Phase relationships and P-T path in NCFMASHTO system of the eclogite from the Tighsi area (Egere terrane, Central Hoggar, Algeria)

The Tighsi area is situated in the northern part of the Egere-Aleksod terrane (Central Hoggar, South of Algeria). The eclogites and the garnet-amphibolites occur as lenses in marble, metapelite and quartzite. The petrological study shows that the high pressure paragenesis is characterized by the assemblage omphacite-garnet-rutile-quartz and epidote. Omphacite contains epidote, rutile and quartz inclusions. Due to the decompression, omphacite is the first mineral to destabilize into very thin symplectites of albite and clinopyroxene poorer of the jadeitic component. The paragenese of the lower pressure is represented by kelyphites and symplectites of amphibole-plagioclase separating garnet from omphacite and quartz. During this evolution, rutile transforms into ilmenite then, in the latest stage sphene replaces ilmenite. The thermodynamic modeling using THERMOCALC has confirmed these textural observations and suggests that the eclogites have experienced three stages of metamorphism. Stage I is the high pressure association, an H2O under-saturated pseudosection adapted to this stage has been constructed in NCFMASHTO system. The association of high pressure corresponds to the field where garnet is in equilibrium with omphacite including epidote-rutile and quartz; The P-T conditions of 19.6 kbar and 694 °C have been estimated using the garnet core compositions. Stage II is the isothermal decompression where the amphibole-plagioclase kelyphites occur at the expense of garnet, omphacite and quartz at 15.5 kbar and 774 °C (conditions estimated using the garnet rim and the plagioclase-amphibole kelyphites compositions). Stage III is the late retrograde evolution; a second pseudosection with H2O in excess has been constructed to model this stage and show the transformation of eclogites into garnet-bearing amphibolites and explains the occurrence of sphene. The P-T conditions of 9.3 kbar and 695 °C have been estimated using the compositions of the plagioclase with the maximum Xan and the amphibole with the minimum concentration of AlVI. Both pseudosections show a clockwise path for eclogite evolution to garnet-amphibolites. This path characterizes the exhumation that succeeded a subduction event; therefore, the P-T path shows an isothermal decompression. Then, the geothermic gradients became normal so the decrease of pressure is accompanied with a decrease in temperature.