Prograde and retrograde evolution of eclogite from Adrar Izzilatène (Egéré-Aleksod terrane, Hoggar, Algeria) determined from chemical zoning and pseudosections, with geodynamic implications

Adrar Izzilatène in the Egéré-Aleksod terrane of the LATEA metacraton (Hoggar, Algeria) exposes one of the best preserved examples of eclogite facies metamorphism in Hoggar. Three distinct stages of metamorphic development are recognised, namely, the pre-peak stage (M1), characterised by garnet, amphibole, epidote, quartz and rutile, the peak eclogite facies stage (M2), consisting of omphacite, garnet, edenite, epidote, quartz and rutile, and the retrograde stage (M3), where initial decompression resulted in the appearance of plagioclase, the development of pargasite+plagioclase kelyphites and finally the formation of anhydrous plagioclase+diopside coronas. Porphyroblastic omphacite has a jadeite content of up to XJd=0.36, which is the highest yet observed for eclogite facies rocks from the Tuareg Shield. Garnet growth zoning patterns are characterised by flat profiles in the cores (XAlm=0.55–0.60; XPrp=0.12–0.16; XGrs=0.26–0.30) before showing a decrease in almandine to XAlm=0.45, coupled to an increase in pyrope to XPrp=0.29 and decrease in grossular to XGrs=0.26 at the rims. Calculated P–T–MH2O pseudosections show that the prograde M1 assemblage equilibrated at 13–14kbar and 580°C, before pressure and temperature increased to 19kbar and 650–700°C at fluid-saturated conditions during peak metamorphism. Retrogression involved near-isothermal decompression to 8–9kbar and 700–750°C at fluid-undersaturated conditions. Prograde-to-peak metamorphism of the Izzilatène eclogite could have involved either oceanic or continental subduction, followed by exhumation as the area was obducted towards the LATEA metacraton during the Pan-African orogeny and the assembly of Western Gondwana.