Garnet hornblendite in the Meatiq Core Complex, Central Eastern Desert of Egypt: Implications for crustal thickening preceding the ∼600 Ma extensional regime in the Arabian-Nubian Shield

Abstract

Dike-like bodies of garnet (Py77.04–84.66) 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 (615–600Ma ago), at P=23–27kbar and T=510–600°C. Diapirism of hot mantle due to decompression caused partial dehydration melting (P=∼12kbar and T=800–>1100°C) of hornblende into pyrope-almandine garnet and diopside (Wo49–51En32–34Fs16–17.5). The clockwise P-T path continued by isothermal decomposition (10–11kbar) of garnet into hercynite-rich spinel (XFe2+=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>5kb) and garnet to chlorite (T=190–345°C). In addition, metasomatic minerals mainly of feldspars and titanite, and minor ilmenite, rutile, topaz and calcite formed, as well as composition of the original amphibole changed having increases of FeO, TiO2 and K2O and decreases of Al2O3, MgO and Na2O. 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.