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 Fe2þ4Na Fe3þ. 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. |
