The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment

Type Article
Date 2020-10
Language English
Author(s) Hodel F.1, Triantafyllou A.2, 3, Berger J.1, Macouin M.1, Baele J.-M.4, Mattielli N.5, Monnier C.6, Trindade R.I.F.7, Ducea M.N.3, 8, Chatir A.9, Ennih N.9, Langlade Jessica10, Poujol M.11
Affiliation(s) 1 : Géosciences Environnement Toulouse (GET), Observatoire de Midi-Pyrénées, CNRS, IRD, Université Toulouse III Paul Sabatier, 14, Avenue Edouard Belin, 31400 Toulouse, France
2 : Department of Geology, University of Liège, Sart Tilman, Liège, Belgium
3 : Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
4 : Geology and Applied Geology Unit – Mining Geology, Université de Mons, 20, Place du Parc, B-7000, Belgium
5 : Department of Earth and Environmental Sciences – G-Time Laboratory, Université Libre de Bruxelles, 50 Avenue Franklin Roosevelt, 1050 Brussels, Belgium
6 : Laboratoire de Planétologie et Géodynamique – Nantes (LPGN), UFR Sciences et Techniques, Université de Nantes, UMR-CNRS 6112, France
7 : Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universitad de São Paulo, 05508-900 São Paulo, Brazil
8 : Faculty of Geology and Geophysics, University of Bucharest, Bucharest, Romania
9 : EGGPG, Département de Géologie, Faculté des Sciences, Université Chouaïb Doukkali, 24000 El Jadida, Morocco
10 : Microsonde Ouest, Ifremer–Centre de Bretagne, Technopole Brest Iroise, BP70, F-29280 Plouzané, France
11 : Univ Rennes, CNRS, Géosciences Rennes, UMR 6118, 35000 Rennes, France
Source Gondwana Research (1342-937X) (Elsevier BV), 2020-10 , Vol. 86 , P. 182-202
DOI 10.1016/j.gr.2020.05.014
WOS© Times Cited 8
Keyword(s) Neoproterozoic, SSZ, Ophiolite, Back-arc, U-Pb dating, Serpentinite, West African Craton
Abstract

The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petro-geochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al2O3: 0.12–1.53 wt%, Ti: 3.5–64.2 ppm and Nb: 0.004–0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr# (0.44–0.81) and low to intermediate Mg# (0.25–0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite.

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Hodel F., Triantafyllou A., Berger J., Macouin M., Baele J.-M., Mattielli N., Monnier C., Trindade R.I.F., Ducea M.N., Chatir A., Ennih N., Langlade Jessica, Poujol M. (2020). The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment. Gondwana Research, 86, 182-202. Publisher's official version : https://doi.org/10.1016/j.gr.2020.05.014 , Open Access version : https://archimer.ifremer.fr/doc/00636/74780/