Petrogenesis of cogenetic silica-oversaturated and -undersaturated syenites of Abu Khruq ring complex, South Eastern Desert, Egypt

Abstract

The upper Cretaceous Abu Khruq ring complex (ARC) is located in the South Eastern Desert of Egypt displays concentric zonation of syenitic rocks from quartz-rich syenite at the margin, through alkali feldspar syenite to nepheline syenite in the centre. The syenitic rocks occur with nepheline monzogabbro, volcanic rocks (phonolite and trachyte) and the quartz- and nepheline-bearing pegmatites. Rocks of contrasting composition (mafic and salic) exhibit sophisticate geometric relationships. The nepheline monzogabbroic rocks have pillowy xenoliths forms within the salic (nepheline syenite and quartz alkali feldspar syenite) rocks, suggesting synchronous emplacement of the mafic and salic magmas. Clinopyroxene analysis of mafic and salic plutonic rocks of the ARC revealed that the overall pyroxene trend suggesting that fractionation involved a late, progressive increase in Na, in a reaction of the type Ca Mg Fe2+↔Na Fe3+. The chemistry of the analysed amphiboles are compositionally similar to those from typical differentiated peralkaline suites. Geochemically, the complex is enriched in the LILE, HFSE and REE. The concentrations of the compatible elements (V, Sr and Ba) generally decrease with increasing silica, consistent with fractional crystallization. A generalised increase in the Nb/Ta from the nepheline monzogabbro to nepheline syenite compositions is attributed to titanite fractionation. All the rock samples show relative increment of the LREE content than the HREE indicating weak to steep fractionated REE patterns (La/Yb) from 9.43 to 10.86, and thus retaining the geochemical characteristics of anorogenic suites. The magma sources of ARC are not derived from normal primitive mantle. The early stages of differentiation involved extensive olivine and pyroxene fractionation, the fractionation of amphibole, titanite, magnetite, apatite and feldspar may have been involved in the genesis of the salic differentiated compositions. The deviation towards silica-oversaturated members attributed to crustal assimilation and associated with fractional crystallization of the basic magma at a relatively early stage during the development of the complex. The whole complex is deduced to have originated from alkali basaltic to basanitic magma of OIB-like character. The general increasing of the amphibole and biotite among the different rocks of the complex reflects the hydrous nature of ARC magmas.