Distinction of different syn- and late orogenic Pan-African sedimentary sequences by metamorphic grade and illite crystallinity (W. Zeidun, Central Eastern Desert, Egypt)

Outcrops of molasse-type sediments and underlying back-arc sedimentary basins in the Pan- African arc terrane of the central Eastern Desert basement of Egypt were studied in the area west of W. Zeidun and its southern tributaries W. Maasar and W. Meesar. Both field and petrographic investigations revealed that the rocks, Eastern and Northeastern of G. El Shalul, are composed mainly of a thick sequence of green, foliated metaconglomerates, pebbly metamudstone with flattened fragments of fine tuffs, mudstone and intermediate to acidic volcanics embedded in a silty matrix. Illite "crystallinity" (IC) is expressed as the width of the 10 Å peak of illite detected by X-ray diffraction. It relies on the fact that the 10 Å peak progressively decreases in width from upper diagenesis to low grade metamorphism. Measurement of changes in IC, therefore, provides a useful indicator of variations within very low grade metamorphism, perhaps related to structure, stratigraphy, and thermal conditions.