NEOPROTEROZOIC OPHIOLITES IN THE ARABIAN SHIELD: FIELD RELATIONS

34 Pages·2004·2.71 MB·English

PrecambrianOphiolitesandRelatedRocks
EditedbyTimothyM.Kusky
DevelopmentsinPrecambrianGeology,Vol.13(K.C.Condie,SeriesEditor) 129
©2004ElsevierB.V. Allrightsreserved.
Chapter4
NEOPROTEROZOICOPHIOLITESINTHE ARABIAN SHIELD:
FIELD RELATIONSANDSTRUCTURE
PETER R. JOHNSONa, FAYEKH. KATTANa AND
AHMEDM. AL-SALEHb
aSaudiGeologicalSurvey,POBox54141,Jiddah21514,SaudiArabia
bGeologyDepartment,KingSaudUniversity,POBox2009,Riyadh,
SaudiArabia
OphiolitesmakeupasmallbuttectonicallyimportantpartoftheArabianshield.Where
mostcomplete,theyconsistofserpentinizedperidotite,gabbro,dikecomplex,basalt,and
pelagic rocks. However, because of folding and shearing, the majority of the ophiolites
lackoneormoreofthesediagnosticlithologies.Nonetheless,theincompleteassemblages
areidentifiedasophiolitesbecausetheyminimallyincludeperidotiteandgabbro,inmany
casesareassociatedwithbasalt,andinallcasesshowevidenceofemplacementbythrust-
ing and shearing rather than intrusion. The ophiolites range in age from ∼870 Ma to
∼695 Ma, documenting a 200-million year period of oceanic magmatism in the Ara-
bian shield, and are caught up in ∼ 780 Ma to ∼ 680 Ma suture zones that reflect a
100-millionyearperiodofterraneconvergence.Alltheophiolitesarestronglydeformed,
metamorphosed,and altered by silicification and carbonatization.Low-gradegreenschist
faciesmetamorphismpredominates,butin placesthe rocksreach amphibolitegrade.Al-
terationresultedinthedevelopmentoflistwaenite,particularlyinshearzones,andlocally
theonlyevidencethatmafic-ultramaficrocksunderlieagivenareaisthepresenceofup-
standingridgesoflistwaenitethatareresistanttoerosion.S/Cfabricsarewidespreadand
indicate that the ophiolites were affected by both strike-slip and vertical displacements.
Variations in senses of shear observedalong and across the strike evidence considerable
strain partitioning during deformation. However, prevailing senses of shear can be dis-
cernedforseveraloftheophiolitesthat,inconjunctionwithotherstructuralobservations,
indicatethemainsheartrajectoriesoftheshearzonescontainingtheophiolites.JabalEss,
Jabal Tharwah, and Bi’r Umq ophiolites were emplaced during periods of dextral trans-
pressionontheYanbuandBi’rUmqsutures,respectively.TheBi’rTuluhahophiolitewas
emplaced during sinistral transpression of the Hulayfah-Ad Dafinah-Ruwah suture, and
theHalabanophiolitewasemplacedduringwest-directedconvergenceontheHalabansu-
ture.
DOI:10.1016/S0166-2635(04)13004-1
130 Chapter4:NeoproterozoicOphiolitesintheArabianShield:FieldRelationsandStructure
1. INTRODUCTION
Neoproterozoicmafic-ultramaficcomplexesmake up less than 1%of the surfacearea
of the Arabian shield but, starting with the pioneering work of Al-Shanti and Mitchell
(1976) and Bakor et al. (1976), they figure prominently in discussions of the origins of
the shield because of their possible tectonic significance as remnants of oceanic crust
and indicators of arc-arc suturing (Pallister et al., 1987; Stoeser and Camp, 1985). Cer-
tainly not all mafic-ultramafic complexes in the region are ophiolites—some are non-
diagnosticlensesofshearedserpentinite,someareintrusionsinthebaseofvolcanicarcs,
andsomearelayeredintrusions—andcaremustbetakentoavoidmisidentification,mis-
interpretation, and spurious correlations (Church, 1988, 1991). Nevertheless, a signifi-
cant number of complexeshave the hallmarks of ophiolites. They are widespread in the
shield along shear zones (Fig. 1) and, together with stratigraphic, geochronologic, and
structuraldata,provideevidenceofactiveocean-floormagmatisminassociationwithde-
velopment of the tectonostratigraphic terranes in the shield and the process of suturing
during terrane amalgamation (Pallister et al., 1988; Johnson and Woldehaimanot, 2003;
Gennaetal.,2002).
The mineralogy, chemistry, and tectonic settings of Arabian and Nubian shield ophi-
olites are reviewed by R.J. Stern and colleagues in a companion chapter in this volume.
Thepurposeofthisreportistodescribethelithology,structure,andfieldrelationsofse-
lectedArabianshieldophiolites,therebyprovidingexamplesofNeoproterozoicophiolites
and illustrating the outcropcharacteristics and degreesof dismembermentand structural
complexitythatmaybeexpectedofNeoproterozoicophioliteselsewhere.Oftheophiolites
selected,JabalEssliesontheYanbusutureatthejoinbetweentheMidyanandHijazter-
ranesinnorthwesternSaudiArabia(Fig.1).TheJabalTharwahandBi’rUmqophioliteslie
ontheBi’rUmqsuturebetweentheHijazandJiddahterranes.TheBi’rTuluhahophiolite
isatthenorthernendoftheHulayfah-AdDafinah-RuwahsuturejoiningtheHijaz-Jiddah-
AsirterranesandtheAfifterrane.TheHalabanandJabalalUwayjahophiolitesarepartsof
theHalabansuturebetweentheAfifandAdDawadimiterranes,andtheJabalTaysophi-
olite is within the Ad Dawadimiterraneeast of the Halabansuture. The Jabal Ess, Jabal
Tharwah,Bi’rUmq,Bi’rTuluhah,Halaban,andJabalalUwayjahophiolitesarebelieved
toberootedintheshearzoneswithwhichtheyareassociatedand,assuch,markthesites
ofconsumptionofoceaniccrust.TheJabalTaysophiolite,incontrast,appearstobepart
ofastructurallydetachedophioliteallochthonfartraveledfromitsrootzone.
2. JABALESSOPHIOLITE
TheJabalEssophiolite(Figs. 2,3)comprisesmantleperidotite,isotropicandlayered
gabbro,adikecomplex,pillowbasalt,andpelagicsedimentsmetamorphosedinthegreen-
schistfaciesandisthemostcompleteophioliteintheArabianshield(Al-Shanti,1982).It
coversanareaofapproximately30kmeast-west,and5kmnorth-south,andhasa mini-
mumestimated thicknessof about3 km. The ophiolite cropsoutin hills thatrise 300 m
2. JabalEssOphiolite 131
Fig.1. Mafic-ultramaficcomplexes andmajor faultsandshear zones intheArabianshield, Saudi
Arabiawithaninsetshowingterranesandinterterraneophiolite-decoratedsuturesandshearzones.
Complexesthatsatisfycriteriaasophiolites,arenamed;othermafic-ultramaficcomplexes(mostly
serpentinite lenses along fault zones) are shown by initials. Complexes: A = Al Amar; D = Ad
Dafinah;H=Hamdah;HA=Hakran;I=Ibran;M=Muklar;N=Nabitah;R=Rahah;RD=
ArRidaniyah;TA=Tabalah-Tarj;TW=Tawilah;UF=UmmFawrah.Terranes:Af=Afif;As=
Asir;D=AdDawadimi;H=Hijaz;Hi=Hail;J=Jiddah;M=Midyan;R=ArRayn.Boxes
outlineareasofFigs.2,9,and11.
abovethesurroundingvalleybottomsandiswelldissectedbyeast-andnortheast-flowing
drainages,whichprovidegoodexposuresoftheunderlyinggeology.Totheeast,thenorth-
ern and southern boundaryfaults of the ophiolite converge,and the ophiolite tapers and
ceasestoberecognizable.Tothewest,theophioliteiscutbythenorthwest-trendingsinis-
tral fault system of the Da’bah and Durr shear zones. Mafic-ultramafic rocks continue
to the south as the Sahluj mélange (named here after Jabal Sahluj) and the Jabal Wask
ophiolite(Fig. 2).Togetherwiththe JabalEssophiolite,these mafic-ultramaficrocksare
132 Chapter4:NeoproterozoicOphiolitesintheArabianShield:FieldRelationsandStructure
Fig. 2. Simplified geologic map and geochronologic data for the Jabal Ess-Jabal Wask ophiolite
zone,whichmarkstheYanbusutureinthenorthwesternArabianshield.MappingafterKemp(1981)
andHadley(1987).GeochronologicdataafterKempetal.(1980),LedruandAugé(1984),Claesson
etal.(1984),andPallisteretal.(1988).BoxshowsareaofFig.3.
parts of the zone of deformed rocks that constitute the Yanbu suture in Saudi Arabia,
and its extension in Northeast Africa, the Allaqi-Sol Hamid suture (Kröner et al., 1987;
AbdelsalamandStern,1995;JohnsonandWoldehaimanot,2003).(FordetailsontheJabal
2. JabalEssOphiolite 133
Fig. 3. Simplified map and cross section of the Jabal Ess ophiolite (after Al-Shanti, 1982;
Chevrèmont andJohan, 1982b; andthisreport).Geochronologic datafromClaessonetal.(1984);
Pallisteretal.(1988).
Wask ophiolite, see Bakor et al., 1976;Chevrèmontand Johan, 1982a;Ledru and Augé,
1984.)
Peridotite is mainly exposed on the southern slope of Jabal Ess in the central part
of the ophiolite shown in Fig. 3. It is strongly altered and is chiefly black, mas-
sive serpentinite in which original textures are rarely preserved although sufficient pri-
mary features remain to locally indicate the presence of harzburgite, subordinate tec-
134 Chapter4:NeoproterozoicOphiolitesintheArabianShield:FieldRelationsandStructure
tonizeddunite,andcumulatewehrlite,orthopyroxenite,andserpentinite(Al-Shanti,1982;
Chevrèmont and Johan, 1982b). The harzburgite and dunite are concentrated in a zone
ofmantleperidotiteasmuchas500mwide inoutcrop.Theharzburgitecontains5–20%
bastitepseudomorphsaftereuhedralorthopyroxeneinaserpentinizedolivinegroundmass
(Shanti and Roobol, 1979). Dunite contains bastatized pyroxene ghosts and local dis-
seminated chromite and podiform chromite lenses 20 cm across. Enstatite banding and
trains of ovoid, stretched chromian spinel define a metamorphic foliation and lineation,
which are suggestive of high-temperature subsolidus deformation possibly as a result
of plastic mantle flow beneath a spreading ridge (Pallister et al., 1988). Olivine in the
peridotite is magnesian rich with fosterite in the range Fo –Fo , orthopyroxene is
91 92.7
close to enstatite, and clinopyroxeneis mainly diopside (Chevrèmontand Johan, 1982b;
Shanti,1983).Chromiferousspinelissimilartothatfrommodernforearcperidotites(Stern
etal.,2004)andispresentasanhedralgrainsenclosingolivineandlesscommonlyaseuhe-
dralgrainsinterstitialtothecumulateolivineintheduniteorenvelopedinorthopyroxene
inharzburgite(ChevrèmontandJohan,1982b).Thecumulaterocks,inferredbyAl-Shanti
(1982)tobeaunitabout400mwideoverlyingtheharzburgite,consistsofserpentinized
wehrlite and orthopyroxenitein layers a meter or so thick intercalated with serpentinite
severalmetersthick.
Gabbro is predominantly a dark, relatively featureless, massive rock, but locally has
well-developedigneouslaminationandrhythmicalternationsofmelanocraticgabbroand
leucocratic anorthosite in layers as much as 20 cm thick (Fig. 4C). The gabbro is meta-
morphosedand,wherestronglydeformed,ismylonitizedandbrecciated,particularlyina
narrow zone southeast of Jabal Ess where gabbro is exposed between peridotite and the
dikecomplexandistectonicallyintercalatedwithperidotite(ShantiandRoobol,1979).
Thesheeteddikecomplexisa unitasmuchas600mwidecomposedofmetadolerite
dikes30cmto2mwide.Itscontactswithgabbro,below,andpillowbasalt,above,aretran-
sitional.Outcropfeaturesofthecomplexarecommonlyobscuredbyextensivedesertvar-
nishbut,whereexposureisfavorable,thecomplexisseentoconsistentirelyofdikesthat
havefine-grainedchilledmarginsandfine-tomedium-grainedcores(ShantiandRoobol,
1979).
The basalt unit includes pillow basalt (Fig. 4D), subordinate massive basalt flows
asmuchas10mthick,andverysparsebasaltbreccia.Itisestimatedtobeupto300mthick
butbecauseoffoldingandfaultrepetitionisexposedovera widthof nearly2.5km(Al-
Shanti,1982).Thin-skinnedpillowlavacharacterizedbyamygdaloidalcoresandrimsof
chloritizedand(or)spheruliticbasaltpredominates.Khaki,locallysiliceousshaleandlam-
inatedchertcropoutasinterbeds50mthickinthepillowbasaltandasisolated,strongly
Fig. 4. Features of the Jabal Ess ophiolite. (A) View of Jabal Ess from the south showing
north-dipping shear surfacesand (along ridgeline)anorth-dipping sheet of carbonated andsilici-
fiedperidotite.Reliefabout150m.(B)South-dippingthrustsinserpentinitemélangeatthesouth-
ernmarginoftheophiolite.(C)Rhythmiclayeringinmetagabbroshowinganorthositeintercalated
withmelanocraticgabbro.(D)Pillowbasalt.(PhotosCandDafterAl-Shanti,1982.)
2. JabalEssOphiolite 135
136 Chapter4:NeoproterozoicOphiolitesintheArabianShield:FieldRelationsandStructure
sheared exposures at the northern boundary fault of the ophiolite (Shanti and Roobol,
1979),andareinterpretedaspelagicsedimentsatthetopoftheophiolitesuccession.
The complex is steeply dipping, and the exposures are effectively a cross-section
through the ophiolite. The gross distribution of rock types suggests an ophiolite succes-
sionyoungingfromsouthtonorthbutthesuccessionisdisruptedbydeformation.Igneous
◦ ◦
layeringintheperidotitedips40 –90 ,mostlytothesouth;thebasaltisfoldedintoaseries
ofanticlinesandsynclines;andthegabbroisrepeatedbyfoldingand/orthrustingnorthof
the basalt. Shear zones, characterizedby serpentiniteschist, secondarylistwaenite, mag-
◦ ◦
nesite,andmélange,areabundantanddipbetween30 and90 southandnorth.Mélange,
composedofangularto subroundedblocksof massiveserpentinite,gabbro,dolerite,and
basaltupto50mindiameterinayellowtoblackserpentiniteschistmatrix,isparticularly
conspicuousasasubverticalshearzoneasmuchas500mwideinthesouthernpartofthe
ophiolite.Anorth-dippingunitofchertandlistwaenite(Fig.4A)markstheshearzoneat
thenorthernboundaryofthe ophioliteonJabalEss. South-dippingshearzonesare com-
moninthenorthernunitofgabbroandinthesouthernpartoftheophiolite(Fig.4B),and
asubverticalshearzoneformsthesouthernmostboundaryoftheophiolite.
S/Cshearfabricsarewidespread.Adextralsenseofhorizontalshearpredominates,but
variationinthesenseofshearalongandacrosstheophioliteindicatesthattherewasalarge
degree of strain partitioning in the ophiolite during deformation.The northern boundary
fault,minorshearzonesinthenortherngabbro,andtheshearzoneatthesouthernmargin
oftheophiolitearedextral(Fig.5A).Ashearzoneabout100–200msouthofthenorthern
boundaryfaultabout1kmeastofJabalEsssummitissinistral(Fig.5B).Thesouthdipping
shearzonesin mélangeinthesouthernpartofthe ophioliteare bothdextralandsinistral
(Figs. 5C, D). Indicators of the sense of vertical movementon the shear zones have not
beenobserved,butitisconceivablethatsouth-dippingshearsthroughouttheophioliteare
north-vergentthrusts. Worldwide, suture zones commonlydisplay combinationsof hori-
zontalshearingandthrustingthatreflectdeformationduringtranspression.Thestructures
at Jabal Ess, suggestive of north-vergentthrusting and regional dextral horizontal shear,
areconsistentwithdevelopmentunderconditionsofdextraltranspression.
TheJabalEssophioliteisdirectlydatedbymeansofa 780±11 MaU-Pbzirconage
obtainedfromgabbrointheeasternpartoftheophiolite(Pallisteretal.,1988)anda782±
36 Ma Sm-Nd mineral model age obtained from the same gabbro sample (Claesson et
al., 1984). A youngerU-Pb zircon age of 706±11 Ma obtained from trondhjemite that
intrudesalreadyserpentinizedandshearedgabbroprovidesa minimumage forophiolite
formation(Pallisteretal.,1988).BothU-Pbagesaremodelages,obtainedbyforcingthe
lower intercept through a fixed point of 15±15 Ma, a procedure commonly applied to
U-PbgeochronologicdataintheArabianshield(Cooperetal.,1979).
3. THARWAHOPHIOLITECOMPLEX
The Tharwah ophiolite complex (Nassief, 1981; Nassief et al., 1984; Pallister et al.,
1988)consistsofmafic-ultramaficrockspreservedasastackofsteeplydipping,northwest-
3. TharwahOphioliteComplex 137
Fig. 5. Typical shear fabrics in the Jabal Ess ophiolite. (A) Dextral shear fabric in shear zone at
the northern marginof the ophiolite. Knifefor scale, about 10 cm long. (B) Sinistralshear fabric
inserpentinitemélangesouthofthenorthernmarginoftheophiolite.Hammerforscale.(CandD)
Dextralandsinistralshearfabrics,respectively,about5mapartalongstrikeinserpentinitemélange
closetothesouthernmarginoftheophiolite.Penforscalein(C),hammerforscalein(D).
andsoutheast-vergentthrustsheetsexposedoveranarea13kmeast-westand6kmnorth-
south (Fig. 6). Togetherwith adjacentpelagic rocks,the ophioliteis partof the Labunah
thrust zone (Ramsay, 1986) and lies in the zone of deformed rocks that constitutes the
southwesternpartoftheBi’rUmqsuture(Pallisteretal.,1988;Johnsonetal.,2002).The
ophioliteisexposedinhillsrising150–200mabovetheadjacentRed Sea CoastalPlain.
Weathering is locally intense and most rock surfaces are coated in desert varnish that,
compoundedbypervasivemetamorphismandshearing,makesrockidentificationdifficult.
The succession is disrupted and locally inverted. Serpentinized depleted-mantle
harzburgite and subordinate dunite together with minor lenses and dike-like bodies of
lherzoliteandgabbromakeupthecentralpartofthecomplex.Harzburgitecontainsrelict
olivine (Fo ) (70–90 mode%), bastite pseudomorphs of orthopyroxene (En )
89.5–93.4 90–92
(15–30%),chromite(<1%),andclinopyroxenes(<1%).Duniteislargelyserpentinized
olivine.Chemically,thechromitesresemblechromitiferousspinelsinmodern-dayforearcs
(Sternetal.,2004).Therocksaretectonizedandhaveastrong,high-temperaturefoliation
138 Chapter4:NeoproterozoicOphiolitesintheArabianShield:FieldRelationsandStructure
Fig.6. SimplifiedmapandcrosssectionoftheJabalTharwahophiolite(afterNassief,1981;Nassief
etal.,1984;Ramsay,1986;Pallisteretal.,1988;Johnson,1998).GeochronologicdataafterPallister
etal.(1988).