El-Nashar, El-Said Ramadan

Hilmy, Mohamed Ezzeldin

Kabesh, Mahmoud Lofty

Salem, Abdel-Karim Ahmed

English

The Samadai granitic complex in the Central Eastern Desert of Egypt is petrochemically evaluated. Major oxides data of 13 new chemical analysis are advanced and processed following several chemical parameters. The serial characters, NIGGL I values and normative minerals are discussed. The rocks of the complex belong to the calc-alkaline series and are dominantly potassic with some sodic tendencies. Chemical classification of the granitic complex is presented

Kabesh Mahmoud Lofty

Salem Abdel-Karim Ahmed

Hilmy Mohamed Ezzeldin

El-Nashar El-Said Ramadan

University of Szeged

Acta Mineralogica-Petrographica, Szeged, XXVjl, 85—96, 1981 S O M E P E T R O C H E M I C A L C H A R A C T E R S O F S A M A D A I G R A N I T I C C O M P L E X , S O U T H E A S T E R N D E S E R T , E G Y P T MAHMOUD LOTFY KABESH, ABDEL-KARIM AHMED SALEM, MOHAMED E . HILMY a n d EL-SAID RAMADAN EL-NASHAR ABSTRACT The Samadai granitic complex in the Central Eastern Desert of Egypt is petrochemically evaluated. Major oxides data of 13 new chemical analysis are advanced and processed following several chemical parameters. The serial characters, N I G G L I values and normative minerals are discussed. The rocks of the complex belong to the calc-alkaline series and are dominantly potassic with some sodic tendencies. Chemical classification of the granitic complex is presented. INTRODUCTION The granitic complex of Samadai is considered by EL-RAMLY and AKAAD [19.60], SABET [1961] and EL-RAMLY [ 1 9 7 2 ] as belonging to the younger group of granitoids. SABET et al., [ 1976] grouped the granitic complex of Samadai with several granitic plutons in the Eastern Desert of Egypt as belonging to the Late-Proterozoic-Early Paleozoic intrusions. The same authors also [op-cit., 1976] assigned these granitic plutons to the "Gattarian group" and to the phase of the coarse-grained biotite granite and the leucocratic granite. The examined rocks of Samadai granitic complex occur along the Eastern border of the basement rocks in the Red Sea Coast, (Fig. 1). The northern part of the complex has been previously examined by ESSAWY, [1967]. He mentioned some details of the field characters and petrography of the complex. According to ESSAWY (op. cit.) the complex consists essentially of medium-grained adamellites and granodiorites associated with tonalites and minor granites. However, ESSAWY (op. cit.) gave no details of the petrochemical characters or the chemical classification of Samadai granitic complex. Later, ESSAWY [1972] in an attempt to reveal the genetic relationships between members of the complex advanced the results of partial analysis for certain elements (Na, K, Rb, Sr) of the rock units of the complex. He concluded that the calc-alkaline granites bear no parental relation to the adamellite and granodiorites. The present study forms part of a continuing investigation of the petrochemistry and petrogenesis of the younger granitoids of Precambrian age in the Eastern Desert of Egypt. GEOLOGIC SETTING OF SAMADAI AREA Samadai area is dominantly formed of Precambrian basement rocks. According to ESSAWY [1967] the basement rocks of the examined area comprise metasediments, serpentinites, metagabbro-diorite complex, granites, adamellites and granodiorites. 85 The present work deals mainly with the granitic complex of Samadai which comprise two field types: pink — red granitic rocks and greyish-white granodiorites. The granitic rocks of Samadai form a rather circular body showing sharp intrusive contacts with older country rocks to the north, west and south and which comprise metasediments, Serpentinites and metagabbro-diorite rocks. The various basement rocks are traversed by few post-granite dykes (Fig. 2). Based on field observations and crosscutting relations as well as the presence of xenoliths of the country rocks within the host granitic rocks, it is believed that the granitic complex is of younger age. Contacts between the various members of the granitic complex may be sharp but more commonly they are gradational. The granitic complex appears to have reached the present position probably as a result of forcible intrusion pushing aside and shoul-dering apart of the country rocks. According to ESSAWY [ 1 9 6 7 ] the emplacement was reasonably accompanied by some kind of the granite tectonics resulting in thrusting, phyllonitisation of metasediments and brecciation of serpentines. PETROGRAPHY The detailed petrography of Samadai granitic complex has already been men-tioned by ESSAWY (op. cit.). Generally rocks of the complex are medium-to rather coarse-grained with holocrystalline hypidiomorphic granular texture. They are usually 86 non-porphyritic aad even-grained. Few types are porphyritic with quartz and plagio-clase megacrysts. Intergrowths such as micrographic and myrmekitic are sometimes striking. Both biotite and hornblende represent the mafic minerals. Rarely biotite is the sole mafic. Secondary chlorite is not uncommon while apatite, zircon, epidote, sphene, magnetite and ilmenite are notable accessories. PETROCHEMICAL INVESTIGATION The petrochemical characters of the granitic rocks were clarified from RITTMAN'S Suite Index, normative minerals, NIGGLI values and variation diagrams as calculated f rom the chemical data obtained. Complete chemical analysis of 13 samples representing the different types of Samadai granitic complex were carried out and the results are given in Table 1 while the location of the analysed samples is shown in Fig. 2. RiTTMAN'S INDEX RITTMAN'S Suite indices, [1957] for Samadai granitic rocks are given in Table 1 and plotted graphically in Fig. 3 against Si02. It is clear from the diagram and according to the quantitative subdivisions of RITTMAN that these granitic rocks range between weak pacific and strong pacific (2.24—4.53) which correspond on the average to calc-alkaline character. The chemical composition of rocks in Table 1 has been recalculated in terms of K 2 0 + N a 2 0 , F e 0 + Fe 2 0 3 and MgO and the results are plotted on an A F M diagram 87 00 00 TABLE 1 Chemical analyses of Samadai granitic rocks 1 2 3 4 5 6 7 8 9 10 11 12 13 SiOa 70.93 71.53 72.14 65.08 70.55 69.75 68.11 70.97 70.16 72.55 71.70 64.30 67.81 Al2Oa 14.10 13.13 13.25 15.68 15.29 14.78 13.55 14.53 15.29 13.32 13.74 17.84 14.50 Fe„03 2.27 1.85 0.24 4.33 0.52 1.18 3.56 0.74 1.52 0.67 0.82 2.88 2.20 FeO 1.19 1.45 2.35 1.06 1.24 1.29 2.30 0.76 1.63 1.27 1.31 3.42 1.79 MnO — — 0.01 — — — 0.03 0.02 MgO 0.43 0.54 0.42 0.50 1.74 1.71 0.27 0.58 0.29 0.50 0.50 0.95 0.89 CaO 0.66 0.60 0.91 2.73 0.42 0.49 0.88 0.49 0.49 1.02 0.49 1.20 0.66 NaaO 4.25 3.88 4.81 4.50 4.00 4:13 4.00 5.00 4.38 4.75 3.94 4.25 4.63 KAO 5.00 4.78 4.88 5.50 4.88 5.25 4:88 5.06 4.50 3.38 5.38 3.13 5.00 TiOa 0.38 0.38 0.03 0.55 0.03 0.03 0.49 0.03 . 0.64 0.15 0.30 0.66 0.40 P2O6 0.31 0.31 — — — ' 0.24 0.24 0.24 HaO 0.98 0.72 0.68 0.46 1.58 0.80 1.00 0.72 1.08 1.50 1.12 1.12 1.07 TOTAL 100.50 99.17 99.72 100.39 100.25 99.41 99.07 99.12 99.98 99.11 99.30 100.01 99.19 R I T T M A N ' S 3.06 2.63 3.22 4.53 2.86 3.31 3.14 3.61 2.89 2.24 3.02 2.56 3.73 Index Agp. Coef. 0.88 0.88 0.96 0.85 0.78 0.84 0.88 0.94 0.79 0.86 0.90 0.58 0.90 Fig. 3. Si02—(Na20 + K20) diagram [after RITTMANN, 1957] (Fig. 4). The diagram shows that the Samadai granitic rocks are enriched in alkalies and impoverished in magnesium and iron. The analyses have been plotted, on the ternary diagram K 2 0 — CaO — NaaO ('Fig. 5.) and follow a calc-alkaline trend [cf. NOCKOLDS and ALLEN, 1953]. Fig. 6 shows the relation between agpaitic coefficient and Si02 percent. The agpaitic coefficient from Zavaritski parameter [cf. BAILEY and M A C D O N A L D , 1 9 7 0 ] is plotted against Si02 to show the nature of Samadai granitic rocks. The diagram shows that all the examined rocks are of miaskitic nature i.e. mol. ratio of N a 2 0 + K 2 0 A1203 is less than unity. The examined granitic rocks have K 2 0 /Na 2 0 ratio >0.6 (Fig. 7). According to R A G U I N [ 1 9 6 5 ] these granitic rocks have potassic character with very few showing sodic tendency. F e O • F e 2 0 3 89 Fig. 5. K20—Na20—CaO diagram of the investigated granitic rocks 1 . 6 -' 1 . 2 0 . 4 -A g p a i t i c Fig. 6. Agpaitic index 6 2 6 4 6 6 silica diagram M i a s k i t i c _J L_ 7 0 7 2 7 4 S i 0 . i The calculated NIGGLI values of the investigated granitic rocks are given in Table 2. The values of al are plotted against fm values of NIGGLI (Fig. 8) showing that the nature of the magma is salic according to NIGGLI 'S classification. The values of al are plotted against alk of NIGGLI (Fig 9.). It is clear that almost all the granitic rocks of Samadai fall within the relatively alkali-rich, with one sample representing granodiorites falling in the intermediate alkaline rocks [BURRI, 1964] . The NIGGLI values are plotted on the appropriate D-diagram of NIGGLI 'S series of (al-alk)-fm—e triangular diagram [1954] which includes the rock categories I and II (alkali-aluminous), III (alkali-calc-aluminous) VI (alkali-calc-femic) and VII (aluminous). It is evident from the diagram on Fig. 10 that all the samples plot in the categories I and II. The calculated norm values of the examined granitic rocks are given in Table 3. Fig. 11 shows normative quartz plotted against normative anorthite. It is evident 90 TABLE 2 N I G G L I values of the investigated granites 5 6 7 1 9 1 0 1 1 1 2 1 3 al 42.24 41.60 40.37 37.07 44.01 41.27 fm 17.00 18.33 14.38 19.62 19.64 21.40 c 3.60 3.46 5.04 11.73 2.20 2.49 alk 37.16 36.61 40.21 31.58 34.15 34.84 si 360.60 384.58 373.02 261.10 344.61 330.52 k 0.44 0.45 0.40 0.45 0.45 0.46 mg 0.19 0.24 0.23 0.15 0.64 0.56 qz + 111.96 + 138.14 + 112.20 + 34.79 + 108.02 + 91.15 38.17 44.55 45.99 43.57 44.17 43.13 39.71 23.90 10.70 15.00 12.83 13.41 26.50 20.92 4.51 2.73 2.68 2.07 2.86 5.27 3.30 33.42 42.01 36.32 37.53 39.56 25.09 35.86 325.60 369.26 358.12 402.73 391.15 263.79 316.75 0.45 0.40 0.40 0.32 0.47 0.33 0.42 0.07 0.42 0.15 0.32 0.30 0.22 0.30 +•91.92 + 101.22 + 112.84 + 152.61 + 132.91 + 63.43 + 73.31 TABLE 3 Norm-values for the examined granites 1 2 3 4 5 6 7 8 9 10 11 12 13 gz 24.28 27.78 20.54 11.94 22.67 20.04 22.59 19.56 24.15 26.97 24.42 21.69 18.25 or 29.85 28.95 29.05 32.60 29.10 31.30 29.91 30.15 27.00 20.45 32.50 18.85 30.10 ab 38.60 35.75 43.55 40.55 36.25 37.47 37.15 45.30 39.90 43.70 36.15 38.85 42.35 an 1.20 1.00 0.15 6.40 2.10 2.45 4.50 0.85 v 2.45 5.20 2.50 4.45 1.75 c 1.40 1.37 — — 2.92 1.56 0.11 0.57 2.58 — 0.60 6.51 0.94 w — 1.76 2.88 — — . — — — — — — — en 1.20 1.52 1.18 1.38 4.84 4.76 0.72 1.62 0.82 1.42 1.42 2.68 2.50 fs 0.44 3.48 — 1.54 1.16 0.46 0.62 0.58 1.32 1.08 2.46 0.70 mt 1.86 1.98 0.25 1.32 0.54 1.24 3,85 0.78 1.62 0.72 0.87 3.06 2.34 il 0.54 0.54 0.04 0.76 0.04 0.04 0.70 0.04 0.90 0.22 0.42 0.94 0.56 ap 0.67 0.66 — — . — — — 0.51 — — — 0.51 0.51 ht 0.40 — — 2.15 — — — — — . — • — . — — Total 100 99.99 100 99.98 100 100 99.99 100 100 ioo 100 100 100 N a 2 0 ( mol ) Fig. 7. Variations in alkalies of the granitic rocks O superimposed point 40-30-f m 2 0 -1 0 -S e m i s a l i c m a g m a Sa l i c m a g m a O 10 Fig. 8. a! — fm diagram 20 30 40 a l 50 60 from the diagram that nearly all the granitic rocks fall within the calc-alkalic series with only one sample plotting in the alkalic-calc series. The normative Ab, Or and Qz are recalculated to 100% for Samadai granitic rocks and plotted on the ternary diagram (Ab—Or—Qz) the residua of TUTTLE and B O W E N [ 1 9 5 8 ] . The plots Fig. 12 show that the granitic rocks are rather enriched in albite. It is clear that all the analysed samples fall within the area of maximum con-92 FI? . 9. Relationships of al and alk in the granitic rocks [after B U R R I and N I G G L I , 1 9 4 5 ] 5 0 I—II: Alk-al silicate rocks III: Alk-calc-al silicate rocks: VI: Alk-calc-femic rocks VII: Al-silicate rocks centration of analyses of the granites of the world according to T U T T L E and B O W E N (pp. cit. 79) "the concentration of the analyses near the centre of the diagram is readily explained if the magmatic history is involved in the origin of most granites". The granitic rocks of Samadai are thus considered to possess a magmatic history. 40 -w. 30 ® > •H o to 20 Caic-alkal ic series 1 0 -Alkali-calcic s e r i e s ^ - - . -L. _1_ " ~ - ' O 10 20 30 40 ¡ f o r m a t i v e An i n w t . % Fig. 11. Normative Qz — An relation for the investigated granites 50 60 Qz Ab Or Fig. 12. Contoured triangular diagram showing the distribution of normative Ab — Or — Qz in all analysed rocks ( 1 2 6 9 ) in WASHINGTON'S Tables, containing 8 0 % or more Ab + Or + + Qz [after TUTTLE and B O W E N , 1 9 5 8 and MARMO, 1 9 7 1 ] 94 Fig. 13. Triangular diagram for An, Ab, Or normative ratio in granitic rocks iaftei HIETANEN 1 9 6 3 ] A Kali-granite C Granite trondhjemite E Quartz-monzonite G Tonalite I Granodiorite K Calci-monzonite M Gabbro B Granite D Trondhjemite F Monzonite H Calci-grariite J Quartz-diorite L Granogabbro N Mafic gabbro O r Ab Fig. 14. Quartz — feldspar rocks [after STRECKEISEN, 1 9 7 6 ] 2 a TAlkali-granite [Alkali-rhyolite 7 . [Alkali-feldspar granite [Rhyolite [(Syeno-) granite d [Rhyolite 3b T(Monzo-) granite [Rhyodacite [Granodiorite Dacite Tonalite Plagidacite Trondhjemite 95 

Some petrochemical characters of Samadai granitic complex, South Eastern Desert, Egypt

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Some petrochemical characters of Samadai granitic complex, South Eastern Desert, Egypt

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