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- Abu El-Rus, Mohamed A., et al.
(författare)
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Mueilha rare metals granite, Eastern Desert of Egypt : An example of a magmatic-hydrothermal system in the Arabian-Nubian Shield
- 2017
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937. ; 294-295, s. 362-382
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Tidskriftsartikel (refereegranskat)abstract
- The Mueilha granite pluton is one of the rare-metals bearing peraluminous granitic plutons in the Arabian-Nubian Shield. It represents the apical part of a highly evolved magma chamber emplaced at a shallow level subsequent to the post Pan-African orogeny. The pluton can be seen as a highly leucocratic medium-grained albite/oligoclase framework infilled with quartz, K-feldspar and muscovite that are variably overgrown by K-feldspar, muscovite, quartz and topaz megacrysts. The increasing number and size of the K-feldspar megacrysts at the expense of the whitened albite/oligoclase framework imparts variably red color to the Mueilha granite. Contacts between the milky white and red granites are usually gradational, but may be locally sharp or may form narrow transition zones resulting from abrupt variations in texture and lithology. Textural relations indicate an initial stage of hydrothermal albitization of magmatic plagioclase and crystallization of topaz megacrysts resulting from infiltration of Na-rich fluorine bearing fluids. A subsequent stage of metasomatic enrichment is characterized by extensive growth of large K-feldspar, quartz and muscovite megacrysts at the expense of the albite/oligoclase crystals as a result of infiltration of K-Si rich hydrous fluids. Post-magmatic infiltration of hydrous fluids along the fault planes is shown by the intense replacement of alkali feldspar megacrysts by quartz, development of myrmekitic intergrowth pockets along the K-feldspar megacrysts and sealing of the micro-fractures by cryptocrystalline mixtures of clay minerals, iron oxides, sericite and chlorite. Compositionally, the red granitic rocks have higher SiO2, Fe2O3total, K2O/Na2O, Σ REE, Y, Th, U, Zr and Zn and lower Al2O3, Ga, Ta, Nb and Mo compared to the milky white granites. LILE and Sn do not show clear variation trends throughout the Mueilha granite pluton, suggesting their immobility during hydrothermal alteration. Microthermometric measurements indicate that the interactions with the hydrothermal fluids started at a minimum temperature > 400°C, most likely during the late-stage crystallization of the Mueilha granite and continued after their complete solidification (i.e. subsolidus conditions) at a temperature as low as 120 °C. The high fertility of Mueilha granite is most plausibly the result of partial melting within the undepleted juvenile crust of the Arabian–Nubian Shield that has formed during the Pan-African orogeny.
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| 2. |
- Khudeir, Ali A, et al.
(författare)
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On the cratonization of the Arabian-Nubian Shield: Constraints from gneissic granitoids in south Eastern Desert, Egypt.
- 2021
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Ingår i: Geoscience Frontiers. - : Elsevier. - 1674-9871. ; 12, s. 1-29
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Tidskriftsartikel (refereegranskat)abstract
- The Shaitian granite complex (SGC) spans more than 80 Ma of crustal growth in the Arabian–Nubian Shield insoutheast Egypt. It is a voluminous composite intrusion (60 km2) comprising a host tonalite massif intruded by subordinate dyke-like masses of trondhjemite, granodiorite and monzogranite. The host tonalite, in turn, encloses several, fine-grained amphibolite enclaves. U-Pb zircon dating indicates a wide range of crystallization ages within the SGC (800 ± 18Ma for tonalites; 754± 3.9 Ma for trondhjemite; 738± 3.8 Ma for granodiorite;and 717±3.2 Ma for monzogranite), suggesting crystallization of independent magma pulses. The high positive εNdi (+6 to +8) indicate that the melting sources were dominated by juvenile material without any significant input from older crust. Application of zircon saturation geothermometry indicates increasing temperatures during the generation of melts from 745±31 °C for tonalite to 810±25 °C for trondhjemite; 840±10 °C for granodiorite; and 868±10 °C for monzogranite. The pressure of partial melting is loosely constrained to be below the stability of residual garnet (<10 kbar) as inferred from the almost flat HREE pattern ((Gd/Lu)N=0.9–1.1), but >3kbar for the stability of residual amphibole as inferred from the significantly lower NbN and TaN compared with LREEN and the sub-chondrite Nb/Ta ratios exhibited by the granitic phases. The inverse relation between the generation temperatures and the ages estimates of the granitoid lithologies argue against a significant role of fractional crystallization. The major and trace element contents indicate the emplacement of the SGC within a subduction zone setting. It lacks distinctive features for melt derived from a subducted slab (e.g. high Sr/Y and high (La/Yb)N ratios), and the relatively low MgO and Ni contents in all granite phases within the SGC suggest melting within the lower crust of an island arc overlying a mantle wedge. Comparison with melts produced during melting experiments indicates an amphibolite of basaltic composition is the best candidate as source for the tonalite, trondhjemite and granodiorite magmas whereas the monzogranite magma is most consistent with fusion of a tonalite protolith. Given the overlapping Sm-Nd isotope ratios as well as several trace element ratios between monzogranite and tonalite samples, it is reasonable to suggest that the renewed basaltic underplating may have caused partial melting of tonalite and the emplacement of monzogranite melt within the SGC. The emplacement of potassic granite (monzogranite) melts subsequent to the emplacement of Na-rich granites (tonalite-trondhjemite-granodiorite) most likely suggests major crustal thickening prior to arc collision and amalgamation into the overthickened proto-crust of the Arabian-Nubian shield. Eventually, after complete consolidation, the whole SGC was subjected to regional deformation, most probably during accretion to the Saharan Metacraton (arc–continent collisions) in late Cryogenian-Ediacaran times (650–542 Ma).
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| 3. |
- Lindh, Anders, et al.
(författare)
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A model for granite evolution based on non-equilibrium magma separation : evidence from the Gharib and Qattar fluorite-bearing granites, Eastern Desert, Egypt
- 2019
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Ingår i: International Journal of Earth Sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 108:4, s. 1201-1232
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Tidskriftsartikel (refereegranskat)abstract
- We present 77 new granite whole-rock analyses from the Qattar and Gharib areas, Eastern Desert, Egypt. Both areas include a “normal” granite and either a hypersolvus (Gharib) or an almost plagioclase-free granite (Qattar) enriched in fluorite. According to earlier results, F influences element distribution in granitic melts forming complexes with specific elements as Nb, Ta, Ga, Hf, Th, Zn, Sn, whereas F excludes Ba and Sr. We use principal component analyses to split the granite into chemical groups allowing an unbiased study of the inter-group element distribution. This adds the heavy REEs and Y to the earlier lists of elements with an affinity for F. The light REEs show a decreasing affinity with decreasing atomic mass; fluorine separates Sm from Nd, whereas Zr follows La. Opposite to some, but in accordance with other earlier results, the ratio Nb/Ta is higher in the fluorite-enriched than in the other granite. Weak tetrad effects are present. Zircon in the hypersolvus granite is high in common lead. We suggest F to be instrumental for separating Pb 2+ from Pb 4+ . Two hypotheses may explain the occurrence of the two contrasting granites: they have either different sources, or they are co-magmatic, but the magma was split into two discrete types. We apply the second hypothesis as our working hypothesis. The liquidus has a gentler slope with pressure than the diapir requiring crystallisation to be most important in the lower part of the magma chamber. Our hypothesis suggests that globules of magma, enriched in volatile components, form during crystallisation due to slow diffusion rates in the crystallizing magma. Elements accompanying F are distributed into this magma batch, which has a lowered density and viscosity than the rest of the magma due to its increased contents of volatile components. A mushroom-formed diapir rises, forming the hypersolvus (or almost plagioclase-free) granite. Due to an edge effect, it is concentrated close to the wall of the magma chamber. The size and form of the outcropping granite depend on the intersection of the diapir with the erosion surface. Fluorine only makes it possible to follow the process. The model may be generalised to explain the diversification of non-F enriched granite, since the buoyancy of a magma batch several thousand m 3 in size has a much larger impact on the system than the small negative buoyancy of crystals or small crystal aggregates. A-type granite classified merely from its trace element content may form from separated F-enriched magma batches. This may be the reason for their high frequency in the Eastern Desert.
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