Metasomatised serpentinite at the contact with anorogenic granite from the Neoproterozoic ophiolites in the Eastern Desert of Egypt

The ophiolitic serpentinites of Neoproterozoic age in the Eastern Desert of Egypt show different types of contact against the country rocks. At the Fawakhir and Gabal Ghadir localities, the contact with the anorogenic granites is distinctly intrusive, with the formation of a limited-scale thermal aureole (up to ~3.15 and ~5.30 m at the two localities, respectively). The metasomatic alterations of the serpentinites caused by flow of hot fluids from the granite led to the formation of meta-ultramafic rocks with abundant dolomite, talc and free silica displaying proximal and distal varieties with respect to the intrusive granite. Combined field, petrographic, whole-rock and microanalytical data help to construct phase equilibria and mineral stabilities along the serpentinite–granite interface based on the μSiO₂ and X_CO2 variables. Phase diagrams for the thermal assemblages at the Ghadir and Fawakhir aureoles were constructed assuming a pressure of 1–3 kbar. The replacement of antigorite by talc becomes more independent on temperature at X_CO2 ≤ 0.2 and the crystallization of dolomite and calcite predominates in the proximal varieties. The upper stability limit of antigorite lies at ~425 °C and ~500 °C at 1 and 3 kbar, respectively. Thus, the granites were emplaced into shallow levels so that they were not able to dehydrate the serpentinites at the two studied localities. Accordingly, the formation of forsterite was not possible at the investigated contact aureoles as the condition to form metamorphic olivine (0.05 < X_CO2 < 0.55) was not attained.