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

Abstract

The Tighsi area is situated in the northern part of the Egere-Aleksod terrane (Central Hoggar, South ofAlgeria). 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 assemblageomphacite–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 ofalbite and clinopyroxene poorer of the jadeitic component. The paragenese of the lower pressure isrepresented by kelyphites and symplectites of amphibole–plagioclase separating garnet from omphaciteand quartz. During this evolution, rutile transforms into ilmenite then, in the latest stage sphene replacesilmenite. The thermodynamic modeling using THERMOCALC has confirmed these textural observationsand suggests that the eclogites have experienced three stages of metamorphism. Stage I is the highpressure association, an H2O under-saturated pseudosection adapted to this stage has been constructedin NCFMASHTO system. The association of high pressure corresponds to the field where garnet is in equi-librium with omphacite including epidote-rutile and quartz; TheP–Tconditions of 19.6 kbar and 694°Chave been estimated using the garnet core compositions. Stage II is the isothermal decompression wherethe amphibole–plagioclase kelyphites occur at the expense of garnet, omphacite and quartz at 15.5 kbarand 774°C (conditions estimated using the garnet rim and the plagioclase–amphibole kelyphitescompositions). Stage III is the late retrograde evolution; a second pseudosection with H2O in excesshas been constructed to model this stage and show the transformation of eclogites into garnet-bearingamphibolites and explains the occurrence of sphene. TheP–Tconditions of 9.3 kbar and 695°C have beenestimated using the compositions of the plagioclase with the maximum Xanand the amphibole with theminimum concentration of AlVI. Both pseudosections show a clockwise path for eclogite evolution togarnet-amphibolites. This path characterizes the exhumation that succeeded a subduction event;therefore, theP–Tpath shows an isothermal decompression. Then, the geothermic gradients becamenormal so the decrease of pressure is accompanied with a decrease in temperature.Ó2014 Elsevier Ltd. All rights reserved.