Distribution and tectonic setting of plutonic rocks of the Arabian Shield

Journal of African Earth Sciences. Vot. 4. pp. 21-46, 1986

11731-3247/86 $3.1~)+I|.IX) Pergamon Press Ltd.

Printed in Great Britain

Distribution and tectonic setting of plutonic rocks of the Arabian Shield DOUGLAS B. STOESER* U.S. Geological Survey, P.O. Box 348, Jiddah. Kingdom of Saudi Arabia

Abstract--Although it is still not possible to model in detail the tectonic evolution of the Arabian Shield, its evolution can be interpreted in terms of the classic pattern of Phanerozoic plate tectonics (the "Wilson cycle'). During the first stage of evolution (900-63(I Ma), plutonism was dominated by intermediate plutonic rocks (diorite, quartz diorite, tonalite and trondhjemite) and involved a progressive evolution from primitive tholeiitic series arc rocks to mature calc-alkaline series rocks. These rocks formed in both ensimatic island arc and continental-marginal arc environments. The magmatic-arc stage was terminated by two collisions (680-630 Ma): between an accreted ensimatic arc terrane and a continental microplate and between the microplate and a continental('?) plate. These collisions resulted in a shift from arc magmatism dominated by intermediate plutonic rocks to collision-related granitic (granodiorite to monzogranite) magmatism (660--610 Ma). The final phase of plutonism within the Shield (610-5 ltl Ma) was the formation of widespread postorogenic intracratonic evolved peraluminous to peralkaline alkali-feldspar granites. Such granites typically form during the terminal relaxation phase of continental collisions. Although minor amounts of evolved peraluminous granites are present in the eastern part of the Shield, well-developed S-type granites (tourmaline- or cordierite-bearing granite) appear to be lacking. Minor amounts of syenitic plutonic rocks were also emplaced throughout the Shield from about 620 to 55(1Ma. The platonic rock assemblage of the Arabian Shield is composed of approximately 37% granite, 19% granodiorite, 17% tonalite and trondhjemite, 1.3% dioritic rocks, 7% alkali-feldspar granite (including 2.3% alkali granite and 1.3% aluminous granite), 6% gabbro, and 1% syenitic rocks. Plutonic rocks compose approximately 55% of the outcrop area of the Shield.

plutonic rocks, intermediate plutonic rocks, granitic plutonic rocks and syenitic plutonic rocks. The intermediate plutonic rocks include tonalite, trondhjemite, quartz diorite, monzodiorite and diorite. The granitic plutonic rocks include granodiorite, monzogranite, syenogranite and alkali-feldspar granite. The syenitic plutonic rocks include monzonite, syenite, alkalifeldspar syenite and feldspathoidal syenite. The tectonic setting of the shield is described in terms of the microplate accretion model of Stoeser and Camp (in press). In their model the Arabian Shield is divided into five terranes, four suture zones and the Nabitah orogenic belt (Fig. 1). In addition, the Asir terrane and the Nabitah orogenic belt are here subdivided into provinces (Fig. 1). The tectonic boundaries of Fig. 1 are shown in the subsequent maps (Figs. 2-11, 13).

INTRODUCTION THIS REPORTbriefly summarizes the distribution, geochronology and tectonic setting of the plutonic rocks of the Arabian Shield, with an emphasis on the granitoid plutonic rocks. It is based on a series of maps that show the distribution of various classes of" plutonic rocks, radiometric ages, and the names of significant plutons. These maps were extracted from the two l:l,000,000scale plutonic rock maps that accompany this volume (Stoeser et al. 1986). In this text occasional references to the units shown on these maps are in parentheses, e.g. monzogranite (gm). Plutonic rock petrographic terminology follows Streckeisen (1976) (see Ramsay et al. 1986b, this volume). This report utilizes 181 radiometric ages for Shield plutonic rocks selected from the literature (Appendix 1). Most early radiometric dating utilized K-Ar and Ar-Ar methods (Fleck et al. 1976, Aldrich 1978). However, after it was established that a major thermal episode about 570 Ma had disturbed these systems throughout much of the Shield (Fleck et al. 1976), Rb-Sr whole-rock and U-Pb zircon methods were employed. Because it is difficult to evaluate the earlier argon work, these data, with a few exceptions, are not used in the present report. In addition, two-point Rb-Sr isochron ages and ages having confidence limits of greater than 100 Ma are not included. The plutonic rocks of the Arabian Shield are described in terms of four main classes of plutonic rocks: gabbroic

* Present address: U.S. Geological Survey, MS 905, Branch of Central Mineral Resources, P.O. Box 25046, Denver Federal Center. Denver, Colorado 80225, U.S.A.

PLUTONIC ROCK ASSEMBLAGE OF THE ARABIAN SHIELD

Tables 1-3 present areal percentages for the map units of Plates 1 and 2. These data were derived from a point-count modal analysis of the maps. A 3-mm grid was used and yielded a total of 46,246 counted points. The data are presented for each terrane and province and shown in Fig. 1. It was not possible to obtain exact data for most specific rock types because composite and undifferentiated map units were used in the map compilation when detailed petrographic data were not available (e.g. an exact estimate of the amount of monzogranite within the Shield is not possible because monzogranite is included in the monzogranite (gm), granite (gr), granodi0rite-granite (gg) and granitoid undifferentiated (gt) map units). 21

D. B. STOESER

22

44o

400 I

36* !

I

28*

28*

R RAY

240

HIJAZ

YANSUSUTURE

240

BIR UMQ SUTURE's,

FAULT 20"

.....

TAIF

~

POSSIBLESUTURE ~

¢

! ,

~

ABHA

NABITAH OROGENICBELT

E

,~

20*

~'1

NABITAH

SUTUREZONE

o

~p

~oo

~,oo

~

~M

1Be 360

40 °

44 °

Fig. 1. Map of the Arabian Shield showing terranes, provinces, suture zones and the Nabitah orogenic belt. T e r r a n e s are identified with large capital letters and provinces with small capital letters.

Table 1. Proportions of major geologic units within the A r a b i a n Shield (in percentages) by terrane and province Asir terrane

Nabitah orogenic belt

"~

"7.

"~

,.'~

,~'

'7.

~,

17.9 10.0 0.3 30.2 41.6

18.9 31.5 -33.3 16.3

18.3 19.5 0.2 31.6 30.4

9.8 31.2 1.9 17.8 39.3

2.3 2.8 -45.9 48.9

28.4 2.5 -54.4 14.8

19.1 2.6 -55.6 22.6

14.3

11.3

25.6

18.4

7.4

5.8

(6602) (5234)11836

8515

~

~

13.8 --49.7 36.5

22.1 --31.4 46.5

I-

=

,.

Unit Alluvium Cenozoicvolcanicroeks Phanerozoic sedimentary rocks Precambrianplutonicrocks Precambrian layered rocks PercentofShield Numberofcounts

3429 (2662)

9.5 0.7 2.5 63.2 24.1

11.5 -2.9 64.0 21.6

17.6 1.5 1.0 58.6 21.3

---74.3 25.7

13.8 11.3 0.7 41.1 33.1

8.8

7.7

2.3

24.5

20.4

2.7

1.0

100.0

(4097)

(3546)

(1059)

11335

9446

1250

435

46246

'Percent of Shield' refers to the areal proportion of each terrane or province within the Shield. N u m b e r of counts refers to n u m b e r of counts from modal analyses of the Plates 1 and 2. N u m b e r of counts for each province shown in parentheses. See text for detailed explanation.

Distribution and tectonic setting of plutonic rocks of the Arabian Shield

23

Table 2. Proportions of plutonic rock units within the A ra bi a n Shield (in percentages) based on the plutonic rock units of Maps 1 and 2 Asir terrane

Nabitah orogenic belt

.=

~

_

~

~=

-

-

.-~

_

•~

~

0.19 3.35 0.46

--0.22

0.70 0,29 5.31 6.16

-

'~

~

'r,

ba ~

~

0.10 1.79 0.35

3.69 0.19 2.92

7.67 0.70 2.84

8,16 0.34 10.78

2.19 11.17 2.07

2.10 0.62 2.95

4.57 4.42 --

3.69 0.80 4.04

0.36 0.47 2.47

-19.64 --

----

2.35 1.34 2.56

--

0.37

5.68

4.88

1.41

0.76

4.57

1,27

1.09

8.53

--

1.64

0.06 5.74 14.06

0,20 5.52 9.83

0.45 8.82 5.113 18.22 9.60 6.04

7.88 25.50 2.90

1.10 2.90 4.16

2.59 16.11 1.78

0.15 14.16 2.80

2.98 13.41 2.95

4.41 14.70 13.70

--0.60

-8.05 --

2.94 11.43 7.63

11,13

1.45 0.04 32.24

0.13 -29.81

--23.16

2.96 0.01 25,47

0.30 11.40 1.51 -24.26 54.96

----

1.34 0.38 22.32

13.33 3.64 0.97 -8.03 .

7.14 2.45 5.00 2.95 10.93

2.21 6.78 7.52 -0.29 --

7.21 3.14 2.79 (I.99 7.21 --

1.34 11.08 4.05 0.114 0.73 11.64

--

11.511

6.64

Mapunit (ag) Alkali granite (mg) Aluminousgranite (gf) Blot. and(or) hbl. alkalifeldspar granite (ga) Alk ali-feldsp argranite and(o r) syenogranite (gs) Syenogranite (gm) Monzogranite (gr) Monzogran ite and(or) sy enogran ite (gh) Granophyre (gu) G r a n i t e , undifferentiated (gg) Mo nzogran ite and(or) g ran odiorite Gran itoid, undifferentiated (gt) Granodiorite (tg) Tonalite (tt) Trondhjemite (tj) Granodioriteandtonalite (tx) Un differen tiated plag.-rich (tu) granitoid Undifferentiateddioritic (dt) and(or) plag.-rich granitoid Quartzdiorite (dg) Dioriteand(or)diabase (di) Monzodioriteorquartz (dm) monzodiorite Un differen tiated dioritic (du) rocks Un differen tiated dioritic and (db) gabbroic rocks Undifferentiatedgabbroic (bu) rocks Syenitic rocks (su, sf, sy, sin, fs) Percent P r e c a m b r i a n o u t c r o p

0.(15 - 0.02 . . . . 4.10 28.01 15.25 0.10 1.89 38.26 3.30 3.61 --

-0.06 5.22 3.45 6.55 23.49 -1.77 2.47 3.08 ---

14.82

18.14

1.31 3.33 0.05

-3.50 0.80 0.26 10.04 1.19

-. 13.50

5.01 2.60 7,48 0.12 0.52 . .

9.19 -1.73 --3.38 .

--0.20 5.36 -0.20

-0.62 12.68 9.91 3.10 0.93

3.28 5.02 7.50 1,05 4.20 0.27

5.32

--

56.96

10.21

15.78

13.00

7.95

10.88

2.23 1.27 0.22

1.75 2.05 2.34 3.43 11.14 0.19

0.06 1.83 --

1.31 0.14 2.49

-3.37 --

2.05 2.41 --

-7.23 --

0.98 2.75 0.54

0.53 1.49 0.98

--2.78

-/I.31 --

0.98 2.22 0.55

0.15

1.27

0.67

0. I3

--

3.32

1.93

--

--

1,39

5.69

0.99

3.10

2,11

8.32

12.54

10.26

1.19

0.37

--

5.61

3.88

4.13

3.66

2.011 11.99

--

3.95

2.96

2.01

2.49

8.611 5.96

1.45

2.54

5.67

6.49

3.76

2.28

5.36

4.34

3.76

1.31

--

11.68

2.50

2.07

1.97

I).67

0.02

1.35

0.58

--

--

11.98

40.3

74.3

67.1

41.2

16.37

2,69 . 20.08

11.28

511.9 31.2

0.37 48.4

78.6

72.4

73.4

74.8

73,5

57.7

55.4

'Percent Precambrian outcrop" is the proportion of plutonic rock to layered rock within each terrane or province, b i o t . - - b i o t i t e , hbl.--hornblende, plag.--plagioclase

Table 3. Estimate of proportions of ma j or classes of plutonic rocks of the A ra bi a n Shield (in percentages) Asir terrane

Nabitah orogenic belt

Rock type Alkali-feldsp ar granite Granite Granodiorite Tonalitic rocks Dioritic rocks G a b b r o i c rocks Syenitic rocks

4.3 14.3 5.8 511.8 16.4 7.1

Total granitic rocks Total i n t e r m e d i a t e rocks

24.4 67.2

1.3

0.2 33.9 20.4 16.9 20.3 8.3

9.6 3/).7 18.6 15.1 14.3 9.2 2.5

13.7 44.8 10.8 15.0 9.2 6.1

--

2.4 23.4 12.6 35.1 18,2 7.6 0.7

11.4

54.5 37,2

38.4 53.3

58.9 29.4

69.3 24.2

2. }

5.1 33.2 27.1 12,1 15,2 5,3 2,0

6.0 39.5 24.6 14.3 7.3 7.6 0.7

11.3 32.1 19.11 13.7 15.3 8.6 tr

9.2 39.3 21.3 11.6 11,7 5.6 1.3

3.8 48.0 23.9 7.9 12,5 3,3 0.6

23.9 32.7 27.5 5.8 4.3 5.8 --

-8.1 2.2 53.5 31.9 4,3 --

7.1 37.3 19.1 16.4 13,4 5.7 1.0

79.4 17, I

65,4 27.3

71}.1 21.6

62.4 29.0

69.8 23.3

75.7 20.4

84.1 10.1

10.3 85.4

63.5 29.8

19.4 52. I 7.9 5.6 1t.5 1,4

Estimates are based on calcalutions presented in A p p e n d i x 2 and data from Table 2. t r - - t r a c e ,

24

D.B.

360

J

STOESER

400 I

J

440

I

I INSET

~ 7 ¢ 17 (271 631 ~7 17371

280

± 5 (Z7)

821 ~ 17 ( Z 7 )

'/ 738~21Z24)

--~

834 ~ 6 IZ37) 629 ~ 7 1Z371

':~

~

280

844,¢ 6 IZ7)

581 ¢8(Z3"/I ~/ 632 26 (Z7l 635:1: B IZ3"/) J

72S±4(Z ~ 1IZlll

" S4SK (Zl0)

688 * 5 ( Z l 0 )

JAR 71~

(NAISTI IQWAO820:1:41 IR6IMURAT UMM QABIR713 :t 29 (RI|I

240

J.SALSJAH 743 :t 10 (Z21

V~

240

t,.

.'-. SHUFFAYAN71S ::1:7IZ231 aUSTAN am ± e ( z ~

B00:1:76 ( R . I

7(1~ ~ 33 ( R 181

821t3(Z~)

828:1" g3 IR2I)-

200

854 :l: 10 1R32) .t

:

~- 782 ± 28 (R28)

779 ~ 49 1 R 2 9 ) ~ 744 ± 22 (R2Sl- j

ALLUVIUM AND PHANEROZOIC COVERROCK

Bll IURSHI PLUTONS ~ EBO± 671A201 ~ / j CENOZOICVOLCANIC ROCKS

/. ~L"

/

AN NIMAS BATHOUTH ~2±5(Z~1

797:1:15(

DIORITIC AND PLUTONIC

TONALITIC ROCKS

0

100

200

300

400

I

I

I

I

I

I 360

I

818

I

I 40 °

732¢3 1Z391

e82 ~: 4 (Z39) "d

500 KM

~±

I

9 cz121

I

I

I

I

44 °

Fig. 2. Map of the Arabian Shield showing intermediate plutonic rocks (tonalite, trondhjemite, quartz diorite and diorite). See Appendix 1 for key to radiometric ages and codes used. Jabal is abbreviated to J in pluton names. Tectonic boundaries are from Fig. I, (Includes tt, tj, tx, dt, dq, dm, di and du map units of Stoeser et aL 1986. Plates I and 2 of present volume).

200

Distribution and tectonic setting of plutonic rocks of the Arabian Shield 36 ° I

400

44 o

I

I

28 c

25

28 o

HUt-AYFAH

24'

24 o

~0

20 °

~"

( covered }

20 °

FAULT

| I t

BELT OF INTERMEDIATE PLUTONISM

SUTURE ZONE

0 i 160~

100 i

200 J

300 i

I

400 i

500KM I I

~

~' ,: I

Fig. 3. Map of the Arabian Shield showing the location and age of magmatic belts containing intermediate plutonic rocks. See Table 4 for additional information.

Table 1 presents the proportions of alluvium, Phanerozoic cover rock, and Precambrian plutonic and layered rocks within the Shield as shown in the Plates 1 and 2. Table 2 presents the results of modal analysis of the plutonic rock assemblage of the Shield based on the map units of Plates 1 and 2. The proportion of plutonic rock to layered rock by terrane and province is also given. The proportion of Precambrian plutonic to layered rock is a maximum value, given the probable tendency of the map compilers to favor plutonic rocks over layered rocks whenever decisions relating to generalization of map units were made. Based on the data from Table 2, Table 3 estimates the proportions of the major plutonic rock types of the Shield. The equations for calculating the estimates of Table 3 are presented in Appendix 2.

GABBROIC AND INTERMEDIATE PLUTONIC ROCKS

Gabbroic plutonic rocks The gabbroic plutonic rocks of the Arabian Shield will not be discussed in detail in this report. Gabbros constitute approximately 6% of the plutonic rock assemblage

of the Shield (Table 3). An exact estimate is not possible because many of the gabbros were combined with diorites in the diorite-gabbro undifferentiated unit (db). The bulk of the gabbros of the Shield appears to be spatially associated with the intermediate plutonic rocks. Significant amounts of gabbros are also associated with the ultramafic complexes of the suture zones. Late to postorogenic gabbros occur throughout the Shield but are only abundant in the southern part where they often occur as layered intrusions (Coleman et al. 1972, Stoeser and Elliott 1980).

Intermediate plutonic rocks Intermediate plutonic rocks constitute about 30% of the plutonic rock assemblage (Table 3) and are found throughout the Arabian Shield (Fig. 2). Radiometric ages for these rocks indicate that the bulk of them formed from about 900 to 630 Ma (Fig. 12). Many of these rocks occur in belts; Figure 3 shows the locations and names of these belts. Table 4 summarizes the lithology, age and reference sources for each belt. Because intermediate plutonic rocks are characteristic of magmatic arcs, these belts potentially indicate the locations of late Proterozoic arcs within the Shield.

D. B. STOESER

26

Table 4. Tabulation of lithologies, ages, and source references for belts of intermediate plutonic rocks shown in Fig. 2 Dominant lithologies

Belt

Age (Ma)

Asir terrane Wadi Tarib

tt, dq, di, gb

AI Qarah

tt, tg, mg, tj, di, bu

680-640

An Nimas

tt, tj, dq, di, bu

8411--7911

Biljurshi

tt, dq, di

Shuqub AI Lith Makkah Jiddah

tt, dq tt, dq di, dq, bu, tg, tt tt, dq

~850 >801) ') 820-770

Hijaz terrane Birak

tt, tg, tj, di, bu

811~-715

Hulayfah

tt, tg

Midyan terrane Nabt

tt. dq, di, bu

820-730

Afif terrane Hail Liban Nuqrah

tt, tg, di dm, di, dq, tt tt, dq, di, bu

~740 ~645 ~830

Siham Damar Tathlith Najran

tt. di, tg, grn di. dq, bu, tt, tg tt, dq, di dq, di, tt

7110-6411 '~ 700-660 ~640

Nasfah Sawda

di, bu, tt, tg dq, tt, di

~645 690-675

Ar Rayn terrane Ar Rayn

tt, tj, dq, di

660-620

>750-720

>900-800

>7211

References Schmidt et al. ( 19791 Stoeser et al. (1984) Schmidt et al. (1979) Stoeser et al. ( 19841 Greenwood et al. (1976, 1982) Cooper et aL (1979) Fleck et al. (1980) Fleck et al. 11980) Greenwood et al. (1976, 1982) Marzouki et al. (1982) Radain et al. (1984) Kr6ner et al. (in press) Skiba ( 19801 Fleck et at. (1980) Skiba (1980) Camp ( 19841 Jackson et al. (1984) Delfour ( 19771 Calvez et al. ( 19831 Kemp et al. ( 19801 Camp (1984) Quick and Doebrich (in press) Johnson and Williams (in press) Delfour (19771 Calvez et al. (1983) Agar (in press a, b) Thieme (in press) Cooper et al. (19791 Sable (in press) Stocser et al. (1984) Thieme (in press) Delfour (1979) Stacey et al. (1984) Cole (in press) Calvez et al. (1983) Stacey et al. (1984) Le Bel ~.nd Laval (1986, this volume)

Lithologies are listed in approximate order of abundance. (Lithology codes: bu, gabbro; di, diorite and diabase: dq, quartz diorite: din, monzodiorite; gm, monzogranite; tg, granodiorite; tj, trondhjemite; and tt, tonalite).

T h e oldest reported intermediate plutonic rocks are from the tonalitic Bidah pluton of the central Asir terrane and have a R b - S r whole-rock isochron age of 901 + 37 Ma (Marzouki et al. 1982). Most o t h e r dioritic and tonalitic plutonic rocks o f the central Asir have ages of 900-790 Ma. Some of these rocks m a y be the oldest intermediate plutonic rocks within the Shield; h o w e v e r , intermediate plutonic rocks older than 800 Ma also occur in the Taif portion of the Asir terrane, in the Hijaz terrane, and possibly in the n o r t h e r n Nabitah o r o g e n i c belt (Fig. 2, Table 4). I n t e r m e d i a t e plutonic rocks that range in age from 770 to 640 Ma are widespread t h r o u g h out much of the Shield, and there is a general t e n d e n c y for these rocks to y o u n g eastward and n o r t h w a r d from the Asir terrane core. T h e early intermediate plutonic and volcanic rocks of the western part of the Shield are chemically primitive and tend to have tholeiitic affinities typical of ensimatic island arcs (Bokhari and K r a m e r s

1981, M a r z o u k i et al. 1982, Schmidt and B r o w n 1982, J a c k s o n 1986, this volume). T h e y o u n g e r intermediate plutonic and volcanic rocks are typically m o r e m a t u r e in character and are of calc-alkaline affinity ( D e l f o u r 1979, 1981, Schmidt and B r o w n 1982, R o o b o l et al. 1983, J a c k s o n et al. 1984). A l t h o u g h at least s o m e of the belts described a b o v e almost.certainly m a r k the site of arc m a g m a t i s m , Figs. 2 and 3 clearly show the v o l u m i n o u s and w i d e s p r e a d nature o f intermediate plutonic rocks within the A r a b i a n Shield and indicate the c o m p l e x task of defining their origin and placing them in the e v o l u t i o n a r y history of the Shield. A l t h o u g h a considerable n u m b e r of radiometric studies ( A p p e n d i x 1) include the intermediate plutonic rocks and associated volcanic rocks, such studies have t e n d e d to be of a reconnaissance nature and only very limited age data are available for most of the plutonic rock assemblages within the belts defined in Fig, 3. In

Distribution and tectonic setting of plutonic rocks of the Arabian Shield general, the ages of the intermediate plutonic rock assemblages are so poorly known that detailed tectonic modelling is not possible. Continued study of these rocks is important because the Arabian-Nubian arc assemblage is one of the most extensive and best exposed terranes of late Proterozoic subduction-related rocks on earth (Gass 1982, Vail 1983, Kr6ner 1985).

GRANITIC PLUTONIC ROCKS The granitic plutonic rocks are here subdivided into three groups: granodiorite and granite-granodiorite complexes, monzogranite and syenogranite, and alkalifeldspar granite. The alkali-feldspar granite group is subdivided into: biotite and(or) hornblende alkalifeldspar granite; alkali granite; and aluminous granite. The distribution of all granitic plutonic rocks of the Shield is shown in Fig. 4. Granitic plutonic rocks constitute about 63% of the plutonic rock assemblage of the Shield (Table 3) and therefore are volumetrically the most important.

Granodiorite and granite-granodiorite complexes Figure 5 shows the distribution of granodiorite and granite-granodiorite complexes within the Shield (granodiorite (tg) and monzogranite and(or) granodiorite (gg) units of Plates 1 and 2). Figure 5 also shows the granitoid undifferentiated (gt) unit; this unit includes some intermediate plutonic rocks but consists predominantly of granitic rocks. Collectively these three units comprise about 30% of all Shield plutonic rocks (Table 2); the granite-granodiorite unit (gg) alone comprises 22%. The granite-granodiorite unit was used in Plates 1 and 2 to indicate complexes composed dominantly of granodiorite and monzogranite; in some cases, however, it was used as an undifferentiated term when further subdivision was not possible. The unit generally indicates major batbolithic complexes that also include migmatite, intermediate to gabbroic plutonic rocks, and remobilized older plutonic and layered rocks. The granite and granodiorite of these complexes commonly have a weak to well-developed tectonic foliation. They in general, therefore, have the characteristics ascribed to synorogenic plutonic rocks. The granite-granodiorite complexes are particularly abundant in the Nabitah orogenic belt, and in the southern part of the belt form the cores of gneiss domes and antiformal gneiss complexes, some of which have outer envelopes of migmatite (Schmidt et al. 1979, Stoeser et al. 1982, 1984). During early work in the Shield, the presence of these complexes led to erroneous reports of older basement throughout much of the southern and eastern Shield (e.g. Schmidt et al. 1973, Delfour 1979, 1981). The question of an older basement is discussed below. The granodiorite and granite-granodiorite complexes formed chiefly from 760 to 580 Ma, with a peak in ages between 660 and 640 Ma (Fig. 12). The emplacement of

27

large amounts of granodiorite to granite after 660 Ma marks a major shift away from the intermediate plutonism typical of the Shield during the previous 240 million years.

Monzogranite and syenogranite Monzogranite and syenogranite occur throughout the Shield but are concentrated in the Nabitah orogenic belt and eastern Shield, with the exception of the Taif province of the Asir terrane and the northern part of the Midyan terrane (Fig. 6). Although these rock types appear to be lacking in the central part of the Nabitah orogenic belt, this deficiency is an artifact of mapping due to a lack of detailed information in that region. They are included in the broader granitoid undifferentiated (gt) and granodiorite-granite (gg) map units and therefore are shown in Fig. 5. The emplacement of granite (stricto senso) within the Shield did not begin until about 680 Ma, after which it became the dominant plutonic rock type. Most of the granite formed between 660 and 610 Ma is mediumgramed leucocratic biotite monzogranite. Figure 6 also includes younger more evolved monzogranite and syenogranite, but it was not possible within the scope of the present study to discriminate these rocks. They are typically younger and often associated with the alkalifeldspar granites discussed below. Minor amounts of rapakivi and microcline-megacrystal granites appeared late in the granite sequence,

Alkali-feldspar granite Alkali-feldspar granite constitutes about 7% of all plutonic rocks of the Shield (Table 3). The alkalifeldspar granites are typically chemically more evolved than the monzogranites and syenogranites and form the most important environment in the Shield for mineral deposits associated with granitic rocks. The alkalifeldspar granites are subdivided on the basis of their mineralogy into three classes: biotite and(or) hornblende alkali-feldspar granite; alkali granite; and aluminous granite (see Ramsay etal. 1986a, this volume, for a more detailed explanation of this classification). The aluminous granites are typically subsolvus, whereas the first two groups generally have hypersolvus textures (with perthitic feldspars). The biotite and(or) hornblende alkali-feldspar granites are metaluminous. Alkali granites are defined as granites containing one or more sodic pyriboles (e.g. katophorite or arfvedsonite amphibole and(or) aegirine or aegirine-augite) and thus must be chemically peralkaline. The aluminous granites are defined as those that contain one or more aluminous minerals, generally muscovite or Fe-Li mica and garnet. The occurrence of these minerals necessitates that these rocks are chemically peraluminous. The aluminous granites are classed with the alkali-feldspar granites because their feldspars are generally microcline and albite.

28

D . B . STOESER

360

400

44 o

I

280 280

240 240

200 •

,

d

200

,~

GRANITIC PLUTONIC ROCKS 0 I

100 I

I 360

200 I

I

300 I

I

400 I

I

500 I

I 400

KM

I

I

I 440

Fig. 4. Map of the Arabian Shield showing all granitic rocks (includes tg, gg, gt, gr, gm, gs, ga, gf, ag and mg map units).

Distribution and tectonic setting of plutonic rocks of the Arabian Shield

360

400

I

I

I

29

440

1

I

I

I

I

I

28 0

280

/

614:1:5 {ZlOI

'.

DHlI31YAH 617 ~:~ 1Zll )

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e41 ± r~ m / : ~"J"~HISHA~ ,

AR RUKHAMA 641 ±25 IZ3$)

240

240 I

r18 :~25 (R21) 637 ± 11 (Z34)

,.p BSB :¢8 (Z34

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'~623 ± ' 8

200

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~.r '

BAGARAH GNEISS 763 ± 4 IZ12)

I

b

~.~,

/I, ~ ~ "'~

!

KHAMIS GNEIS£ 643 ¢ 20 IR20)

GRANODIORITE GRANITE-GRANODIORITE COMPLEXES 0 I

100 I

200 I

AND k" !

300 I

400 I

500 I

•

~31z28,

KM

"~c"

I 360

I

I

I

I 400

I

I

I

I

440

Fig. 5. Map of the Arabian Shield showing granodiorite and gramte-granodiorite complexes (includes gg, gt and tg map units).

30

D.B.

STOESER

40 o

360

I

I

I

I

440

I

I

28 o

I

I

~661 (Z24)

". ~1~

672 ± 30 (R221

28 o

616 - 10 1 Z l l 1

J.ANMAUTH 5 ~ ~ 15 1R25)

LIBAN 621 ¢ 7 {Z22)

-y

676 :i: 4 1Z221 AR AR'AL 636 ¢ 23 (RI

673~: 25 iR251 ~

240

(

•

A L 0 1 ( I A 6 4 0 ± 14 (R25) - -

24 o AD DARA 602 ± 57 (R25) AL WAQQJkS 563:1:7 (R231

~-

ASH SHARAH 573 -+ 22 (RS) ~

580-+9

/ / f

RAMRAM 769 ± 5 (Z$) / I

773 -+ 16 { R I ? I ~

608 ± "•'-"-IR13)9

200 525 ± 2 (R30)

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-

~

AND SYENOGRANITE 0 L

100 I

I 360

200 I

I

300 I

I

\ \ ,.' 400 I

I

500 I

I 400

KM

I

t

-I

I 440

Fig. 6. Map of the Arabian Shield showing monzogranite and syenogranite (includes gin, gs and gr map units).

200

D i s t r i b u t i o n and tectonic setting of plutonic rocks of the A r a b i a n Shield

360 I

I

I

400 I

I

_ I

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31

440 I

I

MARSNAH

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28 © /J SALMA 5~)~: 5 {RilO)

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¢~R RADAD~,H 574 ~: 28 tR7) / ' J . OUTN 579 ~_ 4 IR401 , ASH S H U ' B A H ~

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' ' ~

.

..

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~1Z37 I

LZ34)

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602 ± 4(Z11

I

200

20 ¢ BISHAH " / 678 ~: 10

IZl21 ( "i/"

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Q

BIOTITE AND/OR HORNBLENDE ALKALI-FELDSPAR GRANITE 0

I00

l

I

I 360

200 l

I

300 I

I

400 I

J

•

,r }

500 KM l

J 40 o

-"x.

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i

-1

I

I

44 o

Fig. 7. Map of the Arabian Shield showing biotite and(or) hornblende alkali-feldspar granite (includes gf and ga map units).

32

D, B. STOESER

Biotite and(or) hornblende alkali-feldspar granite

Biotite and(or) hornblende alkali-feldspar granites occur throughout most of the Shield, except the Asir and Ar Rayn terranes, and are particularly abundant in the Nabitah orogenic belt (Fig. 7). Alkali-feldspar granite (gf) and alkali-feldspar granite and(or) syenogranite (ga) are shown on Fig. 7 and represent approximately 4% of all plutonic rocks of the Shield (Table 2). These rocks were emplaced between 670 to 565 Ma and most are younger than 630 Ma. They typically occur in rather homogeneous plutons that rarely contain significant amounts of pegmatite or veining but may have an outer zone of alkali granite. The rocks are mesosolvus to hypersolvus granites that typically consist of coarse-grained perthite and quartz accompanied by brown biotite and(or) dark-green ferroedenitic hornblende. Ferroaugite to ferrohedenbergite pyroxene also occurs in some samples, and fayalite was observed in one sample from the southern Shield (Stoeser, unpublished data).

A Ikali granite

Figure 8 shows the distribution of alkali granites within the Shield. The Arabian Shield contains one of the largest fields of alkali granites in the world, and their occurrence is comparable to those of the Jos Plateau of Nigeria and Niger (Jacobson et al. 1958). They form, along with the aluminous granites, one of the economically important groups of granites in the Shield and are associated with Nb, Th, U, Zr, and REE mineralization (Drysdall et al. 1984, Ramsay et al. 1986a and Jackson 1986, this volume). Forty-nine major and more than a dozen minor alkali granite plutons are concentrated in the Midyan and Hijaz terranes and the Nabitah orogenic belt; alkali granites do not occur in the southwestern and easternmost parts of the Shield (Fig. 8). They are concentrated in the northern half of the Shield; only eight plutons occur south of 23°N latitude. In addition to the alkali granite plutons, major peralkaline rhyolite dike swarms occur in the northeastern Shield (Stoeser and Elliott 1985) (Fig. 8). Stoeser and Elliott (1980) presented an alkali granite map for the Shield that was based on reports from the literature of alkali pyribole-bearing granites. It was subsequently determined (Stoeser, unpublished data) that ferroedenitic hornblende from a number of these granites had been misidentified as an alkali amphibole; therefore, the map of Stoeser and Elliott is partially invalid. The alkali granite map of Fig. 8 is supported by mineralogic or whole-rock chemical data for most of the granites shown. In general, alkali granite is the last major intrusive phase wherever it occurs. Twenty radiometric ages are available for this rock type, and these data show that the alkali granites were emplaced during a span of about 180 million years (686--518 Ma); all but four formed between 630--565 Ma. Although there is no obvious age distri-

bution pattern, there seems to be a slight younging trend eastward across the northeastern part of the Shield. Radiometric ages for the alkali granites vary significantly within clusters of these granites (Fig. 8), although this may be an artifact of technique. The chief exception to alkali granite being the latest intrusive phase is the Subh suite of the southern Hijaz terrane (686 _+ 18 and --660 Ma), which is intruded by a younger series of monzogranites (Jackson et al. 1984). The youngest known pluton within the Arabian Shield is the alkali granite of Jabal Rawda. It has been dated both by the Rb-Sr and by U-Pb zircon methods, giving concordant ages of 517 _+ 10 and 510 _+ 15 Ma, respectively (Kemp et al. 1980, A l e i n i k o f f e t a l . in preparation). The alkali granites are spatially associated only with other granites. They commonly occur in plutons and complexes in which alkali granite forms an outer rim and meta[uminous hypersolvus or subsolvus granite forms the core (Harris and Marriner 1980, Radain 1980, Stoeser and Elliott 1980). In particular, it seems clear that the alkali granites are on the same evolutionary line of descent as the hornblende-bearing alkali-feldspar hypersolvus granites and often occur in the same plutons; there appears to be chemical and mineralogical continuity between them (Stoeser unpublished data). Stuckless et al. (1983) and du Bray (1986, this volume) have proposed that some of the late peraluminous granites of the northeastern Shield are genetically related to the metaluminous and peralkaline hypersolvus alkalifeldspar granites.

A l u m i n o u s granite

Aluminous granites, although not volumetrically important, occur throughout the Shield. Fifty-nine aluminous granites are shown on Fig. 9; 16 occur west of the Nabitah orogenic belt, 18 in the belt, and 25 east of the belt. Eight of the sixteen aluminous granites west of the Nabitah orogenic belt are located in the Asir terrane. The Asir aluminous granites have not been studied in detail and, in general, appear to be postorogenic in style, weakly aluminous, and leucocratic with minor amounts of muscovite and biotite. They probably resulted from partial melting of the volcanic wacke sedimentary rocks associated with the Asir ensimatic volcanic rocks. The only available age for an Asir aluminous granite is 620 + 18 (Fig. 9). Some aluminous granites in the Nabitah belt are synorogenic and formed during the Nabitah orogeny, such as the garnetiferous Madha suite in the southern Shield (Fig. 9) (Stoeser et al. 1984, du Bray 1986a, this volume), Some of those along the eastern margin of the belt, such as the Jabal al Gharrah and Bwana granites, are postorogenic and belong to a more evolved suite (Elliott 1983, Stuckless et aL 1983, du Bray 1986a). Within the Nabitah belt the only available ages are 667 _+ 7 Ma for the Madha granite and 577 + 19 Ma for the Jabal al Gharrah granite (Fig. 9).

Distribution and tectonic setting of plutonic rocks of the Arabian Shield

360 I

S

I

400 I

I

I

L

~,GH 557 ¢ 13 ~Z I ) AYYAH

280

"

630 +- 19 { R 2 5 l

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j KIFANTAH • J RAYOAN •J N U M R A N

625 ± 5 I Z 2 2 ) J MASSAH ~ 629 ¢ 12 IR251

440 I

~

./J.AJA 566 ¢ 4

.QARAQIR

33

•

SHAJARAH~

SHU~RMAH

N I M A R E04 • 26 [ R 4 0 ) ~ J A S SILSILAH / J

TUVVALAH 621B 5E 4 ( R 4 0 )

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/-

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240

H

--~.

(

\

FAH 563 ± 71 [R21)

\ l j

240

J-

WAR JAN H A M R A 686 ¢ IB ( R I 7 1 -

L\

J SUBH ~ 660 (Z U M M AL 8

,,.p

Z U B A Y D A H PIPE J

\

/

J H A D B ASH SHARAR 5B4 - 2{, ( R 5 ) SUMR AL ISHAR " J H A O B AO O A Y A H I N J H A S L A H 625 +- 5 ~Z33l

.

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-7

200

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) t / I

-

200

1

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/ \ : : : : :

PERALKALINE

RHYOLITE

DIKE

SWARM

ALKALI GRANITE 100 I

+

I 36 °

200 I

~

300 I

I

"~'~.,/.

400 I

I

'J

500 KM ,I

I

I

I

1

400 Fig. 8. Map of the Arabian Shield showing alkali granites (ag map unit).

I 44 o

34

D . B . STOESER

360

400

I

I

440

I

I

I

280

280 J AKASH

--J

QUTN 579 ± 4 tR40) ( SILSILAH } FAWWARAH 587 :l: 8 I Z 16

/ f

-BALD At. JIMALAH 571 :¢ 1~ ( Z l 0 )

fj

,,/ ~

KHAZAZ .,"/--d

MINYA 582 d: 22 R4

t\

/

240

240 KHANZIR 576 :¢ 20 (R71

,.p

,9 KEBAO MAHAIL TARBAN

200 20 o

•"

t /. I ~-~'/. \

J AL GAHARRA

ALUMINOUS GRANITE 200

100 I

360

300

I

I

400

]

I

500 KM I

[

I

I 400

I

I

I

I 440

Fig. 9. Map of the Arabian Shield showing aluminous granite (mg map unit).

Distribution and tectonic setting of plutonic rocks of the Arabian Shield

360 I

I

I

40 o I

I

35

440 I

I

280

280

/

I ~COIdpLr~

24 o

q

240

L\

\

f~O I R 31

200

ABUE~ ' IV

.:

)

,(

200

./

gsl AL AJARDA I

B I K19;

_

SYENITIC 0 I

+

I00 I

I 36 °

PLUTONIC 200 I

I

ROCKS

300 I

I

400 I

I

500 l

I 40 o

WADI EARGEARTH STOCKS

~"BANI MALIK d. FAYFA

KM

I

I

I

I

I

440

Fig. I0. Map of the Arabian Shield showing syenitic plutonic rocks. Rock type indicated by the following symbols: su. undifferentiated syenitic rocks; sf, alkali-feldspar syenite; sy, syenite; sin, monzonite; fs, feldspathoidal syenite. "q' indicates rock is quartz bearing.

AJ~S 4 : S1-D

36

D . B . STOESER

Aluminous granites east of the Nabitah suture occur throughout the Afif terrane and the AI Amar suture but have not been found in the Ar Rayn terrane. These rocks generally are postorogenic, and the ages of six dated plutons range from 613 + 11 to 571 + 11 Ma. Typically, they are alkali-feldspar Fe-Li mica-bearing granites (with or without garnet). Unlike the aluminous granites of the western part of the Nabitah belt and the ArabianNubian arc terrane, the eastern aluminous granites are chemically very evolved and constitute one of the most favorable environments in the Shield for tin and tungsten. There is some dispute as to whether the aluminous granites of the Shield are truly magmatic. N. J. Jackson (1985, written communication) considers them to be the product of subsolvus alteration, whereas Elliott (1983), du Bray (1986a,b, this volume), and the author, interpret them as dominantly magmatic in origin.

the terrane. One of these, the Sawda complex, consists of an alkali-feldspar syenite rim and a core complex of various nepheline syenites (Liddicoat et al. 1986, this volume). The age of the Sawda complex has been determined by the U-Pb zircon method at 553 +_ 4 Ma (Liddicoat et al. 1986). The syenitic rocks of the central and northeastern Shield are mostly hornblende and(or) biotite quartzbearing varieties that are probably directly related to the postorogenic A-type granites of those regions; that is, they are the feldspar-rich, low-quartz members of the postorogenic granitic rocks. They include a large syenite pluton in the east-central part of the Shield (Fig. 10) that consists of hornblende-biotite syenite, quartz syenite, alkali-feldspar granite, and syenogranite (Agar in press b).

TECTONIC SETTING SYENITIC PLUTONIC ROCKS Syenitic plutonic rocks are not abundant in the Arabian Shield. Only 65 syenitic plutons are shown on Fig. 10, of which only three are nepheline-bearing syenites. The southern Asir terrane contains a group of syenites that includes quartz-pyroxene syenite and olivineclinopyroxene-syenite (with or without kaersutite and(or) titaniferous biotite) (Stoeser and Elliott 1980, Fairer in press, Stoeser unpublished data). One shonkenite pluton (Jabai Atwid) and a plug have been reported in this area (Coleman 1972, Overstreet et al. 1984); however, recent work by the author (unpublished data) shows that these rocks are sodic melasyenites in which the alkali-feldspar is anorthoclase, not sanidine. Fleck et al. (1976) reported an age of 614 + 7 Ma for the Jabai Atwid syenite. A small group of syenites similar to the southern Asir syenites occurs in the south-easternmost part of the Shield (Fig. 10). The group consists of four pyroxene-quartz syenite plutons and one olivineclinopyroxene syenite (Sable in press). The remarkable AI Ajarda complex of the central Asir terrane (Fig. 10) consists of a core of layered gabbro to pyroxenite and an external rim of sodic aegirine syenite (AI-Koulak 1985). The AI Ajarda syenite has been dated at 557 + 8 Ma using the K-Ar method on biotite. North of AI Ajarda, the Bani Amr syenite group consists of two small syenite plutons, composed of aegirine quartz alkali-feldspar syenite and aegirine nepheline syenite, and the Jabal al Harfa breccia pipe (AI-Koulak 1985). The Hijaz terrane contains a number of syenitic plutons that chiefly contain monzonite and syenite. These include two ring complexes having monzonite cores and gabbroic to dioritic rim units. One of these, the Martabah igneous complex, is described in detail in the present volume (Douch et al. 1986, this volume). Syenitic plutonic rocks are scarce in the Midyan terrane and only four plutons occur in the northern part of

On the basis that the Arabian Shield contains four major belts of ultramafic complexes and the assumption that these represent fundamental tectonic boundaries within the Shield, Stoeser and Camp (in press) subdivided the Saudi Arabian part of the Arabian-Nubian Shield into five terranes separated by four suture zones (Fig. 1). The three terranes (Asir, Hijaz, Midyan) west of the Nabitah suture are of ensimatic character, whereas the eastern two terranes (Afif, Ar Rayn) are of continental-marginal to continental character. The three ensimatic terranes are collectively referred to as the Arabian-Nubian arc terrane (Stoeser et al. 1984).

A r a b i a n - N u b i a n arc terrane

Numerous geologic and isotopic studies of the volcanic and plutonic rocks of the Arabian Shield west of the Nabitah suture zone have demonstrated the similarity of these rocks to those of modern ensimatic island arcs. The geologic studies include Jackaman (1972), Greenwood et al. (1976), Roobol et aL (1983), Camp (1984), and Jackson et aL (1984); the isotopic studies include Fleck et al. (1980), Stacey et al. (1980), Bokhari and Kramers (1981), Duyverman etaL (1982), Marzouki et al. (1982), and Stacey and Stoeser (1984). The three western arc terranes are separated by the Bir Umq and Yanbu sutures, both of which contain ophiolitic complexes (Figs. 1 and 11) that have been studied by Bakor et al. (1976), Shanti and Roobol (1979), Rehaile and Warden'(1980), Claesson et aL (1984), and Nassief et al. (1984). The Asir terrane is extremely complex and can be divided into a number of tectonostratigraphic belts that contain assemblages of primitive to moderately evolved arc-type volcanoclastic sedimentary, basaltic and dacitic volcanic, and dioritic to tonalitic plutonic rocks (Greenwood et al. 1982, Johnson and Vranas 1984). Although the bulk of the terrane appears to be ensimatic-arc related, the western flanks of the terrane contain

Distribution and tectonic setting of plutonic rocks of the Arabian Shield

360 I

I

I

I

I

400 I

I

I

37

440 I

I

280

28*

J, ESS 782 • 38 (N9}

I

761 ± 2 (N9)~

H / U A 882 ± 12

/

240

240 URD 6S4 ± 8 (Z3

~ I~BIDGN. ~1830 I;[341

,O J. THURVVAH

J. KHIDA

1620 ± 201

/ /-'.\ f

-

-

FAULT

. . . .

200

A

THRUST FAULT

20' ______

TECTONIC BOUNDARY, SOLID WHERE COINCIDENT WITH A FAULT

ULTRAMAFIC

COMPLEX

OLDER BASEMENT

0 I

100 I

I 360

200 I

I

300 I

I

400 I

I

500 KM I

I 40*

I

I

I

I

I

440

Fig. 11. Map of the Arabian Shield showing older basement, ultramafic complexes and major faults.

38

D . B . S'rOESER

quartzo-feldspathic sedimentary rocks that appear to be continental in character and possibly derived from the African craton to the west (Ramsay et al. 1981, Fairer in press). Although the Asir terrane may have been a single terrane at the time of accretion with the Hijaz terrane, both its tectonic fabric and distribution of rock assemblages suggest a complex history probably involving one or more arc accretionary events as well as rifting episodes. In particular, the Taif province of the Asir terrane appears to be distinct from the rest of the terrane in that it contains significantly more granitic plutonic rocks (Fig. 4, Tables 2 and 3), and Johnson and Vranas (1984) have proposed that a suture separates the Taif and Abha provinces (Fig. 1). The magmatic arc rock assemblages of the Asir terrane formed between approximately 900+ and 680 Ma (Cooper et al. 1979, Fleck et al. 1980, Bokhari and Kramers 1981, Marzouki et al. 1982, Stoeser et al. 1984). The Hijaz terrane is not as complex as the Asir terrane and is thought to be composed of at least three superimposed magmatic assemblages that formed between 805 and 715 Ma (Camp 1984, Jackson et al. 1984). The Midyan terrane contains at least one suite of intermediate plutonic rocks, which range in age from 725 Ma to less than 680 Ma (Calvez et al. 1983, Hedge 1984). The early history of the Midyan terrane is still poorly understood. The abundance of granites in the northern part of the terrane suggests that the crust in that region may be different from that in the southern part of the terrane, and Ramsay et al. (1986a, this volume) have proposed a terrane boundary in the central part of the Midyan region (Fig. 1). In addition, the occurrence of ultramafic complexes in the northern part of the terrane suggests that a suture may also exist in that region (Figs. I and 11) (Stoeser and Camp in press). The Asir terrane was sutured to the Hijaz terrane along the Bir Umq suture zone no later than 715 Ma (Camp 1984, Stoeser and Camp in press). The time of suturing of the Midyan terrane to the Hijaz terrane along the Yanbu suture zone is poorly constrained, though it probably occurred between 720 and 640 Ma. Subduction prior to suturing is interpreted to have been to the SE for both zones (Camp 1984). Thus sometime after 715 Ma the Arabian-Nubian arc terrane had accreted into a single crustal unit. The accretion events that formed the terrane appear to have been relatively passive because although they involved some compressional deformation, there is no evidence of major orogenesis (Stoeser and Camp in press).

A f i f terrane

The Afif terrane was defined by Stoeser et al. (1984) primarily on the basis of its being bounded on the E and W by the Nabitah and AI Amar suture zones and also by its lead isotope signature, which suggests that the Afif crust contains a continental component and is possibly

underlain by an older cratonic basement (Stacey et al. 1980, Stacey and Stoeser 1984). The existence of an older continental basement within the Arabian Shield has long been a subject of debate. The earliest isotopic evidence for such a basement was presented by Baubron et al. (1976), who found elevated initial XTSr/~6Sr ratios for some plutonic rocks from the eastern Shield. Further evidence was produced by Stacey et al. (1980), who, on the basis of common lead determinations on feldspars and sulfide minerals from the eastern Shield, proposed the possible existence of an early Proterozoic basement with Archean antecedents. These early findings were disputed by the isotopic studies of Bokhari and Kramers (1982), Duyverman and Harris (1982), and Duyverman et al. (1982). A follow-up common lead study by Stacey and Stoeser (1984) supports the earlier interpretation of Stacey et al. (1980) and further refines the distribution of ensimatic vs evolved common lead isotopes within the Shield. These lead isotope data show that rocks of the western arc terrane contain ensimatic lead, that most rocks of the Shield E of the Nabitah suture zone contain a component of evolved continental lead, and that rocks of the southern Afif terrane contain evolved continental lead. The first direct evidence for early Proterozoic rocks in the eastern Shield was the discovery by Calvez et al. (1983, 1985) of inherited 2067 + 74 Ma zircon in a trondhjemite along the AI Amar fault on the eastern boundary of the AI Amar suture zone (Figs. 1 and 11). The first discovery of older evolved sialic rocks within the Shield was by Stacey and Hedge (1984); their Rb-Sr, U-Pb and Nd-Sm isotopic data indicate that a granodiorite gneiss from Jabal Khida in the easternmost part of the Shield (Fig. 11) either formed at 1628 + 200 Ma and was remobilized at about 660 Ma or was derived directly from a protolith of that age about 660 Ma. Subsequent work by Stacey and Agar (in press) shows that a pelitic garnet-sillimanite paragneiss NW of Jabal Khida (Fig. 11) may have been metamorphosed as early as 1830 Ma; its lead isotopic composition requires residence in an upper continental crust for a long period before 1830 Ma. All the above data establish that at least part of the southern Afif terrane is underlain by an early to middle Proterozoic evolved continental basement. Reconnaissance mapping by Thieme (in press) and Agar (in press b) provides the first description of this basement complex, and the approximate limits of exposure of these basement rocks within the Afif terrane are shown in Fig. 11. The basement complex consists of high-grade (sillimanite facies) pelitic and quartzofeldspathic schists and minor quartzite. The associated orthogneisses consist of felsic mica schists, medium- to coarse- grained granite gneiss, granitic migmatite, and banded garnet-biotite mafic gneiss. The complex is extensively intruded by younger granitic plutonic rocks. Thus far, there is no physical or isotopic evidence that the northern part of the Afif terrane is underlain by such rocks; however, the basement in the northern Afif terrane is almost concealed by clastic sediments younger than 640 Ma (Johnson 1983).

Distribution and tectonic setting of plutonic rocks of the Arabian Shield TECTONIC

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A I A m a r s u t u r e a n d A r R a y n terrane

The Afifand Ar Rayn terranes are separated by a broad suture zone, the AI Amar suture, that contains a clastic sedimentary section and intercalated ultramafic complexes. The rocks of the AI Amar suture have been interpreted as representing an accretionary wedge related to a subduction zone (AI-Shanti and Mitchell 1976, AI-Shanti and Gass 1983, Stacey et al. 1984). The Ar Rayn terrane consists chiefly of intermediate plutonic rocks (diorite, tonalite and trondhjemite) and associated volcanic and sedimentary rocks. These have been interpreted as the roots of a magmatic arc that formed from ca 670-630 Ma (Nawab 1979, Coulomb et al. 1981, AI-Shanti and Gass 1983, Stacey etal. 1984, Le Bel and Laval, 1986, this volume). Although the Ar Rayn terrane has been proposed as representing the leading edge of an older continental block that collided along the AI Amar suture with the Arabian crust further to the west (Schmidt et al. 1979, Greenwood et al. 1982), no evidence for an older cratonic basement within the exposed portion of the Ar Rayn terrane has been found (Stacey etal. 1984).

TECTONIC AND PLUTONIC EVOLUTION The preceding discussion on the distribution and ages of Arabian Shield plutonic rocks clearly indicates that the evolution from intermediate plutonic rocks to unevolved granitic rocks to evolved granitic rocks (Fig. 12) is in the broadest terms similar to plutonic rock assemblages formed during the classic stages of orogenesis associated with the Wilson cycle (Dewey and Burke 1973). In recent years, various authors have proposed that the Arabian-Nubian Shield formed through a process of arc and microplate accretion (Sillitoe 1979, Kr6ner et al. 1982, Vail 1983, Johnson and Vranas 1984, Stoeser et al. 1984, Kr6ner 1985, Stoeser and Camp in press). On the basis of this interpretation the evolution of the Arabian Shield will be discussed in terms of three main stages: (1) magmatic arc, (2) continental collision, and (3) intracratonic. This discussion will follow the terminology of Pitcher (1982) as developed in his review of granite types and their tectonic environment.

40

D . B . STOESER

M a g m a t i c arc stage

The early history, tectonic evolution, and relative in situ location of the various older arc segments to one

Although the early history of the Arabian Shield is still obscure, it involved the formation of massive amounts of gabbroic to intermediate plutonic rocks and associated volcanic rocks similar to those found in Phanerozoic magmatic arcs (Jackson 1986, this volume). The many belts of such rocks present within the Shield (Fig. 3) indicate that this magmatic arc development involved multiple arcs, and the available radiometric age data show that these arcs formed during a period of at least 270 million years (900--630 Ma). On this basis, various models of arc accretion have been proposed (e.g. Greenwood et al. 1976, 1982, Delfour 1979, Schmidt et al. 1979, Fleck et al. 1980, Gass 1981, 1982, Camp 1984). Because the western arc terrane intermediate plutonic rock assemblages are strictly of an ensimatic character, these rocks should be the M(mantle)-type granitoids of White (1979), which are found in oceanic island arcs. Phanerozoic ensimatic island arcs typically contain relatively small volumes of plutonic rocks low in silica (gabbro, diorite, and quartz diorite) (Pitcher 1982), whereas the western arc terrane also contains large batholiths of high-silica tonalite and trondhjemite (Greenwood et al. 1976, Jackson et al. 1984). Such high silica rocks are more typical of the Cordillera I-type granitoids (Chappell and White 1974) found in contintental-marginal arcs such as the Andes (Pitcher 1982). These rocks are thought to form through remelting of the base of a thick crust that has been underplated by mafic intrusive rocks derived from the subducting slab and overlying aesthenosphere. Because there is no evidence for the existence of a continental margin close to the western arc terrane, it must be assumed that the arcs of the terrane were able to achieve sufficient thickness and maturity to allow the development of large amounts of Cordillera I-type arc magmatic rocks (Jackson 1986, this volume). Figure 3 and Table 4 show that the cratonic core of the Afif terrane is surrounded by belts of intermediate plutonic rocks similar to Cordillera I-type granitoids (Delfour 1979, 1981, Agar in press a, Cole in press). In the only available detailed studies of these rocks in the Afif terrane, Agar (in press a,b) and Stacey and Agar (in press) have proposed that the Siham belt (Fig. 3) represents an Andean marginal arc to the Afif cratonic basement. Stacey and Agar showed that the Siham rocks contain leads that appear to be mixtures of ensimatic and continental leads. The continental leads could have been added to the Siham magmatic rocks either by direct contamination from a cratonic basement beneath the arc or by contamination of the magmas from sediments containing a continental component derived from the Afif basement. Other lead isotope data show that intermediate plutonic rocks of the Afif terrane, the Ar Rayn terrane, and the Tathlith-Najran region have the same type of lead isotope signature (Stacey and Stoeser 1984). Therefore, all these rocks may belong to continentalmarginal arc assemblages.

another is totally obscure. Based on the ensimatic character of the earlier assemblages, it has been proposed that the western arc terrane was built by a slow process of arc accretion (Gass 1982, Camp 1984, Kr6ner 1985). Such a process is presently occurring in the western Pacific and is producing arc complexes such as the Philipines archipelago or parts of Indonesia (Karig 1983, Silver and Smith 1983). It is unclear, however, as to whether the younger arcs of the Asir terrane, such as the Tarib arc or those present in the Taif region, accreted to the older core of the Asir terrane or formed as marginal arcs to that core. In either case, by sometime after 715 Ma the western arc terrane had accreted into a single crustal unit (Stoeser and Camp in press). C o n t i n e n t a l collision stage

Beginning about 700-680 Ma, the western arc terrane began colliding with the Afif microplate and possibly with a separate microplate farther S along the Nabitah suture zone (Schmidt et al. 1979, Schmidt and Brown 1982, Stoeser et al. 1984, Stoeser and Camp in press). The result of this collision is the Nabitah orogenic belt, a 100-200 km wide zone of orogenesis involving compressional deformation and synorogenic plutonism (Brown 1972, Schmidt et al. 1979, Stoeser et al. 1984). The Nabitah orogenic belt transects the Shield along a N-S belt and within Saudi Arabia it has an exposed length of approximately 1200 km. It contains ultramafic complexes along most of its length (Fig. 11), and at least some of these appear to mark a suture (Frisch and AI-Shanti 1977, Schmidt etal. 1979, Agar in press a, J. E. Quick personal communication 1985). It should be noted that in the southern part of the Nabitah orogenic belt, the pre-orogenic rocks of the AI Qarah province belong to the Asir terrane and the pre-orogenic rocks of the Najran, Tathlith, Zalim, Nuqrah, and Hail provinces to the Afif terrane (Table 4). In the southern part of the Nabitah orogenic belt the Nabitah orogeny occurred approximately during the period 680-640 Ma; it affected the eastern margin of the Arabian-Nubian arc terrane and extended throughout the Afif terrane. In the northern part of the belt, limited radiometric age data indicate similar ages for orogenesis and plutonism (Calvez et al. 1983, J. C. Cole and C. E. Hedge unpublished data cited in Johnson and Williams in press, Stacey and Agar in press). Suturing of the Afif and Ar Rayn terranes occurred between 670 and 630 Ma and thus was partly concurrent with the Nabitah event (Calvez et al. 1983, Stacey et al. 1984). The net result of these two suturing events was the formation of the Arabian neocraton and the termination of arc magmatism throughout the region of the exposed Shield (Stoeser and Camp in press). One of the most distinctive features of the Nabitah orogenic belt is the occurrence of major linear complexes and batholiths of synorogenic plutonic rocks along its length (Figs. 4-6). These complexes are composed of

Distribution and tectonic setting of plutonic rocks of the Arabian Shield granite, granodiorite, and intermediate and gabbroic plutonic rocks, as well as considerable remobilized relic material derived from the host crust. The southern part of the belt also includes extensive areas of migmatite. In addition, both the AI Qarah and Tathlith provinces of the southern Nabitah orogenic belt contain many spectacular examples of gneiss domes cored by leucocratic granodiorite and(or) granite (Schmidt et al. 1979, Stoeser et al. 1982, 1984). The early Nabitah plutonic rocks of the southern Shield (690--650 Ma) are dominantly tonalite, trondhjemite, and granodiorite, whereas the late Nabitah plutonic rocks (650-640 Ma) are dominantly linear batholithic complexes of monzogranite, granodiorite, diorite, and gabbro (Stoeser et al. 1984). Some of the early rocks are undoubtedly pre-Nabitah magmatic arc rocks now enclosed within the orogenic belt, but many are synorogenic. The origin of the early synorogenic rocks within the Nabitah orogenic belt is somewhat problematical, and there are two obvious interpretations for their formation: (1) they represent magmatic-arc plutonic rocks deformed during collision, or (2) they were produced by partial melting related to collision and deformed during emplacement synchronous with collision. The possibility of a magmatic-arc origin arises from the idea that if arc magmatism related to subduction was occurring until the time of collision, then these intrusions would still be in various stages of crystallization and highly susceptible to deformation during compression (Bard 1983). The composition of the early Nabitah plutonic rocks strongly suggests such an origin, and at least some of the plutonic rocks of the AI Qarah, Najran, Tathlith, Liban and Ar Rayn belts shown on Fig. 3 are potentially of the 'synorogenic' magmatic-arc type. The plutonic rocks of the Ar Rayn terrane are a particularly interesting example of the above problem. They have been interpreted to be of synorogenic type (AI-Shanti and Mitchell 1976, Coulomb et al. 1981, Stacey et al. 1984), and good examples of gneiss domes similar to those in the southern Nabitah belt are present in the Ar Rayn terrane. The Ar Rayn plutonic rocks consist of diorite, tonalite and trondhjemite and are the best dated plutonic rock assemblage in the Shield; their ages range from 667 + 17 to 621 + 17 Ma (Calvez et al. 1983, Stacey et al. 1984) (Fig. 2). The Ar Rayn terrane also has been interpreted to be the site of a magmatic arc probably related to an east-dipping subduction zone prior to collision along the AI Amar suture (AI-Shanti and Mitchell 1976, Stacey et al. 1984). Collision along the suture is presumed to have ended by about 630 Ma (Stacey et al. 1984), and it appears that the Ar Rayn magmatic arc was active until and during the time of suturing. As noted above, two plutonic rock assemblages formed during the collisional stage of evolution of the Nabitah orogenic belt, an early arc-type intermediate composition assemblage and a later granodiorite to monzogranite assemblage. Pitcher (1982) proposed that an unevolved (Caledonian) I-type granitoid suite, distinct from the I-type suite of continental-marginal arcs domi-

41

nated by granodiorite to granite, characterizes the plutonic rock assemblage that forms after continentaltype collisions in response to crustal thickening and heating. Much of the granodiorite to monzogranite of Figs. 5 and 6 appears to fit his criteria and is tentatively identified as Caledonian l-type granitoid. An additional complication is that plutonic rocks emplaced in the upper crust of an evolved island arc are not the same as those present in the middle crust where more hydrous peraluminous melts will consolidate. If this deeper arc core is exposed through uplift and erosion related to orogenesis, it would be possible to find plutonic rocks of arc origin but of a type typically assumed to be generated in collisional regimes, i.e. two-mica granite and granodiorite (W. Hamilton 1981, personal communication 1984). Intracratonic stage

The major tectonic events that occurred after suturing along the Nabitah and AI Amar sutures (after 630 Ma) were formation of the Najd transcurrent fault system (Figs. 1 and 11), mild associated local compression and extension, and emplacement of voluminous postorogenic granitic rocks. After the initial phase of collision and suturing, a broad zone of NW-striking transcurrent faults known as the Najd fault system (Brown and Jackson 1960, Brown 1972, Moore 1979) developed within much of the Shield. Although early work suggested that the Najd faulting occurred very late in the evolution of the Shield (580-520 Ma), more recent work suggests that the Najd system began to develop at the end of the Nabitah and AI Amar orogenies after completion of suturing along the Nabitah and AI Amar sutures (Davies 1984, Stoeser and Camp 1984, Agar in press c). This period of transcurrent (wrench) faulting and mild deformation is generally referred to as the Najd orogeny (Brown 1972). Both monzogranite to syenogranite and more evolved peraluminous to peralkaline alkali-feldspar granite formed throughout much of the intracratonic stage (630560 Ma) (Fig. 12). A major pulse of granitic plutonism occurred immediately after the final suturing events, followed by a short period of decline of granitic plutonism about 620-610 Ma, and then by sustained granitic plutonism until 560 Ma. Although evolved granites become proportionately more important after about 610 Ma, their distribution is not homogeneous and they are concentrated in the northern part of the Shield (Figs. 7-9). The distribution of evolved granites may be related in part to the depth of erosion of the postorogenic granites. Several lines of evidence suggest that the level of erosion on these rocks is shallow in the northeastern part of the Shield and that it deepens to the W and S (Stoeser and Elliott 1980, Agar 1986, this volume). This is predictable, in part on the assumption that domal uplift and erosion of the Shield associated with the formation of the Red Sea rift during the Cenozoic would be greatest near the Red Sea and least at its eastern margin. Signifi-

42

D.B.

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I

440

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.......f " ~ t

J. AIIU SARYAH 705 :~ 34 (R2§}

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Fig. 13. Map of the Arabian Shield showing granophyres (gh map unit).

Distribution and tectonic setting of plutonic rocks of the Arabian Shield

43

cant amounts of late rhyolitic volcanic rocks (Shammar Acknowledgements--The author thanks his reviewers, N. J. Jackson group), granitic granophyre intrusions (Fig. 13), and the of the Saudi Arabian Directorate General of Mineral Resources (DGMR) (for his pithy comments) and E. A, du Bray (USGS). Modal shallow root zones of calderas are preserved only in the analytical data were provided by the hard-working Ahmed Hamdan northern and particularly in the northeastern part of the AI-Bazli and drafting support by Riofinex Ltd., Jiddah, and Rich Arabian Shield (Stoeser and Elliott 1980, Delfour 1981, Schoenfeld and Marty Simmons (USGS, Denver). The author particularly wishes to acknowledge all of the unsung quadrangle mapping Roobol and White 1986, this volume, Kellogg in press). geologists who have worked on the Arabian Shield and who make such Because most mineral deposits associated with granitic papers as the present one possible. plutons are located near the apex of the plutons, knowledge of the depth of erosion on the postorogenic granites is important. A shallower level of erosion of the postorogenic granites in the N is supported by the fact that REFERENCES most significant mineral deposits associated with granitic rocks occur in the northern part of the Shield (Jackson Agar, R. A. 1986. Structural geology of felsic plutonic rocks in the Arabian Shield; styles, modes and levels of emplacement. J. Afr. 1986, this volume). Earth Sci. 4,105-121. The evolved alkali-feldspar granites are typical of the Agar, R. A. in press a. Stratigraphy and paleogeography of the Siham group; direct evidence for a Late Proterozoic continental microplatc A-type (anorogenic)granites (Loiseile and Wones 1979, and active continental margin in the Saudi Arabian Shield. J. geol. Collins et al. 1982) that appear late in orogenic cycles Soc, Load. (Dewey and Burke 1973, Pitcher 1982). True S-type Agar, R, A. in press b. Geology of the Zalim quadrangle, sheet 22F. Kingdom of Saudi Arabia. Saudi Arabian Deputy Ministry for granites, i.e. cordierite-, tourmaline-bearing peraluminMineral Resources Geoscience Map, scale 1:250,000. ous granites (Chappell and White 1974) appear to be Agar, R. A. in press c. The Najd fault system revisited; a two-way lacking in the Arabian Shield, although they may occur strike-slip orogen in the Saudi Arabian Shield. Saudi Arabian Deputy Ministry for Mineral Resources Open-File Report DGMRwithin the Afif basement domain. The peraluminous OF-05-I 8. granites that do occur appear to be members of the Aldrich, L. T. 1978. Geochronologic data for the Arabian Shield, A-type suite (Stuckless e t a l . 1983, 1984, du Bray 1986a, Section l--Radiometric age determinations of some rocks from the Arabian Shield. U.S. Geological Survey Saudi Arabian Project b and Ramsay et al. 1986, this volume). Pitcher (1982) Report 24(I. noted that in orogenic belts it is common to find paired Aleinikoff, J, N., Stoeser, D. B. and Fischer. L. B. in preparation. belts of I- and S-type granites. He appears to be using a U-Pb zircon geochronology of seven Pan-African metaluminous and peralkaline granite complexes of the Arabian Shield. Saudi broad definition of S-type granite, and, if his definition is Arabian Deputy Ministry of Mineral Resources Open-File Report. used, the Arabian Shield also contains a paired belt of AI-Koulak, M. Z. M. I. 1985. Geology and petrology of the AI early I-type granitoids along the Nabitah orogenic belt Ajardah ring complex, and Bani Amr syenites of the southern Arabian Shield. Unpublished Ph.D. thesis, Faculty of Earth Sciand somewhat younger evolved peraluminous S-type ences, King Abdulaziz University, Jiddah, Kingdom of Saudi granites to the E (Fig. 9). Arabia. Both the formation of a wrench fault system and the AI-Shanti, A. M. S. and Mitchell, A. H. G. 1976. Late Precambrian subduction and collision in the AI Amar-ldsas region, Arabian formation of syn- and post-collisional plutonic rocks are Shield, Kingdom of Saudi Arabia. Tectonophysics30, T41-T47. events that typically occur after continental collisions AI-Shanti, A. M. S. and Gass, I. G. 1983. The Upper Proterozoic (Dewey and Burke 1973, Tapponnier and Molnar 1979, ophiolite melange zones of the easternmost Arabian Shield. J. geol. Soc. Load. 140,867-876. Tapponnier et al. 1982), In particular, the occurrence Bakor, A. R., Gass, I. G. and Neary, C, 1976. Jabal al Wask. and widespread distribution of the Arabian A-type postnorthwest Saudi Arabia: an Eocambrian back-arc ophiolite. Earth orogenic granites up to 70 Ma after suturing is a predictPlanet. Sei. Lett. 30, I-9, able result of crustal thickening resulting from continen- Bard, J. P. 1983. Metamorphism of an obducted island arc: example of the Kohistan sequence (Pakistan) in the Himalayan collided range. tal collision (Dewey and Burke 1973, Toksoz and Bird Earth Planet. Sci. Lett. 65, 133-144. Baubron, J. C., Delfour, J. and Vialette, Y. 1976. Geochronological 1977, Sillitoe 1979, Buck and Toksoz 1983). measurements (Rb/Sr; K/Ar) on rocks of Saudi Arabia. Saudi After about 560 Ma, significant platonic activity Arabian Directorate General of Mineral Resources Open-File within the Shield waned; some magmatism continued Report 76-JED-22. until as late as about 510 Ma (Fig. 12). Between about Bokhari, F. Y. and Kramers. J. D. 1981. Island arc character and later Precambrian age of volcanics at Wadi Shwas, Saudi Arabia; 560 and 520 Ma, alkali olivine basalt and alkaline andegeochemical and Sr and Nd isotopic evidence. Earth Planet. ScL site were deposited in pull-apart basins within the Najd Lett. 54,409---422. fault system (Delfour 1979). Plutonic activity during this Bokhari, F. Y. and Kramers, J. D. 1982. Lead isotope data from period included the emplacement of saturated and massive sulfide deposits in the Saudi Arabian Shield. Econ. Geol. 77, 1766-1769. undersaturated syenites, the Jabal Radwa peralkaline Brown, G. F. 1972. Tectonic map of the Arabian Peninsula. Saudi granite, and possibly a few metaluminous granites. All Arabian Directorate General of Mhteral Resources Map Arabian of these plutonic rocks occur along the western margin Peninsula AP-2 (with explanation). Brown, G. F. and Jackson, R. O. 1960. The Arabian Shield. htt. Geol. of the Shield. During this waning period of activity, the Congr. XXI, Sec. 9, Precambrian Stratigraphy attd Correlations. Shield was slowly subsiding and being onlapped by Copenhagen, pp. 69-77. epicontinental seas that deposited the Saq Sandstone in Buck, W. R. and Toksoz, M. N. 1983. Thermal effects of continental collisions: thickening a variable viscosity lithosphere. Tecthe north and Wajid Sandstone in the south during the tonophysics 100, 53---69. Cambrian and Ordovician (Powers et al. 1966, Wolfart Calvez, J.-Y., Alsac, C., Delfour, J., Kemp, J. and Pellaton, C. 1983. 1981). Geological evolution of western, central and eastern parts of the northern Precambrian Shield, Kingdom of Saudi Arabia. Saudi Arabian Deputy Ministry for Mineral Resources Open-File Report BRGM-OF-03-17.

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Calvez, J.-Y., Delfour, J. and Feybesse, J. L. 1985. 2000-million-years 17F, Kingdom of Saudi Arabia. Saudi Arabian Deputy Ministry for old inherited zircons in plutonie rocks from the AI "Amar region: Mineral Resources Geoscience Map, scale 1:250,000. new evidence for an early Proterozoic basement in the eastern Fleck, R. J., Coleman, R. G., Cornwall, H. R., Greenwood, W. R., Arabian Shield? Saudi Arabian Deputy Ministry for Mineral Hadley, D. G., Schmidt, D. L., Prinz, W. C. and Ratte, J. C. 1976. Resources Open-File Report BRGM-OF-05-11. Geochronology of the Arabian Shield, western Saudi Arabia: K-Ar Calvez, J.-Y. and Kemp, J. 1982. Geochronological investigations in results• Geol. Soc. Am. Bull 87, 9-21. the Mahd adh Dhahab quadrangle, central Arabian Shield. Saudi Fleck, R. J., Greenwood, W. R., Hadley, D. G., Anderson, R. E. and Arabian Deputy Ministry for Mineral Resources Technical Record Schmidt, D. L. 1980. Rubidium-strontium geochronology and BRGM-TR-02-5. plate-tectonic evolution of the southern part of the Arabian Shield. Calvez, J. Y., Pellaton, C., Alsac, C. and Tegyey, M. 1982. GeoU.S. Geological Survey Professional Paper 1131. chronology and geochemistry of plutonic rocks in the Umm Lajj and Fleck, R. J. and Hadley, D. G. 1982. Ages and strontium initial ratios Jabal al Buwanah area. Saudi Arabian Deputy Ministry for Mineral. of plutonic rocks in a transect of the Arabian Shield. Saudi Arabian Resources Open-File Report BRGM-OF-02-36. Deputy Ministry for Mineral Resources Open-File Report USGSCamp, V. E. 1984. Island arcs and their role in the evolution of the OF-03-38. western Arabian Shield. Bull. geol. Soc. Am. 95,913-921. Frisch, W. and AI-Shanti, A. 1977. Ophiolite belts and the collision of Chappell, B. W. and White, A. J. R. 1974. Two contrasting granite island arcs in the Arabian Shield. Tectonophysics 43,293-306. types. Pacific GeoL 8, 173-174. Gass, I. G. 1981. Pan-African (Upper Proterozoic) plate tectonics of Claesson, S., Pallister, J. S. and Tatsumoto, M. 1984. Samariumthe Arabian-Nubian Shield. In: PreCambrian Plate Tectonics neodymium data on two late Proterozoic ophiolites of Saudi Arabia (Edited by KrOner, A.), pp. 387-405. Elsevier, Amsterdam. and implications for crustal and mantle evolution. Contr. Miner. Gass, I. G. 1982. Upper Proterozoic Pan-African calc-alkaline magPetrol. 85,244-252. matism in northeastern Africa and Arabia. In: Andesites (Edited by Cole, J. C. in press. Geology of the Aban al Ahmar quadrangle, sheet Thorpe, R. S.), pp. 591-609. John Wiley, New York. 25F, Kingdom of Saudi Arabia. Saudi Arabian Deputy Ministry for • Gettings, M. E. 1984. The isostatic gravity anomaly field of southwestMineral Resources Geoscience Map. ern Saudi Arabia and its interpretation. Saudi Arabian Deputy Coleman, R. 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STOESER 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.

Cooperetal. (1979) Darbyshireetal. (1983) Darbyshire, D. P. F. unpublished data cited in Johnson and Vranas (I 984) Delfour (1980) duBray(1984) Duyvermanetal. (1982) Fleck (1981) Flecketal. (1973) Flecketal. (19801 Fleck and Hadley (1982) Hedge (1984) Hedge, C. E., Camp, V. E. and Jackson, N. J., unpublished data reported in Jackson etal. ( 19841 Hedge, C. E., unpublished data reported in Quick and Doebrich (in press) Kempetal. (1980) KrOneretal. (1979) Kr6neretal. (1982) Liddicoatetal. (1986, thisvolumc) Marzoukietal. (1982) Nasseefand Gass (1977) Pallisteretal. (inpress) Radaineta/. (19841 Stacey, J. S., unpublished data, cited in the present report Stacey and Agar (in press) Stacey and Hedge (1984) Staceyetal. (19801 Staceyetal. (1984) Stoeseretal. (1982) Stoeseretal. (1984) Stucklessetal. (1984) Stuckless, J. S., unpublished data, cited in Cole (in press) Stuckless, J. S., unpublished data, cited in du Bray et al. (in preparation).

APPENDIX 1 Key to methods and source references for radiometric age data presented in figures. Each age is suffixed by a letter and number within parentheses; the letter code indicates the method used to obtain the age, and the number of the source reference listed below. Key to methods: A = Ar-Ar; K = K-Ar whole rock or mineral; R = Rb-Sr whole-rock isochron; U = U-Pb zircon. 1. Aleinikoffetal. (inpreparation) 2. Baubronetal. (1976) 3. Brown, G. F., oral communication cited in Greene and Gonzalez (1980) 4. Bokhari and Kramers (1981) 5. Calvez and Kemp (1982) 6. Calvezetal. (1982) 7. Calvezetal. (1983) 8. Calvezetal. (19851 9. Claessonet al. (19841 10. Cole(inpress) 11. Cole, J. C. and Hedge, C. E., unpublished data cited in Johnson and Williams (in press)

APPENDIX

2

Equations used in calculating Table 3. Symbols used in the equations are for the map units of Plates 1 and 2 and are explained in Table 2. Values used are from Table 2. Alkali-feldspargranite = ag + m g + gf + 0.5 ga Granite = gs + gm + gr + gh + gu + 0.5 ga + 0.5 gt + 0.5 gg Granodiorite = tg + 0.5 gg + (1.5 tx + 0.25gt Tonaliticrocks = tt + tj + tu + 0.5 tx + 0.5 dt + 0.125 gt Dioriticrocks = dq + di + dm + du + 0.5 d t + (1.5 db + 0.125 gt Gabbroicrocks = bu + 0.5 db Syeniticrocks = s m + sy + sf + su + fs Total granitic rocks = alkali-feldspargranite + granite + granodiorite Total intermediate rocks = tonalitic rocks + dioritic rocks.