The geochemistry of ferrous biotite and petrogenesis of Wadi-El-Sheikh granitoid rocks Southwestern Sinai, Egypt

Abstract

The granitic plutonism in the area around Wadi: El-Sheikh, Southwestern Sinai, Egypt is mainly represented by the Older and Younger Granitoid Rocks. The younger granitic rocks comprise in turn, two granitic phases, distinguished on a mineralogical and chemical basis.

Biotites separated from the studied granitoid rocks have been examined. The chemical data of 14 new analyzed biotites, from both the Younger and the Older Granitoids show that the biotites are ferrous iron varieties.

The significance of the Fe_t/(Fe_t+Mg) ratio as a relative measure of biotite crystallization suggests that the biotites may be formed under a temperature range of 685° to 742°C which indicate that the more iron rich biotite crystallizing with fall of temperature and with decrease of oxygen fugacity. Relations between the values of iron and magnesium of biotite and the solidification index of the host granitoid rocks show that the values of iron are approximately higher in biotites than in the host granitic rocks and that both granitoid rocks and biotites become gradually iron-rich magnesium poor with decreasing solidification index and as differentiation proceeds. The significance of the oxidation ratio of the granitoid rocks is elucidating the possible conditions of evolution of these rocks.

The biotites in the two granitic cycles can be discriminated by trace elements and show sequential change through the presumed differentiation series.