THE GEOCHEMISTRY OF FERROUS BIOTITE AND PETROGENESIS OF WADI EL-SHEIKH GRANITOID ROCKS SOUTHWESTERN SINAI, EGYPT

The granitic plutonism in the area around Wadi: El-Sheikh, Southwester n Sinai, Egypt is mainly represented by the Older and Younger Granitoi d Rocks. The younger granitic rocks comprise in turn, two granitic pha ses, distinguished on a mineralogical and chemical basis. Biotites sep arated from the studied granitoid rocks have been examined. The chemic al data of 14 new analyzed biotites, from both the Younger and the Old er Granitoids show that the biotites are ferrous iron varieties. The s ignificance of the Fe(t)/(Fe(t)+Mg) ratio as a relative measure of bio tite crystallization suggests that the biotites may be formed under a temperature range of 685-degrees to 742-degrees-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 gra nitoid 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 decreasin g solidification index and as differentiation proceeds. The significan ce of the oxidation ratio of the granitoid rocks is elucidating the po ssible conditions of evolution of these rocks. The biotites in the two granitic cycles can be discriminated by trace elements and show seque ntial change through the presumed differentiation series.