Pb GEOCHRONOLOGY OF CENTRAL HOGGAR (ALGERIA&rpar

18 Pages·2007·1.52 MB·English

TECTONICS, VOL. 5, NO. 7, PAGES 955-972, DECEMBER1 986
STRUCTURE AND U/Pb GEOCHRONOLOGY
OF CENTRAL HOGGAR (ALGERIA):
A REAPPRAISAL OF ITS PAN-AFRICAN
EVOLUTION
Jean-MichelB ertrand,A nnie Michard,
Anne-Marie Boullier, and Danielle Dautel
Centred e RecherchePs(cid:127)t rographiques
et G(cid:127)ochimiquesV, andoeuvre-l(cid:127)s-Nancy,
France
Abstract. Discoveryo f large-scaled eep-seated deepi ntracontinenteavl olutiono f CentraHl oggara nd
thrustsin CentralH oggar,w ith a plurifaciale volution thec ollision-relatetde ctonicos f WesternH oggara nd
ranging from lower amphibolitef acies to upper Iforas suggests a common origin for both
greenschisfat ciesc onditionsa ndl inkedt o a regional phenomena.
refoliation, has led us to reconsider the Pan-African
tectonica ndm etamorphihci storyi n thatr egion.T wo INTRODUCTION
areas are described, and a review of other thrusts
leadst o an interpretativec rosss ectiono f a large Sincet hew orko f Lelubre[ 1952],t heg neisseasn d
portiono f reactivatedc ontinentaclr ust.T he agea nd graniteso f the centralp art of the Hoggarh aveb een
kinematicso f this structurarl eworkingh ave been consideretdo belongt o theo ldests tratigraphuicn ito f
approached using U/Pb zircon dating in the the shield.H igh-gradem etamorphicte rranesc ollec-
Tamanrassetr egion. Despite the difficulty of tively called Suggariana re associatedw ith restricted
estimating the age of the initiation of the assumed belts of low-grade schists defined as Pharusian
intracontinental A-type subduction, the results [Kilian, 1932; Lelubre, 1952]. The high-grade
providea time spano f 30-40 m.y. betweenth ec limax gneissefsr om CentralH oggarh aven everf urnished
of granitoide mplacemenat nd a late retrogressive ages older than the Eburnean (about 2000 Ma
offseta longt he thrustp lanes.S omek ey agesw ere [Bertrand and Lasserre, 1976; Vialette and Vitel,
determined(:1 ) 2075 + 30 Ma is the age given by 1979; Latouchea nd Vidal, 1974]) excepta Pb/Pb
granulitef aciesr emnantsw hich escapedf rom the whole rock isochron( 3480 + 90 Ma) quotedb y
refoliationt,h ec orrespondinlogw eri ntercepat t 530 Latouche[ 1978]. Recent syntheses[B ertranda nd
+ 70 Ma confirms the Pan-African imprint; (2) Caby, 1978; Caby et al., 1981] have proposeda
615+ 5 Ma reflectingt he ageo f syntectonitco late- geotectonica pproach,t rying to recognize which
tectonicg ranitoidse mplacedi n reworkedg neisses structureos f the Hoggars hielda re correlatedw ith the
and in preserved granulites; (3) 580 Ma, the Pan-African Wilson cycle and the well-defined
concordanta ge of sphenesa nd monazitesf rom the collisiona longt hee asternm argino f theW estA frican
same granitoids,w hich is interpreted as corres- craton. Recent field observationsa nd geochro-
pondingt o the endo f medium-gradceo nditionsN. o nologicald atai n the polycyclic CentralH oggar
evidenceh as been found of a ~1000 Ma age: a havel ed to a reappraisaol f its geotectonice volution
Kibaran event does not appeart o exist in Central [Boulliera ndB ertrand,1 981;B ertrande t al., 1984].
Hoggar. The age similarity between the observed
GEOLOGICAL SETTING OF THE CENTRAL
Copyright1 986 HOGGAR
by theA mericanG eophysicaUl nion.
As showni n Figure 1, three main lithologic
Papern umber6 T0397. associationms ay be definedi n Central Hoggar,
0278-7407/86/006T-0397510.00 irrespectiveo f their presumeda ge: (1) quartzo-
956 Bertrande t al.: Structurea ndU /Pb Geochronologyo f CentralH oggar
Amongstt he metasedimenttsw o groupsh ave been
defined: (1) those intimately associatedw ith the
quartzofeldspathicg neisses,t he Arechchoums eries,
and (2) the Aleksod series which lies in structural
unconformity upon group 1 and shows a simpler
structurahl istorys uggestinga youngera ge.
The two groupsa rev ery similari n lithology,b ut in
the Aleksod area a seto f basicd ykesa llows the clear
separationo f the two groups [Bertrand, 1974]. In
places,s tratigraphiuc nconformitieasr ep reservedf,o r
5okra
example,i n the Toukmatines eriesi n the Oumelalen
area [Latouche, 1978] and the Tazat area [Blaise,
1967; Bertrand et al., 1968], but tectonic contactsa re
more usual.I t was from the distinctiono f two groups
(cid:127)TR of metasedimentasn d from the noncriticala cceptance
of the classical Pharusianu nconformityw, hich was
considereda s giving a lower limit for the upper
Proterozoic, that the existence of a Kibaran event at
about 1000 Ma was previouslyp roposedi n Central
Hoggar [Bertrand, 1974]. Some geochronological
resultss eemeda t thatt ime to supportt his assumption
[Bertrand and Lasserre, 1976], but they were
discussedu sing data from other regions [La Boisse
and Lancelot, 1977]. In fact, recent structural
TG investigationsi n the schist belts have shown the
tectonic nature of the classical unconformity (M.
/'/(cid:127) LA . Briedj and J. M. Bertrand,u npublishedo bservations,
1980).
Two major eventsh aveb eenr ecognizedin Central
Hoggar using availableg eochronologicadl ata [Pic-
ciotto et al., 1965; Latouche and Vidal, 1974;
Bertrand and Lasserre, 1976; Bertrand et al., 1978,
and 1984; Vialette and Vitel, 1979; Latouche and
Vidal, 1974]: (1) the Eburnean event at about
Fig. 1. Sketchm ap of CentralH oggar (seei nsetf or 2000 Ma is defined from quartzofeldspathic
location). 1, Granites and quartzofeldspathic gneissesi,n cludingo rthogneissesa nd charnockites,
gneisses2; , high-gradem etasediments3; , low-grade and from the older metasedimentsa nd (2) the Pan-
schistb elts;4 , Ordoviciant o Quaternarys ediments; African event at about 600 Ma.
5, Phanerozoic lavas; 6, major wrench faults; The metamorphic evolution of Central Hoggar
7, thrusts;8 , eclogites.W AC, West African craton; reflects the dual influences of the history of the
TSB, Transaharanb elt. Localitiesq uotedi n the text: Suggaran (Eburnean) basement and of the Pan-
TAM, TamanrassetT; Z, Tin Amzi; IA, In Amguel; African imprint. From their structurea nd radiometric
AL, Aleksod; OUM, Oumelalen; TA, Tazat; age,t he granulitica ssemblageasn dt hep rebasicd yke
TI, Tinef; TT, Tin Teganet; TM, Temasint; gneisses are attributed to the Suggaran event
EG, Egere;T R, Tiririne; AF, Afedafeda;A R, Arefsa; [Latouche,1 978; Vitel, 1979; Ouzegane,1 981]. The
TG, Timgaouine;L A, Laouni. tendency,i n recentp apersa nd theses,i s to attribute
the intermediate pressure metamorphism, often
correspondintgo the mostc onspicuoufso liation,t o a
feldspathicg neissesa nd granites,( 2) bandedh igh- Kibaran event, and the low-pressurem etamorphism
grade metasedimentisn cludingq uartzites,m arbles, to a thermal effect related to granite emplacement
andm etapelitesa, nd (3) low-gradem etavolcanicasn d duringt he Pan-African,t he low-grader etrogression
graywackes. The latter association,s till undated, is beingt her esulto f latew renchf aultsa nd/ora ssociated
known as Pharusian and forms linear belts or small uprightn orth-southfo lding.F rom the point of view
basinsp resumedto be upperP roterozoicin age.I n all of the plutonic evolution,n o Kibaran age has ever
three lithologic associationsth e main metamorphic been found in any granitic body. In the In Amguel
foliation is more or less gently dipping, often region,V itel [1979] hasp roposeda classificationo f
horizontal, and often associatedw ith polyphased the Pan-African granitoids dated by the Rb/Sr
recumbent folds [Bertrand, 1974; Latouche, 1978; isochronm ethod [Vialette and Vitel, 1979] from 670
Vitel, 1979], excepti n the vicinity of the late wrench + 20 Ma to 514 + 20 Ma.
faults and in somel inear beltso f uprightn orth-south To summarize, a polycyclic evolution has been
trendingf olds. recognizedin CentralH oggar,b ut with the exception
Bertrande t al.: Structuraen dU /Pb Geochronologoyf CentraHl oggar 957
!
0 5km
(cid:127)ø(cid:127)i (cid:127) .(cid:127) +
4 +
4 TT; +- SEL
ß,(cid:127) ^ ^ (cid:127)10 (cid:127)11
5 (cid:127) 12
o
o
o o
o o o
o o o
o oe TM
o
Fig.2 . Sketcmh apo f theT inA mzia rea1. ,I herangen eissaens dg ranulite(ws ithm arbleasn d
quartzite2s,) m; igmatitgicra nite3;, r efoliategdn eisse4s, g; ranite5s,; r ecenvto lcanism6, f;a ult;
7, thrust8; , dyke;9 , foliationd ip< 40ø ,4 00-6ø0, > 60ø;1 0,s tretchinlign eation1;1 ,s ample;
12, Transaharan road.
of the granitoide mplacemenatn d associatedlo w twoc ompleteldyi fferentte ctoniuc nitss eparatebdy a
pressurme etamorphisomf e videntlPy an-Africaang e, mylonite belt dipping gently toward the NE
mosto f the tectonica ndm etamorphice volutionw as (Figure2 ). The tectonicu nitsa rea sf ollows.
considereda so lder,b elonginge ithert o an Eburnean Lower tectonic unit: the Iherane gneissesa nd
or a hypotheticalK ibaran event. The new ob- migmatites. The lower tectonicu nit comprises
servationsa nd data presentedi n the following granulitfea ciesm etasedimenwthsi cha rei nvadedb y
sectionsw ere initiateda ftert he discoveryo f flat-lying migmatitesa nd nebuliticg ranitesw ith numerous
retrogressivmey lonitecso ntrastinwgi th zonesw here xenolithsa ndp atcheso f garnet+ cordieriter estite.
olders tructureasr ep reserved[B oulliera ndB ertrand, Two successivme etamorphiacs semblagehsa veb een
1981]. definedb y Ouzegane[ 1981] andc orrespondto the
followingP T conditionsM: 1 = moret han8 kbara nd
THRUST TECTONICS IN CENTRAL HOGGAR 800øC;M 2 = 5 + 1.5 kbar and 600ø + 50øC. M2
associatedm igmatitesa nd migmatiticg ranitesa re
North Tin Amzi Area relatedt o or slighlyp ostdatela rge-scalere cumbent
isoclinalf olds,t he axeso f whicht renda pproximately
Situated 50 km southo f Tamanrasset( Figure 1), east-westt;h ey are in turnr efoldedb y NE trending
thisa reai s characterizebdy a strongc ontrasbt etween upright open folds. No mineral or stretching
958 Bertrande t al.: Structurea ndU /Pb Geochronologoyf CentralH oggar
lineationsh aveb eeno bservedin theI heraneg neisses Them yloniteb eltc ouldb ei nterpreteidn twow ays:
wheret he texturesa re alwaysg ranoblastiacn dt he Se (1) it is a normal fault (NE dip, NNE directiono f
foliation, presumedE burneani n age, is actually a movement of the upper unit), or (2) it is a thrust,
metamorphic banding. Together with a slight originallys ubhorizontaolr SW dipping,t he dip of
deformationo f quartzg rains( undulatorye xtinction, which has beeni nvertedb y later large wavelength
subgrains),a third, retrogressive assemblage is doming;t he vergenceo f the thrusti s thent owardt he
observedin somes amplese, speciallyth osef rom near NNE.
the myloniteb elt: cordieritei s replacedb y muscovite This latter interpretation is preferred for the
andb y smallf lakeso f paler ed biotite. followingm asonsa: southerlyd ippingm yloniticb elt,
The Iheraneg neissesa nd migmatitesa re crosscut structuralleyq uivalentto theN orthT in Amzi mylonite
by the Tin Amzi plutonicc omplex,a n associationo f has been observedi n the North Laouni region by
heterogeneougsr anitesa nd granodioritesw ith some Latouche [1985] and also shows a NNE movement
dioriticx enolithsA. long the contactn, umerousd ykes direction;a nd at Pan-Africant imest he deepeslte vel
of granodiorite and porphyritic granite showing correspondst o the upper tectonic unit (biotite-
granophyrict exturesa re intrudedi nto the gneisses. sillimanite bearing Sp l). This is confirmed by the
The Tin Amzi complexw asd eformedu nderr elatively different emplacemenlte vels of the Anfeg and Tin
low-temperaturceo nditionsn eart he myloniteb elt. Amzi plutons( syntectonigcr anitev ersusc rosscutting
Upper tectonicu nit. In the uppert ectonicu nit all dykes).
the gneissesh ave an intense gently NE dipping Suchf eaturesa rea ctuallyin compatiblwe ith normal
foliationa nd showa conspicuousst retchingli neation faulting and favor the thrust interpretation.A s the
well marked by minerals such as biotite and Anfeg pluton has long been consideredt o be Pan-
sillimanite,t he latter being brokeni nto piecess ealed Africani n age( U/Pb datingb y Picciottoe t al. [1965])
by recrystallizeds illimanite( Figure3 ). This foliation, and is bracketed by the progressive Spl-Sp2
Sp1 becauseit is considereda s the olderP an-African deformation, this main refoliation event and the
foliation,t huso ccurredu nderd eepa mphibolitefa cies thrustinge venta re alsoP an-Africani n age.
conditionsa nd correspondtso a completet ectonica nd Very close to the Tin Amzi area, the Tinef area
metamorphicre workingo f ancientg neissess imilart o shows a similar structure but the refoliation is more
the Iheraneg ranuliticg neissews hicha rer ecognizable widespreada nd more retrogressive(F igure 5). The
as remnants even in this highly deformed unit. main horizontalm ylonitic foliation (S 1 from Boullier
Toward the NE, tight overturned,p ostfoliationf olds and Bertrand [1981]) affects a unit of fine-grained
with NW trendinga xesp erpendiculatro the stretching felsic gneissest, he so-calledT inef series[ Gravelie,
lineationd evelopl ocally a new axial planar cleavage 1969]. These gneissesg rade into augen ortho-
(Sp2) formed at lower metamorphic grades gneissess, howingp reservedi ntrusivec ontactsw ith
(greenschistto epidote-amphibolitefa cies) at a very the In Azarou gneissesw here granuliter emnantsa re
low angle from Spl. The Anfeg and Amsel plutons alsoo bservedA. ccordingly,t he Tinef orthogneisses
crosscutS pl but are deformed later, and the Sp2 may be broadlye quivalentto theA nfeg granite.
foliation affects their outer zone.
Mylonitic belt. A mylonitic belt sharplys eparates In AmguelA rea
the two tectonic units described above and is 50-
100 m thick. The mylonitic surface (Figure 3) is The area is known as the In Amguel mylonite
parallelt o Sp2 andr eflectse pidote-amphiboliftaec ies belt and was interpreted as a major vertical
conditions;i t is characterizedb y fine-grained re- transcurrenst hearz one in the Hoggar shield [Vitel,
crystallized quartz and biotite; some boudins of 1979]. Someo f the mylonitesa ctuallyc orrespontdo
refoliated gneisses( Spl) persisti n an anastomosed an early tangentiale vent and were offset duringl ate
myloniticn et. open folding and shearing[ Boullier and Bertrand,
The mylonitic foliation bearsa stretchingli neation 1981] (Figure 6); a similar interpretation was
orientated 30øN and shows shear sense criteria such previouslyp roposedb y Faur(cid:127) [ 1967].T he lithologies
as rotated porphyroclasts,s hape fabric of quartz are quite differento n both sideso f the myloniteb elt,
grains,p referredo rientationo f quartzC axes,m ica- but the structurals equencesin the two domainsa re
fishes, and C-S structures[ Simpson and Schmid, similar, as shown below.
1983; Bouchez et al., 1983], which indicate a NNE Westernd omain. The westernd omaini s mainly
senseo f movement of the upper unit. The overall composedo f granite gneissesa nd migmatites (the
parallelism between Spl, Sp2 and the mylonitic Arechchoumg neisses)A. n Sm migmatitic banding
foliation suggests a continuum between these appears as relics in a nonpenetrative flat-lying
deformations:t hey are interpreteda s the result of a secondaryf oliation Sml which is in turn folded by
progressive tangential strain occurring while opend issymetricfo lds. As thesef olds becomem ore
metamorphic conditions were vaning (Figure 4). andm ore tightenedo n approachingth e myloniteb elt,
Finally, the thrusti tself was folded by NS to NE-SW a third foliation Sm2 appears. The constant
trendingo pen folds (Fp3), which do not developa dissymetryo f the 350øNt rendingF m2 folds and the
new cleavagein thata rea. fanningd isplayedb y Sm2 suggestth at the mylonite
Bertrande t al.' Structurea ndU /Pb Geochronologyo f Central Hoggar 959
(cid:127);if'A(cid:127).(cid:127) '-(cid:127) :.*(cid:127). ½ ...-,(cid:127)-..-:..,... (cid:127)... .:.(cid:127).(cid:127)
.:.(cid:127)%.'. (cid:127) (cid:127)-.'.(cid:127) ..... /(cid:127):. %-.:---:.i,(cid:127)
'?. .....: :::.-::. ;½. '-
....... .
. '(cid:127)3 ' '(cid:127)-.-.?'. .: ::(cid:127)g:..
::-.ß.- .....(cid:127)::(cid:127).:
:: ::.::.-:-(cid:127)
(cid:127):}x(cid:127) ':(cid:127):(cid:127):-:::':::.:::(cid:127):....,
....
(cid:127)-- ..: :-:- :- (cid:127) (cid:127) '.:-:.. ':%,.' . ..:',..i.:.:..(cid:127),-.- (cid:127)½:..(cid:127) ..'.:. .....;. (cid:127).(cid:127)-(cid:127).. ' ...(cid:127)'., ....o -.(cid:127)'*(cid:127) (cid:127)- ..',:'.:(cid:127);(cid:127):.(cid:127) (cid:127).:..(cid:127),:-':
.. :L...;..: '.x. (cid:127)... -. ... -..-...(cid:127)::.'-&(cid:127)&.'.(cid:127)....-........ -..(cid:127) (cid:127)-(cid:127)-(cid:127)(cid:127)(cid:127)'(cid:127)(cid:127)(cid:127)(cid:127):..:(cid:127):--:(cid:127)..(cid:127).:(cid:127),
.. .::. (cid:127)'.:,.' .,.:.ß.:.'.'(cid:127)- : ::...-.:(cid:127),:...;.'.. (cid:127) . . -'*ß'., .%(cid:127),'.r(cid:127)(cid:127) ;F:'.:.- .(cid:127) ..?....2.:.,*.'*: .-.....,.-.,. '.:,..(cid:127).*o......(cid:127)..-.. . ......:..... . .... . . .-.(cid:127).'.. .*.(cid:127) . :. s.,.. ,..,..s,... ...:...*,...-:...(cid:127),. ..,, .,:. ...,..- ..,..;'....(cid:127) ' ....../...*.. . ...... .....
. '*-. . ... :. ,. ...(cid:127)*. :;.(cid:127)-:'(cid:127) ....... .. .... . -. (cid:127),:...
.(cid:127):. .:,(cid:127)?q:.(cid:127)'.(cid:127).'..,(cid:127) (cid:127)..(cid:127).(cid:127).(cid:127) -..(cid:127):,,,. ß
(cid:127)?.(cid:127),.,..
(cid:127)...
Fig. 3. Somea spectso f the Tin Amzi thrust.( a) Bandedm ylonite;d arkl ayersa re ultramylonites.
(b) Ultramylonitei n XZ sections howingt he movemendt irection;c lastsa re feldsparsg, arnet,a nd
sillimanite.( c) Refoliatedg neisso f the uppert ectonicu nit showingf racturesin a sillimanitec last
sealedb y new transverses illimanite.
96O Bertrande t al.: Structuraen dU /PbG eochronologoyf C entraHl oggar
SSW NNE
( ('(cid:127) (cid:127) -(cid:127) (cid:127)___...(cid:127)/......(cid:127)(cid:127) ANFEGGR ANITE
::::::::::::::::::::::::::::::::::::::::::::::::
Fig.4 . Thet ectoniecv olutiono f theT in Amzi areaS. ei s theE burneafno liationin theg ranulites;
Spl is them ainr efoliationin theg neissesa;n dS p2i s the superimposelodw er-gradfeo liation
parallelt o the mylonites.
belt actually represents the core of a large Assuming the identity of Sml and Snl, the In
anticlinoriumd isruptedb y a late sinistrals trike-slip Amguel mylonite belt appearsa s a large anti-
fault correspondintgo retrogressivger eenschisfat cies clinoriumt, he coreo f whichi s invadedb y post-Sml
conditions.E ven when Sml is still recognizable, a pink granites and characterizedb y high strain
conspicuousL m2 horizontal stretchingl inearion is correspondingt o a later shear zone. The Snl and
developedo n verticalS m2f oliation. Sml surfaceesx hibitm anys imilaritiesw ith the Spl
Eastern domain. In the Eastern domain the surfaced efinedi n the Tin Amzi areae xceptf or the
Amsinassene Formation [Vitel, 1979] consists of less marked stretching linearion in the former.
metapelitic gneisses,m arbles, and some quartzites Furthermorei,n theT in Teganeta rea,8 0 km southo f
with minor plagioclasic gneisses.S ome granulitic the Amsinassenree gion,a n antiformals tructurew as
remnantsh ave also been describedb y Vitel [1979]. recognizedw hoseu pperl ayersc onsisto f refoliated
An Sn migmatiticb andingi s refoldeda nd transposed high-gradeg neissesa ndw hosec orei s composedo f
in most cases into a Snl subhorizontal foliation, a greenschisfat ciesp hyllonitews ith a 230øNs tretching
transpositiown hicho ccurredu ndera mphibolitefa cies lineation. Thus a thrust zone similar to the Tin Amzi
conditionsT. his formationi s intrudedp arallelt o the thrusmt aye xisti n theI n Amguela rea,h iddenb y the
Snl foliation by a heterogeneouso, ften migmatitic, widespreaodc currencoef post-Smli ntrusiveasn db y
monazite-richp orphyritic granite (Aou Zebaouene thel ate high-strains trike-slips hearz one.
granite [Vitel, 1979]). Fold axes and mineral linea-
tions on the Snl foliation are erratic (from 30øN to OtherT hrustsin CentraHl oggar
150øN)p robablya s a resulto f an Fn2 openf olding,
which becomes more and more tighter as the Othert hrustss eeni n the CentralH oggarr egion,
mylonite belt is approached. are described below.
W 4'50 fault
'(cid:127)1[' Tinef orthogneiss (cid:127) I
Pharusianl In Azarou gneiss molasse
2km
I i
Fig. 5. Crosss ectiono f the Tinef (cid:127)ea (Tinef well 22ø20'N,f rom Gravelle [ 1969] and Boullier
andB er(cid:127)and [1981]).A majort hrusts ep(cid:127)ates twor efoliatedte ctonicu nitsi n thisr egion.T he
upperu nit, with numeroums (cid:127)bles, is yew simile, exceptf or ther etromo(cid:127)hic foliation,t o the
Iheraneg ranulites.
Bertrande t al.: Structurea ndU /Pb Geochronologyo f CentralH oggar 961
WSW ENE
I
Sm+Sml Sm2 , Sn+Snl
I (cid:127) (cid:127)C'(cid:127) -
Suggaragnn eiss Branire AmsinasseSnee ries AouZ ebaouenBer anire
(cid:127)km
Fig. 6. Cross sectiono f the In Amguel area (O.Tekchouli 23ø30'N, from Vitel [1979] and
Boullier and Bertrand [ 1981]). The late brittle fault is situatedi n the eatem domain, to the easto f
the main mylonite belt. The post-Sml,p re-Sm2K -granitei s emplaceda t the suspectepdo sition
of the main thrustr ecognizedfa rthers outh.
Aleksod. In the Aleksod mountains [Bertrand, stratigraphicu nconformity correspondinge ither to
1974] two typeso f thrusth ave beend escribed. the bottom of the middle or upper Proterozoic, but
1. The oldest thrustsa re parallel to the main S1 they interpretedo ne of Blaise's unconformitiesa s a
foliation and correspondt o mappedd isconformities thrust on the basis of structural and metamorphic
accompaniedb y a straini ncreaseb ut without a me - breaks.
tamorphicb reak.T he Ouadenki unconformity,p re - Tiririne. The Tiririne thrust forms the eastern
viously interpreteda s having a partly stratigraphic boundaryo f CentralH oggar.T he high-gradeIs salane
origin, correspondsto one of thesee arly thrusts;i t Formation is thrust over the late Proterozoic molassic
marks the contact between the Aleksod Formation and arenaceous sediments of the Tiririne Formation
where S1 is the main and seems the first, foliation, [Bertrande t al., 1978]. The thrustg radest oward the
and the Arechchoum gneisses where S1 is su- south into the 8030' wrench fault but a thrusting
perimposedu pon Ebumean structures[ Bertrand and componenpt robablyr eappearsa longt he easterne dge
Lasserre, 1976]. Contrasting with the Tin Amzi of the Air mountainsin Niger [Black et al., 1967].
thrust, the Aleksod ones are in metamorphic Other areas. Other large-scale thrusts were not
equilibrium and are refolded by isoclinal, postfo- surveyedi n detail. Their interpretationa s thrustsi s
liation recumbent folds accompanied by the re- basedu ponr ecentr econnaissanccer osss ectionsF. or
crystallizationo f the samem etamorphica ssemblage example,in the Egere region [Duplan, 1972], early
(Bi, Mu, Ky, Gt) (Figure 7). Very often the thrusts thrustsa re outlined by eclogite lenses,a nd from a
appear as wide belts of high strain submylonitic crude restoration,a 100-km shorteninge stimate is
gneissesa nd phyllonites.O ne of theseb eltsc ontains proposed by Latouche [1985]. In the Arefsa and
tectonicl enseso f eclogites[ Sautter,1 985]. Afedafeda regions,t he basall ayerso f the so-called
2. The youngert hrusts,t he most obviouso n the Pharusianb asinsm ust be reinterpreteda s mylonites
map, were formed, unlike the older thrusts, under andt he classicasl tratigraphicu nconformityis highly
retrogressivceo nditionsa ndc rosscuat ll the folds. As dubiousi n mostp laces[ Latouche,1 985; M.Briedj et
they merge gradually with the vertical srike-slip al., unpublishedr esults, 1980]. In the Timgaouine
faults, they were interpreteda s a consequencoe f the area [Lapique et al., 1986] high-grade granite
late wrench-fault tectonics[ Bertrand, 1974]. gneissesa nd metasedimentsa re in complext ectonic
Oumelalen. In the Oumelalen region, Latouche contact (wrench fault grading to thrust) with the
[ 1979] describeds everalt hrust sheetsp redating the overlying Pharusianl ow-grade metavolcanics.T he
late F3 upright folding. The thrustsa re related to a age of this tectonic event is bracketed between
SW trending recumbent folding, which occurred 629 Ma and 614 Ma (zircon and-spheneU /Pb dating
undera mphibolitef aciesc onditionsT. hey postdatea of a pretectonicg ranodiorite[B ertrande t al., 1986]).
granulite facies assemblage dated at 2000 Ma Figure 8 summarizest he thrustsd iscussedin this
[Latouche and Vidal, 1974]. Towards the south in the section with their sense of movement as determined
Temasint area, some of the unconformitiesd escribed by microstructuraclr iteria.
by Gugrangg [1966] may actually be thrusts (B.
Gugrangg,p ersonal communication, 1984; J.M.L. An InterpretativeC rossS ectiono f CentralH oggar
Bertrand,u npublishedfi eld observations1, 975).
Tazat. In the Tazat mountains, Blaise [1967] The crosss ectionp resentedh ere( Figure9 ) is very
describeda large synformd eforminga basementa nd different from previouslyp ublisheds ections[ Ber-
a sedimentaryc over characterized by numerous tranda nd Caby, 1978]. Vertical structuresh ave been
intraformational unconformities. Bertrand et al. attenuatedo n purposej,u st keepingt he envelopeso f
[1968] confirmed the existence of one major the large synclinoriaa nd anticlinoria.A lmost all the
962 Bertranedt al.:S tructuraen dU /PbG eochronoloogf Cy entraHl oggar
.'-'OUADEN-iK ß
SERKOUT
OULET
,.
i IO Km i
23ø20 F1 (cid:127) 9
EARLY THRUST (cid:127) 7
F2 (cid:127) 10
LATE THRUST (cid:127) 8
F3 ,....(cid:127)-.,-(cid:127) 1 1
Fig.7 . Structurmala po f theA leksoadr ea[f romB ertran1d9, 74]1. , Arechchogunme isses,
lowePr roterozo2i,cA ;l eksosde riems,i ddlPer oteroz(o?i)c;3 ,g ranit4e,;r ecenptl ateabua sal5t,;
recenttr achytoer p honolit6e,; m ylonite7;, earlyth rust8;, later etogresstihveru s9t;, map
projectioofnF 1a xiapl lane1;0 ,t rajectoarnyd d ipo fL 2l ineatiowni thc orrespenddiinpg;1 1,F 3
antiforman ds ynform(a rrow)1;2 ,e arlyh igh-gratdher us1t;3 ,l ater etrogresstihvreu st.
observationcso nvergteo definea N to NE movement appearst o have been overestimatedin previous
direction,e xcepti n the Tinef and Egere areas papers[B ertranda ndC aby,1 978;C abye t al., 1981].
[Duplan1, 972;L atouche1,9 85],a ndi n manyc ases, In the caseo f the largestf ault in the Hoggar,t he
thrustms aye volveg radualltyo wrenchfa ults( e.g., 4ø50'Ef ault,i t wass hown[ Boulliera ndB ertrand,
T'nidnea ndT imgaouine). 1981]t hatm osto f them ylonitebse longto ane arlier,
The importancoef theN S trendingw renchfa ults tangentiald eformation,s imilar to thosed escribedi n
Bertrande t al.: Structuraen dU /Pb Geochronologoyf C entraHl oggar 963
the Tin Amzi area. The deformation related to the fault
itself and, associated ,with an estimated dextral
movement of about 100 km, is restricted to a narrow
\ CENTRAL zone of retrogressionin ductile conditions,f ollowed
WES T %WH. OGaqR by brittle behavior correspondingto a Phanerozoic
uplift of the easternb lock.
Instead of permanent, long-lived, deep vertical
crustal breaks, the NS trending faults are thus
interpreteda s the resultso f the brittleb ehavioro f the
(cid:127).l .
upperc rust,a ssociatedp erhapsw ith an indentation-
relatedl aterale jectiono f blocks,s imilart o that seeni n
Asia during the collision process [Molnar and
Tapponier, 1975; Peltzer et al., 1982]. The succes-
sion of severalt hrustinge ventsw hich are more and
more retrogressive with time suggests such a
continuume ndingi n brittlec onditionsw ith dominant
lateralm ovement.T his interpretationis in agreement
with the flattening at depth of vertical structures
shown by the results of deep seismic profiles
(MOIST, COCORP, etc.) acrossc omparableo ro-
fi./ genicb elts( Appalachiana ndC aledoniano rogens).
U/Pb GEOCHRONOLOGY
....'.'....;:.
With the structural study having shown, for
granitoide mplacementt,h e difficultieso f precisinga
regional chronology compared to the tectono-
metamorphice volution of high-gradeg neisses,t he
first problemt o be solvedb y radiometricd atingw as
to find out what stage, if any, of the structural
sequencceo uldr epresenat time break.F or thiss tudy,
1 2 3 4 5 6 7 both highly foliated granites parallel to the
surrounding gneisses (Anfeg and Tinef) and
Fig. 8. Sketchm ap of the Transaharanb elt showing crosscuttingu ndeformedg ranitoids( Tin Amzi) were
the known movement directions; question marks choosenA. further goal was to define the age of the
outline the structurallyu nknownd omain. 1, Western initiation of the refoliation process;t he syntectonic
Pharusian belt; 2, In Ouzzal and Iforas Ebumean Aou Zebaouenem igmatiticg ranitew as choosenf or
granulites; 3, Eastern Pharusianb elt showing the this purpose.
known and presumed extent of the pre-840 Ma
Pharusian I assemblage; 4, Central Hoggar Analytical Methods
polycyclic gneisses,g ranites, and schistbelts;
5, Eastern Hoggar; 6, direction and sense of Zircon fractions were separatedu sing a com-
movement of early Pan-African nappe tectonics; binationo f superpanerh, eavy liquids and magnetic
7, direction and sense of movement of the late Pan- separationH. eavy mineralc oncentratews erep urified
Africanf orelandn appes. by handpicking under a microscope to obtain
sw NE
4ø50 FAULT
TINEF TIN AMZI AREFSA TIN TARABINE SERKOUT TEMASINT ISSALANE TIRIRINE
©(cid:127)(i) (3 ,,(i) (i),© (i)',©
-'(cid:127)? + ++ 10K0M I -I,4
I I
Fig. 9. Interpretedc rosss ectionf rom Tinef to Tiririne. The height scalei s exaggerated.
Granitoidasr eu ndifferentiatferdo mp retectontico syntectonaicn dp osttectonic.
96(cid:127)4 Bertrande t al.: Structurea ndU /Pb Geochronologoy f CentralH oggar
homogeneousfr actionsi n size, color, morphology, from the contact,w here gneissicx enoliths are dis-
and magnetismN. onmagneticf ractionsw ere favored persed in the granite (1009). Ten zircon fractions
for analysisa, carefuls tudyu ndert he microscopaen d were analyzed (Figure 10). The P59 fractions are
a SEM study having shown that in most casest he groupedc lose to the Concordiac urve, four of them
magnetic fractions are rich in small inclusions of being almost concordant between 580 Ma and
magnetic minerals. The external and internal mor- 600 Ma. The 1009 fractions and three fractions of the
phology of the zirconsw as studiedu sing a Cam- P59 define a Discordia with a 615 + 5 Ma upper
bridgeS EM. intercept and a 27 + 25 Ma lower intercept. We
Analyses were carried out using a variant of the interpret the 615 + 5 Ma age as the time of
technique described by Mahnes et al. [1978]. A emplacement,a nd the concordantP 59 resultsa s the
mixed2 øSpb/235sUpi kew asa ddedto thes mallt eflon effecto f metamorphismon moreo r lessa bundantti ny
bombs before the HF attack. Pb and U were Ce, Th- and U-rich inclusions in the outer zones of
separatedo n anionic resin columns.P b was loaded the zircons, which are undetected by the SEM
with H3PO4 ands ilicag el on a singleR e filament.U [Kosztolanyi et al., 1986]. This interpretation is
suggestedb y the identity of the youngesto f these
wasr un asU 20 3 on a singleW filamentw ith Ta and
agesw ith the concordanat geg iven by the T9 sphene.
H3PO4 . Isotopicc ompositionw asd eterminedu singa
AouZ ebaouenge ranite. One sampleo f porphyritic
CAMECA masss pectrometerT. otal lead processing
adamellite was studied (T57). Three zircon fractions
blanksa re smallert han 1 ng. Calculationo f agesa nd
and one monazite from T57 were analyzed. The
errors was achieved with the method of Minster et al.
experimental points of the zircon fractions are
[1979].
grouped despite their size difference, and their
2ø7*pb/23a5pUp arenatg esa res ligthlyo ldert han6 00
Results Ma (Figure 10). Includingt he concordanmt onazitei n
the agec alculationa, lower intercepot f 592 + 6 Ma is
The analyticadl ataa reg iveni n Table 1. obtainedw ith a correspondingu pper intercepto f
Iherane gneissesa nd migmatites. Two samples 2500 + 400 Ma. Although no clear inherited cores
(T11, T16) were studied( see Figure 2 for location) have been seeni n thesez ircons,a n old component
from a restitic metasediment and a nearly homo (>2000 Ma) seemst o be present, as is the case in
geneousg ranite,r espectively.T he analyticalr esults, Pan-Africang ranitesfr om theI forasr egion[ Lancelot
obtainedo n six zirconf ractions,a re groupedf or each et al., 1983].
sample.T hey are moderatelyd iscordant,e specially
the granitic sample, and intercept the Concordia at Discussiono f the Results
2075 + 20 Ma and 530 + 70 Ma (Figure 10).
Tinef orthogneisses. Two samplesw ith similar Pan-African event. The structural data show the
texture and composition were studied. T22 was close links betweent he main tectonometamorphic
sampledi n the type area, closet o the Tinef well and evolutiono f the CentralH oggara ndt he emplacement
IC1023 (courtesyR .Caby) is from Abalessaa bout of the largeg raniticp lutonss ucha st he Anfegp luton.
100 km to the north. Three zircon fractions from the The 615 Ma ageo btainedfo r the emplacemenotf this
T22 samplew ere analyzed.T hey are groupedc loset o pluton confirms previous data by Picciotto et al.
the Concordiac urvei n the vicinity of 580 Ma and do [ 1965]:6 15 and6 19 Ma 2ø7pb/2ø6mpbo dela gesa s
not define a good intercept (Figure 10). The three recalculatedb y Cahene t al. [ 1984].I n the light of this
complementarfyr actionsfr om the sampleI C1023 are age, the significanceo f the gneissic assemblages
almostc oncordanat ndd efinea preciseu pperi ntercept which dominate the domain must be reconsidered.
at 604 + 10/-6 Ma. The age calculatedw ith the T22 Instead of an old gneissic block of Eburnean (or
pointsi s not significantlyd ifferentf rom this, but the older) age, affectedt o a limited extent by the Pan-
errori s largerd ue to the scattero f the points. African event [Bertranda nd Caby, 1978; Latouche,
Tin Amzi complex. Two samplesw ere studied.T 9 1978], it is now suggestedth at an almostc omplete
is a granodioritef orming a stockw ithin the domi- tectonometamorphrice workingo ccurred;t hisr ewor-
nantly granitic complex and situated close to the king is, however,n onpenetrativoen the crustals cale,
deformed zone of the NE contact. T18 is also a and domainso f all sizes( tectonicl enses)w here older
granodioriteb ut was sampledf rom a 10 m thick dyke structureasr es till recognizablhe avep artlye scapedit s
which sharply crosscutst he surroundingg neisses. effects.
The T18 results are not well aligned and the age In order to set up the chronology of this Pan-
cannot be precisely defined (612 +50/-20 Ma). African reworking, the following must be pointed
Furthermore, the T9 fraction does not lie on the out:
referenceli ne. The T9 spheneis concordanat ndg ives 1. The Tin Amzi and Anfeg plutons,a ndt he Tinef
an ageo f 578 + 6 Ma (Figure 10). orthogneissesp, redatet he last deformation,i .e., the
Anfeg granite. Two samplesw ere studied, one last retromorphic thrusting and the upright NNE
from the deformedm argin of the pluton (P59), and trendingf oldingi n the area.
the other from a heterogeneousa rea situated 1 km 2. The time span betweent he main refoliation