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Jean-Paul Liégeois
Earth Sciences
Geodynamics and mineral resources
Geodynamics and mineral resources
Publication details
Doukkari, S.A., Ouzegane, K., Arab, A., Kienast, J.R., Godard, G., Drareni, A., Zetoutou, S. & Liégeois, J.P. 2014. ‘Phase relationships and P-T path in NCFMASHTO system of the eclogite from the Tighsi area (Egere terrane, Central Hoggar, Algeria)’. Journal of African Earth Sciences. Amsterdam : Elsevier. DOI: http://dx.doi.org/10.1016/j.jafrearsci.2014.02.016. I.F. 1.219.
Article in a scientific Journal / Article in a Journal
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.