Petrography, geochemistry and petrogensis of the Pan-African basement rocks in the area around Gabal Hamrat Salma – Wadi Shait terrain,
South Eastern Desert, Egypt.
Mehanna, M. A., Rashwan, A. A., Abu El-Farah and Abd El-Nasser, A.
Geology Department, Faculty of Science, Benha University, Egypt
Abstract
The present work is a field, petrographic, geochemical and petrogenetic study of the rocks forming the area around Gabal Hamrat Salma, immediately west of Migif-Hafafit tectonic window - Wadi Shait domain, (SED).
Two distinct rock terrains were identified in the studied area; the contact between these two terrains is marked by a low-angle thrust. The footwall of this thrust or the lower tectonic unit comprises from the para- and ortho-gneisses and associated mafic-ultramafic rocks. On the other hand, the hangingwall or the upper tectonic unit includes the low-grade rocks such as serpentinites, metapyroclastics and calc-alkaline metagabbros. The rocks of the lower tectonic unit are; metapyroxenites and their derivatives have ultimate relation with the studied foliated metagabbros which pertaining to an ophiolite assemblage. As well as their mode of occurrence and petrographic studies revealed that, they are quit similar to those of Hafafit which described previously as dismembered ophiolite.
The foliated metagabbros which have tholeiitic affinity and are MORB-island-arc tholeiite in character, that were generated in a back-arc basins environment. They are overlain by psammitic gneisses of sedimentary origin which were deposited in an active continental margin. The magmatic activity in the present area started with the calc-alkaline granitic intrusions. These rocks were developed under an environment of compression, above a subduction zone, and evolved in a volcanic-arc setting.
These rocks have been subjected to the (T & P) conditions of amphibolite facies which overprinted by retrograde metamorphism within the greenschist facies conditions.
The rocks of the upper tectonic unit are the serpentinite rocks have been emplaced tectonically and do not show intrusive contacts with the surrounding rocks. They were derived from harzburgite nearly similar to the alpine type peridotite which generated in supra-subduction zone environment. The metapyroclastic rocks constitute the major component of the ophiolitic mélange area. The non-foliated metagabbros are intruded within the metapyroclastic rocks; they have calc-alkaline affinity and were developed in an island-arc setting. These rocks have been affected by low-grade metamorphism within the greenschist facies conditions.
Accepted and published in the 9th International conference on Geological Arab World (GAW 9), Cairo University, Cairo, Egypt. (March 2008)
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