Geochemistry and petrogenesis of El Gezira ring complex, Egypt: a monzosyenite cumulate derived from fractional crystallization of trachyandesitic magma

Abstract

El Gezira ring complex (GRC) is a high-level Permo-Triassic intrusion related to the alkaline province of southern Egypt. It comprises an older, outer ring of alkali basalt flows and agglomerates and an inner ring of monzosyenite. Crescent-shaped bodies of alkali gabbros separate the two rings. The core of the complex is dominated by monzosyenite and volcanic flows which range in composition from trachyandesite to trachydacite. The monzosyenite constitutes a major part of the complex and rarely grades to mafic-rich segregations. Mg-rich augites are the dominant mafic minerals in most rock types of GRC. Olivine is common in basic rocks, whereas secondary amphiboles and annitic biotite are more abundant in intermediate-felsic rocks. The rocks of GRC are metaluminous and exhibit geochemical features of A-type anorogenic magmatism such as high Fe/Mg ratio, total alkalis, Nb, Zr and Y contents with concomitant low values of Ca, Mg and Sr. The basic rocks have flat primitive REE patterns with no Eu anomalies suggesting that they probably represent the parental basaltic melt from which the complex was evolved. The trachyandesite, trachydacite and the monzosyenite all have steep LREE-enriched patterns with no enrichment in HREE. Pronounced positive Eu anomalies (Eu/Eu *=1.83–3.50) are characteristic for the monzosyenites on which, together with remarkably high Ba contents indicate the role of feldspar accumulation. The trachyandesite and trachydacite exhibit large negative Eu anomalies (Eu/Eu *=0.45–0.75; 0.72) indicating the dominant separation of feldspars. Field and geochemical features of the complex supported by mass-balance calculations of major elements and REE modelling suggest two distinct stages of fractional crystallization in the evolution of the GRC. The first stage includes crystal-melt fractionation dominated by clinopyroxene, calcic plagioclase and olivine from the basaltic source to yield the trachyandesitic magma. Further, fractional crystallization of the trachyandesitic melt with the alkali feldspar as dominant separating phase and minor contribution of clinopyroxene, biotite, Fe–Ti oxides and apatite yielded the more evolved felsic trachydacite (stage II). The cumulus mineralogy and its modelled REE abundances of stage II closely match the modal and REE compositions of the monzosyenite, which reinforces the cumulate origin of the rock.