Le métamorphisme hercynien dans les formations tardi et post-panafricaines de l'Anti-Atlas occidental (Maroc). Données isotopiques Rb/Sr et K/Ar des fractions fines / The hercynien metamorphism in the late to post-panafrican units of the Western Anti-Atlas (Morocco). Rb/Sr and K/Ar isotopic datas

Abstract

The very slightly metamorphosed sedimentary rocks of the end of the Upper Proterozoic and of the beginning of the Paleozoic, near the Kerdous area, in the occidental part of the Anti-Atlas, have been submitted to Rb-Sr and K-Ar isotopic studies. The results obtained are able to define two events, respectively dated at 370 and 290 My, whose signification in the hercynian evolution is discussed.

Post‐Variscan exhumation of the Central Anti‐Atlas (Morocco) constrained by zircon and apatite fission‐track thermochronology

“…before the deposition of the unconformable infra Cambrian limestones (Adoudounian) onto the Precambrian terranes (Leblanc, ; Ouanaimi and Soulaimani, ), (ii) sampled rocks have been heated to minimum temperature 210–240 °C corresponding to the upper temperatures of the zircon partial annealing zone (Tagami et al ., ) and most probably to T = 300 ± 20 °C according to the epizonal recrystallization of the Cambrian rocks of Central and Western Anti‐Atlas (Ruiz et al ., ) at 321 ± 21 Ma (average ZFT age; Table ). The latter value compares with the K‐Ar ages (357 ± 9–291 ± 7 Ma, mean age 315 ± 8 Ma) obtained by Bonhomme and Hassenforder () on white micas from the Kerdous region (Precambrian and Cambrian) and with the 40 Ar‐ 39 Ar ages (345 ± 8 and 316 ± 7 Ma, mean age 330 ± 8 Ma) measured by Margoum () on biotite from the Kerdous basement; (iii) basement rocks were close to the surface 145–125 Ma ago as unconformable Lower Cretaceous redbeds are observed above them everywhere around the Anti‐Atlas (Ifni, Ouarzazate, Tineghir and Rissani areas) and within the Siroua massif itself (Hollard et al ., ; Zouhri et al ., ). Equivalent redbeds are dated from the Barremian (i.e.…”

“…). Likewise, as mentioned above, they are consistent with the K/Ar and 40 Ar‐ 39 Ar results from the Kerdous area (Bonhomme and Hassenforder, ; Margoum, ). It appears that: (i) a single thermal event is responsible for resetting the various dating systems about 310–330 Ma ago (Late Visean‐Bashkirian); (ii) the thermal event was followed by rapid cooling below 240 ± 20 °C (closure temperature for zircon fission‐track dating).…”

Folding and inversion tectonics in the Anti‐Atlas of Morocco

“…Dolerite dikes such as the famous dike of Foum-Zguid and sills clearly postdate AntiAtlas folding [Sebai et al, 1991], but their lower Jurassic age does not put any tight time constraint on the deformation age of Anti-Atlas folding either, since folding might still be anything between ''middle'' Carboniferous and Late Triassic. Some attempts at a direct age determination of the tectonic cleavage have been made using radiometric methods, applied to fine grained illite-muscovite assemblages of the westernmost Anti-Atlas [Bonhomme and Hassenforder, 1985]. Isotopic ages of 370 Ma and 290 Ma were attributed to a pre-Variscan phase of deformation and the peak of regional metamorphism, respectively.…”

Illite crystallinity patterns in the Anti-Atlas of Morocco

“…No large-scale thrusting or duplexing is known. the Variscan metamorphism is generally of low to very low degree (Faure-Muret 1971;Michard 1976;Piqué & Michard 1989;Piqué et al 1991;Piqué 2001;soulaimani et al 1997choubert 1963;bonhomme & Hassenforder 1985).…”