-
Precambrian Research 140 (2005) 157182
Contribution to a geodynamic reconstruction of the
Anti-Atlas(Morocco) during Pan-African times with the emphasis
on
a U
Abstract
New geodome (Oug531 5 Maand from th(>2100 Ma)and
Eastern143Nd/144Ndfrom the lo
A recentland those frof the Eburthe Anti-Atshoshonitic
The accrextension, s
CorrespoE-mail ad
0301-9268/$doi:10.1016/inversion tectonics and metallogenic
activity at thePrecambrianCambrian transition
Dominique Gasquet a,, Gilles Levresse b, Alain Cheilletz
c,Moulay Rachid Azizi-Samir d, Abdellah Mouttaqi e
niversite de Savoie, CISM-EDYTEM, CNRS-UMR 5204, Campus
Scientique, 73376 Le Bourget du Lac Cedex, Franceb UNAM, Centro de
Geociencias, Campus Juriquilla, 76230 Santiago de Queretaro,
Mexico
c CRPG-CNRS UPR A2300 and ENSG-INPL, BP 20, 54501
Vandoeuvre-le`s-Nancy Cedex, Franced Reminex, Groupe ONA, 26 Av.
Allal El Fassi, Tissir 36-40, Marrakech, Morocco
e ONHYM, 5 Charia My Hassan, BP 99, Rabat, Morocco
Received 15 October 2004; received in revised form 22 April
2005; accepted 7 June 2005
chronological analyses (UPb SIMS zircon ages) have yielded ages
of 552 5 Ma for the Bou Madine rhyoliticnat, eastern Anti-Atlas),
543 9 Ma for the Tachkakacht rhyolitic dyke (SaghroImiter, eastern
Anti-Atlas), andfor the Aghbar trachytic sill (Bou Azzer, central
Anti-Atlas). Inherited zircon cores from the Aghbar trachytic sille
Bou Madine rhyolitic dome have been shown to be of Statherian age
(ca. 16001800 Ma) and Paloproterozoicage, respectively, suggesting
that a significantly older protolith underlies the Pan-African
rocks in the CentralAnti-Atlas. Granodiorites and rhyolites from
the SaghroImiter area have similar low 87Sr/86Sr (0.7020.706)
and(0.51160.5119) initial ratios, suggesting a mixture of mantle
and lower crust sources. This can also be inferred
w 187Os/188Os ratios obtained on pyrite crystals from the
rhyolites.y published lithostratigraphic framework has been
combined with these new geochemical and geochronological data,om
the literature to produce a new reconstruction of the complex
orogenic front that developed at the northern edgenian West African
craton during Pan-African times. Three Neoproterozoic magmatic
series can be distinguished inlas belt, i.e., high-K calc-alkaline
granites, high-K calc-alkaline to shoshonitic rhyolites and
andesites, and alkaline-trachytes and syenites, which have been
dated at 595570, 570545 and 530 Ma, respectively.
etion of the Pan-African Anti-Atlas belt to the West African
super continent (WAC) was a four-stage event, involvingubduction,
moderate collision and extension. The calc-alkaline magmatism of
the subduction stage was associated
nding author. Tel.: +33 3 79 75 86 45; fax: +33 3 79 75 87
77.dress:[email protected] (D. Gasquet).
see front matter 2005 Elsevier B.V. All rights
reserved.j.precamres.2005.06.009
-
158 D. Gasquet et al. / Precambrian Research 140 (2005)
157182
with large-scale base metal and gold mineralisation.
Metallogenic activity was greatest during the final extensional
stage, atthe PrecambrianCambrian boundary. It is characterised by
world-class precious metal deposits, basemetal porphyry
andSEDEX-type occurrences. 2005 Else
Keywords: N rocco;
1. Introdu
The geois extremeopment ofwith metal-the lower cdevelop laral.,
1999; Kstruct comunderstandlithospherimineral conical data foand
geochelogical andNeoprotero(AA) of Mthat develoAfrican Craclass ore
drecently being extensiof the ambifunded by tOne of thebeen to
imactivity dutory of theproduced ddeposit (stoAg0) clearlelements
fcrustal leveto draw uptions through the orogenic belt, taking into
account newgeochronological data, in order to accurately definethe
architenew U/Pbfrom the B
ne ints pubarerThe dynamias thisnorth
ity.
eologiocco
e Moern edn (Figesetia
. Thee Herg et aleseta
mleltffner,(Bao
), blocdefor
originbate atheticaornee
ih and(Piquent c
te Protaran t
mentary Palaeozoic rocks. The Precambrian basementoutcrops in
several inliers within late Ediacaran andcture of the chain. This
paper combinesgeochronological and geochemical data
ou Azzer, SaghroImiter and OugnatBou
younger units (Bas Draa, Ifni, Kerdous, Akka, Igherm,Sirwa,
Zenaga, Bou Azzer, Saghro and Ougnat; Fig. 1)distributed along two
major fault zones (South Atlasvier B.V. All rights reserved.
eoproterozoic; Zircon UPb dating; SrNd isotopes; Anti-Atlas;
Mo
ction
dynamic reconstruction of ancient beltsly important for
understanding the devel-ore deposits, particularly those
associatedconcentration systems that involve parts ofrust and
mantle and are therefore able to
ge-tonnage, high grade ore deposits (Ord etay and Mpodozis,
2001). In order to recon-
plex and long-lived orogenic fronts and tosome of the
fundamental processes (on a
c scale) involved in the production of largecentrations, it is
necessary to have geolog-r a whole region, particularly
geophysicalmical data, supported by solid geochrono-structural
data. This paper focuses on thezoic evolution of the Anti-Atlas
Mountainsorocco, a segment of the Pan-African chainped along the
northern edge of the Westton (WAC). The region houses large
world-eposits (Fig. 1) and known reserves haveen increased by new
discoveries made dur-ve exploration programs carried out as
parttious National Geological Mapping Projecthe Moroccan Ministry
of Energy and Mines.most important results of this project hasprove
our understanding of metallogenicring the 300 Ma-long
Neoproterozoic his-Anti-Atlas. For example, geochemical datauring a
recent study of the Imiter AgHgck metal estimated at 8000 metric
tonnes
y show a large-scale transfer of chalcophilerom the mantle to
superficial (km-deep)ls (Levresse et al., 2004). It is also
essentialprecise and reliable lithostratigraphic sec-
Madiresula clebelt.geodbelt,of theactiv
2. GMor
ThnorthCratothe MFaultby thCaritthe Mat
TaHoepAtlas2002atelyTheto dehypoand C(Ennranes
basemto LaEdiacWest African Craton
liers (Eastern AA) with a discussion oflished by other workers
in order to proposepicture of this Neoproterozoic orogeniciscussion
focuses on the final stages of thec evolution of the Pan-African
Anti-Atlasperiod is fundamental to the understandingern border of
the WAC and its metallogenic
cal setting of the Anti-Atlas belt of
roccan Anti-Atlas belt is situated on thege of the Eburnian (ca.
2 Ga) West African. 1); it is separated from the High Atlas andn
domains to the north by the South AtlasAnti-Atlas region was
slightly reworked
cynian and Alpine orogenies (Pique, 1994;., 2004), which had a
profound influence on-Atlas domains. In the High Atlas domain,in
the East (Du Dresnay, 1976; Houari and2003) and in the western part
of the Highuch, 1984; Ouazzani et al., 2001; Eddif,ks of
Precambrian materials were moder-
med by the Variscan and Alpine tectonics.al position of these
blocks is still subjectnd it is uncertain whether they belong to al
North Moroccan continent (Villeneuve, 1994), to AvalonianCadomian
terranesLiegeois, 2001), or to CarolinaIberia ter-
e, 2003). In the Anti-Atlas, the Precambrianomprises several
units of Paloproterozoicerozoic age, unconformably overlain by
lateo Cambrian rocks and then by mostly sedi-
-
D. Gasquet et al. / Precambrian Research 140 (2005) 157182
159
Fig. 1. (a) Th edrawnsketch map of eposits
Fault, andthe extent ostructure aAnti-Atlasconsensus
been reachmagmatichave beencorrespond(ii) a
NeoPan-AfricaLancelot,Moussine-P1994; El AFekkak et aGasquet etof
the Panthree-stagefollowed bsion stage.and break-uthe succesan
island ablue-schist
; Heffassoc
or soe Anti-Atlas belt at the northern limit of the West African
craton, rthe Anti-Atlas belt in southern Morocco and location of
main ore d
Central Anti-Atlas Fault; Fig. 1). Due tof its sedimentary
cover, for a long time thend tectonic evolution of the
Pan-African
1974planenorthbelt was poorly understood, but now a basicon the
major stages of its development hased. Two main periods of
tectono-thermalactivity, associated with crustal
accretion,recognised: (i) a Paloproterozoic period,ing to the
Eburnean (Birimian) orogeny,proterozoic period, corresponding to
then orogeny. Most authors (Leblanc and1980; Bassias et al., 1988;
Leblanc and
ouchkine, 1994; Villeneuve and Cornee,ouli et al., 2001; Ennih
and Liegeois, 2001;l., 2001; Thomas et al., 2002; Pique, 2003;al.,
2004) consider that the development-African orogen can be explained
by aprocess controlled by crustal extension,
y compression, and then by a second exten-(1) The first stage
was related to the riftingp of the West African Craton and
involved
sive development of an oceanic plateau,rc and a marginal basin,
with associatedmetamorphism (ca. 800690 Ma; Clauer,
690605 Mward direcophiolite ocalc-alkalinwas associplane.
(3)(605530 Mtation andforeland dethe south.(ca. 300 Mthe AA
ex2004), whby polyhasedimentaronly subjec2004).
The newfor three sedescriptionthe Pan-Afafter Dallmeyer and
Lecorche (1991). (b) Geological. SAF: South Atlas Fault.
eran et al., 2002). Whether the subductioniated with island arc
development dipped
uth is still unclear. (2) The second stage (ca.
a) is characterised by basin closure, south-ted arccontinent
collision (arc accretion),bduction, deformation, metamorphism ande
magmatism. This convergence episode
ated with a southward dipping subductionThe final,
post-collision extension stagea) is characterised by molasse
sedimen-magmatism, which was followed by thevelopment of the
Saharan cratonic basin toThe Statherian (ca. 1750 Ma) and
Variscana) events did not have a major effect oncept in its western
part (Gasquet et al.,
ere the Variscan shortening is expressedrmonic buckle folding in
the Palozoicy cover. The Precambrian basement wast to faulting
(Helg et al., 2004; Caritg et al.,
geochronological data we have obtainedlected inliers have
allowed us to clarify theof the post-collision extensional stage
of
rican orogeny.
-
160 D. Gasquet et al. / Precambrian Research 140 (2005)
157182
3. Geological setting of the Ougnat, Saghro andBou Azzer
inliers
3.1. Ougna
Cryogengreywackein the centFreton, 198facies andbearing
grgeochronolThis basemvolcanic fomostly conwith basaltBou Madinby
severalOne of thewith polymsations. Then-echelonscurrent tefault
zoneet al., 199sequence urocks.
3.2. Saghr
In the Itary rocksof black shing formed(Marini androcks
werefacies metevent. Theby an Edia(Ouarzazatfrom andetop. This
swhich is n(Cheilletzsilver mineof felsic vdome), whtectonics
(L
dated at 550 3 Ma by ion-probe U/Pb on zircons(Cheilletz et al.,
2002). This epithermal event postdatesan earlier discrete base
metal episode associated with
sive grat 5
alkalinlusite-e counet al.,tly trant aloe felsmilarambr
oic rough aoughuir et
Bou A
e Boulex p; Saqubasemts, is in formntinenii) a mlite (Lntinites,
andtonesPan-A
are u
e mozazate00 m)ites, aaced d, 1998Tarou
sills oar). Tg theul et ar Ebuoped,tBou Madine
ian to early Ediacaran schistoses and black shales (Saghro
Group) outcropre of the Bou Madine inlier (Fig. 2, after8). They
are metamorphosed to greenschistintruded by quartz-diorites and
garnet-
anites, the ages of which have not beenogically established
(Abia et al., 2003).ent is unconformably overlain by
Ediacaranrmations (Ouarzazate Supergroup), whichsist of ignimbrites
and minor andesites,ic lava flows at the top. Moreover, in thee
area, the ignimbrites have been intruded
rhyolitic domes and numerous felsic dykes.Ediacaran rhyolitic
domes is associatedetallic ZnCuPbSnAgAu minerali-
ese mineral deposits occur in NNWSSEvein arrays related to
Ediacaran tran-
ctonics represented by N30E strike-slips (Freton, 1988; At
Saadi, 1992; Abia9, 2003). A Cambrian sedimentary covernconformably
overlies the Neoproterozoic
oImiter
miter area (Fig. 3), the oldest sedimen-are represented by
Cryogenian sequencesales (Saghro Group), interpreted as hav-in an
extensional back-arc environmentOuguir, 1990; Ouguir et al., 1996).
Thesefolded and subject to low greenschist
amorphism during the main Pan-Africanblack shales are
unconformably overlain
caran volcanic and volcanoclastic sequencee Supergroup), with
compositions that rangesitic at the bottom to ignimbritic at
theequence hosts the Imiter AgHg deposit,ow considered to be of
epithermal originet al., 2002; Levresse et al., 2004).
Theralisation is genetically related to a seriesolcanics (Takhatert
calc-alkaline rhyoliticich were coeval with regional
extensionalevresse, 2001). These volcanics have been
intrudatedcalc-andain thWall(mosprese
Thare sidle Cterozalthoare th(Oug
3.3.
Thcomp1981zoicschisgeniaepicoand (ophioserpecanicsandsmainand(i)
thOuar(10imbremplet al.zoicrare
AghbdurinBelfomajotelescanodiorites (Taouzzakt, Igoudrane; Fig.
3),72 5 Ma (Cheilletz et al., 2002). Thesee intrusive rocks
produced a cordierite-
biotite contact-metamorphism assemblagetry rocks (Leistel and
Qadrouci, 1991; De2001). Dykes of intermediate composition
chyandesite) and of various relative ages areng the Ediacaran
Imiter normal fault zone.ic dykes, including the Tachkakacht
dyke,in composition to the Takhatert dome. Mid-ian sedimentary
rocks overly the Neopro-cks. The Variscan and Alpine
orogenies,ctive in various sections of the AA belt,t to have had
little effect in the Imiter areaal., 1994).
zzerAghbar
Azzer inlier (Fig. 4) is structurally the mostart of the whole
Anti-Atlas (see Leblanc,aque et al., 1989, 1992). A
Paloprotero-ent, including gneisses, amphibolites, andntruded by
granites and overlain by Cryo-ations (Bou Azzer Group) comprising
(i)
tal sedimentary and volcanoclastic rocksaficultramafic complex,
interpreted as aneblanc, 1975; Brabers, 1988), comprisings,
gabbros, quartz-diorites and mafic vol-associated with limestones,
quartzites and
. These formations were deformed by thefrican shortening
tectonic event (thrusts)
nconformably overlain by, successively:lassic Tiddiline Group,
(ii) the Ediacaran
Supergroup, which consists of a thickvolcano-clastic pile
(dacitic to rhyolitic ign-ndesites, conglomerates, sandstones,
etc.)uring strong transtensive tectonics (Maacha), and (iii) the
late Ediacaran to early Palo-dant Group (dolomites, sandstones andf
syenite and trachyte at Jbel Boho andhe regional NS shortening that
occurredVariscan orogeny (Soulamani et al., 1997;l., 2001; Caritg
et al., 2004) reactivated the
rnean and Pan-African faults. A complex,polymetallic
(CoNiAsFeCuAuAg)
-
D. Gasquet et al. / Precambrian Research 140 (2005) 157182
161
Fig. 2. Main geological units of the OugnatBou Madine inlier
(after Freton, 1988) and concordia diagram for zircons from the Bou
Madinerhyolitic dome. Weighted mean ages, ellipses are 2 errors.
Star represents the position of the dated rhyolite.
-
162 D. Gasquet et al. / Precambrian Research 140 (2005)
157182
Fig. 3. Mainfrom the TachStars represen
mineral deand Variscathe mineraUPb datinat 218
8muscovitesgeological units of the SaghroImiter inlier (after
Leistel and Qadrouci, 199kakacht rhyolitic dyke and (inset) the
rhyolitic Takhatert dome (Cheilletz ett the position of the dated
rhyolites.
posit was formed during these Pan-Africann events (Maacha et
al., 1998). The age of
lisation is poorly constrained at 550 Ma byg on brannerite
(En-Naciri et al., 1997) andMa by 40Ar/39Ar dating on
hydrothermal(Levresse, 2001).
4. UPb gand Bou A
4.1. Analy
UPb dfrom Ouarz1; Levresse, 2001) and concordia diagram for
zirconsal., 2002). Weighted mean ages, ellipses are 2 errors.
eochronology in the Ougnat, Saghrozzer inliers
tical procedures
ata were obtained for single zircon grainsazate and Taroudant
Group volcanic rocks.
-
D. Gasquet et al. / Precambrian Research 140 (2005) 157182
163
Fig. 4. Main geological units of the Bou Azzer inlier
(simplified after Leblanc, 1981) and concordia diagram for zircons
from the Aghbartrachyte. The 206Pb/238U age of the younger
concordant point corresponds to the emplacement age of the trachyte
sill (see text). Ellipses are2 errors. The age of the Jbel Boho
syenite has been recalculated with Isoplot (Ludwig, 2003) from data
of Ducrot and Lancelot (1977). Errorscorresponds to the error
average given by the authors; the error correlation value is 0.98.
Star represents the position of the dated trachyte andsyenite.
-
164D.G
asquetet
al./Preca
mbrian
Research140(2005)157182
Table 1UPb isotopic data (SIMS-CAMECA IMS 1270, Nancy, France)
for zircon grains from the Ediacaran magmatism from the
SaghroImiter, OugnatBou Madine and Bou AzzerinliersSample Measured
data Concentration
(ppm)Tera-WasserburgConcordia data
Conventional concordiadata
Ages (Ma)
Is206Pb 207Pb/206Pb
204Pb/206Pb
206Pb/238U
UO/U Pb U Th 238U/206Pb
207 Pb/206Pb
207 Pb/235U
Error 206 Pb/238U
Error Correlationerror
206
Pb/238UError 207
Pb/235UError
BM7 (OugnatBou Madine, rhyolite, limpid grains)1218 4531 0.0644
0.00035 0.208 7.51 32 425 174 11.2938 0.1526 0.0644 0.0003 0.7356
0.0116 0.0885 0.0012 0.859 547 7 560 73814 3837 0.0648 0.00059
0.204 7.44 36 472 223 11.2687 0.2268 0.0648 0.0024 0.7020 0.0360
0.0887 0.393 548 11 540 21315 11314 0.1099 0.00361 0.127 7.77 80
1975 1099 21.3385 0.4168 0.1099 0.0006 0.3738 0.0117 0.0469 0.0009
0.623 295 6 322 940-3 5639 0.0603 0.00011 0.219 7.74 41 548 238
11.4911 0.1560 0.0603 0.0002 0.7131 0.0107 0.0870 0.0012 0.902 538
7 547 643-8 7893 0.0865 0.00187 0.175 7.55 62 1009 787 13.9941
0.2939 0.0865 0.0031 0.5909 0.0457 0.0715 0.0015 0.272 445 9 471
2943-9 2070 0.0734 0.00135 0.184 7.10 25 322 123 11.1924 0.3062
0.0734 0.0008 0.6858 0.0349 0.0893 0.0024 0.538 552 14 530 2144-4
5489 0.0591 0.00002 0.235 7.83 40 517 177 10.9875 0.1559 0.0591
0.0002 0.7475 0.0111 0.0910 0.0013 0.952 562 8 567 64-10 5948
0.0656 0.00041 0.214 7.64 50 662 266 11.4408 0.1538 0.0656 0.0003
0.7272 0.0132 0.0874 0.0012 0.743 540 7 555 84-7 6803 0.0754
0.00113 0.246 8.03 50 656 234 11.3496 0.1665 0.0754 0.0004 0.7244
0.0167 0.0881 0.0013 0.637 544 8 553 107-1 26581 0.0681 0.00063
0.380 9.41 62 850 356 11.8578 0.1007 0.0681 0.0003 0.6870 0.0088
0.0843 0.0007 0.665 522 4 531 57-12 11053 0.0660 0.00045 0.410 9.91
27 384 112 12.1114 0.0678 0.0660 0.0002 0.6793 0.0056 0.0826 0.0005
0.678 511 3 526 37-16 30385 0.0670 0.00038 0.400 9.48 63 834 320
11.3788 0.0846 0.0670 0.0008 0.7475 0.0413 0.0879 0.0007 0.389 543
4 567 87-19 19993 0.0599 0.00004 0.430 9.59 49 610 285 10.7606
0.0746 0.0599 0.0001 0.7629 0.0057 0.0929 0.0006 0.931 573 4 576
37-20 9929 0.0595 0.00004 0.450 9.98 24 315 174 11.0989 0.0705
0.0595 0.0003 0.7350 0.0057 0.0901 0.0006 0.814 556 3 559 37-21
12822 0.0604 0.00010 0.480 10.35 33 428 123 11.1435 0.0761 0.0604
0.0001 0.7328 0.0052 0.0897 0.0006 0.971 554 4 558 37-25 13215
0.0644 0.00038 0.450 9.84 32 399 166 10.8728 0.1277 0.0644 0.0004
0.7495 0.0109 0.0920 0.0011 0.809 567 6 568 67-3 17284 0.0595
0.00002 0.450 9.84 39 486 186 10.8019 0.0586 0.0595 0.0001 0.7587
0.0043 0.0926 0.0005 0.965 571 3 573 27-4 17866 0.0592 0.00001
0.420 9.59 43 551 239 11.0108 0.0622 0.0592 0.0001 0.7422 0.0043
0.0908 0.0005 0.981 560 3 564 2
BM7 (OugnatBou Madine, rhyolite, inherited core)7-2 9175 0.1800
0.00054 1.980 9.84 21 62 39 2.4780 0.0220 0.1800 0.0004 9.6636
0.0971 0.4035 0.0036 0.882 2185 16 2403 9
IM74 (SaghroImiter, Tachkakacht rhyolite, limpid grains)74-1
29146 0.1896 0.00920 0.566 10.08 59 797 196 11.5425 0.1204 0.1896
0.0018 0.6654 0.0450 0.0866 0.0009 0.154 536 5 518 2774-10 108462
0.2527 0.00108 0.552 11.01 245 3333 2540 11.6809 0.0965 0.2527
0.0034 2.8428 0.0485 0.0856 0.0007 0.484 530 4 1367 1374-11 23921
0.0786 0.00138 0.510 10.30 52 694 148 11.3672 0.0968 0.0786 0.0013
0.7132 0.0259 0.0880 0.0007 0.235 544 4 547 1574-12 79577 0.1716
0.00802 0.508 9.54 219 2887 1216 11.3506 0.0997 0.1716 0.0006
0.6647 0.0174 0.0881 0.0008 0.335 544 5 517 1174-13 15705 0.0873
0.00192 0.523 10.35 38 505 126 11.2928 0.0766 0.0873 0.0009 0.7296
0.0191 0.0886 0.0006 0.259 547 4 556 1174-14 33336 0.0604 0.00010
0.500 10.14 104 1332 341 11.0171 0.0626 0.0604 0.0001 0.7393 0.0044
0.0908 0.0005 0.957 560 3 562 374-15 71200 0.0766 0.00132 0.455
9.84 210 2876 1126 11.7491 0.0986 0.0766 0.0004 0.6744 0.0095
0.0851 0.0007 0.596 527 4 523 674-17 7509 0.1247 0.00452 0.736
11.76 41 492 88 10.4086 0.1085 0.1247 0.0015 0.7885 0.0402 0.0961
0.0010 0.205 591 6 590 2374-3 91970 0.1057 0.00327 0.481 10.16 202
2861 959 12.1917 0.1282 0.1057 0.0039 0.6598 0.0749 0.0820 0.0009
0.093 508 5 514 4574-5 39739 0.0604 0.00011 0.505 10.11 96 1217 362
10.8683 0.0656 0.0604 0.0002 0.7475 0.0063 0.0920 0.0006 0.714 567
3 567 4
BA06 (Bou Azzer, Aghbar trachyte, limpid grains)6-1 19113 0.0626
0.00018 0.490 10.51 61 779 195 10.9699 0.1355 0.0626 0.0002 0.7553
0.0103 0.0912 0.0011 0.904 562 7 571 66-11a 26251 0.0677 0.00061
0.460 10.17 79 972 451 10.5442 0.1338 0.0677 0.0009 0.7705 0.0212
0.0948 0.0012 0.460 584 7 580 126-11b 24248 0.0720 0.00089 0.460
10.19 74 924 406 10.6724 0.1324 0.0720 0.0018 0.7634 0.0368 0.0937
0.0012 0.257 577 7 576 216-15 12454 0.0620 0.00019 0.670 12.16 56
795 129 12.1024 0.1549 0.0620 0.0003 0.6761 0.0093 0.0826 0.0011
0.927 512 6 524 66-2 16074 0.0594 0.00003 0.400 10.3 49 720 149
12.5365 0.1219 0.0594 0.0001 0.6485 0.0064 0.0798 0.0008 0.979 495
5 508 46-4a 9940 0.0639 0.00021 0.550 10.64 30 351 120 10.1710
0.2004 0.0639 0.0007 0.8255 0.0227 0.0983 0.0019 0.715 605 11 611
136-4b 10041 0.0671 0.00040 0.550 10.61 30 351 119 10.1148 0.1602
0.0671 0.0014 0.8370 0.0303 0.0989 0.0016 0.437 608 9 618 176-9
1758 0.1400 0.00503 0.580 10.56 6 76 37 10.3185 0.2223 0.1400
0.0034 0.9071 0.0818 0.0969 0.0021 0.239 596 12 655 43
BA06 (Bou Azzer, Aghbar trachyte, inherited grains)6-12 1808
0.1100 0.00011 1.950 10.96 6 21 11 3.1350 0.0457 0.1100 0.0003
4.7745 0.0733 0.3190 0.0047 0.951 1785 23 1780 136-14 19449 0.1200
0.00033 2.030 12.18 104 488 12 4.0211 0.1036 0.1200 0.0010 3.9660
0.1091 0.2487 0.0064 0.937 1432 33 1627 22
-
D. Gasquet et al. / Precambrian Research 140 (2005) 157182
165
A CAMECA IMS 1270 ion microprobe (CRPG-CNRS, Nancy, France) was
used following the ana-lytical methods described in Deloule et al.
(2002).Detailed options reveagrains. Thcussed belograms
were2003).
4.2. Ougna
One samMadine meuhedral, b50150mcracks andmicas andtory
zonincores.
Eleven sage of 552the crystallnat inlier.EdiacaranPaloprotecore
may bolder under
4.3. Saghr
The Ta(2100 Ma) suggesting that thee an inherited xenocryst from
a significantlylying basement.
oImiter
chkakacht rhyolitic dyke contains smallimpid zircons. They are
euhedral and bipyra-
well-developed prisms. Primary cracksons (apatite, quartz,
occasional micas) arelatory zoning of magmatic origin is seeny and
none of the grains had inherited
b-concordant points gave a weighted mean 9 Ma (Fig. 3). This is
in good agreement0 3 Ma age of the neighbouring Takhatertme and
with the stratigraphic position ofolcanism, which belongs to the
EdicaranSupergroup. Three other zircons yieldedages of between 560
and 600 Ma. They
epresent xenocrysts inherited from grani-Ediacaran volcanic
rocks. Their ages arence with the inherited ages obtained by
4.4.
Olyzedverysionszonin
SiteredTheycoun
an agmentwas i1952the Jband L(ca. 1xeno
teroz
5. GCam
Th(OuaGrouusingAA,OugnTable
5.1.
(i)zzerAghbar
ple of the Aghbar trachytic sill was ana-zircons are limpid and
sub-euhedral, with
prisms (5080m). They contain rare inclu-atite, quartz and micas
and show oscillatory
agmatic origin and rare inherited cores.small (5060m) zircon
grains are scat-the concordia between 560 and 615 Ma.
obably inherited zircons from the basementks. The youngest
concordant zircon gives
531 5 Ma (Fig. 4). This age is in agree-the stratigraphic
position of the sill, whichd into an early Cambrian series
(Choubert,
anc, 1981), and with the 529 3 Ma age ofho syenite (Fig. 4)
(recalculated from Ducrotot, 1977). Two zircons gave Statherian
ages800 Ma; Table 1). They probably representinherited from the
underlying Palopro-
sement.
mistry of the Ediacaran andmagmatism in the AA area
chemical characteristics of the Ediacarane Supergroup) and
Cambrian (Taroudantst-orogenic magmatism were determinediously
published UPb data for the wholeer with analyses from the
SaghroImiter,u Madine and Bou Azzer inliers (Figs. 57;d 3).
and trace elements
95570 Ma magmatism is represented byeter-scale intrusive bodies
of granites, gra-rites and tonalities, with minor dioritesabbros,
and rare basalts and andesites.ock types show mostly
metaluminous< SiO2 < 75%), high-K calc-alkaline affini-ig.
5a). Na2O/K2O ratios range from 0.287, REE patterns (Fig. 6a) are
moderatelynated [6.43 < (La/Yb)N < 27.63] with rel-low YbN
contents (10) and low nega-
-
166D.G
asquetet
al./Preca
mbrian
Research140(2005)157182
Table 2Representative major (wt.%) and trace (ppm) element
analyses from SaghroImiter inlierSample type B. Madine Imiter B.
Azzer
BM99-01 rhyolite SJ127 granite IM99-50 granite IM00-03 diorite
IM99-72 rhyolite IM99-05 rhyolite IM99-74 rhyolite IM99-75
ignimbrite IM01-01 rhyolite BA99-6 trachyteSiO2 77.06 66.28 68.72
55.17 73.26 72.66 77.30 75.51 76.29 64.33Al2O3 12.33 15.45 14.53
17.14 13.53 13.56 11.99 12.98 12.51 13.87Fe2O3 0.83 4.00 3.10 8.36
2.31 1.80 1.13 1.36 1.26 7.63MnO 0.10 0.08 0.08 0.14 0.07
-
D. Gasquet et al. / Precambrian Research 140 (2005) 157182
167
Table 3Available Sr and Nd isotopic compositions of (595570 Ma)
granite-granodiorites and (570545 Ma) rhyolites from the Imiter
InlierAge/sample Type Sr (ppm) Rb (ppm) 87Rb/86Sr 87Sr/86Sr (2)
(87Sr/86Sr)tRb-Sr data
595570 MIM 99-50 2.6IM 00-03 0.6SJ 127 1.0
570545 MIM 99-72 3.1IM 99-74 5.3IM 99-75 4.0IM 01-01 3.2
Age/sample 4Nd (Sm-Nd data
595570 MIM 99-50 9 (11)IM 00-03 0 (21)SJ 127
570545 MIM 99-72IM 99-74IM 99-75IM 01-01
Analytical meSr, Sm and Nspectrometer.and 143Nd/144for Sr and
JMare negligiblethe last two dito the deplete
tive Emostlyseriesmetalu
(ii) The 5thickignimintrustop ofbasaltet al.,atationof themon.
U>2.04aGranodiorite 115.22 106.7Granodiorite 482.48 101.0Granite
349.57 125.5
aRhyolite 103.98 113.5Rhyolite 73.01 133.3Ignimb. rhyolite 87.05
120.3Rhyolite 101.96 112.8
Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/14
a3.26 16.8 0.1172 0.512053.42 16.8 0.1228 0.51222
3.39 19.4 0.1059 0.512188 (11)
a4.55 27.4 0.1005 0.512267 (15)3.38 18.4 0.1110 0.512096 (9)2.75
18.0 0.0922 0.512158 (25)5.05 30.1 0.1015 0.512197 (18)
thod: Separation of RbSr and SmNd was made according to the
methodd concentrations were determined by isotope dilution. Rb was
analyzed bSr and Nd isotopic ratios and Sm concentration were
measured using a FinnNd ratios were normalized to 86Sr/88Sr =
0.1194 and 146Nd/144Nd = 0.7219,for Nd gave average ratios of
87Sr/86Sr = 0.710205 23 (2) and 143Nd/1(
-
168 D. Gasquet et al. / Precambrian Research 140 (2005)
157182
Fig. 5. K2OScalc-alkaline tCambrian Agtered (LOI < 3Freton
(1988)(1992), Cama(1997), Ouguet al. (2000),(2001), GasquOuazzani
et a
5.2. Isotop
5.2.1. RbSrNd
tonic eventSaghroImFig. 7. The(0.7020.7
tial ratios, which attest to a mixing of mantle andlower crustal
sources, based on the mean geochem-
haracteristics of these reservoirs as determinedaure (9557s
(Tab1561
in thare thea youn
34ical cby Fthe 5canic1161foundageswithiO2 plots (Le Maitre et
al., 1989) of selected Ediacarano alkaline plutonic (a) and
volcanic (b) rocks and of thehbar trachytic sill (star) of the
Anti-Atlas belt. Unal-%) rocks are from this study and from Lebrun
(1982),, Ighid et al. (1989), Azizi Samir et al. (1990), At Saadira
(1993), Mokhtari (1993), At Isha (1996), At Malekir (1997), Ikenne
(1997), Eddif et al. (2000), ThomasBajja (2001), Barbey et al.
(2001), Chalot-Prat et al.et et al. (2001), Karl et al. (2001),
Levresse (2001),
l. (2001) and Thomas et al. (2002).
e geochemistry
Sr, SmNd, S and rare gasisotopic data for the 595570 Ma plu-and
570545 Ma volcanic episode in the
iter district are presented in Table 3 andtwo groups display the
same low 87Sr/86Sr
06) and 143Nd/144Nd (0.51160.5119) ini-
The event indicing fluids, atopic reservet al., 2004gangue
miratios rangiwith the absions, suggwith the ep
5.2.2. ReThe sulp
sured 187Ocating a dand, by inIn order todata basedals at
ImitReOs geometal depodifferent g(Table 4).epithermalChinkuashShen
andmal and poof measurethe degreetaminated bmantle: 0.1nental
crusresults obtaconcur wit(see above)tial 187Os/1from the 5tive Sr
sign2001). The calculated Nd model ages for0 Ma granites and 570545
Ma felsic vol-le 3) fall within the rather restricted range ofMa.
No Mesoproterozoic rocks have beene AA, so it seems probable that
these TDMresult of the mixing of an older 2 Ga sourceger
Neoproterozoic one.
SCDT values for the silver mineralisationate preferential
degassing of SO2 in ascend-s well as mixing between the magmatic
iso-oir and the country rock reservoir (Levresse). Helium isotope
analyses of sulphides andnerals yield similar results, with
3He/4Heng from 0.76 to 2.64Ra. These data, togethersence of 20Ne in
the analyzed fluid inclu-est a mantle origin for the fluids
associatedithermal silver event (Levresse et al., 2004).
Oshide phases and Ag0 minerals show mea-
s/188Os ratios of 0.1440.197, clearly indi-ominantly mantle
source for the osmiumference, the silver (Levresse et al.,
2004).
assess the validity of this ReOs isotopeinterpretation of the
source of the met-
er, we have compared our data with thechemical signatures of a
variety of precioussits from around the world, formed
undereodynamic regimes and at different timesThe measured
187Os/188Os of the Imiterdeposit (0.1440.197) is similar to the
ih AuCu epithermal deposit (0.130.24;Yang, 2004). Other gold
bearing epither-rphyry-type deposits show a larger range
d 187Os/188Os ratios (0.2190.2), indicatingto which the
primitive magma was con-y crustal and/or host country rocks
(mean20.13; Chen et al., 1998; mean conti-
t: 1.21.3; Esser and Turekian, 1993). Theined for the Imiter
ReOs isotope system
h the data from other geochemical proxies. They are also
consistent with the low ini-88Os ratios (0.280.62) obtained on
pyrites45 Ma volcanics and their relatively primi-atures,
reinforcing the probable genetic link
-
D.G
asquetet
al./Preca
mbrian
Research140(2005)157182
169
Table 4ReOs isotopic compositions of the Epithermal AgHg Imiter
deposit and a variety of precious metal deposits in the world
formed in various geodynamic setting and differentepochs
Type of deposit Localization [Re] [Os] 187Re/188Os 187Os/188Os
187Os/188Os(i) Age ReferenceEpithermal-Ag Imiter, Morocco 00.06 ppb
0.0060.162 ppb 014.2 0.1440.197 0.0130.197 Late Proterozoic
Levresse et al. (2004)Epithermal-Au Low-sulphidation,
Bucaramanga,Colombia
43153 ppb 2130 ppt 2441299 1.518.48 Paleocene Mathur et al.
(2003)
High sulphidation,Bucaramanga,Colombia
0.833 ppb 1934 ppt 865156136 1.858.11 1.2 Paleocene Mathur et
al. (2003)
AuCu, Chinkuashih,Taiwan
1.42.9 ppb 0.22.1 ppb 3.734.5 0.130.24 1.152.07 Pliocene Shen
and Yang (2004)
Gold shear-zone Kimberley, Zimbabwe 3142378 ppt 28.488 ppt
50.4456.5 2.014.83 2 Late Archean Frei et al. (1998)Porphyry
Base-metal PC, large,
Chile0.3320 ppb 4244 ppt 96-94730 0.2178.46 0.1551.23 Eocene
Mathur et al. (2000a,b)
Base-metal PC, small,Chile
0.8464 ppb 632 ppt 1202120921 4.390.2 3.84.68 Eocene Mathur et
al. (2000a,b)
CuMo PC,Bagdad-vein 1, USA
3.866.76 ppt 817 ppt 2.419.9 5.132.1 2.12 Maastrichtian Barra et
al. (2003)
CuMo PC,Bagadad-vein 2, USA
1.684.1 ppt 612 ppt 1914 1.0610.8 0.130.83 Maastrichtian Barra
et al. (2003)
CuAu PC stage 2,Grasberg, Irian Jaya
11140 ppb 321024 ppt 7515653 1.031.31 0.791.26 Pliocene Mathur
et al. (2000a, 2000b)
-
170 D. Gasquet et al. / Precambrian Research 140 (2005)
157182
Fig. 6. Chondarea (a, this stEvensen et al.
Fig. 7. 87Sr/8lites from the187Os/188Os sSaghroImite
between thmineralisat
The gedeposit app
Fig. 8. Structuand a propose(2002); CentrSamson et al.(2001),
Cheillthree studiedrite-normalized REE patterns of the Ediacaran
and Cambrian plutonic andudy) and from the Central and Eastern
Anti-Atlas (b), after Bajja (2001) an(1978).
6Sr vs 143Nd/144Nd plot of late granites and rhyo-SaghroImiter
area. The mantellic initial (550 Ma)
ignature of euhedral pyrite of rhyolitic intrusion fromr
(Levresse et al., 2004) is also shown.is volcanic event and the
epithermal silverion.ochemical characteristics of the Imiterear to
be dominated by metals and fluids
ral map and reliable UPb radiometric ages (new and previous) of
the Neoprd timetable. Western Anti-Atlas data are from Hassenforder
(1987), At Mal Anti-Atlas from Ducrot and Lancelot (1977), Leblanc
and Lancelot (19(2003), Inglis et al. (2004); Thomas et al. (2004);
Eastern Anti-Atlas frometz et al. (2002). International
stratigraphic ages (left, in red) are after Amthinliers.
of mostly mcentrationtonnes), isChilean basa direct
relaconcentratihas been eldemonstrattectonic regevent (570a
relativelymantle to ttion of othecurrently in
6. Discuss
6.1. Synthegeochronol
Three liup for threevolcanic rocks from the SaghroImiter and Bou
Azzerd Chalot-Prat et al. (2001). Normalization values from
antle origin. This result, and the huge con-of silver at Imiter
(more than 8000 metric
consistent with observations made onemetal porphyry deposits
(Table 4), wheretionship between the volume of total metalon and
the participation of a mantle sourceucidated (Mathur et al.,
2000a,b). This alsoes the need to invoke a major extensionalime at
the time of the Imiter mineralizing545 Ma) to account for the rapid
transfer oflarge quantity of metals directly from the
he surface. The geochemical characterisa-r precious metal
deposits in the AA belt isprogress.oterozoic plutonic and volcanic
rocks in the Anti-Atlasalek et al. (1998), Gasquet et al. (2001),
Walsh et al.
80), Mifdal and Peucat (1985), Thomas et al. (2002),Magaritz et
al. (1991), Landing et al. (1998), Levresseor et al. (2003) and
Gradstein et al. (2004). In bold the
ion
sis of the lithostratigraphic andogical data for the Pan-African
AA
thostrathigraphic columns have been drawnsections of the
Western, Central and East-
-
D. Gasquet et al. / Precambrian Research 140 (2005) 157182
171
-
172 D. Gasquet et al. / Precambrian Research 140 (2005)
157182
ern AA (Fig. 8). The lithostratigraphic subdivisions arebased on
the latest results obtained by the MoroccanNational Geological
Mapping Project and other recentcontributio2002; Bou2004). By chave
been amore compbelt.
The matribution oproterozoicmentary plterozoic bawhich supption
that thparts separcated by Cmarked hetsequencesand suggesranes (the
Pan-Africaat 580 Ma,genic seriefinally to anous (up to(upper
Oua(Fig. 8), thinliers andanomaly atsition
betwsedimentartransgressicanic eventTata and Taunconformwith a
lowpatible wit(Benssaoudynamic bOuarzazateTaroudantextensiona
Two Panthe three swas the larghas been in
ciated magmatism (Mifdal and Peucat, 1985) and at663 14 Ma from
metamorphic rims on zircons fromthe Iriri migmatite (Thomas et al.,
2002). The second,
ler evee the Oa).e AAationmbria, 1991analy
is no dthostraternatipston
, 2004
PrecisAtlas
tentaatic
ozoiceochr
8. Sevo majhe tec. Thes deves propoupedage 1:ified a;
Admevidenhari e). Hoiitic mou Ae emed to
een dLancee of 7gneissp inthe e2 Mans (Thomas et al., 2002,
2004; Walsh et al.,ougri and Saquaque, 2004; Gasquet et
al.,ombining all this geochronological data weble to produce a
relatively homogenous andlete lithostrathigraphic synthesis of the
AA
in points are: (i) Uniformity in the dis-f the four major units
from the Palo-
basement to the Middle Cambrian sedi-atform series. (ii) A
crystalline Palopro-sement underlying the entire AA region,orts
Ennih and Liegeoiss (2001) proposi-e old division of the AA into
two distinctated by the Central AA fault as advo-houbert (1963) be
abandoned. (iii) A
erogeneity of Cryogenian to Ediacaran rockthat reflects their
probable distinct originsts the presence of a collage of exotic
ter-Anti-Atlas terranes, AAT) during the mainn tectonic events.
(iv) A transition, datedfrom a Cryogenian/early Ediacaran oro-
s to an Ediacaran post-orogenic series andCambrian anorogenic
series. (v) A volumi-
2 km thick) Ediacaran volcanic sequencerzazate Supergroup),
deposited over 30 Maat covers most of the surface of the AAsuggests
the existence of a huge thermaldeep lithospheric levels. (vi) A
rapid tran-een a dominantly volcanic regime to a
y marine epicontinental carbonate platformon at ca. 550 Ma,
based on the last dated vol-(Benssaou and Hamoumi, 1999, 2001).
Theroudant Groups often unconformably/para-ably overly the edges of
the different inliers,-angle angular discordance. This is com-h a
progressive transgressive marine eventand Hamoumi, 1999, 2001). No
major geo-reak exists between the upper part of the
Supergroup and the bottom of Tata andGroups; both of which are
coeval with anl tectonic event.-African tectonic events are
recognised in
ections of the AA. The first event, whicher of the two (Leblanc
and Lancelot, 1980),
directly dated at 661 23 Ma from an asso-
smalbefor580 M
ThinternPrecaet al.threetherethe lito in(Comet al.
6.2.Anti-
AmagmPalThe gFig.ing tand ttifiedbelt istagebe gr
Stident1980lessBouk2002tholeAt Bto thassum
has bandan agorthoGroudated762 nt (Leblanc and Lancelot, 1980)
took placeuarzazate Supergroup magmatic event (ca.
belt from Morocco has been used to testal stratigraphic
correlations related to then/Cambrian boundary problem (Magaritz;
Landing et al., 1998). However, for thezed sections (Fig. 8), it
can be seen thatirect correlation between the major limits
oftigraphic groups, as they do not correspond
onally recognised stratigraphic boundarieset al., 1992; Amthor
et al., 2003; Gradstein).
e timing of Pan-African events in the(Figs. 9 and 10)
tive synthetic log of the tectonic andevents that characterise
the Precambrianevolution of the AA is shown in Fig. 9.onological
information is reproduced fromen major magmatic episodes
correspond-or changes in geochemical characteristicstonic
environment have been clearly iden-geodynamic evolution of the
Pan-Africanloped in Fig. 10 according to the four majorosed in Fig.
9. These magmatic events caninto four stages.Ocean opening. This
stage has been clearlyt Bou Azzer (Leblanc, 1975, 1981; Church,ou,
2000; Fekkak et al., 2003), but it ist in the Sirwa massif
(Leblanc, 1975; Elt al., 1991; Chabane, 1991; Thomas et al.,wever,
the precise age of the associatedagmatism has not been
well-constrained.zzer, the thermal metamorphism linkedplacement of
gabbroic dykes, which is
belong to the tholeiitic magmatic event,ated at 788 8 Ma
(Clauer, 1976; Leblanclot, 1980). Thomas et al. (2002) obtained43
14 Ma for a tonalite protolith from an
of the ophiolitic sequence of the Bleidathe Sirwa inlier. Samson
et al. (2003)mplacement of a tholeiitic plagiogranite at.
-
D. Gasquet et al. / Precambrian Research 140 (2005) 157182
173
Fig. 9. Synthetic log of the Anti-Atlas series with the main
tectonic events and UPb ages of magmatic episodes, obtained in this
study orcompiled from the literature: Juery (1976), Charlot (1976),
Ducrot and Lancelot (1977), Ducrot (1979), Leblanc and Lancelot
(1980), Mifdaland Peucat (1985), Magaritz et al. (1991), Mrini
(1993), At Malek et al. (1998), Landing et al. (1998), Gasquet et
al. (2001), Walsh et al. (2002);Cheilletz et al. (2002), Thomas et
al. (2002, 2004); Barbey et al. (2004); Inglis et al. (2004). Note
that the number of geochronological data isroughly proportional at
the volumetric importance of the magmatisms.
-
174 D. Gasquet et al. / Precambrian Research 140 (2005)
157182
Fig. 10. Pan-African geodynamic evolution of the Anti-Atlas
during Pan-African times. WAC: West African Craton, NAA: Northern
Anti-Atlas(after Levresse, 2001 modified). See text.
-
D. Gasquet et al. / Precambrian Research 140 (2005) 157182
175
Stage 2: Subduction. The calc-alkaline magmatismrelated to
arc-subduction processes in Saghro and BouAzzer has not yet been
precisely dated (Saquaque etal., 1989, 1whether thethe south
(LHefferan eEnnih andIf viewed amargin (afProterozoicseveral
micplanes within the AA bexplain thedifferent hythe high P-the Bou
Amately 5 kb
Stage 3:
3a. This e(Leblaresponallochtrelated(6606low-grgneissetectoninot
beeof this
3b. This eptectonifore-ar
Stage 4:
4a. This imat 595to felsrelatedLevresstitutesdistens
4b. The Edmajor hIt is intdeposi
and to base metal porphyry type mineralisations(Cheilletz and
Gasquet, 2001; Abia et al., 2003).Although it has not been dated
precisely, the Tiouit
esothlkalinelong997).n alk
he Cazzeroho (iselyelatedo whic
e Palarand bya (Hu330
tion irecen
7 Matotaln to Pthe
an tim
Inversambria
A magorogeg thenic an550 MThis enic toical anition, wd by eni
et, 2001of no
l depor worl, 2002CuP992). There is currently no consensus
onsubduction plane dipped to the north or toeblanc, 1975; Leblanc
and Lancelot, 1980;
t al., 1992; Villeneuve and Cornee, 1994;Liegeois, 2001, 2003;
Thomas et al., 2002).s an analogue to the modern western pacificter
Lagabrielle, in Pomerol et al., 2005),
subduction zones in the AA might boundroplates, so there could
have been severaldifferent dip directions. If this was the caseelt
during early Pan-African times, it wouldconflicting interpretations
and reconcile thepotheses. Stage 2 is also characterised by
low T metamorphism of the metabasites ofzzer inlier (T 350 C and
P of approxi-ar; Hefferan et al., 2002).Arccontinent collision and
ocean closure:
pisode was first recognised at Bou Azzernc, 1981; Hefferan et
al., 1992). It cor-ds to the tectonic emplacement of thehonous
oceanic ophiolitic slivers and isto the main Pan-African tectonic
event
90 Ma). Accompanying features includeade metamorphism, partially
anatectics and calc-alkaline intrusives. The mainc event in the
western and eastern AA hasn precisely dated, which makes
correlationevent over the whole AA difficult.isode, which is coeval
with the later, minorc event, corresponds to the closure of thec
and back-arc basin.
Extension and marine basin development:
portant magmatic episode has been dated570 Ma. It is represented
by intermediateic (mainly high-K calc-alkaline) intrusives
to base metal ore deposits (CuPbZn;se, 2001; Cheilletz et al.,
2002) and con-
a transition towards the main Ediacaranive tectonic
event.iacaran (570545 Ma) is characterised by aigh-K calc-alkaline
to alkaline magmatism.imately linked to the largest precious
metalts (AuHg Imiter, Cheilletz et al., 2002)
m
a
b1
4c. AtABc
r
t
ThEdiacterise400 Mity atralisabeen301
AgeniaalongAfric
6.3.Prec
Afromdurinpluto605ism.orogetologtranstrolleChbaet
al.tancemetaImiteet al.brianermal Au deposit is related to high-K
calc-e magmatism and has been interpreted asing to the same event
(Al Ansari and Sagon,
aline volcanic episode interbedded withinmbrian platform
sedimentary series at BouAghbar (this work) and at Bou
AzzerJbelDucrot and Lancelot, 1977) has been pre-dated at 530 Ma.
This alkaline volcanism isto the general anorogenic tectonic
regimeh the whole AA was subjected at this time.
ozoic geodynamic cycle started after theearly Cambrian
evolution. It is charac-the injection of felsic dykes at 470 andch,
1988), followed by hydrothermal activ-
and 300 Ma. At least part of the Au mine-n the Iourirn deposit
(Western AA) hastly attributed to a late Variscan event at(Gasquet
et al., 2004).
thickness of approximately 13 km of Cryo-alozoic rocks were
produced and accretednorthern border of the WAC during Pan-es.
ion tectonics at thenCambrian boundary
matic rocks present a progressive transitionnic magmatic suites,
episodically emplacedperiod 690605 Ma, to high-K calc-alkalined
volcanic rocks, formed during the perioda, and then, 20 Ma later,
to alkaline volcan-volution clearly suggests a change from anan
anorogenic tectonic regime. Sedimen-d structural evidence also
indicates such aith the development of molasse basins con-
xtensional tectonics (Benziane et al., 1983;al., 1999; Fekkak et
al., 1999; Soulamani; Soulaimani and Pique, 2004); the impor-rmal
faulting in the localisation of precioussits, particularly in the
epithermal AgHgd-class mine (Ouguir et al., 1994; Cheilletz); the
formation of Sedex-type early Cam-bZn deposits (Viland, 1988;
Benssaou and
-
176 D. Gasquet et al. / Precambrian Research 140 (2005)
157182
Hamoumi, 1999). On the whole, the Ediacaran periodin the AA
strongly reflects a tectonic inversion froma
compressivetranstensive regime to an extensionalregime
thaPan-Africaactive marggin of the Wtransition isuccessiveticularly
trufaults in thSouth AtlaLiegeois, 2
6.4. Geodyactivity in t
It is notecesses of ththe large-scthis area atary. Stagesclearly
corrduring theterised in theralisations690605 Mcollision, wening.
Theobduction,ity, calc-alkand collageto the nortTerranes (Aduring
thiscise geochrbe attributeinvolved thcoupled wment of mran
cratonithere was ametallogenImiter gianLevresse eparte lowervolume
ofcophile ele
The post-collisional features, related extensive
high-Kcalc-alkaline magmatism and marine basin develop-ment,
together indicate a high heat flow contribution
r due tllingmic elopmein thesits (e.l transextenseepes). Thir
crus
nifical foras Imtel and
iter sanalo
onclu
e evoised bion-ex, the cever, w
e Pans (Avfor exhy, 20mainan maled bynd genn is
mssionhism.rked
enic cisatioic act4 atcterissits (Imoccurrt is characteristic of
the final stage of then orogeny, resulting in a change from anin to
a passive margin at the northern mar-AC. The precise structural
pattern of this
s not yet fully understood, mainly due toVariscan and Alpine
overprints. This is par-e for the kinematic interpretation of
majore AA, for example the Central AA or thes faults (Saquaque et
al., 1992; Ennih and001; Oudra et al., 2003).
namic reconstruction and metallogeniche Pan-African AA (Fig.
10)
worthy that the geodynamic stages and pro-e AA Pan-African belt
(Fig. 10) controlledale metallogenic activity that took place inthe
PrecambrianLower Palozoic bound-1 and 2 are not developed here as
they do notespond to a major metallogenic event exceptocean opening
stage 1, which is charac-e Bleida inlier by base metal and gold
min-(Mouttaqi, 1997; Belkabir et al., 2004). Thea stage 3
corresponds to an arccontinentith a moderate collision and crustal
thick-main tectonic events were ocean floor
development of a regional NESW schistos-aline to high-K
calc-alkaline magmatism,of meta-sedimentary and -magmatic rocks
hern border of the WAC to form the AAAT). A typical active
margin developedstage 3. However, due to a lack of pre-
onological data no specific ore deposit cand to this event. The
605530 Ma stage 4e transition to a passive continental margin,ith
extensional tectonics and the develop-arine basins to the North and
the Saha-c basin to the South. During this period,long association
between magmatism and
ic activity (e.g. felsic volcanism and thet AgHg deposit,
Cheilletz et al., 2002;
t al., 2004). They have mantellic and procontinental crust
signatures. Thus, a large
juvenile material, precious metals and chal-ments was added to
the continental crust.
eitheupwedynadevetionsdepometa(e.g.the d2004loweis
sigmodesuch(Leisthe Imbrian
7. C
ThactercollisHereHowof thchainetc.,MurpthreeAfricrevea
ing aorogesucce
morpis maallogorganlogenstagecharadepotypeo continental
underplating and/or to mantle(Kearey and Vine, 1992). This type of
geo-nvironment is particularly suitable for thent of superficial
hydrothermal mineralisa-form of epithermal or base metal porphyryg.
Cheilletz et al., 2002). Furthermore, hugefers suggest the
existence of vertical drainsional fault zones) that were able to
mobiliset parts of the lithosphere (Levresse et al.,s model, which
involves a large part of thet and mantle (Kay and Mpodozis,
2001),ntly different to the earlier re-mobilisationthe origin of
giant precious metal deposits,iter, from local superficial
convective cellsQadrouci, 1991; Barodi et al., 1998). Thus,ilver
deposit can be regarded as a Precam-gue to modern epithermal
deposits.
sion: the northern margin of the WAC
lution of the Pan-African AA belt is char-y the successive
development of extension-tension events, as in all modern
orogens.
ollision event is subdivided into two stages.hen comparing the
geodynamic evolution
-African AA belt with other Pan-Africanalonia, South-America,
Carolina, Hoggar,ample: Murphy and Nance, 1989, 2002;02; Abdesalam
et al., 2002; Neves, 2003),differences can be noted: (i) The AA
Pan-in collisional stage (stage 3) is moderate, asthe absence of
large-scale crustal thicken-
eralised thrusting. (ii) The AA Pan-Africanostly composed of a
shallow-crust terranethat has been subject to low-grade meta-(iii)
The end of the Pan-African evolutionby strong extensional
tectonics. The met-onsequences of this specific orogenic beltn are
highlighted. The most intense metal-ivity occurred during the late
extensionalthe PrecambrianCambrian boundary. It ised by world-class
precious metal epithermal
iter) and base metal porphyry- and Sedex-ences (Bou Madine;
western AA early Cam-
-
D. Gasquet et al. / Precambrian Research 140 (2005) 157182
177
brian mineralisations). It is noteworthy that orogenicgold
mineralisations are lacking in the Pan-African AAbelt. This might
be related to the moderate crustal thick-ening that tpermit the
iand related
The tectined on thby Villeneutimes the wsubject
toconsequenteastern mathe westernTherefore,(Rogers etson and Dis in
a posifaults withdirection (L1994; Cabycan be conPan-AfricaEnnih
andthis type othe Imiter fextensionaregime (Oumatic charaAA fault,
Swithin thisattempt to rPan-Africaunderlie th
Acknowled
This stuation grantMinistry o0002). Weand M. Chties, C.
Spaanalyses ana careful rethank the twtive criticis
numerous improvements in the quality and the clarityof the
English text.
rence
alam, Metacrato
E.H.,neralisalOugn
ronologE.H., Ne polymgnat, eion of a, 2512u,
H.,nafricaipublisha, M.,
rtementaghro orsity ofalek, HPb dalay (M(Maro
267.alek, Hochronnafricairdous ead. Sciaadi, 1leovolcne`se deBou
M
hed Thesari, Ael Sagulfures
age pror, J.EmezaniCloudiundarySamir,lcanismntral (MlAfriq
A.,oprotergeodyntic evo
ogenic Irschunsook place during this orogeny, which did
notnstallation of large-scale hydrothermal cellsgold
re-mobilisations.onic evolution of the AA belt can be exam-e scale
of the WAC. As clearly reportedve and Cornee (1994), during
Pan-Africanestern and eastern margins of the WAC werecompletely
different tectonic regimes andly exhibited very different
behaviours Thergin was subject to a compressive regime;margin was
subject to an extensive regime.within the West African super
continent
al., 1995; Dalziel, 1997; Unrug, 1997; Sam-Lemos, 1999; Hefferan
et al., 2000), the AAtion where the main faults acted as
wrenchrespect to the main Pan-African shorteningapique et al.,
1986; Liegeois et al., 1987,et al., 1989; Caby, 2003). Thus, the AA
belt
sidered to have been a transfer zone duringn times (similar to
the transpression belt ofLiegeois, 2001). The abundant evidence
forf structural behaviour in this area includesault, which evolved
from a pure NNWSSEl regime to a NWSE sinistral transtensionalguir
et al., 1994; Levresse, 2001). The kine-cteristics of the other
major faults (CentralAF, NAF) have not yet been well defined
late Pan-African framework. Therefore, anyeconstruct the plate
tectonic geometry of then terranes, and the probable WAC limits
thatem, would be highly hypothetical.
gements
dy was supported by two scientific cooper-s awarded to A.C. and
D.G. by the Frenchf Industry (# 98 2 24 00 30 and # 00 224would
like to thank E. Deloule, D. Manginampenois for the Cameca IMS1270
facili-tz and L. Reisberg for the SmNd and RbSrd J.M. Bertrand for
fruitful discussions andview of early versions of the paper. We
alsoo anonymous reviewers for their construc-
m. The comments of P. Henderson led to
Refe
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Contribution to a geodynamic reconstruction of the Anti-Atlas
(Morocco) during Pan-African times with the emphasis on inversion
tectonics and metallogenic activity at the Precambrian-Cambrian
transitionIntroductionGeological setting of the Anti-Atlas belt of
MoroccoGeological setting of the Ougnat, Saghro and Bou Azzer
inliersOugnat-Bou MadineSaghro-ImiterBou Azzer-Aghbar
U-Pb geochronology in the Ougnat, Saghro and Bou Azzer
inliersAnalytical proceduresOugnat-Bou MadineSaghro-ImiterBou
Azzer-Aghbar
Geochemistry of the Ediacaran and Cambrian magmatism in the AA
areaMajor and trace elementsIsotope geochemistryRb-Sr, Sm-Nd, S and
rare gasRe-Os
DiscussionSynthesis of the lithostratigraphic and
geochronological data for the Pan-African AAPrecise timing of
Pan-African events in the Anti-Atlas (Figs. 9 and 10)Inversion
tectonics at the Precambrian-Cambrian boundaryGeodynamic
reconstruction and metallogenic activity in the Pan-African AA
(Fig. 10)
Conclusion: the northern margin of the
WACAcknowledgementsReferences