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Petrology and chemistry of Jebel Tanumah complex, Khamis Mushayt, Southern Arabian shield, Saudi Arabia
Journal of African Earth Sciences, Vol. 10, No. 4, pp. 625-631, 1990 Printed in Great Britain
0899-5362/90 $3.00 + 0.00 © 1990 Pergamon Press plc
Petrology and chemistry of Jebel Tanumah Complex, Khamis Mushayt, Southern Arabian Shield, Saudi Arabia M. O. NASSIEF, H. M. Au and F. A. ZAKIR Facultyof EarthSciences,KingAbdulazizUniversity,Jeddah, Saudi Arabia
Abstract- The mafic intrusive complex at Jebel Tanumah is located 15 km north-west of Khamis Mushayt in the southern Arabian Shield and includes olivine-bearing gabbro as well as ampbibole-diopside-homblende gabbro cumulates. These rocks have been generally metamorphosed to upper greeenschist-lower ampbibolite facies. Fourteen white rock silicate analyses indicate that the majority of the rocks a r e calc-alkaline to tholeiitic in composition. The two major structural units in the Kharnis Mushayt region identified by Coleman consist of the basement complex of Asir Mountains and the younger metamorphic rocks. Syntectonic granitic rocks intruded the antiforms characterizing the younger rocks whereas the lower parts of the synforms are intruded by post-tectonic intrusions of layered gabbros such as the one studied at Jebel Tanumah.
Very few studies of mafic-ultramafic bodies within the Arabian Shield have b e e n u n d e r t a k e n so it is difficult to relate these rock types to either The studied area (18 ° 24' N; 42 ° 37'E) lies some oceanic or continental environments. The present 15 k m NW of Khamis M u s h a y t (Fig. l) It consists of work h a s been u n d e r t a k e n to identify the different a n oval maflc intrusion of 3.5 k m in m a x i m u m characteristics of the complex, which m a y assist diameter, and rising to 1829 -1890 m asl any further s t u d y of the area or of similar bodies (Coleman, 1973a) which h a s been intruded into within the shield. b a s e m e n t rocks of Precambrian age. Analyses of the samples collected h a s been The Precambrian rocks of the Khamis Mushayt carried out in the laboratories of the Faculty of region have b e e n divided by Coleman (1973b) into Earth Sciences, King Abdulaziz University, Saudi the b a s e m e n t complex and a series of y o u n g e r Arabia and the GAPS (Geological Consultants) metamorphic rocks. The former includes the laboratories in England. Khamis M u s h a y t Gneiss comprising mainly orthogneiss with minor intercalations of partially PETROLOGY AND GEOCHEMISTRY migmatitic paragneiss a n d amphibolites. The y o u n g e r m e t a m o r p h i c rocks include interlayered Fourteen representative rock samples of the pelitic s e d i m e n t a r y and mafic volcanic rocks of T a n u m a h gabbroic body were petrographicaUy various m e t a m o r p h i c grades, which are equivalent studied. From t h e m it h a s been possible to define to the Hali, Baish and B a h a h groups. Intrusive two types ofmafic-ultramaflc associations ofbroadrocks include syntectonic granites and post- ly olivine bearing gabbro and c u m u l a t e gabbros tectonic gabbroic and granitic intrusions. and showing various degrees of metamorphism. The present s t u d y c o n c e m s a n intrusive gabMagmatic zonation a n d / o r layering is not clear broic body (Fig. I) within the y o u n g e r metamorphic due to post-intrusion deformation and metarocks (Coleman, 1973a), other intrusive rocks morphism. It is also difficult to recognize the include granites and granodiorites. Biotite from relationship between mafic a n d ultramaflc rock the gabbro gave a K-Ar age of 690 + 22 M.a. units or the extent of each. (Coleman, 1973b). Biotite from a gabbroic plug The olivine bearing gabbro (KI and K3) is a north of Khamis Mushayt which m a y be related to medium-grained holocrystalline rock with an the oval gabbroic intrusions gave a n age of 640 allotriomorphic texture (Fig. 2). The m a i n mineral M.a. (Coleman et al., 1972). These rocks m a y be components are hornblende (25 - 50 %), plagiorelated to dyke s w a r m s recorded in the Red Sea clase (25 - 30%) and olivine which is about 5 % but Hills of the S u d a n and dated at 660 M.a. Waft and in samples m a y reach as m u c h as 35 % (K1) giving Hughes, 1977). It is not certain w h e t h e r these ages the rock a troctolitic affinity. Minor constituents are of m e t a m o r p h i s m or crystallization. include diopside and opaques. INTRODUCTION
625
626
M. O. NASSIEF,H. M. ALI and F. A. ZAKIR --\ 250
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Fig. I. Location and general geology of the Jebel Tanumah area. Olivine occurs as skeletal grains ranging from 1 to 4 m m in m a x i m u m diameter with slight serpentinization along its cracks. Hornblende exists as anhedral prisms up to 3 m m in length and poikilitically enclosing olivine and plagioclase. Its pleochroism is deep reddish brown to pale reddish brown to buff. Plagioclase occurs in the form of subhedral to anhedral twinned laths ranging in composition from An4o to Anso. The c u m u l a t e rocks include amphibole-plagioclase cumulate, diopside cumulate, hypersthenediopside cumulate, hyperthene-diopside-hornblende cumulate, and plagioclase (?)cumulate. The amphibole-plagioclase c u m u l a t e s (K2A, K33) are medium- to coarse-grained holocrystalline rocks with hypidiomorphic to allotriomorphic granular textures (Fig. 3). The m a i n mineral constituents are hornblende (60 %) and plagioclase (30 %). The h o r n b l e n d e o c c u r s as elongated s u b h e d r a l crystals showing simple twinning and pleochroism similar to that encountered in the olivine bearing gabbro. The plagioclase occurs in the form of subhedral laths reaching 3 m m in length and showing lammelar twinning. The An content of plagioclase is 50 to 54 which may show partial saussuritization. The minor constituents include interstitial opaques, apatite and epidote.
The larger amphibole and plagioclase crystals represent c u m u l u s p h a s e s and m a y reflect igneous laminations where they show preferred orientation. The hatched cores a n d perfect rims of the amphibole suggest possible m e t a m o r p h i s m from pyroxene (Simonian, personal com.). Fine intergrowth of hornblende, plagioclase and opaques could well be the i n t e r c u m u l u s phases. Sample K34 is a medium-grained rock consisting essentially of plagioclase (about 90 %) wlth an average of An42. This plagioclase is highly saussuritized. Alteration products also include chlorite and biotite. This rock is considered to be a plagioclase c u m u l a t e with preferred orientation probably representing igneous lamination. The diopside-bearing rocks (K2B and K30 ) are medium- to coarse-grained holocrystalline rocks with hypidiomorphic to allotriomorphic granular texture. The major constituents are diopside (20 - 35 %), hornblende (35 - 55 %) and plagioclase (+ 20 %). The diopside occurs as colourless anhedral to subhedral crystals which in some places are partially altered to amphibole. The hornblende exists as large anhedral to subhedral crystals enclosing part of the pyroxene. It displays simple twinning and is pleochroic from deep reddish brown to buff. The h a t c h e d cores and the well
Petrology and chemistry of Jebel Tanumah Complex, Khamis Mushayt
627
Fig. 2. Olivine, white with curved crack (A): hornblende, brown, pleochroic (B): and plagioclase, white (C), forming a n allotrimorphic granular aggregate, (x 45).
Fig. 3. The two essential mineral components, brown hornblende (A) and plagioclase (B) are in the field of view, as well as accessory titanomagnetite or ilmenite (C). Apatite is also present (D). The directed texture is apparent, (x 45).
628
M. O. NASSIEF,H. M. ALl and F. A. ZAKIR
defined fringes s u g g e s t r e p l a c e m e n t from a former mineral, probably pyroxene. The plagioclase o c c u r s as small t w i n n e d crystals interstitial to the pyroxene a n d amphibole. It r a n g e s in average composition from An3~(K2B) to An43 (K30). Partial s a u s s u r i t i z a t i o n of plagioclase is c o m m o n . The m i n o r c o n s t i t u e n t s include o p a q u e s a n d accessory s p h e n e a n d apatite. We c o n s i d e r this r o c k to be a c u m u l u s p h a s e a n d suggest t h a t it w a s originally a diopside c u m u l a t e with i n t e r c u m u l u s plagioclase a n d t h a t the hornblende is t h e p r o d u c t of amphibolite facies m e t a morphism. The h y p e r s t h e n e - b e a r i n g rock (K8, K9, K22, K31) are m e d i u m - to c o a r s e - g r a i n e d holocrystalllne with hypidiomorphic to allotrimorphic t e x t u r e s (Fig. 4). The m a i n m i n e r a l c o n s t i t u e n t s are h y p e r s t h e n e (5 - 20%), h o r n b l e n d e (15 - 50 %) a n d plagioclase ( 15 - 40 %). Diopside is the m a j o r c o n s t i t u e n t of K8 w h e r e it m a y r e a c h 60 % while it m a k e s up a b o u t 10 % of K9 a n d is not e n c o u n t e r e d in K22 a n d K31. The h y p e r s t h e n e exists as x e n o m o r p h i c crystals some of w h i c h s h o w lamellar intergrowth, with diopside existing a s s m a l l e r discrete grains. Some x e n o m o r p h i c h o r n b l e n d e crystals, u p to 2 m m long, s o m e t i m e s s h o w a poikilitic texture a n d pleochroism from deep-pale greenish b r o w n to buff. The plagioclase o c c u r s as a n h e d r a l crystals reaching 1 m m in length, with a n average composition of An43. The m i n e r a l a s s e m b l a g e s observed in the J e b e l
T a n u m a h a r e a indicate conditions of m e t a m o r p h i s m of u p p e r g r e e n s c h i s t to lower amphibolite facies probably within t h e biotite zone. This g r a d e of m e t a m o r p h i s m is c o m p a r a b l e to t h a t observed in the a d j a c e n t c o u n t r y rocks. Major a n d trace e l e m e n t s of f o u r t e e n s a m p l e s (Table I) were d e t e r m i n e d b y X-ray fluorescence a n d atomic absorption t e c h n i q u e s . Although t h e results are not sufficient for detailed geochemical interpretation, t h e d a t a plotted on t h e AFM d i a g r a m (Fig. 5) s h o w s t h a t s o m e of t h e fall into the calc-alkaline field while t h e o t h e r s s h o w tholeiitic affinities. MgO s h o w s two g r o u p s of c o n c e n t r a t i o n s , t h e first r a n g e s from 4.82 to 9.26 while the s e c o n d r a n g e s f r o m 10.64 to 17.29. This is reflected in the FeOt~/MgO ratios (0.70 to 2.80) a n d s u g g e s t s a bimodal c o m p o s i t i o n for t h e complex, with s o m e of the r o c k s being tholeiitic while the o t h e r s are calc-alkaline. The high TiO 2 r e a c h e s up to 3.22 % a n d the relatively high FeO 2, Al203 a n d P2Os in s o m e s a m p l e s clearly e m p h a s i z e the tholeiitic affinity. On FeO(, against F e O I J M G O d i a g r a m (Fig. 6), the rocks m o s t l y fall within t h e s k a e r g a a r d field with two falling within the abbyssal tholeiites. SiO 2, K20 a n d FeOctl/Mg v a l u e s are v e r y c o m - p a r a b l e with o t h e r gabbroic r o c k s within the Arabian Shield (Coleman et al., 1972). The high Ni c o n c e n t r a t i o n (e.g. K i- i a n d K22) m a y be d u e to olivine e n r i c h m e n t . High Sr v a l u e s (770-172 ppm) indicate the Ca-plagioclase a b u n d a n c y in t h e s e
Fig. 4. Hyperthene (A), brown hornblende (B) and plagioclase (C) are the essential minerals. Sparse accessory opaques are also evident (D), (x 45).
Petrology and T a b l e 1. M a j o r e l e m e n t
KI-1
K2A
SiO2 A1203 Fe203 F¢O M.nO MgO CaO NaaO I~O TiO 2 P20s LOI
42.15 15.03 1.71 10.31 0.21 17.29 9.65 1.21 0.20 1.11 0.01 0.60
Total
99.48
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Cr Nb Ni Rb Sr Zr
139 243 0 285 3 186 14 30
chemistry o f Jebel Tanumah
a n a l y s i s (In w t . % ) a n d t r a c e e l e m e n t a b u n d a n c e from the Jebel Tanumah Complex. K3
K4
K8
K9
K31
K33
K34
K38 K39
39.21 48.11 45.19 38.27 51.53 47.08 44.62 16.53 7.47 17.00 20.87 4.61 16.95 18.23 7.28 3.12 1.86 7.26 2.34 1.81 3.11 9.13 8.08 8.74 7.30 6.49 8.60 6.97 0.14 0.23 0.17 0.14 0.16 0.19 0.13 6.67 13.13 9.26 5.20 15.07 9.16 12.26 11.50 15.22 12.83 12.74 16.77 11.30 10.62 2.73 1.66 2.19 1.83 1.08 2.10 1.66 0.66 0.43 0.2(3 0.85 0.33 0.24 0.31 2.96 1.38 1.46 1.98 0.64 1.59 1.26 1.96 0.10 0.03 1.56 0.08 0.18 0.04 0.79 0.79 0.51 1.66 0.63 0.64 0.36
45.90 8.89 4.42 7.96 0.21 12.50 14.59 1.81 0.61 1.68 0.22 0.90
46.04 17.11 1.92 7.63 0.13 10.64 12.15 2.01 0.35 1.34 0.02 0.57
40.34 16.58 6.37 9.07 0.14 8.68 11.13 2.44 0.67 3.22 0.01 1.19
50.07 20.25 1.27 5.51 0.11 4.82 7.01 3.32 0.78 2.07 0.45 4.28
40.48 17.61 5.98 9.75 0.13 8.37 11.27 2.28 0.45 3.20 0.00 0.44
40.12 17.83 6.49 9.53 0.13 7.67 11.23 2.35 0.54 2.90 0.04 0.79
99.56
99.69 99.90
99.84! 99.93
99.97
99.63
99.71 99.43
99.67
99.73
99.85
220 445 3 46 4 224 39 94
406 26 3 1 10 1723 34 60
157 1142 2 132 7 130 23 73
162 251 5 74 3 275 30 101
88 311 2 89 3 223 23 62
K22
(In p p m ] o f r o c k s
K30
303 48 4 9 5 1066 46 49
K2B
629
Complex, Khamis Mushayt
99.57
72 142 4 187 4 250 22 72
264 201 3 28 3 345 38 80
136 309 2 143 5 205 21 62
250 52 3 8 7 779 28 47
294 71 2 26 13 781 7 24
145 47 5 2 2 770 28 50
219 53 3 3 5 817 30 48
1. V a l u e s b e l o w t h e (2s] d e t e c t i o n l i m i t a r e q u o t e d a s zero, 2. LOI is t h e l o s s o n i g n i t i o n a t 1050 ° C.
F'e O*
.
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00
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10
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Mmah a m for the different
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F i g6.. FeO m - FeO[t)/mgO of different rocks ofJebel Tanumah, tholeiltic and calc-alkalic series observed in the Skaergaarcl intrusion (Greenland], abyssal tholefltea (mid-oceanic ridges), MiyakeJirna (Izo-Bonin Arc), Tofua Island (Tonga Arc), Asama Volcano (Japan), and Amagl Volcano (Japan). The boundary between the two series is shown by a broken llne (after Miyashiro, 1975).
_ 01
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i 1
I 2
,
I 3
FeO*/MoO
,
I 4
,
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630
M. O. NASSmF,H. M. ALl and F. A. ZAKIR
rocks or reflects t h e a l t e r a t i o n d u e to m e t a m o r p h i s m , AI203 i n c r e a s e s with the decrease of SiO 2 a n d MgO. However, as m e n t i o n e d before, the area n e e d s f u r t h e r s a m p l i n g a n d m o r e detailed geochemical s t u d y to differentiate rock g r o u p s a n d t h e n relate t h e i r petrogenesis. REGIONAL T E C T O N I C S E T T I N G
Different h y p o t h e s i s o n t h e c r u s t a l evolution of the A r a b i a n Shield have b e e n proposed. B a s e d on petrological, geochemical a n d geochronological studies, a n i n t r a o c e a n i c i s l a n d arc accretion model with convergent plate m a r g i n s w a s i n t r o d u c e d by Greenwood e t al. ( 1 9 8 2 ) , Stoeser a n d C a m p (1984) a n d J o h n s o n a n d V r a n a s (1984). On the o t h e r h a n d , for local a r e a s in the shield including the K h a m i s M u s h a y t area, a n older sialic b a s e m e n t to the layered v o l c a n o - s e d i m e n t a r y rock above w a s p r e s u m e d to be p r e s e n t (Stacey a n d Hedge, 1983; Stacey a n d Stoeser, 1983; a n d Stacey a n d Agar, 1985). C o l e m a n (1973a) divided the K h a m i s M u s h a y t Q u a d r a n g l e into two m a j o r s t r u c t u r a l units: the b a s e m e n t complex (infra-structure) w h i c h s h o w s a complex d e f o r m a t i o n a l history, a n d the y o u n g e r m e t a m o r p h i c rocks ( s u p r a s t r u c t u r e ) m a r k e d by steep isoclinal folds. He a d d e d t h a t the steep folds in t h e s u p r a s t r u c t u r e m u s t have developed d u r i n g the rise of t h e remobilized b a s e m e n t complex into the y o u n g e r m e t a m o r p h i c rocks. Antfforms a n d s y n f o r m s were developed d u r i n g t h i s period. The a n t i f o r m s were i n t r u d e d by syntectonic granite w h e r e a s the s y n f o r m s b e c a m e the locus point for later i n t r u s i o n s of gabbro. Detailed s t r u c t u r a l s t u d i e s were carried out in two small adjoining a r e a s to the n o r t h of K h a m i s M u s h a y t t o w n by Amlas (1983) a n d Qari (1985). The two a u t h o r s d i s t i n g u i s h e d four successive p h a s e s of d e f o r m a t i o n a l s t r u c t u r e s in the y o u n g e r m e t a m o r p h i c rocks a n d the K h a m i s M u s h a y t gneiss which p r o d u c e d a r e m a r k a b l e complex s t r u c t u r a l p a t t e r n a n d f o u n d t h a t the gneiss h a s a n intrusive relationship to the y o u n g e r m e t a m o r p h i c rocks. The gabbroic b o d y which is the focus of the p r e s e n t s t u d y is enclosed in i n t e r b a n d e d amphibolite a n d quartzo-feldspathic schist with m i n o r i n t e r c a l a t i o n of semipelites. The body h a s irregular b o u n d a r i e s a n d looks massive in m o s t of the outcrops. It is, however, characterized locally by vague layering. Generally this layering is not clear probably due to later tectonic a n d metam o r p h i c effects. In the n o r t h e r n a n d n o r t h e a s t e r n parts, the layering is subhorizontal. The gently inclined layers generally strike n o r t h w e s t (330 ° - 345 °) a n d dip t o w a r d s the s o u t h - w e s t (20 ° - 30°).
CONCLUSIONS
The p r e l i m i n a r y s t u d y of J e b e l T a n u m a h Complex s u g g e s t s t h a t t h i s m a f i c / u l t r a m a f i c vaguely layered i n t r u s i o n is a n intrusive complex of distinct p e t r o c h e m i c a l characteristics. The Complex is formed m a i n l y of olivine-gabbro, amphibole-plagioclase c u m u l a t e , h y p e r s t h e n e diopside c u m u l a t e , h y p e r s t h e n e - d i o p s i d e - h o m blende c u m u l a t e a n d plagioclase c u m u l a t e . The rocks are m e t a m o r p h o s e d to u p p e r green-schistlower amphibolite facies. Limited m a j o r a n d trace e l e m e n t d a t a m a y indicate b i m o d a l c o m p o s i t i o n of the complex s h o w i n g calc-alkaline c h a r a c t e r i s t i c s with few tholeiites. More detailed s t u d i e s are needed to u n d e r s t a n d t h e c h a r a c t e r i s t i c s a n d tectonic setting of the m a f i c / u l t r a m a f i c i n t r u s i o n of t h e Arabian Shield which are not related to ophlolites. Acknowledgements - We are grateful to Mr. M. Qari for his assistance in the field and to M. Bin Garboo AIQahtani for his klnd assitance with transport. Also, we acknowledge Dr. Mac Coy and Dr. K. Simonian for their useful discussions on the paper.
REFERENCES.
Amlas, M. M. A. 1983. Geology and structures of the Precambrlan rocks north of Khamls Mushayt, southern Arabian Shield. Unpub. M. Sc. thesis, Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia, 141 p. Coleman, R. G., Brown, G. F. and Keith, E. C. 1972. Layered Gabbros in Southwest Saudi Arabia, U.S.G.S. Prof Paper 800D, D 143-D 150. Coleman, R. G. 1973a. Reconnaissance geology of the DI43-DI50. Khamis Mushayt quadrangle, Kingdom of Saudi Arabia. Saudi Arabian Dir. Gen. Min. Res., Geol. Map GM-4. Coleman, R. G. 1973b. Reconnaissance geology of the DI43-D150. Khamis Mushayt quadrangle, Kingdom of Saudl Arabia. Saudi Arabian Dir. Gen. Min. Res., Geol. Map GM-5. Gill, J. B. 1981. Orogenic Andesite and Plate Tectonics. Springer-Verlag, Berlin. Heidleberg and New York, 390 p. Greenwood, W. R., Stoeser, D. B., Fleck, R. J. and Stacey, J. S. 1982. Late Proterozoic island-arc complexes and tectonic belts in the southern part of the Arabian Shield. Saudi Arabian Dir. Gen. Min. Res., Jeddah, USGS-OF-02-8, 46 p. Johnson, P. R. and Vranas, G. J. 1984. The evolution of ideas concerning the stratigraphy and tectonic development of the Arabian Shield. Saudi Arabian Dir. Gen. Min. Res., Jeddah, FR-OF-04-27, 52 p. Miyashiro, A. 1975. Classification, characteristics and origin of ophiolites, J. Geol. 83, 249-281. Klemenic, P. M. and Poole, S. 1983. The Geology and Geochemistry of the Kadaweib Igneous Complex, Red Sea Hills, Sudan. Bull. Fac. Earth Sci, King Abdulaziz University, 6, 277-288.
Petrology and chemistry of Jebel Tanumah Complex, Khamis Mushayt Qari, M. Y. 1985. S t r u c t u r a l Analysis of the Proterozolc Rocks n e a r J a n f o o r Village northwest of Khamls M u s h a y t , S o u t h e r n A r a b i a n Shield. U n p u b . M.Sc. thesis, Faculty of E a r t h Sciences, King A b d u l a z i z University, J e d d a h , S a u d i Arabia, 153 p. Stacey, J. S. a n d Hedge, C. E. 1983. Direct evidence for early Proterozolc C r u s t in the Eastern Arabian Shield. S a u d i A r a b i a n Dir. Gen. Min. Res., J e d d a h , USGS-OF03, 63, 17 p. Stacey, J. S. and Agar R. A. 1985. U-Pb Isotopic evidence for Accretion of a Continental microplate in the Zalrn region of the S a u d i A r a b i a n Shield. Dir. Gen. Min. Res., J e d d a h USGS-OF-05-24, 24 p.
631
Stacey, J. S. and Stceser, D. S. 1983. Distribution of oceanic a n d Continental leads in the Arabian Shield. Saudl Arabian Dir. Gen. Min. Res., J e d d a h , USGS-OF03-55, 36 p. Stoeser, D. B. and Camp, V. E. 1984. Pan-African mlcroplate accretion of the Arabian Shield. Saudi Arabian Dir. Gen. Min. Res., J e d d a h , USGS-TR-04-17, 26 p. Vail, J. R., and Hughes, D. J. 1977. Tholeiite derivative dyke swarms n e a r Erkowit, Red Sea Hills, Sudan, Geologlsche Rundschau, 8 8 ( 1 ) , 228-237.
0899-5362/90 $3.00 + 0.00 © 1990 Pergamon Press plc
Petrology and chemistry of Jebel Tanumah Complex, Khamis Mushayt, Southern Arabian Shield, Saudi Arabia M. O. NASSIEF, H. M. Au and F. A. ZAKIR Facultyof EarthSciences,KingAbdulazizUniversity,Jeddah, Saudi Arabia
Abstract- The mafic intrusive complex at Jebel Tanumah is located 15 km north-west of Khamis Mushayt in the southern Arabian Shield and includes olivine-bearing gabbro as well as ampbibole-diopside-homblende gabbro cumulates. These rocks have been generally metamorphosed to upper greeenschist-lower ampbibolite facies. Fourteen white rock silicate analyses indicate that the majority of the rocks a r e calc-alkaline to tholeiitic in composition. The two major structural units in the Kharnis Mushayt region identified by Coleman consist of the basement complex of Asir Mountains and the younger metamorphic rocks. Syntectonic granitic rocks intruded the antiforms characterizing the younger rocks whereas the lower parts of the synforms are intruded by post-tectonic intrusions of layered gabbros such as the one studied at Jebel Tanumah.
Very few studies of mafic-ultramafic bodies within the Arabian Shield have b e e n u n d e r t a k e n so it is difficult to relate these rock types to either The studied area (18 ° 24' N; 42 ° 37'E) lies some oceanic or continental environments. The present 15 k m NW of Khamis M u s h a y t (Fig. l) It consists of work h a s been u n d e r t a k e n to identify the different a n oval maflc intrusion of 3.5 k m in m a x i m u m characteristics of the complex, which m a y assist diameter, and rising to 1829 -1890 m asl any further s t u d y of the area or of similar bodies (Coleman, 1973a) which h a s been intruded into within the shield. b a s e m e n t rocks of Precambrian age. Analyses of the samples collected h a s been The Precambrian rocks of the Khamis Mushayt carried out in the laboratories of the Faculty of region have b e e n divided by Coleman (1973b) into Earth Sciences, King Abdulaziz University, Saudi the b a s e m e n t complex and a series of y o u n g e r Arabia and the GAPS (Geological Consultants) metamorphic rocks. The former includes the laboratories in England. Khamis M u s h a y t Gneiss comprising mainly orthogneiss with minor intercalations of partially PETROLOGY AND GEOCHEMISTRY migmatitic paragneiss a n d amphibolites. The y o u n g e r m e t a m o r p h i c rocks include interlayered Fourteen representative rock samples of the pelitic s e d i m e n t a r y and mafic volcanic rocks of T a n u m a h gabbroic body were petrographicaUy various m e t a m o r p h i c grades, which are equivalent studied. From t h e m it h a s been possible to define to the Hali, Baish and B a h a h groups. Intrusive two types ofmafic-ultramaflc associations ofbroadrocks include syntectonic granites and post- ly olivine bearing gabbro and c u m u l a t e gabbros tectonic gabbroic and granitic intrusions. and showing various degrees of metamorphism. The present s t u d y c o n c e m s a n intrusive gabMagmatic zonation a n d / o r layering is not clear broic body (Fig. I) within the y o u n g e r metamorphic due to post-intrusion deformation and metarocks (Coleman, 1973a), other intrusive rocks morphism. It is also difficult to recognize the include granites and granodiorites. Biotite from relationship between mafic a n d ultramaflc rock the gabbro gave a K-Ar age of 690 + 22 M.a. units or the extent of each. (Coleman, 1973b). Biotite from a gabbroic plug The olivine bearing gabbro (KI and K3) is a north of Khamis Mushayt which m a y be related to medium-grained holocrystalline rock with an the oval gabbroic intrusions gave a n age of 640 allotriomorphic texture (Fig. 2). The m a i n mineral M.a. (Coleman et al., 1972). These rocks m a y be components are hornblende (25 - 50 %), plagiorelated to dyke s w a r m s recorded in the Red Sea clase (25 - 30%) and olivine which is about 5 % but Hills of the S u d a n and dated at 660 M.a. Waft and in samples m a y reach as m u c h as 35 % (K1) giving Hughes, 1977). It is not certain w h e t h e r these ages the rock a troctolitic affinity. Minor constituents are of m e t a m o r p h i s m or crystallization. include diopside and opaques. INTRODUCTION
625
626
M. O. NASSIEF,H. M. ALI and F. A. ZAKIR --\ 250
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Fig. I. Location and general geology of the Jebel Tanumah area. Olivine occurs as skeletal grains ranging from 1 to 4 m m in m a x i m u m diameter with slight serpentinization along its cracks. Hornblende exists as anhedral prisms up to 3 m m in length and poikilitically enclosing olivine and plagioclase. Its pleochroism is deep reddish brown to pale reddish brown to buff. Plagioclase occurs in the form of subhedral to anhedral twinned laths ranging in composition from An4o to Anso. The c u m u l a t e rocks include amphibole-plagioclase cumulate, diopside cumulate, hypersthenediopside cumulate, hyperthene-diopside-hornblende cumulate, and plagioclase (?)cumulate. The amphibole-plagioclase c u m u l a t e s (K2A, K33) are medium- to coarse-grained holocrystalline rocks with hypidiomorphic to allotriomorphic granular textures (Fig. 3). The m a i n mineral constituents are hornblende (60 %) and plagioclase (30 %). The h o r n b l e n d e o c c u r s as elongated s u b h e d r a l crystals showing simple twinning and pleochroism similar to that encountered in the olivine bearing gabbro. The plagioclase occurs in the form of subhedral laths reaching 3 m m in length and showing lammelar twinning. The An content of plagioclase is 50 to 54 which may show partial saussuritization. The minor constituents include interstitial opaques, apatite and epidote.
The larger amphibole and plagioclase crystals represent c u m u l u s p h a s e s and m a y reflect igneous laminations where they show preferred orientation. The hatched cores a n d perfect rims of the amphibole suggest possible m e t a m o r p h i s m from pyroxene (Simonian, personal com.). Fine intergrowth of hornblende, plagioclase and opaques could well be the i n t e r c u m u l u s phases. Sample K34 is a medium-grained rock consisting essentially of plagioclase (about 90 %) wlth an average of An42. This plagioclase is highly saussuritized. Alteration products also include chlorite and biotite. This rock is considered to be a plagioclase c u m u l a t e with preferred orientation probably representing igneous lamination. The diopside-bearing rocks (K2B and K30 ) are medium- to coarse-grained holocrystalline rocks with hypidiomorphic to allotriomorphic granular texture. The major constituents are diopside (20 - 35 %), hornblende (35 - 55 %) and plagioclase (+ 20 %). The diopside occurs as colourless anhedral to subhedral crystals which in some places are partially altered to amphibole. The hornblende exists as large anhedral to subhedral crystals enclosing part of the pyroxene. It displays simple twinning and is pleochroic from deep reddish brown to buff. The h a t c h e d cores and the well
Petrology and chemistry of Jebel Tanumah Complex, Khamis Mushayt
627
Fig. 2. Olivine, white with curved crack (A): hornblende, brown, pleochroic (B): and plagioclase, white (C), forming a n allotrimorphic granular aggregate, (x 45).
Fig. 3. The two essential mineral components, brown hornblende (A) and plagioclase (B) are in the field of view, as well as accessory titanomagnetite or ilmenite (C). Apatite is also present (D). The directed texture is apparent, (x 45).
628
M. O. NASSIEF,H. M. ALl and F. A. ZAKIR
defined fringes s u g g e s t r e p l a c e m e n t from a former mineral, probably pyroxene. The plagioclase o c c u r s as small t w i n n e d crystals interstitial to the pyroxene a n d amphibole. It r a n g e s in average composition from An3~(K2B) to An43 (K30). Partial s a u s s u r i t i z a t i o n of plagioclase is c o m m o n . The m i n o r c o n s t i t u e n t s include o p a q u e s a n d accessory s p h e n e a n d apatite. We c o n s i d e r this r o c k to be a c u m u l u s p h a s e a n d suggest t h a t it w a s originally a diopside c u m u l a t e with i n t e r c u m u l u s plagioclase a n d t h a t the hornblende is t h e p r o d u c t of amphibolite facies m e t a morphism. The h y p e r s t h e n e - b e a r i n g rock (K8, K9, K22, K31) are m e d i u m - to c o a r s e - g r a i n e d holocrystalllne with hypidiomorphic to allotrimorphic t e x t u r e s (Fig. 4). The m a i n m i n e r a l c o n s t i t u e n t s are h y p e r s t h e n e (5 - 20%), h o r n b l e n d e (15 - 50 %) a n d plagioclase ( 15 - 40 %). Diopside is the m a j o r c o n s t i t u e n t of K8 w h e r e it m a y r e a c h 60 % while it m a k e s up a b o u t 10 % of K9 a n d is not e n c o u n t e r e d in K22 a n d K31. The h y p e r s t h e n e exists as x e n o m o r p h i c crystals some of w h i c h s h o w lamellar intergrowth, with diopside existing a s s m a l l e r discrete grains. Some x e n o m o r p h i c h o r n b l e n d e crystals, u p to 2 m m long, s o m e t i m e s s h o w a poikilitic texture a n d pleochroism from deep-pale greenish b r o w n to buff. The plagioclase o c c u r s as a n h e d r a l crystals reaching 1 m m in length, with a n average composition of An43. The m i n e r a l a s s e m b l a g e s observed in the J e b e l
T a n u m a h a r e a indicate conditions of m e t a m o r p h i s m of u p p e r g r e e n s c h i s t to lower amphibolite facies probably within t h e biotite zone. This g r a d e of m e t a m o r p h i s m is c o m p a r a b l e to t h a t observed in the a d j a c e n t c o u n t r y rocks. Major a n d trace e l e m e n t s of f o u r t e e n s a m p l e s (Table I) were d e t e r m i n e d b y X-ray fluorescence a n d atomic absorption t e c h n i q u e s . Although t h e results are not sufficient for detailed geochemical interpretation, t h e d a t a plotted on t h e AFM d i a g r a m (Fig. 5) s h o w s t h a t s o m e of t h e fall into the calc-alkaline field while t h e o t h e r s s h o w tholeiitic affinities. MgO s h o w s two g r o u p s of c o n c e n t r a t i o n s , t h e first r a n g e s from 4.82 to 9.26 while the s e c o n d r a n g e s f r o m 10.64 to 17.29. This is reflected in the FeOt~/MgO ratios (0.70 to 2.80) a n d s u g g e s t s a bimodal c o m p o s i t i o n for t h e complex, with s o m e of the r o c k s being tholeiitic while the o t h e r s are calc-alkaline. The high TiO 2 r e a c h e s up to 3.22 % a n d the relatively high FeO 2, Al203 a n d P2Os in s o m e s a m p l e s clearly e m p h a s i z e the tholeiitic affinity. On FeO(, against F e O I J M G O d i a g r a m (Fig. 6), the rocks m o s t l y fall within t h e s k a e r g a a r d field with two falling within the abbyssal tholeiites. SiO 2, K20 a n d FeOctl/Mg v a l u e s are v e r y c o m - p a r a b l e with o t h e r gabbroic r o c k s within the Arabian Shield (Coleman et al., 1972). The high Ni c o n c e n t r a t i o n (e.g. K i- i a n d K22) m a y be d u e to olivine e n r i c h m e n t . High Sr v a l u e s (770-172 ppm) indicate the Ca-plagioclase a b u n d a n c y in t h e s e
Fig. 4. Hyperthene (A), brown hornblende (B) and plagioclase (C) are the essential minerals. Sparse accessory opaques are also evident (D), (x 45).
Petrology and T a b l e 1. M a j o r e l e m e n t
KI-1
K2A
SiO2 A1203 Fe203 F¢O M.nO MgO CaO NaaO I~O TiO 2 P20s LOI
42.15 15.03 1.71 10.31 0.21 17.29 9.65 1.21 0.20 1.11 0.01 0.60
Total
99.48
Ba
Cr Nb Ni Rb Sr Zr
139 243 0 285 3 186 14 30
chemistry o f Jebel Tanumah
a n a l y s i s (In w t . % ) a n d t r a c e e l e m e n t a b u n d a n c e from the Jebel Tanumah Complex. K3
K4
K8
K9
K31
K33
K34
K38 K39
39.21 48.11 45.19 38.27 51.53 47.08 44.62 16.53 7.47 17.00 20.87 4.61 16.95 18.23 7.28 3.12 1.86 7.26 2.34 1.81 3.11 9.13 8.08 8.74 7.30 6.49 8.60 6.97 0.14 0.23 0.17 0.14 0.16 0.19 0.13 6.67 13.13 9.26 5.20 15.07 9.16 12.26 11.50 15.22 12.83 12.74 16.77 11.30 10.62 2.73 1.66 2.19 1.83 1.08 2.10 1.66 0.66 0.43 0.2(3 0.85 0.33 0.24 0.31 2.96 1.38 1.46 1.98 0.64 1.59 1.26 1.96 0.10 0.03 1.56 0.08 0.18 0.04 0.79 0.79 0.51 1.66 0.63 0.64 0.36
45.90 8.89 4.42 7.96 0.21 12.50 14.59 1.81 0.61 1.68 0.22 0.90
46.04 17.11 1.92 7.63 0.13 10.64 12.15 2.01 0.35 1.34 0.02 0.57
40.34 16.58 6.37 9.07 0.14 8.68 11.13 2.44 0.67 3.22 0.01 1.19
50.07 20.25 1.27 5.51 0.11 4.82 7.01 3.32 0.78 2.07 0.45 4.28
40.48 17.61 5.98 9.75 0.13 8.37 11.27 2.28 0.45 3.20 0.00 0.44
40.12 17.83 6.49 9.53 0.13 7.67 11.23 2.35 0.54 2.90 0.04 0.79
99.56
99.69 99.90
99.84! 99.93
99.97
99.63
99.71 99.43
99.67
99.73
99.85
220 445 3 46 4 224 39 94
406 26 3 1 10 1723 34 60
157 1142 2 132 7 130 23 73
162 251 5 74 3 275 30 101
88 311 2 89 3 223 23 62
K22
(In p p m ] o f r o c k s
K30
303 48 4 9 5 1066 46 49
K2B
629
Complex, Khamis Mushayt
99.57
72 142 4 187 4 250 22 72
264 201 3 28 3 345 38 80
136 309 2 143 5 205 21 62
250 52 3 8 7 779 28 47
294 71 2 26 13 781 7 24
145 47 5 2 2 770 28 50
219 53 3 3 5 817 30 48
1. V a l u e s b e l o w t h e (2s] d e t e c t i o n l i m i t a r e q u o t e d a s zero, 2. LOI is t h e l o s s o n i g n i t i o n a t 1050 ° C.
F'e O*
.
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F i g6.. FeO m - FeO[t)/mgO of different rocks ofJebel Tanumah, tholeiltic and calc-alkalic series observed in the Skaergaarcl intrusion (Greenland], abyssal tholefltea (mid-oceanic ridges), MiyakeJirna (Izo-Bonin Arc), Tofua Island (Tonga Arc), Asama Volcano (Japan), and Amagl Volcano (Japan). The boundary between the two series is shown by a broken llne (after Miyashiro, 1975).
_ 01
0
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i 1
I 2
,
I 3
FeO*/MoO
,
I 4
,
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630
M. O. NASSmF,H. M. ALl and F. A. ZAKIR
rocks or reflects t h e a l t e r a t i o n d u e to m e t a m o r p h i s m , AI203 i n c r e a s e s with the decrease of SiO 2 a n d MgO. However, as m e n t i o n e d before, the area n e e d s f u r t h e r s a m p l i n g a n d m o r e detailed geochemical s t u d y to differentiate rock g r o u p s a n d t h e n relate t h e i r petrogenesis. REGIONAL T E C T O N I C S E T T I N G
Different h y p o t h e s i s o n t h e c r u s t a l evolution of the A r a b i a n Shield have b e e n proposed. B a s e d on petrological, geochemical a n d geochronological studies, a n i n t r a o c e a n i c i s l a n d arc accretion model with convergent plate m a r g i n s w a s i n t r o d u c e d by Greenwood e t al. ( 1 9 8 2 ) , Stoeser a n d C a m p (1984) a n d J o h n s o n a n d V r a n a s (1984). On the o t h e r h a n d , for local a r e a s in the shield including the K h a m i s M u s h a y t area, a n older sialic b a s e m e n t to the layered v o l c a n o - s e d i m e n t a r y rock above w a s p r e s u m e d to be p r e s e n t (Stacey a n d Hedge, 1983; Stacey a n d Stoeser, 1983; a n d Stacey a n d Agar, 1985). C o l e m a n (1973a) divided the K h a m i s M u s h a y t Q u a d r a n g l e into two m a j o r s t r u c t u r a l units: the b a s e m e n t complex (infra-structure) w h i c h s h o w s a complex d e f o r m a t i o n a l history, a n d the y o u n g e r m e t a m o r p h i c rocks ( s u p r a s t r u c t u r e ) m a r k e d by steep isoclinal folds. He a d d e d t h a t the steep folds in t h e s u p r a s t r u c t u r e m u s t have developed d u r i n g the rise of t h e remobilized b a s e m e n t complex into the y o u n g e r m e t a m o r p h i c rocks. Antfforms a n d s y n f o r m s were developed d u r i n g t h i s period. The a n t i f o r m s were i n t r u d e d by syntectonic granite w h e r e a s the s y n f o r m s b e c a m e the locus point for later i n t r u s i o n s of gabbro. Detailed s t r u c t u r a l s t u d i e s were carried out in two small adjoining a r e a s to the n o r t h of K h a m i s M u s h a y t t o w n by Amlas (1983) a n d Qari (1985). The two a u t h o r s d i s t i n g u i s h e d four successive p h a s e s of d e f o r m a t i o n a l s t r u c t u r e s in the y o u n g e r m e t a m o r p h i c rocks a n d the K h a m i s M u s h a y t gneiss which p r o d u c e d a r e m a r k a b l e complex s t r u c t u r a l p a t t e r n a n d f o u n d t h a t the gneiss h a s a n intrusive relationship to the y o u n g e r m e t a m o r p h i c rocks. The gabbroic b o d y which is the focus of the p r e s e n t s t u d y is enclosed in i n t e r b a n d e d amphibolite a n d quartzo-feldspathic schist with m i n o r i n t e r c a l a t i o n of semipelites. The body h a s irregular b o u n d a r i e s a n d looks massive in m o s t of the outcrops. It is, however, characterized locally by vague layering. Generally this layering is not clear probably due to later tectonic a n d metam o r p h i c effects. In the n o r t h e r n a n d n o r t h e a s t e r n parts, the layering is subhorizontal. The gently inclined layers generally strike n o r t h w e s t (330 ° - 345 °) a n d dip t o w a r d s the s o u t h - w e s t (20 ° - 30°).
CONCLUSIONS
The p r e l i m i n a r y s t u d y of J e b e l T a n u m a h Complex s u g g e s t s t h a t t h i s m a f i c / u l t r a m a f i c vaguely layered i n t r u s i o n is a n intrusive complex of distinct p e t r o c h e m i c a l characteristics. The Complex is formed m a i n l y of olivine-gabbro, amphibole-plagioclase c u m u l a t e , h y p e r s t h e n e diopside c u m u l a t e , h y p e r s t h e n e - d i o p s i d e - h o m blende c u m u l a t e a n d plagioclase c u m u l a t e . The rocks are m e t a m o r p h o s e d to u p p e r green-schistlower amphibolite facies. Limited m a j o r a n d trace e l e m e n t d a t a m a y indicate b i m o d a l c o m p o s i t i o n of the complex s h o w i n g calc-alkaline c h a r a c t e r i s t i c s with few tholeiites. More detailed s t u d i e s are needed to u n d e r s t a n d t h e c h a r a c t e r i s t i c s a n d tectonic setting of the m a f i c / u l t r a m a f i c i n t r u s i o n of t h e Arabian Shield which are not related to ophlolites. Acknowledgements - We are grateful to Mr. M. Qari for his assistance in the field and to M. Bin Garboo AIQahtani for his klnd assitance with transport. Also, we acknowledge Dr. Mac Coy and Dr. K. Simonian for their useful discussions on the paper.
REFERENCES.
Amlas, M. M. A. 1983. Geology and structures of the Precambrlan rocks north of Khamls Mushayt, southern Arabian Shield. Unpub. M. Sc. thesis, Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia, 141 p. Coleman, R. G., Brown, G. F. and Keith, E. C. 1972. Layered Gabbros in Southwest Saudi Arabia, U.S.G.S. Prof Paper 800D, D 143-D 150. Coleman, R. G. 1973a. Reconnaissance geology of the DI43-DI50. Khamis Mushayt quadrangle, Kingdom of Saudi Arabia. Saudi Arabian Dir. Gen. Min. Res., Geol. Map GM-4. Coleman, R. G. 1973b. Reconnaissance geology of the DI43-D150. Khamis Mushayt quadrangle, Kingdom of Saudl Arabia. Saudi Arabian Dir. Gen. Min. Res., Geol. Map GM-5. Gill, J. B. 1981. Orogenic Andesite and Plate Tectonics. Springer-Verlag, Berlin. Heidleberg and New York, 390 p. Greenwood, W. R., Stoeser, D. B., Fleck, R. J. and Stacey, J. S. 1982. Late Proterozoic island-arc complexes and tectonic belts in the southern part of the Arabian Shield. Saudi Arabian Dir. Gen. Min. Res., Jeddah, USGS-OF-02-8, 46 p. Johnson, P. R. and Vranas, G. J. 1984. The evolution of ideas concerning the stratigraphy and tectonic development of the Arabian Shield. Saudi Arabian Dir. Gen. Min. Res., Jeddah, FR-OF-04-27, 52 p. Miyashiro, A. 1975. Classification, characteristics and origin of ophiolites, J. Geol. 83, 249-281. Klemenic, P. M. and Poole, S. 1983. The Geology and Geochemistry of the Kadaweib Igneous Complex, Red Sea Hills, Sudan. Bull. Fac. Earth Sci, King Abdulaziz University, 6, 277-288.
Petrology and chemistry of Jebel Tanumah Complex, Khamis Mushayt Qari, M. Y. 1985. S t r u c t u r a l Analysis of the Proterozolc Rocks n e a r J a n f o o r Village northwest of Khamls M u s h a y t , S o u t h e r n A r a b i a n Shield. U n p u b . M.Sc. thesis, Faculty of E a r t h Sciences, King A b d u l a z i z University, J e d d a h , S a u d i Arabia, 153 p. Stacey, J. S. a n d Hedge, C. E. 1983. Direct evidence for early Proterozolc C r u s t in the Eastern Arabian Shield. S a u d i A r a b i a n Dir. Gen. Min. Res., J e d d a h , USGS-OF03, 63, 17 p. Stacey, J. S. and Agar R. A. 1985. U-Pb Isotopic evidence for Accretion of a Continental microplate in the Zalrn region of the S a u d i A r a b i a n Shield. Dir. Gen. Min. Res., J e d d a h USGS-OF-05-24, 24 p.
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Stacey, J. S. and Stceser, D. S. 1983. Distribution of oceanic a n d Continental leads in the Arabian Shield. Saudl Arabian Dir. Gen. Min. Res., J e d d a h , USGS-OF03-55, 36 p. Stoeser, D. B. and Camp, V. E. 1984. Pan-African mlcroplate accretion of the Arabian Shield. Saudi Arabian Dir. Gen. Min. Res., J e d d a h , USGS-TR-04-17, 26 p. Vail, J. R., and Hughes, D. J. 1977. Tholeiite derivative dyke swarms n e a r Erkowit, Red Sea Hills, Sudan, Geologlsche Rundschau, 8 8 ( 1 ) , 228-237.