干旱区生态环境知识资源中心

In an effort to understand the genesis of the pegmatitic rocks of El-Hudi and Um Hibal areas, detailed field, petrographic, and geochemical investigations were conducted. These investigations revealed evidence of two distinct modes of emplacement of these rocks. El-Hudi pegmatites are emplaced concordantly in a belt of folded gneisses and migmatites that mostly fit well with a metasedimentary protolith, which are later cataclased or even mylonitized. El-Hudi pegmatites can be classified into (1) plagioclase or sodic and (2) k-feldspar or potassic pegmatites. Their mineral assemblage, the variable forms of albite bodies in perthitized k-feldspars, and the myrmekite and graphic intergrowths are conclusive evidence indicating metasomatic and later stages of replacement. Geochemically, the peraluminous character of these pegmatites, compressional setting, partial melting parentage, and S-type affinity confirm their metasedimentary source and support a linkage with their host gneisses. The following stages explained the most accepted scenario for the development of these pegmatites "partial melting from metasedimentary protoliths, minimal fractionation and differentiation, injection into foliation, fractures and/or tectonic planes of the host gneisses, and invading the gneiss and migmatite hosts reflecting metasomatic exchanges, segregated earlier or contemporaneous with the genesis of garnet-bearing granites." In Um Hibal pegmatites, their vein-and miarolitic-type emplacements with the characteristic chilled margin of the later, the recorded hydrothermal nature of the well-preserved silica gels, and the well-crystallized accessory zircons revealed their magmatogenic origin. The similar behavior in depletion or enrichment of the rare earth element contents of Um Hibal pegmatites and their host granites indicate that they are genetically related. They have a mild to peralkaline affinity, emplaced in an extensional regime and shallow depth environment, of within plate and A(1)-type granite characters. However, the pegmatites seem to be fractionally crystallized from the residual melt of the host granites.