extension-related exhumation Research Papers - Academia.edu (original) (raw)
Dike-like bodies of garnet (Py 77.04–84.66) hornblendites were recorded, for the first time, in the structurally lowest parts of the gneiss of Meatiq Core Complex in the Central Eastern Desert of Egypt. They are composed mainly of... more
Dike-like bodies of garnet (Py 77.04–84.66) hornblendites were recorded, for the first time, in the structurally lowest parts of the gneiss of Meatiq Core Complex in the Central Eastern Desert of Egypt. They are composed mainly of hornblende (pargasite to pargasitic hornblende), [depleted in LREE with flat HREE segments (normalized to CI-chondrite), and enriched in LILE (normalized to N-MORB)] and crystallized from sub-arc tholeiitic melt that derived from wet mantle wedge (615–600 Ma ago), at P = 23–27 kbar and T = 510–600 °C. Diapirism of hot mantle due to decompression caused partial dehydration melting (P = $12 kbar and T = 800–>1100 °C) of hornblende into pyrope-almandine garnet and diopside (Wo 49– 51 En 32–34 Fs 16–17.5). The clockwise P-T path continued by isothermal decomposition (10–11 kbar) of garnet into hercynite-rich spinel (XFe 2+ = 0.46–0.64) and quartz. By continuous cooling and probably by introduction of the Abu Ziran granite-related solutions, the hornblendites followed a retrograde greenschist facies hydration path, including transformation of diopside to tremolite (T = 280–420 °C and pressure > 5 kb) and garnet to chlorite (T = 190–345 °C). In addition, metasomatic minerals mainly of feld-spars and titanite, and minor ilmenite, rutile, topaz and calcite formed, as well as composition of the original amphibole changed having increases of FeO, TiO 2 and K 2 O and decreases of Al 2 O 3 , MgO and Na 2 O. Deep source of the Meatiq hornblendites implies for a probable local crustal thickening of Meatiq Core Complex by crustal shortening, while their exhumation was most probably accompanied the NW-SE extension and thinning of the previously thickened crust that occurred during oblique island arc convergence with the closure of the Mozambique ocean and the collision of East and West Gondwanaland.