Geochronological constraints on the polycyclic magmatism in the Bou Azzer-El Graara inlier (Central Anti-Atlas Morocco)
Descripción
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
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
26 novembre 2013
1
Geochronological constraints on the polycyclic magmatism in the
2
Bou Azzer-El Graara inlier (Central Anti-Atlas Morocco)
3
O. Blein1*, T. Baudin1, P. Chèvremont1, A. Soulaimani 2, H. Admou2, P. Gasquet3, A.
4
Cocherie1, E. Egal1, N. Youbi2, P. Razin4, M. Bouabdelli5, P. Gombert1.
5
1
BRGM, BP 6009, 45060 Orléans Cédex, France
6
2
Faculty of Science, Cadi Ayyad University, Marrakech, Morocco
7
3
Laboratoire EDYTEM, Université de Savoie, CNRS, Campus Scientifique, 73376 Le Bourget du Lac
8
Cedex, France.
9
4
EGID, Université de Bordeaux 3, 33607 Pessac Cedex, France.
10
5
GEODE Terre et Patrimoine, B.P. 7004, 40014 Marrakech, Morocco.
11
*
Corresponding author.
12
Keywords. Anti-Atlas, Cryogenian, Ediacaran, Bou Azzer, Pan-African, U-Pb geochronology.
13
ABSTRACT
14
New U-Pb SHRIMP zircon ages from the Bou Azzer-El Graara onlier constrains the Neoproterozoic
15
evolution of the Anti-Atlas during Pan-African orogenesis. Within the Central Anti-Atlas, the Bou Azzer-
16
El Graara inlier exposes a dismembered ophiolite, long considered to mark a late Neoproterozoic
17
suture between the West African Craton in the south, and Neoproterozoic arcs to the north. From
18
north to south, this inlier includes four main geological units: a volcanic-arc, an ophiolite, a
19
metamorphic complex and a continental platform. Several plutons intrude the volcanic-arc, the
20
ophiolite, the metamorphic complex, and post-orogenic volcanic and sedimentary deposits
21
unconformably cover these terranes.
22
The age of the volcanic-arc is reported here for the first time. Analyses of zircon of two rhyolites
23
provide ages of 761 ± 7 Ma and 767 ± 7 Ma. Zircons from two gneisses provide dates of 755 ± 9 Ma
24
and 745 ± 5 Ma. Both dates are considered best estimates of the crystallization ages of their igneous
25
protoliths. Analyses of zircon from two granitic bodies, which crosscut gneisses, provide younger dates
26
of 702 ± 5 Ma and 695 ± 7 Ma. The age of an aplitic body of the ophiolite is reported here for the first
27
time, as 658 ± 8 Ma (SHRIMP U-Pb on zircons). Theses ages suggest the existence of three distinct
28
orogenic events during Cryogenian times: (i) 770-760 Ma Tasriwine-Tichibanine orogeny with rollback
29
of the subducting oceanic plate, leading to the formation of back-arc basins; (ii) 755-695 Ma Iriri-
30
n’Bougmmane orogeny; and (iii) the 660-640 Ma Bou Azzer orogeny involving the formation and the
31
emplacement of the Bou Azer ophiolite.
32
During Ediacaran times, the Bou Azzer-El Graara inlier is characterized with the development of a
33
continental volcanic arc between 630 and 580 Ma (Bou Lbarod Group, 625 ± 8 Ma ; Bleïda
34
granodiorite, 586 ± 15 Ma), and strike-slip pull-apart basins (Tiddiline Group, 606 ± 4 Ma and
35
606 ± 5 Ma). These volcanic and sedimentary Lower Ediacaran sequences are deformed before the
36
felsic pyroclastic deposits of the Ouarzazate Group (567 ± 5 Ma and 566 ± 4 Ma). Finally, the
37
Ouarzazate Group is overlain by early Cambrian volcanic deposits of the Jbel Boho Formation
38
(541 ± 6 Ma).
39
1. INTRODUCTION
40
Located on the northern edge of West African Craton (WAC), the Anti-Atlas belt of Morocco is
41
characterised by a Proterozoic basement unconformably underlying by late Ediacaran to Paleozoic
42
sedimentary rocks in several inliers (Bas Dra, Ifni, Kerdous, Akka, Igherm, Sirwa, Zenaga, Bou Azzer-
43
El Graara, Saghro and Ougnat; Figure 1). This Proterozoic basement consists of: (i) Paleoproterozoic
44
metamorphic and igneous rocks; (ii) Cryogenian rocks affected by the main Pan African orogenic
45
events; (iii) Early Ediacaran sedimentary and volcanic rocks deformed by the latest tectonic event of
46
the Pan African orogeny; and (iv) and late Ediacaran volcanic rocks (Ouarzazate Group). However,
47
two main structural domains have been recognised part of the NW-SE Anti-Atlas Major Fault (AAMF;
48
Choubert, 1963).
49
In Western Anti-Atlas (Figure 1), Paleoproterozoic basement (2030-2200 Ma) has been recognised
50
and recently confirmed by U–Pb zircon dating in several inliers (Ait Malek et al., 1998; Charlot-Prat et
51
al., 2001; Thomas et al., 2002; Walsh et al., 2002; Barbey et al., 2004, Gasquet et al. 2004). This
52
Paleoproterozoic basement, consisting of schists, gneisses, migmatites and plutonic rocks, is
53
unconformably overlain by Early Ediacaran sedimentary and volcanic rocks and/or Late Ediacaran
54
volcanic rocks.
55
The Eastern Anti-Atlas domain, including Sirwa, Bou Azzer-El Graara and Saghro inliers (Figure 1), is
56
characterised by the lack of Paleoproterozoic rocks (except as relics in the inherited cores of zircon
57
from Ediacaran volcanic rocks, Gasquet et al; 2005; Pelleter et al. 2007), and by Cryogenian rocks
58
affected by the main Pan African orogenic events, Early Ediacaran sedimentary and volcanic rocks
59
affected by the latest stage of Pan African orogeny, and late Ediacaran volcanic rocks. In the Sirwa
60
inlier, two dismembered ophiolite sequences (Tasriwine and N’Qob ophiolites) include ultramafic
61
cumulates, gabbros, a sub-vertical sheeted dyke complex and plagiogranite intrusions (Admou, 2000).
62
Samson et al. (2004) dated two plagiogranite intrusions within the Tasriwine ophiolite, 761 Ma and
63
762 Ma, which were interpreted to date formation of oceanic crust. In the central part of the Sirwa
64
inlier, medium to high grade metamorphic rocks are thought to represent the roots of an arc complex
65
estimated to be 743 Ma (Thomas et al., 2002). The relationships of the ophiolites in the Sirwa inlier to
66
the main Bou Azzer–El Graara ophiolite are unclear.
67
Recent mapping in the Bou Azzer-El Graara inlier and the western edge of the Saghro massif, within
68
the framework of the Moroccan National Project of Geological mapping (1/50 000-scale sheet maps of
69
Bou Azzer, Alougoum, Aït Ahmane and AlGlo’a) gave opportunities to acquire new field, geochemical
70
and geochronological data. Therefore, the aims of this paper are: (i) a re-appraisal of the lithological
71
units of the Bou Azzer-El Graara inlier; (ii) to present new geochronological data on magmatic rocks;
72
and (iii) to discuss the geodynamical evolution of the West African Craton during Proterozoic times.
73
The new geochronological data have allowed us to clarify the timing of polycyclic magmatism
74
observed in several terranes of Pan-African belt.
75
2. LITHOLOGICAL UNITS OF THE BOU AZZER-EL GRAARA INLIER
76
The Bou Azzer-El Graara inlier is a key element of the Anti-Atlas area for the understanding of Pan-
77
Africa events. It is believed to expose the dismembered relics of a Neoproterozoic suture zone
78
(Leblanc, 1981; Saquaque et al., 1989; Hefferan et al., 2000) marking the boundary between the
79
Paleoproterozoic Eburnean basement of the WAC to the south, and Neoproterozoic accreted arcs to
80
the north. Composed of tectonic blocks separated by oblique slip faults, the Bou Azzer-El Graara inlier
81
is structurally the most complex part of the whole Anti-Atlas (Leblanc, 1981; Saquaque et al., 1989,
82
1992).
83
Nine main lithological units were identified in the Bou Azzer-El Graara inlier: (i) a gneissic basement
84
composed of deformed meta-igneous and metasedimentary rocks, including augen granite gneiss,
85
and leucogranite (Assif n’Bougmmane gneiss complex); (ii) a plateform sequence of quartzite and
86
stromatolitic limestones overlain by mafic lavas and volcano-sedimentary rocks, interpreted as a
87
Tonian and/or Cryogenian passive margin cover sequence (Tachdamt-Bleida Group); (iii) a
88
Cryogenian mafic–ultramafic complex, interpreted as a dismembered ophiolite fragment (Bou Azzer
89
Group); and (iv) a Cryogenian metasedimentary sequence with subordinate volcanic units (Tichibanine
90
Group). These pre-Ediacaran units are deformed by the main Pan-African shortening tectonic event,
91
and are intruded by granitoid bodies, (v) the Ousdrat Suite, which are believed to have been emplaced
92
syn-tectonically during the main period of Pan-African orogenesis (Saquaque et al., 1989). This
93
collisional event is generally considered to have been the major Pan-African orogenic phase (PA1),
94
with the southward obduction of the Bou Azzer ophiolite onto the WAC at about 685 Ma (Leblanc,
95
1975; Leblanc and Lancelot, 1980), 663 Ma (Thomas et al., 2002), or 653-640 Ma (Inglis et al., 2004).
96
The above units are unconformably overlain or intruded by successively: (vi) mafic volcanic rocks (Bou
97
Lbarod Group); (vii) the terrigeneous sediments with local pyroclastic rocks (Tiddiline Group); (viii)
98
undeformed intrusions such as the Bleïda granodiorite (Bleïda Suite); (ix) a late Ediacaran volcano-
99
clastic sequence (Ouarzazate Group).
100
The lithostratigraphic system used for the Anti-Atlas orogen is summarised in Table 1, where it is
101
compared to the previous chronostratigraphic nomenclature.
102
2.1. Tonian and/or Cryogenian lithological units
103
The Tachdamt-Bleïda Group is characterized by the following succession from the base to the top: a
104
stromatolitic limestone and quartzite unit; a tholeiitic basalt unit; a schist unit; and a schist and
105
sandstone unit (Leblanc and Billaud, 1978; Mouttaqi and Sagon, 1999). The limestone and quartzite
106
unit comprises: an alternation of sandstones and limestones; massive quartzites; and an alternation of
107
sandstones and pelites. This group is interpreted as the local Neoproterozoic passive margin of the
108
West African Craton (Saquaque et al., 1989; Leblanc and Moussine-Pouchkine, 1994; Hefferan et al.,
109
2002; Bouougri and Saquaque, 2004).
110
2.2. Cryogenian lithological units
111
The southern part of the Bou Azzer-El Graara inlier consists of a variety of deformed igneous, meta-
112
igneous and metasedimentary rocks, including augen granite gneiss, paragneisses, low-grade
113
amphibolites, muscovite pegmatite and leucogranite. The five main outcrops occur at Bou Azzer,
114
Oumlil, Tazigzaout, Ightem and assif n’Bougmmane. On the basis of their deformational state and
115
lithological similarities with rocks of the nearby Zenaga Massif these units have been considered to be
116
2 Ga Eburnean WAC basement by all previous workers (Leblanc, 1981; Saquaque et al., 1989;
117
Saquaque, 1991).
118
Recently determined ages by SHRIMP on zircons have been performed on metagabbro, augen
119
granite gneiss, and crosscutting leucogranites bodies of the Tazigzaout complex (D’Lemos et al.,
120
2006). Three concordant U–Pb analyses of zircon from an augen granite gneiss provide a date of c.
121
753 Ma. Zircons from a nearby metagabbro provide a similar age date of c. 752 Ma. Both dates are
122
considered best estimates of the crystallization ages of their igneous protoliths. Analyses of zircon
123
from two crosscutting leucogranite bodies provide younger dates of 701 and 705 Ma.
124
The ophiolite defined by Leblanc (1975, 1981a) is a composite terrane, involving three separate units:
125
(i) an oceanic crust and upper mantle, i.e. the true ophiolite; (ii) a northern belt made up of calc-
126
alkaline volcanic rocks (Saquaque et al., 1989b), the Tichibanine Group; and (iii) late Cryogenian syn-
127
kinematic granitoïd plutons (Ousdrat Suite), intruding both oceanic and arc units (Beraaouz et al.,
128
2004).
129
In this paper, we restricted the term of Bou Azzer ophiolite to the oceanic-type crust and upper mantle.
130
This ophiolite comprises the following rock units: upper mantle tectonite peridotites, mafic-ultramafic
131
cumulates, submarine basaltic pillow lava, and volcano-sedimentary sequence (Leblanc, 1976;
132
Bodinier et al., 1984). This mafic-ultramafic complex is interpreted as a dismembered ophiolite
133
fragment (Leblanc, 1975, 1981) later on interpreted as a mélange complex (Saquaque et al., 1989;
134
Hefferan et al., 2002), taking place at blueschist metamorphic conditions (Hefferan et al., 2002).
135
Until recently no precise radiometric dates exist for any magmatic rocks of the Bou Azzer ophiolite.
136
Admou (2000) considered on extensive mapping and structural studies the Sirwa inlier as a westward
137
extension of the Bou Azzer-El Graara inlier. Within the Sirwa inlier, crop out two small highly
138
tectonized ophiolites, the N’qob and Tasriwine ophiolites (Thomas et al., 2002). Samson et al. (2004)
139
obtained two ages of 762 Ma on plagiogranites. El Hadi et al. (2010) place the age of the formation of
140
the Bou Azer ophiolite at 697 Ma.
141
The Tichibanine Group outcrops on the northern part of the Bou Azzer-El Graara inlier, and
142
corresponds to the northern terrane of Saquaque et al. (1989b). This group consists of a complex
143
tectonic assemblage of metagraywackes with basalts, andesites, rhyolites and tuffs (Tekiout et al.,
144
1991), bearing calc-alkaline and island arc tholeiitic signatures (Naidoo et al., 1991). A thick series of
145
layered cinerites has been evidenced. The rocks are metamorphosed at greenschist-facies to lower
146
epidote-amphibolite facies conditions.
147
The late Cryogenian granitoïds of the Ousdrat Suite are organized in massifs and stocks that are
148
either roughly circular in plan (Ousdrat) or, more commonly, WNW-ESE elongated (Ait Ahmane and
149
Bou Frokh), and intrude sedimentary and volcano-sedimentary Cryogenian rocks. The intrusions,
150
generally more intermediate than felsic rocks, include diorites, quartz-diorites and monzodiorites.
151
Published ages for these granitoïds are 667 ± 11 Ma (Mrini, 1993) and 653 ± 1.5 Ma (Inglis et al.,
152
2003) for quartz-diorites from Bou Frokh, 640 ± 1.5 Ma for quartz-diorites from Ousdrat (Inglis et al.,
153
2003), 650 ± 2 Ma for diorite from Ait Ahmane (Samson et al., 2004), and 646 ± 8 Ma and 646 ± 6 Ma
154
for diorites from Tamellalet and Tafrawt (Yazidi et al., 2008). These late Cryogenian (650-670 Ma)
155
medium-K calc-alkaline diorites were produced by partial melting of subducted oceanic crust followed
156
by interaction of the melt with the overlying mantle wedge (Beraaouz et al., 2004).
157
2.3. Ediacaran lithological units
158
Three volcano-sedimentary or volcanic groups occurred during that Ediacaran period: the Bou Lbarod,
159
the Tiddiline and the Ouarzazate groups.
160
The Bou Lbarod Group outcrops in the Bou Azer-El Graara inlier and in the western edge of the
161
Saghro massif (Figure 2). This group is composed of volcanic rocks with andesitic to rhyolitic
162
compositions, and corresponds to the lower member of the Ouarzazate Group defined by Choubert
163
(1953b). In the Saghro massif, the Issougri complex (the lower member of the Ouarzazate Group), a
164
great thickness of andesitic volcanic rocks, is folded with local sub-vertical bedding (Choubert and
165
Faure-Muret, 1970). The middle member of the Ouarzazate Group unconformably overlies the Issougri
166
complex.
167
The Tiddiline Group is a clastic sedimentary succession characterized by a coarsening upwards
168
sequence of siltstones to conglomerates with local diamictites interpreted as marine tilloids with
169
dropstones (Leblanc, 1975). This group unconformably overlies Cryogenian rocks deformed by the
170
Pan African orogeny. The Tiddiline Group is tilted, faulted and folded with local development of axial-
171
plane cleavage. In the eastern part of the Bou Azzer-El Graara inlier, the two main outcrops of the
172
Tiddiline Group are the Dwaïssa basin on the northern border and the Trifya basin on the southern
173
border. Hefferan et al. (1992) suggest that the upper members of the Tiddiline Group contain clasts
174
derived from the underlying tectonic blocks, including boulders of the Ediacaran granodioritic and
175
dioritic intrusions (such as Bleïda granodiorite), and thus the tectonic blocks were thrust over the
176
Tiddiline basins. On this basis Hefferan et al. (1992) argued for a continuation in collision within the
177
Bou Azzer region during deposition of the Tiddiline Group.
178
Recent mapping permit to observe that the conglomerates of the upper members are not stratified and
179
unconformably overlain typical stratified conglomerates of the Tiddiline Group. In the ultimate
180
conglomerates, the lithology of the boulders varies from west to east, from gneisses, to quartzite and
181
diorite. Such lithological variation reflects variations in local Cryogenian rocks and is frequently
182
observed at the base of the Ouarzazate Group. This formation is characterized by basal sedimentary,
183
volcano-sedimentary or volcanic breccias with boulders of local Cryogenian to Ediacaran basement.
184
These petrographic observations and U-Pb ages obtained on Bleïda granodiorite (Inglis et al., 2004)
185
suggest that ultimate conglomerates of the Tiddiline Group are in fact basal conglomerates of the
186
Ouarzazate Group.
187
The Ediacaran granodioritic and dioritic intrusions (Bleïda Suite) occur within each of the tectonic
188
blocks of the Bou Azzer-El Graara inlier and several of the intrusions carry a weak to moderately
189
developed foliation that is co-planar to the main regional fabric observed in the surrounding host-rocks
190
(Admou, 2000).
191
The Bleïda granodiorite is exposed in the eastern wedge of the inlier, emplaced within the Tachdamt-
192
Bleïda Group. The bulk of the intrusion is a medium grained granodiorite consisting of plagioclase,
193
quartz, hornblende, alkali-feldspar, apatite and zircon. Tonalitic and dioritic compositions occur within
194
the intrusion; without definite boundaries, they result from variation in the relative proportion of quartz,
195
hornblende and plagioclase. A fine-grained granodiorite phase is evident in the south of the intrusion,
196
forming sheets running parallel to the southern boundary. The granodiorite exhibits primary igneous
197
features. Internal magmatic fabrics are weak and discontinuous, defined by the alignment of lath
198
shaped hornblende, and possess no predominant structural orientation across the intrusion. In the
199
south-east of the body the magmatic fabrics are sub-vertical and trend to the WNW. In the north-west
200
fabrics rotate into parallelism with the boundary of intrusion. Ducrot (1979) provided the first U–Pb age
201
(615 ± 12 Ma) of the Bleïda granodiorite. Recently, Inglis et al. (2004) provided a younger age of
202
597 ± 2 Ma.
203
The relationship of the Bleïda Suite to deformation in the Bou Azzer inlier has remained open to
204
question. Leblanc (1981) argued that the lack of penetrative deformation within the intrusion implied
205
that its emplacement post-dated thrusting and pervasive greenschist facies metamorphism. Saquaque
206
et al. (1989) regarded the intrusion as having been emplaced and deformed during the greenschist
207
facies deformation event. Inglis et al. (2004) argued that the granodiorite was emplaced after the end
208
of greenschist facies metamorphism and the development of the pervasive regional fabrics in the Bou
209
Azzer inlier. However, they suggested that the intrusion could conceivably have been emplaced during
210
late brittle transcurrent faulting, and in this regard the granodiorite is better identified as being syn-
211
tectonic only with late brittle deformation in the region, rather than pervasive regional fabric
212
development (Inglis et al., 2004).
213
The Ouarzazate Group consists of potassic to high-potassic basaltic andesites, andesites, dacites,
214
and rhyolites interstratified with chaotic breccia, polygenic conglomerates and arkosic sandstones.
215
Felsic volcanics were dated in several inliers: 565 Ma in the Tagragra of Tata (Walsh et al., 2002);
216
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
218
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
227
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-
268
feldspathic sandstones. Sample ALDG4 is a trachytic dyke which cross-cutting the Tachdamt-Bleïda
269
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
271
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
273
members of the Tiddiline Group. Hefferan et al. (1992) suggest that they came from the Bleïda
274
granodiorite. Recently, Inglis et al. (2004) obtained two well constrained ages of 579.4 ± 1.2 Ma and
275
578.5 ± 1.2 Ma for two samples of the Bleïda granodiorite.
276
Two rhyolitic welded tuffs of the Ouarzazate Group have been sampled in the Bou Azzer-El Graara
277
inlier and the Saghro massif. In the Bou Azzer-El Graara inlier, the ignimbrite (ALDG20)
278
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)
280
occurs closer to the top of the ignimbritic sequence of the Ouarzazate Group. To the north of the Bou
281
Azzer mine, basalts and volcanic breccias are interlayered with dolomies at the base of the Adoudou
282
formation. Sample BOPC343 is one of these volcanic clasts.
283
Finally, we will present a geodynamic reconstruction of the Anti-Atlas during Cryogenian and
284
Ediacaran times.
285
3. ANALYTICAL METHOD
286
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
290
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)
302
and Cocherie et al. (2009).
303
The SHRIMP II was used for the thirteen others analyses. The SHRIMP analyses were
304
performed following the analytical procedure described by Claoué-Long et al. (1995) and
305
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
309
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).
312
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.
317
4. GEOCHRONOLOGICAL RESULTS
318
4.1. Tichibanine Group
319
Two samples of rhyolites (AGEE298 and AGEE094) from the Tichibanine Group were dated. Sample
320
AGEE298 is a rhyolite with quartz phenocrysts. The outcrop is a succession of felsic pyroclastic tuffs
321
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).
327
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,
337
Tazigzaout, Ightem and Tamaliout in the Bou Azzer-El Graara inlier (Figure 2). In the Bou Azzer area,
338
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
373
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 (
Lihat lebih banyak...
Comentarios