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Abstract
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Article Information:
Estimation of Temperature, Pressure and Oxygen Fugacity of the Cal-Alkaline Basin-Type Granitoids in the Winneba Area, Ghana
Nyarko Saah Esther, D.K. Asiedu, Dampare Samuel, Osae Shiloh, Sakyi Patrick and Hanson John
Corresponding Author: Nyarko Saah Esther
Submitted: 2011 May, 09
Accepted: 2011 June, 28
Published: 2012 January, 01 |
Abstract:
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In Ghana, the granitoids rocks are in two different groups, thus the basin type granitoid and the belt
type granitoids. These granitoids have been studied petrographycally mineralogically and geochemically,
especially the belt-type granitoids since it is the major host of gold occurrences in Ghana, however there are
little studies on the petrogenesis and emplacement conditions of this important rock. In this paper we present
the lacking knowledge on the emplacement conditions (temperature, pressure and the oxygen fugacity) of the
basin-type granitoids in the winneba area of Ghana.The calc-alkaline basin-type granitoids in the Winneba area
is composed of quartz+ plagioclase +potash feldspar+ alkali feldspar+ Biotite+ Hornblende +amphibole+
titanite. The plagioclase composition ranges from Ab58 An1.7 Or0.45 to Ab98 An41 Or41 and mainly in the field of
albite and oligoclase. The amphiboles however have a compositional range of Mg/(Mg +Fe) ranging from 0.52
to 0.62 and a Si content of 7.1 to 7.4 atom per formula unit (afu). Their end-member compositions in the
classification diagram are controlled by Magnesio-Hornblend, tschermakitic-Hornblend, and tschemakite
substitution types. Scanning Electron Microscope (SEM) analyses of coexisting hornblende and plagioclase
(hornblende-plagioclase thermometry), Al content in hornblende (aluminum-in-hornblende barometry) and the
assemblage titanite-magnetite-quartz were used to constrain the P, T and fO2 during the crystallization of the
parent magmas. The estimated temperature indicated an average temperature of crystallization of 677ºC
reflecting late crystallization from highly oxidized magma (log fO2 -20 bars).The rocks were emplaced at an
average pressure of 2.2 kbars corresponding to approximately 8 km depth of below the crust.
Key words: Amphibole, barometry, crystallization, hornblende, magma, plagioclase, thermometry
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Cite this Reference:
Nyarko Saah Esther, D.K. Asiedu, Dampare Samuel, Osae Shiloh, Sakyi Patrick and Hanson John, . Estimation of Temperature, Pressure and Oxygen Fugacity of the Cal-Alkaline Basin-Type Granitoids in the Winneba Area, Ghana. Research Journal of Environmental and Earth Sciences, (01): 41-47.
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ISSN (Online): 2041-0492
ISSN (Print): 2041-0484 |
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