Ta-Ky-Spr-Crd-Crn ± Qtz

1. Quartz Talc Kyanite Cordierite (XMg = 0.99) Corundum Sapphirine (XMg = 0.99) Spinel-bearing serpentinite Olivine-bearing Marble The contact between Serpentinite and marble Serpentinite Eclogite Whiteschists Ta-Ky-Spr-Crd-Crn ± Qtz Whiteschists and associated rocks from the Serouenout terrane (Central Hoggar, Algeria) Z. Adjerid (1,3,4), A. Derridj (2,4), G. Godard, (3) K. Ouzegane (4) & J-R. Kienast (3) (1): ENS- Algeria; (2): UMBB- Algeria, (3): IPGP and UNIV. Paris 7- France, (4): USTHB- Algeria The Ti-N-Eggoleh area is situated in the southern part of the Serouenout terrane (Central Hoggar, Algeria). It consists predominantly of orthogenesis and eclogite-facies rocks, which include talc–kyanite-bearing whiteschists, clinohumite-bearing marbles, serpentinite, chlorite-bearing quartzites and eclogites. These rocks are regarded as the remnants of an old oceanic lithosphere affected by a high-pressure metamorphism. Central Hoggar The Ti-N-Eggoleh area Serpentinite Whiteschists Marble Chlorite-bearing quartzites Clinohumite-bearing marble Clinohumite Ol-Spl-CC-Clinohumite Eclogite consists of omphacite, garnet, green amphibole, plagioclase and orthopyroxene. Common accessory minerals are rutile, ilmenite and apatite. Eclogites show a complex polymetamorphic history that indicates the passage of the rock by different stages, during the metamorphic event, consistent with two important metamorphic cycles. Garnet Cpx + plg + mt Amp Amp Grt Cpx (Omphacite) Opx plg Grt Eclogite Amp Plg The second cycle is characterised the growth of orthopyroxene (Opx) between amphibole (Hbl) and Garnet (Grt), following the reaction Hbl + Grt  Opx + plg, is consistent with an important increase of temperature, corresponding to the granulite facies conditions. The first cycle is characterised by the presence of omphacite (jadeite  35%), which provides evidence of ultrahigh-pressure (UHP) metamorphism in the Ti-N-Eggoleh area. Later, a partial amphibolitisation of the rock allows also the appearance of fine symplectites, including clinopyroxene (Cpx) and zoned plagioclase (Plg), according to the reaction Omp + Qtz  Cpx + Plg. The whiteschist consists of coarse grained matrix of quartz and talc and minor phengite, chlorite, kyanite and gedrite. Rutile is a common accessory. The study of the whiteschists evidenced the same polymetamorphic evolution, observed in eclogites. So, on the basis of textural observations, two stages of metamorphic recrystallization are distinguished: The eclogite-facies stage is characterised by the development of the high pressure talc (Ta)-kyanite (Ky) assemblage, which appears at the expense of the primary chlorite (Chl)-Quartz (Qtz) paragenesis, following the reaction Qtz + Chl  Ta + Ky. sapphirine In the FMASH system High pressure Talc-Kynaite Qtz Ta Spr + Crd Ta Ky Spr Crd Crn Spr + Crd Crn Ky/Sil Ta Qtz Highly alumina-rich sapphirine Dravite Cordierite-Sapphirine During the granulite-facies stage, sapphirine (Spr), corundum (Crn) and cordierite (Crd) grew as fine symplectites and coronas, around the previous minerals, following the reactions Ta + Ky  Crd + Crn and Ta + Ky  Crd + Spr ± Crn. The chemical composition of sapphirine is exceptional. It approaches the 13(Mg, Fe2+)O:9Al2O3:5SiO2 composition. The metamorphic evolution of the Ti-N-Eggoleh region is consistent with two metamorphic cycles. The peak of the first one coincides with eclogite-facies conditions (P>15 kbar), coeval with the omphacite-garnet and talc-kyanite parageneses, in eclogite and whiteschists, respectively. After a hydrated retrogression, the second granulite-facies metamorphic peak produced orthopyroxene-plagioclase in eclogite and sapphirine-corundum-bearing symplectites in whiteschists. This event would be related to the emplacement of gabbroic rocks during a probable extensional stage.