干旱区生态环境知识资源中心

The Meatiq basement, which is exposed beneath late Proterozoic nappes of supracrustal rocks in the Central Eastern Desert of Egypt, was affected by three metamorphic events. The ophiolite cover nappes show only the last metamorphic overprint. The M1 metamorphic event (T greater than or equal to 750 degrees C) is restricted to migmatized amphibolite xenoliths within the Um Ba’anib orthogneiss in the structurally lowest parts of the basement. Typical upper amphibolite facies M2 mineral assemblages include Grt-Zn-rich Spl-Qtz+/-Bt, Grt-Zn-rich Spl-Ms-Kfs-Bt-Sil-Qtz and locally kyanite in metasedimentary rocks. The mineral assemblages Ms-Qtz-Kfs-Sil in the matrix and Sil-Grt in garnet cores indicate that peak M2 P-T conditions exceeded muscovite and staurolite stabilities. Diffusional equilibration at M2 peak temperature conditions caused homogeneous chemical profiles across M2 garnets. Abundant staurolite in garnet rims and the matrix indicates a thorough equilibration during M2 at decreasing temperature conditions. M2 P-T conditions ranged from 610 to 690 degrees C at 6-8 kbar for the metamorphic peak and 530-600 degrees C at about 5.8 kbar for the retrograde stage. However, relic kyanite indicates pressures above 8 kbar, preceeding the temperature peak. A clockwise P-T path is indicated by abundant M2 sillimanite after relic kyanite and by andalusite after sillimanite. M2 fluid inclusions, trapped in quartz within garnet and in the quartz matrix show an array of isochores. Steepest isochores (water-rich H2O-CO2+/-CH4/N-2 inclusions) pass through peak M2 P-T conditions and flatter isochores (CO2-rich H2O-CO2+/-CH4/N-2 inclusions) are interpreted to represent retrograde fluids which is consistent with a clockwise P-T path for M2. The M3 assemblage Grt-Chl in the uppermost metasedimentary sequence of the basement limits temperature to 460 to 550 degrees C. M3 temperature conditions within the ophiolite cover nappes are limited by the assemblage Atg-Trem-Tlc to (540 degrees C and the absence of crysotile to >350 degrees C. The polymetamorphic evolution in the basement contrasts with the monometamorphic ophiolite nappes. The M1 metamorphic event in the basement occurred prior to the intrusion of the Um Ba’anib granitoid at about 780 Ma. The prograde phase of the M2 metamorphic event took place during the collision of an island are with a continent. The break-off of the subducting slab increased the temperature and resulted in the peak M2 mineral assemblages. During the rise of the basement domain retrograde M2 mineral assemblages were formed. The final M3 metamorphic event is associated with the updoming of the basement domain at about 580 Ma along low-angle normal faults.