The First Example of Kyanit-Staurolite-Garnet–Bearing Metapelites from the Hoggar (Egéré Terrane, South Algeria)

Abstract

Metapelites containing kyanite, garnet, staurolite, biotite cordierite, and spinel are relatively rare. However, they have been reported from a numerous metamorphic terranes around the world. kyanite-garnet-staurolite– bearing metapelites have been identified among the various lithologies featuring the Egéré area (Central Hoggar). These rocks contain mineral assemblages consisting of garnet, kyanite, quartz, staurolite, spinel, cordierite, biotite, ± plagioclase and muscovite. These rocks display complex symplectites and coronas reaction textures comprises spinel and cordierite resulted from the breakdown of the peak paragenesis formed by garnet, kyanite and staurolite. Garnets porphyroblasts with inclusions of quartz, biotite, plagioclase, and rutile reveal an obvious compositional core–rim–outr-rim zonation, particularly in grossular, and pyrope, which is related to the metamorphic evolution of their host metapelitic rocks. Phengitic white micas display high-Si content that indicates crystallization under relatively high pressure conditions. The textural observations and mineral chemical compositions described in this contribution are interpreted via the application of P–T pseudosections, which were calculated in the system MnO–Na2O–CaO– K2O–FeO–MgO–Al2 O3–SiO2–H2 O–TiO2 (MnNCKFMA SHT). The obtained results indicate that the metamorphic evolution recorded in these rocks started at pic conditions of 12 kbar and 700 °C from amphibolitic facies and pursued via retrograde path toward granulitic facies conditions around 5 kbar and 850 °C. The combination of the observed mineral paragenesis, microstructural relationships and mineral chemistries, coupled with thermodynamic modeling of representative metapelitic samples, provide evidences that the Egéré kyanite-staurolite–bearing metapelites experienced relatively high pressure conditions, followed by exhumation to the earth’s surface through a clockwise P–T path during the Pan-African orogeny.