V.7.S

Rubidium -Strontium Geochronology and 

Plate -Tectonic Evolution of the 

Southern Part of the Arabian Shield

GEOLOGICAL SURVEY PROFESSIONAL PAPER 113

Prepared in cooperation with the Saudi Arabian 

Ministry of Petroleum and Mineral Resources, 

Directorate .General of Mineral Resources
Rubidium-Strontium Geochronology and 

Plate-Tectonic Evolution of the 

Southern Part of the Arabian Shield

By ROBERT J. FLECK, WILLIAM R. GREENWOOD, DONALD G. HADLEY, 

R. ERNEST ANDERSON, and DWIGHT L. SCHMIDT

GEOLOGICAL SURVEY PROFESSIONAL PAPER

Prepared in cooperation with the Saudi Arabian 

Ministry of Petroleum and Mineral Resources, 

Directorate General of Mineral Resources

A study of the age of rocks and 

the geologic history of 

southwestern Saudi Arabia

1131

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY

H. WiHiam Menard, Director

Library of Congress Cataloging in Publication Data

United States. Geological Survey.

Rubidium-strontium geochronology and plate-tectonic evolution of the southern part of the Arabian Shield.

(Geological Survey professional paper ; 1131)

Bibliography: p.

1. Rubidium-strontium dating. 2. Plate tectonics. 3. Geology Arabia. I. Fleck, Robert J. II. Title. III. Series:

United States. Geological Survey. 

Professional paper ; 1131. 

QE508.U5 1979 555.3 79-13596

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, B.C. 20402
CONTENTS

Abstract _____________________________________

Introduction __________________________

Acknowledgments ____________________

General geologic setting ___________________________

Volcanic and sedimentary rocks ___________

Basaltic assemblage ______________

Andesitic assemblage ________________

Plutonic rocks _____________________________

Dioritic batholiths ________________

Granodiorite gneiss ___________

Late-orogenic or postorogenic plutons _______

Layered gabbro and associated rocks __ 

Granodiorite to granite intrusions ____

Najd fault system ______________________

Previous geochronologic studies ____________________

Analytical techniques _____________________

Results of rubidium-strontium analyses __________

Basaltic assemblage _________________________

Page 

1

1

1

3

3

3

5

5

5

6

7

7

7

9

15

Results of rubidium-strontium analyses—Continued 

Andesitic assemblage ____ — — _ — — _________

Undisturbed isotopic systems _____________

Disturbed isotopic systems ________________

Foliated diorite to trondhjemite batholiths _____

Granodiorite gneiss domes ___________________

Late-orogenic or postorogenic granodiorite to 

granite plutons ______——— — _ — ———— — ___

Tindahah batholith ____________________

Bani Thuwr pluton ____________________

Wadi Shuwas ____———__—_————-

Jabal Shada and Jabal Ibrahim ___________

Jabal Qal and Jabal 'Aya ________________

Wadi Halal _______________———- 

Wadi al Miyah and Wadi Musayrah ______

Other late-orogenic or postorogenic plutons

Discussion of the results —_—————————————— 

Conclusions __________________ — — _ — — _____

References cited ___________ — __ — — — _ —

Page

16

16

17

19

22

24

24

24

25

25

25

26

26

28

29

36

36

ILLUSTRATIONS

Page

FIGURE 1. Map of the western part of the Arabian Peninsula showing the area studied and the exposed part

of the Arabian Shield ________________________________——__————————————— 2

2. Chart showing a generalized chronology of igneous and sedimentary units of the southern part of

the Arabian Shield and of tectonic events affecting them ________—__——___ —— _———— 3

3. Geologic map of the southern part of the Arabian Shield in the Kingdom of Saudi Arabia ___ 4

4. Index map of the location of analyzed samples from the Arabian Shield and of quadrangles _——__ 10 

5-25. Total-rock rubidium-strontium isochron diagrams:

5. Basalt of Wadi al Faqh ___________________________ ______________ 15

6. Arphan Formation of Hadley and meta-andesite of the Junaynah quadrangle _______ 16

7. Khadrah Formation metavolcanic rocks, rhyolite of the Bi'r Juqjuq quadrangle, and Arphan

Formation of Hadley ____________—__—_______________—__ 16

8. Metavolcanic rocks of Wadi bin Dwaynah _ —————————————— —————————————— 17

9. Metavolcanic rocks of Hishat al Hawi and Wadi Shuklalah _———————_————______ 17

10. Rhyolite of the Murdama Group _—————————————————————————————————— 18

11. Juqjuq Formation of Hadley _________- —— --_ ——————— — __ — ——— — ——————— 18

12. Quartz diorites of Wadi Khadrah and Wadi ash Shaqah ash Shamiyah, Al Lith area, and

of Biljurshi and Wadi Qanunah, Biljurshi area ________________________ 19

13. Quartz diorites of An Nimas, Wadi Tarj, and Al Mushirah and the trondhjemite of Wadi

Asmak, An Nimas batholith _____________________ ______________ 20

14. Quartz diorites of Wadi Makhdhul and Simlal and the diorite and trondhjemite of the

Wadi Tarib batholith, Wadi Malahah quadrangle ______________________ 21

15. Quartz diorites of Jabal Umm al Hashiyah and the southwestern Al Qarah quadrangle __ 22

16. Quartz diorite of Malahah dome and granodiorite of Wadi Malahah _____________ 22

17. Granodiorite gneiss of Jabal Mina ___________..__________________—__ 23

18. Granodiorite gneiss of Wadi Bagarah and granitic gneiss of Jiddah Airport _________ 23

ill
IV CONTENTS

Page

FIGURES 5-2.5. Total-rock rubidium-strontium isochron diagrams—Continued

26. 

27, 28.

29.

30.

19. Granitic gneisses of Harisi dome and Wadi Bishah —————————————————————————— 23

20. Tindahah batholith and the Bani Thuwr pluton, quartz monzonites of Wadi Shuwas, and

granites of Jabal Shada and Jabal Ibrahim ————————————————————————— 25

21. Quartz monzonites of Jabal Qal and Jabal 'Aya ————————————————————————— 26

22. Granodiorite of Wadi Halal ________——————————————————————————————— 26

23. Granodiorites of Wadi al Miyah and Wadi Musayrah and an aplite dike of the southwestern

Al Qarah quadrangle ____-- — — — — — - — - ——————— — -——— — -———————— 27

24. Hornblende diorite of Hadley __—————————————— T ——————————————————— 28

25. Quartz monzonite and gneissic granodiorite of the Bi'r Juqjuq quadrangle and the quartz

monzonite of Jabal Tarban ___—_———-—-————---—----—--—-—————— 28 

Diagrams showing variation of S7Sr/ S6Sr and of Rb/Sr with age —————————————————————— 30 

Sketch maps showing geographic distribution of ages:

27. Dioritic complexes and metavolcanic rocks older than 680 million years ———————————— 32

28. Volcanic rocks and late-orogenic or postorogenic plutonic rocks younger than 680 million

years ______________________ 33

Diagram of a plate-tectonic model of the origin and deformation of the Arabian Shield ———————— 34 

Map showing a comparison of continental-collision-related structures of Asia with erogenic structures of the Arabian Shield __________————————————————————————————— 36

TABLES

Page

TABLE 1. Analytical results for rocks of the Arabian Shield —————————————————————————————— 11 

2. Summary of Rb-Sr ages and isochron data _____ — — ______ — ____ — — _ — ____ — __—_——————— 14

CONVERSION FACTORS

Metric unit Inch-Pound equivalent

Length

millimeter (mm) 

meter (m) 

kilometer (km)

= 0.03937 inch (in) 

= 3.28 feet (ft) 

= .62 mile (mi)

Area

square meter (m2 ) 

square kilometer (km2 ) 

hectare (ha)

= 10.76 square feet (ft2 ) 

= .386 square mile (mia ) 

= 2.47 acres

Volume

cubic centimeter (cm3 ) 

liter (L) 

cubic meter (m3 ) 

cubic meter 

cubic hectometer (hm3 ) 

liter 

liter 

liter 

cubic meter

cubic meter

= 0.061 cubic inch (in3 ) 

= 61.03 cubic inches 

= 35.31 cubic feet (ft3 ) 

= .00081 acre-foot (acre-ft) 

= 810.7 acre-feet 

= 2.113 pints (pt) 

— 1.06 quarts (qt) 

= .26 gallon (gal) 

= .00026 million gallons (Mgal or 

106 gal) 

= 6.290 barrels (bbl) (1 bbl = 42 gal)

Weight

gram (g) 

gram 

metric tons (t) 

metric tons

= 0.035 ounce, avoirdupois (oz avdp) 

= .0022 pound, avoirdupois Ub avdp) 

= 1.102 tons, short (2,000 Ib) 

= 0.9842 ton, long (2,240 Ib)

Specific combinations

kilogram per square 

centimeter (kg/cm2 ) 

kilogram per square 

centimeter 

cubic meter per second 

(m3/s)

= 0.96 atmosphere (atm)

— .98 bar (0.9869 atm) 

= 35.3 cubic feet per second (ft3/s)

Metric unit

Specific

liter per second (L/s) 

cubic meter per second 

per square kilometer 

[(m3/s)/km2 ] 

meter per day (m/d)

meter per kilometer 

(m/km) 

kilometer per hour 

(km/h) 

meter per second (m/s) 

meter squared per day 

(m2/d) 

cubic meter per second 

(ma/s)

cubic meter per minute 

(nvymin)

liter per second (L/s) 

liter per second per 

meter f(L/s)/m] 

kilometer per hour 

(km/h) 

meter per second (m/s) 

gram per cubic 

centimeter (g/cm3 ) 

gram per square 

centimeter (g/cm2 ) 

gram per square 

centimeter

Inch-Pound equivalent

combinations — Continued

= .0353 cubic foot per second 

= 91.47 cubic feet per second per 

square mile [(ft3/s)/mi2 ]

= 3.28 feet per day (hydraulic 

conductivity) (ft/A) 

= 5.28 feet per mile (ft/mi)

— .9113 foot per second (ft/s)

— 3.28 feet per second

= 10.764 feet squared per day (ft2/d) 

( transmissivity )

=r 22.826 million gallons per day 

(Mgal/d) 

= 264.2 gallons per minute (gal/min)

= 15.85 gallons per minute 

= 4.83 gallons per minute per foot 

[(gal/min) /ft]

= 62.43 pounds per cubic foot (lb/ft3 ) 

= 2.048 pounds per square foot (lb/ft2 ) 

= .0142 pound per square inch (lb/in2 )

Temperature

degree Celsius (°C) 

degrees Celsius 

(temperature)

= 1.8 degrees Fahrenheit (°F) 

= [(1.8x°C)+32] degrees Fahrenheit
RUBIDIUM-STROlVTIUM GEOCHRONOLOGY AND PLATE-TECTONIC 

EVOLUTION OF THE SOUTHERN PART OF THE ARABIAN SHIELD

By ROBERT J. FLECK, WILLIAM R. GREENWOOD, DONALD G. HADLEY, 

R. ERNEST ANDERSON, and DWIGHT L. SCHMIDT

ABSTRACT

Rubidium-strontium studies of Precambrian volcanic and 

plutonic rocks of the Arabian Shield document an early 

development of the Arabian craton between 900 and 680 

m.y. (million years) ago. Geologic studies indicate an islandarc environment characterized by andesitic (dioritic) magmas, volcaniclastic sedimentation, rapid deposition, and contemporaneous deformation along north- or northwest-trending 

axes. Magmatic trends show consistent variation in both 

composition and geographic location as a function of age. 

The oldest units belong to an assemblage of basaltic strata 

exposed in western Saudi Arabia that yield an age of 

1165±110 m.y. The oldest andesitic strata studied yield an 

age of 912±76 m.y. The earliest plutonic units are diorite 

to trondhjemite batholiths that range from 800 to 900 m.y. 

in age and occur along the western and southern parts of 

Saudi Arabia. Younger plutonic units, 680 to 750 m.y. in 

age, range from quartz diorite to granodiorite and become 

more abundant in the central and northeastern parts of the 

Arabian Shield. Initial ^Sr/^Sr ratios for both dioritic groups 

range from 0.7023 to 0.7030 and average 0.7027. The absence 

of sialic detritus in sedimentary units and the evidence for 

an island-arc environment suggest the early development of 

the Arabian craton at a convergent plate margin between 

plates of oceanic lithosphere. Active subduction apparently 

extended from at least 900 m.y. to about 680 m.y.

Subsequent to this subduction-related magmatism and tectonism, called the Hijaz tectonic cycle, the Arabian craton 

was sutured to the late Precambrian African plate in a 

collisional event. This period of orogeny, represented in 

Arabia and eastern Africa by the Mozambiquian or PanAfrican event, extended from some time before 650 m.y. to 

at least 540 m.y. and perhaps 520 m.y. B.P. Although the 

tectonic processes of subduction and continental collision 

during the 900+ to 500-m.y. period require similar directions 

of plate convergence, the differences in magmatic and tectonic styles of Hijaz orogenesis from those of the PanAfrican and the temporal break between them in much of 

the southern part of the Arabian Shield support division into 

at least two events. As defined by the ages of major plutonic 

units, the axis of magmatic and tectonic activity migrated 

eastward or northeastward during the Hijaz cycle, the predominantly dioritic plutonic rocks becoming younger and 

more siliceous to the east. Granodiorite to granite plutonism 

of the Pan-African event, however, shows no geographic 

bias, being distributed throughout the Arabian Shield. 

Although the Hijaz diorites and Pan-African granitic rocks

exhibit strong contrasts in composition and age differences 

as great as 250 m.y. in the westernmost parts of the area, 

the two groups are less distinct compositionally and nearly 

the same age in the eastern part.

INTRODUCTION

Geologic and geochronologic studies of the southern part of the Arabian Shield present a complex 

but surprisingly clear view of late Precambrian island-arc development and crustal evolution. The 

geology of the Arabian Shield between lat 21°30' N. 

and the Saudi Arabian-Yemen border (fig. 1) is discussed by Schmidt and others (1973) and by Greenwood and others (1973, 1976, 1977) ; geologic mapping was compiled by Greenwood and others (1974). 

Geochronologic studies of the shield include those by 

Fleck and others (1972, 1973, and 1976). Results of 

geologic, geochemical, and preliminary geochronologic studies have been interpreted as indicating formation of the Arabian Shield in an intraoceanic island arc by andesitic volcanism, associated plutonism, and cannibalistic sedimentation (Greenwood 

and others, 1976, 1977). This report presents Rb-Sr 

results that document a detailed chronology for the 

development of the shield area. These results were 

obtained by the U.S. Geological Survey as part of 

the program of geologic investigations of the Directorate General of Mineral Resources, Ministry of 

Petroleum and Mineral Resources, Kingdom of Saudi 

Arabia.

ACKNOWLEDGMENTS

We thank A. Berry, J. Saburomaru, and W. Doering for laboratory assistance with the rubidiumstrontium analyses reported here. We also thank 

T. H. Kiilsgaard, W. C. Prinz, R. G. Coleman, J. C. 

Ratte, and H. R. Cornwall for assistance of various 

kinds. M. A. Lanphere and F. C. W. Dodge made 

many useful suggestions.
Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

36° 39° 42° 45° 48°

Massawa •' :•' 

Asmara^

200 KILOMETERS

FIGURE 1.—Western part of the Arabian Peninsula showing the area studied and the exposed part of the Arabian 

Shield (shaded). Hachures are on Paleozoic side of contact between Paleozoic units and units of the shield. Only 

representative major faults of the Najd fault system are shown.
GENERAL GEOLOGIC SETTING

GENERAL GEOLOGIC SETTING

The Arabian Shield represents a terrane of 

slightly to intensely deformed stratified units and 

undeformed to partly remobilized plutonic units of 

late Precambrian (Proterozoic) age. The shield is 

exposed over an area of 610,000 km2 in the western 

part of the Arabian Peninsula (fig. 1). On the north 

and east the shield is overlain by a veneer of nearly 

flat-lying Phanerozoic sedimentary rocks. On the 

west the Red Sea rift, containing "oceanic" or 

"basaltic" crust of Tertiary and Quaternary age, 

separates the Arabian Shield from its previously 

continuous counterpart, the Nubian (or African) 

Shield in Egypt, Sudan, and Ethiopia. A summary of 

the major sedimentary and igneous units of the

southern part of the shield and a generalization of 

the type of tectonic environment are shown in figure 

2. The distribution of these units in the area studied 

is shown on the geologic map (fig. 3), which is modified from Greenwood and others (1974).

VOLCANIC AND SEDIMENTARY ROCKS

BASALTIC ASSEMBLAGE

The oldest rocks exposed in the southern part of 

the Arabian Shield constitute an assemblage of 

clastic and volcaniclastic marine sedimentary rocks, 

basalt and basaltic andesite flows, flow breccias, 

tuffs, and, in lesser amounts, chert and impure limestone (marble). Sedimentary units exhibit graded 

bedding, slump structures, intraformational breccias,

Age(m.y.) Plutonic rocks Stratified rocks Tectonic setting

-600- Granodiorite 

to granite

f / r—T- —

Hiatus?

Layered gabbro

700-

Diorite to 

granodiorite

800-

Diorite to 

trondhjemite

900

— >900 Dikes

(Includes the 

Halaban and 

Ablah Groups)

Continental 

collision

Neocontinental

Juvenile

? (Includes the 

Basaltic \ jjddah 

assemblage \ 

(Includes \ 

the Baish and ? 

Bahah Groups) \

a

3 

Q_

Group)

Oceanic

FIGURE 2.—Generalized chronology of igneous and sedimentary units of the southern part of the Arabian Shield and of tectonic events affecting them. Periods indicated by shading were of decreased igneous activity or deposition and are shown 

as time transgressive because of a general southwest to northeast decrease in age of the early shield-forming units.
Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

22o 39°

20° —

18° —

PHANEROZOIC

CAMBRIAN

AND 

PRECAMBRIAN

PROTEROZOIC

EXPLANATION

Igneous and sedimentary rocks and alluvium

Granodiorite to granite circular plutons, ring dike complexes, sheets, and irregular bodies—Diapirically to passively intruded

Gneissic quartz diorite to quartz monzonite domes and complex diapirs

Diorite to trondhjemite-Subordinate gabbro and serpentinized ultramafic rocks

Andesitic assemblage—Metamorphosed andesitic to dacitic flows, breccia, and tuff, associated volcaniclastic rocks, and subordinate 

carbonaceous rocks and marble; includes units assigned to the Jiddah, Ablah, Halaban, and Murdama Groups

Basaltic assemblage—Metamorphosed basaltic flows, breccia, and tuff, graywacke, chert, and minor marble; includes units 

assigned to the Baish and Bahah Groups

FIGURE 3.—Geologic map of the southern part of the Arabian Shield in the Kingdom of Saudi Arabia. Modified from Greenwood and others (1974).
GENERAL GEOLOGIC SETTING

and similar features characteristic of turbidites. 

Algal structures have been identified in some carbonaceous units (T. Kiilsgaard, oral commun., 

1977). Neither potassium feldspar clasts nor terrigenous lithic fragments have been reported from 

these strata. Contacts between this "basaltic assemblage," as we shall refer to these units, and any other 

rock unit within the area studied are either tectonic 

or indicate this unit to be the older. No evidence has 

been found within the southern part of the Arabian 

Shield to suggest deposition of the basaltic assemblage on sialic crust, and no older rock units are 

known. The basaltic assemblage includes units assigned by various workers to the Baish and Bahah 

Groups. Bakor and others (1976) identify a maficultramafic complex at Jabal al Wask in the northern 

part of the shield (fig. 1) as "back-arc ophiolite." 

Although serpentinite occurs within the southern 

part of the shield, no relation to rocks of the basaltic 

assemblage has been demonstrated. The sedimentary 

aspects of the "ophiolite" described by Bakor and 

others are similar to those described here, but none 

of the volcanic rocks studied in the southern shield 

could be classified as spilitic. Any relation between 

this "ophiolite" and the basaltic assemblage, however, remains to be demonstrated.

ANDESITIC ASSEMBLAGE

The stratified units of the southern part of the 

Arabian Shield mot part of the basaltic assemblage 

are classified here as the "andesitic assemblage." 

This generalization of previously published stratigraphic nomenclature is necessary for several reasons. Correlation of units of an extremely thick, 

laterally variable, highly deformed, and slightly to 

highly metamorphosed sequence of lithologically reDetitious strata over 610,000 km2 must be described 

as difficult at best. This remains true when the study 

area is restricted to the southern one-third of the 

shield area. Available age determinations are inadequate for regional correlation of strata at a formational level and probably questionable at the group 

level. Alteration, sedimentary reworking, and varying degrees of metamorphism further complicate the 

geochronologic interpretation and, as will be shown, 

invalidate many of the age determinations made on 

fine-grained volcanic or volcaniclastic units. The 

andesitic assemblage consists predominantly of calcalkalic andesite, basaltic andesite, dacite, associated 

volcaniclastic rocks, and subordinate carbonaceous 

rocks and marble. The volcanic rocks include flows, 

breccias, and large amounts of tuff, including welded 

ash-flow tuff. Locally, younger units of this assemblage rest unconformably on older units of the assemblage, on older plutonic rocks, or on the basaltic 

assemblage. In some localities, the younger units 

contain dioritic to granitic detritus; in other areas, 

the rocks cannot be subdivided easily on this basis. 

The relation between the oldest units of the andesitic assemblage and the basaltic assemblage is uncertain because all contacts between these two groups 

of strata are tectonic. Because the two assemblages 

are chemically and lithologically distinct, never occur in the same tectonic block, and contain units 

whose characteristics require significantly different 

environments of deposition, we conclude that these 

assemblages accumulated at different times or in 

areas originally remote one from another and were 

juxtaposed subsequently by faulting along northsouth shear zones. Although the nature of this displacement is inadequately known, it appears to have 

occurred during deposition of the andesitic assemblage and therefore must be consistent with the island-arc origin of that sequence.

PLUTONIC ROCKS

DIORITIC BATHOLITHS

The basaltic assemblage and the older units of the 

andesitic assemblage are complexly deformed, metamorphosed, and intruded by diorite, quartz diorite, 

and trondhjemite batholithic complexes. The intrusive sequence within these complexes generally follows a differentiation trend, beginning with hornblende diorite and gabbro followed by quartz diorite 

and trondhjemite. In places, however, the sequence 

was repeated, the trondhjemite being intruded by 

younger quartz diorite or hornblende diorite. Chemir 

cal variations within the batholiths include ranges in 

Si02 from 55 to 73 percent, K2 0 from less than 0.4 

to more than 1.7 percent, and CaO from less than 2 

to more than 8.5 percent (Greenwood and Brown, 

1973). Normative orthoclase commonly makes up 

less than 10 percent, especially in the western part 

of the area. As are the older strata, the diorite 

batholiths are cut by zones of intense shearing that 

segment the bodies into narrow north-trending slices, 

at many places separated by similarly sheared 

blocks of stratified units. Unlike the two older stratified assemblages, which never occur in the same 

tectonic slices, the batholiths retain a general coherence and, though cut by the same shear zones, 

exhibit no significant lateral displacement. This relation suggests that batholithic intrusion was probably 

concurrent with much of the deformation but that 

the major translation required to juxtapose the older
Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

units of the andesitic assemblage with the basaltic 

terrane occurred earlier.

Diorite-trondhjemite complexes (fig. 3) in the 

south-westernmost part of the shield have been assigned to three large batholiths—Biljurshi, An 

Nimas, and Wadi Tarib—that exhibit an en echelon 

pattern from northwest to southeast. The northwesternmost complex, the Biljurshi batholith (Greenwood, 1975a), includes dioritic bodies between Al 

Lith and the major north-trending shear zone at 

approximately long 41°45' E. South and east of the 

Biljurshi batholith, dioritic units are assigned to the 

An Nimas batholith (Greenwood, 1979a; Anderson, 

1977), which extends north from the town of An 

Nimas between long 41°45' and long 42°30' E. 

Dioritic intrusive rocks south and east of the An 

Nimas batholith are assigned to the Wadi Tarib 

batholith, which occupies a broad area in the southeastern part of the Arabian Shield (Greenwood, 

1979c, d). The Wadi Tarib batholith includes both 

dioritic and granodioritic units mapped previously 

as Khamis Mushayt Gneiss (Coleman, 1973a).

Northeast of the en echelon batholiths, exposures 

of diorite are generally less extensive, as younger 

andesitic units cover and younger granodiorite to 

granite bodies intrude the older terrane. East of the 

town of Bishah, discontinuous exposures suggest 

that a large dioritic batholith may once have occupied a large area but this has been reduced by emplacement of a younger granitic body, the granodiorite of Wadi Musayrah.

Subsequent to emplacement of the early dioritic 

batholiths, younger units of the andesitic assemblage, including conglomerates containing diorite 

and trondhjemite cobbles, were deposited on both 

older volcanic assemblages and on erosionally exposed plutons of the batholiths themselves. The 

younger units of the andesitic assemblage indicate a 

continuation of andesitic to dacitic volcanism and 

deposition of andesitic volcaniclastic, fine-grained 

terrigenous, and carbonate sediments, but the plutonic detritus in the abundant breccias and intraformational conglomerates indicates local uplift and 

erosion of the early dioritic bodies. Because the 

younger andesitic strata were deposited on both 

older andesitic units and the basaltic assemblage, 

juxtaposition of these two older sequences must 

have been largely complete before that time. Because these younger andesitic units are also displaced by the north-south shear zones and range in 

metamorphic grade up to almandine-amphibolite 

facies (including kyanite and sillimanite in some 

localities), deformation and nietamorphism clearly

must have continued for a significant period of time. 

In some areas, the younger units of the andesitic 

assemblage are intruded by a second, or younger, 

generation of diorite batholiths that may in turn be 

overlain by even younger graywacke, andesite, volcanic tuff, breccia, and agglomerate (fig. 2). Plutonic 

detritus increases upward in the assemblage but is 

clearly controlled by local exposures of older diorites. 

As lateral variations of detrital components are 

common within these strata, the presence or absence 

of dioritic detritus at a given locality is unreliable 

as a means of correlation.

GRANODIORITE GNEISS

The first major shift in plutonic rock type occurred in the western part of the Arabian Shield after 

the major dioritic complexes were emplaced in that 

area. Compositionally variable gneiss domes, ranging 

from biotite-bearing trondhjemite to biotite-muscovite granite, were emplaced with both internal and 

host-rock foliations concordant with the intrusive 

margins. Greenwood (1979a) describes the granodiorite orthogneiss of Jabal Mina as "... a nested 

to interconnected series of phacoidal and lensoidal 

bodies separated by thin septae of metamorphic 

layered rocks. The bodies appear to be regionally 

conformable with the structure of the metamorphic 

rocks but have slightly to moderately discordant 

contacts when viewed on an outcrop scale." Gneiss 

domes of this general description, though not abundant, are common in the southwestern part of the 

Arabian Shield. The rocks range from quartz diorite 

to quartz monzonite and generally contain both 

biotite and muscovite but no hornblende. The fabric 

of these rocks is strongly planar, defined primarily 

by oriented micaceous minerals and stretched inclusions. This foliation is commonly antiformal or 

domical, paralleling that of the deformed metamorphic rocks at the margins. Contacts between the 

gneiss domes and adjacent stratified units vary in 

definition, some being determinable to within a millimeter, as where the intruded strata are amphibolite-grade metavolcanic rocks, and others being 

almost completely gradational from sillimanite-bearing metapelites into biotite-muscovite granodiorite 

gneiss. Locally, garnet is an accessory mineral.

The metamorphic grade of stratified units adjacent 

to these gneiss domes is generally high, and kyanite 

and sillimanite are common where AL03 contents are 

adequate for their formation. Grade of nietamorphism decreases away from the domes to that of an 

average greenschist facies, the minimum metamorphic grade of all older strata in this area of the
GENERAL GEOLOGIC SETTING

Arabian Shield. This metamorphic gradient adjacent 

to the domes indicates a clear genetic relationship 

between the gneiss domes and high-pressure, hightemperature metamorphism, but designation of 

either as cause or effect is generally uncertain.

The gneiss domes are generally elongate, doubly 

plunging antiforms whose axes and longest dimensions are concordant with the generally north-south 

trend of schistosity, axes of folds, and faults in the 

strata intruded. Although the gneiss domes may be 

bounded by the north-trending shear zones, few are 

actually truncated by the zones. Minor fractures offset the domes, but in general, most field evidence indicates that emplacement of the domes was simultaneous with movement on the north-south shear 

zones and with the most intense deformation of both 

the basaltic and andesitic assemblages. These characteristics have led some workers to refer to the 

domes as "syntectonic" granites (see, for example, 

Coleman, 1973a, b).

The emplacement of the gneiss domes marks a 

significant point in the geologic evolution of the 

Arabian Shield. Strata younger than the domes may 

be included within the andesitic assemblage but are 

generally only broadly folded, except near or in large 

northwest-trending wrench-fault zones of the Najd 

fault system (Brown and Jackson, 1960; Delfour, 

1970). Metamorphism of post-gneiss-dome units 

does not exceed the greenschist-facies except in contact metamorphic aureoles around younger, lateorogenic or postorogenic granitic plutons. These 

youngest units of the andesitic assemblage commonly contain large amounts of plutonic detritus 

but are included with the assemblage because of the 

dominance of andesitic to dacitic volcanic and volcaniclastic detritus. The more variable distribution 

of flow rocks in the post-gneiss-dome strata compared to the older units suggests that volcanic centers were more widely spaced than before, giving 

rise to broad areas of largely detrital units bounded 

by areas with interbedded flows and clastic strata 

with fewer areas of predominantly volcanic units.

LATE-OROGENIC OR POSTOROGENIC PLUTONS

Volumetrically, the units discussed represent the 

primary constructional phase of the southern part 

of the Arabian Shield. These units were emplaced or 

deposited during a period of active orogenesis, and 

most are deformed and metamorphosed, exhibiting 

isoclinal folding, cataclasis, and local migmatization. 

Early plutonic rocks are less obviously deformed 

than are stratified units, but they are generally 

foliated and commonly highly sheared. Late-orogenic

or postorogenic units, however, show little evidence 

of deformation, or metamorphism, although those 

bodies adjacent to such younger structures as faults 

of the Najd fault system may exhibit local effects. 

Two major groups of plutonic rock units occur as 

late-orogenic or postorogenic bodies: (1) layered 

gabbro and associated rocks and (2) granodiorite 

to granite intrusions.

LAYERED GABBRO AND ASSOCIATED ROCKS

Gabbroic masses occur as mafic phases of the diorite batholiths, as small masses of metagabbro intruding metavolcanic units, and as largely unmetamorphosed, undeformed, subcircular, layered bodies. 

The first two of these groups are associated with 

the active orogenic andesite and diorite magmatism 

of the island arc and probably represent the plutonic 

equivalents or more mafic differentiates of island-arc 

basalts. The layered gabbros are younger than the 

other groups, being intruded after the gneiss domes 

but before the last thermal events affecting the 

Arabian Shield. The layering of these gabbros dips 

inward from the margins, where it is commonly 

steep, and flattens toward the center (Coleman and 

others, 1972, 1973). Compositionally, the layering 

is rhythmic, controlled largely by variations in 

amount of plagioclase, and represents crystallization 

and settling from a subalkaline tholeiitic magma.

GRANODIORITE TO GRANITE INTRUSIONS

Late-orogenic or postorogenic granodiorite to 

granite plutons that yield K-Ar ages between 520 

and 620 m.y. (Fleck and others, 1976) intrude the 

pre-gneiss-dome units and either intrude the youngest andesitic strata or are their plutonic equivalents. 

These plutons occur throughout the Arabian Shield, 

representing the last major shield-forming event, 

which has been equated with the Pan-African event 

or orogeny of the African continent (Fleck and 

others, 1976). The intrusions commonly deflect and 

invade the north-trending shear zones of the shield, 

at only a few places showing small displacements 

produced by late movement. The plutons are affected, 

however, by left-lateral offsets, some of which may 

be large, along northwest-trending fractures of the 

Najd fault system and by small, generally rightlateral displacements having an east-west trend that 

may be conjugate to that of the Najd (Fleck and 

others, 1976). In composition, the plutons range 

from granodiorite to granite and follow a calcalkaline differentiation trend (Greenwood and 

Brown, 1973).
8 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

Although the granodiorite to granite bodies occur 

as batholiths in some areas, especially in the eastern 

part of the Arabian Shield, the more common occurrence is as subcircular ring structures or arcuate, 

lensoidal cone-sheets ernplaced between similarly 

shaped plates of host rock. Many have steeply dipping primary foliations that parallel their margins. 

Some of these bodies are interpreted as subvolcanic 

intrusions, whereas cleavage in others suggests diapiric emplacement. With the exception of volumetrically insignificant mafic and siliceous dikes that may 

cut these plutons, the granodiorite to granite bodies 

represent the youngest plutonic units of the Arabian 

Shield.

NAJD FAULT SYSTEM

Strata younger than the late-orogenic or postorogenic plutons are confined to the northern part 

of the shield and are generally related to downfaulted areas of the Najd fault system (Hadley, 

1974; Delfour, 1970). Although volcanic units occur 

within these strata, the units are dominantly clastic 

sedimentary rocks derived by erosion from adjacent 

fault blocks. The Najd fault system trends northwest-southeast across the entire exposure of the 

Arabian Shield (fig. 1). Brown (1972) suggests a 

composite left-lateral offset of as much as 240 km 

along the system. Only the easternmost part of the 

area studied by us (fig. 3) is within the Najd fault 

system.

PREVIOUS GEOCHRONOLOGIC STUDIES

Most of the previously reported age determinations from the Arabian Shield are either K-Ar analyses or Rb-Sr model ages on minerals. The Rb-Sr ages 

were calculated from assumed initial 87Sr/86Sr ratios. 

These results provide a reasonably accurate view of 

the most recent thermal or tectonic event but generally "telescope" older ages toward that event as 

mineral ages are reset. In this paper the term "apparent age" is used to describe those ages that are 

analytically correct but are not necessarily representative of the time of formation or "true age" of 

the unit studied. "Indicated age" is used in a similar 

sense by some authors. Many workers—Brown and 

others (1963a, b), Bramkamp and others (1963a, b), 

and Jackson and others (1963)—report both K-Ar 

and Rb-Sr apparent ages on 1:500,000 scale maps 

of the shield. Other age determinations are reported 

by Goleman and others (1972), Brown (1970,1972), 

and Lenz and others (1973). Fleck and others 

(1976) studied rocks from the southern part of the

shield in order to investigate the effectiveness of the 

K-Ar technique in age studies of this area.

Because of the obvious inadequacies of K-Ar 

studies in the older units, studies reported here were 

begun using Rb-Sr whole-rock isochrons. Fleck and 

others (1976) conclude that a pervasive orogenic 

event with at least two pulses or thermal maxima 

affected the Arabian Shield between 510 and about 

610 m.y. (B.P.) and correlate this event with the 

Pan-African (Mozambiquian) orogeny or event of 

eastern Africa. Potassium-argon ages of amphiboles 

showed that the diorite-granodiorite (here called 

diorite to trondjhemite) batholiths and the tonalite 

to granite (here called granodiorite) gneiss domes 

were at least 765 m.y. old. Preliminary results of the 

Rb-Sr studies (Fleck and others, 1973) suggested 

ages of 958 ±22 m.y. and 759 ±36 m.y. (A0 = 1.39X 

10- 11 yr- 1 ), respectively, for one of the diorite 

batholiths and one gneiss dome. Preliminary interpretations of results presented in this report are 

cited by Greenwood and others (1976, 1977), who 

suggest a general chronology of orogenesis and 

plate-tectonic evolution of the Arabian Shield from 

before 960 m.y. to 550 m.y. The present report, 

presenting Rb-Sr whole-rock isochrons for the major 

rock groups, modifies that chronology and extends 

the plate-tectonic model to explain the late-stage 

orogenesis of the Pan-African event.

ANALYTICAL TECHNIQUES

Most rock samples collected for Rb-Sr analyses 

were heavier than 10 kg, although locations and access occasionally obviated the collecting of samples 

of this size. Weathered material was removed. In all 

cases the volume of material powdered for analysis 

was greater than that of a cube whose edges were at 

least 10 times the maximum dimension of any contained mineral grain. In no case, however, was the 

weight of material used less than 1 kg. In foliated 

rocks, these dimensions were arbitrarily increased 

such that the maximum width of mineral bands was 

used instead of grain size to determine sample size. 

Most crushed samples averaged 3 kg or more. This 

material was crushed so that the entire amount 

would pass 150-mesh Tyler sieves. This sieve size 

was changed to 200 mesh in the latter part of the 

study. After sieving, the powder was mixed 

thoroughly and an aliquant of approximately 30 g 

was taken for analyses.

Rubidium and strontium concentrations used in 

this study were measured either by mass-spectrometric isotope dilution or by X-ray fluorescence
ANALYTICAL TECHNIQUES

analysis calibrated by standards measured by isotope dilution. For isotope-dilution analyses, standard 

silicate-digestion (HF + HC104 ) and ion-exchange 

techniques were followed. Because of the use of an 

87Rb spike, rubidium and strontium were analyzed 

in separate splits to permit correction of 87Sr/86Sr 

ratios for even very low levels of rubidium. X-ray 

fluorescence analyses were made using the technique 

of Norrish and Chappell (1967), modified slightly 

to correct for short-term instrumental drift. Uncertainties in isotope-dilution analyses were generally 

less than 1 percent, as determined by replicate measurements. The principal uncertainty appears to have 

been sample inhomogeneity, as data for both blank 

runs and spike calibrations indicate errors of less 

than 0.5 percent (one standard deviation). Standard deviations of isotope-ratio measurements were 

generally less than 0.0002, with standard errors of 

the mean of less than 0.0001. Uncertainties in concentrations determined by X-ray fluorescence were 

calculated as the standard deviation of the calibration curve determined on standards analyzed with 

the unknowns, using three to six standards with 

each set of unknowns. In most cases, at least one 

unknown from each data set was analyzed by isotope 

dilution and included in the calibration curve. Standard deviations varied from less than 1 ppm (part per 

million) to about 6 ppm for rubidium (unknowns 

from 1 to 200 ppm) and from about 2 to 15 ppm for 

strontium (unknowns from 10 to 1,200 ppm). Except those for concentrations below 20 ppm, these 

values generally represented uncertainties between 

1 and 4 percent (one standard deviation).

Isochron data were calculated using the leastsquares regression technique of York (1969) with 

provision for correlated errors. No correlation of 

errors was used, however, because none of the 

87Sr/86Sr values exceeded 1.0. Studies of two-error 

regression techniques by Brooks and others (1972) 

found no significant correlation of errors in samples 

with 87Sr/86Sr below 1.0; their recommendation to 

use a correlation coefficient of zero in these cases 

has been followed here. Uncertainties in age and 

strontium initial-ratio reported in this study represent values calculated by the York (1969) regression technique using estimates of analytical errors 

and do not reflect the actual fit of data points to, or 

degree of scatter about, the best-fit line. As shown 

by York (1969), to incorporate a measure "of the 

actual degree of scatter of the data points about the 

best line, the uncertainties reported here are multiplied by the index of fit, (SUMS/(»-2)) % , given 

here simply as "Index." Because Index values are

less than 1.0 in all but three of the isochrons in this 

study, uncertainties reflecting this degree of fit are 

much lower than those reported. As discussed by 

Brooks and others (1972), this Index may be used 

as an indicator of geologic error on the basis of its 

comparison of the observed dispersion of the data 

with that calculated from assigned experimental 

errors. They suggest a cutoff between analytical and 

geologic error at an Index level of 1.58 1 . With the 

exception of those for two rock units, Index values 

obtained in this study are consistently below this value, even when geologic error is implied by other evidence. Because the estimated experimental errors in 

the concentrations of rubidum and strontium are 

based not on replicate isotope-dilution analyses of 

similar samples but on regressions of standards for 

X-ray fluorescence and on only limited numbers of 

strictly similar duplicates, we use the Index only as 

a comparative device, not as a quantitative measure 

of geologic error. As used in this paper, "isochron" 

refers only to the type of diagram or the best-fit line 

for a given set of points and does not imply any 

conclusion regarding presence or absence of geologic error.

The decay constant of 87Rb used here is x,s=1.42x 

1C-" yr-i (Neumann and Huster, 1976). A rubidium abundance ratio 85Rb/87Rb=2.59265 and 

atomic weight of Rb = 85.46776 (Catanzaro and 

others, 1969) were used. Strontium isotope-ratios 

were normalized to the Nier (1938) 86Sr/88Sr value 

of 0.1194. Measurements of 87Sr/86Sr in National 

Bureau of Standards SrC03 987 during the period of 

analyses (n = 69) yield a weighted mean of 0.71024± 

0.00002 (standard error of the mean).

RESULTS OF RUBIDIUM-STRONTIUM ANALYSES

Locations of samples used in total-rock Rb-Sr 

studies reported here are shown in figure 4. Locality 

numbers shown correspond to those in table 1, which 

presents the analytical results. Samples represent all 

of the major geologic units shown in figure 2 with 

the exception of the quartz-porphyry and gabbro 

bodies. Rb-Sr ages obtained from these data (table 

2) are discussed in the context of the generalized 

geologic units (fig. 2). In all cases these ages are 

calculated from whole-rock Rb-Sr analyses. Those

following Brooks and others (1972), most authors, including us, have 

used (SUMS/(n 2)) % as the equivalent of the mean square of weighted 

deviates (MSWD) of Mclntyre and others (1966). As pointed out by 

Roddick (1978), however, the MSWD is actually equivalent to SUMS/ 

(n—2) and not its square root. Values reported here as Index should be 

squared to be compared directly with MSWD and the cutoff value of 2.5 

recommended for that quantity by Brooks and others (1972).
10 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

40°

18° —

16° —

30 MINUTE QUADRANGLES

Jiddoh

A. Bi'r Juqjuq

B. Wadi Sadiyah

C. Wadi al Miyah

D. Jabal Yafikh

E. Al Lith

F. Jabal 'Afaf

G. Jabal Ibrahim

H. Al 'Aqiq

I. Junaynah

J. Al Qarah

K. Jabal Shada

L. Biljurshi

M. Wadi Tarj

N. Wadi Harjab 

P. Wadi Yiba 

Q. An Nimas 

R. Khadrah 

S. Wadi Mali 

T. Jabal 'Aya 

U. Khaybar 

V. KhamisMushayt 

W. Wadi Malahah 

X. Wadi Wassat 

Y. Wadi 'Atf 

Z. AAayza

FIGURE 4.—Location of analyzed samples from the Arabian Shield (unpatterned areas) and of quadrangles. Cross-ruled areas represent areas of Paleozoic strata. Tertiary and Quaternary units are 

not shown.
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 11

TABLE 1.—Analytical results for rocks of the Arabian Shield

L°Na0lity Sample No.

1 __ 71-8-9 A

2_ _ 71-8-9B _ _

3 __ 71-8-9C _

4_ _71-8-9D _____

71-8-9E _ _

71-8-9F _ _

5_. _ 71-8-11K ____

6 __ 71-8-13H __ 

7 _ 71-8-131 _

8 _ 71-8-13J _

9 __ 71-8-14A ____ 

10 __ 71-8-14B ____

11 __ 71-8-15A ____ 

12 __ 71-8-15C ____ 

13 __ 71-8-15D __ 

14 __ 71-8-15E __

15 __ 71-8-18M ____ 

71-8-18N ____ 

71-8-1 8P __

16 __ 724-28J

724-28K _ _

724-28L __

17— _724-28N _____

18 _ 724-28P __

19_ _ 724-29H _ __

724-291 _ _ _

20__ _725-6L

21 __ 725-6M _ _

725-6N _ __

22 __ 725-6P __ _

23_ _725-6Q _ _

24_ _ 725-7C

25 _ _725-7D _ _

26 -725-7E _ _ _

27__ 725-7F __ _

28_ _ 725-8N

29. _742-15A ____

742-1 5B __ _

30 __ 742-15C __ _

31 __ 742-15D _

32 _ 742-19R __ _

742-19S

742-1 9T _ __

33 __ 742-19U _ __

742-1 9V __

742-1 9 W ____ 

34 __ 742-20 A _ __

742-20B _ _

35_ _742-20C _____

742-20D _ _

742-20E _ __

742-20F

742-20G

36 __ 742-201 _ __

742-20J _ _ _

742-20K __ _

742-20L _____

37 _ 742-21F

742-21 G __

742-21H __ _

38 _ 742-21A __ _

742-21 B _ __

40 __742-22A _____

742-22B _ __

742-22C

742-22D

742-22E _ _

41 __ 742-22F __ _

North 

lat

18°18.6'

18°20.5'

18°23.5'

18°26.7'

18°26.7'

18°26.7'

18°40.7'

19°55.0' 

19°51.0'

19°50.0'

19°50.5' 

20°25.5'

19°48.0' 

19°44.5' 

20°07.0' 

20°07.5'

20°48.6' 

20°48.6' 

20°48.6'

20°08.2'

20° 08.2' 20°08.2'

20°03.6'

20°02.7'

19°46.5'

19°46.5'

18° 52.0' 18°43.7'

18°43.7'

18°48.2'

18°44.0'

18°57.9'

18°58.5'

18°49.0'

18°45.6'

18°56.0'

21°31.3'

21°31.3'

21°31.1'

21°30.7'

17°40.8'

17°40.8'

17°40.8'

17°41.2'

17°41.2'

17°41.2' 

17°36.2'

17° 36.2' 17°36.3'

17°36.3'

17°36.3'

17°36.3'

17°36.3'

18°23'

18°23'

18°23'

18°23'

18°22'

18°22'

18°22'

18°05'

18°05'

19°01'

19°01'

19°01'

19°01'

19°01'

19°00'

East 

long

42° 52.1' 42°53.4'

4.OOCK q/

40040 1 '

42°40.1'

42° 40.1'

42°39.5'

41°37.0' 

41°36.0'

41°34.5'

41°19.0' 

41°08.5'

41° 52.5' 

41°52.5' 

41°55.0' 

41°54.0'

43°48.9' 

43°48.9' 

43° 48.9'

42°51.0'

42°51.0'

42°51.0'

42° 54.3'

42° 54.2'

42° 55.4'

400CC 4'

42°19.0'

4OOOQ 1 '

42°09.1'

42° 07.1'

42° 02.0'

4OOA2 7'

42°03.4'

4O001 O'

42°01.6'

41°59.0'

39°16.0'

39°16.0'

39°16.0'

39°16.3'

43°45.1'

43°45.1'

43°45.1'

4Qo 4 r n,

43°45.0'

43°45.0' 

43°49 1'

43°4Q 1 '

43049 Q'

43°4Q 0'

43°49.0'

43°49.0'

4Q<MQ 0'

44°04'

44°04'

44°04'

44° 04'

44°00'

44°0ifl'

44°0fl'

44° 05'

44° 05'

42°13'

42°13'

42°13'

42°13'

42°13'

42°13'

Rb 

(ppm)

345.7

1 f^O Q

156.1

107.6

112.8

113.7

294.6

39.9 

17.6

58.5

163.0 

131.9

123.2 

24.68 

31.71 

49.22

57.00 

60.95 

52.05

20.8

23.1

3.7

54.2

41.5

60.2

Q2 1

202

150

160

131

56.1

34.1

33.2

53.7

21 fi

35.0

66.6

62.3

68.6

66.9

12.5

129

22.1

2fi 7

22.9

28.7 

38.6

Q4 2

P;I Q

40.7

45.6

55.9

94.6

25.5

35.9

26.2

16.4

44 1

31.5

37.2

29.5

18.4

60.2

CO C

63.6

57.3

69.8

64.4

Sr 

(ppm)

253.5

34Q Q

283.6

163.1

71.17

CO O^

235.4

377

QC.8

345

85.70 

222.4

209.3 

493.5 

218.3 

52.94

121.0 

51.02 

75.19

24s;

187

208

692

680

534

545

101

116

28.4

396

388

167

370

qoc

366

415

214

239

226

214

114

136

150

212

180

138 

570

157

304

602

396

379

207

526

562

704

626

530

615

767

496

537

62.5

62.9

96.8

61.9

92.0

132

s'Rb/ssSr

q QKQ

1 24Q

1.594

1 Q11

4.603

6.608

3.631

.3058 

.1417

.4838

5.528 

1.718

1.705 

.1447 

.4131 

2.6955

1.364 

3.466 

2.006

.2450

.3584

.0518

.2266

.1766

.3262

.4887

5.830

3.745

16.50

QK7Q

.4178

.5903

.2600

.4036

.1723

.2438

.9017

.7532

.8802

.9035

.3169

2fi(VQ

.4282

.3648

.3683

.6013 

.1958

1.736

.4936

.1955

.3332

.4261

1.324

.1403

.1844

.1075

.0760

.2405

.1482

.1401

.1715

.0989

2.797

2.746

1.902

2.686

2.200

1.410

87Sr/s«Sr

0.73905

.71487

71 788

.72153

7484fi

.76607

.73467

.70652 

.70442

.70877

.75365 

.71930

.71888 

.70485 

.70709 

.72859

.71711 

.73383 

.72244

.70528

.70656

.70312

.70486

.70439

.70587

.70760

.75419

.73651

.84947

.71188

.70773

.70960

.70600

.70783

.70548

.70583

.71251

.71081

.71219

.71237

.70615

.70618

.70761

.70710

.70717

.70908 

.70489

.71905

.70719

.70472

.70586

.70678

.71493

.70427

.70467

.70394

.70378

.70521

.70412

.70402

.70455

.70387

.73433

.73318

.72477

.73335

.72724

.71638

Rock type

Granodiorite-quartz monzonite

Do.

Do.

Garnet-bearing granite

Do.

Do.

Quartz monzonite

Quartz diorite 

Do.

Do.

Biotite, muscovite granite 

Do.

Quartz monzonite 

Do. 

Perthitic quartz monzonite 

Do.

Rhyolite tuff 

Do. 

Do.

Andesite tuff

Do.

Do.

Quartz diorite

Do.

Do.

Do.

Quartz monzonite

Do.

Granite

Granodiorite

Granodiorite gneiss

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Metadacite

Do.

Do.

Do.

Do.

Do. 

Quartz diorite

Granodiorite

Do.

Quartz diorite

Granodiorite

Do.

Do.

Quartz diorite

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Granodiorite gneiss

Do.

Do.

Do.

Do.

Do.
12 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

TABLE 1.—Analytical results for rocks of the Arabian Shield—Continued

Lo£a0Hty Sample No.

42 __ 742-23F

742-23G

742-23H

742-231 _

43 — 742-24C

742-24D

742-24E

742-24F __ .

742-24G

742-24H _ _

742-241 _ _ .

44 __ 742-24 J __ .

742-24K

742-24L

742-24M

45- _ 742-25 J _

46 742-25K

742-25L

47_ —742-25M

742-25N _ .

742-25P

742-25Q

742-25R

742-25 S

742-25T

742-25U

48_ _ 742-26A _ _

742-26B

49 _ 742-26C

742-26D

742-26E

50 742-26F

742-26G _ _

51 _ 742-26M __

742-26N

742-26P

742-26Q

742-26R _ _

52 __ 742-2 7D

742-27E _

742-27F

53 _ 742-271 _

742-27J _

742-27K

54 742-27L

742-27M

55_ 742-27N

742-27P _ __

742-27Q

742-27R

742-27S

56 _ 742-28A

742-28B _

742-28C

57 _ 742-28D _ __

742-28E

58 742-28F

742-28G

742-28H _ _

59 _ 742-281 _

742-28J

742-28K

60 • 742-28L

742-28M

61- 742-28N _ .

742-28P

North 

lat

. 19°03.0'

. 19°03.0'

. 19°03.0'

. 19°07'

. 19°15'

1 Q°1 £,'

1Q°1 V

. 19°15'

1Q°1 V

19°15'

, 19°15'

. 19°08'

. 19°08' iQ°os'

. 19°08'

20°40.7'

20°40.1'

. 20°40.1'

20°38.4'

20°38.4'

. 20°38.4' 20°38.4'

20°38.4'

20°38.4'

20°38.4'

20°38.4'

20°16.7'

20°16.7'

9ft°1 7 Q'

20°17.9'

20°17.9'

20°17.2'

20°17.2'

20°08.9'

20° 08.9' 20°08.9'

20°08.9'

20°08.9'

21°04.0'

21°04.0'

21°04.0'

91 °9<^ 9'

21°25.2'

91 °9<^ 9'

. 21°23.7'

. 21°23.7'

21°23.5'

21°23.5'

21°23.5'

21°23.5'

21°23.5'

2l°142'

91 °1 A 9'

. 21-14.2'

. 21°16.8'

. 21°16.8'

. 21°16.5'

91 °1 fi fi'

. 21°16.5'

. 21°28.1'

. 21°28.1'

. 21°28.1'

91 °9Q fi'

. 21°29.6'

. 21°19.5'

. 21°19.5'

East 

long

49 ° 1 9 0 '

42°12.0'

42°12.0'

49°OQ'

49°0<V

49°OQ'

49°0<V

42°09'

49°OQ'

49°OQ'

42°09'

42°58'

42°58'

42°58'

42°58'

42° 42.9' 42°43.2'

49°4Q o/

42°37.2'

42°37.2'

42°37.2'

42°37.2'

42°37.2'

42°37.2'

42°37.2'

42°37.2'

42°57.2'

42°57.2'

43°01.1'

43 0 01.1'

43°01.1'

43°05.0'

43°05.0'

43°08.5'

43°08.5'

43°08.5'

43°08.5'

43°08.5'

43°53.6'

43°53.6'

43° 53.9'

43°43.5'

43°43.5'

43°43.5'

43°37.9'

43°37.9'

43°37.6'

43°37.6'

43°37.6'

43°37.6'

43°37.6'

43°56.9'

/iq°C£ Q'

43°56.9'

43°54.9'

43°54.9'

43°59.4'

43°59.4'

43°59.4'

43°58.6'

43°58.6'

43°58.6'

43°52.5'

43°52.5'

43°50.6'

43°50.6'

Rb 

(ppm)

46.6

36.2

1.1

qc A

KA Q

14.0

34.1

36.8

12.9

8.5

22.2

21.4

Cft C

Cft C

99 0

1 1 S

1 x>

9 °.

1.0

6.2

1.1

20.8

11.7

8.8

fiQ 7

Q7 fi

1 1 C

65.0

7C o

107

OC A

94. 9

82.2

1Q 1

74.8

81

82

92

14

3Q

40.8

87.0

90.6

36.6

13.0

13.0

32.2

42.4

QO 4

89.7

93.7

107

94.2

6.8

1 C 7

11.7

147

1 ^7

148

110

112

108

110

Sr 

(ppm)

qoo

499

AQQ

365

170

OOQ

OftQ

q/ic

£CO

409

occ

i an

KC 7

CO ft

1 OQ

149

165

141

43.4

CO C

46.1

41.0

46.7

97ft

007

OqO

4.01

000

970

9&A

340

262

7Q4

886

316

764

304

qi o

qco

OftQ

650

390

con

82.1

qc o

698

358

358

465

filQ

qq/(

qqQ

350

9Qfi

346

77°.

726

662

01 O

332

306

338

322

1Q£.

177

^Rb/80Sr

0.4097

947Q

.0063

.2807

qqco

1 77Q

4791

.3085

.0565

.0601

.2521

.3704

9 4Q4

2 81 S

.5837

2.312

2.066

2.481

1 (V94

1999

.0607

.4347

.0695

.2224

1 ft°.^

.1090

.5030

Q43Q

1 9°.c.

.6048

1.505

.6404

1.182

.0925

.0790

.7537

.0723

.7119

.7500

.6743

1.2814

.0648

.2899

.1904

3.073

2.758

.1516

.1050

.1200

.2000

.1979

.7823

.7667

.7752

1.044

.7865

.0254

.0625

.0511

1.364

1.371

1.400

.9427

1.045

1.603

1.808

87Sr/8(iSr

0.70765

.70559

.70282

.70616

.71142

.70477

7fl7QQ

.70662

.70320

.70358

.70575

.70623

79Q9^

.73438

.70839

.72418

.72176

794.Q9

.70463

7040,7

.70483

.70806

.70445

.70508

.70439

.70435

.70769

71 1 £,<}

.71435

.70866

.71665

.70903

.71347

.70359

.70380

.70945

.70347

.70908

.70997

.70911

.71572

.70397

.70596

.70508

.73137

.72862

.70449

.70412

.70414

.70558

.70556

.71019

.71024

.71003

.71222

.71007

.70352

.70380

.70370

.71619

.71553

.71540

.71157

.71213

.71764

.71949

Rock type

Quartz diorite

Do.

Do.

Do.

Meta-andesite or dacite

Do.

Do.

Do.

Do.

Do.

Do.

IWiO^Q €1 Tl f\ O'Sll'l'A

Do.

Do.

Do.

Do.

Do.

IWo^SI 51 Ylf\ AQl't'.f*

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do. '

Do.

Do.

Do.

Quartz diorite

Do.

Aplite

Quartz diorite

Aplite

Rhyolite ash-flow

Do.

Do.

Pyroxene andesite

Do.

Rhyolite ash-flow

Do.

Basaltic andesite

Basaltic andesite

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Granodiorite

Do.

Do.
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 13

TABLE 1.—Analytical results for rocks of the Arabian Shield—Continued

Lo£ao1Hy Sample No.

62 743-3B _

743-3C _ _ _

743-3D

63 _ -743-3E _ _ _

743-3F ___ .

743-3G ___ _

64 _ 743-3H _ __

743-31 _

743-3J _ __ _

65_ -743-4G _

743-4H ___ _

743-41 _

743-4 J

66 _ 743-4K

743-4L _ _ _

743-4M

743-4N

67 _ 743-6A _ _

68 743-6B _

69 -743-6C

70 _ 743-6D _

71 743-6E

743-6F

72 _ -743-6G

73 _ .7411-9A _

74 _ 7411-9B

75 __ 7411-9C

76_ J-653 __

77 __ J-659 __

78 J-667 _

J-669 _ _

79 __ J-606

80 -J-625

81__ J-632 —

82 __ 725-9A

83_ _ 725-9B _ _

85_ 742-26H _ _

742-261 _ _

86 _ 742-26J _ _

87 _ 742-26K _ _

88 7411-7A _ _

89 7411-7B

7411-7C

7411-7D

90_ _7411-7E _ _

7411-7F

7411-7G

7411-7H

91 741 1-8 A

7411-8B

92 _7411-8C

7411-8D

93 __7411-8H _

7411-81 _ _

94 -7411-8J _

North 

lat

. 20°30.9' 20°30.9'

20°30.9'

20°29.5'

20°29.5'

20°29.5'

20°22.6'

20°22.6'

20°22.6'

20°20.6'

20°20.6'

20°20.6'

20°20.6'

20°19.0'

20°19.0'

20°19.0'

9n°iq 0'

1Q°4fi 8'

19°44 0'

19°43.7'

19°43.3'

19°43.3'

19°43.0'

17°31.0'

17°30.8'

17°30.9'

19°36.0'

19°33.0'

19°31.0'

19°31.0'

19°54.4'

19°51.3'

. 19°52.6'

19°29.0'

19°23.6'

20°20.1'

20°20.1'

20°19.8'

20°05.6'

18°21.7'

18°24.3'

18°24.3'

18°24.3'

18°23.3'

18°23.3'

18°23.3'

18°23.3'

18°21.9'

18°21.9'

18°28.3'

18°28.3'

18°08.2'

18°08.2'

18°07.0'

East 

long

40°08.5'

40°08.5'

40°08.5'

40°20.6'

40°90 fi' 40°20.6'

40°15 9'

40°15.9'

40°15 9'

40°56.2'

40°56.2'

40°56.2'

40°52.3'

40°52.3'

42°17.4'

40010 4/

42°19.4'

4901 q o>

42°21.5'

49001 c,

42°21.8"

43°03.5'

43°04.1'

43°04.0'

41°58.5'

41°57.6'

41°59.3'

41°59.3'

41 °34 9' 41°34.6'

41°34.8'

41° 37.0' 41°35.3'

43°10.0'

43°10.0'

43°08.5'

43°04.0'

43°59.5'

43°45.6'

43°45.6'

43°45.6'

43°46.8'

43°46.8'

43°46.8'

43°46.8'

43°42.3'

43°42.3'

43° 44.8'

4.3044 8' 43°38.5'

43°38.5'

43°37.8'

Rb 

(ppm)

OQ C

9fi 4

18.4

16.3

10.1

137

4 89

0 49

19 Q

OK O

1 4 Q

15.4

11.7

99 a

44.5

38.2

43 3

4.87

188

98 fl

22.8

39.9

n

o

17.3

1 7Q

62.8

18.5

18.1

19.3

6.6

16.3

11.5

1 Q 0

26.2

23.6

91 7

91 4

13.1

52.1

33.9

18.2

54.4

39.8

93 O

33.4

26.9

1Q 7

72.7

74.3

43.3

24.4

3.02

2.72

2.8

Sr 

(ppm)

c.r\r\

ri o

609

4Q9

548

416

347

516

620

591

512

584

458

409

433

510

408

rqq

rqq

448

701

744

Q14

286

411

400

43Q

roq

370

381

3Q4

376

407

703

798

Q4S

867

1214

858

440

1019

821

862

Q7Q

518

508

723

509

636

1294

1048

«Rb/«Sr

0.16^6

1 4QO

O879

AQI o

.0593

0255

.0030

.1077

1 Q73

.0694

.0656

1 1 98

981 9

.2754

OSQ3

097fi

1 39Q

1 O CC

1 99^

9^78

.0486

.0674

3 Qf\3

1 QSfi

1 91Q

.1290

1 9Qfi

.1258

.0316

.1277

.0874

.1395

.2014

.1686

.0891

.0776

.0401

.2655

1 1 31

.0433

.1834

.2622

.0653

.1177

.0901

.0581

.4062

.4236

1 739

.1388

01 38

.0061

.0077

"Sr/^Sr

0.70469

.70448

.70344

70384

70337

.70346

7033^

709Q9

.70468

.70471

70497

7(1380

.70487

.70678

rrn/jQO

7fiqoo

.70448

.70443

70449

.70571

.70333

.70348

.74638

704Q9

.71404

.70420

.70417

.70407

.70312

.70450

.70401

.70464

.70552

.70490

.70387

.70327

.70329

.70547

703Q4

.70314

.70465

.70539

.70351

.70395

.70375

.70347

.70684

.70701

.70457

70451

.70232

.70242

.70256

Rock type

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Quartz diorite

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Quartz diorite

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Do.

Granodiorite

Do.

Do.

Quartz diorite

Granodiorite

Do.

Do.

Do.

Do.

Trondhjemite
14 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

TABLE 2.—Summary of Rb-Sr ages and isochron data

[See table 1 for listing of localities]

Rock unit Locality

No.

Fig. 

No.

Number Mean 

of points Rb/Sr

Apparent 

Age (m.y.) (Index)

Basaltic assemblage:

Basalt of Wadi al Faqh ______ 64 53 0.016

Andesite assemblage:

Rhyolite of the Murdama Group _ 15 10 3 .786 

Meta-andesite, Junaynah

quadrangle ________—__ 16 63 .076 

Metavolcanic rocks of Wadi

Shuklalah _____________ 32,33 9 6 .135 

Metavolcanic rocks of Wadi bin

Dwaynah 1 ____________ 43 84 .070 

Khadrah Formation metavolcanic

rocks _________________ 44 74 .569 

Metavolcanic rocks of Hishat al

Hawi _______________ 47 94 .062 

Arphan Formation of Hadley

(1976) _______________ 52 73 .312 

Arphan Formation of Hadley

(1976) _______________ 55 62 .047 

Juqjuq Formation of Hadley

(1976) _______________ 53 11 3 .063 

Rhyolite of the Bi'r Juqjuq

quadrangle ____________ 54 72 1.005

Diorite to trondhjemite batholiths,

Al Lith and Biljurshi areas: 

Quartz diorite of Biljurshi

(set A) _______________ 6-8 12 3 .107 

Quartz diorite of Biljurshi

(set B) _______________ 79-81 12 3 .041 

Quartz diorite of Wadi Khadrah __ 62-63 12 6 .035 

Quartz diorite of Wadi ash

Shaqah ash Shamiyah _____ 65-66 12 8 .059 

Quartz diorite of Wadi Qanunah 2 _ 82-83 12 2 .064

Diorite to trondhjemite batholiths,

An Nimas batholith:

Quartz diorite of An Nimas ___ 42 13 4 .082 

Trondhjemite of Wadi Asmak __ 67, 68 13 2 .028 

Quartz diorite of Wadi Tarj ___ 69, 71, 72 13 4 .044 

Quartz diorite of Al Mushirah _ 76-78 13 4 .036

Diorite, trondhjemite, and granodiorite

batholiths, central and eastern

areas: 

Quartz diorite of Jabal Umm al

Hashiyah _____________ 17-19 15 4 .105 

Quartz diorite of the Al Qarah

quadrangle ____________ 51,87 15 4 .044 

Hornblende diorite of Hadley

(1976) _______________ 58 24 3 .016 

Quartz diorite of Wadi Makhdhul 37 14 3 .061 

Quartz diorite of the Malahah

Dome _________________ 89 16 2 .053 

Granodiorite of Wadi Malahah _ 36, 88-92 16 13 .057 

Quartz diorite of Simlal _____ 38 14 2 .047

Granodiorite gneiss-domes

Granodiorite gneiss of Jabal

Mina ________________ 40 17 5 .851 

Granitic gneiss of Wadi Bishah _ 4 19 3 1.506 

Granodiorite gneiss of Wadi

Bagarah ______________ 23-28 18 6 .120 

Granitic gneiss of Jiddah Airport _ 29-31 18 4 .297 

Granitic gneiss of Harisi Dome _ 73-74 19 2 .716

Late-orogenic, or postorogenic,

granodiorite to granite plutons: 

Tindahah batholith of Coleman

(1973a) _______________ 1-3 20 3 .782 

Bani Thuwr pluton of Coleman

(1973b) 3 _______________ 5 20 1 (1.251) 

Granite of Jabal Shada 3 _____ 9 20 1 (1.902) 

Granite of Jabal Ibrahim 3 ____ 10 20 1 (.593) 

Quartz monzonite of Wadi

Shuwas _______________ 11-14 20 4 .428

1,165±110 0.7029±0.0001

568± 29 

785± 96 

593± 53 

912± 76 

746± 16 

660± 43 

761± 23 

775(T) 

620± 95 

612 (T)

848±282 

895±173

853± 72 

(846)

837± 50 

838(T) 

818± 95

747±178

723±107 

724± 93

522±429 

843 ±273

720(T) 

684± 43 

815(T)

746±114 

664± 9

763± 53 

763±159 

773(T)

(608) 

(640) 

(657)

636± 21

.7061 ± .0007 

.7025± .0003 

.7040± .0003 

.7024± .0002 

.7021 ± .0002 

.7039±. 0001 

.7018± .0003 

.7028(T) 

.7034± .0003 

.7045 (T)

890± 67 .7026± .0003

.7030 ± .0005 

.7025 ± .0003

.7030± .0002 

(.7028)

.7027± .0002 

.7029(T) 

.7028± .0002 

.7028± .0003

.7025± .0005 

.7027± .0002

.7033± .0003 

.7023± .0006

.7027 (T) 

.7029± .0001 

.7027 (T)

.7043± .0036 

.7035± .0007

.7033 ± .0003 

.7026± .0019 

.7027(T)

626± 17 .7037± .0004

(.7032) 

(.7032) 

(.7032)

.7035± .0003

0.28

.47 

.02 

.15 

.40 

.38 

1.67 

.15

.01

.05

.01 

.81

1.61

.48

~42 

.22

.10 

.42

.07 

.01

".14

.40 

1.01

.90 

.25

.16

.43
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 

TABLE 2.—Summary of Rb-Sr ages and isochron data—Continued

15

Rock unit

Late-orogenic, or postorogenic, 

granodiorite to granite 

plutons — Continued 

Quartz monzonite of Jabal Qal _ _ 

Granodiorite of Wadi Halal __

Granodiorite of Wadi al Miyah _ 

Granodiorite of Wadi Musayrah _ 

Aplite dike, southwestern Al Qarah 

quadrangle __ _ _

Gneiss granodiorite; Bi'r Juqjuq 

quadrangle 2 __ _ _

Quartz monzonite, Bi'r Juqjuq 

quadrangle 2 _ _ _

Quartz monzonite of Jabal 

Tarban _ _ _ _ _ _ _

Locality 

No.

21-22 

34-35

45-46 

48-50

51

56-57

59

61

Fig.

No.

21 

22

23 

23

23

25

25

25

Number 

of points

3

7

3

7

0

5

3

2

Mean 

Rb/Sr

2.429

000

.789 

.326

.253

.287

.476

.589

Apparent 

Age ( m.y. )

620± 18 

643 ± 20

587± 99 

623± 18

621 (T)

(620)

(636)

635(T)

(*Sr/seSr)o < SU]

0.7034+0.0005 

.7028+ .0002

.7045± .0030 

.7033± .0003

.7028(T)

(.7032)

(.7032)

.7031(T)

T/nfc^*

0.03 

.52

.77 

.42

1 Only data points for older isochron summarized here.

2 Data have insufficient spread in Rb/Sr. Age reported represents model age, using weighted mean of all points and assumed initial 87Sr/S8Sr.

3 Data are for single point. Age reported represents model age using assumed initial ^Sr/^Sr.

ages in table 2 with cited uncertainties were calculated by least-squares regression. Ages and initial 

ratios followed by (T) are 2-point "isochrons" and 

no uncertainties are quoted because their validity 

cannot be evaluated. Ages shown in parentheses are 

"model ages," calculated for single samples by assuming the initial 87Sr/86Sr ratio shown in parentheses. The assumed values are the mean initial 

87Sr/86Sr ratios (here designated rt for convenience) 

obtained from isochrons of similar rock units. Because of the high degree of uniformity in r* values 

of a given rock type (table 2), the uncertainty attached to this assumption is probably only slightly 

greater than estimates cited for many of the isochrons themselves (c.a. <0.0005 at one standard 

deviation). As in the case of 2-point "isochrons," 

uncertainties are not assigned. These data not only 

provide a chronology for the evolution of the Arabian Shield but also support a plate-tectonic model 

for that evolution.

BASALTIC ASSEMBLAGE

Although stratified rocks of the basaltic assemblage occur over large areas of the southern part of 

the shield, only one locality (loc. 64, fig. 4) has been 

sampled sufficiently to yield a total-rock Rb-Sr isochron. Basaltic flow rocks (now in greenschist 

facies) from this locality, near Wadi al Faqh, were 

very low in rubidium, averaging about 6 ppm. For 

this reason, all analyses of rubidium and strontium 

were done by isotope dilution. The isochron diagram 

for the three basalts (fig. 5) indicates an age of 

1165 ±110 m.y. and rt of 0.7029 ±0.0001. The calculated Index of 0.28 suggests that dispersion of the 

points is easily within analytical uncertainty.

Rubidium concentrations of the basalts of Wadi al 

Faqh are similar to those of tholeiitic basalts from 

either island-arc or ocean-floor environments but are 

significantly less than those of alkali basalts (Hart 

and others, 1970). Strontium concentrations, however, are significantly greater than those of oceanfloor basalts and comparable to those of island arcs 

such as New Britain or Japan. Similar conclusions 

regarding magma genesis were drawn by Greenwood

0.705

0.704

0)

0.703

Basalt of Wadi al Faqh 

(loc. 64)

31

3H

-0.7029 ±0.0001 

I I I

Index =0.28

0.04

Rb

0.08 

6 Sr

0.12

FIGURE 5.—Total-rock Rb-Sr isochron diagram of the basalt 

of Wadi al Faqh. Sample location is shown on figure 4 and 

in tables 1 and 2.
16 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

and others (1976) from major-element concentrations in rocks of the basaltic assemblage and by 

Greenwood and others (1977) from trace-element 

concentrations.

ANDESITIC ASSEMBLAGE

Age determinations of volcanic rocks of the andesitic assemblage (table 2) exhibit significant variation. Geologic controls provided by K-Ar minimum 

ages or Rb-Sr ages of plutonic bodies that intrude 

the volcanic units indicate that some of the apparent 

ages of fine-grained volcanic strata have been reset 

by low-temperature events. Five of the units sampled, however, yield ages that are consistent with 

geologic constraints and are interpreted as times of 

extrusion.

UNDISTURBED ISOTOPIC SYSTEMS

Isochron diagrams of data from andesitic strata 

that reflect closed isotopic systems, shown in figures 

6 and 7, yield ages between 746 and 785 m.y. for 

four of the five groups. Samples of the Arphan 

Formation of Hadley (1976) (Halaban Group) from 

two different localities (Iocs. 52, 55; tables 1 and 2; 

fig. 4) yield ages of 761 ±23 m.y. and 775 m.y., although the latter is a two-point "isochron." Metaandesites from the Junaynah (loc. 16) and Khadrah 

(loc. 44) quadrangles yield ages of 785 ±96 m.y. and 

746 ±16 m.y., respectively, with excellent indices of 

fit (table 2). The generally low initial ratios of these 

groups (0.7018 and 0.7028) support the interpretation of these ages as times of eruptions, although 

the low Rb/Sr ratios of some of the samples would

0.706

0.704

0.702

•0.7028(T)

-0.702510.0003

EXPLANATION

O Arphan Formation of

Hadley (1976) (loc. 55)

O Meta-andesite of the

Junaynah quadrangle 

(loc. 16)

Index = 0.02

0.1 0.2 

87 Rb / "Sr

0.3 0.4

FIGURE 6.—Total-rock Rb-Sr isochron diagram of the Arphan 

Formation of Hadley (1976) and of the meta-andesite 

of the Junaynah quadrangle. Sample locations correspond 

to those in figure 4 and table 1.

0.730 -

0.720 -

0.710 -

0.700

O Rhyolite of the Bi'r Juqjuq 

quadrangle (loc. 54)

O Arphan Formation of 

Hadley (1976) (loc. 52 

isochron not shown)

O Khadrah Formation 

(loc. 44)

0.7021 ±0.0002

4.0

FIGURE 7.—Total-rock Rb-Sr isochron diagram of the Khadrah 

Formation metavolcanic rocks (Greenwood, 1979b), rhyolite of the Bi'r Juqjuq quadrangle, and the Arphan Formation of Hadley (1976). Where isochrons are so nearly simultaneous as to add confusion to the diagram, one or more of 

them may have been omitted. Data for all isochrons are 

shown in table 2. Sample locations are shown in figure 4 

and listed in table 1.

still permit an interpretation of isotopic homogenization of slightly older volcanic rocks as an undeniable 

alternative. Even if the calculated ages were reset, 

however, the data do not permit the times of eruption to be much older.

The youngest andesitic unit studied, discussed 

here as the rhyolite of the Bi'r Juqjuq quadrangle 

(loc. 54), was mapped as the youngest unit of the 

Arphan Formation of Hadley (1976) and considered 

to be part of the Halaban Group (Hadley, 1976). 

The apparent age of the strata, as determined from 

only two points, is 612 m.y., however, considerably 

younger than the andesitic units upon which it rests 

(Iocs. 52 and 55; figs. 6, 7). Hadley (1976) mapped 

sills of a similar rhyolite porphyry that intrude the 

older units nearby; this relationship suggests that 

the rhyolite may be a late-orogenic or postorogenic 

shallow intrusive or ash flow related to the sills. Although the high rt of 0.7045 for this "isochron" (fig. 

7) would permit interpretation of the age as having 

been reset by metamorphism, the high Rb/Sr ratio 

of the rocks would require an rt below meteorite 

values (less than 0.699) in order for the rhyolite to 

have been contemporaneous with the other units of 

the Arphan Formation studied (ca. 770 m.y.). We 

now consider this rhyolite to be a younger, post-
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 17

metamorphic unit, possibly equivalent to the Shammer Rhyolite of Brown and Jackson (1960) or to the 

Shammer Formation of Delfour (1970).

DISTURBED ISOTOPIC SYSTEMS

Units of the andesitic assemblage discussed although folded and faulted are less deformed than 

many sequences of andesitic strata within the Arabian Shield. The discussed localities are confined to 

the central and eastern parts of the area studied and 

probably represent younger parts of the assemblage 

(Halaban Group). Older units of the assemblage, 

however, are exposed to the west, such as in Wadi 

bin Dwaynah, where metavolcanic rocks mapped as 

Jiddah Group by Greenwood (1979a) are part of a 

large roof pendant in the An Nimas batholith. This 

batholith is one of the older quartz diorite to 

trondhjemite batholiths of the southern part of the 

shield and predates much of the andesitic assemblage (see fig. 2). A Rb-Sr isochron diagram 

(fig. 8) of all samples from the Wadi bin Dwaynah 

locality (loc. 43, fig. 4) suggests open-system behavior in some samples. The data points define two intersecting linear arrays that yield apparent ages of 

912±76 m.y. (rt = 0.7024±0.0002) and 641±28 m.y. 

(n = 0.7030±0.0002). All seven samples fit one array 

or the other, with Index Values of 0.40 and 0.42 for 

the older and younger groups, respectively. As a 

whole, however, the data set yields a composite age 

of 669 ±25 m.y., r, of 0.7030 ±0.0001, and an Index 

of 1.67. The Index is equal to the highest obtained 

in this study and exceeds the cutoff of 1.58, suggesting geologic error. Where it intrudes these units of 

the andesitic assemblage near Wadi bin Dwaynah, 

the An Nimas batholith yields an age of 837 ±50 

m.y., a minimum age for the metavolcanic rocks. 

The apparent age obtained from the composite of all 

Wadi bin Dwaynah samples is significantly younger 

than the age of the batholith at all confidence levels 

up to 99.7 percent, and the strontium isotopic system of the metavolcanic rocks is considered disturbed.'The 912-m.y. apparent age obtained from 

one grouping of samples would be consistent with 

the 837 ± 50 m.y. age of the batholith, but this grouping cannot be justified by any independent criteria. 

We suggest that the 912 ±76 m.y. age may represent 

the time of extrusion of the strata, but we are aware 

of the tenuous nature of this conclusion.

Fine-grained metavolcanic rocks sampled near 

Hishat al Hawi (loc. 47, fig. 4) yield an elevated 

Index of fit and young apparent age of 660 ± 43 m.y. 

(fig. 9). As do the Indices for the Wadi bin Dwaynah 

rocks, the Index of 1.67 itself indicates geologic

error; the clearly open-system behavior of some of 

the Wadi bin Dwaynah rocks that yield an identical 

age and index of fit leaves open the possibility of 

partial or complete isotopic homogenization of 

strontium in the Hishat al Hawi strata as well. The 

rt of these rocks of 0.7039 ±0.0001 is high relative to 

other andesitic units but is not sufficiently elevated 

to indicate unambiguously a resetting of isotopic 

systems. Because the strata are correlated with the

0.712

0.710

0.708

0.706

0.704

0.702

0.700

Metavolcanic rocks of

Wadi bin Dwaynah

(loc. 43)

Index (912) =0.40 

Index (641) = 0.42

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

•TRb/»»Sr

FIGURE 8.—Total-rock Rb-Sr isochron diagram of the metavolcanic rocks of Wadi bin Dwaynah. See figure 7 for explanation of isochron data.

I I

0.710 —

0.704

0.703

-0.7040 ±0.0003

S 0.7039 ±0.0001 

I I

EXPLANATION

Metovolconic rocks of Hishot al 1 

Hawi (lac. 47) 

Index = 1.67

\/ Metavolcanic rocks of Wadi 

Shuklalah (Iocs. 32, 33)

Index-0.15

I I__ I

'0.1 0.2 0.3 0.4 0.5

87 Rb / 8BSr

0.6 0.7

FIGURE 9.—Total-rock Rb-Sr isochron diagram of metavolcanic rocks of Hishat al Hawi and Wadi Shuklalah. See 

figure 7 for explanation of isochron data.
18 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

meta-andesite of the Junaynah quadrangle (loc. 16), 

which yields an age of 785 ±96 m.y., however, we 

conclude that the 660-m.y. age does not represent 

the time of extrusion of the metavolcanic rocks of 

Hishat al Hawi.

Several other sequences of fine-grained metavolcanic strata yield isochrons having acceptable indices 

of fit (Brooks and others, 1972) that probably reflect the time of metamorphism rather than of extrusion. The metavolcanic rocks of Wadi Shuklalah 

(Iocs. 32, 33, fig. 4), plotted in figure 9, are of this 

type. These strata, found in the southeastern part of 

the shield, are deformed and metamorphosed by a 

quartz diorite to granodiorite body (granodiorite of 

Wadi Halal) that yields an age of 643 ±20 m.y., but 

the total-rock isochron for the metavolcanic rocks 

yields an age of 593 ±53 m.y., ri of 0.7040 + 0.0003, 

and Index of 0.15. Although the age difference is significant only at confidence levels less than 62.3 percent, the geologic control and slightly elevated rt suggest a metamorphic age even though the original 

ash-flow tuff fabric of some units is nearly undisturbed.

Fine-grained rhyolite within young strata of the 

andesitic assemblage (the Murdama Group) in the 

Jabal Yafikh quadrangle (loc. 15) also appears to 

yield a reset total-rock isochron, with an age of 

568±29 m.y. (fig. 10). Although the strata are 

tightly folded, metamorphic effects are not detectable 

in the volcanic rocks studied. Deformation appears 

to increase with proximity to one of the major shear 

zones of the Najd fault system (Brown and Jackson, 

1960). Discordant K-Ar ages on hornblende are 

reported by Fleck and others (1976) for quartz diorite upon which the strata were deposited, but no 

geologic control of minimum age is available within 

the area. The interpretation of the age as reset is 

based on the elevated r{ of 0.7061 ±0.0007 but must 

be considered tenuous because of the lack of independent control.

Andesitic strata in the Bi'r Juqjuq quadrangle assigned to the Juqjuq Formation of Hadley (1976) 

(Halaban Group) are intruded by the quartz monzonite of Jabal Tarban, which yields an apparent 

age of about 635 m.y. (table 2, loc. 61). As in the 

Jabal Yafikh area (loc. 15), the stratified units are 

folded, but metamorphism is not represented by 

petrographically detectable textural or mineralogical 

changes. Where sampled (loc. 53), the Juqjuq Formation contains detrital, volcaniclastic, and volcanic 

units. Three andesites from the unit yield a totalrock isochron age of 620 ±95 m.y., r{ of 0.7034 ± 

0.0003, and an Index of 0.01, indicating extremely

low dispersion about the isochron (fig. 11). All have 

very fine grained mesostases, although two samples 

from these andesites contain coarse phenocrysts. 

The quartz monzonite of Jabal Tarban truncates 

both bedding and fold axes in the Juqjuq Formation, 

indicating a late-deformation or postdeformation 

emplacement. The apparent age of the intruded 

strata is essentially the same as that of the intrusive 

body, raising the possibility of isotopic homogenization during intrusion. The ri of the andesitic unit

0.740

0.730

0.720

0.710

Rhyolite, Murdama Group 

(loc. 15)

Index = 0.47

-0.706110.0007

1.0 2.0 

87 Rb / "Sr

3.0 4.0

FIGURE 10.—Total-rock Rb-Sr isochron diagram of the rhyolite of the Murdama Group. See figure 7 for explanation 

of symbols and isochron data.

0.706-

0.705-

0.704 -

0.703

Juqjuq Formation of Hadley (1976) 

(loc. 53)

0.3

FIGURE 11.—Total-rock Rb-Sr isochron diagram of the Juqjuq 

Formation of Hadley (1976). See figure 7 for explanation 

of isochron data.
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 19

(0.7034) is not significantly elevated to support a 

homogenization hypothesis, however, as magmas 

related to the late-orogenic or postorogenic plutonism 

commonly have values in that range. Rb/Sr ratios 

of the young plutons, however, are commonly much 

higher than the 0.063 value (table 2) from the Juqjuq Formation. Hadley (1976) considered the Juqjuq to be older than his Arphan Formation, determined here to be 760-775 m.y. For these reasons, 

we interpret the apparent age obtained from these 

strata to be reset but consider this assignment 

tentative.

Ages interpreted as times of extrusion of volcanic 

units of the andesitic assemblage range from 612 to 

912 m.y. This range of ages does not represent 

magma generation of constant rate, common source, 

or similar mechanism, but rather includes a variety 

of volcanic and volcaniclastic units whose average 

composition is andesitic. The majority of ages, probably representing as much as 50 percent of the total 

volume of andesitic strata, are between 740 and 790 

m.y. In the southern part of the shield, andesitic 

units younger than about 700 m.y. probably represent a volumetrically small proportion of the assemblage. Some of the less deformed strata, as well as 

some of the most deformed ones, yield ages younger 

than plutonic units that intrude them, indicating disturbance and, commonly, rehomogenization of the 

strontium isotopic systems within them. The timing 

of this disturbance is inadequately known. The 

data indicate repeated, possibly episodic, deposition 

and metamorphism of andesitic strata over broad 

areas of the Arabian Shield and severely limit correlation by rock type and degree of deformation. 

Conversely, the evidence of disturbance of isotopic 

systems in these strata emphasizes the tenuous nature of interpretation of isotopic ages in fine-grained 

rocks without additional geologic and geochronologic 

control.

FOLIATED DIORITE TO TRONDHJEMITE BATHOLITHS

Plutonic rocks represent about 40 percent of rocks 

exposed in the Arabian Shield (Greenwood and 

Brown, 1973). In the western part of the shield, 

major batholiths range in composition from diorite 

to trondhjemite, but granodiorite increases in abundance eastward. In the western and southern parts 

of the area (fig. 3), dioritic batholiths represent 

60-80 percent of the intrusive units. In the eastern 

part, granodiorite gneiss and younger granitic rocks 

are more abundant. Rb-Sr total-rock isochron ages 

of diorite to trondhjemite bodies provide the most 

complete and reliable chronology of the evolution of

the southern part of the Arabian Shield. Field evidence, particularly in the Biljurshi and al 'Aqiq 

quadrangles (Greenwood, 1975a, b), demonstrates 

repeated andesitic volcanism, volcaniclastic sedimentation, dioritic intrusion, deformation, and erosion. 

Ages of dioritic bodies shown in table 2 support a 

model of continued, though possibly episodic, igneous 

activity between about 900 and 680 m.y. ago.

The oldest plutonic rocks studied occur in the Al 

Lith and Biljurshi areas (figs. 3 and 4), where they 

intrude rocks of both the basaltic and andesitic assemblages. Five suites of samples from these areas 

were studied; Rb-Sr data are presented in tables 1 

and 2 and figure 12. Quartz diorites of Wadi 

Khadrah and Wadi ash Shaqah ash Shamiyah (Iocs. 

62, 63, 65, and 66) in the area of Al Lith yield ages 

of 895 ±173 m.y. and 853 ±72 m.y. and r, values of 

0.7025 ±0.0003 and 0.7030 ±0.0002, respectively. 

Rb/Sr values for these rocks are very low, rubidium 

concentrations averaging about 25 ppm. Index 

values of 0.81 and 1.61 are larger than many values

0.708

<P 0.706

0.704

0.702

0.708

0.706

0.704

0.702

-A

•0.7030 ±0.0002 

•0.7025±0.0003

EXPLANATION

O Quartz diorite of Wadi 

Khadrah (Iocs. 62, 63)

O Quartz diorite of Wadi ash 

Shaqah ash Shamiyah 

(Iocs. 65, 66)

"7 Rb Sr

B

632

625

•0.7030 ± 0.0005

•0.7026+0.0003 

J_______l_

13J

13H

EXPLANATION

O Quartz diorite of Biljurshi _ 

(Set A, Iocs. 6-8)

O Quartz diorite of Biljurshi — 

(SetB, Iocs. 79-81)

A Quartz diorite of Wadi — 

Qanunah (Iocs. 82, 83)

I_______|_______I

0.1 0.2 0.3 

"Rb / "Sr

0.4 0.5

FIGURE 12.—Total-rock Rb-Sr isochron diagrams. A, Al Lith 

area, quartz diorities of Wadi Khadrah and Wadi ash 

Shaqah ash Shamiyah. B, Biljurshi area, quartz diorites of 

Biljurshi and Wadi Qanunah. Data for all isochrons are 

shown in table 2. Sample locations are shown in figure 4 

and listed in table 1.
20 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

obtained from the diorite units studied, although 

neither is significantly above the 1.58 cutoff suggested for distinguishing between analytical and 

geologic error. Data shown in figure 12, however, 

reveal the much greater scatter of these sets when 

compared with other sets reported here, and this is 

reflected in the uncertainties in the apparent ages.

Data for samples from the Biljurshi batholith 

yield ages similar to those from the Al Lith area, although the intrusive complexes are spatially distinct. 

Quartz diorite of Biljurshi (Iocs. 6-8 and 79-81), 

mapped by Greenwood (1975a) as older quartz diorite, intrudes units of both the basaltic and andesitic 

assemblages and generally is cut by the major northor northeast-trending fracture systems. Other exposures, however, indicate that some of these faults 

formed prior to diorite emplacement and were 

truncated by the plutons. The presence within the 

fracture system of faults older than the diorite and 

others that are younger indicates contemporaneous 

deformation and dioritic intrusion. The ages indicated by the two data sets, 890 ±67 m.y. and 

848 ±282 m.y. and n values of 0.7026 ±0.0003 and 

0.7030 ±0.0005, are almost identical with those of 

the Al Lith samples. In contrast with the Al Lith 

data, however, the Biljurshi sets show extraordinarily linear arrays with extremely low Index

values. Based solely on dispersion of the data, uncertainties calculated for the first and second groups, 

respectively, are 3 and 4 m.y. In the southern part 

of the Biljurshi batholith, the quartz diorite of Wadi 

Qanunah (Iocs. 82, 83) yields similar isotopic results, but the two samples from this body are similar in Rb/Sr ratio. Assuming an initial ratio of 

0.7028, the average of the other Biljurshi quartz 

diorites, an apparent age of 846 m.y. is calculated 

for this body.

Diorite to trondhjemite bodies of the An Nimas 

batholith (Greenwood, 1979a; Anderson, 1977) fall 

into two age groups. The older group, which represents the oldest plutonic rocks of that region, yields 

ages of 818 to 838 m.y. (Iocs. 42, 67-69, 71, 72), as 

shown in figure 13. The quartz diorite of An Nimas 

(loc. 42) intrudes older units of the andesitic assemblage; at Wadi bin Dwaynah, the metavolcanic 

rocks yielded an age of 912 ±76 m.y. The 837 ±50 

m.y. age (table 2) of the quartz diorite body is consistent with the age interpretation discussed for the 

metavoleanic rocks, thereby making improbable an 

alternative hypothesis of a contact metamorphic origin for the 912 m.y. isochron. Quartz diorites in the 

batholith in the Wadi Tarj quadrangle (Anderson, 

1977) yield ages and rt values similar to those of 

rocks near An Nimas. Trondhjemite of Wadi Asmak

0.708

0.704

0.702

0.7027 ±0.0002

EXPLANATION 

D Quartz diorite of Wodi Torj (Iocs. 69, 71, 72)

O Trondhjemite of Wadi Asmok (Iocs. 67, 68) 

O Quortz diorite of An Nimas (loc. 42)

<^> Quartz diorite of Al Mushirah (Iocs. 76, 78)

0.1 0.2 0.3 0.4

Rb / 88Sr

FIGURE 13.—Total-rock Rb-Sr isochron diagram of the quartz diorites of An Nimas, Wadi 

Tarj, and Al Mushirah and the trondhjemite of Wadi Asmak, An Nimas batholith. 

See figure 12 for explanation of isochron data.
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 21

(Iocs. 67, 68) yields a two-point "isochron" age of 

838 m.y.; the quartz diorite of Wadi Tarj (Iocs. 69, 

71, 72), which is closely related to the trondhjemite, 

defines an isochron of 818 ±95 m.y., with an r{ of 

0.7028 ±0.0002. In the eastern part of the Al 'Aqiq 

(Greenwood, 1975b) and Biljurshi (Greenwood, 

1975a) quadrangles, the younger group of dioritic 

units of the An Nimas batholith intrude strata of 

the andesitic assemblage (the Ablah Group of 

Greenwood, 1975b) that were deposited nonconformably on the Biljurshi batholith. One of these, the 

quartz diorite of Al Mushirah (Iocs. 76-78), though 

petrologically indistinguishable from older diorites, 

yields an age of 747 + 178 m.y. and rt of 0.7028 ± 

0.0003 (fig. 13). The age of the younger unit does 

not differ significantly, however, from that of the 

quartz diorite of An Nimas (loc. 42) at levels of 

confidence above 37 percent because three of the 

four data points have nearly identical Rb/Sr ratios. 

Dioritic complexes in the central and eastern parts 

of the area studied yield results similar to those for 

the Biljurshi and An Nimas batholiths, although 

ages are commonly younger. Data presented in table 

2 define an age range from 522 to 843 m.y., although 

the youngest age is not considered representative of 

a time of emplacement. The oldest ages of plutonic 

rocks from the eastern parts of the region are part 

of the Wadi Tarib batholith in the Wadi Wassat and 

Wadi Malahah quadrangles (Greenwood, 1979c, d), 

where they intrude units of the andesitic assemblage. 

The diorite complex near Wadi Makhdhul was found 

to be a composite of plutons of several different ages. 

The oldest of these probably represents the earliest 

phase of plutonism in the eastern part of the Arabian Shield, yielding an age of 843 ±273 m.y. and ri 

of 0.7023±0.0006 (loc. 37, fig. 14). Data from 

diorite and trondhjemite of the Wadi Tarib batholith sampled west of this locality in the southern 

part of the Wadi Malahah quadrangle (Iocs. 93, 94) 

show greater dispersion, as might be expected from 

the extremely low Rb content (table 1), but fall on 

the same isochron as those of the Wadi Makhdhul 

samples (fig. 14). When these three points are included, the isochron age and r< are virtually unchanged, 846 ±82 m.y. and 0.7023 ±0.0001, but their 

uncertainties are greatly reduced. The Index changes 

from 0.01 to 0.61, reflecting the increased scatter, 

but still indicates that the dispersion is well within 

the range of analytical variation. Quartz diorite from 

the area of Simlal, south of Wadi Wassat in the 

Wadi Qatan area (loc. 38, figs. 4, 14), yields an apparent age of 815 m.y. but is represented by only 

two points and consequently a large uncertainty.

0.705

0.704

0.703

0.702

21F

i

.0006

EXPLANATION

O Quartz diorite of Simlal 

(loc. 38)

O Diorite and trondhjemite of 

the Wadi Tarib batholith 

(Iocs. 93, 94)

O Quartz diorite of Wadi 

Makhdhul (loc. 37)

0.1

7 Rb

0.2 

"Sr

0.3

FIGURE 14.—Total-roek Rb-Sr isochron diagram of the quartz 

diorites of Wadi Makhdhul and Simlal and the diorite and 

trondhjemite of the Wadi Tarib batholith, Wadi Malahah 

quadrangle. See figure 12 for explanation of isochron data.

The Simlal data do not fit the Wadi Makhdhul diorite isochron (fig. 14) but fall above that line, yielding an rt of 0.7027. The quartz diorite of the Simlal 

area is thought to postdate ore deposits in the 

andesitic assemblage in the Wadi Qatan area (Dodge 

and Rossman, 1975). Because of its significance for 

models of evolution of ore deposits, we cannot overemphasize the uncertainty of the two-point "isochron."

Most batholithic units sampled in the central and 

eastern parts of the area of study yield ages significantly younger than the 815-850 m.y. range and 

87Sr/86Sr initial ratios greater than 0.7023. Many of 

these units have more siliceous phases, ranging from 

quartz diorite to granodiorite with significant 

amounts of potassium feldspar and Rb/Sr ratios 

10-50 times those of the older diorites. Dioritegranodiorites samples from two different units east 

of Bishah that intrude andesitic units, quartz diorites of Jabal Umm al Hashiyah (Iocs. 17-19) and 

southwestern Al Qarah quadrangle (Iocs. 51, 87), 

yield ages of 723 ±107 m.y. and 724 ±93 m.y., as 

shown in figure 15. Initial ratios obtained are 

0.7025 ±0.0005 and 0.7027 ±0.0002, respectively. 

Strata intruded include those of the andesitic assemblage (loc. 16, table 2) that yield an age of 785 ±96 

m.y. and represent the oldest unit exposed in the 

Bishah area.

Diorite and granodiorite units in the Wadi 

Malahah quadrangle show the greatest age range 

within the Arabian Shield. As stated, trondhjemite
22 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

and hornblende diorite in the southernmost part of 

the quadrangle represent very primitive units consistent with the 843 m.y. age of the Wadi Makhdhul 

diorites. Results from other units including the 

quartz diorite and granodiorite plotted in figure 16, 

define significantly different isochrons. Quartz diorite to granodiorite gneiss forms a somewhat sigmoidal east-west-trending body in the Wadi Malahah 

and western Wadi Wassat quadrangles (fig. 3). 

North of this body, quartz diorite petrologlcally 

similar to the quartz diorite of Wadi Makhdhul is 

cut by dikes of the granodiorite. These units form 

the southern margin of a large structural high called 

the Malahah dome by Greenwood (1979d). Although 

the regional structure may be domical, a domical 

foliation is not obvious in the individual plutonic

0.708

0.706

0.704

0.702

-0.7027 ± 0.0002 

V 0.7025 ± 0.0005

EXPLANATION

O Quartz diorite, southwestern Al Qarah quadrangle (Iocs. 51. 87)

O Quartz diorite of Jabal 

Umm al Hashiyah 

(Iocs. 17-19)

0.1 0.2 0.3 

•7Rb/"Sr

0.4 0.5

FIGURE 15.—Total-rock Rb-Sr isochron diagram of the quartz 

diorites of Jabal Umm al Hashiyah and the southwestern 

Al Qarah quadrangle. See figure 12 for explanation of isochron data.

0.708h

0.706

0.704

0.702

•O7D

'7B

7029 ± 0.0001 

0.7027,______

0.1 0.2

EXPLANATION

O Quartz diorite 

of Malahah 

dome (loc. 89)

O Granodiorite of 

Wadi Malahah 

(Iocs. 36, 88-92)

0.3 0.4 0.5

87 Rb/"Sr

FIGURE 16—Total-rock Rb-Sr isochron diagram of the quartz 

diorite of Malahah dome and granodiorite of Wadi Malahah. 

See figure 12 for explanation of isochron data.

units as it is in many of the smaller gneiss domes. 

The younger of the two major units of the dome, 

referred to here as the granodiorite of Wadi Malahah, is represented by 13 samples (Iocs. 36, 88-92) 

that define an excellent isochron (fig. 16). The age 

of the unit is 684 ±43 m.y., with an ri of 0.7029 

±0.0001 and Index of 0.14, making this body one 

of the youngest deformed units studied. The older 

phase of the dome, the quartz diorite, is represented 

by only two samples (loc. 89), and although the 

two differ greatly in Rb/Sr ratio, the uncertainties 

in age and initial ratios easily include those of the 

younger granodiorite. The apparent age of the 

quartz diorite is 720 m.y., with an r{ of 0.7027 (fig. 

16). Although the uncertainties cannot be evaluated, 

the similiarity in age and r{ to those of quartz diorite-granodiorite bodies in the Bishah area (table 2; 

fig. 15) offers additional support to the results.

Diorite-trondhjemite batholithic complexes occur 

in a northwest-trending en echelon pattern in the 

southwestern part of the shield. These bodies range 

in age from about 750 to 900 m.y. Diorite-granodiorite complexes are more common in the central 

and eastern parts of the region than diorite-trondhjemite. They range in age from about 680 to 750 m.y. 

Rb/Sr ratios of the dioritic phases of these two 

groups are similar, but the granodiorite units show 

marked increases in Rb/Sr relative to the diorites, 

whereas trondhjemites are characterized by sharp 

decreases in both Rb and Sr from diorite values but 

with little difference in the Rb/Sr ratio.

GRANODIORITE GNEISS DOMES

Total-rock Rb-Sr isochrons were obtained from 

five deformed granodiorite to granite gneiss domes 

whose foliations are generally concordant with their 

intrusive margins. The granodiorite gneiss of Jabal 

Mina (loc. 40) intrudes a highly deformed amphibolite-grade sequence of the andesite assemblage 

south and east of An Nimas. Data obtained from the 

gneiss define a 746±114 m.y. isochron (fig. 17), with 

an rt of 0.7043 ±0.0036 and Index of 0.40. At the 

scale of figure 3, the Jabal Mina body is shown as 

contiguous with the granodiorite gneiss of Wadi 

Bagarah (Iocs. 23-28), whereas small pendants of 

the andesitic assemblage or diorite of the An Nimas 

batholith actually separate the two. The age of the 

Wadi Bagarah body was reported earlier by Fleck 

and others (1973) as 759 m.y. The effects of additional analyses and revised 87Rb decay constants offset each other such that all data for the gneiss now 

yield an isochron age of 763 ±53 m.y., with an rt of 

0.7033±0.0003 and Index of 0.90 (fig. 18). Because
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 23

0.750

0.740

0) 0.730

0.720

0.710

0.700

Granodiorite gneiss of Jabal Mina 

(loc. 40)

3.0

RIGURE 17.—Total-rock Rb-Sr isochron diagram of the granodiorite gneiss of Jabal Mina. Data for all isochrons are 

shown in table 2. Sample locations are shown in figure 4 

and listed in table 1.

15A / 

150^15 

15B/-" ,. D

^0.7033 ± 0.0003 

^0.7026 ± 0.0019

EXPLANATION

O Granitic gneiss of Jiddah 

Airport (Iocs. 29-31)

O Granodiorite gneiss of 

Wadi Bagarah (Iocs. 

23-28)

0.2 0.4 0.6 

87Rb/"Sr

0.8

FIGURE 18.—Total-rock Rb-Sr isochron diagram of the granodiorite gneiss of Wadi Bagarah and the granitic gneiss of 

Jiddah Airport. See figure 17 for explanation of isochron 

data.

the Rb/Sr ratios of the Jabal Mina and Wadi 

Bag-arah bodies differ significantly (table 2), reflecting the lower content of potassium feldspar in the 

Wadi Bagarah body, the two are considered separate 

intrusions. The similar ages, tectonic styles, and twomica mineralogy, however, suggest a similar origin. 

Samples of biotite-muscovite granitic gneiss exposed 10 km east of Jiddah were analyzed previously by L. T. Aldrich, quoted by Brown and others 

(1962), who reports a Rb-Sr apparent age on biotite 

of 1,050 m.y. Brown (1970, 1972) referred to these 

rocks and more widespread exposures of granodiorite and quartz diorite near Mecca as the oldest 

radiometrically dated units in the Arabian Shield. 

The rocks exposed near Jiddah, here called the 

granite gneiss of Jiddah Airport, were resampled

by us for whole-rock Rb-Sr analysis (Iocs. 29-31). 

Results from these samples yield a whole-rock isochron age of 763 ±159 m.y., an r, of 0.7026 ±0.0019, 

and Index of 0.25. These data are inconsistent with 

an age of 1,050 m.y., at all levels of confidence up 

to 92.9 percent. They do not invalidate ages reported 

for dioritic rocks closer to Mecca but suggest a 

greater similarity to two-mica granitic rocks such 

as those of Wadi Bagarah or Jabal Mina.

The granite gneiss of Harisi Dome in the southwestern part of the Wadi 'Atf quadrangle (Iocs. 73, 

74) is represented by only two samples and the uncertainties associated with the "isochron" obtained 

cannot be adequately evaluated (fig. 19). These 

rocks, like the other gneiss domes, are biotitemuscovite gneisses with foliation concordant with 

both the intrusive margins and the schistosity in 

the adjacent andesitic assemblage rocks. The Harisi 

dome, like the Wadi Bagarah and Jabal Mina gneiss 

domes, intrudes and deforms rocks assigned to the 

Ablah Group (Anderson, 1978a). The apparent age 

(773 m.y.) and r, (0.7027) obtained from the two 

samples are consistent with results from the other 

two intrusives.

Rubidium-strontium studies of the garnet-bearing 

granitic gneiss of Wadi Bishah (loc. 4) indicate an 

apparent age of 664 ± 9 m.y., significantly less than 

the 761-m.y. average of the other four domes. The 

Index value of 1.01 is higher but cannot be interpreted as indicating disturbance of the Rb-Sr system 

(fig. 19). The n of 0.7035 ±0.0007 is in agreement

EXPLANATION

Granitic gneiss of Harisi 

dome (Iocs. 73, 74) 

fj= 0.7027

O Granitic gneiss of Wadi 

Bishah (loc. 4) 

i = 0.7035+0.0007 

I I 0.700

87Rb/ 88Sr

FIGURE 19.—Total-rock Rb-Sr isochron diagram of the granitic 

gneisses of Harisi dome and Wadi Bishah. See figure 17 for 

explanation of isochron data.
24 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

with values obtained for the other domes. The three 

sample sites within the body were separated by at 

least 100 m but represent the same general location. 

Because the body is strongly deformed and contains 

a metamorphic mineral assemblage, the possibility 

of metamorphic homogenization of strontium cannot 

be entirely disregarded. Amphibolites of the andesite unit that were intruded by the gneiss dome have 

high strontium concentrations and low 87Sr/86Sr 

ratios. Because of the relatively low strontium (95 

ppm, average) in the gneiss, exchange between the 

gneiss and amphibolites during metamorphism could 

reset the 87Sr/86Sr ratio to the low value of 0.7035, 

even though the body has a Rb/Sr ratio of 1.5. 

Although we consider this possibility unlikely and 

regard the 664-m.y. age as a reasonable value for the 

age of intrusion, the data do not permit a more 

definitive conclusion. Regardless of whether the apparent age represents the time of intrusion or of 

metamorphism, major deformation occurred in the 

southern part of the Arabian Shield at or subsequent 

to 664 ±9 m.y. ago.

LATE-OROGENIC OR POSTOROGENIC GRANODIORITE 

TO GRANITE PLUTONS

Fleck and others (1976), reporting K-Ar and 

40Ar/39Ar ages for late-orogenic or postorogenic (socalled "Pan-African") granodiorite to granite plutons (fig. 3), conclude that most of the plutons were 

emplaced between about 610 m.y. and about 560 

m.y., as determined by concordant mineral ages. 

Recalculating these data to the decay constants of 

Beckinsale and Gale (1969) and potassium abundance values of Garner and others (1975) changes 

this range to about 620 m.y. to 570 m.y. Rb-Sr results for 14 different granodiorite-granite plutons 

or intrusive complexes are shown in table 2. Totalrock isochron ages obtained for 8 bodies range from 

587 to 643 m.y., but the youngest of these, from 

Wadi al Miyah (Iocs. 45, 46), has a very large uncertainty (one standard deviation is 17 percent) and 

high rt . The remaining isochron ages cluster between 

620 m.y. and 643 m.y., averaging 629 m.y. with a 

mean rt of 0.7032. Model ages obtained for five other 

bodies by assuming this initial ratio (0.7032) range 

from 608 m.y. to 657 m.y., with a mean of 632 m.y., 

nearly identical with the mean value of isochron 

ages.

The Rb-Sr ages indicate that the oldest K-Ar ages 

within the granodiorite to granite bodies are between 15 and 45 m.y. too young. Because many of 

these bodies contain no amphiboles and K-Ar ages 

reported are for micas, some of the differentials

might be explained by slow cooling. As discussed by 

Fleck and others (1976, p. 20), however, a later 

thermal pulse or phase between about 550 and 520 

m.y. (recalculated values) is required by the irregular regional pattern of discordance in K-Ar ages. 

Studies using 40Ar/39Ar ages indicate that even the 

oldest apparent ages have been affected to some degree. As shown by the Rb-Sr ages, the timing of the 

last major intrusive phase of shield formation was 

earlier than that defined by K-Ar ages, commencing 

about 645 m.y. and continuing for at least 45 m.y., 

and probably longer.

TINDAHAH BATHOLITH

The Tindahah batholith of Coleman (1973a), in 

the eastern Khamis Mushayt quadrangle, intrudes 

deformed units of the andesite assemblage and 

gneissic to migmatitic bodies of quartz diorite to 

granodiorite, called the Khamis Mushayt Gneiss by 

Coleman (1973a), which represent the western part 

of the Wadi Tarib batholith. The Tindahah batholith, which ranges in composition from granodiorite 

to quartz monzonite, exhibits little or no evidence 

of deformation and forms an elongate body whose 

largest dimension parallels the fold axes and foliation in adjacent units. In contrast to many other 

late-orogenic or postorogenic bodies, the margins 

of the Tindahah batholith are irregular, invading 

adjacent stratified units both across and along 

structural trends. Rubidium-strontium total-rock 

data for the Tindahah batholith, shown in figure 20, 

form an excellent isochron, with an age of 626 ±17 

m.y., rt of 0.7037 ±0.0004, and Index of 0.16. Potassium-argon ages reported by Fleck and others 

(1976) for this body range from 567 to 579 m.y. 

when recalculated using the new constants. Although 

the differences between K-Ar ages are not significant at the 95-percent level of confidence, they are 

clearly discordant with the Rb-Sr age and indicate 

either slow cooling or subsequent reheating. As discussed by Fleck and others (1976), however, discordant 40Ar/39Ar age-spectra and the irregular regional pattern of K-Ar ages indicate argon loss due 

to a thermal event subsequent to initial cooling.

BANI THUWR PLUTON

The Bani Thuwr pluton of Coleman (1973b) is a 

complex of coalescing ring structures that intrudes 

the same rock units as the Tindahah batholith approximately 20 km northwest of that body. It is a 

coarse-grained quartz monzonite containing large 

phenocrysts of pink microcline in some facies. Potassium-argon ages reported by Fleck and others
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 25

0.760

<*

& 0.740

Crt

&

0.720

0.700

I""IIII

EXPLANATION 

O Quartz monzonites of Wodi Shuwos (Iocs. 11-14)

O Granite of Jabal Ibrahim (loc. 10) 

A Granite of Jabal Shodo (loc. 9) 

O Tindohoh batholith (Iocs. 1-3) 

D Bom Thuwr pluton (loc. 5)

>| ^x-"UA

-0.703510.0003 

I I I

"Rb / *Sr

FIGURE 20.—Total-rock Rb-Sr isochron diagram of the Tindahah batholith and 

the Bani Thuwr pluton, quartz monzonites of Wadi Shuwas, and the granites 

of Jabal Shada and Jabal Ibrahim. Data for all isochrons are shown in 

table 2. Sample locations are shown in figure 4 and listed in table 1.

(1976) range from 529 m.y. to 590 m.y., with obvious discordance. Because only a single sample was 

suitable for Rb-Sr study, no isochron could be obtained. As shown in figure 20, the single data point 

falls somewhat below the isochron for the Tindahah 

batholith but is not significantly different at the 

95-percent level of confidence. A model age of 608 

m.y. is obtained by assuming an r; of 0.7032, but 

its uncertainty could be as much as 30 m.y., because 

of analytical errors or an incorrect assumption of rit

WADI SHUWAS

Coalescing arcuate lenses or sheets of quartz monzonite, granodiorite, and quartz-bearing syenite 

(Greenwood, 1975a, 1975b) invade a dominantly 

diorite to quartz diorite terrane in the southeastern 

part of the Aqiq quadrangle and northeastern part 

of the Biljurshi quadrangle along Wadi Shuwas 

(Iocs. 11-14). Data from all of these units, called 

the quartz monzonites of Wadi Shuwas, form an 

isochron (fig. 20) with an age of 636 ±21 m.y., rt of 

0.7035 ±0.0003, and Index of 0.43, supporting the 

postulate of cogenesis. Potassium-argon ages reported by Fleck and others (1976) are discordant, 

yielding apparent ages from 618 m.y. to 545 m.y. 

and having hornblende values greater than those of 

coexisting biotite in all but one sample (71-8-15D, 

loc. 13). This single hornblende concentrate was 

examined by *°Ar/39Ar incremental-heating (Fleck 

and others, 1976), evolving a discordant age-spectrum with minimum age of 588 m.y. We conclude 

that the discordance cannot be due to slow cooling 

but rather is the result of argon loss subsequent to

the 636-m.y. time of intrusion. The age of the event 

causing argon loss is poorly defined but is presumed 

to be less than 545 m.y. old.

JABAL SHADA AND JABAL IBRAHIM

Two small subcircular quartz monzonite to granite 

plutons occur in quadrangles adjacent to the Wadi 

Shuwas bodies at Jabal Shada (loc. 9) and Jabal 

Ibrahim (loc. 10). The single analyses reported 

(table 1) for each pluton represent the only samples 

from these locations, and sampling was made originally for K-Ar studies (Fleck and others, 1976). 

Recalculated to new constants, the K-Ar ages are 

from Jabal Shada, biotite 602 m.y. and muscovite 598 

m.y. and for biotite from Jabal Ibrahim, 610 m.y. 

Rubidium-strontium model ages, shown in table 2, 

were calculated for these bodies assuming the mean 

r( of total-rock isochrons of the granodiorite to 

granite plutons (0.7032). Compared with the 

isochron for the nearby Wadi Shuwas bodies 

(rt = 0.7035), however, the value for the Jabal Shada 

sample fits precisely and the Jabal Ibrahim data 

point is only slightly above (fig. 20). We conclude 

that the granite of Jabal Shada may have been 

cogenetic with the quartz monzonites of Wadi 

Shuwas (at 636 m.y.) and that the Jabal Ibrahim 

body may have been emplaced slightly earlier, between 640 and 657 m.y. ago.

JABAL OAL AND JABAL 'AYA

Quartz monzonite to granite plutons in the western part of the Jabal 'Aya quadrangle (Iocs. 21, 22) 

at Jabal Qal and Jabal Barkouk intrude metasedi-
26 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

mentary and metavolcanic strata of both the basaltic 

and andesitic assemblages. Major north-south shear 

zones are truncated by the intrusives, and foliation 

in the metamorphic units adjacent to the bodies is 

deflected from the regional trend. The plutons are 

nearly circular bodies with arcuate screens of mafic 

rocks subparallel to the margins. They are undeformed and clearly postdate all major deformation 

within the area. As shown in figure 21 and table 2, 

sample values from these units form a linear array 

on the isochron diagram, yielding an age of 620 ±18 

m.y., n of 0.7034 ±0.0005, and Index of 0.03. Data 

from a sample of a small closely related pluton at 

Jabal 'Aya (loc. 20) 15 km east of the Jabal Qal body 

also fit the isochron, suggesting that the Jabal 'Aya 

intrusive was coeval with Jabal Qal. Adding the 

data for Jabal 'Aya would reduce the isochron age 

slightly, yielding 614 ±13 m.y., an n of 0.7035 

±0.0004, and an Index of 0.34. The 614±13-m.y. 

value represents a minimum age for the last movements on major north-trending shear zones in this 

area, which extend for distances as great as 300 km 

(see, for example, Greenwood and others, 1974).

WADI HALAL

The granodiorite of Wadi Halal intrudes a sequence 

of deformed ash-flow tuffs of the andesitic sequence 

in the Mayza quadrangle (Anderson, 1978b). The 

body is essentially circular but is itself cut by a 

younger reddish granitic phase. Foliations in the 

andesitic unit are deflected around the intrusion,

0.860

0.820

0.780

0.740

0.700

I I I I I I 

EXPLANATION

O Quartz monzonite of Jabal Qal 

(Iocs. 21, 22)

O Quartz monzonite of 

Jabal Aya (loc. 20)

6N

Index = 0.03

6M

,0.7034 ± 0.0005

I I I

8 

»7 Rb

12 16

/ MSr

FIGURE 21.—Total-rock Rb-Sr isochron diagram of the quartz 

monzonites of Jabal Qal and Jabal 'Aya. See figure 20 for 

explanation of isochron data.

which has metamorphosed some of these strata to 

the amphibolite facies. The metavolcanic rocks of 

Wadi Shuklalah (Iocs. 32, 33) are from this metamorphosed sequence, and as stated, yield a wholerock isochron age of 593 ±53 m.y. Values for samples of the granodiorite are plotted in figure 22, 

where the array defines a good isochron showing an 

age of 643 ±20 m.y., rt of 0.7028 ±0.0002, and Index 

of 0.52. Samples 742-20A and 742-20B were collected approximately 0.5 km east of the other samples in what Anderson maps as a separate "border" 

phase of the body. Both samples appear to be displaced slightly above the isochron, indicating a 

slightly higher rt (0.7031) but an identical (2-point 

isochron) age of 644 m.y. The younger "red-granite" 

phase that intrudes the granodiorite may well be 

responsible for homogenization of strontium isotopes in the metavolcanic rocks at about 593 m.y. 

That phase was not collected as part of the current 

study, however. Because much of the granodiorite 

of Wadi Halal is foliated, it is hypothesized that 

emplacement may have been diapiric, as the magma 

of the younger, granitic phase drove a nearly circular "piston" of metavolcanic rocks and granodiorite upward. This hypothesis is strengthened by the 

presence of subcircular fractures that are generally 

concentric with arcuate sheets of red granite.

WADI AL MIYAH AND WADI MUSAYRAH

Granodiorite units in the area north and east of 

the town of Bishah (fig. 3) show a vague, possibly 

primary, foliation but are structurally, chemically,

0.718

0.714

0.710

0.706

0.702

20Br

Granodiorite of Wadi Halal 

(Iocs. 34, 35)

20G

Index =0.52

"20D

-0.7028 ± 0.0002

j____1___|___i_

0.4 0.8 1.2 

87 Rb / "Sr

1.6 2.0

FIGURE 22.—Total-rock Rb-Sr isochron diagram of the granodiorite of Wadi Halal. See figure 20 for explanation of 

isochron data.
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 27

and isotopically distinct from the highly deformed 

(gneissic) granodiorite and quartz diorite bodies 

such as those of the southwestern part of the Al 

Qarah quadrangle (Iocs. 51, 87) that give ages of 

724 m.y. Samples of the granodiorite of Wadi Musayrah (Iocs. 48-50, fig. 4) define a precise isochron 

(fig. 23) of 623± 18 m.y. with an rt of 0.7033±0.0003 

and Index of 0.42. The average Rb/Sr ratio of the 

samples is significantly greater than those of the 

older batholiths (table 2), while Sr concentrations 

are less than half (table 1). The granodiorite of 

Wadi al Miyah (Iocs. 45, 46) intrudes metavolcanic 

strata of the andesite unit (loc. 47), but its relation 

to dioritic units in the region is not known. Feldspars in samples of the Wadi al Miyah body are 

somewhat altered, but it is not clear whether this 

alteration accompanied deformation or was significantly later. A statistically acceptable isochron may 

be drawn through the three data points, giving an 

apparent age of 587±77 m.y. (fig. 23), but the large 

uncertainty reflects both the dispersion of the data 

and the inadequate spread in Rb/Sr ratio. Only one 

point from the Wadi al Miyah set, 742-25L, diverges 

significantly from the Wadi Musayrah isochron. 

That point has the highest Rb/Sr ratio and falls 

below the isochron, a characteristic pattern of geologic error in Rb-Sr systems (see, for example, 

Brooks and others, 1972). The data are consistent 

with a 623-m.y. age for both bodies, although stron0.724

CO 0.716

CO

0.708

0.700

EXPLANATION

Granodiorite of Wadi al Miyah 

(Iocs. 45, 46)

> Aplite dike (loc. 51)

9 Granodiorite of Wadi Musoyrah

(Iocs. 48-50) 

I______I______I____

87 Rb / 88Sr

FIGURE 23.—Total-rock Rb-Sr isochron diagram of the granodiorites of Wadi al Miyah and Wadi Musayrah and an 

aplite dike of the southwestern Al Qarah quadrangle. See 

figure 20 for explanation of isochron data.

tium concentrations in the Wadi al Miyah samples 

are much lower (table 1).

On its southern and eastern margins, the granodiorite of Wadi Musayrah intrudes a broad area of 

quartz diorite discussed previously (Iocs. 17-19, 51, 

87; fig. 15) that gives an age of 723 m.y. At one of 

these localities (loc. 51) an aplite dike was sampled 

that cuts across foliation and compositional banding 

in the gneissic quartz diorite but is tightly folded 

coaxially with isoclinal folds in the banding of the 

gneiss. As the dike is less tightly folded than is the 

banding in the quartz diorite, its emplacement 

clearly postdated initial deformation. The folding, 

however, does indicate significant deformation subsequent to intrusion, probably at temperatures sufficiently elevated for plastic flow. Rb-Sr data for 

two samples of the dike are shown in figure 23. The 

points fall slightly below the isochron of samples of 

the granodiorite of Wadi Musayrah located 15 to 

20 km to the north, but a line through the two points 

is clearly parallel to the isochron. The apparent age 

of the two-point "isochron" is 621 m.y., essentially 

identical with the Wadi Musayrah value of 623 ± 18 

m.y. The rt of the dike, 0.7028, is equal to the lowest 

value obtained from any of the late-orogenic or postorogenic bodies but is exactly the value to be expected if the aplite were derived by partial melting 

(mobilization) of the 723-m.y.-old gneissic host rock 

at that location (southwestern Al Qarah quadrangle) 623 m.y. ago.

The weak foliation of the granodiorites of Wadi 

Musayrah and Wadi al Miyah and the obvious deformation of the aplite dike of the same age show that 

deformation and, undoubtedly, metamorphism continued in the Bishah area after 620 m.y. ago. Because this deformation involved folding and the development of foliation along the major north-south 

trend at a time when all similar deformation in the 

area of Jabal Qal (loc. 21, 22) had ceased, deformation in the Bishah area must have occurred, or at 

least continued, after that to the west.

Although the granodiorites described are among 

the youngest intrusive bodies in the shield, a still 

younger "red granite" represents the youngest intrusive unit within this region. Potassium-argon 

studies by Fleck and others (1976) indicate concordant biotite and hornblende ages of about 605 

m.y. from these red granites, although some biotite 

and plagioclase ages are as young as 536 m.y and 

indicate subsequent loss of radiogenic argon. The 

red granites intrude the granodiorite of Wadi al 

Miyah, apparently disturbing the Sr isotopic systems to some degree. Because of the low strontium
28 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

concentrations and high Rb/Sr ratios of the Wadi al 

Miyah granodiorite, small disturbances of the system could be apparent even though they may occur 

within 30-50 m.y. after intrusion.

OTHER LATE-OROGENIC OR POSTOROGENIC 

PLUTONS

Late-orogenic or postorogenic intrusions in the 

northeastern part of the region studied (fig. 3) show 

the same range in age as those in the southern and 

western parts, but in the northeast the bodies are 

generally deformed by the Najd fault system (see, 

for example, Greenwood and Brown, 1973; Hadley, 

1976). The system is represented not only by northwest-trending, apparently left-lateral strike-slip 

faults but also by penetrative deformation of units 

adjacent to the major shear zones, forming strong 

planar and linear fabrics. These structures, however, have distinctly northwesterly trends, whereas 

the generally older regional trends are more nearly 

north-south or slightly northeasterly.

Effects of deformation after 600 m.y. ago are well 

displayed in the Bi'r Juqjuq quadrangle (Iocs. 

52-61), studied by Hadley (1976). Although the 

resetting of Rb-Sr ages of units of the airdesitic assemblage can probably be related to emplacement of 

the granodiorite to granite plutons, the age of hornblende diorite in the eastern part of the quadrangle 

(loc. 58) cannot. As seen on the isochron diagram 

for samples of this unit, figure 24, the three samples, 

all with Rb/Sr ratios less than 0.03, yield an apparent age of 522 ±429 m.y., rt of 0.7033 ±0.0003, 

and Index of 0.07. The samples all show petrographic evidence of greenschist-facies metamorphism, and the unit is intruded by a granodiorite to 

quartz monzonite complex that occurs within much

0.704

0.703

I I I 

Hornblende diorite of Hadley (1976) 

(loc. 58)

Index =0.07

0.02 0.04 

7 Rb / "Sr

0.06 0.08

FIGURE 24.—Total-rock Rb-Sr isochron diagram of the hornblende diorite of Hadley (1976). See figure 20 for explanation 

of isochron data.

of the eastern one-third of the Bi'r Juqjuq quadrangle. Of the Rb-Sr results from samples from four 

different locations (Iocs. 56, 57, 59, 61) within this 

complex intrusion, shown in the isochron diagram of 

figure 25, only the two from the Jabal Tarban area 

(loc. 61) have sufficient variation in Rb/Sr ratio to 

give an independent indication of age for a single 

locality. That value is 635 m.y., with an intercept of 

0.7031, close to the average n (0.7032) of isochrons 

from rocks of this group. The remaining granodiorite or quartz monzonite samples show significant 

dispersion that may be the result of metamorphism 

but also may indicate differences in age and (or) rt 

within the intrusive complex. Model ages generated 

by using the weighted mean 87Sr/86Sr and 87Rb/86Sr 

values for each area and and assuming an rt of 

0.7032 yield apparent ages of 620 m.y. for the 

gneissic granodiorite (loc. 56, 57) and 636 m.y. for 

quartz monzonite to the northeast (loc. 59). If all 

samples are presumed to be coeval, they yield an 

isochron age of 651 + 25 m.y., rt of 0.7029 ±0.0004, 

and Index of 0.92. These values indicate a significantly greater age than the age of the hornblende 

diorite (522 m.y.) intruded by the complex. In addition to showing evidence of greenschist-facies metamorphism, the hornblende diorite body is cut by en 

echelon fractures of the Najd fault system, is the 

host for a closely spaced system of mafic dikes that 

parallel the trend of the Najd system, and contains 

a pervasive foliation with a similar northwest trend. 

We suggest that the apparent age of the diorite may 

represent the time of homogenization of strontium 

as a result of deformation within the Najd fault 

system, although the uncertainty in the age is ex0.720

0.710

0.700

I I I I I Till

EXPLANATION

Gneissic granodiorite of the Bi'r Juqjuq quadrangle (Iocs. 56, 57) 

Quartz monzonite of the Bi'r Juqjuq

28I/\ 

sXOw

m

quadrangle (loc. 59)

Quartz monzonite of Jobol Torbon

(loc. 61)

-0.7031

0.4 0.8

7Rb /

1.2 

•Sr

1.6 2.0

FIGURE 25.—Total-rock Rb-Sr isochron diagram of the quartz 

monzonite and gneissic granodiorite of the Bi'r Juqjuq quadrangle and the quartz monzonite of Jabal Tarban. See figure 

20 for explanation of isochron data.
RESULTS OF RUBIDIUM-STRONTIUM ANALYSES 29

ceptionally large because of the small range of the 

Rb/Sr ratio. By this interpretation, the 522-m.y. 

value would reflect the closing of strontium isotopic 

systems at the end of major deformation and greenschist metamorphism along the Najd fault system 

and is quite similar to that suggested by Fleck and 

others (1976) for that event. .

DISCUSSION OF THE RESULTS

Rubidium-strontium ages reported here represent 

all major rock units of the southern part of the Arabian Shield with the exception of the layered gabbros 

and several exposures of serpentinite. The layered 

bodies are considered to be largely late-orogenic or 

postorogenic but older than most of the granodiorite 

to granite plutons (Coleman and others, 1972). In the 

Khaybar quadrangle (Coleman, 1973b), hornblende 

from the Jabal Shayi gabbro (Coleman and others, 

1973) yields an 40Ar/39Ar age of 626±8 m.y. (corrected to new decay constants) and is intruded by 

the Bani Thuwr pluton (608 m.y., table 2). The gabbro postdates the granitic gneiss of Wadi Bishah 

(loc. 4), which yields an age of 664±9 m.y. (table 2). 

These data and those for other gabbros reported by 

Fleck and others (1976) are consistent with an aere 

range of 620-660 m.y. for the layered gabbros. With 

the exception of the serpentinite, then, the Rb-Sr 

data reported here provide a detailed chronology for 

the evolution of the shield.

The Arabian Shield is not Archean continental 

crust remobilized during one or more periods of 

intracratonic events. There is at this time no evidence to support the presence of any sialic crust 

before 1,000 m.y. ago. The oldest plutonic rocks 

studied aie about 900 m.y. old. Units of the basaltic 

assemblage represent the oldest rocks in the southern part of the shield and the constituent rock types 

indicate marine sedimentation and volcanism remote or isolated from a continental landmass. 

Whether part of the andesitic assemblage might 

be as old as the 1,165-m.y. age obtained from the 

basaltic assemblage cannot be resolved at this time. 

The oldest rocks of the andesitic assemblage studied 

yield an age of 912 ±76 m.y., almost precisely the 

age of the oldest plutonic rocks studied.

Although none of the major rock units has been 

so adequately sampled as to represent the entire 

shield, the similar age ranges, initial 87Sr/86Sr 

ratios, chemical compositions and Rb/Sr ratios of 

flow units of the andesitic assemblage and the dioritic batholiths are strong evidence of a common 

genesis. The greatest time spans not represented by

either andesitic or dioritic units during the period 

from 920 to 720 m.y. are 37 m.y. (between 890 and 

853 m.y.) and 30 m.y. (between 815 and 785 m.y.). 

Considering the problem of representative sampling 

of an area the size of the Arabian Shield, as well as 

the uncertainties in the ages themselves, however, 

these "gaps" cannot be assigned much significance 

at present.

The model of Proterozoic evolution of the Arabian 

Shield presented by us previously (Greenwood and 

others, 1976) is consistent with data presented here, 

although the earlier results have been modified to 

reflect the newly recommended decay constant for 

87Rb as well as the analyses of additional samples. 

Although the tectonic episodes delineated by Greenwood and others (1976), which represent specific 

deformational periods in the areas where they were 

defined, may eventually be recognized more widely, 

we cannot extend these "events" shieldwide. At 

given periods of time, deformation, uplift, and erosion occurred in some areas while others were the 

sites of rapid accumulation of volcanic, volcaniclastic, and even carbonate strata. This situation was 

reversed later, providing evidence of repeated local 

cycles of deposition, deformation, intrusion, and erosion. Folded schistosities in the oldest strata demonstrate the repetitive nature of deformation, although 

folds from later events may be coaxial with those 

developed earlier. In short, multiple tectonic and 

magmatic phases are detectable within specific areas 

during the period between 920 and 680 m.y. ago, but 

some of these episodes were both temporally and 

areally restricted and did not affect broad areas of 

the Arabian Shield. This period of andesitic volcanism, diorite intrusion, island-arc sedimentation, 

and deformation with generally north-south structural-trends is temporally defined by the ages of the 

diorite batholiths. We favor treatment of this orogenesis as a single geologic event, the Hijaz tectonic 

cycle or orogeny (Brown, 1972; Greenwood and 

others, 1973, 1976), which represents the major 

period of development of the Arabian subcontinent 

and formation of continental crust within, an 

"oceanic" geologic environment.

Whereas plutonism and island-arc volcanism and 

sedimentation of the Hijaz tectonic cycle spanned a 

period of about 200 m.y., the culminating plutonic 

phase of orognesis was comparatively brief, covering approximately 40 m.y. between about 650 and 

610 m.y. ago. Magmas of this late-orogenic phase 

were much more evolved than those in earlier 

phases; plutons average over 70 percent Si02 ,
30 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

whereas diorite bodies, including trondhjemite, 

average only 64 percent (Greenwood and Brown, 

1973). As shown by Greenwood and Brown (1973), 

the differentiation trends of the two groups diverge 

significantly, suggesting derivation of the magmas 

from different source materials.

Rubidium-strontium studies not only support the 

interpretation of differences in the sources of Hijaz 

and Pan-African magmas but also provide data that 

reflect the evolution of sialic crust in Saudi Arabia. 

From correlation diagrams of ( 87Sr/86Sr) 0 and 

(Rb/Sr) with age (fig. 26), it is evident that both 

ratios are universally low in the early magmas. 

Even the metavolcanic rocks of Wadi bin Dwaynah 

(912 m.y.), which contain 74 percent Si02 , have low 

Rb/Sr ratios and probably represent trondhjemitic 

magma. Between 770 and 780 m.y., however, magmas enriched in rubidium (Rb/Sr ratios greater 

than 0.3) appeared for the first time. Magmas in 

which Rb/Sr ratios were below 0.2 were still common, but the average value was distinctly greater

90° 800 700 

AGE, IN MILLION YEARS

600

2.5

2.0

fa 1.5

o

ir 1.0

0.5

0.2

0

B

—

_

—

~

i i i

A Volcanic rocks

O Diorite-trondhjemite

D Granodiorite gneiss

O Diorite-granodiorite

O Granodiorite-granite

AtfV CfS^ O> I

900 800

AGE, IN

1 1 1 V 1

1 1

1 1 ~ 1 1 /\

1 1

1 d 1 1

1 1 V

1 1 ^ ~

D D 00

A O O

fj> — •

^ <x^

^A r^i fi> i O I I

700 600

MILLION YEARS

FIGURE 26.—Variation of (A) initial wSr/"Sr ratio and (B) 

Rb/Sr with age. Vertical dashed lines are drawn at two 

periods of major change in strontium isotope composition 

and Rb/Sr ratio of magmas. Strontium-evolution line (solid 

line) in A represents single-stage evolution from a meteorite initial 8T Sr/88Sr ratio of 0.699 to the average modern 

island-arc value of 0.7037 (Faure and Powell, 1972; Dickinson, 1970). Dashed evolution line is least-squares fit of data 

points shown here and listed in table 2 having Rb/Sr ratio 

less than 0.2, yielding wSr/*Sr values of 0.7036 for the 

present and 0.6989 at 4,500 m.y.

than that of the more primitive diorite-trondhjemite 

suite. Although many of the plutons having Rb/Sr 

ratios greater than 0.2 occur as gneiss domes, volcanic units are also represented in this group. We 

suggest that the change in magma type reflects an 

evolution of the source from one dominated by 

oceanic lithosphere and perhaps mantle to one including previously differentiated (that is, sialic) 

crust. Because this sialic component is juvenile, 

however, no increase in initial 87Sr/86Sr ratios is observed. As shown by Kistler and Peterman (1973), 

in places where magma was derived at least in part 

from ancient sialic crust 87Sr/86Sr ratios are elevated 

above those of magma derived solely from mantle 

or oceanic lithosphere.

Initial 87Sr/86Sr ratios obtained in this study 

(table 2) are compared to an average oceanic basalt 

strontium-evolution line (solid line, fig. 26A) representing a "single-stage" or constant Rb/Sr-ratio 

model for strontium evolution (Faure and Powell, 

1972; Gast, 1967). The evolution line was constructed using an average 87Sr/86Sr ratio of 0.7037 

for modern oceanic island-arc andesites and basalts 

(Faure and Powell, 1972; Dickinson, 1970; Peterman and Hedge, 1971) and an initial 87Sr/86Sr ratio 

(r{ ) of 0.699 and age of 4.5 XlO9 yr for the Earth. 

As shown by Peterman and Hedge (1971) and 

Brooks and others (1976), rt values of modern 

magmas show significant variation, much of which 

is probably related to source heterogeneities. Data 

presented in figure 26 show similar ranges, as would 

be expected. The evolution line is considered to be 

only an average and- depends critically upon the 

assumption of constant average Rb/Sr ratio (0.025) 

in the source region. The close agreement between 

this line and initial ratios of early shield-forming 

units from Saudi Arabia, however, is impressive 

(fig. 26). Not only does the best-fit strontiumevolution line for primitive Arabian magmas, 

as defined by data from all units studied having 

Rb/Sr ratios less than 0.20 (dashed line, fig. 26A), 

demonstrate the agreement of initial 87Sr/86Sr with 

a linear island-arc strontium-evolution model, but 

also the slope of this line indicates a similar Rb/Sr 

ratio for the source area (0.025). Extrapolated to 

4.5 xlO9 yrs, this line yields a 87Sr/86Sr ratio of 

0.6989, where its zero-age (modern) value would be 

0.7036. We conclude that the early arc-building 

magmas in Saudi Arabia were both compositionally 

and isotopically identical to those that would have 

been formed during the period from 900 to 780 m.y. 

ago in intraoceanic island arcs and were distinctly 

different from those that would have formed ensiali-
DISCUSSION OF THE RESULTS 31

cally from ancient continental crust. Magmas 

formed between 770 and 680 m.y. ago were more 

variable in 87Sr/86Sr, but the values for these ratios 

nevertheless average near the evolution line. Because these average values are consistent with the 

model, no significant amounts of older sialic material 

could have been involved, although involvement of 

very juvenile "granitic crust" cannot be excluded.

Units studied that formed after 670 m.y. are significantly enriched in rubidium relative to older 

rocks, and their Rb/Sr ratios are greater than 0.20, 

without exception. As stated, however, no analyses 

were made of layered gabbro that formed between 

about 660 and 620 m.y. ago, but the presence of 

these mafic rocks suggests a bimodal distribution of 

the Rb/Sr ratio during that period. Volumetrically, 

gabbro constitutes only a small proportion of the 

younger magmas, whereas almost all Pan-African 

magmas had compositions ranging from granodiorite to granite and Rb/Sr ratios greater than 0.20. 

Initial 87Sr/86Sr values for most of the Pan-African 

plutons fall somewhat above the best-fit strontiumevolution line of the older rocks (fig. 26), although 

in most cases the differences from the line are not 

significant at the 95-percent level of confidence. 

The slight elevation of the average initial ratio of 

these younger magmas above the strontium-evolution line, however, when combined with the marked 

increase in Rb/Sr ratio, suggests a contribution 

to the melts from a more highly differentiated 

source than that from which earlier units were 

derived. Because the rt values of the older units of 

the area are inconsistent with the presence of ancient sialic crust, the only possible source of sialic 

material in Arabia would have been the early arcforming units themselves. Although still low, the 

average Rb/Sr ratio of analyzed rocks older than 

670 m.y. is 0.15, well above the 0.025 value indicated 

for their source by their strontium-evolution line 

(fig. 26). Partial melting at 650-600 m.y. of this 

thickened, more siliceous, and higher Rb/Sr lower 

crust formed between 900 and 700 m.y., could 

produce both the higher Rb/Sr ratios and the 

slightly elevated rf values of the Pan-African plutons. The fact that the rt values of these plutons fall 

only slightly above the theoretical strontium-evolution line for oceanic island arcs indicates that this 

sialic component in the source of Pan-African magmas was juvenile, with insufficient time for significant enrichment in 87Sr.

When the age determinations of dioritic bodies 

(table 2) are plotted geographically (fig. 27), a

general decrease in age is seen from west to east 

(or southwest to northeast) across the southern part 

of the shield, although the sparcity of data does not 

justify any attempt to delineate age provinces or 

gradients. As will be noted from the small variation 

in Rb/Sr ratios of the diorite bodies, little if any 

systematic variation in rock type occurs, even though 

the ages have a range of almost 200 m.y. These data 

suggest an eastward migration of the axis of magmatism and presumably the volcanic arc itself with 

time. West of the major north-trending fault system 

that extends from north of Jizan toward Ranyah 

along Wadi Bishah at about long 42°30' E, most 

dioritic complexes are older than 800 m.y. As shown 

in figure 26, only low Rb/Sr magmas formed before 

about 775 m.y. ago, when granodiorite gneiss domes 

such as those at Jabal Mina and Wadi Bagarah were 

emplaced. These more highly differentiated bodies 

not only constitute the youngest plutonic units of 

the Hijaz cycle in each region but also suggest an 

evolution of the source of orogenic magmas. To the 

southeast, dioritic magmas range from 843 to 684 

m.y., but only the oldest are associated with trondhjemite. East of Bishah, units representing the greatest volume of magma range from diorite to granodiorite with ages between 750 and 680 m.y. These 

data document an eastward or northeastward migration of magmatic activity with the earliest magmas 

in each area being dioritic, but evolving to granodiorite as the volume of sialic material in the source 

regions increased.

In contrast to the geographic distribution of plutonic rocks of the Hijaz cycle, that of late-orogenic 

or postorogenic granitic rocks is shieldwide (fig. 

28). The migration of plutonic activity recorded by 

the decrease in ages of dioritic complexes from west 

to east does not continue in plutons younger than 

about 660 m.y. The youngest apparent age, 587 ±99 

m.y., granodiorite in the Wadi Miyah quadrangle, 

is from the north-central part of the area; but, as 

noted, this rock may have been affected by later 

alteration. Ages reported here (table 2) indicate a 

shieldwide period of largely postorogenic plutonism 

between 660 m.y. and about 610 m.y. with some indication of subsequent late-stage emplacement of red 

granite, some of which may be peralkaline.

Although the dioritic magmas are associated with 

the early evolution of an island arc, the granodiorite 

to granite suite differs from the diorites in age, 

composition, degree of deformation, and intrusive 

style, suggesting to us a genetic difference as well. 

This younger suite is contemporaneous with largely 

detrital sedimentary strata containing much less
32 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD 

40° 42° 44°

22° —

AMakkah

(Mecca)

Al Qunfudhah

Al Birk AAbha

Rb-Sr ages >800 m.y.

• Dioritic complexes

• Volcanic rocks 

Rb-Sr ages 680-800 m.y. 

O Dioritic complexes 

D Volcanic rocks

75 KILOMETERS

16° —

FIGURE 27.—Distribution of dioritic complexes and metavolcanic rocks older than 680 m.y.

primary volcanic material dominated by the products of presumably subaerial eruptions, including 

ash-flow tuff. Even though the older units of the 

andesitic assemblage were deposited in an area remote or isolated from eroding continental masses, 

by 540 m.y. ago the Arabian Shield had become part 

of the African plate, and the granodiorite to granite 

suite represents the last major magmatic event. We

suggest that this event, known as the Pan-African 

event or orgeny (Kennedy, 1964; Holmes, 1965; 

Clifford, 1967, 1968, 1970) and expressed not only 

in Saudi Arabia but throughout much of the eastern 

part of the African continent, represents a phase of 

continental collision or suturing of Arabia to 

the Proterozoic African plate. Subduction would 

have ceased as the continental margin of Africa
DISCUSSION OF THE RESULTS 33

40°

A

Makkah 

(Vecco,

Qunfudhah

Birk A 

Abha

EXPLANATION 

Rb-Sr ages <680 m.y.

• Plutonic rocks

• Volcanic rocks

75 KILOMETERS

18° —

16° —

FIGURE 28.—Distribution of volcanic rocks and late-orogenic or postorogenic plutonic rocks less than 680 m.y. old.

entered the trench, making this event the culminating phase of a prolonged period of plate convergence.

Our model suggests that prior to the collisional 

phase, consumption of a margin of oceanic crust on 

the east side of the Proterozoic African plate by 

subduction along an eastward-dipping oceanicoceanic plate-boundary (island-arc-type subduction

zone) generated new sialic crust along the Arabian 

island arc (fig. 29). As subduction proceeded, the 

distance between the Proterozoic African Continent 

and the Arabian island arc decreased, and progressively larger amounts of terrigenous detritus were 

added to andesitic volcanic and sedimentary sequences in the arc (see fig. 2). Because the areas of 

active volcanism, plutonism, and sedimentation mi-
34 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

WEST EAST

Trench Volcanic arc

SI. ==

85O m.y.

Proterozoic African Continent

Continental crust •**"- 

78O m.y.

Asthenosphe

SEA LEVEL 

Oceanic crust

Volcanic arc

; I !

SI.

Transitional 

crust

Volcanic arc

S I -M. *_*_* J. S_*_f-»- 

.L. .»...,«>.

7OO m.y. ^ Lithosphere 

'"""

62O m.y.

Molasse

Asthenosphere

FIGURE 29.—Plate-tectonic model of the origin and deformation of the Arabian Shield.
DISCUSSION OF THE RESULTS 35

grated to the eastern part of the Arabian island arc 

before collision, however, exposure of the older parts 

of the arc may have provided both a source of 

juvenile plutonic detritus and a barrier limiting 

amounts of African continental detritus. Because 

subduction would have continued until the time of 

actual collision of the African continent with the 

island arc and because the axis of subduction-related 

volcanism and plutonism (that is, andesite and diorite) was migrating eastward, the hiatus between 

island-arc volcanism and intrusion of the Hijaz cycle 

and the onset of collision-related (Pan-African) intrusion and tectonism would decrease from west to 

east. In the easternmost parts of the Arabian arc, 

subduction-related tectonism and plutonism would 

have progressed directly into those related to collision with no hiatus at all. Deformation of 640-630- 

m.y.-old granodiorite in the Bi'r Juqjuq and Mayza 

quadrangles and the presence of 684-m.y.-old 

gneissic quartz diorite in Wadi Malahah may be 

evidence of this transition.

As collision began, deformation, metamorphism, 

and granodiorite to granite plutonism became shieldwide. The collisional phase (the Pan-African event) 

probably began prior to 650 m.y. ago, subsequent to 

emplacement of the major diorite batholiths. Location of a suture or sutures between the Proterozoic 

African plate and the island arc or arcs is currently 

uncertain, although the report of an ophiolite body 

at Jabal al Wask (Bakor and others, 1976) raises 

the possibility that part of such a structure may lie 

within the Arabian Peninsula. Study of both diorite 

and granodiorite to granite intrusive bodies in 

northeastern Sudan (Neary and others, 1976), however, revealed the presence of intrusive rock types 

characteristic of the Arabian Shield. Darracott 

(1972) reports a gravity "signature" of the edge of 

the Pan-African orogen in East Africa, but this may 

well represent the limit of deformation within the 

African plate and could be well west of an actual 

suture. We would locate the westernmost parts of 

the Proterozoic island arc west of all occurrences of 

900- to 700-m.y.-old diorite but within the area affected by the Pan-African event. The presence of 

subsidiary or even imbricate zones of subduction 

within the Arabian Peninsula cannot be discounted, 

but such zones are inadequately documented at present. Possible locations along the major north-trending shear zones, such as that at about long 42°30' E., 

or the major contact between the basaltic and older

andesitic assemblage (fig. 3) might be suggested as 

areas for investigation.

In addition to explaining the early evolution of 

the present Arabian craton, this model accounts for 

intraplate deformations subsequent to subduction. 

Tapponierand Molnar (1976) presented an indentation-deformation model for continental-collision tectonics that has application to the Arabian Peninsula. 

Similar, though less rigorously developed, collision 

models have been presented by Sengor (1976), 

Dewey and Burke (1973), and Dewey and others 

(1973). During the Pan-African event, the Arabian 

Peninsula was subjected to simultaneous plutonism, 

folding, graben formation, and transcurrent faulting with large displacement (Fleck and others, 

1976; Delfour, 1970; Brown, 1972; Al-Shanti and 

Mitchell, 1976). Folding about nearly north-southtrending axes, such as that of 621-m.y.-old aplite 

dikes in the Al Qarah Quadrangle (loc. 51), must be 

accommodated by the same tectonic model invoked 

to explain the northwest-trending Najd fault system. We suggest that a continental-collision model 

such as that proposed for the Himalayan deformation by Tapponnier and Molnar (1976) provides a 

unifying explanation of Arabian intraplate tectonics. As shown by those workers, left-lateral 

strike-slip faults of large displacement, such as the 

Altyn Tagh fault north of the Himalayas, would be 

predicted by analogy to indentation of a semiinfinite rigid-plastic medium (Asia) by a rigid indenter (India). The angular relations of the northwest-trending Najd fault system to the nearly 

north-south trend of compressional structures in 

Saudi Arabia are nearly identical to those between 

the Altyn Tagh fault and the Himalayan fold system 

(fig. 30). These collisionlike intraplate tectonics 

combined with the strong evidence of pre-PanAfrican isolation from continental areas, the chemiical evidence cited by Greenwood and Brown (1973) 

and Greenwood and others (1976) for an eastdipping subduction zone, and the extended period 

of eastward-migrating, subduction-related magmatism suggest that the Pan-African event represents 

the culmination of orogenesis in Arabia during 

which time the juvenile Arabian subcontinent or 

island arc was sutured to the Proterozoic African 

Continent. This last tectonic episode probably extended from before 650 m.y. to at least 540 m.y. 

ago, and the cooling of rocks now exposed was completed after about 520 m.y. ago.
36 Rb-Sr GEOCHRONOLOGY, PLATE-TECTONIC EVOLUTION, SOUTHERN ARABIAN SHIELD

16«

300 KM

^^ ::S f- v^^jar^v^ »Canton 

c 1

* u*4 ^^OUTH C»JN* * *

iOO«E ^10«N 110«E 20«N 120«E 

0 500 KM

FIGURE 30.—A comparison of continental-collision-related structures of Asia (after Tapponnfer and Molnar, 1976) with orogenic structures of the Arabian Shield. Large arrows indicate implied direction of subduction prior to collision.

CONCLUSIONS

Basaltic volcanism with island arc affinities rather 

than mid-ocean-rift affinities occurred prior to 1,100 

m.y. ago in the area presently incorporated into the 

Arabian Shield. Island-arc volcanism, sedimentation, and plutonism began prior to 900 m.y. ago; 

magmas were dioritic to trondhjemitic, probably derived by partial melting of oceanic lithosphere or, 

less probably, upper mantle. After about 775 m.y. 

ago, more alkali-rich melts appeared with quartz 

diorite-granodiorite replacing diorite-trondhjemite. 

The youngest dioritic units occur in the eastern parts 

of the shield, emplaced between 680 and 720 m.y. 

ago when the north- or northwest-trending zone of 

magmatism migrated to its most easterly position. 

After this period of subduction-related orogenesis, 

called the Hijaz tectonic cycle, the newly formed arc 

was sutured to the Proterozoic African plate during 

a collisional phase recognized in Arabia and eastern 

Africa as the Pan-African orogeny. Plutonism related to this event began between 660 and 640 m.y. 

ago and continued after the major deforxnational 

period was concluded. The youngest, possibly postorogenic, plutonic rocks of the shield were probably

formed between about 610 and 570 m.y. ago, but 

low-grade metairorphism and wrench-fault tectonism continued until about 520 m.y. ago.

REFERENCES CITED

Al-Shanti, A. M. S., and Mitchell, A. H. G., 1976, Late Precambrian subduction and collision in the Al Amar-Idsas 

region, Arabian Shield, Kingdom of Saudi Arabia: Tectonophysics, v. 30, p. T41-T47.

Anderson, R. E.. 1977, Geology of the Wadi Tarj quadrangle, 

Sheet 19/42A, Kingdom of Saudi Arabia: Saudi Arabian 

Directorate General Mineral Resources Geologic Map 

GM-29, scale 1:100,000, with text.

____ 1978a, Geology of the Wadi 'Atf quadrangle, Sheet 

17/43A, Kingdom of Saudi Arabia: Saudi Arabian 

Directorate General Mineral Resources Geologic Map 

GM-30, scale 1:100,000, with text.

1978b, Geology of the Mayza quadrangle, Sheet

17/43B, Kingdom of Saudi Arabia: Saudi Arabian Directorate General Mineral Resources Geologic Map 

GM-31, scale 1:100,000, with text.

Bakor, A. R., Gass, I. G., and Neary, C. R., 1976, Jabal al 

Wasik, northwest Saudi Arabia—An Eocambrian backarc ophiolite: Earth and Planetary Science Letters, v. 

30, p. 1-9.

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it U.S. GOVERNMENT PRINTING OFFICE: 1980 O— 341-6U/194