Metamorphic evolution and mineral assemblages of fassaite calc-silicate and sapphirine-quartz Al-Mg granulites from the Tekhamalt area (In Ouzzal terrane, Western Hoggar): Implications for ultra-high-temperature conditions and petrogeneses

Abstract

Tekhamalt area (In Ouzzal terrane Western Hoggar) is remarkable for the occurrence of quartz-sapphirine granulites associated with fassaite-bearing calc-silicate granulitesand alkaline orthogneisses. The clinopyroxenes in these calc-silicate granulites contain various contents of aluminum, with concentrations reaching up to 11.69 wt%. The Al-Mg granulites are particularly notable for the presence of ultra-high temperature (UHT) assemblages, such as sapphirine-quartz and high-Al orthopyroxene. This sapphirine-quartz association has enabled a reevaluation of the peak metamorphic conditions in the region, which were previously studied using a simpler system. The new and more realistic systems (MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3) have been employed for this purpose and allowing us to constrain the peak metamorphic conditions at 7.5 ± 1 kbar and 980 ± 50 °C. Furthermore, the study of alkaline orthogneiss shows the assemblage of perthite-quartz-clinopyroxene with a notable addition of the rare mineral chevkinite. The alkaline orthogneiss provides valuable information for understanding the formation of the calc-silicates in Tekhamalt area, which result from metasomatic processes involving dolomitic limestones and fluids in equilibrium with protolith of alkaline orthogneiss.