PRE-POST COLLISIONAL PLUTONITES OF ARAIS, Eastern Desert of Egypt

PRE-POST COLLISIONAL PLUTONITES OF ARAIS AREA, EASTERN DESERT, EGYPT: GEOCHEMICAL 

CONCEPT

M. F. Ghoneim*, E. M. Lebda** and M.Z. Khedr**

*Geology department, Faculty of Science, Tanta University.

**Biological and Geological Department-Faculty of Education at Kafr El-Sheikh-Tanta University.

ABSTRACT

            Petrological and geochemical studies of Neoproterozoic rocks of Arais area, south Eastern Desert of Egypt has been carried out. They include both, metagabbros and Younger Granites of NE Pan-African belt. Geochemical studies using major, trace and REE aim to follow variation in their composition and genesis throughout their tectonic evolution. The studied metagabbros show magma of tholeiitic composition together with minor calc-alkaline affinity. They are belonging to low Ti-ophiolite and show transitional between ocean floor and island arc regime (back-arc tectonic setting). Spider diagrams of the studied metagabbros exhibit relative enrichment of HFSE, LILE, Sr and Ba. The Sr and Ba enrichment are due to plagioclase, biotite and amphibole accumulation. On the contrary, depletion of Ni and Cr suggests significant olivine and pyroxene fractionation.

         The Younger Granites in the present area are represented by Abu Bayt, Al- Hindusi and Arais granites. They are characterized mainly by peraluminous with slightly metaluminous compositions, originated from relatively potassic-sodic magma. They have alkaline to calc-alkaline magma affinity of A-type that developed during post orogenic granite at the initial stage of plate accretion. Abu Bayt and Al- Hindusi granites have been emplaced at depths ranging from 20 km to30 km, while Arais granite have emplaced at a relatively shallower depth than other types. The chondrite-noramalized of REE shows that, Arais granite has higher fractionated REE and HREE values than Al-Hindusi and Abu Bayt granites. The studied granites show enriched LREE, depleted HREE and large negative Eu anomaly indicating removal of plagioclase due to fractionation.

            Using Rayleigh fractional crystallization model of REE revealed that monzogranitic melt may be derived through 20% partial melting of lower crust followed by 60-80% fractional crystallization. Meanwhile, Arais alkali-feldspar granitic melt is derived through 55-75% fractional crystallization of monzogranitic parent melt.

To download full paper click here