Geological, Geochemical, and Petrogenetic Aspects of Late Neoproterozoic Younger Granites at Wadi Um Sidra-Wadi Um Asmer Area, North Eastern Desert, Egypt.

Abstract

The late Neoproterozioc younger granites in the study area are classified into monzogranites, syenogranites and alkali feldspar granites according to the field, petrography and geochemical investigations. Monzogranites are mainly exposed along W. Um Sidra at G. Abu Mesaid in the eastern parts of the study area, intruding the Dokhan volcanic with sharp contacts. Syenogranites are exposed at G. El Shagola, G. Abu Harba, G Abu Mesaid, and some parts along W. Um Sidra intrude the metavolcanics and Dokhan volcanics with sharp contacts and contain enclaves of them. Alkali feldspar granites are exposed at G. El Shaylah, G. Urf El Eir, G. El Hurus, and the western exposures of G. Abu Harba intruding the metavolcanics and Dokhan volcanics with sharp and irregular contacts. The K/Rb, Ba/Rb and Rb/Sr ratios for the studied granites indicate that they were originated from highly differentiated magma and have been formed by partial melting of granitic crustal rocks. These granites were generated in possible extension-related environment and in a post-collision granites environment. The post-collision granites have calc-alkalic affinites, metaluminous to slightly peraluminous, and exhibit most of the characteristics of highly fractionated I-type granites derived from tonalites (intermediate magmas) of crustal sources. The studied monzogranites and syenogranites have generated at moderate/high water pressure in the 3 -8 kbar and temperatures of about 660ºC -685ºC, while the alkali feldspar granites were crystallizated at low water vapor pressure about 1 kbar and temperature abour 660 ºC., suggesting crystallization at relatively shallow depth between 20 and 30 km. The crystal-liquid equilibrium was the dominant mechanism involved in the genesis of these granites. The monzogranites and syenogranites have been generated at greater depth >30 km of the lower crust.The overall decreasing trends in FeO t , MgO, CaO, TiO 2 , and P 2 O 5 with increasing SiO 2 coupled with increasing Rb, Y,Th and Nb and the fractionated nature of the REE [(La/Lu) n = 3.67-14.21] suggesting that the fractional crystallization was played the major role during the evolution of the studied granites.