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Estimation of Temperature, Pressure and Oxygen Fugacity of the Cal-Alkaline Basin-Type Granitoids in the Winneba Area, Ghana
並列摘要
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 Ab_58 An_1.7 Or_0.45 to Ab_98 An_41 Or_41 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 677℃ 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.