PETROGENESIS AND TECTONIC EVOLUTION OF THE RING COMPLEXES IN SOUTH EASTERN DESERT, EGYPT: A CASE STUDY FROM EL-GEZIRA ALKALINE ASSOCIATION

Abstract

El-Gezira ring complex represents an important alkaline association in the south Eastern Desert, Egypt display crescent-shaped bodies and vast diversity of lithologies. It consists of several concentric rings separated by arch-shaped bodies of alkali gabbros, pyroclastics and hybrid rocks. The outer ring of El Gezira is composed of metavolcanics and basaltic andesitic flows. The inner part comprises carbonate, pyroxene bearing syenite, monzosyenite, trachyte, trachy-andesite and trachydacite. Clinopyroxenes are the prevailing mafics components in most rocks varieties. Amphiboles and olivine are common in most basic units. El Gezira ring complex comprises a high fractures frequency. El Gezira ring complex is one of a major cluster of ring complexes aligned along an ENE transform fault in the extreme southern part of Egypt. Chemically, El Gezira rock-associations are mostly metaluminous, and show low concentrations of CaO, MgO and Sr. Moreover alkalis, Rb, Nb, Y, and REE are relatively high which is distinctive of anorogenic affinity. The alkali gabbros have flat REE patterns with slight positive Eu anomalies. The trachyandesite, trachydacite and monzosyenite all have weakly to moderately fractionated patterns with a general enrichment in LREE, and mostly possess a pronounced positive and negative Eu anomalies. Typically, El-Gezira alkaline associations are classified as within plate granites. It is suggested that the primitive mantle mafic magmas were emplaced into intracrustal magma chambers during extensional regime. The primitive magma underwent fractionation processes to give the highly evolved liquids. Influx of more hot primitive magma may promote dynamic convection, crustal assimilation and eruption of the volcanic alkaline association filling fractures during post orogenic regimes.