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The Carboniferous volcano-sedimentary depocentre of Tazekka Massif (Middle-Atlas, Morocco): new observations and geodynamic implications
Journal of African Earth Sciences 39 (2004) 359–368 www.elsevier.com/locate/jafrearsci
The Carboniferous volcano-sedimentary depocentre of Tazekka Massif (Middle-Atlas, Morocco): new observations and geodynamic implications Azzeddine Bennouna a,*, Mohamed Ben Abbou a, Christian Hoepffner b, Fatima Kharbouch b, Nasrrddine Youbi c a
Faculty of Sciences Dher Mehraz, Department of Earth Sciences, University Mohamed Ben Abdellah, B.P. 1796 Atlas, Fe`s, Morocco b Faculty of Sciences, Department of Earth Sciences, University of Mohamed V, B.P. 1014, Rabat, Morocco c Laboratory of Petrology, Department of Geology, University Cadi Ayyad Semlalia, B.P. 2390, Marrakech, Morocco Available online 25 September 2004
Abstract The integrated analysis of thrust structures and facies allows us to interpret the Carboniferous volcano-sedimentary complex of Tazekka massif as a compressive sub-basin controlled mainly by the Hajra Sbaa–Merja Caı¨Õd northwest-verging thrust-propagation fold. The tectono-sedimentary sequence (conglomerates, greywacke and shale) is associated with an extrusive magmatism comprising basalts, andesites and rhyolites, under effusive and volcanoclastic facies (or rhyolites with exotic blocks), of orogenic calk-alkaline nature, which is consistent with a context of continental subduction. These results, and the comparison of regional contraction ages in the Moroccan Meseta, integrate the Carboniferous volcano-sedimentary depocentre of Tazekka massif in the wedgetop depozone of a foreland basin system generated by two north-west thrusted piggy-back sequences (aged from Fameno-Tournaisian to upper Visean-lower Westphalian) from the eastern Meseta to the western Meseta of Morocco. Ó 2004 Published by Elsevier Ltd. Keywords: Foreland basin system; Propagation of thrust-and-fold system; Thrust-controlled deposition; Depocentre migration; Calk-alkaline volcanism; Carboniferous; Eastern Meseta; Morocco
1. Introduction The Hercynian Tazekka massif situated in the centre of the Middle Atlas mountains (Morocco) forms the transition between two mesetian domains of different structural evolution: the Eastern Meseta characterised by a first deformation of successive synschistose folding and shearing whose age is post-Frasnian and anterior to upper Visean (Eovariscan phase; Hoepffner, 1987), and the Western Meseta with a major post-Viseo-Namurian deformation (Asturian phase; Pique´ and Michard, 1981;
*
Corresponding author. Tel.: +212 55 75 40 81; fax: +212 55 72 34
05. E-mail address: [email protected] (A. Bennouna). 0899-5362/$ - see front matter Ó 2004 Published by Elsevier Ltd. doi:10.1016/j.jafrearsci.2004.07.032
Pique´, 1989; Pique´ et al., 1993). The latter is preceded by the formation of turbiditic basins opened since the upper Devonian, in response to the ‘‘Eovariscan phase’’ of the Eastern Meseta. This structural zonation and the general vergence towards the west or southwest indicate a structural polarity between the internal (eastern) and external zones (western) of the Moroccan mesetian chain (Hoepffner, 1987) (Fig. 1). The Carboniferous volcano-sedimentary complex, up to 1000 m thick, occupies the NE extremity of Tazekka massif (almost 20 km South of Taza city) and unconformably rests upon the Ordovician Schists that constitute the essential outcroppings of Tazekka massif (Agard et al., 1958; Rauscher et al., 1982) (Fig. 1). It is represented by argillaceous and calcareous greso-conglomeratic deposits of Viseo-Namurian age (Marhoumi,
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Fig. 1. Geological map of the Carboniferous volcano-sedimentary depocentre of Tazekka massif (after Huvelin, 1986; Chalot-Prat, 1990; Essamawal, 1999) and localisation of the cross-sections of Fig. 2. (1) Ordovician substratum, (2) andesitic flows and associated volcano-sedimentary rocks, (3) basaltic flows, (4) rhyolites with reworked blocks, (5) microdiorites, (6) Tazekka granite, (7) Secondary terranes, (8) Miocene. Insert: place of Tazekka massif in Hercynian septentrional Morocco.
1985; Chalot-Prat and Vachard, 1989) associated with volcanic rocks where two superimposed units are distinguished: an andesite with olivine, hornblende and/or pyroxene that is surmounted by another unit constituted of effusive and ignimbritic rhyolites (Chalot-Prat, 1986, 1990; Kharbouch, 1994). This volcano-sedimentary complex was often interpreted as a Viseo-Namurian extensional basin inverted by a NW–SE compressive deformation (Hoepffner, 1987; Essamawal, 1999) or as a post-orogenic volcanotectonic depression, like a caldera (Chalot-Prat, 1986, 1990). Huvelin (1986, 1992) underlines the importance of resedimentation phenomena contemporaneous with
deposition of the volcanic rocks, suggesting a fast infilling of a basin bounded by active faults. Some new tectonic, sedimentary and magmatic observations allow us to illustrate the tectonic control of the sedimentation, the genesis of the volcanic rocks and to propose a new model of the dynamics of this volcanosedimentary depository.
2. Tectonism/sedimentation relationships The volcano-sedimentary depression of Tazekka massif corresponds to a sedimentary depocentre where
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Fig. 2. Structural cross-sections in the Carboniferous volcano-sedimentary depocentre of Tazekka massif, (A) Hajra Sbaa, (B) Merja Caı¨d. (1) Ordovician substratum, (2) andesitic flows and associated sedimentary and volcano-sedimentary rocks, (3) basaltic flows, (4) rhyolites, (5) Secondary terranes.
volcanic extrusions were predominant, especially of andesitic and rhyolitic nature (Fig. 1). Regarding the sedimentary filling, the Carboniferous series shows some facies variations from the Eastern to the Western boundaries of the depocentre. The Eastern boundary series, which outcrops only at the level of the Hajra Sbaa mound in the SE extremity of the complex (Fig. 1), is oriented NE–SW, dipping 70°– 80° to the West (Fig. 2A). It comprises catastrophic sedimentation that is unconformable on an Ordovician slate substratum, organised in a megasequence (100– 150 m), composed at the bottom of a decametric conglomeratic bed with andesitic flows, evolving towards the top into sandstones (Fig. 3A). The conglomerates are unorganised and exclusively constituted of angular to sub-rounded blocks, cobbles and pebbles, derived solely from the Ordovician basement (slates, sandstones, quartz veins). This retrogressive megasequence is localised at the top of a flat overthrust (NE–SW, 10–20°E), associated with an amortisation anticline in the Ordovician series outcropping from Hajra Sbaa to Merja Caı¨d (Figs. 1 and 2) (Ordovician slice of Merja Caı¨d; Hoepffner, 1987). This disposition allows interpretation of this megasequence as a tectonogenic prism, reflecting the fast creation of available space and the destruction and erosion of the eastern boundary to the depocentre. The western series of the volcano-sedimentary complex (200–300 m) is also oriented NE–SW with a weak dip (10–30°) toward the East. It reflects volcano-sedimentary deposition: arkosic sandstones or argillaceous greywackes which alternate with numerous metric to
decametric andesitic flows (Figs. 1 and 2). At the small scale, the argillaceous greywackes exhibit erosive bases and load casts. The thin argillaceous sandstones interbeds show, most often, convoluted and/or parallel lamination (Fig. 3B). The whole succession reflects storm dynamics associated with some andesitic flows, on a relatively stable, shallow platform. From the magmatic point of view, the volcano-sedimentary lower series is surmounted by basaltic extrusions (300 m) in the sector Hajra Sbaa–Merja Caı¨d (Figs. 1 and 2a). The whole is overlain unconformably by porphyritic effusive, or crystal-rich, pyroclastic rhyolitic flows (ignimbrites in the sense of Chalot-Prat, 1986, 1990; Hoepffner, 1987; Kharbouch, 1994) (Figs. 2 and 3). The pyroclastic rhyolitic flows constitute the main outcrops of the Carboniferous volcano-sedimentary complex of Tazekka massif (Fig. 1), and are characterised by the ubiquitous presence of transported exotic blocks, with variable volumes, from cm3 to several hundred m3, derived essentially from the Ordovician substratum (slates and sandstones) and from the underlying Carboniferous series (conglomerates and siltstones, volcano-sedimentary beds, bioclastic limestones, rhyodacites, andesites, basalts). Rhyolitic blocks with porphyroclastic or pyroclastic structure are reworked from the same pyroclastic rhyolitic flows. Blocks of granite and microdioritic rocks are also present. This important reworking within pyroclastic rhyolitic massflows was described locally as a resedimentation phenomenon (Huvelin, 1986).
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The Western margin of the Carboniferous depocentre is structured by westverging slices, implying essentially the lower and middle Ordovician (Hoepffner, 1987; Amaouin, 1991). They constitute an antiformal stack (Boyer and Elliot, 1982) situated between a ‘‘floor thrust’’ located in the lower part of the Ordovician and a ‘‘roof thrust’’ situated at the base of the Upper Ordovician and/or in the Silurian shales. The whole structure is complicated at the south-western extremity of Tazekka massif by many back thrusts toward the SE, emplaced in the ‘‘roof thrust’’. Therefore, the different structures observed in the Tazekka massif can be integrated in a continuous progression of thrust sequences determining duplex culmination in the Ordovician substratum, and controlling at the same time the Carboniferous sedimentation and magmatism in a transported depocentre (Fig. 2). This model differs from the discontinuous succession of short tectonic phases previously proposed (Hoepffner, 1987; Essamawal, 1999).
3. Magmatism
Fig. 3. Lithostratigraphic successions of the active (A) and passive (B) boundaries of the Carboniferous volcano-sedimentary depocentre of Tazekka massif, (la) detrital series composed of catastrophic conglomerates and sandstones of the active margin, (1b) volcano-sedimentary series composed of greywacke and argillaceous silts of passive margin, with numerous andesitic flows, (2) Basaltic flows, (3) pyroclastic and effusive rhyolites.
The genesis of these basaltic and rhyolitic flows was related to the progression of the thrusting at Hajra Sbaa–Merja Caı¨d, as attested to by some metric basaltic veins and by a rhyolitic vein in the order of 30 m width at the level of Merja Caı¨d (Fig. 2). On the whole, the Viseo-Namurian volcano-sedimentary depocentre developed at the top of an amortisation anticline of the main thrust of Hajra Sbaa–Merja Caı¨d that is itself covered toward the NE by secondary deposits (Fig. 1). The disposition is here clearly the result of a ‘‘CAS’’ system (Specht et al., 1991; De´ramond et al., 1996) that realises the synchroneity of tectonism, sedimentation and magmatism, and that are controlled by the development of a thrust (C = chevauchement), the formation of an amortisation anticline (A = anticlinal) and the consecutive creation of a frontal syncline (S = synclinal) (Fig. 2A and B).
The Carboniferous volcanism of Tazekka depocentre corresponds to a calk-alkaline association typical of orogenic zones (Hoepffner, 1981; Kharbouch et al., 1985). Chalot-Prat and Cabanis (1989) and Chalot-Prat (1990) distinguish two groups of volcanic rocks. The first is composed of basic to andesitic homogeneous or hybrid rocks with an intraplate affinity and/or associated with an active continental margin; the second is essentially rhyolitic, of crystal origin (Chalot-Prat, 1995). On the basis of petrographical and geochemical studies, we regroup the different volcanic rocks of the Carboniferous volcano-sedimentary depocentre of Tazekka massif in three superimposed petrographic entities: andesitic (basaltic andesites, andesites and rhyodacites), basaltic (olivine basalts) and porphyritic rhyolites (effusive and homogeneous at the bottom and crystal-rich pyroclastic flows with reworked blocks at the top). 3.1. Petrography The basic, intermediate and acidic Carboniferous volcanic rocks of the Tazekka massif depocentre are flow-textured microlitic porphyric lavas or are porphyroclastic with some variable phenocryst assemblages as a function of the SiO2 content. In addition, plagioclase, clinopyroxene and olivine are common, with some variable proportions reflecting the transition from the basalts to the andesites. The amphibole (green hornblende) is found in the basaltic andesites to andesites and rhyodacites. The biotite is present in the andesites, rhyodacites and rhyolites. The quartz and alkali feldspar
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
are found in the rhyodacites. The accessory mineral phases are represented by apatite, zircon and opaque minerals. The mineral assemblages are the same, from the first homogeneous effusive rhyolitic flows with a porphyritic texture and variable modal phenocryst contents, to the uppermost crystal-rich pyroclastic rhyolitic flows with reworked blocks. Furthermore the reworked blocks of different types (shales and quartzitic sandstones, basalts, andesites, granites and microdiorites,. . .), the cognate enclaves of the crystal-rich pyroclastic rhyolitic flows (with angular to sub-rounded shape and a volume lower than m3), show up to 40% of phenocrysts; the latter are large (15–50 mm), frequently broken and well oriented in a vitreous flow-textured mesostase. These cognate enclaves, present exclusively in the uppermost rhyolites containing reworked blocks, are similar to less differentiated porphyric homogeneous effusive rhyolites with variable modal phenocrysts that outcrop at the base of the rhyolitic group. The hypothesis of hybrid magma proposed by Chalot-Prat and Cabanis (1989) and Chalot-Prat (1990) appears unlikely. Indeed, no structure indicative of magma mixing was observed, either at the outcrop or at the microscopic scales, although some xenocrysts of quartz, plagioclase and/or of amphibole are present in the basic and intermediate volcanic rocks. The acidic group is equally homogeneous and does not show crystalline phases of the precedent group that appear rather as reworked blocks. The mineral phases of these volcanic rocks are generally altered. The olivine is either silicified and/or chloritised and iddingsitised. The pyroxene and amphibole are generally altered to chlorite, epidote, calcite and oxides. The biotite is transformed into muscovite and oxides in the andesites, and to chlorites, epidote and oxides in the rhyodacites and rhyolites. The plagioclase feldspar is altered to epidote, calcite and/or sericite. The matrix of these rocks is generally devitrified and recrystallised to a felsitic homogeneous assembly associated with chlorite, calcite and oxides. These transformations are thought to be the result of hydrothermal activities which accompany antimony polymetallic mineral deposits. 3.2. Geochemistry The geochemistry of the volcanic rocks (major elements, trace elements and REE) of the Carboniferous depocentre of the Tazekka massif was established through 30 analyses, 15 of which were obtained from the previous work (Chalot-Prat and Cabanis, 1989). The new chemical analyses were performed at the University of Bretagne (Brest, France). The effusive rocks of the volcano-sedimentary depocentre of Tazekka massif are spilitised and generally show some contents in LOI, often above 2%, in particular in the basic and
363
andesitic lithologies, and reach values in some samples in the order of 9% (Table 1). The C.I.P.W norm allows us to infer the chemical modifications that affected original magmas. It shows silica and sodium enrichment and loss of calcium and potassium revealed by a variation of the contents in quartz, albite, orthoclase, anorthite and by the appearance of corundum in the CIPW norm. The mobility of the other elements is difficult to establish; we note, however, iron oxidisation and MgO depletion. The least weathered basalt and andesite samples show a supersaturation in silica and fairly elevated contents of A12O3, weak TiO2 and elevated K2O and in CaO contents (Table 1). The syn- to post-magmatic mobility of the chemical elements previously cited reduces the potential of these elements for the chemical characterisation of the Carboniferous magmatism (Cabanis, 1986). It is however possible to note an absence of enrichment in iron and in titanium and some weak contents of TiO2 (0.5–1.16%) in the basic and intermediate rocks, that are characteristic features of calk-alkaline magmatism. The normalisation multi-element diagrams permit the visualisation of the more discriminating chemical element contents for the basic and intermediate magmas (Pearce, 1983; Cabanis, 1986). The andesites show a very marked negative anomaly in Nb, characteristic of an orogenic volcanism (Fig. 4). This orogenic affinity is also attested by the generally weak contents in transition elements (V, Cr, Ni and Sc); the samples with high contents in Cr, Ni and Sc would be in relation to the elevated concentration in ferromagnesian minerals, notably pyroxene. The high values of La/Nb, above 1.8 (2–5.82), associated with high values of Zr/Y, above 3 (4.63–12.29) and weak values of Ti/Y, lower than 350 (151.60–247.73), are characteristic of those of an orogenic magmatism of a continental active margin setting (Condie, 1989). The elevated contents in hygromagmaphile elements (Sr, K, Rb, Ba, Th, La and Ce) and in lithophile elements (Sr, K, Rb and Ba) confirm the orogenic character of the Carboniferous andesitic rocks of Tazekka massif, with a possible involvement of a crustal component, as testified to by the relatively high values of ratios, such as (Ce/Yb)N (4.48–12.20). The basic volcanic rocks are distinguished from the andesitic rocks by relatively less important concentrations in hygromagmaphile elements, showing the less differentiated and contaminated nature of these rocks (Fig. 4) affirmed by relatively weak values of (Ce/ Yb)N ratios (3.35 and 6.10). The negative anomaly in Nb and the high values of La/Nb ratios, above 1.8 (2.36–2.70), associated with high values of Zr/Y ratios, above 3 (5.61–7.40) and weak values of Ti/Y ratios, less than 350 (239.80–315.83), are analogous to those of an orogenic magmatism of continental active margin rather
364
Table 1 Representative chemical analysis (W/R) of extrusive magmatic rocks from the Carboniferous volcano-sedimentary depocentre of Tazekka massif Sample Basalts HS
B21
Basic andesite and andesite B29
Porph. Rh.
K8475 K8476 K8468 K8470 K8460 K8843 K8470a K8467 B63
B12
CBc6
Porphyrocl. Rh.
Enc.
CBc39 CBc41 CBc51 CBc53 CMa1 CMa4 CMa10 CMa11 CMa46 CMa61 CRL14 CRL23 CRII12 CRI25 K8483 B44
SiO2
54.30 53.40 54.00 52.10
53.50
53.60
56.50
57.00
57.90
59.30
62.50
55.40 58.20 55.37
67.47
59.48
54.85
62.72
50.92
57.02
57.22
57.75
65.95
50.98
73.07
73.20
73.57
75.24
70.00
70.50
TiO2
0.97
1.02
0.96
0.73
0.68
0.75
0.69
0.70
1.00
0.78
0.95
1.16
0.91
1.12
1.02
0.80
0.99
0.53
1.09
0.17
0.16
0.36
0.37
0.40
0.43
Al2O3
17.20 16.40 16.95 15.70
0.97
1.02
0.99
0.75
0.99
15.20
15.80
15.80
16.20
14.70
14.80
14.92 15.50 14.18
17.93
16.70
16.90
17.20
16.86
16.42
15.74
18.29
15.77
17.21
13.90
13.41
12.97
12.28
11.80
13.60
7.70
7.30
7.30
9.00
6.40
6.60
6.30
6.00
4.65
5.80
7.85
5.94
7. 39
5.83
6.27
7.71
5.49
7.64
6.70
5.97
5.40
3.51
7.74
1.61
1.85
2.75
3.41
3.40
3.75
MnO
0.14
0.14
0.21
0.47
0.14
0.19
0.16
0.17
0.16
0.18
0.12
0.13
0.13
0.12
0.13
0.10
0.10
0.06
0.10
0.16
0.12
0.12
0.08
0.15
0.07
0.01
0.08
0.00
0.14
0.10
MgO
4.82
6.22
4.39
7.69
7.20
2.30
2.46
2.38
2.25
4.03
2.60
5.03
2.07
5.38
1.75
2.54
2.99
2.37
5.71
4.37
3.82
2.06
1.53
6.74
0.18
0.16
0.48
0.68
0.66
0.80
CaO
4.45
7.30
2.96
5.80
5.90
6.70
4.50
5.60
5.30
4.28
3.70
4.90
4.60
5.06
0.43
4.97
7.04
1.43
7.96
2.77
5.96
3.31
3.66
7.47
0.59
0.01
1.18
0.12
2.80
1.50
Na2O
6.10
2.90
6.61
2.68
3.99
2.24
2.29
4.22
2.23
3.43
0.07
3.60
3.21
4.81
2.71
252
2.73
4.49
3.54
3.30
4.61
5.04
4.20
2.53
3.64
2.23
4.09
1.53
2.21
3.27
K2O
0.90
0.22
0.34
0.62
1.94
2.81
3.10
1.86
3.00
0.89
3.72
2.44
3.10
2.16
1.96
2.83
4.36
3.46
0.33
2.21
1.11
1.87
1.51
0.33
4.04
4.79
3.88
3.69
3.41
4.40
P2O5
0.20
0.19
0.26
0.43
0.30
0.42
0.20
0.37
0.62
0.34
0.22
0.20
0.15
0.18
0.11
0.21
0.10
0.05
0.09
0.08
LOI
2.96
4.48
5.65
5.20
2.21
7.58
6.17
5.87
6.28
6.77
5.81
3.68
5.73
3.14
3.37
5.44
3.43
2.38
5.49
5.23
5.40
4.67
4.15
4.76
1.31
1.45
1.51
1.61
3.97
1.50
Total
99.89 99.92 99.69 98.45
98.50
97.98
98.31
99.88
100.07 99.24
99.82
99.38 99.53 99.02
99.40
101.00
99.68
101.00
99.21
98.68
97.32
100.96
99.01
98.79
99.98
Ba
265
152
290
230
664
1463
604
266
817
617
1164
1100 880
1071
1085
1409
1039
182
143
183
625
542
103
579
577
337
1160
438
555
Rb
285
6
17
14.40
23.80
112
124
89
34
159
73
135
69.80
71.80
103.14 106
116
12.30
75.10
37.90
82.80
56.56
14.86
172.50
216
168
158
159
172
Sr
620
371
412
344
714
317
813
316
324
193
675
565
847
337
611
1016
585
580
353
293
416
499
294
43
90
77
143
136
Y
21
20.50 21
25.10
24.60
24.20
25.70
26
26
29.50 24
Zr
115
120
155
277
292
262
138
259
210
170
Nb
5.80
5.60
7.60
13
12
6.45
7.20
Th
3.30
3.20
Ni
122
V
160
Cr
286
123
104
273
4
3.10
2.30
12.70
17.80
16.30
124
52
196
111
35
50
46
77
155
130
178
131
130
132
265
76
78.80
82.20
82.80
88.30
13
102.66 102.17 101.89 100.95 99.99 1843 1520
0.13
25.80
39 76
272
187
294
388
293
148
135
147
166
110
178
129
106
182
179
217
13
6.60
12
10.60
13.40
10.70 14
11.27
9.69
13.09
14.63
12.81
3.66
4.86
6.88
5.46
7.61
3.57
10.70
10.46
13.60
13.60
14.60
15.60
36
43
218
204
40.43
45.42
20.52
33.94
148
156
62.70
55.30
23
171
1.88
1.50
4.65
10.10
11
13
105
85
180
85
73.70
485
38 66
614.00 85.94
111.10 65.06
85.14
245
453
47
33 36
163
271
42
234
42
7.60
6.50
17
20.90 5.90
Hf
2.90
3
6.50
7.30
6.60
6.60
6.32
4.52
7.15
8.94
7.05
3.53
3.65
3.57
3.55
3.16
3.51
3.27
3.20
4.46
5.10
Cs
6.20
0.18
0.90
8.90
6.60
15
0.71
2.95
5.98
5.87
4.32
655
17.32
5.97
2.67
3.59
6.00
8.06
6.80
3.63
12.50
6.80
40
19.90
15.20
14.60
24.60
12.11
16.69
22.79
15.72
21.14
22.78
16.71
24.16
8.33
21.02
4.76
4.90
7.24
7.70
9.90
0.57
0.57
0.60
0.78
0.74
0.50
0.63
0.73
0.81
0.67
0.56
0.40
0.47
0.47
0.52
0.55
1.32
1.32
1.57
1.40
1.32
32.80
44.10
23.10
20.20
19.70
35.43
9.60
17.23
21.36
15.55
29.78
28.67
21.38
14.04
11.51
31.06
1.62
1.00
4.30
5.10
7.20
1.10
0.47
4.90
6.20
6.50
4.14
2.55
2.90
5.39
5.46
1.07
1.63
2.73
2.06
2.70
1.17
4.48
3.60
6.12
4.20
4.70
Sc
23
22
15.70 20.50
Ta Co
31
30
24
U
15.30 21
13.80 13.20 20.50 15.20
8.60
41.30
40.30
36.80
41
30
27.90
20.60
94
79
82
74
Pr
16.50 15.80 26.50
Sm
3.70
3.40
4.90
3.90
3.50
9.80
9
8.10
Eu
1.08
1.08
1.34
1.22
1.01
2.60
1.80
1.75
Gd
3.85
3.50
4.40
Tb
20
15.00
28.50
0.57
0.83
0.74
34
18
1.45
0.68
33
34
30.63
24.27
31.03
48.49
29.42
15.69
16.27
18.28
23.27
21.40
16.77
24.33
15.10
26.98
25
34.80
37.50
73
67
66.83
44
65
64.41
56.32
25.53
27.48
30.23
32.01
37.24
26.61
38.60
30.50
52.54
47
74
79
39
34.50
7.50
8.10
6.80
5.19
4.76
6.52
10.05
6.76
3.57
3.47
2.40
4.67
3.67
3.86
4.30
2.60
6
5.10
8
7.25
2.10
1.95
1.63
2.07
1.14
1.74
136
1.24
1.07
1.45
1.01
0.82
0.60
0.74
0.75
1.04
1.07
7.25
5.80 0.84
0.53
0.74
0.59
0.55
0.48
0.79
0.38
0.59
0.57
0.98
0.83
0.94
0.74
3.55
3.30
3.90
4.50
5.30
4.25
Er
2
1.85
2
2.50
2.90
2.20
Yb
1.88
1.79
2.05
2.40
2.08
2.20
1.70
2.27
7
6.90
Dy
2.10
9.50
35.30
30
0.53
14
4.60
Ce
45
26 0.70
La
28
15
2.20
34.50
7.30 0.91
0.85
0.58
6.70 3.50 2.46
1.69
2.38
1.81
3.49
1.81
1.97
The samples noted CBc, CRL and CRI are from Chalot-Prat and Cabanis (1989); En: cognate enclave of rhyolitic pyroclastic flows (Fe2O3* = total Fe as Fe2O3).
2.12
1.70
1.58
1.70
2.97
1.91
4.92
3.15
3.50
3.34
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
13.60
Fe2O3* 7.85
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
365
Fig. 4. MORB normalised patterns (after Pearce, 1983) for basaltic and andesitic flows from Carboniferous volcano-sedimentary depocentre of Tazekka massif.
Fig. 6. REE normalised to Chondrites (after Nakamura, 1974) for basaltic, andesitic and rhyolitic flows from Carboniferous volcanosedimentary depocentre of Tazekka massif.
than to those of an intraplate magmatism contaminated by a subduction component (Chalot-Prat, 1990). The eruption in the Carboniferous depocentre of Tazekka massif of andesitic lavas, rich in K2O and in Th and therefore more differentiated than the basaltic lavas can be explained by a magmatic differentiation of a basaltic denser liquid generating a less dense andesitic liquid, capable of reaching the surface more easily (Masson et al., 1996; Wilson, 1989). The multi-element normalization diagram for the available analyses of the basal homogeneous porphyric rhyolitic facies, represented by a cognate enclave of crystal-rich pyroclastic rhyolites, compared to ‘‘ORG’’ (Pearce et al., 1984) suggests that these rhyolites resemble to the calk-alkaline granites of volcanic arc affinity (Fig. 5), with an enrichment in K, Rb, Ba, Th and in Ce and Sm, with respect to the contents of Nb, Hf, Zr and Y.
The volcanic rocks of the Carboniferous depocentre of Tazekka massif present rather fractionated Chondrite-normalised REE patterns (Nakamura, 1974) with (La/Yb)N varying from 4.91 to 10.93 (Fig. 6). The negative Eu anomalies become more and more marked from the basalts to the basal rhyolites (cognate enclaves) (0.95–0.45), indicating more and more fractionation of plagioclase feldspar from basaltic to rhyolitic rocks. In the same sense the relatively elevated content of REE in the rhyolites, particularly in HREE, attests to a continuous differentiation by fractional crystallisation from the basalts to rhyolites. In the (Ce/Yb)N versus Th diagram, permitting distinction between the rate of crustal participation and the degree of magmatic differentiation by fractional crystallisation, the basalts are least differentiated and contaminated, with weak values for (Ce/Yb)N ratios and Th contents. The andesites and rhyolites would
Fig. 5. ORG normalised patterns (after Pearce et al., 1984) for rhyolitic samples from Carboniferous volcano-sedimentary depocentre of Tazekka massif.
Fig. 7. Evolution of volcanic rocks from Carboniferous volcanosedimentary depocentre of Tazekka massif in a (Ce/Yb)N versus Th diagram. Arrow indicates the sense of magmatic evolution by fractional crystallisation with or without crustal assimilation (ACF).
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have been associated with the basalts in the same magmatic series that would have evolved by fractional crystallisation with assimilation of a crustal component, which is distinctly marked in the andesitic lavas (Fig. 7).
The Th/Yb versus Ta/Yb diagram of Pearce (1983) (Fig. 8) also shows that the Carboniferous Tazekka volcanics belong to the orogenic calk-alkaline magmas of continental active margin settings.
4. Conclusion
Fig. 8. Th-Ta-Yb diagram (after Pearce, 1983) for basaltic and andesitic flows from Carboniferous volcano-sedimentary depocentre of Tazekka massif.
The Carboniferous volcano-sedimentary depocentre of Tazekka massif illustrates the tectono-sedimentary evolution of a flexural basin in relation to progression towards the northwest of folds associated with the Hajra Sbaa–Merja Caı¨d thrusts (Fig. 9). The tectonogenic sequence of its active side (Eastern) is represented by immature catastrophic conglomerates, indicating an active escarpment. On the other hand it is represented on its passive side (Western) by an argillaceous volcanosedimentary sedimentation on a marine platform subject to storm dynamics. Compared to the different Carboniferous depocentres of the Moroccan Meseta, the Viseo Namurian (and lower Westphalian?) Tazekka depocentre is integrated in the recently proposed model of Ben Abbou et al. (2001) and Ben Abbou (2001). According to these authors the Moroccan Hercynides resulted from a propagation of two sequences of piggy back thrusts, determining
Fig. 9. Diagrammatic evolutionary stages of Carboniferous volcano-sedimentary depocentre. Continuous thrusting toward the West controlling the Carboniferous sedimentation and extrusive magmatism. A. deposition of detrital and volcano-sedimentary series with numerous andesitic flows, B. development of basaltic flows, C. extrusion of effusive and pyroclastic rhyolitic flows. (1) Ordovician, (2) Silurian, (3) Devonian, (4) sedimentary and/ or volcano-sedimentary rocks with andesitic flows, (5) basaltic flows, (6) rhyolitic flows.
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
depocentres or sub-basins where a sandy conglomeratic, turbiditic and carbonate sedimentation succession accumulated. These depocentres migrated towards the west or the southwest, from the Eastern Meseta towards the Western Meseta. These sub-basins belong to the wedge top depozone of a foreland basin system, underfed (in the sense of DeCelles and Giles, 1996) in relation to continental subduction of an oceanic African crust under the ‘‘Moroccan plate’’. The magmatism associated with these sedimentary sequences is constituted by volcanic rocks dominated by andesites and rhyolites (and rare basalts) which present the features of calk-alkaline magmas of active continental margins [La/Nb above 1.8 (2–5.82), Zr/Y above to 3 (4.63–12.29) and Ti/Y lower than 350 (151.60–247.73)] like the magmatism of Carboniferous depocentres of the Moroccan Meseta (Roddaz et al., 2002); however an elevated enrichment in LILE, particularly in the andesites, and an elevation of (Ce/Yb)N ratios, reflect crustal assimilation. The magmatic differentiation of the different volcanic facies resulted therefore in fractional crystallisation with or without crustal contamination.
Acknowledgments The manuscript greatly benefited from a critical reading by N. Ennih, M. L. Ribeiro and an anonymous reviewer whom I would like to thank for their constructive criticism. I would also like to extend my gratitude to Dr. Lhoussine Asserraji for translating this article into English.
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The Carboniferous volcano-sedimentary depocentre of Tazekka Massif (Middle-Atlas, Morocco): new observations and geodynamic implications Azzeddine Bennouna a,*, Mohamed Ben Abbou a, Christian Hoepffner b, Fatima Kharbouch b, Nasrrddine Youbi c a
Faculty of Sciences Dher Mehraz, Department of Earth Sciences, University Mohamed Ben Abdellah, B.P. 1796 Atlas, Fe`s, Morocco b Faculty of Sciences, Department of Earth Sciences, University of Mohamed V, B.P. 1014, Rabat, Morocco c Laboratory of Petrology, Department of Geology, University Cadi Ayyad Semlalia, B.P. 2390, Marrakech, Morocco Available online 25 September 2004
Abstract The integrated analysis of thrust structures and facies allows us to interpret the Carboniferous volcano-sedimentary complex of Tazekka massif as a compressive sub-basin controlled mainly by the Hajra Sbaa–Merja Caı¨Õd northwest-verging thrust-propagation fold. The tectono-sedimentary sequence (conglomerates, greywacke and shale) is associated with an extrusive magmatism comprising basalts, andesites and rhyolites, under effusive and volcanoclastic facies (or rhyolites with exotic blocks), of orogenic calk-alkaline nature, which is consistent with a context of continental subduction. These results, and the comparison of regional contraction ages in the Moroccan Meseta, integrate the Carboniferous volcano-sedimentary depocentre of Tazekka massif in the wedgetop depozone of a foreland basin system generated by two north-west thrusted piggy-back sequences (aged from Fameno-Tournaisian to upper Visean-lower Westphalian) from the eastern Meseta to the western Meseta of Morocco. Ó 2004 Published by Elsevier Ltd. Keywords: Foreland basin system; Propagation of thrust-and-fold system; Thrust-controlled deposition; Depocentre migration; Calk-alkaline volcanism; Carboniferous; Eastern Meseta; Morocco
1. Introduction The Hercynian Tazekka massif situated in the centre of the Middle Atlas mountains (Morocco) forms the transition between two mesetian domains of different structural evolution: the Eastern Meseta characterised by a first deformation of successive synschistose folding and shearing whose age is post-Frasnian and anterior to upper Visean (Eovariscan phase; Hoepffner, 1987), and the Western Meseta with a major post-Viseo-Namurian deformation (Asturian phase; Pique´ and Michard, 1981;
*
Corresponding author. Tel.: +212 55 75 40 81; fax: +212 55 72 34
05. E-mail address: [email protected] (A. Bennouna). 0899-5362/$ - see front matter Ó 2004 Published by Elsevier Ltd. doi:10.1016/j.jafrearsci.2004.07.032
Pique´, 1989; Pique´ et al., 1993). The latter is preceded by the formation of turbiditic basins opened since the upper Devonian, in response to the ‘‘Eovariscan phase’’ of the Eastern Meseta. This structural zonation and the general vergence towards the west or southwest indicate a structural polarity between the internal (eastern) and external zones (western) of the Moroccan mesetian chain (Hoepffner, 1987) (Fig. 1). The Carboniferous volcano-sedimentary complex, up to 1000 m thick, occupies the NE extremity of Tazekka massif (almost 20 km South of Taza city) and unconformably rests upon the Ordovician Schists that constitute the essential outcroppings of Tazekka massif (Agard et al., 1958; Rauscher et al., 1982) (Fig. 1). It is represented by argillaceous and calcareous greso-conglomeratic deposits of Viseo-Namurian age (Marhoumi,
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Fig. 1. Geological map of the Carboniferous volcano-sedimentary depocentre of Tazekka massif (after Huvelin, 1986; Chalot-Prat, 1990; Essamawal, 1999) and localisation of the cross-sections of Fig. 2. (1) Ordovician substratum, (2) andesitic flows and associated volcano-sedimentary rocks, (3) basaltic flows, (4) rhyolites with reworked blocks, (5) microdiorites, (6) Tazekka granite, (7) Secondary terranes, (8) Miocene. Insert: place of Tazekka massif in Hercynian septentrional Morocco.
1985; Chalot-Prat and Vachard, 1989) associated with volcanic rocks where two superimposed units are distinguished: an andesite with olivine, hornblende and/or pyroxene that is surmounted by another unit constituted of effusive and ignimbritic rhyolites (Chalot-Prat, 1986, 1990; Kharbouch, 1994). This volcano-sedimentary complex was often interpreted as a Viseo-Namurian extensional basin inverted by a NW–SE compressive deformation (Hoepffner, 1987; Essamawal, 1999) or as a post-orogenic volcanotectonic depression, like a caldera (Chalot-Prat, 1986, 1990). Huvelin (1986, 1992) underlines the importance of resedimentation phenomena contemporaneous with
deposition of the volcanic rocks, suggesting a fast infilling of a basin bounded by active faults. Some new tectonic, sedimentary and magmatic observations allow us to illustrate the tectonic control of the sedimentation, the genesis of the volcanic rocks and to propose a new model of the dynamics of this volcanosedimentary depository.
2. Tectonism/sedimentation relationships The volcano-sedimentary depression of Tazekka massif corresponds to a sedimentary depocentre where
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
361
Fig. 2. Structural cross-sections in the Carboniferous volcano-sedimentary depocentre of Tazekka massif, (A) Hajra Sbaa, (B) Merja Caı¨d. (1) Ordovician substratum, (2) andesitic flows and associated sedimentary and volcano-sedimentary rocks, (3) basaltic flows, (4) rhyolites, (5) Secondary terranes.
volcanic extrusions were predominant, especially of andesitic and rhyolitic nature (Fig. 1). Regarding the sedimentary filling, the Carboniferous series shows some facies variations from the Eastern to the Western boundaries of the depocentre. The Eastern boundary series, which outcrops only at the level of the Hajra Sbaa mound in the SE extremity of the complex (Fig. 1), is oriented NE–SW, dipping 70°– 80° to the West (Fig. 2A). It comprises catastrophic sedimentation that is unconformable on an Ordovician slate substratum, organised in a megasequence (100– 150 m), composed at the bottom of a decametric conglomeratic bed with andesitic flows, evolving towards the top into sandstones (Fig. 3A). The conglomerates are unorganised and exclusively constituted of angular to sub-rounded blocks, cobbles and pebbles, derived solely from the Ordovician basement (slates, sandstones, quartz veins). This retrogressive megasequence is localised at the top of a flat overthrust (NE–SW, 10–20°E), associated with an amortisation anticline in the Ordovician series outcropping from Hajra Sbaa to Merja Caı¨d (Figs. 1 and 2) (Ordovician slice of Merja Caı¨d; Hoepffner, 1987). This disposition allows interpretation of this megasequence as a tectonogenic prism, reflecting the fast creation of available space and the destruction and erosion of the eastern boundary to the depocentre. The western series of the volcano-sedimentary complex (200–300 m) is also oriented NE–SW with a weak dip (10–30°) toward the East. It reflects volcano-sedimentary deposition: arkosic sandstones or argillaceous greywackes which alternate with numerous metric to
decametric andesitic flows (Figs. 1 and 2). At the small scale, the argillaceous greywackes exhibit erosive bases and load casts. The thin argillaceous sandstones interbeds show, most often, convoluted and/or parallel lamination (Fig. 3B). The whole succession reflects storm dynamics associated with some andesitic flows, on a relatively stable, shallow platform. From the magmatic point of view, the volcano-sedimentary lower series is surmounted by basaltic extrusions (300 m) in the sector Hajra Sbaa–Merja Caı¨d (Figs. 1 and 2a). The whole is overlain unconformably by porphyritic effusive, or crystal-rich, pyroclastic rhyolitic flows (ignimbrites in the sense of Chalot-Prat, 1986, 1990; Hoepffner, 1987; Kharbouch, 1994) (Figs. 2 and 3). The pyroclastic rhyolitic flows constitute the main outcrops of the Carboniferous volcano-sedimentary complex of Tazekka massif (Fig. 1), and are characterised by the ubiquitous presence of transported exotic blocks, with variable volumes, from cm3 to several hundred m3, derived essentially from the Ordovician substratum (slates and sandstones) and from the underlying Carboniferous series (conglomerates and siltstones, volcano-sedimentary beds, bioclastic limestones, rhyodacites, andesites, basalts). Rhyolitic blocks with porphyroclastic or pyroclastic structure are reworked from the same pyroclastic rhyolitic flows. Blocks of granite and microdioritic rocks are also present. This important reworking within pyroclastic rhyolitic massflows was described locally as a resedimentation phenomenon (Huvelin, 1986).
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The Western margin of the Carboniferous depocentre is structured by westverging slices, implying essentially the lower and middle Ordovician (Hoepffner, 1987; Amaouin, 1991). They constitute an antiformal stack (Boyer and Elliot, 1982) situated between a ‘‘floor thrust’’ located in the lower part of the Ordovician and a ‘‘roof thrust’’ situated at the base of the Upper Ordovician and/or in the Silurian shales. The whole structure is complicated at the south-western extremity of Tazekka massif by many back thrusts toward the SE, emplaced in the ‘‘roof thrust’’. Therefore, the different structures observed in the Tazekka massif can be integrated in a continuous progression of thrust sequences determining duplex culmination in the Ordovician substratum, and controlling at the same time the Carboniferous sedimentation and magmatism in a transported depocentre (Fig. 2). This model differs from the discontinuous succession of short tectonic phases previously proposed (Hoepffner, 1987; Essamawal, 1999).
3. Magmatism
Fig. 3. Lithostratigraphic successions of the active (A) and passive (B) boundaries of the Carboniferous volcano-sedimentary depocentre of Tazekka massif, (la) detrital series composed of catastrophic conglomerates and sandstones of the active margin, (1b) volcano-sedimentary series composed of greywacke and argillaceous silts of passive margin, with numerous andesitic flows, (2) Basaltic flows, (3) pyroclastic and effusive rhyolites.
The genesis of these basaltic and rhyolitic flows was related to the progression of the thrusting at Hajra Sbaa–Merja Caı¨d, as attested to by some metric basaltic veins and by a rhyolitic vein in the order of 30 m width at the level of Merja Caı¨d (Fig. 2). On the whole, the Viseo-Namurian volcano-sedimentary depocentre developed at the top of an amortisation anticline of the main thrust of Hajra Sbaa–Merja Caı¨d that is itself covered toward the NE by secondary deposits (Fig. 1). The disposition is here clearly the result of a ‘‘CAS’’ system (Specht et al., 1991; De´ramond et al., 1996) that realises the synchroneity of tectonism, sedimentation and magmatism, and that are controlled by the development of a thrust (C = chevauchement), the formation of an amortisation anticline (A = anticlinal) and the consecutive creation of a frontal syncline (S = synclinal) (Fig. 2A and B).
The Carboniferous volcanism of Tazekka depocentre corresponds to a calk-alkaline association typical of orogenic zones (Hoepffner, 1981; Kharbouch et al., 1985). Chalot-Prat and Cabanis (1989) and Chalot-Prat (1990) distinguish two groups of volcanic rocks. The first is composed of basic to andesitic homogeneous or hybrid rocks with an intraplate affinity and/or associated with an active continental margin; the second is essentially rhyolitic, of crystal origin (Chalot-Prat, 1995). On the basis of petrographical and geochemical studies, we regroup the different volcanic rocks of the Carboniferous volcano-sedimentary depocentre of Tazekka massif in three superimposed petrographic entities: andesitic (basaltic andesites, andesites and rhyodacites), basaltic (olivine basalts) and porphyritic rhyolites (effusive and homogeneous at the bottom and crystal-rich pyroclastic flows with reworked blocks at the top). 3.1. Petrography The basic, intermediate and acidic Carboniferous volcanic rocks of the Tazekka massif depocentre are flow-textured microlitic porphyric lavas or are porphyroclastic with some variable phenocryst assemblages as a function of the SiO2 content. In addition, plagioclase, clinopyroxene and olivine are common, with some variable proportions reflecting the transition from the basalts to the andesites. The amphibole (green hornblende) is found in the basaltic andesites to andesites and rhyodacites. The biotite is present in the andesites, rhyodacites and rhyolites. The quartz and alkali feldspar
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
are found in the rhyodacites. The accessory mineral phases are represented by apatite, zircon and opaque minerals. The mineral assemblages are the same, from the first homogeneous effusive rhyolitic flows with a porphyritic texture and variable modal phenocryst contents, to the uppermost crystal-rich pyroclastic rhyolitic flows with reworked blocks. Furthermore the reworked blocks of different types (shales and quartzitic sandstones, basalts, andesites, granites and microdiorites,. . .), the cognate enclaves of the crystal-rich pyroclastic rhyolitic flows (with angular to sub-rounded shape and a volume lower than m3), show up to 40% of phenocrysts; the latter are large (15–50 mm), frequently broken and well oriented in a vitreous flow-textured mesostase. These cognate enclaves, present exclusively in the uppermost rhyolites containing reworked blocks, are similar to less differentiated porphyric homogeneous effusive rhyolites with variable modal phenocrysts that outcrop at the base of the rhyolitic group. The hypothesis of hybrid magma proposed by Chalot-Prat and Cabanis (1989) and Chalot-Prat (1990) appears unlikely. Indeed, no structure indicative of magma mixing was observed, either at the outcrop or at the microscopic scales, although some xenocrysts of quartz, plagioclase and/or of amphibole are present in the basic and intermediate volcanic rocks. The acidic group is equally homogeneous and does not show crystalline phases of the precedent group that appear rather as reworked blocks. The mineral phases of these volcanic rocks are generally altered. The olivine is either silicified and/or chloritised and iddingsitised. The pyroxene and amphibole are generally altered to chlorite, epidote, calcite and oxides. The biotite is transformed into muscovite and oxides in the andesites, and to chlorites, epidote and oxides in the rhyodacites and rhyolites. The plagioclase feldspar is altered to epidote, calcite and/or sericite. The matrix of these rocks is generally devitrified and recrystallised to a felsitic homogeneous assembly associated with chlorite, calcite and oxides. These transformations are thought to be the result of hydrothermal activities which accompany antimony polymetallic mineral deposits. 3.2. Geochemistry The geochemistry of the volcanic rocks (major elements, trace elements and REE) of the Carboniferous depocentre of the Tazekka massif was established through 30 analyses, 15 of which were obtained from the previous work (Chalot-Prat and Cabanis, 1989). The new chemical analyses were performed at the University of Bretagne (Brest, France). The effusive rocks of the volcano-sedimentary depocentre of Tazekka massif are spilitised and generally show some contents in LOI, often above 2%, in particular in the basic and
363
andesitic lithologies, and reach values in some samples in the order of 9% (Table 1). The C.I.P.W norm allows us to infer the chemical modifications that affected original magmas. It shows silica and sodium enrichment and loss of calcium and potassium revealed by a variation of the contents in quartz, albite, orthoclase, anorthite and by the appearance of corundum in the CIPW norm. The mobility of the other elements is difficult to establish; we note, however, iron oxidisation and MgO depletion. The least weathered basalt and andesite samples show a supersaturation in silica and fairly elevated contents of A12O3, weak TiO2 and elevated K2O and in CaO contents (Table 1). The syn- to post-magmatic mobility of the chemical elements previously cited reduces the potential of these elements for the chemical characterisation of the Carboniferous magmatism (Cabanis, 1986). It is however possible to note an absence of enrichment in iron and in titanium and some weak contents of TiO2 (0.5–1.16%) in the basic and intermediate rocks, that are characteristic features of calk-alkaline magmatism. The normalisation multi-element diagrams permit the visualisation of the more discriminating chemical element contents for the basic and intermediate magmas (Pearce, 1983; Cabanis, 1986). The andesites show a very marked negative anomaly in Nb, characteristic of an orogenic volcanism (Fig. 4). This orogenic affinity is also attested by the generally weak contents in transition elements (V, Cr, Ni and Sc); the samples with high contents in Cr, Ni and Sc would be in relation to the elevated concentration in ferromagnesian minerals, notably pyroxene. The high values of La/Nb, above 1.8 (2–5.82), associated with high values of Zr/Y, above 3 (4.63–12.29) and weak values of Ti/Y, lower than 350 (151.60–247.73), are characteristic of those of an orogenic magmatism of a continental active margin setting (Condie, 1989). The elevated contents in hygromagmaphile elements (Sr, K, Rb, Ba, Th, La and Ce) and in lithophile elements (Sr, K, Rb and Ba) confirm the orogenic character of the Carboniferous andesitic rocks of Tazekka massif, with a possible involvement of a crustal component, as testified to by the relatively high values of ratios, such as (Ce/Yb)N (4.48–12.20). The basic volcanic rocks are distinguished from the andesitic rocks by relatively less important concentrations in hygromagmaphile elements, showing the less differentiated and contaminated nature of these rocks (Fig. 4) affirmed by relatively weak values of (Ce/ Yb)N ratios (3.35 and 6.10). The negative anomaly in Nb and the high values of La/Nb ratios, above 1.8 (2.36–2.70), associated with high values of Zr/Y ratios, above 3 (5.61–7.40) and weak values of Ti/Y ratios, less than 350 (239.80–315.83), are analogous to those of an orogenic magmatism of continental active margin rather
364
Table 1 Representative chemical analysis (W/R) of extrusive magmatic rocks from the Carboniferous volcano-sedimentary depocentre of Tazekka massif Sample Basalts HS
B21
Basic andesite and andesite B29
Porph. Rh.
K8475 K8476 K8468 K8470 K8460 K8843 K8470a K8467 B63
B12
CBc6
Porphyrocl. Rh.
Enc.
CBc39 CBc41 CBc51 CBc53 CMa1 CMa4 CMa10 CMa11 CMa46 CMa61 CRL14 CRL23 CRII12 CRI25 K8483 B44
SiO2
54.30 53.40 54.00 52.10
53.50
53.60
56.50
57.00
57.90
59.30
62.50
55.40 58.20 55.37
67.47
59.48
54.85
62.72
50.92
57.02
57.22
57.75
65.95
50.98
73.07
73.20
73.57
75.24
70.00
70.50
TiO2
0.97
1.02
0.96
0.73
0.68
0.75
0.69
0.70
1.00
0.78
0.95
1.16
0.91
1.12
1.02
0.80
0.99
0.53
1.09
0.17
0.16
0.36
0.37
0.40
0.43
Al2O3
17.20 16.40 16.95 15.70
0.97
1.02
0.99
0.75
0.99
15.20
15.80
15.80
16.20
14.70
14.80
14.92 15.50 14.18
17.93
16.70
16.90
17.20
16.86
16.42
15.74
18.29
15.77
17.21
13.90
13.41
12.97
12.28
11.80
13.60
7.70
7.30
7.30
9.00
6.40
6.60
6.30
6.00
4.65
5.80
7.85
5.94
7. 39
5.83
6.27
7.71
5.49
7.64
6.70
5.97
5.40
3.51
7.74
1.61
1.85
2.75
3.41
3.40
3.75
MnO
0.14
0.14
0.21
0.47
0.14
0.19
0.16
0.17
0.16
0.18
0.12
0.13
0.13
0.12
0.13
0.10
0.10
0.06
0.10
0.16
0.12
0.12
0.08
0.15
0.07
0.01
0.08
0.00
0.14
0.10
MgO
4.82
6.22
4.39
7.69
7.20
2.30
2.46
2.38
2.25
4.03
2.60
5.03
2.07
5.38
1.75
2.54
2.99
2.37
5.71
4.37
3.82
2.06
1.53
6.74
0.18
0.16
0.48
0.68
0.66
0.80
CaO
4.45
7.30
2.96
5.80
5.90
6.70
4.50
5.60
5.30
4.28
3.70
4.90
4.60
5.06
0.43
4.97
7.04
1.43
7.96
2.77
5.96
3.31
3.66
7.47
0.59
0.01
1.18
0.12
2.80
1.50
Na2O
6.10
2.90
6.61
2.68
3.99
2.24
2.29
4.22
2.23
3.43
0.07
3.60
3.21
4.81
2.71
252
2.73
4.49
3.54
3.30
4.61
5.04
4.20
2.53
3.64
2.23
4.09
1.53
2.21
3.27
K2O
0.90
0.22
0.34
0.62
1.94
2.81
3.10
1.86
3.00
0.89
3.72
2.44
3.10
2.16
1.96
2.83
4.36
3.46
0.33
2.21
1.11
1.87
1.51
0.33
4.04
4.79
3.88
3.69
3.41
4.40
P2O5
0.20
0.19
0.26
0.43
0.30
0.42
0.20
0.37
0.62
0.34
0.22
0.20
0.15
0.18
0.11
0.21
0.10
0.05
0.09
0.08
LOI
2.96
4.48
5.65
5.20
2.21
7.58
6.17
5.87
6.28
6.77
5.81
3.68
5.73
3.14
3.37
5.44
3.43
2.38
5.49
5.23
5.40
4.67
4.15
4.76
1.31
1.45
1.51
1.61
3.97
1.50
Total
99.89 99.92 99.69 98.45
98.50
97.98
98.31
99.88
100.07 99.24
99.82
99.38 99.53 99.02
99.40
101.00
99.68
101.00
99.21
98.68
97.32
100.96
99.01
98.79
99.98
Ba
265
152
290
230
664
1463
604
266
817
617
1164
1100 880
1071
1085
1409
1039
182
143
183
625
542
103
579
577
337
1160
438
555
Rb
285
6
17
14.40
23.80
112
124
89
34
159
73
135
69.80
71.80
103.14 106
116
12.30
75.10
37.90
82.80
56.56
14.86
172.50
216
168
158
159
172
Sr
620
371
412
344
714
317
813
316
324
193
675
565
847
337
611
1016
585
580
353
293
416
499
294
43
90
77
143
136
Y
21
20.50 21
25.10
24.60
24.20
25.70
26
26
29.50 24
Zr
115
120
155
277
292
262
138
259
210
170
Nb
5.80
5.60
7.60
13
12
6.45
7.20
Th
3.30
3.20
Ni
122
V
160
Cr
286
123
104
273
4
3.10
2.30
12.70
17.80
16.30
124
52
196
111
35
50
46
77
155
130
178
131
130
132
265
76
78.80
82.20
82.80
88.30
13
102.66 102.17 101.89 100.95 99.99 1843 1520
0.13
25.80
39 76
272
187
294
388
293
148
135
147
166
110
178
129
106
182
179
217
13
6.60
12
10.60
13.40
10.70 14
11.27
9.69
13.09
14.63
12.81
3.66
4.86
6.88
5.46
7.61
3.57
10.70
10.46
13.60
13.60
14.60
15.60
36
43
218
204
40.43
45.42
20.52
33.94
148
156
62.70
55.30
23
171
1.88
1.50
4.65
10.10
11
13
105
85
180
85
73.70
485
38 66
614.00 85.94
111.10 65.06
85.14
245
453
47
33 36
163
271
42
234
42
7.60
6.50
17
20.90 5.90
Hf
2.90
3
6.50
7.30
6.60
6.60
6.32
4.52
7.15
8.94
7.05
3.53
3.65
3.57
3.55
3.16
3.51
3.27
3.20
4.46
5.10
Cs
6.20
0.18
0.90
8.90
6.60
15
0.71
2.95
5.98
5.87
4.32
655
17.32
5.97
2.67
3.59
6.00
8.06
6.80
3.63
12.50
6.80
40
19.90
15.20
14.60
24.60
12.11
16.69
22.79
15.72
21.14
22.78
16.71
24.16
8.33
21.02
4.76
4.90
7.24
7.70
9.90
0.57
0.57
0.60
0.78
0.74
0.50
0.63
0.73
0.81
0.67
0.56
0.40
0.47
0.47
0.52
0.55
1.32
1.32
1.57
1.40
1.32
32.80
44.10
23.10
20.20
19.70
35.43
9.60
17.23
21.36
15.55
29.78
28.67
21.38
14.04
11.51
31.06
1.62
1.00
4.30
5.10
7.20
1.10
0.47
4.90
6.20
6.50
4.14
2.55
2.90
5.39
5.46
1.07
1.63
2.73
2.06
2.70
1.17
4.48
3.60
6.12
4.20
4.70
Sc
23
22
15.70 20.50
Ta Co
31
30
24
U
15.30 21
13.80 13.20 20.50 15.20
8.60
41.30
40.30
36.80
41
30
27.90
20.60
94
79
82
74
Pr
16.50 15.80 26.50
Sm
3.70
3.40
4.90
3.90
3.50
9.80
9
8.10
Eu
1.08
1.08
1.34
1.22
1.01
2.60
1.80
1.75
Gd
3.85
3.50
4.40
Tb
20
15.00
28.50
0.57
0.83
0.74
34
18
1.45
0.68
33
34
30.63
24.27
31.03
48.49
29.42
15.69
16.27
18.28
23.27
21.40
16.77
24.33
15.10
26.98
25
34.80
37.50
73
67
66.83
44
65
64.41
56.32
25.53
27.48
30.23
32.01
37.24
26.61
38.60
30.50
52.54
47
74
79
39
34.50
7.50
8.10
6.80
5.19
4.76
6.52
10.05
6.76
3.57
3.47
2.40
4.67
3.67
3.86
4.30
2.60
6
5.10
8
7.25
2.10
1.95
1.63
2.07
1.14
1.74
136
1.24
1.07
1.45
1.01
0.82
0.60
0.74
0.75
1.04
1.07
7.25
5.80 0.84
0.53
0.74
0.59
0.55
0.48
0.79
0.38
0.59
0.57
0.98
0.83
0.94
0.74
3.55
3.30
3.90
4.50
5.30
4.25
Er
2
1.85
2
2.50
2.90
2.20
Yb
1.88
1.79
2.05
2.40
2.08
2.20
1.70
2.27
7
6.90
Dy
2.10
9.50
35.30
30
0.53
14
4.60
Ce
45
26 0.70
La
28
15
2.20
34.50
7.30 0.91
0.85
0.58
6.70 3.50 2.46
1.69
2.38
1.81
3.49
1.81
1.97
The samples noted CBc, CRL and CRI are from Chalot-Prat and Cabanis (1989); En: cognate enclave of rhyolitic pyroclastic flows (Fe2O3* = total Fe as Fe2O3).
2.12
1.70
1.58
1.70
2.97
1.91
4.92
3.15
3.50
3.34
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
13.60
Fe2O3* 7.85
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
365
Fig. 4. MORB normalised patterns (after Pearce, 1983) for basaltic and andesitic flows from Carboniferous volcano-sedimentary depocentre of Tazekka massif.
Fig. 6. REE normalised to Chondrites (after Nakamura, 1974) for basaltic, andesitic and rhyolitic flows from Carboniferous volcanosedimentary depocentre of Tazekka massif.
than to those of an intraplate magmatism contaminated by a subduction component (Chalot-Prat, 1990). The eruption in the Carboniferous depocentre of Tazekka massif of andesitic lavas, rich in K2O and in Th and therefore more differentiated than the basaltic lavas can be explained by a magmatic differentiation of a basaltic denser liquid generating a less dense andesitic liquid, capable of reaching the surface more easily (Masson et al., 1996; Wilson, 1989). The multi-element normalization diagram for the available analyses of the basal homogeneous porphyric rhyolitic facies, represented by a cognate enclave of crystal-rich pyroclastic rhyolites, compared to ‘‘ORG’’ (Pearce et al., 1984) suggests that these rhyolites resemble to the calk-alkaline granites of volcanic arc affinity (Fig. 5), with an enrichment in K, Rb, Ba, Th and in Ce and Sm, with respect to the contents of Nb, Hf, Zr and Y.
The volcanic rocks of the Carboniferous depocentre of Tazekka massif present rather fractionated Chondrite-normalised REE patterns (Nakamura, 1974) with (La/Yb)N varying from 4.91 to 10.93 (Fig. 6). The negative Eu anomalies become more and more marked from the basalts to the basal rhyolites (cognate enclaves) (0.95–0.45), indicating more and more fractionation of plagioclase feldspar from basaltic to rhyolitic rocks. In the same sense the relatively elevated content of REE in the rhyolites, particularly in HREE, attests to a continuous differentiation by fractional crystallisation from the basalts to rhyolites. In the (Ce/Yb)N versus Th diagram, permitting distinction between the rate of crustal participation and the degree of magmatic differentiation by fractional crystallisation, the basalts are least differentiated and contaminated, with weak values for (Ce/Yb)N ratios and Th contents. The andesites and rhyolites would
Fig. 5. ORG normalised patterns (after Pearce et al., 1984) for rhyolitic samples from Carboniferous volcano-sedimentary depocentre of Tazekka massif.
Fig. 7. Evolution of volcanic rocks from Carboniferous volcanosedimentary depocentre of Tazekka massif in a (Ce/Yb)N versus Th diagram. Arrow indicates the sense of magmatic evolution by fractional crystallisation with or without crustal assimilation (ACF).
366
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
have been associated with the basalts in the same magmatic series that would have evolved by fractional crystallisation with assimilation of a crustal component, which is distinctly marked in the andesitic lavas (Fig. 7).
The Th/Yb versus Ta/Yb diagram of Pearce (1983) (Fig. 8) also shows that the Carboniferous Tazekka volcanics belong to the orogenic calk-alkaline magmas of continental active margin settings.
4. Conclusion
Fig. 8. Th-Ta-Yb diagram (after Pearce, 1983) for basaltic and andesitic flows from Carboniferous volcano-sedimentary depocentre of Tazekka massif.
The Carboniferous volcano-sedimentary depocentre of Tazekka massif illustrates the tectono-sedimentary evolution of a flexural basin in relation to progression towards the northwest of folds associated with the Hajra Sbaa–Merja Caı¨d thrusts (Fig. 9). The tectonogenic sequence of its active side (Eastern) is represented by immature catastrophic conglomerates, indicating an active escarpment. On the other hand it is represented on its passive side (Western) by an argillaceous volcanosedimentary sedimentation on a marine platform subject to storm dynamics. Compared to the different Carboniferous depocentres of the Moroccan Meseta, the Viseo Namurian (and lower Westphalian?) Tazekka depocentre is integrated in the recently proposed model of Ben Abbou et al. (2001) and Ben Abbou (2001). According to these authors the Moroccan Hercynides resulted from a propagation of two sequences of piggy back thrusts, determining
Fig. 9. Diagrammatic evolutionary stages of Carboniferous volcano-sedimentary depocentre. Continuous thrusting toward the West controlling the Carboniferous sedimentation and extrusive magmatism. A. deposition of detrital and volcano-sedimentary series with numerous andesitic flows, B. development of basaltic flows, C. extrusion of effusive and pyroclastic rhyolitic flows. (1) Ordovician, (2) Silurian, (3) Devonian, (4) sedimentary and/ or volcano-sedimentary rocks with andesitic flows, (5) basaltic flows, (6) rhyolitic flows.
A. Bennouna et al. / Journal of African Earth Sciences 39 (2004) 359–368
depocentres or sub-basins where a sandy conglomeratic, turbiditic and carbonate sedimentation succession accumulated. These depocentres migrated towards the west or the southwest, from the Eastern Meseta towards the Western Meseta. These sub-basins belong to the wedge top depozone of a foreland basin system, underfed (in the sense of DeCelles and Giles, 1996) in relation to continental subduction of an oceanic African crust under the ‘‘Moroccan plate’’. The magmatism associated with these sedimentary sequences is constituted by volcanic rocks dominated by andesites and rhyolites (and rare basalts) which present the features of calk-alkaline magmas of active continental margins [La/Nb above 1.8 (2–5.82), Zr/Y above to 3 (4.63–12.29) and Ti/Y lower than 350 (151.60–247.73)] like the magmatism of Carboniferous depocentres of the Moroccan Meseta (Roddaz et al., 2002); however an elevated enrichment in LILE, particularly in the andesites, and an elevation of (Ce/Yb)N ratios, reflect crustal assimilation. The magmatic differentiation of the different volcanic facies resulted therefore in fractional crystallisation with or without crustal contamination.
Acknowledgments The manuscript greatly benefited from a critical reading by N. Ennih, M. L. Ribeiro and an anonymous reviewer whom I would like to thank for their constructive criticism. I would also like to extend my gratitude to Dr. Lhoussine Asserraji for translating this article into English.
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