Geochronological constraints on the polycyclic magmatism in the Bou Azzer-El Graara inlier (Central Anti-Atlas Morocco)

Accepted Manuscript Geochronological constraints on the polycyclic magmatism in the Bou AzzerEl Graara inlier (Central Anti-Atlas Morocco) O. Blein, T. Baudin, P. Chèvremont, A. Soulaimani, H. Admou, P. Gasquet, A. Cocherie, E. Egal, N. Youbi, P. Razin, M. Bouabdelli, P. Gombert PII: DOI: Reference:

S1464-343X(14)00127-7 http://dx.doi.org/10.1016/j.jafrearsci.2014.04.021 AES 2033

To appear in:

African Earth Sciences

Received Date: Revised Date: Accepted Date:

1 July 2013 5 April 2014 24 April 2014

Please cite this article as: Blein, O., Baudin, T., Chèvremont, P., Soulaimani, A., Admou, H., Gasquet, P., Cocherie, A., Egal, E., Youbi, N., Razin, P., Bouabdelli, M., Gombert, P., Geochronological constraints on the polycyclic magmatism in the Bou Azzer-El Graara inlier (Central Anti-Atlas Morocco), African Earth Sciences (2014), doi: http://dx.doi.org/10.1016/j.jafrearsci.2014.04.021

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26 novembre 2013

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Geochronological constraints on the polycyclic magmatism in the

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Bou Azzer-El Graara inlier (Central Anti-Atlas Morocco)

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O. Blein1*, T. Baudin1, P. Chèvremont1, A. Soulaimani 2, H. Admou2, P. Gasquet3, A.

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Cocherie1, E. Egal1, N. Youbi2, P. Razin4, M. Bouabdelli5, P. Gombert1.

5

1

BRGM, BP 6009, 45060 Orléans Cédex, France

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2

Faculty of Science, Cadi Ayyad University, Marrakech, Morocco

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3

Laboratoire EDYTEM, Université de Savoie, CNRS, Campus Scientifique, 73376 Le Bourget du Lac

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Cedex, France.

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4

EGID, Université de Bordeaux 3, 33607 Pessac Cedex, France.

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5

GEODE Terre et Patrimoine, B.P. 7004, 40014 Marrakech, Morocco.

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*

Corresponding author.

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Keywords. Anti-Atlas, Cryogenian, Ediacaran, Bou Azzer, Pan-African, U-Pb geochronology.

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ABSTRACT

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New U-Pb SHRIMP zircon ages from the Bou Azzer-El Graara onlier constrains the Neoproterozoic

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evolution of the Anti-Atlas during Pan-African orogenesis. Within the Central Anti-Atlas, the Bou Azzer-

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El Graara inlier exposes a dismembered ophiolite, long considered to mark a late Neoproterozoic

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suture between the West African Craton in the south, and Neoproterozoic arcs to the north. From

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north to south, this inlier includes four main geological units: a volcanic-arc, an ophiolite, a

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metamorphic complex and a continental platform. Several plutons intrude the volcanic-arc, the

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ophiolite, the metamorphic complex, and post-orogenic volcanic and sedimentary deposits

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unconformably cover these terranes.

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The age of the volcanic-arc is reported here for the first time. Analyses of zircon of two rhyolites

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provide ages of 761 ± 7 Ma and 767 ± 7 Ma. Zircons from two gneisses provide dates of 755 ± 9 Ma

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and 745 ± 5 Ma. Both dates are considered best estimates of the crystallization ages of their igneous

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protoliths. Analyses of zircon from two granitic bodies, which crosscut gneisses, provide younger dates

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of 702 ± 5 Ma and 695 ± 7 Ma. The age of an aplitic body of the ophiolite is reported here for the first

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time, as 658 ± 8 Ma (SHRIMP U-Pb on zircons). Theses ages suggest the existence of three distinct

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orogenic events during Cryogenian times: (i) 770-760 Ma Tasriwine-Tichibanine orogeny with rollback

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of the subducting oceanic plate, leading to the formation of back-arc basins; (ii) 755-695 Ma Iriri-

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n’Bougmmane orogeny; and (iii) the 660-640 Ma Bou Azzer orogeny involving the formation and the

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emplacement of the Bou Azer ophiolite.

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During Ediacaran times, the Bou Azzer-El Graara inlier is characterized with the development of a

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continental volcanic arc between 630 and 580 Ma (Bou Lbarod Group, 625 ± 8 Ma ; Bleïda

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granodiorite, 586 ± 15 Ma), and strike-slip pull-apart basins (Tiddiline Group, 606 ± 4 Ma and

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606 ± 5 Ma). These volcanic and sedimentary Lower Ediacaran sequences are deformed before the

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felsic pyroclastic deposits of the Ouarzazate Group (567 ± 5 Ma and 566 ± 4 Ma). Finally, the

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Ouarzazate Group is overlain by early Cambrian volcanic deposits of the Jbel Boho Formation

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(541 ± 6 Ma).

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1. INTRODUCTION

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Located on the northern edge of West African Craton (WAC), the Anti-Atlas belt of Morocco is

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characterised by a Proterozoic basement unconformably underlying by late Ediacaran to Paleozoic

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sedimentary rocks in several inliers (Bas Dra, Ifni, Kerdous, Akka, Igherm, Sirwa, Zenaga, Bou Azzer-

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El Graara, Saghro and Ougnat; Figure 1). This Proterozoic basement consists of: (i) Paleoproterozoic

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metamorphic and igneous rocks; (ii) Cryogenian rocks affected by the main Pan African orogenic

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events; (iii) Early Ediacaran sedimentary and volcanic rocks deformed by the latest tectonic event of

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the Pan African orogeny; and (iv) and late Ediacaran volcanic rocks (Ouarzazate Group). However,

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two main structural domains have been recognised part of the NW-SE Anti-Atlas Major Fault (AAMF;

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Choubert, 1963).

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In Western Anti-Atlas (Figure 1), Paleoproterozoic basement (2030-2200 Ma) has been recognised

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and recently confirmed by U–Pb zircon dating in several inliers (Ait Malek et al., 1998; Charlot-Prat et

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al., 2001; Thomas et al., 2002; Walsh et al., 2002; Barbey et al., 2004, Gasquet et al. 2004). This

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Paleoproterozoic basement, consisting of schists, gneisses, migmatites and plutonic rocks, is

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unconformably overlain by Early Ediacaran sedimentary and volcanic rocks and/or Late Ediacaran

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volcanic rocks.

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The Eastern Anti-Atlas domain, including Sirwa, Bou Azzer-El Graara and Saghro inliers (Figure 1), is

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characterised by the lack of Paleoproterozoic rocks (except as relics in the inherited cores of zircon

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from Ediacaran volcanic rocks, Gasquet et al; 2005; Pelleter et al. 2007), and by Cryogenian rocks

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affected by the main Pan African orogenic events, Early Ediacaran sedimentary and volcanic rocks

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affected by the latest stage of Pan African orogeny, and late Ediacaran volcanic rocks. In the Sirwa

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inlier, two dismembered ophiolite sequences (Tasriwine and N’Qob ophiolites) include ultramafic

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cumulates, gabbros, a sub-vertical sheeted dyke complex and plagiogranite intrusions (Admou, 2000).

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Samson et al. (2004) dated two plagiogranite intrusions within the Tasriwine ophiolite, 761 Ma and

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762 Ma, which were interpreted to date formation of oceanic crust. In the central part of the Sirwa

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inlier, medium to high grade metamorphic rocks are thought to represent the roots of an arc complex

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estimated to be 743 Ma (Thomas et al., 2002). The relationships of the ophiolites in the Sirwa inlier to

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the main Bou Azzer–El Graara ophiolite are unclear.

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Recent mapping in the Bou Azzer-El Graara inlier and the western edge of the Saghro massif, within

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the framework of the Moroccan National Project of Geological mapping (1/50 000-scale sheet maps of

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Bou Azzer, Alougoum, Aït Ahmane and AlGlo’a) gave opportunities to acquire new field, geochemical

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and geochronological data. Therefore, the aims of this paper are: (i) a re-appraisal of the lithological

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units of the Bou Azzer-El Graara inlier; (ii) to present new geochronological data on magmatic rocks;

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and (iii) to discuss the geodynamical evolution of the West African Craton during Proterozoic times.

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The new geochronological data have allowed us to clarify the timing of polycyclic magmatism

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observed in several terranes of Pan-African belt.

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2. LITHOLOGICAL UNITS OF THE BOU AZZER-EL GRAARA INLIER

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The Bou Azzer-El Graara inlier is a key element of the Anti-Atlas area for the understanding of Pan-

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Africa events. It is believed to expose the dismembered relics of a Neoproterozoic suture zone

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(Leblanc, 1981; Saquaque et al., 1989; Hefferan et al., 2000) marking the boundary between the

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Paleoproterozoic Eburnean basement of the WAC to the south, and Neoproterozoic accreted arcs to

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the north. Composed of tectonic blocks separated by oblique slip faults, the Bou Azzer-El Graara inlier

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is structurally the most complex part of the whole Anti-Atlas (Leblanc, 1981; Saquaque et al., 1989,

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1992).

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Nine main lithological units were identified in the Bou Azzer-El Graara inlier: (i) a gneissic basement

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composed of deformed meta-igneous and metasedimentary rocks, including augen granite gneiss,

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and leucogranite (Assif n’Bougmmane gneiss complex); (ii) a plateform sequence of quartzite and

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stromatolitic limestones overlain by mafic lavas and volcano-sedimentary rocks, interpreted as a

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Tonian and/or Cryogenian passive margin cover sequence (Tachdamt-Bleida Group); (iii) a

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Cryogenian mafic–ultramafic complex, interpreted as a dismembered ophiolite fragment (Bou Azzer

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Group); and (iv) a Cryogenian metasedimentary sequence with subordinate volcanic units (Tichibanine

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Group). These pre-Ediacaran units are deformed by the main Pan-African shortening tectonic event,

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and are intruded by granitoid bodies, (v) the Ousdrat Suite, which are believed to have been emplaced

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syn-tectonically during the main period of Pan-African orogenesis (Saquaque et al., 1989). This

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collisional event is generally considered to have been the major Pan-African orogenic phase (PA1),

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with the southward obduction of the Bou Azzer ophiolite onto the WAC at about 685 Ma (Leblanc,

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1975; Leblanc and Lancelot, 1980), 663 Ma (Thomas et al., 2002), or 653-640 Ma (Inglis et al., 2004).

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The above units are unconformably overlain or intruded by successively: (vi) mafic volcanic rocks (Bou

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Lbarod Group); (vii) the terrigeneous sediments with local pyroclastic rocks (Tiddiline Group); (viii)

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undeformed intrusions such as the Bleïda granodiorite (Bleïda Suite); (ix) a late Ediacaran volcano-

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clastic sequence (Ouarzazate Group).

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The lithostratigraphic system used for the Anti-Atlas orogen is summarised in Table 1, where it is

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compared to the previous chronostratigraphic nomenclature.

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2.1. Tonian and/or Cryogenian lithological units

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The Tachdamt-Bleïda Group is characterized by the following succession from the base to the top: a

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stromatolitic limestone and quartzite unit; a tholeiitic basalt unit; a schist unit; and a schist and

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sandstone unit (Leblanc and Billaud, 1978; Mouttaqi and Sagon, 1999). The limestone and quartzite

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unit comprises: an alternation of sandstones and limestones; massive quartzites; and an alternation of

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sandstones and pelites. This group is interpreted as the local Neoproterozoic passive margin of the

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West African Craton (Saquaque et al., 1989; Leblanc and Moussine-Pouchkine, 1994; Hefferan et al.,

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2002; Bouougri and Saquaque, 2004).

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2.2. Cryogenian lithological units

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The southern part of the Bou Azzer-El Graara inlier consists of a variety of deformed igneous, meta-

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igneous and metasedimentary rocks, including augen granite gneiss, paragneisses, low-grade

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amphibolites, muscovite pegmatite and leucogranite. The five main outcrops occur at Bou Azzer,

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Oumlil, Tazigzaout, Ightem and assif n’Bougmmane. On the basis of their deformational state and

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lithological similarities with rocks of the nearby Zenaga Massif these units have been considered to be

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2 Ga Eburnean WAC basement by all previous workers (Leblanc, 1981; Saquaque et al., 1989;

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Saquaque, 1991).

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Recently determined ages by SHRIMP on zircons have been performed on metagabbro, augen

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granite gneiss, and crosscutting leucogranites bodies of the Tazigzaout complex (D’Lemos et al.,

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2006). Three concordant U–Pb analyses of zircon from an augen granite gneiss provide a date of c.

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753 Ma. Zircons from a nearby metagabbro provide a similar age date of c. 752 Ma. Both dates are

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considered best estimates of the crystallization ages of their igneous protoliths. Analyses of zircon

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from two crosscutting leucogranite bodies provide younger dates of 701 and 705 Ma.

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The ophiolite defined by Leblanc (1975, 1981a) is a composite terrane, involving three separate units:

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(i) an oceanic crust and upper mantle, i.e. the true ophiolite; (ii) a northern belt made up of calc-

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alkaline volcanic rocks (Saquaque et al., 1989b), the Tichibanine Group; and (iii) late Cryogenian syn-

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kinematic granitoïd plutons (Ousdrat Suite), intruding both oceanic and arc units (Beraaouz et al.,

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2004).

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In this paper, we restricted the term of Bou Azzer ophiolite to the oceanic-type crust and upper mantle.

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This ophiolite comprises the following rock units: upper mantle tectonite peridotites, mafic-ultramafic

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cumulates, submarine basaltic pillow lava, and volcano-sedimentary sequence (Leblanc, 1976;

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Bodinier et al., 1984). This mafic-ultramafic complex is interpreted as a dismembered ophiolite

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fragment (Leblanc, 1975, 1981) later on interpreted as a mélange complex (Saquaque et al., 1989;

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Hefferan et al., 2002), taking place at blueschist metamorphic conditions (Hefferan et al., 2002).

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Until recently no precise radiometric dates exist for any magmatic rocks of the Bou Azzer ophiolite.

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Admou (2000) considered on extensive mapping and structural studies the Sirwa inlier as a westward

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extension of the Bou Azzer-El Graara inlier. Within the Sirwa inlier, crop out two small highly

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tectonized ophiolites, the N’qob and Tasriwine ophiolites (Thomas et al., 2002). Samson et al. (2004)

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obtained two ages of 762 Ma on plagiogranites. El Hadi et al. (2010) place the age of the formation of

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the Bou Azer ophiolite at 697 Ma.

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The Tichibanine Group outcrops on the northern part of the Bou Azzer-El Graara inlier, and

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corresponds to the northern terrane of Saquaque et al. (1989b). This group consists of a complex

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tectonic assemblage of metagraywackes with basalts, andesites, rhyolites and tuffs (Tekiout et al.,

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1991), bearing calc-alkaline and island arc tholeiitic signatures (Naidoo et al., 1991). A thick series of

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layered cinerites has been evidenced. The rocks are metamorphosed at greenschist-facies to lower

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epidote-amphibolite facies conditions.

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The late Cryogenian granitoïds of the Ousdrat Suite are organized in massifs and stocks that are

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either roughly circular in plan (Ousdrat) or, more commonly, WNW-ESE elongated (Ait Ahmane and

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Bou Frokh), and intrude sedimentary and volcano-sedimentary Cryogenian rocks. The intrusions,

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generally more intermediate than felsic rocks, include diorites, quartz-diorites and monzodiorites.

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Published ages for these granitoïds are 667 ± 11 Ma (Mrini, 1993) and 653 ± 1.5 Ma (Inglis et al.,

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2003) for quartz-diorites from Bou Frokh, 640 ± 1.5 Ma for quartz-diorites from Ousdrat (Inglis et al.,

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2003), 650 ± 2 Ma for diorite from Ait Ahmane (Samson et al., 2004), and 646 ± 8 Ma and 646 ± 6 Ma

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for diorites from Tamellalet and Tafrawt (Yazidi et al., 2008). These late Cryogenian (650-670 Ma)

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medium-K calc-alkaline diorites were produced by partial melting of subducted oceanic crust followed

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by interaction of the melt with the overlying mantle wedge (Beraaouz et al., 2004).

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2.3. Ediacaran lithological units

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Three volcano-sedimentary or volcanic groups occurred during that Ediacaran period: the Bou Lbarod,

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the Tiddiline and the Ouarzazate groups.

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The Bou Lbarod Group outcrops in the Bou Azer-El Graara inlier and in the western edge of the

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Saghro massif (Figure 2). This group is composed of volcanic rocks with andesitic to rhyolitic

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compositions, and corresponds to the lower member of the Ouarzazate Group defined by Choubert

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(1953b). In the Saghro massif, the Issougri complex (the lower member of the Ouarzazate Group), a

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great thickness of andesitic volcanic rocks, is folded with local sub-vertical bedding (Choubert and

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Faure-Muret, 1970). The middle member of the Ouarzazate Group unconformably overlies the Issougri

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complex.

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The Tiddiline Group is a clastic sedimentary succession characterized by a coarsening upwards

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sequence of siltstones to conglomerates with local diamictites interpreted as marine tilloids with

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dropstones (Leblanc, 1975). This group unconformably overlies Cryogenian rocks deformed by the

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Pan African orogeny. The Tiddiline Group is tilted, faulted and folded with local development of axial-

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plane cleavage. In the eastern part of the Bou Azzer-El Graara inlier, the two main outcrops of the

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Tiddiline Group are the Dwaïssa basin on the northern border and the Trifya basin on the southern

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border. Hefferan et al. (1992) suggest that the upper members of the Tiddiline Group contain clasts

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derived from the underlying tectonic blocks, including boulders of the Ediacaran granodioritic and

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dioritic intrusions (such as Bleïda granodiorite), and thus the tectonic blocks were thrust over the

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Tiddiline basins. On this basis Hefferan et al. (1992) argued for a continuation in collision within the

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Bou Azzer region during deposition of the Tiddiline Group.

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Recent mapping permit to observe that the conglomerates of the upper members are not stratified and

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unconformably overlain typical stratified conglomerates of the Tiddiline Group. In the ultimate

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conglomerates, the lithology of the boulders varies from west to east, from gneisses, to quartzite and

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diorite. Such lithological variation reflects variations in local Cryogenian rocks and is frequently

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observed at the base of the Ouarzazate Group. This formation is characterized by basal sedimentary,

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volcano-sedimentary or volcanic breccias with boulders of local Cryogenian to Ediacaran basement.

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These petrographic observations and U-Pb ages obtained on Bleïda granodiorite (Inglis et al., 2004)

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suggest that ultimate conglomerates of the Tiddiline Group are in fact basal conglomerates of the

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Ouarzazate Group.

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The Ediacaran granodioritic and dioritic intrusions (Bleïda Suite) occur within each of the tectonic

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blocks of the Bou Azzer-El Graara inlier and several of the intrusions carry a weak to moderately

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developed foliation that is co-planar to the main regional fabric observed in the surrounding host-rocks

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(Admou, 2000).

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The Bleïda granodiorite is exposed in the eastern wedge of the inlier, emplaced within the Tachdamt-

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Bleïda Group. The bulk of the intrusion is a medium grained granodiorite consisting of plagioclase,

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quartz, hornblende, alkali-feldspar, apatite and zircon. Tonalitic and dioritic compositions occur within

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the intrusion; without definite boundaries, they result from variation in the relative proportion of quartz,

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hornblende and plagioclase. A fine-grained granodiorite phase is evident in the south of the intrusion,

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forming sheets running parallel to the southern boundary. The granodiorite exhibits primary igneous

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features. Internal magmatic fabrics are weak and discontinuous, defined by the alignment of lath

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shaped hornblende, and possess no predominant structural orientation across the intrusion. In the

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south-east of the body the magmatic fabrics are sub-vertical and trend to the WNW. In the north-west

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fabrics rotate into parallelism with the boundary of intrusion. Ducrot (1979) provided the first U–Pb age

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(615 ± 12 Ma) of the Bleïda granodiorite. Recently, Inglis et al. (2004) provided a younger age of

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597 ± 2 Ma.

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The relationship of the Bleïda Suite to deformation in the Bou Azzer inlier has remained open to

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question. Leblanc (1981) argued that the lack of penetrative deformation within the intrusion implied

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that its emplacement post-dated thrusting and pervasive greenschist facies metamorphism. Saquaque

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et al. (1989) regarded the intrusion as having been emplaced and deformed during the greenschist

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facies deformation event. Inglis et al. (2004) argued that the granodiorite was emplaced after the end

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of greenschist facies metamorphism and the development of the pervasive regional fabrics in the Bou

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Azzer inlier. However, they suggested that the intrusion could conceivably have been emplaced during

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late brittle transcurrent faulting, and in this regard the granodiorite is better identified as being syn-

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tectonic only with late brittle deformation in the region, rather than pervasive regional fabric

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development (Inglis et al., 2004).

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The Ouarzazate Group consists of potassic to high-potassic basaltic andesites, andesites, dacites,

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and rhyolites interstratified with chaotic breccia, polygenic conglomerates and arkosic sandstones.

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Felsic volcanics were dated in several inliers: 565 Ma in the Tagragra of Tata (Walsh et al., 2002);

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563 Ma and 580 Ma in the Central Anti-Atlas (Mifdal and Peucat, 1985); between 560 and 575 Ma in

217

the Sirwa massif (Thomas al., 2002); 550 Ma in the Imiter inlier (Cheilletz et al., 2002) and 552 ± 5 Ma

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in the Bou Madine inlier (Gasquet et al., 2005). The Ouarzazate Group has not recorded the Pan-

219

African deformations (PA1 and/or PA2), but was deposited on a highly variable basement topography,

220

which, coupled with the important and rapid thickness variations of the volcano-sedimentary deposits,

221

suggests that this group was deposited during active tectonics, most probably transtensional

222

movements (Maacha et al., 1998; Gasquet et al., 2005). This late Neoproterozoic magmatic activity

223

constitutes the Ediacaran Atlasic Volcanic Chain that built across the whole Anti-Atlas (Pouclet et al.

224

2007). This chain extends from the Atlantic coast to the border of Algeria, with a total length reaching

225

850 km and the width, 80-150 km.

226

2.4. Paleozoic sedimentary cover and intercalated volcanics

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The Ediacaran-Cambrian transition is recognized in the Moroccan Anti-Atlas throughout a carbonate-

228

dominated succession (Adoudou and Lie-de-vin formations; Choubert, 1952, 1953a, b) that

229

unconformably overlies the late Ediacaran Ouarzazate Group.

230

Volcanic ashes and flows also occur interbedded with the Adoudou and Lie-de-vin strata (Choubert

231

and Faure-Muret, 1970). One source for these ashes is preserved in the Alougoum volcanic complex

232

located on the Al Glo’a area (Choubert, 1952; Boudda et al., 1979), in which volcanic

233

paleotopographies cover directly the Adoudou dolostones and were progressively onlapped by the

234

breccias, dolostones, variegated shales and sandstones of the Lie-de-vin Formation. An early U/Pb

235

date of 529 ± 3 Ma from the Boho volcano (Recalculated from Ducrot and Lancelot, 1977) and

236

531 ± 5 Ma (Gasquet et al., 2005) suggests that deposition of the uppermost part of the Adoudou

237

Formation took place in the pre-trilobite earliest Cambrian. As a result, the location of the Ediacaran-

238

Cambrian boundary (defined by the first appearance of the ichnospecies Treptichnus (former

239

Phycodes pedum; Narbonne et al., 1987) remains problematical in Morocco because it lies within the

240

thick carbonate-dominated Adoudou Formation, extremely poor in shelly metazoans and ichnofossils.

241

The boundary has been tentatively correlated with carbon isotope signatures (Tucker, 1986; Latham

242

and Riding, 1990; Kirshvink et al., 1991; Magaritz et al., 1991) above the medusoid-like imprints of the

243

‘Série de base’ or Basal series (Adoudou Formation; Houzay, 1979) and below the occurrence of

244

Atdabanian (sensu Spizharski et al., 1986), shelly metazoan fossils in the Tiout Member (Sdzuy, 1978;

245

Schmitt, 1979; Debrenne and Debrenne, 1995).

246

The carbonate to clastic sedimentary cover comprises from bottom to top: (i) the transgressive late

247

Proterozoic to early Paleozoic Taroudannt Group; (ii) the Cambrian Tata Group; and (iii) the Cambrian

248

to Ordovician transgressive groups of internal Feijas, Tabanite, external Feijas, first and second Bani

249

and Ktaoua.

250

2.5. Main objectives and sampling

251

This paper provides new high-precision zircon U-Pb ages for Cryogenian and Ediacaran magmatic

252

rocks of the Bou Azzer-El Graara inlier to constrain the magmatic events and deformation phases of

253

Pan-African events in the Anti-Atlas area.

254

In a first stage, we will focus on Cryogenian period. Recent U-Pb analyses of zircon of an augen

255

granite gneiss and metagabbro exposed in the Tazigzaout area provide dates of 753 and 752 Ma, and

256

dates of 701 and 705 Ma for crosscutting leucogranite bodies (D’Lemos et al., 2006). We selected two

257

samples of orthogneisses (BOPC028 and AADG4) in Bou Azzer and assif n’Bougmmane areas, and

258

two samples of crosscutting granitic bodies (BOPC076 and AAPC140) in Oumlil and assif

259

n’Bougmmane areas to confirm these new ages in others outcrops of metamorphic rocks.

260

Bodinier et al. (1984) and El Hadi et al. (2010) suggested that the Tichibanine volcanic island-arc may

261

be contemporaneous from the Bou Azzer ophiolite. Then, we selected two rhyolites in the Tichibanine

262

Group, and a leucogranodiorite within the Bou Azzer ophiolite to test this hypothesis.

263

In a second stage, we will try to precise timing of Ediacaran volcanic rocks, intrusions and deformation

264

phase recorded in part of Ediacaran rocks. The sample AAYN55a represents a small diorite pluton

265

within the Bou Lbarod Group composed of volcano-sedimentary and volcanic rocks of andesitic

266

compositions. Three samples were selected in the Tiddiline Group. Sample ALDG21 is a rhyolitic

267

welded tuff observed at the top of the Tiddiline Group conformably overlaying cross-bedded quartzo-

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feldspathic sandstones. Sample ALDG4 is a trachytic dyke which cross-cutting the Tachdamt-Bleïda

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Group. And the sample AGEE24 is a sandstone sampling in the Dwaissa basin.

270

The sample AGDG1 is a quartz diorite which outcrops in the Trifya basin of Tiddiline Group. The

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relationships between conglomerates of the Tiddiline Group and dioritic intrusions are unclear.

272

However, cobbles and boulders of granodiorite and diorite occur within conglomerates of upper

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members of the Tiddiline Group. Hefferan et al. (1992) suggest that they came from the Bleïda

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granodiorite. Recently, Inglis et al. (2004) obtained two well constrained ages of 579.4 ± 1.2 Ma and

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578.5 ± 1.2 Ma for two samples of the Bleïda granodiorite.

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Two rhyolitic welded tuffs of the Ouarzazate Group have been sampled in the Bou Azzer-El Graara

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inlier and the Saghro massif. In the Bou Azzer-El Graara inlier, the ignimbrite (ALDG20)

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unconformably overlies folded ignimbrite (ALDG21) of the Tiddiline Group, and occurs at the local

279

base of the Ouarzazate Group. In the western edge of the Saghro massif, the ignimbrite (BODG6)

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occurs closer to the top of the ignimbritic sequence of the Ouarzazate Group. To the north of the Bou

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Azzer mine, basalts and volcanic breccias are interlayered with dolomies at the base of the Adoudou

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formation. Sample BOPC343 is one of these volcanic clasts.

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Finally, we will present a geodynamic reconstruction of the Anti-Atlas during Cryogenian and

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Ediacaran times.

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3. ANALYTICAL METHOD

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Two different ion microprobes were used for this study: a Neptune MC-ICP-MS at BRGM

287

(Orléans, France) and a SHRIMP

288

(Australia). For each sample, about 30 grains were mounted in epoxy and polished for U and

289

Pb isotopes to be analysed by the ion microprobe. Spot analyses were carried out on zircon

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grains with both instruments. Only the most homogeneous parts of the zircons, without any

291

cracks, were investigated after a careful checking of cathodoluminescence images and

292

reflected light photomicrographs of the sectioned zircon grains. Within the zircons,

293

patches showing alteration domains were avoided.

294

Three analyses on single grains were made using the Neptune MC-ICP-MS (ThermoElectron,

295

Bremen, Germany) at BRGM (Orléans, France) equipped with a multi-ion counting system, allowing a

296

very high sensitivity (Cocherie and Robert, 2007), and a laser ablation system (New Wave frequency-

297

quintupled Nd:YAG UV laser, distributed by VG, UK) operating at 213 nm. The ablation pit was 20 µm

298

in diameter and 15-20 µm deep. Argon gas was used as carrier gas. Zircon standard used is 91500

299

(Wiedenbeck et al., 1995). Standard bracketing was applied in order to correct both elemental

300

fractionation during the ablation process and mass bias originating from the MC-ICP-MS itself.

301

Detailed instrumentation and analytical accuracy descriptions are given in Cocherie and Robert (2008)

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and Cocherie et al. (2009).

303

The SHRIMP II was used for the thirteen others analyses. The SHRIMP analyses were

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performed following the analytical procedure described by Claoué-Long et al. (1995) and

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Willliams (1998). The zircon standards used to calibrate the U–Pb ratio were the 91500

306

zircon from Ontario (Canada) for the CAMECA ion microprobe (1062.4 ± 0.4 Ma; Wiedenbeck et

307

al., 1995), and the Duluth gabbro (USA) for the Australian microprobe (1099.1 ± 0.5 Ma; Paces

308

and Miller, 1993).

II

at

the

Australian

National

University,

Canberra

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Whatever analytical approach was used, all the uncertainty calculations were made at the

310

2σ level (95% confidence limit) using the ISOPLOT/EX program (version 3.6) of Ludwig

311

(2008).

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Two methods were used to build the histogram diagram. For ages younger than 1000 Ma, the plotted

313

data correspond to

314

Terra-Wasserburg Inverse Concordia diagram. For ages older than 1000 Ma, the plotted data

315

correspond to the

316

mixing line.

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4. GEOCHRONOLOGICAL RESULTS

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4.1. Tichibanine Group

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Two samples of rhyolites (AGEE298 and AGEE094) from the Tichibanine Group were dated. Sample

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AGEE298 is a rhyolite with quartz phenocrysts. The outcrop is a succession of felsic pyroclastic tuffs

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and rhyolitic flows. Sample AGEE094 is a dacitic coarse pyroclastic tuff with feldspar clasts. The

322

outcrop is a succession of lapilli, coarse and fin pyroclastic tuffs.

323

Zircons in AGEE298 are limpid, euhedral crystals with little well-developed 50-100 µm prisms.

324

Thirteen spot analyses were carried out on 12 zircon grains. All the analyses show a low common lead

325

contribution, and no loss of radiogenic lead. Thirteen sub-concordant points gave a weighted mean

326

age of 767 ± 7 Ma for the rhyolite AGEE298 (Figure 3a, Table 2).

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Zircons in AGEE094 are quite similar in size and morphology to those of AGEE298, but are light pink

328

to pink in colour. Fourteen spot analyses were carried out on 13 zircon grains. The analysis of grain 10

329

shows a level in common lead significative and a radiogenic Pb-loss. It will not be taken into

330

consideration. Two other analyses, 4.1 and 9.1, close to the Concordia curve, have not been

331

considered in the calculation of the age. Ten sub-concordant points gave a weighted mean age of

332

761 ± 7 Ma for the rhyolite AGEE094 (Figure 3b, Table 2).

333

These ages of 761 and 767 Ma correspond to the emplacement age of the rhyolites in the Tichibanine

334

volcanic arc.

335

4.2. Assif n’Bougmmane gneiss complex

206

Pb*/238U ages of the analyses involved in the calculation of obtained ages in the

207

* 206

Pb /

*

Pb ages for concordant analysis or extrapolated ages in case of Discordia

336

The five main outcrops of the assif n’Bougmmane gneiss complex occur at Bou Azzer, Oumlil,

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Tazigzaout, Ightem and Tamaliout in the Bou Azzer-El Graara inlier (Figure 2). In the Bou Azzer area,

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foliated gneisses are associated with augen gneisses, amphibolites and metagabbros. Sample

339

BOPC028 is a foliated fine- to coarse grained gneiss consisting of 3-5 mm-sized crystals of

340

plagioclase, quartz and hornblende. Sample AADG4 is an orthogneiss collected in the assif

341

n’Bougmmane area, where orthogneisses are associated with paragneisses, amphibolites,

342

metagabbros. Sample AADG4 is an augen gneiss with large plagioclase feldspar and/or quartz and

343

muscovite. Gneisses are crosscutted by leucogranites bodies.

344

Zircons in BOPC028 are small (~100 µm), limpid, and rounded. Fifteen spot analyses were carried

345

out on 15 zircon grains. The analyses show low U contents (20 to 30 ppm) and the lack of

346

common lead. Two analyses (9.1 and 10.1) with significant younger ages will not be taken

347

into consideration. Thirteen sub-concordant points gave a weighted mean age of 755 ± 9 Ma

348

(Figure 4a, Table 2).

349

Separated zircons in AADG4 are small to coarse-grained (~100 to 400 µm). They show two zircon

350

populations. The first one consists of colored, massive and metamict zircons, and the second one of

351

clear, colorless and small zircons. This second population has been selected for datation. Thirteen

352

spot analyses were carried out on 12 zircon grains. These thirteen sub-concordant points gave a

353

weighted mean age of 745 ± 5 Ma (Figure 4b, Table 2).

354

These ages of 745 and 755 Ma correspond to the emplacement age of the granitic protoliths of these

355

orthogneisses.

356

Samples AAPC140 and BOPC076 are porphyric granitic bodies exposed in the assif n’Bougmmane

357

and Oumlil areas respectively. Sample AAPC140 is a coarse-grained granodiorite consisting of

358

crystals of potassic feldspar, plagioclase feldspar and quartz. Sample BOPC076 is a coarse-grained

359

granite consisting of crystals of potassic feldspar, plagioclase feldspar, quartz, muscovite and biotite.

360

Zircons in AAPC140 are rare and small (~100 to 150 µm). They are sometimes elongated with a

361

concentric zonation. Fifteen spot analyses were carried out on 15 zircon grains. The analyses do not

362

show common lead, but the data show two distinct populations. Four analyses give a significative old

363

age of 743 ± 9 Ma (Figure 4c, Table 2) observed in orthogneiss AADG4. The main groups of eleven

364

zircons gave a weighted mean age of 702 ± 5 Ma (Figure 4c, Table 2). This age corresponds to the

365

emplacement of the granodiorite, whereas the older ages reflect the crustal inheritance.

366

Zircons in BOPC076 are rare and small (~50 to 150 µm). They are limpid with a concentric zonation.

367

Thirteen spot analyses were carried out on 13 zircon grains. The analyses show very low common

368

lead. Four analyses give a significative old age of 752 ± 10 Ma (Figure 4d, Table 2) observed in

369

orthogneiss BOPC028. The main groups of eight zircons gave a weighted mean age of 695 ± 7 Ma

370

(Figure 4d, Table 2). This age corresponds to the emplacement of the granite, whereas the older ages

371

reflect the crustal inheritance.

372

4.3. Bou Azzer Group

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Sample AADG12 is a light granodiorite dyke in a small exposure in cumulative gabbro of the ophiolitic

374

complex. This dyke is two hundred meters in length and fifteen meters in width, and has a foiled

375

contact with a cumulative gabbro. Sample AADG12 is a coarse-grained granodiorite consisting of 1-

376

4 mm sized crystals of potassic feldspar, plagioclase feldspar, quart and biotite. According to the

377

presence of potassic feldspar, sample AADG12 is not a plagiogranite, however potassic feldspar is not

378

abundant.

379

Zircons in AADG12 are clear, thick, often broken and small (