i grönstensbälten och
metasedimentära bassänger som har blivit intruderade av två generationer granitoider. Den Birimiska terrinen utgör
en stor del av den västafrikanska kratonen. Den bildades under en period med omfattande juvenil magmatism som
skapade stora volymer med kontinental jordskorpa. Många frågor angående den tektoniska miljön i vilken denna
process ägde rum är fortfarande obesvarade. De två generationer av granitoider - grovt indelade i äldre bält- och
ocu yngre bassäng-typer - utgör en viktig nyckel till förståelsen för den Birimiska terranens geodynamiska utveckling.
Syftet med detta examensarbete var att bestämma i vilken... (More)
Den Paleoproterozoiska Birimiska terrinen består av vulkaniska och sedimentära bergarter i grönstensbälten och
metasedimentära bassänger som har blivit intruderade av två generationer granitoider. Den Birimiska terrinen utgör
en stor del av den västafrikanska kratonen. Den bildades under en period med omfattande juvenil magmatism som
skapade stora volymer med kontinental jordskorpa. Många frågor angående den tektoniska miljön i vilken denna
process ägde rum är fortfarande obesvarade. De två generationer av granitoider - grovt indelade i äldre bält- och
ocu yngre bassäng-typer - utgör en viktig nyckel till förståelsen för den Birimiska terranens geodynamiska utveckling.
Syftet med detta examensarbete var att bestämma i vilken tektonisk miljö granitoider - både bält- och bassängtyper
- från den Birimiska terranen i södra Ghana bildades.
Bält-granitoiderna bildades i en subduktions-miljö mellan 2232-2169 Ma. De har många likheter med TTGs såsom
att dem är Na-rika och uppvisar låga HREE-värden vilket indikerar att de bildades genom smältning av en subducerande
oceanskorpa. Dock uppvisar de också tecken på interaktion med mantel samt ett calc-alkalint beteende. De
verkar därför som om granitoiderna bildats genom en slab-smälta men där subduktionsvinkeln på oceanskorpan var
tillräckligt stor för att smältan skulle kunna reagera med mantel-kilen.
Bassäng-granitoiderna bildades genom krustal anatexis i samband med den Eburniska orogenesen och har åldrar
mellan 2134-2098 Ma. Undantaget Winneba-graniten så skedde smältningen i vatten-mättade förhållanden vilket
ledde till preferentiel smältning av plagioklas. Smältning kan ha skett i diagonal-förkastningar där vatten har tillförts
från dehydrerande sediment. Winneba-graniten bildades genom dehydreringssmältning. Detta kan förklara
varför den är yngreän närliggande bassäng-granitoider då dehydrerings-smältning äger rum vid högre temperaturer
än vatten-mättad smältning.
Metamorfos i Birimiska terrinen under den Eburniska orogensen uppgick till grönskiffer-facies. Två granitoider
från Sefwi- och Ashanti-bältena har blivit kraftigt omvandlade av hydrothermala fluider, möjligen i anslutning med
bildandet av hydrotermala guld-mineraliseringar. (Less)
Abstract
The Paleoproterozoic Birimian terrane consists of volcanic and sedimentary rocks occurring in greenstone belts and
metasedimentary basins that have been intruded by two generations of granitoids. The Birimian terrane comprises a
large part of the West African craton. It formed during a period of extensive juvenile magmatism that led to the
creation of a large area of continental crust. Many questions remain unanswered regarding the tectonic setting in
which this crust-forming event occurred. The two generations of granitoids—broadly divided into older belt and
younger basin types - are an important key to understanding the geodynamic evolution of the Birimian terrane. The
purpose of this thesis has been to determine in what tectonic... (More)
The Paleoproterozoic Birimian terrane consists of volcanic and sedimentary rocks occurring in greenstone belts and
metasedimentary basins that have been intruded by two generations of granitoids. The Birimian terrane comprises a
large part of the West African craton. It formed during a period of extensive juvenile magmatism that led to the
creation of a large area of continental crust. Many questions remain unanswered regarding the tectonic setting in
which this crust-forming event occurred. The two generations of granitoids—broadly divided into older belt and
younger basin types - are an important key to understanding the geodynamic evolution of the Birimian terrane. The
purpose of this thesis has been to determine in what tectonic setting granitoids - both belt and basin types - from the
Birimian terrane in southern Ghana were emplaced in.
The belt type granitoids were emplaced in a subduction setting between 2232-2169 Ma. They share many similarities
with TTGs such as being sodic and HREE-depleted indicating that they were derived from a slab melt. However,
they also show variable interaction with the mantle as well as a calc-alkaline behavior. It would therefore appear
that the granitoids originated as slab melts but were the angle of the subducting slab was steep enough to allow the
melt to interact with the mantle wedge.
The basin granitoids formed through crustal anatexis in association with the Eburnean orogeny and have ages between
2134-2098 Ma. With the exception of the Winneba granitoid melting occurred in water-saturated conditions
leading to preferential melting of plagioclase. Melting may have initiated in transcurrent deformation zones where
water was supplied from dehydrating sediments. The Winneba granitoid formed through dehydration melting. This
may explain why it is younger then adjacent basin granitoids given that dehydration melting occurs at higher temperatures
then water-saturated melting.
During the Eburnean orogeny the Birimian terrane was subjected to greenschist facies metamorphism. Two granitoids
from the Sefwi and Ashanti belt has been extensively altered by hydrothermal fluids, possibly in association
with the formation of hydrothermal gold mineralizations. (Less)
Please use this url to cite or link to this publication:
author
Grenholm, Mikael
supervisor
organization
alternative title
Petrologi på Birimiska granitoider i södra Ghana : petrografi och petrogenes
year
type
M2 - Bachelor Degree
subject
keywords
Ghana, Birimian, granitoids, subduction, TTG, water-saturated melting, transcurrent deformation
publication/series
Dissertations in Geology at Lund University
report number
285
language
English
id
2278009
date added to LUP
2012-01-09 12:09:00
date last changed
2014-10-24 03:44:45
@misc{2278009,
  abstract     = {The Paleoproterozoic Birimian terrane consists of volcanic and sedimentary rocks occurring in greenstone belts and
metasedimentary basins that have been intruded by two generations of granitoids. The Birimian terrane comprises a
large part of the West African craton. It formed during a period of extensive juvenile magmatism that led to the
creation of a large area of continental crust. Many questions remain unanswered regarding the tectonic setting in
which this crust-forming event occurred. The two generations of granitoids—broadly divided into older belt and
younger basin types - are an important key to understanding the geodynamic evolution of the Birimian terrane. The
purpose of this thesis has been to determine in what tectonic setting granitoids - both belt and basin types - from the
Birimian terrane in southern Ghana were emplaced in.
The belt type granitoids were emplaced in a subduction setting between 2232-2169 Ma. They share many similarities
with TTGs such as being sodic and HREE-depleted indicating that they were derived from a slab melt. However,
they also show variable interaction with the mantle as well as a calc-alkaline behavior. It would therefore appear
that the granitoids originated as slab melts but were the angle of the subducting slab was steep enough to allow the
melt to interact with the mantle wedge.
The basin granitoids formed through crustal anatexis in association with the Eburnean orogeny and have ages between
2134-2098 Ma. With the exception of the Winneba granitoid melting occurred in water-saturated conditions
leading to preferential melting of plagioclase. Melting may have initiated in transcurrent deformation zones where
water was supplied from dehydrating sediments. The Winneba granitoid formed through dehydration melting. This
may explain why it is younger then adjacent basin granitoids given that dehydration melting occurs at higher temperatures
then water-saturated melting.
During the Eburnean orogeny the Birimian terrane was subjected to greenschist facies metamorphism. Two granitoids
from the Sefwi and Ashanti belt has been extensively altered by hydrothermal fluids, possibly in association
with the formation of hydrothermal gold mineralizations.},
  author       = {Grenholm, Mikael},
  keyword      = {Ghana,Birimian,granitoids,subduction,TTG,water-saturated melting,transcurrent deformation},
  language     = {eng},
  note         = {Student Paper},
  series       = {Dissertations in Geology at Lund University},
  title        = {Petrology of Birimian granitoids in southern Ghana : petrography and petrogenesis},
  year         = {2011},
}