U-Pb zircon geochronology and geological evolution of the Halaban-AI Amar region of the Eastern ...

13 Pages·2001·1.02 MB·English

J. geol. Soc. London, Vol. 141, 1984, pp. 1043-1055, 5 figs, 3 tables. Printed in Northern Ireland.
U-Pb zircon geochronology and geological evolution of the
Halaban-AI Amar region of the Eastern Arabian Shield,
Kingdom of Saudi Arabia
J. S. Stacey, D. B. Stoeser, W. R. Greenwood & L. B. Fischer
SUMMARY:U -Pb zircon model ages for eleven major units from the Halaban-AI Amar
region of the eastern Arabian Shield indicate three stages of evolution: (1) plate convergence,
(2) plate collision, and (3) post-orogenic intracratonic activity.
Convergenceo ccurredb etween the western Afif ande asternA r Raynp lates that were
separated by oceanic crust. Remnants of oceanic crust now comprise the ophiolitic complexes
of the Urd group. Theo ldest plutonic unit in the study is from one of these complexes and gave
an age of 694 k 8 Ma. Detrital zircons from the sedimentary Abt formation of the Urd group,
which is intercalated with the ophiolitic rocks, were derived from source rocks with a mean age
of 710 Ma. The Abt formation may be an accretionary wedge on the western margin of the Ar
Rayn plate. Plate convergence was terminated by collision of the Afif and Ar Raynp lates
during the AI Amar orogeny which began about 670 Ma. During collision, the Urd groupr ocks
were deformed and in part obducted on to one obr oth plates. Synorogenic leucogranitoid rocks
were intruded from 670 to 640 Ma. From about 640 to 630 Ma, widespread unfoliated dioritic
plutons were emplaced in the Ar Rayn block, and represent the end of orogenesis related to
collision. There is no definitive evidence for a significantly older basement beneath the study
region
The Saudi Arabian Shield is a region for which to explain evolution of that part of the Saudi Arabian
considerable evidence has accumulated for the opera- Shield.
tion of late Proterozoic plate tectonics. Western and
southerpn o rtions of the shielwd erfe o rmed in Geological setting
ensimatic island arc typee nvironmentsd uring the
period1 000-680Maa g o (Greenwood et al. 1976; On the basis of different lithologies, Delfour (1980)
Fleck et al. 1980; Bokhari & Kramers, 1982; Roobol et recognized three geological provinces within the
al. 1983; Stoeser et al. 1984). Inc ontrast,a n older easternA rabian Shield, all of which occur in the
Proterozoic oAr rchaean crystalline basemenht as Halaban-A1 Amar region (Fig. 1). These are: (1) the
been reported to underlie parts of the eastern shield Afif province in the W, (2) the Ad Dawadimi province
(Baubron et al. 1976; Delfour, 1979a,b, 1980; Nawab, in the centre, and (3) the Ar Rayn province in the E.
1979; Schmidt et al. 1979). In these studies, the ages of Within the study areath , teh ree provinces are
the basement units were mainly inferred on geological bounded by faultz ones. In particular, the boundary
grounds and the few available radiometric age deter- between the Ad Dawadimi and Ar Rayn provinces is
minationws ere inconclusive (Baubron et al. 1976; the A1 Amar fault zone which is one of the most
Kroner et al. 1979). prominent features of the region (Moore, 1976; Fig.
More recently, Calvez et al. (1983) report -2000 Ma 2). This zone has been interpreted to be an arc-arc or
zircons in a trondhjemite associated with the A1 Amar arc-continent suture (Brown & Coleman, 1972; Al-
fault in the eastern shield (Figs 1 and 2). Further S in Shanti & Mitchell, 1976; Nawab, 1979; Schmidt et al.
the Jabal Khida region (Fig. 1) Stacey & Hedge (1984) 1979).
report U-Pb zircon, Sm-Nd, Rb-Sr, and common Pb The Afif province is covered by a thick accumula-
dataf rom a granodiorite,t hat all indicate an Early tion of shallow marine to continental type sediments,
Proterozoic (1600-1800 Ma) crustal history for this the Murdama group of Delfour (1979a). Underlying
rock or its protolith. However, in neither of these two rocks are exposedo nly locally, and comprise (1)
areash ast hee xtent of theo lder rocks been deter- marine types ediments of theA jalg roup (2) a few
mined. dacitic to andesitic volcanics that may be an early
The present geochronology study is located in the phase of theM urdama,a nd (3) al arge complex of
Halaban-A1 Amar region of the eastern shield (Fig. 1). foliatedt o gneissic dioriticp lutonicr ocks, the As
It was conducted to determine the ages of the main Sawda domain (Fig. 2) that Delfour (1979a) assigned
plutonic rock units, and to place limits on hypotheses to an older basementu nit.R ocksy ounger than the
1044
L,>
l 7
YEMEN
FIG.1 . Index map of the Arabian Shield showing the study area, geological provinces, localities referred to in the
text, and the Halaban (A) and Ar Rika (B) 1:2.50,000 scale quadrangles. Volcanic rocks dated by Darbyshire et al.
(1983): (1) Arfan 608 f 9 Ma, (2) Juqjuq 612 22 Ma, (3) Jahhad 616 f 13 Ma, (4) Hummah 572 f 23 Ma. Fleck &
Hadley (1983) dated the Uyaijah monzogranite (5) at 59.5 f 1.5 Ma.
Murdama include the sedimentary Jibalah group and the authors concluded that these rocks formed either
granites which intrudes the Murdama sediments. in the vicinity of an oceanic transform fault zone and
Twom ainu nitso ccur within theA d Dawadimi were then deposited in a trench, or that they were the
province: mafic-ultramafic ophioliticc omplexes and product of fore-arc igneous activity.
clastic sediments of theA btf ormation.T ogether, The Abt formation, or Abts chist, is a tightly folded
thesec onstitutet heU rdg roupt hatf orms a broad andm onotonous series of sericitea nd/ocr hlorite
synclinorium, with the ophioliticc omplexes att he schists occupying ab elta pproximately 40 km wide
base,a ndt heA btf ormationa tt hec ore.( Delfour, between the Urd mafic belt at Halaban and the A1
1979a). Where in contact,t he two units are mainly Amar fault (Delfour, 1979a, 1981). The schists grade
fault bounded. to fine-grained graywackce o, nglomerate, arkosic
The ophioliticc omplexes arep resent in several sandstone and dolomitic marble (Delfour, 1979a) that
major belts within the study area (Fig. 2). In a recent are generally metamorphosed to the upperg reenschist
examination of the ophiolitic zones along the eastern facies.
portion of theA btf ormation (Fig. 2), Al-Shanti & Int h eA r Raynp rovince Calvez et al. (1983)
Gass (1983) conclude thatt h e zonecs o mprisae subdivide the layered rocks of the area into an older
mklange of dyked gabbro, dolerite, plagiogranite and A1 Amarg roupa nd ay ounger Ahrmer group. The
basic volcanic blocks in a serpentinite matrix. On the layeredr ocks,h owever, ares ubordinate in areat o,
basis of the Ti, Zr, Y and Nb chemistry of the blocks, and intercalatedw ithaw, idespread and complex
Late Proterozoic plate tectonics 1045
30'
FIG. 2. Simplified geological map of a part of the Halaban-AI Amar region showing sample localities (locality
numbers from Table 1). C indicates trondhjemite with 2000 Ma zircons of Calvez er al. (1983). The deep seismic line
of Healy er al. (1982) is shown by the dashed line.
Units ymbols: v, metavolcanic and metasedimentary rocks with hypabyssal diorite; t, tonalite; 0,Ur d group
ophioliticr ocks; 0,Ab t schist; a, syntectonicp lutonicr ocks; m, Murdama group clastic sedimentaryr ocks;g ,
m,
post-tectonic granitic plutonic rocks; j, Jibalah group clastic sedimentary rocks; Phanerozoic sedimentary cover
rocks; heavy line, fault.
series of syntectonic gneisses thaat re tonalitic to confidence limits. Theya rec omputed using them ethod of
granodioritic in composition. Ludwig (1982). In all cases where we havce o mputed
All of the units discussed above, except the Jibalah, regression lines for small numbers of data points, four or less,
arei n truded by graniticp lutons (Delfour, 1979a). scatter was assumed tob e entirely duet o analytic error.
Correlation coefficients for z06Pb/238Ua nd 207Pb/230Uw ere
These plutonsi nclude hornblende-biotite and biotite
determined mainly by the commonl e ad content of the
monzogranite andg ranodiorite, as well as two-mica
zircons and range from 0.98 to 0.85. Decay constants used for
leucogranite. Baubron et al. (1976) and Delfour
age calculations are those of Jaffey er al. (1971).
(1979a) report Rb-Sr ages of 640-518 Ma for granites
of the Halaban-A1 Amar region.
Results
Analytical methods
Samples were selected from major plutonic bodies in
Chemistry and mass spectrometry of lead and uranium from the Halaban-AI Amar region, locations are shown in
zircons follow thep rocedures of Krogh (1973) with minor the map of Fig. 2, and the zircon analytical data are
modifications. Laboratory blank levels forl eadw ere moni- listed in Table 1. Concordia plots are in Figs 3 and 4,
tored within ther angef rom 0.3 to2 .0 n. Correctionsf or and as ummary of model ages is shown in Table 2.
common lead inherent in the zircons were made by using the
Whole rock Rb-Sr data are listed in Table 3.
composition of leadf rom feldspar in thes ame samples,
The zircon dataf orm two differentg roups,t hose
reported by Stacey & Stoeser (1983).
from the least evolved rocks that have comparatively
Analytical precisions att he 95% level of confidence are
f1.2'% fort h e concentrations of lead andu ranium,a nd low uranium contents (60-600ppm)a ndt hosef rom
+0.1%f or 2"7Pb/Z'fiPb.U ncertainties quoted fort he inter- the monzogranites of the Ad Dawadimi province that
cepts betweern e gression lines and concordia are 95% have high uranium contents (11004300 ppm).F rom
1046 J. S. Stucey et al.
TABLE1 : U-Pb analytical data for zircons from the Halaban-A1 Amar region of the eastern Saudi Arabian Shield
Zircon fraction: M magnetic, NM, non-magnetic.
Concentrations ppm
andL o cality Fraction Estima teA ge R atioAs t omic Ma Blank corrected
pb' 2#p2b0/72p3 b8lU23 5u 207Pb21"2PWb Pi2b0 4Pb
sizem n esusamh m b pelre U
~ ~
Afif province
1. 128935 +l00 NM 134 14.9 0.10203 0.8709 67 1 3553
- 325 176 19.5 0.10116 0.8670 680 6868
Ad Dawadirni province
2. 128955 All sizes 317 22.2 0.06455 0.5578 697 1523
3. 128924 +l50 7.83 63.2 0.11192 0.9641 690 4600
- 150 7.24 59.6 0.10847 0.9370 696 2495
4. 128942 +250 170 1 237 0.06397 0.5431 659 410
+
-200 250 NM 1791 132 0.07064 0.6093 693 749
+
-250 400 2236 173 0.07052 0.6255 753 485
+
5. 128937 NM1 5 0 1125 86.5 0.07271 0.6015 604 664
+
- 150 215409 M 2312 0.05815 0.4737 570 500
-+ 325 164 2188 0.06620 0.5430 585 489
6. 175607 200 254 3962 0.05969 0.4821 552 497
- 200 3489 232 0.05892 0.4786 564 357
+
7. 128941 100 408M 4172 0.05525 0.4441 541 110
+
-100 135201 N M 2698 0.08754 0.7214 596 255
+250 458M 5267 0.06321 0.5101 550 231
-250 402M 4986 0.05558 0.4437 526 186
+
8. 128959 N2M 0 0 388 4495 0.08409 0.6892 583 1110
- 200 335M 3951 0.08043 0.6579 579 799
Ar Rayn province
9. 128921 All sizes NM 304 29.8 0.09564 0.8080 649 1370
+
10. 128919 200 NM 177 18.4 0.09812 0.8228 633 7080
400 NM 164 15.8 0.08881 0.7450 633 5410
+
11. 128944 N1M 00 491 48.4 0.09557 0.8009 63 1 6143
NM -325 653 65.4 0.09557 0.8020 634 2351
+
12. 128946 200 NM 276 26.5 0.08796 0.7368 630 1058
+
-325 400 NM 272 24.8 0.08718 0.7289 626 3989
+
13. 175612 150 NM 322 32.3 0.09752 0.8155 627 2520
+
-200 325 358 32.2 0.08419 0.7025 622 1182
-325 47.4 M 583 0.07189 0.6030 633 556
* Radiogenic Pb.
almost all the samples zircon yields were quite small, time was computed for 20 zircon fractions as 15 f 20
and inm ost cases this limited then umber of sized Ma. The loss of lead was attributed to hydrothermal
fractions that could be prepared. activity during regional uplift prior to opening of the
Data from the low uranium zircons were almost Red Sea. For consistency, we too use a common lower
concordant but for most samples the data points were intercept of 15 f 20 Ma for regression lines of the low
not sufficiently separated for meaningful chords to be uraniums amples, localities 1-3 and 9-13. Table 2
drawn. However, the zircons from the dioritic rocks of shows these as model I1 ages.
the Ar Raynp rovinceh ave very similar 207Pb/206Pb Zirconfs r om the monzogranites (localities 4-8)
ages of -630 Ma (Table 1, localities 1G13). A single havec onsiderablyh igheur raniumc ontents (1100-
regression line for all nine data points from the four 5300 ppm), their data are generally more discordant
samples has upper and lower intercepts of 631 f 6 Ma and they have higher common lead contents than those
and 2 f 39 Ma, respectively. We conclude that zircons from the less evolved rocks (Table 1, Fig. 4). In three
in all these rocks crystallized at about the same time cases there is sufficient spread in the data points to
and all lost lead in a comparatively recent event. Work establisha line ont he concordia diagram and the
by Cooper et al. (1979) further substantiated by intercepts are listed as model ages in Table 2. Note
Stoeser et al. (1984) shows that zircons from different that in all three instances elevated lower intercepts of
rock types over la a rge portion of thes o uthern about 110Ma are obtained. To check the validity of
Arabian Shield all lost lead att hes ame time.T his the elevated lower intercept for the pluton from the
o.lol plaPter ote ro zoLica te tectonics 1047
I I I l l ARr ukhamahd omaina, n additionasl a mple was
HIGH URANIUM ZIRCONS taken (locality 6, 5 km from locality 5). Sufficient
zircon was separated for just two sized fractions that
unfortunately yielded almost coincident data points
(Table 1, Fig. 4). However, these data do apparently
confirm an elevated lower intercept as obtained from
locality 5. Additionael vidence for elevated lower
0.09 -
intercepts comes from 100 to 200 km farther S in the
shield. Dataf or granites atJ abal Sitaraha nd Jabal
3 Dahul (Fig. 1) yield loweri ntercept ages of approx-
02 - imately 140 and 170 Ma, respectively, (J. S. Stacey and
0.08 J. D. Aleinikoff,u npublished data). Apparently by
0'
N Mesozoic time, the high uranium zircons from mon-
zogranitesw ere sufficiently metamic(t damaged by
radiation) to be particularly susceptible to lead loss.
-
0.07 LoMAcgoae dli teyl Ms To t?,, 2 of the study area, episodes of Mesozoic uplift
' 4. Abu lsnun 0 < 738 aild erosion are indicated by thick sections of upper
5. Ar Rukhamah +A 641 225 Triassic and middle Cretaceous sandstone (Powers et
-/ 6. Ar Rukhamah -623 al. 1963) andt he middle tou pper CretaceousA z
n nfi 7. J. Sabhah 0 607t19 Zibirah bauxite (Bowden 1981). Moreover, uplift was
8 J Khurs A 598 235
doubtless accompanied by some thermal activity, both
I I I of which mechanisms have been invoked for causing
0.5 0.6 0.7 0.8
lead loss in metamict zircons (Goldrich & Mudrey
207Pb/ 235U
1972; Gebauer & Grunenfelder, 1976). In this study,
FIG.3 . Concordia plot for the low uranium content for the two monzogranite samples where the data did
zircons in this study. Regression lines are shown
not permit computation of regression lines, approxi-
for model I1 that utilizes a common lower intercept
mate upper intercept ages were computed using lower
of 15 rt 20 Ma, shown to be appropriate for a large
region of the southern shield (Cooper et al. 1979; intercepts of -110 Ma (model 111, Table 2).
Stoeser & Stacey 1983). If significant lead loss occurred in the high uranium
zircons during Cretaceous time,i t is probable that they
werei m pervious tof urther loss inT ertiaryt i me.
However, the complex thermal history of the region
may well account for some of the discrepancies
between zircon and Rb-Sr ages in the monzogranites.
0.1 1 -
Nontheless, the zircon model ages should be reliable
within the uncertainties quoted, provided that inheri-
tance of older zircon fromt he sedimentarys ource
rocks has not affected our data.
0.10-
2
c: Samples with zircons of low uranium
2 - content
0.09
a
N The only rock from the Afif province that we have
Locality Model Age M:
1. As Sawda ln 0 677*9 analysed is as omewhat altered, biotite-hornblende
2. Abt schist + -710 tonalite from the As Sawda domain (Fig. 2, locality 1).
3. Urd gabbro 694*8 Two zircon fractions gave almost ide*n tical data points
9. Unnamed In 0 65029 (Fig. 3) and a model I1 age of 677 9 Ma (Table 2).
10. Unnamed @ 0 635'6 Therefore, the As Sawda dioritic rocks are a plutonic
11. Unnameddi 0 634*6 complex that was emplaceda pproximately 680 Ma
12.J aiaragd v 631*7 ago. The initial 87Sr/86Srv alue of 0.7030 (Table 3) does
13. Unnamed tn A 629*7
noin t dicadte e rivatiofnr o m sialic basement of
I I significantly older age.
0.6 0.7 0.8 0.9 1
207pb/235U Sufficient zircon for only a single fraction was
obtained from approximately 3 kg of the Abt schist in
FIG. 4. Concordia plot for highu ranium content
the Ad Dawadimi province, locality 2. These detrital
zircons from granites that intrude the Abt forma-
tion. Regression lines throughd atah ave lower zircons (Table 1) yield a rather discordant data point
intercepts of -110 Ma, Models I and 111. with am odel I1 age of -710Ma (Table 2, Fig. 3).
1048 J. S. Stucey et al.
TABLE2 : Descriptions and U-Pb model ages for 13 samples discussed. Model I uses intercept ages for regression
lines for data; Model II estimates upper intercept ages using a common lower intercept of 15 f 20 Ma; Model III
estimates approximate upper intercepta ge for a lower intercept of1 10 Ma. Uncertainties quoteda re model dependent,
computed for 95% confidence levels
Locality number Sample Regression line through data
Sample number description Model I Preferred
Lower intercept Upper intercept Model age
(N Ist, E long) (Mal (Mal (Mal
Afif province
1. 128935 Biotite-hornbletnodn ea liteS-aA wsd a domain. 21 677 f 9 (11)
23"36.5', 44O05.8' Plagioclase saussuritized, hornblende and
biotite replaced by chlorite.
Ad Dawadimi province
2. 128955 Abt Schist-Urd group l1 710 (11)
23"52', 44"54' Detrital zircons, low U content.
+
3. 128924 Hypersthene gabbro-Urd group ophiolite 21 694 8 (11)
23"32.0', 44O19.2 complex. Foliated rock containing 2
percent quartz, 1% biotite.
4. 128942 Bioletiu tceo nomzografnroi tme the 31 - 738 (111)
23"21.5', 44O29.1' Abu Isnun pluton. Weak gneissic, partially
recrystallized cataclastic texture.
5. 128937 Two-mmioc an zogranite-Ar Rukhamah domain 109 f 37 641 f 25 641 f 25 (I)
23"48.9', 44O24.0' 6% primary muscovite alL<lh iotite.
Two generations of plagioclase. Fiobably
partial melting product of Abt sediments.
6. 175607 Two-mica monzogranite-Ar Rukhamah domain 21 - 623 (111)
23"52.6', 44'21.3' Probably partial melting product of
Abt sediments.
7. 128941 Two-mmic oan zogranfrito em JaSbaa bl bah 115 f 42 607 f 19 607 f 19 (I)
23"15.3', 44O36.0' Probably partial melting product of
Abt sediments.
8. 128959 Biotite monzogranite from Jabal Khura 110 f 160 598 f 35 598 f 35 (I)
23"35.9', 44O42.9' Probably partial melting product of
Abt sediments.
Ar Rayn province
9. 128921 Bioletui tceo tonalite gneiss (syntectonic) l1 - 650 f 9 (11)
23"17.5', 45O18.3' 5% biotite, 1% muscovite,
no K-feldspar.
10. 128919 Biotite-hornblende quartz diorite -2 f 48 633 f 5 635 f 6 (11)
2351.3',45"11.0' 7%q uartz,3 % biotite, 30%
hornblende. Biotite and hornblende
extensively replaced by chlorite. Intrudes
AI Amar volcanics.
11. 128944 Hornblende diorite 21 - 634 f 6 (11)
23"36.0', 45"06.8' 2%q uartz,n ob iotite, 30%
hornblende. Biotite and hornblende extensively
replaced by chlorite. Intrudes AI Amar
volcanics.
12. 128946 Biotite leucogranodiorite from Jabal al Jabara 21 - 631 f 7 (11)
23"53.5,45"07.2' weakly foliateds y ntectonibc odyi n truding
AI Amar volcanics.
13. 175612 Hornblende tonalite -24 f 23 623 f 5 629 f 7 (11)
23"35.1', 47O07.1' 10%h ornblende,n ob iotite.Q uartz
recrystallized. Intrudes AI Amar volcanics.
11, Only 1 fraction analysed; 21, virtually coincident data points; 31, data too scattered to fit meaningful~ line.
plaPtero te ro zoLica te tectonics 1049
TABLE3: Rubidium-strontium data for some of the samples of this study, by kind permission of C. E. Hedge.
Data are normalized to 86Sr/88Sr= 0.1194. Analytical precision for8 7Sr/86Sr isf 0.00006 (20) between runs
Locality and Preferred ConcentratioAntso pm pimc Ratios
nsmauommd pbelele r age Rb Sr 87Rb186Sr "SrIs6Sr (87SrdSiea8sm6c Srprilp)eit ion
Ma (zircon)
Afif province
1. 1627879 35 16f. 3 9 1432 00.0.730 02.739 03 350 3 . Tonalite gneiss-As Sawda
domain
Ad Dawadimi province
7102 . 1 28955 42.8 292 0.424 0.7007.47 60 330 Abt Schist-Urd group
3.6 9142 8 924 +X 4.0 915 00..0710 2351 2 0.70300 Hypersthene gabbro-Urd
group
Ar Rayn province
9. 615208 921 7.0 f 9 564 000T..7.7o000n 3 3 3a56l29i29t e gneiss
10. 128919 635 f 6 8.8 321 0.0792 0.70393 0.70322 Quartz diorite
11. 128944 634 f 6 13.8 273 0.1461 0.70453 0.70321 Diorite
12. 128946 631 f 7 20.4 490 0.1203 0.70433 0.70325 Granodiorite from Jabal
Jabara
Analyst K. Futa.
Because of the low uraniumc ontent of the zircons samples from localities 10-13 are only weakly foliated
(317 ppm) and the whole rock initial 87Sr/86Srv alue of and all intrude the A1 Amar volcanic host rocks. As
0.7033 (Table 3) the most reasonable interpretation of previously discussed, the zircon dataa r e nearly
this single analysis is thatt he zircons wered erived concordanta nd give similar model I1 ages between
from comparatively unevolved rocks with a mean age 635 f 6 and 629 f 7 as shown in Tables 1 and 2, and
of 710 Ma. Figs 2 and 3. Initial 87Sr/86Srv alues are in the range
The oldest rock in this study is apparently from the 0.70329-0.70321 (Table 3).
ophiolitic complex of the Urd group thatw as sampled
NW of the village of Halaban in the Ad Dawadimi
Samples with zircons of high U content
province (Fig. 2, locality 3). The rock is a hypersthene
gabbro, whose foliation may be of cumulate or Three zircon fractions from theR ukhama granite
tectonico rigin.T wof ractions are almostc oncordant (locality 5) yield a regression line for which upper and
and give a model I1 age of 694 f 8 Ma, (Table 2, Fig. lower intercept ages are 641 f 25 and 109 f 37 Ma,
3). We interpret this to be the crystallization age for respectively (model I, Table 2; Fig. 4)A. s econd
theg abbroa ndt husf ort he age of the associated sample was collected at locality 6, 5 km to the NW.
ophioliticc o mplex. The initial s7Sr/86Sr is 0.7030 Sufficient zircon was obtained for only two fractions
(Table 3). The time o6f 94 f 8 Ma also setsa whose data yield coincident data points. These
maximum age for the overlying Abt formation at this indicate a slightly younger model 111 age of -623 Ma.
locality. South of the village of Halaban we sampled the
In the Ar Rayn province, the sample from locality 9 Jabal Sabhah pluton, a two-mica leucogranite. Figure
is a biotite leucotonalite from a large gneissic massif E 4 shows good separation of the four data points, and
of the A1 Amar fault. Such gneisses are the dominant the chord yields uppera nd loweri ntercept ages of
rock type E of thfe a ulta ,n d thehy a vbe e en 607 f 19 and 115 f 42 ma, respectively (model I, Table
considered to be a remobilized older basement (Kahr 2). Although these zircons have the highest common
et al. 1972; Overstreet et al. 1972; Coulomb et al. 1981; lead content of any in the study, the computed age is
AbdelM onem et al. 1982) or youngers yntectonic not particularly sensitive to the correcting ratios. This
intrusions (Eijkelboom et al. 1969, 1971; Nebert, 1970; is apparently because the most concordant data point
A1 Shanti & Mitchell, 1976). At locality 9, it can be is from the fraction with lowest common lead content.
clearly seen that the tonalitew as thrust westward over The zircon age is, however, appreciably older than* t he
the A1 Amar group and the intrusion is interpreted as Rb-Sr age obtained by Delfour (1979a) of 556 23
syntectonic in origin. Sufficient zircon was obtained Ma, although the initial 87Sr/86Sv alue of 0.7032i s
for only a single analysis, but it gives a model I1 age of consistent with derivation from the Abt formation.
650 f 9 Ma (Table 2, Fig. 3). The initial 87Sr/s6Sr is A biotite monzogranite from the Khurs laccolith of
0.70329 (Table 3). Delfour (1979a) was sampled at locality 8. Data from
Thge r oup of dioritipc l utons represented by two zircon fractionsh aves ome separation in Fig. 4
1050 J. S. Stucey et al.
and yield ar e gression line with u*p pera nd lower & Stoeser, 1983). This we interprete to result from a
intercepts of 598 f 35 Ma and 110 35 Ma, respec- considerable period of residence in the lower crust (or
tively. This is considerably older than the Rb-Sr age of continental mantle) for at least part of the lead prior
518 5 12 Ma, initial 87Sr/86Sr of 0.7033 f 0.0008, to 600 Ma ago. Furthermore, galena data from the Ad
obtained by Delfour (1979a). Dawadimi and Ar Rayn provinces form a linear array
Three zircon fractions fromt heA bu Isnunm on- that was interpreted by Stacey et al. (1980) to be due
zogranite (locality 4) havec omparativelyd iscordant to a radiogenic lead component from lower continen-
data points (Fig. 4). Consequently, inheritance of tal crust- 2100Ma old. Bokhari & Kramers (1982)
older zircon is considered likely. The fraction with the obtainedl e adi s otope dataf rom massive sulphide
lowest 207Pb/2"Pb age, has a model 111 upper intercept deposits in the A1 Amar region and their data were
age of -738 Ma. We consider that this monzogranite is similar to some of the galena data of Stacey et al.
primarily ap roduct of partial melting of theA bt (1980). Bokhari and Kramers did not acceptt h e
formationa ndt h at it inherited zircons from that interpretation of an older lower crustal component in
source. It does not appear to be an older basement thle e ad, buth t ey did not offer an alternative
plutonic unit as proposed by Delfour (1979a), but is explanation fotr h e 208Pb/204Pbd ata. In addition,
probably a post-tectonic intrusive rock, similar in age Duyverman et al. (1982) did not observe a significant
to the others that intrude the Urd group. older crustal signature in their Sm-Nd data from the
Ar Ridaniyah gneiss to the north of the study area
Discussion (Fig. 1). At this time, it has to be admitted that we do
not understand the role played by older crust in the Al
Objectives of this study were: (1) to establish the age Amar region of the Shield. Further investigations are
of the major rock units in the Halaban-AI Amar area, clearly needed to resolve this problem.
some of which have been identified as older basement, More definitive evidence for older sialic crust has
and (2) to examine the overall geological evolution of been found in the Jabal Khida area, 200 km to the S of
the region. the presenst t udy area (Fig. l), whereaf o liated
granodiorite contains zircons -1630 Ma old. In addi-
tion, common Pb, Sm-Nd, and Rb-Sr data all confirm
Older basement
an early Proterozoic crustal history for this rock or its
U-Pb zircon agehs ave beeno btained fofr o ur protolith (Stacey and Hedge 1984). Commonl e ad
plutonirc o cuk n its previously identified as older isotope studies indicate that some plutonic rocks of the
basement. These are the As Sawda tonalite from the Afif province contain lead derived at least in part from
Afif province (locality l), the Abu Isnunl eucomon- an EarlyP roterozoicc ontinental crust with isotopic
zogranite (locality 4), the Ar Rukhamah leucomono- characteristics similar to that found in the Jabal Khida
zogranite from the Ad Dawadimi province (localities 5 sample (Stacey & Stoeser, 1983; J. S. Stacey,
and 6) and a leucotonalite gneiss from the Ar Rayn unpublished data). Also initial 87Sr/s6Sr ratios are
province (locality 9). Three of these units are in the somewhat elevated (0.704-0.708) in some late Precam-
age range 677-643 Maa nd,t herefore, they aren ot brian plutonic rocks from the Afif province (Baubron
part of an older basement. The fourth sample, from et al., 1976; Fleck & Hadley, 1983; Kroner et al., 1979;
the Abu Isnun pluton, is probably of similar age, but C. E. Hedge, unpublished data).I n contrast tot he
scatter in the zircon data indicates inheritance of older crust of thes outhernp art ofA fif province,l ead in
zircons from another source. This source may be the feldspars from plutonic rocks of the Ad Dawadimi and
Abt schist or the Afif crust that may underliet his Ar Rayn provinces lacks the isotopic signature of an
locality, see Fig. 5. older upper crust.
Discovery of zircons 2000 Ma old in a trondhjemite
in the AI Amar fault zone is the only direct evidence
for older crust within the study area (location C of Fig.
Geological evolution
2, Calvez et al. 1983). The trondhjemite is identified as
belonging to one of the ophiolitic complexes of the The Halaban-A1 Amar region of the Arabian Shield
Urd group, but no detailesdt udy of the occurrence has has been interpreted to contain a suture zone, in which
been published. If an older crust does indeed occur in sa e dimentaray c cretionary wedge and associated
the region of the A1 Amar fault, then its effects on the oceanf l oor, represented by theU rdg roup, was
isotopic systems of the rocks of the region that have so compressed between two plates. Four different plate
far been analysed, seem to be rather subdued. Lead tectonic models have been proposed:
isotope ratiosf rom galenas and fromf eldspars of (1) AI-Shanti & Mitchell (1976) first proposeda
intrusive rocks in theA r Rayn rovince haves ome- plate tectonic model which interpreted the Ar Rayn
what retarded values of 206Pb/2g4Pb,a nd their data block as an oceanic island arc, underlain by an
consequently plot above the 208Pb/204Pbv ersus 206Pb/ eastwardd i ppinsg u bduction zone, with thAe bt
204Pba verage growth curve (Stacey et al. 1980; Stacey sediments as an accretionary wedge on the W flank of
Late Proterozoic plate tectonics 1051
W E
MANTLE
l
MANTLE
FIG.5 . Generalized geological cross section of the Halaban-AI Amar region. (A) At 680 Ma, shortly before plate
collision, and (B) at 640 Ma, the end of collision. Unit symbols: a, Abt schist; U, ophiolitic complexes of the Urd
group; v, volcanic and sedimentary rocks of the AI Amar group; S, syntectonic plutonic rocks.
the island arc. Subduction was terminated by collision subduction of the back-arc oceanic crust beneath the
of the arc with a continental block to the W. easterna rc segment. Collision of the segments de-
(2) Nawab (1979) interpreted the Ar Rayn block as formed and tectonically intercalated theU rdg roup
the site of an island arc located on the eastern margin ophiolitic complexes andA bt sediments that were
of a continental block. The Urd group was thought to deposited in the back-arc basin.
have developed in a back arc extensional basin, that In addition tto h ae b oveD , elfour (1981) has
was subsequentlyc ompressedb etween thea rc seg- proposed without elaboration, that the Ad Dawadimi
ment and the continental block. province is a suture zone between two older basement
(3) Schmidt et al. (1979) reversed the interpretation blocks represented by the ArR ayn and Afif provinces.
of Al-Shanti and Mitchell and called for a westward If theU rdg roupf ormed by somep latet ectonic
dippisnu gb duction zone which generatteh de process, then a fundamental question is whether the
Hulayfah arc complex in the central Shield. Subduc- crustal blocks on each side of the Urd group rocks are
tion was terminated by collision with the continental similar. If the blocks ared ifferent,t hen suturing of
Ar Rayn block from the E. allocthonous plates, preceded by subduction could be
(4) Al-Shanti & Gass (1983) call for the formation inferred. However, if they are similar then a back arc
of an ensimatic island arc that was rifted to forma rift situation is also possible.
basin floored by oceanicc r usb te tween thae r c Because thpe re-Murdama geology of the Afif
segments. Thea rc segments then converged with province immediately W of the A1 Amar region is
1052 J. S. Stucey et al.
poorly known, and both the Afif and Ar Rayn crusts the Ar Rayn province during the period 635 f 6 to
have been extensively remobilized, it is very difficult 629 f 7 Ma (localities 10-13). Calvez et al. (1983) have
toc omparet heir geology. However, results from a also dated four tonalites and trondhjemites within the
deep seismic refraction profile (Healy et al. 1982) Ar Rayn province ranging in age from 644 to 632 Ma.
indicate that the crust in the Halaban-A1 Amar region Wce o nclude that pa e riod of compressional
is approximately 43 km thick and that a major crustal orogenesis, which we will informally refer to as the AI
discontinuityo ccurs beneatht heA d Dawadimi pro- Amar orogeny, and which represents the proposed
vince, 25 f 15 km W along the profile from the A1 collision and suturing of the Afif, Ad Dawadimi and
Amar fault( Fig. 5). Althought he geometry of the Ar Raypn r ovinceos ,c curred during the period
discontinuity could not be resolved, the crust to the W 670-640 Ma. Previous work, as well as our own field
undert he Afif province is seismically distinct from, observations,i ndicates thatt he direction of tectonic
and less denset han,t hatt ot he E (Gettings et al. transporta nd thrusting was westward directed (A1
1983). Shanti & Mitchell, 1976; Delfour, 1979a). Given the
The foregoing evidence,t ogether with them ore apparent continuity of the Afif and Ar Rayn crusts
continental character of the Afif province to the SW, beneath the Ad Dawadimi province, and their sharp
supportts h e concept thatt h e Halaban-A1 Amar interface (Gettings et al. 1983) we conclude that the
regionc ontains two distinct crustalb locks thata re Urd group was obducted onto the Afif and Ar Rayn
separated by a zone of highly deformed sediments and plates, with possible concurrent crustal underthrusting
ophiolitic rocks. Therefore, we prefer previous inter- (Fig. 5). Immediately following this deformation, the
pretations that call for a suture in this region. By this suite of dioritic and tonaliticp lutons was emplaced
modetlh ,A e bt sediments are a clastic wedge during the period 640-630 Ma, as a consequence of the
associated with a subduction zone and the ophiolitic crustahl eating and remobilization which occurred
complexes represent segments of associatedo cean during active suturingT. hise pisode of orogenesis,
floor. plate suturing, and magmatism corresponds closely in
Two of our ages bear directly on the age of the Urd time with regional compression and gneiss doming in
group. The gabbro from the ophiolitec omplex near the southern and central shield, as represented by the
Halaban (locality 3) gives a model I1 age of 694 f 8 Nabitah mobile belt, 68M40 Ma ago (Fig. 1; Schmidt
Ma, and detrital zircons from the Abt schist indicate a et al. 1979; Stoeser et al. 1984). The Nabitah mobile
mean age of 710Ma for the source rocks of the Abt belt is interpreted to be a suture zone between the Afif
sediments at locality 2. We conclude therefore, that microplate and an island arc terrain to the W.
theU rdg roup was still forming upt oa t least Following the suturing event, three majorg eological
700-690 Ma ago. Wen otet he occurrence of the episodes are recognized. These are: deposition of the
pre-orogenic A1 Amarg roup of island arc volcanic Murdama group, emplacement of widespread granitic
rocks in the Ar Rayn province and the lack of such plutons, and formation of the left-lateral Najd wrench-
rocks in the Afif province. Therefore, if subduction fault system. The Najd orogeny of Brown (1972) could
occurredp rior to collision, it musth aveb eene ast- ber easonably extendedt o include all thesee vents
wardsd irected beneatht heA rR aynp late. Conse- collectively.
quently, of the models listed above, we prefer that of From our data, we can infer only that the Murdama
Al-Shanti & Mitchell (1976) which was the first to sediments are younger thant he6 77Ma As Sawda
utilize this idea. tonalite. However,t ot he S in the Wadi Ar Rika
After formation of the Urd group, these rocks were quadrangle (Delfour 1980; Fig. l), Fleck & Hadley
deformeda ndi n truded by sa u ite of syntectonic (1983) give a 595 f 15 Ma Rb-Sr age for the Uyaijah
leucogranitoids. We have zircon data for two of these monzogranite which intrudes the Murdamag roup.
monzogranite plutons, the Abu Isnun (locality 4) and Also,i n theA rR ikaq uadrangle, Darbyshire et al.
the Ar Rukhamah (localities 5 and 6). The Abu Isnun (1983) have dated, by Rb-Sr, the underlying Jahhad
data was scattered, but that for Ar Rukhamah gave a volcanic rocks at 616 f 13 Ma. If the Rb-Sr systems in
model I age of 641 f 25 Ma, which places a lower limit these rhyolitic volcanics have really remained closed,
on the age of the Abt schist. and this age is correctt, h en it follows thatt h e
Other syntectonic plutonic rocks occur throughout Murdamag roup in the study area was deposited
the Ar Rayn province, and one of these, a leucotona- during the period 615-595 Ma. Nevertheless, the
lite gneiss (locality 9) gave ma odel I1 age of Rb-Sr isochron age for the Jahhadv olcanics should be
approximately 650 MaT t.o h N e , in thAe r regarded as am inimum, since such ages for acid
Ridayniyah area, Baubron et al. (1976) have dated by volcanic rocks are often too young in spite of their
Rb-Sr, a two mica granitic gneiss at 645 f 35 Ma. In apparently well defined isochrons (e.g., Schleicher et
the same region Calvez et al. (1983) obtained a U-Pb al. 1983). Following deposition, the Murdama sedi-
zircon age of 667 f 17 Ma for a trondhjemite. ments were deformed into broad open folds. This mild
Aftert hef ormation of the syntectonicp lutons,a compressional deformation occurrepd r ior too ,r
series of diorites to granodiorites was emplaced within contemporaneous with, early Najd faulting.