Nd, Pb, Sr, and O isotopic characterization of Saudi Arabian Shield terranes

26 Pages·2006·1.5 MB·English

ChemicalGeology226(2006)163–188
www.elsevier.com/locate/chemgeo
Nd, Pb, Sr, and O isotopic characterization of Saudi Arabian
Shield terranes
Douglas B. Stoeser a,*, Carol D. Frost b
a UnitedStatesGeologicalSurvey,MS973,DenverFederalCenter,DenverCO80225,USA
bDepartmentofGeologyandGeophysics,UniversityofWyoming,LaramieWY82071,USA
Accepted5September2005
Abstract
NewNd,SrandOisotopicdataforgranitoidrocksoftheSaudiArabianShieldarepresentedtogetherwithpublishedNd,Pb,Sr
and O isotopic data and all available geologic and geochronologic information to re-evaluate the terranes defined for the Saudi
ArabianpartoftheArabian–NubianShield.Threegroupsofterranesareidentified:1)thewesternarcterranes,2)theeasternarc
terranes, and3) the Khidaterrane. The Khidaterrane isthe only terrane composed ofpre-Neoproterozoic continental crust.The
westernarcterranesareofoceanicarcaffinity,andhavetheleastradiogenicPbandSrandmostradiogenicNdisotopiccompositions
andsomeofthelowesty18OvaluesofanyrocksoftheSaudiArabianShield.Althoughsomepreviousstudieshavecharacterizedthe
easternarcterranesasofcontinentalaffinity,thisstudyshowsthattheytooarecomposedofNeoproterozoicoceanicarcs,although
theirsourceshaveslightlyelevated208Pb/204Pb, ,Sr,andy18Ovaluescomparedtothewesternarcterranes.Thesedatasuggest
Nd i
thateithertheisotopiccompositionofthemantlesourceforthewesternarcterranesismoredepletedthanthatoftheeasternarc
terranesortheeasternarcterraneshavebeenmixedwithasmallamountofcratonicsourcematerial,orboth.
WefurtherelaborateontheHulayfah-AdDafinahfaultzoneasamajorboundarywithintheSaudiArabianportionoftheEast
AfricanOrogen.Withfurtherstudy,itsnorthernextensionmaybeshowntopassthroughwhathasbeendefinedastheHailterrane,
anditssouthernextensionappearstolieundercovereastoftheTathlith-MalahahterraneandextendintoYemen.Itmayrepresent
thecollisionzonebetweenEastandWestGondwana,andattheveryleastitisanimportantsuturebetweengroupsofarcterranes
of contrasting isotopiccomposition caughtbetween twoconvergingcontinents.
Published byElsevier B.V.
Keywords:SaudiArabia;Precambrian;ArabianShield;ArabianCraton;Nd-isotopes;Pb-isotopes;Sr-isotopes;O-isotopes;Terranes
1. Introduction also includes northeastern Africa, Sinai, Israel, and
Jordan. The ANS comprises a vast expanse of Neo-
The Arabian Shield is the exposed Precambrian proterozoic juvenile oceanic island arc crust, flanked
basement region of the Arabian Peninsula of Saudi on the west and east by older cratonic crust. The ANS,
Arabia, Yemen, and Oman, with the bulk of the ex- which represents one of the largest such tracts of
posed Precambrian lying within western Saudi Arabia. juvenile crust on Earth occupies the northern part of
It is part of the Arabian–Nubian Shield (ANS) that the East African Orogen (EAO) of Stern (1994, 2002),
a major Neoproterozoic orogen that resulted from the
collision of the East and West Gondwana continents to
form the Gondwana supercontinent near the end of the
* Correspondingauthor.Fax:[phone].
E-mailaddress: [email protected](D.B.Stoeser). Neoproterozoic.
0009-2541/$-seefrontmatter.PublishedbyElsevierB.V.
doi:10.1016/j.chemgeo.2005.09.019
164 D.B.Stoeser,C.D.Frost/ChemicalGeology226(2006)163–188
Fig.1.(a)TerranemapoftheSaudiArabianShield(seeAppendixBfordetails).ProtolithagesaregiveninMaandwherethoseinparentheses
indicateyoungerarcassemblageemplacedintoanolderprotolith.FZ—faultzone.(b)ArabianShieldterranesasdefinedbyStoeserandCamp
(1985).
Table1
NdandSrisotopicdataforgraniticrocksoftheSaudiArabianShield
Sample Intrusion Latitude Longitude Age Terrane Prov Rb Sr 87Rb/86Sr 87Sr/86Sr Initial Sm Nd 147Sm/144Nd 143Nd/144Nd Initial Initial Nd model age, Gab
Ma (ppm) (ppm) 87Sr/86Sra (ppm) (ppm) 143Nd/144Nd eNd Model 1c Model 2d
184417 JZa’abah 23.81 44.77 ~600e AdDawadimi EAT 93.15 136.04 1.99 0.73 0.710 6.42 28.85 0.135 0.512549 0.51202 3.02 1.16 0.95
184418 JZa’abah 23.81 44.77 ~600e AdDawadimi EAT 117.2 16.02 21.59 0.91 18.42 84.43 0.132 0.512565 0.51205 3.52 1.09 0.90
184435 JKhazaz 25.33 43.60 584f AdDawadimi EAT 1.47 4.28 0.208 0.512818 0.51202 2.71
155019 JabanalAhmar 25.58 42.79 574g Sawdah EAT 59.70 4.55 39.25 1.06 9.38 43.38 0.131 0.512566 0.51207 3.43 1.08 0.88
155025 JabanalAhmar 25.50 42.77 574g Sawdah EAT 67.62 23.63 8.35 0.79 5.98 30.46 0.119 0.512535 0.51209 3.71 0.99 0.83
155029 JabanalAhmar 25.45 42.75 574g Sawdah EAT 69.91 24.79 8.22 0.78 4.83 26.92 0.108 0.512501 0.51209 3.80 0.94 0.80
155016 JabanalAhmar 25.75 42.96 574g Sawdah EAT 101.6 24.37 12.22 0.85 17.26 89.96 0.116 0.512509 0.51207 3.41 1.00 0.84
184420 JKhinzir 23.16 43.79 567f Sawdah EAT 491.5 7.12 240.08 2.78 8.66 21.59 0.243 0.512986 0.51208 3.45 D
155034 JQutn 26.004 42.334 579g Sawdah EAT 204.8 16.14 37.74 0.99 7.29 29.18 0.151 0.512670 0.51210 3.99 .B.
175612h Unnamed 23.085 45.118 629i ArRayn EAT 2.50 10.22 0.148 0.512655 0.51205 4.27 Sto
175608h JSitarah 22.098 44.720 613j Khida KD 9.27 30.62 0.183 0.512706 0.51197 2.40 es
175609h Unnamed 22.208 44.748 ~645j Khida KD 5.07 23.25 0.132 0.512426 0.51187 1.21 1.35 1.14 er,
155101 JAja 27.27 41.31 566k Hail WAT? 94.95 5.15 56.32 1.27 6.25 29.37 0.129 0.512625 0.51215 4.67 0.94 0.76 C.D
155116 JAja 27.41 41.35 566k Hail WAT? 12.93 68.01 0.115 0.512586 0.51216 4.91 0.87 0.72 .
155128 JAja 27.54 41.53 566k Hail WAT? 84.49 7.39 33.92 0.97 4.46 21.57 0.125 0.512594 0.51213 4.35 0.99 0.78 Fro
184374 JAbha 24.36 40.68 584l Hijaz WAT 8.06 1440.10 0.02 0.70 0.703 3.14 14.80 0.128 0.512631 0.51214 4.99 0.93 0.75 st
155083 J al Kurayziyah 25.28 40.62 ~610e Hulayfah WAT 10.93 1.04 8.69 44.54 0.118 0.512575 0.51210 4.92 0.92 0.76 /
C
155639 JAn 21.29 41.18 ~22e Bidah WAT 26.43 16.12 4.75 0.71 0.708 6.08 28.28 0.130 0.512939 0.51292 6.05 0.39 0.23 h
e
155648 JBurgatinah 20.80 39.86 ~22e Bidah WAT 15.20 34.11 1.29 0.70 0.704 5.32 25.21 0.128 0.512899 0.51288 5.28 0.45 0.30 m
155630 JIbraham 20.42 41.13 610m Bidah WAT 46.71 521.93 0.26 0.71 0.704 2.75 15.31 0.109 0.512544 0.51208 5.58 0.88 0.74 ica
l
155633 Turabah 20.93 41.36 ~620e Bidah WAT 50.35 353.70 0.41 0.71 0.703 6.97 39.62 0.106 0.512538 0.51211 5.21 0.87 0.74 G
155541 Najran 17.56 44.00 617f Malahah WAT 77.96 54.36 4.17 0.76 14.84 80.29 0.112 0.512533 0.51208 4.66 0.92 0.78 eo
155544 JAshirah 18.01 44.20 637k Malahah WAT 129.4 93.59 4.02 0.75 4.47 24.24 0.112 0.512535 0.51207 4.93 0.92 0.76 log
155548 JalGaharra 18.063 44.000 575f Malahah WAT 226.9 204.05 4.16 3.69 7.61 17.11 0.269 0.513169 0.51216 5.07 y2
155591 alAjarda 19.17 41.97 ~620e AnNimas WAT 34.10 95.33 1.04 0.72 0.706 4.21 26.03 0.098 0.512466 0.51207 4.49 0.90 0.77 26
155671 JSawdah 20.80 40.22 ~620e AnNimas WAT 102.8 93.19 3.20 0.74 0.709 0.93 4.55 0.123 0.512579 0.51208 4.67 0.97 0.79 (2
155701 Jazirah 19.05 42.91 ~620e AlQarah WAT 24.68 24.67 2.90 0.74 0.712 5.60 27.77 0.122 0.512575 0.51208 4.69 0.96 0.79 00
6
)
Provinceabbreviations:EAT—Easternarcterranes,KD—Khidaterrane,WAT—Westernarcterranes. 1
a Initial87Sr/86SrofProterozoicsamplescalculatedonlyforsampleswith87Rb/86Srb4. 63
–
b Modelagescalculatedonlyforsampleswith147Sm/144Ndb0.14. 18
8
c ModelofGoldsteinetal.,1984.
d ModelofDePaolo,1981.
e Estimatebyauthors.
f Stucklessetal.,1987.
gStucklessetal.,1984.
h UnpublishedTIMSNddatacourtesyofJ.VervoortandP.J.Patchett,1999.
iStaceyetal.,1984.
j Unpublishedzirconisochronage,J.S.Stacey,1983.
k AleinikoffandStoeser,1989.
l Calvezetal.,1983.
m Flecketal.,1976.
1
6
5
166 D.B.Stoeser,C.D.Frost/ChemicalGeology226(2006)163–188
Table2
PublishedandunpublishedoxygenisotopicdatafortheArabianShield
Sample Intrusion Latitude Longitude Rock Quartz Kspar A/CNK
155019* JabalAbanalAhmar 25.5800 42.7883 8.5 9.7 8.6 0.90
155020* JabalAbanalAhmar 25.5656 42.7819 8.8 0.90
155023* JabalAbanalAhmar 25.5181 42.7756 9.7 9.9 0.97
155024 JabalAbanalAhmar 25.5033 42.7761 9.1 0.98
155026 JabalAbanalAhmar 25.4828 42.7608 9.1 9.5 1.02
155028 JabalAbanalAhmar 25.4558 42.7558 9.6 0.96
155030* JabalQutn 26.0672 42.3458 10.8 10.8 1.07
155031* JabalQutn 26.0483 42.3469 10.1 10.9 1.04
155032* JabalQutn 26.0344 42.3447 10.5 10.7 1.03
155033* JabalQutn 26.0194 42.3400 10.8 11.1 1.08
155034* JabalQutn 26.0039 42.3344 10.4 10.8 1.01
155035 JabalQutn 25.9928 42.3300 10.9 11.9 1.06
155036 JabalQutn 25.9806 42.3192 9.4 11.8 1.07
155037 JabalQutn 25.9719 42.3311 10.7 11.2 1.06
155038 JabalQutn 25.9581 42.3183 10.6 11.2 1.03
155039 JabalQutn 25.9383 42.3000 9.7 10.5 1.02
155052 JabalTuwalah 25.5608 41.0717 7.9 8.0 7.6 1.01
155062* JabalAwja 25.8542 40.9031 4.3 7.6 0.88
155063* JabalAwja 25.8469 40.9208 7.0 8.6 0.85
155064* JabalAwja 25.8344 40.9267 8.3 8.2 0.88
155065* JabalAwja 25.8267 40.9319 8.3 8.1 0.86
155066 JabalAwja 25.8156 40.9386 7.3 7.9 6.8 0.81
155067 JabalAwja 25.8156 40.9386 7.6 7.8 0.86
155068 JabalAwja 25.8067 40.9447 7.7 7.7 0.93
155079 Kurayziyah 25.2725 40.5739 8.8 0.82
155097 JabalBidayah 25.1319 41.1475 6.8 6.6 0.92
155106* JabalAja 27.3108 41.2917 6.8 7.1 0.81
155107* JabalAja 27.3453 41.4222 7.4 7.4 1.04
155108* JabalAja 27.3453 41.4222 6.6 1.02
155111* JabalAja 27.3244 41.3258 5.3 4.4 1.06
155115 JabalAja 27.3178 41.2861 6.3 7.1 1.04
155129 JabalAja 27.5422 41.5292 5.7 6.3 0.95
155136 JabalAja 27.6275 41.3628 6.7 7.8 4.0 0.84
155159 JabalAja 27.6342 41.5136 7.9 6.4 0.86
155151 JabalarRuman 26.7661 41.4372 5.1 6.8 4.4 0.84
155171 JabalSalma 27.0583 42.0425 7.9 1.00
155177 JabalSalma 27.1128 42.1392 7.9 8.2 7.8 1.01
155192 Ba’gham 26.9250 40.8569 6.7 1.19
155196 Ba’gham 26.9261 40.9181 7.2 8.0 1.01
155204 Ba’gham 26.9319 40.8761 6.5 1.20
155205 Ba’gham 26.9450 40.8739 8.1 1.20
155500 JabalTarban 21.2583 43.9708 8.6 9.2 7.7 1.13
155504 JabalTarban 21.2444 44.0000 8.7 9.2 1.21
155509 JabalasSukkah 21.7250 43.6875 8.0 7.6 1.15
155512 JabalSabhah 23.2958 44.5986 10.3 11.4 1.12
155515 JabalSabhah 23.2861 44.6292 10.3 11.3 10.0 1.14
155516 JabalSabhah 23.2528 44.6417 10.3 11.4 1.14
155526 JabalSahah 22.2028 44.8042 8.3 8.9 7.7 1.14
155528 JabalSahah 22.1514 44.8250 8.5 9.0 1.16
155536 JabalHuqban 21.7764 43.8250 8.1 9.1 1.10
155537 JabalHuqban 21.7764 43.8250 8.1 8.7 7.4 1.08
155540 Najran 17.5583 44.0000 8.1 9.0 6.5 1.06
155543 Najran 17.5458 44.1542 8.2 9.3 1.09
155546 JabalalGaharra 18.0542 44.0042 11.4 11.9 1.14
155548 JabalalGaharra 18.0625 44.0000 11.7 10.8 1.18
155549 JabalalGaharra 18.0917 44.0083 7.9 10.8 1.23
155553 BaniBwana 18.5306 43.9361 8.7 10.1 1.09
155555 BaniBwana 18.4417 43.9333 8.9 8.1 1.13
D.B.Stoeser,C.D.Frost/ChemicalGeology226(2006)163–188 167
Table2(continued)
Sample Intrusion Latitude Longitude Rock Quartz Kspar A/CNK
155556 BaniBwana 18.4917 43.9292 9.3 10.2 1.26
155558 JabalHubbah 18.5292 43.2958 7.7 9.3 1.97
155561 JabalMadhah 18.6958 43.2139 8.7 9.2 1.34
155562 JabalZayd 18.8208 43.3292 8.6 13.4 1.40
155563 JabalZayd 18.8250 43.3222 9.1 8.8 1.61
155568 JabalHassir 19.6500 43.0944 9.0 8.9 1.08
155570 JabalHassir 19.5000 43.0653 8.7 1.23
155571 JabalHassir 19.4236 43.0764 7.8 7.5 1.06
181800 JabalSilsilah 25.08 4267 10.5 10.7 –
181878 JabalSilsilah 25.08 4267 10.1 11.1 –
181904 JabalSilsilah 25.08 4267 10.2 10.0 –
181929 JabalSilsilah 25.08 4267 9.9 10.0 –
181856 JabalSilsilah 25.08 4267 10.5 10.6 –
184405 JabalSagrah 23.0550 43.0000 9.2 10.1 8.5 1.19
184421 JabalKhinzir 23.1489 43.8083 8.4 9.2 8.1 1.25
184424 JabalMinya 24.9653 43.3492 10.1 10.8 1.23
184425 JabalMinya 24.9586 43.3292 7.7 10.7 6.4 1.38
184430 MiskaRing 24.5908 43.1686 8.8 9.1 7.9 1.15
184432 Dukhnah 24.8739 43.2156 9.4 10.1 8.4 1.06
184434 JabalKhazaz 25.3847 43.5831 9.8 10.3 9.3 1.21
184439 JabalShiib 24.0839 42.5250 9.2 9.3 8.8 1.11
889494 BaidalJimalah 25.1486 42.6875 10.3 1.15
893039 BaidalJimalah 25.1486 42.6875 10.6 1.19
893235 BaidalJimalah 25.1486 42.6875 11.0 1.32
HD141 HadbadDayahin 23.54 41.17 8.3 8.6 –
HD154B HadbadDayahin 23.54 41.17 8.7 9.0 –
JS40 JabalSayid 23.92 40.90 9.8 10.1 –
JS29 JabalSayid 23.92 40.90 9.8 11.6 –
Asterisk(*)indicatesdatapublishedbyStucklessetal.,1984.A/CNKindicatesmolarAl O /(CaO+NaO+KO)value.
2 3 2
TheNeoproterozoic evolution oftheArabian Shield the closure of ocean basins (e.g. Stoeser and Camp,
occurred in three main stages: (1) formation and accre- 1985; Johnson and Woldehaimanot, 2003) and (2) lat-
tionofislandarcsprimarilyduringtheperiod~870–620 eraltranspositionalaccretionsuchashasbeenproposed
Ma, (2) continental orogenesis resulting from the colli- for the western margin of North America (Samson and
sion with the northwestern margin of East Gondwana, Patchett, 1991; Patchett and Chase, 2002). In addition,
~660–620Ma, and (3) post-collisional extension, mag- although most models primarily involve the accretion
matismandsedimentation~620–540Ma(Greenwoodet of oceanic island arcs, others have proposed that the
al.,1976;Al-ShantiandMitchell,1976;Schmidtetal., accretion of oceanic plateaus has also played a signif-
1979; Stoeserand Camp, 1985; Kro¨ner,1985; Johnson icant part in the evolution of the Arabian–Nubian
and Woldehaimanot, 2003).The agesfor arcformation Shield (e.g. Stein and Goldstein, 1996).
andcollisionoverlapbecausewhilecollisionwasoccur- Correct identification and interpretation of terranes
ringinthesouth,subductionofoceaniccrustappearsto requiresknowledgeofbasicfieldrelations,geochronol-
havecontinuedinthenorthuntilapproximately620Ma ogy, and isotopic compositions of major units. Al-
(Doebrichetal.,2004).Duringcollisionalorogenesisa though these basic descriptions of the Saudi Arabian
major left-lateral fault system, the Najd fault system, Shield are far from complete, it is now possible to
developed throughout the northern part of the Arabian scrutinize and redefine Saudi Arabian terranes more
Shield. The Najd has been interpreted as the result of confidentlythaninpaststudies.Onenotabledeficiency
escape tectonics similar to those resulting from the has been the paucity of Nd isotopic data for granitoid
India–Asia collision (Schmidt et al., 1979; Fleck et al., rocks of the Shield, and we augment the limited data-
1980a,b; Burke and Sengor, 1986). Three of the most base with 27 additional analyses (Table 1). Other Nd
prominentoftheNajdfaultzones,Ruwah,ArRika,and isotopic data have not been widely available; we pres-
Halaban-Zarghat,are shown on Fig. 1a. ent this in Appendix A along with updated age and
Interpretive models for terrane accretion of the Ara- geologic information. In addition, we present 70 un-
bian Shield include (1) terrane accretion resulting from published oxygen isotopic analyses to augment the
168 D.B.Stoeser,C.D.Frost/ChemicalGeology226(2006)163–188
sparse data in the literature (Table 2). This study also terranes are simply considered as domains in the
relied on the geochronologic and isotopic compilation descriptive senseQ. We endorse this perspective and
of Johnson et al. (1997). Based upon a comprehensive view most of the terranes identified in this paper and
compilation of this new data and all published infor- asshown onFig. 1 asbeing primarilya division ofthe
mation, we present an investigation of the Arabian Saudi Arabian Shield into domains consisting of ob-
Shield with the following goals: vious crustal blocks that appear to have a common
history but whose boundaries are drawn rather arbi-
1. re-evaluationanddefinitionofArabianShieldterranes, trarily and whose relationship to adjoining terranes in
2. refinement of the distribution of oceanic vs. conti- most cases remains to be determined. All of these of
nental crust, and these terranes except one have been interpreted to
3. discrimination of isotopic provinces within the arc represent oceanic arc assemblages (Stoeser and Stacey,
terranes,especiallyinregardstoidentifying terranes 1988; Pallister et al., 1988; Harris et al., 1990), either
oneitherside ofthenominal maincollisional suture as primary volcanic arcs or flanking sedimentary
within the Saudi Arabian portion of the Arabian– basins, which may be either fore-arc or back-arc in
Nubian Shield. origin. The one exception is the Khida terrane of the
eastern Shield, which is interpreted be a microplate of
2. Saudi Arabian Shield terranes reworked older continental crust (Stacey and Agar,
1985; Stoeser and Stacey, 1988). Much of our subdi-
2.1. Terrane identification vision into terranes is based on the apparent protolith
age of the primary arc assemblage(s) within each
ItiswidelyacceptedthattheArabian–NubianShield terrane, and the terrane boundaries are drawn where
formed by terrane accretion and a number of different faults appear to bound crustal blocks of distinctly
terranes have been identified within the Saudi Arabian different protolith age. Without good geochronologic
Shield (e.g. Johnson and Vranas, 1984; Stoeser and data even the tentative subdivision presented here
Camp, 1985). In the current context of bterrane ana- would be difficult because oceanic arc assemblages
lysisQ it is implied that terranes properly identified will are similar in their stratigraphy and lithology and
represent crustal blocks that are allochthonous relative thus very difficult to distinguish based purely on
toeach other andthat have beenaccreted toeachother field relations.
through some tectonic process (Jones et al., 1983;
Howell, 1995). Although it is quite easy to identify 2.2. Terranes
and name terranes by simply recognizing blocks of
crust separated by major fault zones, it is quite another Our re-evaluation of geologic, petrologic, geochro-
to definitely prove that these bterranesQ are in fact nologic and structural data of the Saudi Arabian
allochthonous crustal units relative to one another par- Shield has led to the terrane map of Fig. 1a. Spe-
ticularly for Precambrian terranes. Once terrane maps cific information on how terranes were defined is
have been drawn, it is also relatively easy to construct provided in Appendix B. We also refer the reader to
plate tectonic models with regards to location and the earlier primary sources that defined Saudi Arabian
polarity of subduction zones and the formation of terranesorstructuralprovincesbyDelfour(1981),Cal-
fore-arc, main arc, and back-arc crustal units. It is vez et al. (1983), Johnson and Vranas (1984), Stoeser
difficult, however, to support such interpretations and and Camp (1985), Stoeser and Stacey (1988), Johnson
for the Arabian–Nubian Shield, we note the words of and Kattan (2001), and Johnson and Woldehaimanot
caution inthisregard by Church (1988), Quick(1991), (2003).
AbdelsalamandStern(1996),Al-Saleh(1998),andthe StoeserandCamp(1985)subdividedtheShieldinto
basically arbitrary nature of terrane identification and five terranes: Midyan, Hijaz, Ar Rayn, Asir and Afif
analysisasawholeasdiscussedbySengo¨randDewey (Fig. 1b). Stoeser and Stacey subsequently referred to
(1990). the Asir and Afif as bcomposite terranesQ based on the
In one of the seminal papers on terrane analysis, conceptthatbothcrustalunitswereinfactcomposedof
Coney et al. (1980) make the following statement in multipleterraneswhichhadaccretedintounifiedcrustal
regards to recognizing terranes: b...that identification blocks or terranes prior to collision along the bNabitah
of a terrane is based primarily on its stratigraphy and suture zoneQ. The designation of these two large ter-
need not carry any genetic or even plate tectonic ranes has been widely accepted and applied in the
implication.Atthestartofinvestigations,theidentified literature typically without the bcompositeQ qualifying
D.B.Stoeser,C.D.Frost/ChemicalGeology226(2006)163–188 169
Fig.2.(a)Mapofinitiale forPrecambriansamples,(b)mapofprotolithage,(c)mapshowingwesternarcterranes,easternarcterranes,and
Nd
Khidaterrane,(d)histogramofT ages(DePaolo,1981,depletedmantlemodelages).ComparetoT histogramsofStern(2002)fortheentire
dm dm
Arabian–NubianShield.
170 D.B.Stoeser,C.D.Frost/ChemicalGeology226(2006)163–188
term, but the fundamental basis for their designation is The suture zone cited most commonly in the liter-
arbitrary and we do not accept them in the present ature for the Arabian Shield is the Nabitah suture. The
work.ThebAsircompositeterraneQasdefinedbyStoe- Nabitah suture was proposed by Schmidt et al. (1979)
serandCampiscomposedofatleastfourensimaticarc who interpreted the Nabitah fault zone that has nu-
assemblages,theAnNimas,Bidah,andJiddahterranes, merous serpentinite bodies along its suture and ex-
which are older than about 800 Ma and the Al Qarah tended it to the north to correspond with the ophiolite-
terrane which is less than 740 Ma (Fig. 1). More bearing Hulayfah-Ad Dafinah zone (Fig. 1a). The
recently Johnson and Kattan (2001) and Johnson and Nabitah suture as originally proposed also formed
Woldehaimanot (2003) have continued the use of the the core of the Nabitah orogenic belt (Schmidt et
Asir terrane, but with the Jiddah as a separate terrane al., 1979; Stoeser and Camp, 1985; Stoeser and Sta-
and including the Tathlith-Malahah terrane of Fig. 1. cey, 1988), which was interpreted to result from
The bAfif composite terraneQ is composed of a Paleo- collision between the accreted arc terranes of the
proterozoic cratonic terrane, the Khida terrane, in the Arabian Shield and a cratonic block to the east (the
south and at least two or three Neoproterozoic ensi- Arabian Craton, Stoeser et al., 2004). The Nabitah
matic arc terranes in the north (Fig. 1). suture has been viewed as the northern part of the
Inthispaper,wewillrefertothearcdomainswestof main suture between East and West Gondwana (e.g.
theHulayfah-AdDafinahlineanditsinterpretedexten- Shackleton, 1996; Windley et al., 1996). Because
sionsouthwardsasthebwesternarcterranesQandthose these fault zones all locally contain ophiolites and
to the east as the beastern arc terranesQ (Fig. 1b). The large bodies of serpentinite, they do appear to repre-
arcterranes havealsobeensubdivided intotwoclasses sent suture zones (Stoeser and Camp, 1985; Quick,
(Fig. 2a), bvolcanic arc assemblagesQ and bbasinal arc 1991). Johnson and Kattan (2001), however, have
assemblagesQ such that the first represents volcanic challenged the correlation of the Nabitah suture with
assemblages formed on or near the main line of arc the Hulayfah-Ad Dafinah suture and interpreted the
magmatism,whereasthelatterismorelikelyformedin extension of the Hulayfah-Ad Dafinah zone south of
fore- or back-arc environments. the Ruwah fault zone to lie concealed beneath Phan-
Fig. 1c shows the protolith age distribution for the erozoic sedimentary cover to the east of the Tathlith-
Arabian Shield. By protolith we mean the age of the Malahah terrane. As will be seen in the present study,
basement arc assemblage for the arc terranes and the the Hulayfah-Ad Dafinah zone does represent the
cratonic basement for the Khida terrane. Based on this boundary between major isotopic provinces within
compilation it can be seen (Fig. 2b) that the Shield has the Shield and appears to represent a fundamental
an older ensimatic arc core in the west (N800 Ma) that structural zone within the Shield. We also further
is flanked on the east and north by younger ensimatic examine the issue of its location within the southern
arc terranes (N740 Ma). The youngest arc assemblages Shield.
(b700 Ma) lie along the eastern flank of the Arabian
Shield and appear to have formed as late as about 620 3. Sample selection and analytical methods
Ma.Thusthereappearstobeaprogressiveyoungingof
oceanic arcs eastwards across the Shield. 3.1. Neodymium and strontium
2.3. Suture zones Twelve Neoproterozoic granitoids from the eastern
arc terranes and ten Neoproterozoic granites plus two
The interpretation that the Arabian Shield is com- Tertiary syenites from the western arc terranes were
prised of accreted terranes in turn requires that prop- selected for Sr and Nd isotopic analysis. Samples
erly identified terranes will be bounded by suture were chosen to provide good geographic coverage
zones. Some fault zones of the Shield are widely andtorepresentavarietyofgeochemicalcompositions.
accepted as sutures, in that they incorporate ophiolites Thesuiteincludesmultiplesamplesfromseveralzoned
(Kro¨ner, 1985; Pallister et al., 1988). These include plutonic complexes in the eastern arc terranes, includ-
the Bir Umq and Yanbu sutures that bound the Hijaz ingfoursamplesfromJabalAbanalAhmar,threefrom
terrane (Nassief et al., 1984; Johnson et al., 2002) and Jabal Aja, and two from Jabal Za’abah. All samples
the Ad Dawadimi which contains an ophiolitic me- were obtained from the U.S. Geological Survey collec-
lange and the Halaban ophiolite along its western tions courtesy of John S. Stuckless. Major and trace
margin (Al-Shanti and Gass, 1983; Al-Saleh and elementgeochemicaldataonthesamplesarepresented
Boyle, 2001). in Stuckless et al. (1982a,b, 1983, 1985, 1986, 1987).
D.B.Stoeser,C.D.Frost/ChemicalGeology226(2006)163–188 171
The silica content of the samples varies from 62% to thousand). Accuracy of reported values is F0.1x
78%, plus one hornblende gabbro with SiO =45%. (2j) (Stuckless et al., 1984).
2
Most samples are metaluminous, although the suite
extends from peralkaline to peraluminous composi- 4. Results
tions. The samples are mainly ferroan and alkali-calcic
according to the classification scheme of Frost et al. Sm–Nd and Rb–Sr isotopic data are presented in
(2001). On trace element discrimination diagrams Table 1. REE abundances vary substantially: Nd con-
(Pearce et al., 1984), the samples are defined mainly tents of the granitoid samples range from 4 to 90 ppm,
as within-plate granites, with the exception of several and Sm concentrations from 1.5 to 18 ppm. The range
that lie within the volcanic arc granite field. Rare-earth in147Sm/144Ndfrom0.098to0.269reflectsdifferences
patterns show pronounced negative europium anoma- in the LREE-enrichment of these rocks, the ones with
lies and little to no light rare-earth enrichment (Stuck- highest 147Sm/144Nd also displaying bgull-wingQ REE
less et al., 1986). patterns in which Sm is greater than La . Despite
N N
Samples were powdered and 80 to 100 mg of each these variations, the initial are all positive, and
Nd
sample were dissolved in HF–HNO . After conversion range from =+2.7 to +6.1. The initial for eastern
3 Nd Nd
to chlorides, one-third of the sample was spiked with arc terrane granitoids overlaps those of the western arc
mixed 87Rb–84Sr- and 149Sm–146Nd-enriched tracers. granitoids,butthoseoftheeasternShieldextendtoless
Rb, Sr and REE were separated by conventional cat- positive values. Depleted mantle model ages for the
ion-exchange procedures. Sm and Nd were further Neoproterozoic samples vary from 0.9 to 1.2 Ga
separated in di-ethyl-hexyl orthophosphoric acid col- (model of Goldstein et al., 1984) and from 0.7 to 1.0
umns. All isotopic measurements were made on a VG Ga (model of DePaolo, 1981).
Sector multi-collector mass spectrometer at the Uni- Rb contents are typically higher than Sr contents in
versity of Wyoming. An average 87Sr/86Sr isotopic this sample suite, leading to high 87Rb/86Sr ratios (up
ratio of 0.710251F20 (2j) was measured for 15 to 240). Thin section petrography reveals that most
analyses of NBS 987 Sr, and an average 143Nd/ samples are sericitized or otherwise altered to varying
144Nd ratio of 0.511846F11 (2s) was measured on extents,andmobilityofRband/orSrislikely.Because
20 analyses of the La Jolla Nd standard. Uncertainties agivenpercentageofmobilitychangestheRb/Srratio
in individual Sr isotopic ratio measurements are and thus the calculated initial 87Sr/86Sr most for sam-
F0.00002 and uncertainties in Nd isotopic ratio mea- ples with high Rb/Sr, we have entered initial 87Sr/86Sr
surements are F0.00001 (2j). Blanks are b50 pg for ratios on Table 1 only for those Proterozoic samples
Rb, Sr, Nd, and Sm and no blank correction was with 87Rb/86Sr less than 4 and therefore that experi-
made. Uncertainties in Rb, Sr, Nd, and Sm concentra- encedonlymodestradiogenicgrowthsinceintrusionin
tions are F2% of the measured value based upon Late Proterozoic time. These initial ratios range from
replicate dissolutions and analyses of unknown and valuesclosetocontemporarydepletedmantletohigher
of USGS standard rock samples; uncertainties on ini- ratios up to 87Sr/86Sr=0.7121.
tial are F0.3 epsilon units. Nd model ages are Whole rock y18O (Table 2) varies from 5.3x to
Nd
calculated based upon the depleted mantle models of 11.7x. The results show a fairly wide range in values
DePaolo (1981) and Goldstein et al. (1984) (Table 1; for individual plutons, even for plutons with a limited
Appendix A). range in silica content. For example, samples from
Jabal Qutn in the Sawdah terrane of the eastern arcs
3.2. Oxygen composite terrane vary from y18O of 9.4x to 10.8x,
butsilicarangesonlyfrom74.9%to79.4%.Mosty18O
Granitoid samples for the eastern part of the Shield ofquartzareintherange of0.5xto1.0xhigherthan
were analyzed for oxygen isotopic composition (Table the corresponding whole rocks, suggesting that both
2).Oxygenisotopedatawereobtainedonwholerocks, whole rock and quartz values reflect magmatic compo-
quartz and feldpar at the USGS by fluorine extraction sitions (Taylor, 1978). However, in some samples the
and analysis in a VG Micromass mass spectrometer, differencesaregreaterorthevaluesarereversed.Some
model602C,modifiedwithaPSCDataControlSystem ofthespreadinwholerockoxygenisotopevaluesmay
(Stuckless et al., 1984). The 18O/16O values are be caused by secondary alteration: brick-red, altered
reported relative to Standard Mean Ocean Water feldspars are common in the granites of the Arabian
(SMOW), and are calibrated relative to NBS-28 stan- Shield. However, D(quartz–feldspar) values are all
dard that has a defined value of 9.61x (parts per b2.5 except for two samples from Jabal Aja and Jabal
172 D.B.Stoeser,C.D.Frost/ChemicalGeology226(2006)163–188
Minya at 3.8 and 4.3, respectively, which suggest hy- McGuire and Stern (1993) analyzed the Nd isotopic
drothermal alteration (Taylor, 1978; Criss and Taylor, composition of mafic and intermediate composition
1983). In general, we consider the y18O data from lower crustal xenoliths that were collected from Ceno-
quartz separates listed on Table 2 to be most reliable; zoic alkali basalts in the Midyan, Hijaz, Jiddah and
these vary from 4.4x to 11.9x. Bidah terranes. The initial compositions of these
Nd
rocks are positive, and they have late Precambrian
5. Isotopic terrane mapping depleted mantle Nd model ages. These data suggest
that the lower crust of the western arc terranes is
5.1. Nd isotopic terrane mapping composed of juvenile, late Precambrian crust isotopi-
cally similar to the upper crust of those terranes.
Nd isotopic data for granitoid rocks of the Saudi Gabbros from the Midyan ophiolite (Claesson et al.,
Arabian Shield are limited, consisting of the 27 analy- 1984) also have positive initial compositions (Fig.
Nd
ses presented here (Table 1), and 32 analyses from the 3b), again confirming that both mafic and felsic rocks
literature (Appendix A). The latter include data for from the western arc terranes have similar depleted
granitic gneisses from the Al Lith area of the western mantle sources.
arc terranes (Hegner and Pallister, 1989), a variety of
granitoids and granitic gneisses from the eastern and 5.2. Pb isotopic terrane mapping
western arc terranes (Duyverman et al., 1982), and a
variety of intrusive rocks and paragneisses from the Early common lead studies of the Arabian Shield
Khida terrane and adjacent areas (Stacey and Hedge, recognized two Pb groups within the Arabian Shield, a
1984; Agar et al., 1992). In Appendix A, we have typeIofoceaniccharacteristicsandatypeIIinterpreted
providedadditional,updatedageestimatesanddescrip- to have a component of older continental lead (Dele-
tions of these samples as the literature permitted. vaux et al., 1967; Stacey et al., 1980; Stacey and
These Nd data are plotted relative to age on Fig. Stoeser, 1983) (Fig. 4). Rocks with type I Pb isotope
3a,b. Immediately evident is the difference in Nd characteristics occur throughout thewesternand south-
isotopic character for rocks of the Khida terrane rel- ern part of the Shield in the western arc terranes,
ative to the rest of the Saudi Arabian Shield: the whereas type II leads occur in the eastern arc terranes
Khida terrane is identified by negative initial , (Figs. 1 and 5). The primitive oceanic character of the
Nd
whereas all western and eastern arc terrane samples, western arc terranes was also affirmed in lead isotopic
whether Proterozoic or Tertiary, have positive initial studies by Bokhari and Kramers (1982), and Ellam et
. These results imply that only the Khida terrane is al. (1990).
Nd
composed of continental crust that pre-dates the late StaceyandStoeser(1983)alsorecognizedthatsam-
Precambrian episode of ensimatic arc formation and ples from the Khida region had radiogenic 207Pb/204Pb
accretion and supports previous similar conclusions and 208Pb/204Pb characteristic of evolved continental
by Stacey and Hedge (1984), Stacey and Agar crust. Further studies by Stacey and Hedge (1984)
(1985), Stoeser and Stacey (1988), and Harris et al. and Stacey and Agar (1985) further amplified this
(1991). In Fig. 3b, the western arc terrane samples observation. On this basis Stoeser and Stacey (1988)
(shaded) define a tighter array of Nd isotopic compo- recognized an additional type III lead group to account
sitions than do the eastern arc terranes. Most eastern forthosewithcontinentalcharacteristics(Fig.5).These
arc terrane samples plot below the western arc array. studies led to the proposal that the Khida region was
The affinity of the Hail terrane is still unclear, but the underlain by evolved pre-Neoproterozoic continental
available data suggests that the Hail terrane is akin to crust, an interpretation sustained and amplified by
the western arc terranes. Agar et al. (1992). This block of continental crust,
For those zoned intrusions for which there are mul- which has been identified and spatially delineated pri-
tiple Nd isotopic analyses, the within-pluton variations marily on the basis of the lead isotopic data, has been
inNd isotopic composition aresimilartothereproduc- referred to as the Khida microplate (Stacey and Agar,
ibility of individual analyses (F0.3 units), yet plu- 1985),Khidabasement(StoeserandStacey,1988),and
Nd
tons from different domains have different Khidaterrane(Stoeseretal.,2001).Weretainthelatter
Nd
compositions. This suggests that the geochemical zo- term in this study.
nation is a function of magmatic processes rather than The type III leads have been modeled as resulting
multiplemagmasources,andthattheNdisotopiccom- fromseparationofnewcontinentalcrustfromtheman-
position of the source varies from terrane to terrane. tle around 2600–2800 Ma and with a 238U/204Pb (A)