The alkaline–peralkaline granitic post-collisional Tin

21 Pages·1998·0.82 MB·English

Lithos45(cid:14)1998.223–243
The alkaline–peralkaline granitic post-collisional Tin Zebane
dyke swarm (cid:15)Pan-African Tuareg shield, Algeria/: prevalent
mantle signature and late agpaitic differentiation
Zakia Hadj-Kaddour a,1, Jean-Paul Lie´geois b,2, Daniel Demaiffe a,), Renaud Caby c,3
a Ge´ochimieIsotopique,CP160r02,Uni˝ersite´LibredeBruxelles,50a˝F.Roose˝elt,B-1050Brussels,Belgium
bDpt.deGe´ologie,SectiondeGe´ologieIsotopique,Muse´eRoyaldel’AfriqueCentrale,B-3080Ter˝uren,Belgium
c LaboratoiredeTectonophysique,Uni˝ersite´deMontpellier2,F-34095Montpellier,France
Received18December1997;accepted30June1998
Abstract
The Tin Zebane dyke swarm was emplaced at the end of the Pan-African orogeny along a mega-shear zone separating
two contrasting terranes of the Tuareg shield. It is located along the western boundary of the Archaean In Ouzzal rigid
terrane, but inside the adjacent Tassendjanet terrane, strongly remobilized at the end of the Precambrian. The Tin Zebane
swarm was emplaced during post-collisional sinistral movements along the shear zone at 592.2"5.8 Ma (cid:14)19WR Rb–Sr
isochron..Itisadyke-on-dykesystemconsistingofdykesandstocksofgabbrosanddykesofmetaluminousandperalkaline
granites. All rock types have Sr and Nd isotopic initial ratios (cid:14)Srs0.7028 and « sq6.2. typical of a depleted mantle
i Nd
source,similartotheprevalentmantle(cid:14)PREMA.atthatperiod.NocrustalcontaminationoccurredinthegenesisoftheTin
Zebane swarm. Even the samplesshowingevidenceof fluid interaction(cid:14)essentiallyalkali mobility. have the same isotopic
signature. The peralkaline granites have peculiar geochemical characteristics that mimic subduction-related granites: this
geochemicalsignature is interpreted in terms of extensive differentiationeffects due to late cumulates comprisingaegirine,
zircon, titanite, allanite and possibly fergusonite, separated from the liquid in the swarm itself due to magmatic flow
turbulence. The Tin Zebane dyke swarm is thus of paramount importance for constraining the differentiation of mantle
products to generate highly evolved alkaline granites without continental crust participation, in a post-collisional setting.
q1998ElsevierScienceB.V. All rightsreserved.
Keywords: Alkalinegranite;Post-collisional;Pan-African;Prevalentmantle;Tuaregshield;Algeria
1. Introduction
In a post-collisional setting, alkaline magmatism
commonly occurs in the form of high-level plutons
and dyke swarms that intrude after calc-alkaline
)Correspondingauthor.E-mail:[email protected]
magmatism. However, its origin and the reason for
1E-mail:[email protected].
2E-mail:[email protected]. its emplacement at that period of an orogenic belt
3E-mail:[email protected]. history are still highly debated. The relations be-
0024-4937r98r$-seefrontmatterq1998ElsevierScienceB.V.Allrightsreserved.
PII: S0024-4937(cid:14)98.00033-4
224 Z.Hadj-Kaddouretal.rLithos45(1998)223–243
tween the peralkaline and metaluminous alkaline se- variously composed of Pan-African juvenile rocks
ries are also the subject of discussion as are the with reactivated andror preserved Archaean–
possiblesourcesoftheseseries,eitherthecontinental Palaeoproterozoicrocks.Emplacementofwidespread
crust (cid:14)Clemens et al., 1986. or the mantle (cid:14)e.g., calc-alkaline batholiths has accompanied the relative
Bonin, 1996.. movements (cid:14)Black et al., 1994.. The end of these
Shear zone related dyke complexes from colli- movements was often marked by the intrusion of
sional orogens are an important target since they high-levelplutons,dykeswarmsandplateaulavasof
were mostly emplaced during the strike-slip motion alkaline composition (cid:14)Lie´geois and Black, 1987. or
(cid:14)tens or even hundreds of km. of the lateral move- of transitionalaffinity(cid:14)Azzouni-Sekkaland Boisson-
ment of the terranes. Such large shear zones can be nas, 1993.. Terrane boundaries are often marked by
considered as lithospheric faults that may be the mafic–ultramafic bodies, potentially ophiolitic, and
locus of drainage of mantle-derived magmas sam- by late elongated molassic basins (cid:14)Fig. 1..
pled at variable depths. Such shear zones are The post-collisional alkaline to peralkaline mag-
widespread in the Pan-African rocks of the western matism is mainly located to the west of the shield
partoftheTuaregshield,atabout150kmeastofthe (cid:14)Fourcade and Javoy, 1985; Lie´geois and Black,
main suture with the 2 Ga old West African craton. 1987.. Alkaline magmatism in the central and east-
Several alkaline dyke complexes have been de- ern parts of the shield is either very limited in
scribed in this domain (cid:14)Dostal et al., 1979.. Among volume (cid:14)some facies of the ‘Taourirt’-type plutons,
them, the post-collisional group comprises mafic, Azzouni-Sekkal and Boissonnas, 1993. or much
silica-undersaturated and alkaline–peralkaline rock younger than the Pan-African orogeny such as the
types. A¨ır ring-complexes at ca. 410 Ma (cid:14)Moreau et al.,
This study focuses on the Tin Zebane alkaline– 1994. or recent volcanism (cid:14)Atakor, Girod, 1971;
peralkaline dyke swarm that intruded after the colli- southern A¨ır, Moreau et al., 1991.. Immediately to
sion-related HT–HP Pan-African regional metamor- the west of the main suture, within the West African
phism.Itwasemplacedduringstrike-slipmovements craton, the 270–160 Ma Tadhak carbonatite-bearing
along N–S trending mega-shear zones that are re- silica-undersaturatedring-complexesarealsoyounger
sponsible for the important relative movements of (cid:14)Lie´geoisetal.,1991..Moreover,thepost-collisional
the several terranes constituting the Tuareg shield. late Pan-African alkaline magmatism is mainly con-
The Tin Zebane swarm is thus typically post-colli- centrated within the Kidal and Tassendjanet terranes
sional. west of the Archaean granulitic In Ouzzal terrane
(cid:14)Caby, 1996; Fig. 1..
The western branch of the Pan-African Trans-
2. Geological setting Saharan belt (cid:14)Kidal terrane in Mali and Tassendjanet
terrane in Algeria; Black et al., 1994; Fig. 1. com-
TheTuaregshieldiscomposedofseveralterranes prises spectacular alkaline dyke swarms in northern
welded together during the Pan-African orogeny Mali (cid:14)Boullier et al., 1986; Lie´geois and Black,
(cid:14)Black et al., 1994; Fig. 1.. It formed during two 1987.. These swarms were emplaced in the rigid,
major collisional periods, an early intense collision already cooled, Pan-African crust; they are penecon-
with the East Saharan craton at 750–660 Ma, in the temporaneous to the deposition of molassic series.
East (cid:14)Black and Lie´geois, 1993; Lie´geois et al., Their Rb–Sr ages are bracketed between 560 and
1994. and a late more gentle oblique collision with 540 Ma (cid:14)Lie´geois and Black, 1987; Lie´geois et al.,
the WestAfricancratonat 630–580 Ma, in the West 1996.. In Western Hoggar, a few alkaline–peral-
(cid:14)Blacketal., 1979;Cabyetal., 1981;Lie´geoisetal., kaline plutons, sometimes associated with calderas,
1987.. These collisions induced large movements are known (cid:14)e.g., the In Zize complex; Fourcade and
along mega-shear zones and mega-basal thrusts in- Javoy, 1985 and references therein.. On the other
ducing in turn important relative displacements be- hand, older, shear zone related and deformed dyke
tween the different terranes (cid:14)several hundred km; complexes, considered as late with respect to the
Caby, 1968; Lie´geois et al., 1994.. The latter are regional metamorphism (cid:14)Caby, 1970. have been de-
Z.Hadj-Kaddouretal.rLithos45(1998)223–243 225
Fig.1.TheTuaregshield.(cid:14)A.SketchmapoftheTuaregShieldwithterranesboundaries(cid:14)Blacketal.,1994..(cid:14)B.WesternpartoftheTuareg
shield. Slightly and strongly reworked terranes are made of Archaean and Proterozoic sequences. The suture separates the West African
Craton (cid:14)WAC. from the Tuareg Shield. Ki: Kidal, Ou: In Ouzzal, Tas: Tassendjanet, Til: Tilemsi, Tim: Timetrine, Tir: Tirek, Za: Tin
Zaouatene,Ugi:Iforasgranuliticunit.
scribed in the same area (cid:14)Dostal et al., 1979.. The while in the Tassendjanet terrane, it is much larger
Tin Zebanedyke swarmbelongsto these complexes. and pervasive (cid:14)Fig. 2.. It had a post-metamorphic
The Tin Zebane dyke swarm was emplaced at the sinistral movement of about 150 km (cid:14)Caby, 1968..
eastern margin of the Tassendjanet terrane, along a The Tassendjanet and In Ouzzal terranes behaved
major fault separating it from the In Ouzzal terrane. differently during the Pan-African orogeny. The
This sinuous several hundreds km long West Ouzza- Pan-African imprint was very slight (cid:14)T8-2808C. in
lian fault is marked in the In Ouzzal only by low- the In Ouzzal terrane which preserved its Palaeopro-
temperature cataclasites and mylonites of several terozoic ultrahigh temperature (cid:14)10008C, Ouzegane
metres to several tens of metres of width at most, and Boumaza, 1996. and granulite facies metamor-
226 Z.Hadj-Kaddouretal.rLithos45(1998)223–243
Fig.2.GeologicalsketchmapoftheTinZebanearea(cid:14)Dostaletal.,1979..(cid:14)1.DykeswarmofTinZebane:alkaline–peralkalinegranitesand
pyroxene gabbros and dolerites; (cid:14)2. main bodies of olivine gabbros; (cid:14)3. country rocks of the swarm: metasediments and gneisses of
Tassendjanetseries;(cid:14)4.ArchaeangranuliticformationofInOuzzal.
phic parageneses. This terrane behaved as a rigid dated at 1.75 Ga (cid:14)Caby and Andreopoulos-Renaud,
cratonic block during the Neoproterozoic; it was 1983.; (cid:14)2. a ca. 1 Ga Stromatolite series made of
most probably attached to its Palaeoproterozoic quartzites and limestones intruded by (cid:14)3. thick ultra-
lithosphere. In contrast, the Pan-African evolution of mafic and gabbroic sills (cid:14)pre-Pan-African oceanic
the Tassendjanet terrane is marked by major thrust domain at ca. 790 Ma; Dostal et al., 1996. and
sheets, HT–HP metamorphism, abundant magma- quartz diorites and granites; (cid:14)4. thick basaltic to
tism and volcanism (cid:14)Caby, 1970.. andesitic series (cid:14)active margin; Chikhaoui et al.,
The Tassendjanet terrane, host of the Tin Zebane 1978; Caby, 1987.. These volcanics were reworked
dyke swarm, recorded nearly the whole Pan-African to form a thick marine, juvenile Neoproterozoic
Wilson cycle. It is composed of (cid:14)1. an Eburnian grauwacke unit with volcanic intercalations (cid:14)‘Se´rie
gneissic basement in the north, covered by ortho- verte’, 690–630 Ma; Caby et al., 1989.. The
quartzites interleaved with alkaline metarhyolites tectono-metamorphic evolution of monocyclic
Z.Hadj-Kaddouretal.rLithos45(1998)223–243 227
metasedimentsalongthe In Ouzzalterraneis charac- gashesinawideningtranstensionalsite(cid:14)Caby,1970..
terized by an early system of west-verging recum- The large-scale foldrgash is thus considered as syn-
bent folds synchronous with prograde medium tem- to late-magmatic; it is probably connected with the
perature mineral assemblages (cid:14)chloritoid, kyanite, large-scale sinistral displacement along the West
staurolite. in metapelites with the occurrence of ret- Ouzzalian fault. Most felsic rocks were ductilely
rogressedeclogitesingarnetamphibolites,consistent deformed and partially recrystallized under deep
with T8 around 550–6008C. (cid:14)5. Syn-kinematic calc- greenschist conditions. This recrystallization oc-
alkaline plutons correlated with the Iforas batholith curred during and after the final consolidation of the
(cid:14)630–580 Ma; Lie´geois et al., 1987. intruded to the magma and uplift of the complex.
westwiththedevelopmentofyoungerhigh-tempera-
3.2. Petrography of the dyke swarm
ture, low-pressure conditions (cid:14)Caby, 1987.. (cid:14)6. Ex-
humation of the root of the Tassendjanetterrane was TheTinZebanedykeswarmisbimodal.Rocksof
coeval with infill of continental molassic graben intermediate composition are very rare. The basic
(cid:14)‘Se´rie pourpre´e’. between 570 and 530 Ma. rocks are olivine gabbros or pyroxene gabbros and
dolerites. Some are completely transformed into
greenschist mineral assemblages (cid:14)calcite–albite–
3. The Tin Zebane dyke swarm
chlorite–actinote–epidote paragenesis.. Primary
magmatic associations are however preserved in the
3.1. Field data
central part of the main body. Oli˝ine gabbro com-
The Tin Zebane dyke swarm post-dates the re- prises elongated subhedral plagioclase (cid:14)An .,
70-61
gional HP–HT Pan-African metamorphic event af- large anhedral olivine (cid:14)Fo . rimmed by orthopyrox-
66
fecting its country-rocks (cid:14)mainly kyanite-bearing ene (cid:14)En ., interstitial augite, and abundant Fe–Ti
71
quartzites and schists deposited during the Palaeo- oxides (cid:14)ilmenite and magnetite. with green spinel.
proterozoic.. It wasthusemplacedpriorto the brittle Augite and Fe–Ti oxides are systematically rimmed
tectonics associated with graben development and by hornblende, which also occurs as individual late
emplacement of undeformed dyke swarms of alka- stage grains. Pyroxene gabbros and dolerites are
line to peralkaline composition (cid:14)generation V of composed of euhedral labradorite laths, subophitic
Dostal et al., 1979.. These field relationships indi- augite, rare orthopyroxene and hornblende, Fe–Ti
cate an age bracket of 630–540 Ma for the Tin oxides, titanite and rare apatite. These pyroxene
Zebane dykes. gabbros and dolerites do not present an igneous
The Tin Zebane dyke complex is a 10 km wide lamination.
sigmoiddenseswarmlocatedalongthe WestOuzza- The coarse-grained felsic rocks commonly dis-
lian fault at lat. 22820XN (cid:14)Fig. 2.. It is composed of play variously deformed magmatic assemblages.
fine-grained alkaline and peralkaline granites and of Gradual transition into cataclastic to mylonitic tex-
gabbros, being then typically bimodal. Intermediate tures can be frequently seen on a few cm scale. The
rocks and few rhyolites are recorded but are rare. largecrystalsaredeformedandhavegranulatedmar-
The dykes are several km in length and from less gins leading to the formation of augen; the fine-
than one metre to several metres in width. This is a grained rocks often show alternating layers or bands
dyke-on-dyke swarm, without any screen of of felsic and mafic minerals. Typical porphyroclastic
country-rocks. Gabbros from the main body (cid:14)30 and mylonitic microtextures indicate that solid state
km2, ca. 5 km thick. display a magmatic layering deformation occurred after consolidation. However,
defined by variable amounts of plagioclase and dykes with undeformed magmatic fabrics have also
olivine. The felsic dykes constitute a complex net- beenobservedcrosscuttingdeformeddykes.Thesyn-
work: they apparently root in stocks and become to late-kinematic growth of brown or green biotite,
thinner towards the east, along the fault, where they riebeckite and aegirine took place during plastic
grade to ultramylonites. deformation. The secondary mineral associations ob-
The swarm shows a large open sigmoidal fold. served in the retrogressed mafic dykes suggest that
This feature is in agreement with opening tension the syn-kinematic metamorphism reached deep
228 Z.Hadj-Kaddouretal.rLithos45(1998)223–243
greenschistfaciesconditionsfollowingmagmasolid- with quartz and accessory minerals (cid:14)Fe–Ti oxides,
ification. zircon and allanite. in a groundmass deeply trans-
The acid rocks can be separated into two groups: formed to greenschist mineral assemblage (cid:14)clinozoi-
a peralkaline series characterized by sodic pyroxene site, chlorite, quartz, titanite..
andamphibole,andametaluminousseriescomposed One reddish granophyricleucocraticsample (cid:14)531.
of biotite-bearing fine-grained granite, sometimes contains abundant brick-red microcline both as phe-
with granophyric texture. The peralkaline granites nocrysts and in the quartz–feldspar mesostasis. Al-
are composed of two feldspars (cid:14)K-feldspar and al- bite is absent. This rock has suffered subsolidus
bite., quartz, sodic clinopyroxene and amphibole transformationandfluidinteraction,itisclassifiedas
with accessory titanite, zircon, apatite, allanite, ru- ‘metasomatic’. Two other samples share unusual
tile, ilmenite and magnetite. Rarely preserved abundant microcline relative to the other samples.
perthitic alkali feldspar occurs either as phenocryst Thefirstisadevitrifiedrhyolite(cid:14)531.2.withslightly
or within the mesostasis; it is often partially trans- perthitic microcline and quartz phenocrysts in a
formed to microcline with albite rims. The latter is mainly felsic mesostasis. The second (cid:14)183. is of
also present as small anhedral to subhedral grains. felsic composition and contains Fe–Mg mineral
The quartz, in large or small granulated grains, is ghosts (cid:14)formerly aegirine?.; it was probably peralka-
always anhedral. The clinopyroxene is an aegirine line before these subsolidus modifications. Although
often with an aegirine–augite core; it forms late having different primary features, these three sam-
interstitial, sometimes poikilitic, elongated crystals pleshavebeenputtogetherina‘metasomatic’group
or fine needles. It is partly transformedto riebeckite, to reflect their common, although at different de-
which occurs also as individual subhedral grains grees, subsolidus transformations.
with arfvedsonite cores or as needles. Biotite is rare Locally, some dark fine-grained rocks enriched in
andpresentas thinelongatedcrystalsorrimmingthe accessory minerals occur as diffuse layers (cid:14)few cm
sodic amphibole or pyroxene. The metaluminous thick. in felsic rocks. Sample 538.1 contains abun-
granites have porphyroclastic texture marked by dant quartz, actinolite, rare microcline and over 30%
feldspars. The K-feldspar is often a perthitic micro- of accessory minerals: mainly zircon (cid:14)either colour-
cline elongated and granulated. Primary plagioclase less and inclusion-poor or strongly metamict and
is an oligoclase; late albite occurs as rims around Th-rich.butalsotitanite,allanite,monaziteandother
microcline. Most samples are subsolvus, sometimes REE-rich phosphates and Fe–Ti oxides.
granophyric and some samples contain only albite in
the mesostasis.Quartzis late and anhedral. Biotiteis
4. Geochronology and isotope geochemistry
ubiquitous but in small amounts; it appears as inter-
stitial elongated crystals associated with the acces-
4.1. Analytical techniques
sory and secondary minerals. Muscovite is well de-
veloped in the granophyres as large, presumably After acid digestion of the sample and separation
primary, elongated crystals while, in other granites, on ion exchange resin, Sr and Nd isotopic measure-
it is scarce and replaces biotite. Only one sample ments have been carried out on a Fisons VG Sector
(cid:14)534.1. contains abundant hornblende. Accessory 54 mass spectrometer. All errors are given at the 2s
mineralsaretitanite,magnetiteandilmenite,primary level. Repeated measurements of Sr and Nd stan-
allanite and clinozoisite, zircon, apatite and rutile. dards have shown that between-run error is better
Garnetisrareinbiotitegranite(cid:14)Sp Gr Alm Pyr .; than 0.00002. Systematic measurements of the
42 44 12 1
it can be abundant in the granophyres where it has a NBS987 Sr standard during this study gave an aver-
spessartine composition and a clear magmatic habit age 87Srr 86Sr value of 0.710267"0.000007 (cid:14)nor-
(cid:14)Sp Gr Alm Pyr .. Pistachite and fluorine are malised to 86Srr 88Srs0.1194.. The MERCK Nd
50 26 22 2
rare. Secondary chlorite and calcite are common. standard gave an average 143Ndr 144Nd value of
Intermediate rocks are uncommon: one sample of a 0.512742"0.000008 (cid:14)normalised to 146Ndr 144Nd
porphyritic quartz–monzonite is available. It is com- s0.7219.. Used decay constants (cid:14)Steiger and Ja¨ger,
posed of relict andesine and orthoclase phenocrysts 1977. are 1.42=10y11 yry1 (cid:14)87Rb. and 6.54 10y12
Table1
SrandNdisotopicratios
Sample Rb Sr 87Rbr86Sr 87Srr86Sr 2s Sr Sm Nd 147Smr144Nd 143Ndr144Nd 2s « T
m i592 m Nd592 DM
a b c
G 188.2 0.50 432 0.0033 0.702834 0.000015 0.702806 0.63 2.20 0.17320 0.512636 0.000045 1.7 1478 2015 1519
G 533 6.35 442 0.0415 0.703151 0.000017 0.702801 1.80 6.80 0.16011 0.512836 0.000009 6.6 675 939 617
QM 532 17.88 183 0.2831 0.705275 0.000011 0.702885 7.00 35.95 0.11777 0.512677 0.000006 6.7 634 776 598
Z
PG 545.1 50.5 10.14 14.596 0.826752 0.000010 0.703539 5.30 23.00 0.13938 0.512709 0.000005 5.7 740 938 701 .
H
PG 540.1 56.6 5.90 28.207 0.875178 0.000016 0.637060 5.80 39.00 0.08995 0.512555 0.000005 6.5 641 752 614 a
d
PG 546 53.5 7.64 20.599 0.870594 0.000016 0.696698 7.40 51.00 0.08776 0.512542 0.000005 6.4 646 754 619 j-K
PG 545.2 63.8 4.52 42.257 1.049651 0.000013 0.692927 10.20 66.00 0.09347 0.512482 0.000004 4.8 745 867 721 ad
d
PG 269.6 62.0 15.28 11.853 0.799935 0.000013 0.699878 4.60 27.00 0.10305 0.512671 0.000005 7.7 563 680 530 o
u
PG 269.4 54.4 20.70 7.648 0.765028 0.000009 0.700465 7.68 53.50 0.08682 0.512586 0.000004 7.3 592 696 563 re
t
PG 545 51.4 9.26 16.281 0.835439 0.000018 0.697998 2.10 9.20 0.13806 0.512700 0.000008 5.7 745 939 707 a
PG 269.2 51.6 4.68 32.621 0.947144 0.000011 0.671760 2.70 14.50 0.11262 0.512589 0.000006 5.4 727 870 697 l.r
L
MG 534.1 92.3 81.2 3.299 0.731801 0.000007 0.703952 5.20 26.00 0.12097 0.512664 0.000007 6.2 673 825 639 ith
MG 526 96.2 38.0 7.358 0.766182 0.000012 0.704063 7.39 35.87 0.12461 0.512700 0.000006 6.7 642 796 605 os
MG 538.2 72.8 37.6 5.619 0.751109 0.000011 0.703671 7.90 24.00 0.19910 0.512966 0.000005 6.2 959 2167 847 45
MG 525.1 84.2 13.9 17.725 0.857611 0.000012 0.707982 11.74 56.01 0.12678 0.512758 0.000005 7.6 564 714 522 1(
9
MGr 531.1 94.0 43.9 6.227 0.757677 0.000009 0.705111 4.80 24.00 0.12097 0.512635 0.000005 5.7 717 873 684 98
)
MGr 527.1 74.4 37.4 5.780 0.748905 0.000043 0.700111 9.50 43.00 0.13363 0.512682 0.000005 5.6 356 475 309 2
2
MGr 180 67.4 26.2 7.496 0.764841 0.000023 0.701559 8.16 40.87 0.12077 0.512874 0.000006 10.4 739 922 702 3
–
mG 183 166 18.73 26.184 0.932882 0.000013 0.711839 9.08 33.65 0.16322 0.512945 0.000010 8.5 429 663 349 24
3
mGr 531 235 72.0 9.503 0.780244 0.000011 0.700021 2.49 13.53 0.11131 0.512620 0.000005 6.1 675 812 644
mR 531.2 105 30.2 10.176 0.791142 0.000011 0.705239 5.10 22.00 0.14021 0.512757 0.000006 6.6 659 848 614
Nd and Sr isotopic results. Rb, Sr, Sm and Nd contents in ppm. Gsgabbro; QMsquartz monzonite; PGsperalkaline granite; MGsmetaluminous granite; MGrs
metaluminousgranitewithgranophyrictexture;mGsgranitefromthemetasomaticgroup;mGrsgranitewithgranophyrictexturefromthemetasomaticgroup;mRsrhyolite
fromthemetasomaticgroup.Sr (cid:14)87Srr86Srinitialratio.and« arecalculatedat592Ma,theageoftheRb–Srisochron.VariabilityandtoolowvaluesinSr (cid:14)-0.7.are
i592 Nd592 i
explainedbythehighRbrSrratioofseveralsamples.TheonlyreliableSr valueforTinZebaneisgivenbytheisochron.Ndmodelages(cid:14)T .werecalculatedfollowingthe
i DM
depletedmantleparametersfromMichardetal.,1985(cid:14)a.,Goldsteinetal.,1984(cid:14)b.andNelsonandDePaolo,1985(cid:14)c..
2
2
9
230 Z.Hadj-Kaddouretal.rLithos45(1998)223–243
Fig.3.(cid:14)A.Rb–Srisochrondiagram.(cid:14)B.Sm–Ndisochrondiagram.
yry1 (cid:14)147Sm.. Isochron calculations were made fol- ites and 3 ‘metasomatic’ rocks. have been analyzed
lowingWilliamson(cid:14)1968..ResultsaregiveninTable for the Nd and Sr isotopes (cid:14)Table 1.. Nineteen
1. samples define an isochron in the Rb–Sr isotopic
system giving the following parameters: 592.2"5.8
4.2. Results: a PREMA Pan-African mantle produc- Ma, Sr (cid:14)initial 87Srr 86Sr.s0.70281"0.00001
i
tion (cid:14)MSWDs2.87,19WRr21;Fig.3A..IntheSm–Nd
isotopic system, despite the narrow range of SmrNd
Twenty-one samples (cid:14)2 gabbros, 1 quartz mon- ratios, 18 samples form nearly an isochron, giving
zonite, 7 metaluminous granites, 8 peralkaline gran- within error limits an age similar to the Rb–Sr
Fig.4.PositionoftheTinZebane(cid:14)TZ.isotopicSr(cid:14)A.andNd(cid:14)B.signaturescomparedtoevolutioncurvesofreferenceddepletedmantle,
prevalentmantle(cid:14)PREMA.andchondriticuniformreservoir(cid:14)CHUR..FromAlle`greandRousseau(cid:14)1985.,BenOthmanetal.(cid:14)1984.,De
Paolo(cid:14)1981.,Goldsteinetal.(cid:14)1984.,LiewandMcCulloch(cid:14)1985.,McCullochandBlack(cid:14)1984.,TaylorandMcLennan(cid:14)1985.andZindler
andHart(cid:14)1986.;seeageneraldiscussiononthesevariousparametersinRollinson(cid:14)1993..
Z.Hadj-Kaddouretal.rLithos45(1998)223–243 231
geochronometer: 580"55 Ma, « sq6.2"0.4 These isotopic results, in particular the existence
Nd
(cid:14)MSWDs3.97, 18WRr21; Fig. 3B..There is no of a single Rb–Sr isochron, indicate moreover no or
obvious explanation for the outliers in both isotopic veryslightelementmobilityduringpenecontempora-
systems. neous metamorphism. In the following geochemistry
These data show that the different petrographic section, metamorphic effect can then be neglected.
types of the Tin Zebane dyke swarm, that is the
mafic and the felsic rocks, the peralkaline and meta-
luminousgranites,areallcogenetic:theyallhavethe 5. Major and trace elements geochemistry
same age and the same source. This source is un-
usual for granites: the isotopic compositions (cid:14)Sr s 5.1. Analytical techniques
i
0.7028 and « sq6.2. are indeed characteristic of
Nd
a rather depleted mantle, even more depleted than Major elements have been measured by X-ray
the least crust-contaminated sample of the subcon- fluorescencespectrometry(cid:14)XRF.attheUniversityof
temporaneous Laouni tholeiitic layered intrusion in Lie`ge (cid:14)CGR-Lambda 2020 instrument. on Li borate
Central Hoggar (cid:14)Sr s0.7029 and « sq5.1; Cot- discs except Na which was measured on pressed
i Nd
tin et al., 1998.. When compared to different mantle powder pellets (cid:14)Bologne and Duchesne, 1991.. Rb
reservoir evolution curves (cid:14)Fig. 4A and B., the Tin andSrhavebeenmeasuredbyhighprecisionXRFat
Zebane Nd and Sr initial ratios are only slightly less ULB for concentrations above 10 ppm and by iso-
depleted than the different depleted mantle evolu- tope dilution (cid:14)MRAC-ULB. below 10 ppm. Other
tions. This is reflected by the T model ages trace elements have been measured at MRAC (cid:14)Sec-
DM
(cid:14)Table 1. which are very close to the emplacement tion de Ge´ochimie. by ICP-MS (cid:14)VG PQ2q.: the
age: 646"37 Ma (cid:14)mean of 18r21 values. with the result of the alkaline fusion (cid:14)0.3 g of sampleq0.9 g
parametersofNelsonandDePaolo(cid:14)1985.orslightly oflithiummetaborateat10008Cduring1h.hasbeen
older(cid:14)815"43Ma,meanof18r21values.withthe dissolved in 5% HNO . The calibrations were set
3
parameters of Goldstein et al. (cid:14)1984.. using both synthetic solution (cid:14)mixture of the consid-
The Tin Zebane Nd and Sr initial ratios fall close ered elements at 2, 5 and 10 ppb. and international
to the Prevalent Mantle (cid:14)PREMA. curve. The rock standards (cid:14)BHVO-1, W1, GA, ACE.. For all
PREMA component corresponds to a source match- these elements, the precision varies from 5 to 10%
ing a large proportion of the isotopic characteristics (cid:14)for details, see Navez, 1995.. Results are given in
ofoceanicbasalts(cid:14)mainlyoceanislands., althoughit Table 2.
does not correspond necessarily to a physical reser-
voir (cid:14)Zindler and Hart, 1986.. The gabbros, the 5.2. Results
metaluminous and peralkaline granites of the Tin
Zebane dyke swarm come then from a source corre- Thirty samples representative of the different pet-
sponding to the depleted end-member of the ocean rographical types have been analyzed. Mafic rocks
island basalts(cid:14)OIB. and thus of the alkalinemagma- are often retrogressed andror weathered, only two
tism. This indicates the absence of any contribution fresh gabbros have been selected. One intermediate
of the continental crust. Even a small crustal partici- rock only, a quartz monzonite, was available. This
pation can be ruled out as the Tin Zebane dyke study is thus mainly focused on the metaluminous
swarm has intruded along the Archaean In Ouzzal and peralkaline granites. The three samples from the
terrane which had, at 600 Ma, « and Sr values metasomatic group will be also considered.
Nd i
between y20 and y30 and )0.720, respectively Both metaluminous and peralkaline series have
(cid:14)calculatedfromthedataofPeucatetal.,1996..This mean normative compositions close to thermal mini-
indicates that the northwards motion of the rigid In mum Ab –Or –Q , suggesting a temperature of
35 25 40
Ouzzal terrane along the sigmoidal fault created a 7208C for a pressure of 1 to 2 kbar (cid:14)Tuttle and
transtensional site within the rocks of the less rigid Bowen, 1958.. All samples have a NarK molar
Tassendjanet terrane that allows mantle melts to ratio higher than 1, except the metasomatic group
intrude vertically without interaction with the crust. (cid:14)Fig. 5A.. The peralkaline and metaluminous gran-
232 Z.Hadj-Kaddouretal.rLithos45(1998)223–243
Table2
Majorandtraceelementcompositions
Sample Gabbros Q-mz Peralkalinegranites
188.2 533 532 545.1 536 540.1 546 269.5 545.2 269.6 269.4 544 545 540.2 269.2
SiO 47.08 47.47 67.09 75.22 76.13 76.16 76.58 76.70 76.78 76.80 76.86 76.88 76.88 77.76 78.08
2
TiO 0.93 1.39 0.46 0.15 0.38 0.20 0.21 0.20 0.20 0.13 0.16 0.22 0.17 0.20 0.17
2
Al O 19.41 17.65 15.31 10.92 10.64 11.36 11.21 11.30 11.05 11.30 11.37 11.16 11.07 10.84 11.26
2 3
Fe O 10.72 10.48 4.08 3.22 4.24 2.43 2.31 2.54 2.44 2.53 2.29 2.19 2.62 2.68 1.91
2 3
MnO 0.15 0.15 0.06 0.18 0.16 0.10 0.10 0.07 0.09 0.08 0.08 0.10 0.10 0.11 0.07
MgO 7.54 6.92 0.85 0.25 0.09 0.10 0.11 0.08 0.11 0.12 0.05 0.10 0.10 0.08 0.05
CaO 11.63 12.50 1.99 0.26 0.20 0.13 0.21 0.05 0.09 0.20 0.07 0.12 0.11 0.18 0.04
Na O 2.26 2.19 7.31 4.54 4.09 4.31 4.00 4.31 4.26 3.59 4.33 3.91 4.19 4.70 3.72
2
K O 1.06 0.23 1.51 4.13 4.00 4.60 4.93 4.52 4.62 5.10 4.43 4.62 4.20 3.06 4.66
2
P O 0.03 0.11 0.10 0.05 0.03 0.04 0.02 0.03 0.04 0.04 0.03 0.03 0.03 0.02 0.01
2 5
H O 0.14 1.09 1.06 0.05 0.34 0.02 0.02 0.05 0.05 0.02 0.40 0.02 0.02 0.02 0.31
2
Total 99.95 100.2 99.82 98.97 100.3 99.45 99.70 99.85 99.73 99.91 100.1 99.35 99.49 99.65 100.3
A.I. 0.19 0.22 0.89 1.09 1.04 1.06 1.06 1.06 1.09 1.01 1.05 1.02 1.03 1.02 0.99
V 179.0 302.0 41.9 6.1 1.1 1.1 0.7 0.6 0.8 2.1 2.4 3.0 3.3 0.4
Rb 0.5 6.4 17.9 50.6 32.4 56.6 53.5 61.6 63.9 62.0 54.4 46.9 51.5 36.0 51.7
Sr 432.2 442.4 182.7 8.8 10.7 5.9 7.6 23.8 4.5 15.3 20.7 2.1 9.3 7.4 4.7
Y 3.4 8.6 43.7 14.8 5.9 8.1 6.5 7.4 9.8 10.8 12.4 5.4 5.8 4.7
Zr 6.8 27.0 637.7 204.0 24.7 346.0 252.0 376.0 276.0 379.0 324.8 71.0 73.0 282.0
Nb 0.1 0.6 10.1 3.5 1.1 4.8 4.3 6.2 4.7 2.4 5.4 2.0 2.8 3.0
Ba 33 244 634 117 430 217 196 315 148 200 379 131 116 188
La 1.5 4.6 32.7 15.3 33.9 38.0 53.0 42.0 65.0 28.0 58.8 5.5 5.7 14.0
Ce 3.3 9.4 72.3 43.0 69.6 86.0 120.0 89.0 128.0 65.0 124.5 14.5 16.7 30.0
Pr 0.4 1.4 9.0 5.6 7.9 10.5 14.0 12.5 17.0 6.8 14.5 2.0 2.3 3.8
Nd 2.2 6.8 36.0 23.0 29.0 39.0 51.0 46.0 66.0 27.0 53.5 9.2 10.5 14.5
Eu 0.48 0.83 2.03 1.02 0.58 0.93 1.23 1.04 1.65 0.95 1.31 0.45 0.51 0.48
Sm 0.63 1.80 7.00 5.30 3.71 5.80 7.40 6.60 10.20 4.60 7.68 2.10 2.50 2.70
Gd 0.78 2.10 6.42 4.80 2.70 3.60 4.90 3.90 6.90 3.80 5.77 1.90 2.10 2.20
Dy 0.86 2.10 6.54 4.10 1.36 2.40 2.50 2.40 3.30 2.80 2.89 1.58 1.77 1.35
Ho 0.16 0.41 1.31 0.80 0.17 0.42 0.38 0.41 0.54 0.50 0.45 0.26 0.29 0.23
Er 0.46 1.19 3.92 2.40 0.62 1.45 1.07 1.36 1.48 1.45 1.38 0.83 0.92 0.78
Yb 0.43 1.09 4.03 3.90 0.94 2.40 1.34 2.40 2.20 1.56 1.76 1.55 1.48 1.27
Lu 0.06 0.15 0.58 0.75 0.17 0.47 0.25 0.50 0.42 0.24 0.32 0.31 0.28 0.24
Hf 0.29 0.99 14.74 7.90 0.66 6.60 4.90 8.10 5.80 5.90 5.98 2.00 2.60 4.90
Ta 0.10 0.10 0.46 0.24 0.10 0.15 0.10 0.20 0.10 0.10 0.11 0.10 0.10 0.10
W 0.10 0.22 0.10 0.25 0.10 0.59 0.84 0.73 0.50 3.04 0.10 0.96 0.65 0.10
Pb 0.27 5.30 9.19 1.74 2.75 1.88 1.95 0.74 0.69 3.20 2.39 2.20 4.50 1.84
Th 0.12 0.33 5.30 2.00 0.36 2.70 1.59 2.40 1.99 1.97 2.22 0.24 0.41 1.61
U 0.05 0.05 2.00 0.54 0.05 0.49 0.22 0.41 0.36 0.36 0.74 0.09 0.39 0.54
Majorandtraceelementcompositions.Allmajorelementsinwt.%,alltraceelementsinppm.SeeTable1fortheaffiliationofeachsample.
Q-mzsquartz monzonite. M.S.smineral segregation (cid:14)‘magmatic placer’ enriched in accessory minerals such as zircon, allanite and
titanite..A.I.sAgpaiticindex.
itesplotin the samefield, the metaluminousgranites trend, implying that an alkaline-rich mineral, proba-
being slightly more sodic. The peralkaline granites bly a sodic pyroxene, fractionatedfrom the melt as a
are actually more depleted in Al than enriched in latecumulusphase.Themostsilicaenrichedperalka-
alkalis(cid:14)Table2..Theyhaveindeedanagpaiticindex line granite(cid:14)269.2. has AI slightlylower than 1: this
(cid:14)AI. above 1 (cid:14)Fig. 5B. and show a clear negative may be marked by the late crystallisation of particu-