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Volume :3 Issue : 1 1976      Add To Cart                                                                    Download

The behaviour of some major and trace elements in feldspar phenocrysts from Zaker granitic rocks of Zanjan area, northwest Iran

Auther : ADEL M. REFAAT

Teachers Institute of Education, El-Odylia, Kuwait

 

ABSTRACT

 

Broadly speaking, the irregularity of K content in the investigated alkali feldspars and plagioclases is mainly due to the kaolinization and sericitization processes. The alkali feldspars examined have lost most of their Na content while Ca is not expelled from its host. The myrmekitic intergrowths around the alkali feldspars have resulted mainly from the interaction between the recrystallized albite and perthitic albite from the free Na with the excess silicons which are expelled from the alkali feldspars during exsolution. The Ga content in the feldspars varies to a large extent. It is interesting to note that the disordered alkali feldspars incorporate less Ga than the ordered albite crystals during the post-magmatic recrystallization. It is suggested that most of the Rb and K content is extracted from the sodic plagioclases during the sericitization process. The rate of this process decreases with the increase of bridge numbers of SiO2, which do not break down easily towards the end of the felsic member. Great similarity has been observed between the examined feldspars and Grimstad feldspars (Christie et al. 1970), in which the weight distribution coefficient of Sr is rather similar. It varies from 0.3 to 1.0 in the Zaker and Grimstad feldspars. The ratios of Sr in the coexisting feldspars indicate the temperature of crystallization at which these ratios increase as the temperature becomes high. According to Barth (1961), the ratio equals 1 at 450°C. The Sr ratios of the feldspars examined vary from 0.33 to 1.04, showing that these feldspars began to crystallize at 450°C, falling gradually towards the felsic end member. It is concluded also that the weight distribution coefficient of Ba in these feldspars (D = 7 to 20) decreases with the An content. The pink and red feldspars have high contents of iron oxides (haematite) which resulted mainly from the effects of late hydrothermal solutions. The strong alteration of biotite in the host rocks gave rise to ferric oxides which were redeposited along the fractures and cleavage planes of the studied feldspars.

 

 

 

 

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