研究業績

The Neoproterozoic Gharib Granitoid Complex (GGC), in the North Eastern Desert (NED) of Egypt, is characterized by the occurrence of varieties of granitoids. Monzogranite, the most abundant granite variety, contains abundant magmatic microgranular enclaves (MMEs) of mafic to intermediate composition. The MMEs are more mafic than their host and marked by a greater amount of amphibole and biotite. They are finer grained and have porphyritic/poikilitic textures. The sharp, crenulated, and fine-grained quenched contacts of the elongated, rounded, and ellipsoid form of the MMEs with their felsic host indicate rapid cooling during a magma mixing/mingling event. Micro-textural evidence supports the mechanical transfer of minerals that crystallized in felsic magma to the mafic melt (e.g., feldspar phenocrysts cross-cutting the MME-host boundaries, stubby apatite, quartz ocelli, and poikilitic outermost rims of K-feldspar phenocrysts), supercooling/quenching process (e.g., acicular apatite and elongated hornblende and biotite crystals), and disequilibrium growth (e.g., hornblende-biotite clots replaced early crystallized clinopyroxene), which also favour a magma mingling/mixing origin. Linear to curvilinear chemical variations of the MMEs and the host monzogranites on Harker diagrams, highly similar trace element contents, and Sr-Nd isotopic systematics together with mineral-chemical features also support mixing/mingling as the main process in magma genesis and strongly suggest that the MMEs were supercooled hybrid globules within cooler, partially crystallized host felsic magma. The crust-like geochemical signatures (i.e., SiO2 content, metaluminous nature, and lack of upper crustal xenoliths and typical peraluminous minerals), along with low [MMEs: (87Sr/86Sr)initial = 0.70136?0.70373; monzogranites: (87Sr/86Sr)initial = 0.70202?0.70320)] along with zircon and apatite U-Pb age data indicate the possible involvement of juvenile lower continental crust (LCC) of the Arabian-Nubian Shield (ANS) in their genesis. The medium- to high-K calc-alkaline affinity, the LILE enrichment, and the HFSE depletion along with low Nb/La ratios and positive εNd(t) (MMEs = +4.01 - +5.62, monzogranites = +2.29 - +6.03) indicate the contribution of depleted lithospheric mantle in the magma genesis, whereas the low δEu and fairly high δCe values, stable La/Sm, and variable Sr/Th ratios in apatites from the monzogranites and MMEs indicate the contribution of volatile/fluids from the remnants of the oceanic slab in facilitating melting in the source region. The genesis of the monzogranites and MMEs started with asthenosphere upwelling, followed by melting of the remnants of lithospheric mantle, and underplating of mantle magma, which led to partial melting of the LCC. Multi-element modelling postulates that the low degree mixing between partially crystallized mantle magma and LCC-derived felsic melt (Fmix ? 0.1) could produce the parent magma for the monzogranites. Further mixing/mingling of the crystal-charged monzogranite parent magma with new pulses of mafic melts (Fmix ? 0.25) developed a hybrid zone and formed the hybrid MMEs. Thus, the final episode of the ANS evolution was associated with a complex post-collisional interplay between asthenosphere, lithospheric mantle, and the LCC caused by the removal of delaminated lithospheric root and dense lower crust following the collision between East and West Gondwana in the Neoproterozoic time.