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

摘要

Adrar Izzilatene in the Egere-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 X-Jd = 036, 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 (X-Alm = 0.55-0.60; X-Prp = 0.12-0.16; X-Grs = 0.26-0.30) before showing a decrease in almandine to X-Alm = 0.45, coupled to an increase in pyrope to X-Prp = 0.29 and decrease in grossular to X-Grs = 0.26 at the rims. Calculated P-T-M-H2O pseudosections show that the prograde M1 assemblage equilibrated at 13-14 kbar and 580 degrees C, before pressure and temperature increased to 19 kbar and 650-700 degrees C at fluid-saturated conditions during peak metamorphism. Retrogression involved near-isothermal decompression to 8-9 kbar and 700-750 degrees C at fluid-undersaturated conditions. Prograde-to-peak metamorphism of the Izzilatene 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. (C) 2014 Elsevier B.V. All rights reserved.