The petrogenesis of late Neoproterozoic gabbro ...

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Abstract The late Neoproterozoic gabbro/diorite intrusion. (~11.7 km2) at Sheikh El-Arab area represents the only mafic exposure in the basement rocks of ...
Arab J Geosci DOI 10.1007/s12517-014-1652-z

ORIGINAL PAPER

The petrogenesis of late Neoproterozoic gabbro/diorite intrusion at Sheikh El-Arab area, central Sinai, Egypt Michael Dawood Samuel & Doris Sadek Ghabrial & Hilmy Essa Moussa & Mohamed Wahbi Ali-Bik

Received: 23 April 2014 / Accepted: 22 September 2014 # Saudi Society for Geosciences 2014

Abstract The late Neoproterozoic gabbro/diorite intrusion (~11.7 km2) at Sheikh El-Arab area represents the only mafic exposure in the basement rocks of Katharina province of central Sinai largely occupied by granitoids and their volcanic equivalents. The field relations indicate that the intrusion is younger than the lower unit (630–615 Ma) of the volcanics and clastics of the Rutig Formation, and older than the surrounding granodiorites of Sheikh El-Arab and Rahaba (~610 Ma) plutons. It is not affected by regional metamorphism or ductile deformation, but recorded petrographic uralitization/amphibolitization signature. It is composed chiefly of pyroxene-hornblende gabbro, diorite, and quartz diorite. The chemical composition of the mafic minerals indicated that the suite was derived from calc-alkaline magma. Geochemically, the studied rocks are characterized by enrichment of LILE relative to HFSE and LREE relative to HREE. The gabbros are notably low in total REE (38–56 ppm) with (La/Yb)n =2.7–4.8, while the dioritic rocks are high in ΣREE (142–161 ppm) with high (La/Yb)n values (12.5–15.8); both are characterized by the absence of Eu anomaly, their Eu/Eu* ratios are close to unity (0.93–1.11). The studied intrusion evolved from mafic mantle magma into different types by assimilation fractional crystallization process (AFC) and/or gradual decrease in oxygen fugacity. The initial magma corresponds, most probably, to pyroxene-hornblende gabbro and the crystallization of hornblende was ascribed by slight H2O increase in magma after crystallization of near-liquidus clinopyroxene with high Ca content (Wo42–52) and Ca-rich plagioclase (An80–83). The intrusion is related to the later calcalkaline substage of post-collisional tectonic setting. M. D. Samuel : D. Sadek Ghabrial (*) : H. E. Moussa : M. W. Ali-Bik Geological Sciences Department, National Research Centre, 12662, Dokki Cairo, Egypt e-mail: [email protected]

Keywords Neoproterozoic . Sinai . Arabian-Nubian shield . Gabbro/diorite intrusion . Post-collisional setting

Introduction The basement rocks of Sinai are located in the northernmost outcrops of the Arabian-Nubian Shield (ANS) (Fig. 1). The Shield is a prominent example of juvenile crustal province of Neoproterozoic age (1000–540 Ma) (e.g. Bentor 1985; Stern 1994; Johnson and Woldehaimanot 2003). It is the northern part of the East African Orogen, which is viewed as a collage of island arc complexes accreted during the closure of the Mozambique Ocean between East and West Gondwana (Kröner 1985; Stern 1994; Meert 2003; Jarrar et al. 2003; Stoeser and Frost 2006). In Sinai, the ANS juvenile crust comprises the island arc complexes (IAC) of pre-collisional stage (~820–740 Ma) and formed of metamorphosed volcano-sedimentary rocks, paraand orthogenesis, associated with migmatites and amphibolites. This is followed by the collisional stage (~670–630 Ma) characterized by the presence of variably deformed calcalkaline granodiorites, diorites, and gabbros. The Dokhan Volcanics, the mollase sediments, together with mostly undeformed calc-alkaline granitoids, quartz diorites and minor gabbros (~630–590 Ma), and alkaline/peralkaline A-type granites and their equivalent volcanics (~610–580 Ma), are formed during late- to post-collisional stage of the ANS crust evolution (e.g. Stern and Hedge 1985; Stern 1994; Beyth et al. 1994; Abdel-Rahman 1995; Jarrar et al. 2003; Moussa et al. 2008; Be’eri-Shlevin et al. 2009a; Eyal et al. 2010; Farahat et al. 2011). Recently, however, Eyal et al. (2014) recorded shift of ages of similar stages in the northeastern and southern areas of Sinai. The Neoproterozoic gabbroic rocks constitute one of the distinctive rock units in the Precambrian basement of Egypt.

Arab J Geosci

Fig. 1 Simplified geological map of late Neoproterozoic rocks in south Sinai showing the location of the study area (modified after Eyal et al. 1980). Inset shows the location of Sinai in the northernmost ANS, along with Neoproterozoic exposures of eastern Africa and western Arabia

Previous studies on these rocks created controversy on their age, origin, and tectonic environment. This controversy exists as a result of the differences in their classification. Basta and Takla (1974), Takla et al. (1981, 2001), Takla (2002) classified these gabbros into older and younger gabbroic suites. The older suite (Older Gabbros) is ophiolitic metagabbros, while the younger suite (Younger Gabbros) is unmetamorphosed, alkali and calc-alkali mafic-ultramafic intrusions of posttectonic intraplate setting. Bentor (1985) related the gabbroic rocks of the Arabo-Nubian Massif to four phases: the oceanic,

island arc, calc-alkaline batholithic, and final alkaline phase. El-Gaby et al. (1988, 1990) and El-Gaby (2007) differentiated these gabbros into (1) ophiolitic metagabbros with tholeiitic affinity, (2) intrusive subduction-related calc-alkaline regionally metamorphosed gabbros (=metagabbro/diorite complexes) which intruded the island arc metavolcanics and metasediments, and (3) tholeiitic olivine gabbro and related rocks intruded at the late cordillera stage. All the abovementioned classifications negate the presence of ophiolitic mafic-ultramafic assemblages in Sinai. However, several

Arab J Geosci

Fig. 2 Geologic map of the gabbro/diorite intrusion at Sheikh El-Arab area, central Sinai (modified after Eyal et al. 2013)

authors interpreted some mafic-ultramafic complexes in south Sinai as fragments of an ophiolitic sequence. The criteria given for their interpretation cannot be substantiated. Table 1 Modal composition of gabbro/diorite intrusion of Sheikh El-Arab area

Soliman (1996) described Gebel Sheikh El-Arab itself as diorite-granodiorite complex, younger than the gabbro/diorite complex exposed to the north and separated from it by Wadi

Rock type

Sample

Qz

Pl

K-feld

Amp

Cpx

Bt

Chl

Opaques

Py-Hb gabbro

11 12 13 14 15 16 17A 17B 1 2

– – – – – – 5.05 3.64 7.44 8.72

58.42 56.58 57.51 54.34 59.59 50.49 68.73 70.81 65.83 63.90

– – – – – – 2.01 0.99 2.58 3.84

17.31 25.10 20.83 31.06 25.55 37.17 11.92 12.97 12.88 9.49

13.01 8.33 12.25 6.45 4.31 6.08 2.22 2.32 1.14 1.92

6.80 6.33 6.13 4.83 7.12 3.65 7.48 5.56 4.87 6.04

1.12 0.77 0.83 1.81 0.62 0.61 1.03 0.66 1.55 2.88

3.35 2.83 3.19 3.32 2.81 2.92 2.63 3.04 3.72 3.21

3 4A 4B 4C 4D 4E 6 7

9.48 8.62 9.32 8.92 8.99 8.61 7.70 10.19

64.96 66.64 67.87 70.02 68.65 69.34 68.36 69.94

1.42 1.63 1.80 1.42 1.88 1.91 2.25 1.62

13.75 11.09 8.51 7.19 10.03 8.32 9.44 7.04

0.95 1.86 1.35 1.83 1.25 3.11 2.89 1.62

4.98 6.05 6.33 5.60 5.64 4.54 5.20 6.02

0.95 0.70 0.90 1.22 0.42 0.48 1.28 1.11

3.51 3.40 3.89 3.82 3.14 3.68 2.89 2.45

Diorite Qz-diorite

0.8 Byt

0.8 Byt

0.4 Byt

0.5 And

Byt bytownite, And andesine, Olig oligoclase, Anth anorthoclase, Orth orthoclase

0.2 Byt

2.567 0.001 1.454 0.008 0.000 0.001 0.364 0.613 0.005 5.013 62.4 37.1

2.089 0.000 1.929 0.005 0.000 0.003 0.829 0.172 0.004 5.031 17.1 82.5

Or Name

7.66 7.14 0.08 0.00 100.95

17.06 1.95 0.07 0.00 101.42

57.94 0.03 27.88 0.21 0.00 0.01

CaO 16.96 16.93 21.07 Na2O 2.22 2.18 2.77 K2O 0.04 0.14 0.17 P2O5 0.04 0.00 0.06 Total 102.14 101.38 99.20 Number of cations on the basis of 8 oxygens Si 2.136 2.109 2.302 Ti 0.001 0.000 0.000 Al 1.865 1.895 1.452 Fe2+ 0.005 0.008 0.007 Mn 0.002 0.001 0.001 Mg 0.001 0.004 0.005 Ca 0.818 0.824 1.066 Na 0.194 0.192 0.254 K 0.002 0.008 0.010 Total 5.024 5.041 5.097 Ab 19.1 18.8 19.1 An 80.7 80.5 80.2

46.41 0.00 35.41 0.22 0.03 0.06

46.05 0.00 36.11 0.14 0.00 0.04

47.47 0.02 35.21 0.13 0.04 0.01

Diorite 48.76 0.00 26.11 0.17 0.02 0.07

SiO2 TiO2 Al2O3 FeO MnO MgO

Py-Hb gabbro

Plagioclases

Table 2 Representative microprobe analyses of feldspars of the studied intrusion

1.5 And

2.492 0.005 1.515 0.010 0.000 0.000 0.439 0.545 0.015 5.021 54.6 43.9

9.16 6.29 0.27 0.00 100.64

55.73 0.16 28.77 0.26 0.00 0.00

3.7 And

2.468 0.001 1.556 0.007 0.001 0.006 0.443 0.499 0.036 5.017 51.0 45.3

8.89 5.53 0.60 0.02 96.76

53.03 0.02 28.39 0.17 0.03 0.08

0.7 And

2.536 0.001 1.490 0.002 0.002 0.000 0.370 0.624 0.007 5.032 62.3 37.0

7.61 7.10 0.12 0.01 98.77

55.91 0.02 27.89 0.05 0.06 0.00

1.0 Olig

2.689 0.000 1.328 0.002 0.000 0.001 0.246 0.734 0.010 5.010 74.1 24.8

5.21 8.60 0.18 0.06 100.86

61.10 0.01 25.63 0.05 0.00 0.02

Qz-diorite

1.8 Olig

2.708 0.001 1.315 0.003 0.001 0.000 0.221 0.747 0.018 5.014 75.8 22.4

4.62 8.65 0.32 0.01 99.54

60.77 0.02 25.05 0.08 0.02 0.00

16.8 Anth

2.669 0.002 1.399 0.012 0.001 0.013 0.103 0.691 0.160 5.050 72.4 10.8

2.16 7.98 2.80 0.04 99.94

59.77 0.05 26.60 0.32 0.03 0.19

K-feldspars

92.7 Orth

2.928 0.003 1.100 0.001 0.001 0.001 0.002 0.068 0.884 4.988 7.1 0.2

0.03 0.75 14.84 0.06 98.53

62.71 0.08 20.01 0.03 0.02 0.01

94.1 Orth

2.943 0.000 1.084 0.002 0.000 0.001 0.000 0.057 0.907 4.994 5.9 0.0

0.00 0.64 15.36 0000 99.58

63.61 0.01 19.90 0.05 0.00 0.01

89.4 Orth

2.924 0.002 1.107 0.004 0.000 0.001 0.001 0.101 0.861 5.001 10.5 0.1

0.03 1.12 14.47 0.00 98.65

62.71 0.05 20.17 0.09 0.00 0.01

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50.16 0.54 4.73 9.47 0.28 14.37 19.77 0.60

51.43 0.19 2.76 6.69 0.21 14.24 24.00 0.28

51.99 0.26 2.20 6.85 0.12 15.10 23.26 0.31

Aug

Dio

Aug augite, Dio diopside

Name

Dio

K2O 0.24 0.02 0.26 P2O5 0.00 0.03 0.00 Total 100.16 99.85 100.35 Number of cations on the basis of 6 oxygens Si 1.851 1.903 1.908 Al 0.206 0.120 0.095 Ti 0.015 0.005 0.007 Fe3+ 0.115 0.084 0.108 Fe2+ 0.177 0.123 0.102 Mn 0.009 0.007 0.004 Mg 0.791 0.785 0.826 Ca 0.782 0.951 0.915 Na 0.043 0.020 0.022 K 0.011 0.001 0.012 Total 4.000 3.999 3.999 Wo 41.73 48.78 46.79 En 42.20 40.27 42.26 Fs 16.07 10.95 10.95

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O

Py-Hb gabbro

Dio

1.919 0.085 0.005 0.086 0.133 0.007 0.813 0.935 0.018 0.000 4.001 47.40 41.19 11.41

1.907 0.088 0.006 0.103 0.120 0.006 0.821 0.932 0.017 0.001 4.001 47.05 41.45 11.50 Dio

0.01 0.00 100.22

52.06 0.18 1.95 7.08 0.22 14.79 23.68 0.25

0.02 0.01 99.36

51.32 0.21 2.01 7.15 0.18 14.82 23.41 0.23

Dio

1.944 0.052 0.003 0.066 0.151 0.007 0.812 0.952 0.012 0.001 4.000 47.91 40.83 11.26

0.02 0.00 100.31

52.70 0.12 1.19 7.04 0.21 14.76 24.10 0.17

Dio

1.951 0.028 0.002 0.075 0.140 0.008 0.792 0.993 0.010 0.000 3.999 49.47 39.43 11.10

0.01 0.02 99.88

52.55 0.08 0.65 6.91 0.26 14.30 24.96 0.14

Table 3 Representative microprobe analyses of pyroxenes of the studied intrusion

Dio

1.923 0.026 0.017 0.103 0.114 0.008 0.753 1.047 0.009 0.000 4.000 51.69 37.18 11.13

0.01 0.00 99.68

51.47 0.62 0.58 6.96 0.25 13.52 26.15 0.12

Dio

1.824 0.207 0.013 0.168 0.078 0.005 0.788 0.860 0.045 0.006 3.994 45.12 41.70 13.18

0.12 0.16 98.97

48.98 0.48 4.70 7.90 0.16 14.31 21.54 0.62

Aug

1.846 0.209 0.013 0.121 0.143 0.004 0.796 0.818 0.043 0.007 4.000 43.48 42.27 14.25

0.14 0.03 99.51

49.82 0.48 4.78 8.53 0.12 14.40 23.61 0.60

Dio

1.913 0.107 0.010 0.065 0.158 0.007 0.783 0.940 0.017 0.000 4.000 48.12 40.10 11.79

0.00 0.11 100.63

51.92 0.35 2.47 7.26 0.21 14.26 23.81 0.24

Dio

1.934 0.069 0.005 0.067 0.169 0.009 0.778 0.956 0.013 0.000 4.000 48.35 39.31 12.34

0.01 0.00 99.27

51.70 0.18 1.58 7.53 0.27 13.95 23.87 0.18

Dio

1.928 0.080 0.005 0.074 0.169 0.008 0.777 0.941 0.019 0.000 4.001 47.80 39.48 12.72

0.01 0.08 99.28

51.54 0.17 0.82 7.75 0.25 13.93 23.47 0.26

Dio

1.912 0.120 0.007 0.076 0.177 0.006 0.796 0.871 0.030 0.005 4.000 45.22 41.32 13.47

0.11 0.04 99.09

51.18 0.26 2.74 8.11 0.19 14.29 21.76 0.41

Dio

1.922 0.068 0.003 0.094 0.136 0.009 0.783 0.970 0.013 0.001 3.999 48.69 39.30 12.01

0.02 0.10 99.68

51.62 0.11 1.56 7.38 0.30 14.10 24.31 0.18

Dio

1.919 0.082 0.014 0.078 0.187 0.010 0.776 0.907 0.026 0.001 4.000 46.35 39.63 14.02

0.02 0.03 98.96

51.08 0.51 1.84 8.43 0.30 13.85 22.54 0.36

Diorite

Dio

1.884 0.060 0.006 0.185 0.067 0.008 0.785 0.979 0.022 0.003 3.999 48.34 38.75 12.91

0.06 0.11 96.29

48.86 0.20 1.32 7.84 0.26 13.65 23.69 0.30

Aug

1.859 0.177 0.021 0.133 0.163 0.008 0.769 0.802 0.040 0.029 4.001 42.79 41.01 16.21

0.61 0.01 98.24

49.27 0.73 3.96 9.36 0.26 13.66 19.83 0.55

Arab J Geosci

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O

0.121 0.120 0.144 Tsch Mg-Hb Tsch Py-Hb gabbro Secondary amphiboles 50.56 51.07 51.16 0.43 0.21 0.20 5.14 4.59 4.25 12.79 12.37 12.03 0.27 0.26 0.28 14.55 15.31 15.63 12.65 12.56 12.47 0.57 0.54 0.43 0.26 0.18 0.12 50.86 0.29 5.26 12.80 0.25 14.98 12.74 0.59 0.19

0.107 Mg-Hb

46.98 0.51 8.28 12.36 0.22 13.45 12.59 0.92 0.39

0.126 Tsch

46.88 0.6 8.66 13.69 0.26 13.51 12.59 1.05 0.33

0.120 Mg-Hb

6.525 1.475 0.451 0.121 0.290 1.282 0.026 2.817 2.012 0.370

6.443 1.557 0.488 0.109 0.320 1.508 0.032 2.630 1.913 0.406

K Name

0.21 12.87 12.79 1.30 0.64 0.00 97.31

0.26 11.98 12.12 1.42 0.67 0.07 97.88

MnO 0.31 0.26 0.26 0.32 MgO 11.58 13.37 12.58 12.75 CaO 11.92 12.07 12.21 12.43 Na2O 1.47 1.35 1.30 1.24 K2O 0.64 0.64 0.76 0.57 P2O5 0.00 0.04 0.00 0.00 Total 96.99 97.63 96.91 97.31 Number of cations on the basis of 23 oxygens Si 6.380 6.589 6.485 6.554 AlIV 1.620 1.411 1.515 1.446 AlVI 0.521 0.404 0.423 0.367 Ti 0.115 0.106 0.105 0.104 3+ Fe 0.323 0.292 0.366 0.410 Fe2+ 1.534 1.353 1.361 1.321 Mn 0.039 0.032 0.033 0.040 Mg 2.568 2.919 2.777 2.797 Ca 1.900 1.894 1.937 1.960 Na 0.424 0.383 0.373 0.354

44.45 1.10 11.14 12.81

43.75 0.98 11.79 14.84

43.80 0.94 11.12 13.94

44.53 0.94 10.46 14.07

SiO2 TiO2 Al2O3 FeO

Py-Hb gabbro Primary amphiboles 42.89 44.99 1.03 0.96 12.22 10.52 14.93 13.43

47.57 0.53 8.12 13.40 0.33 13.18 12.78 0.89 0.24

0.118 Tsch

6.512 1.488 0.456 0.114 0.313 1.486 0.035 2.664 1.932 0.373

0.28 12.16 12.27 1.31 0.63 0.00 97.86

44.31 1.03 11.23 14.64

Table 4 Representative microprobe analyses of amphiboles of the studied intrusion

50.10 0.81 4.82 14.26 0.27 12.41 12.68 1.12 0.47

0.020 Mg-Hb

6.985 1.015 0.013 0.061 0.653 0.394 0.012 3.185 2.682 0.207

0.10 14.80 17.34 0.74 0.11 0.10 96.87

0.031 Mg-Hb Diorite Prim. amph. 47.12 4.14 2.54 12.81 0.52 13.68 16.92 0.44 0.12

7.096 0.904 0.224 0.004 0.423 1.127 0.035 3.277 1.909 0.218

0.29 15.28 12.39 0.78 0.17 0.04 97.87

Secondary amphiboles 48.39 49.33 0.56 0.04 6.05 6.66 8.68 12.89

49.27 0.47 3.74 12.65 0.30 14.52 12.88 0.39 0.21

7.696 0.278 0.000 0.006 0.261 0.942 0.031 3.898 1.888 0.047

0.26 18.40 12.40 0.17 0.05 0.00 97.28

54.16 0.06 1.66 10.12

Secondary amphiboles 54.18 53.02 52.43 0.11 0.11 0.30 1.34 1.54 2.27 10.95 10.70 11.95 0.41 0.30 0.30 16.82 16.67 15.88 12.97 12.93 12.58 0.25 0.25 0.33 0.07 0.07 0.15

7.617 0.383 0.101 0.006 0.163 1.114 0.035 3.676 1.905 0.089

0.29 17.28 12.46 0.32 0.10 0.00 97.45

53.37 0.06 2.88 10.69

0.009 Act

0.044 Mg-Hb

7.170 0.830 0.315 0.000 0.261 1.233 0.032 3.268 1.892 0.210

0.26 15.20 12.24 0.75 0.24 0.00 97.52

49.71 0.00 6.74 12.38

0.018 Act

0.037 Mg-Hb

7.257 0.743 0.261 0.009 0.240 1.172 0.028 3.357 1.953 0.188

0.23 15.60 12.50 0.67 0.20 0.03 97.20

50.28 0.08 5.91 11.70

48.05 0.71 5.21 14.17 0.39 13.77 12.55 0.64 0.41

0.015 Act

7.666 0.334 0.056 0.016 0.171 1.066 0.030 3.727 1.934 0.061

0.25 17.59 12.70 0.22 0.08 0.00 97.67

53.94 0.15 2.33 10.41

50.29 0.67 4.52 13.30 0.34 14.87 12.45 0.69 0.36

0.015 Act

7.626 0.374 0.023 0.017 0.225 0.970 0.025 3.801 1.939 0.077

0.21 17.89 12.70 0.28 0.08 0.15 97.37

53.51 0.16 2.37 10.02

53.09 0.10 0.96 10.83 0.34 16.69 12.46 0.20 0.09

0.007 Act

7.817 0.183 0.024 0.012 0.087 1.083 0.028 3.785 1.981 0.041

0.23 17.84 12.99 0.15 0.04 0.06 97.41

54.92 0.11 1.24 9.83

50.15 0.23 3.96 12.92 0.24 14.68 13.13 0.40 0.22

0.018 Mg-Hb

7.504 0.496 0.101 0.034 0.187 1.129 0.029 3.536 1.985 0.123

0.24 16.47 12.86 0.44 0.10 0.00 96.96

52.10 0.31 3.52 10.92

51.76 0.16 2.85 12.01 0.29 15.32 13.81 0.30 0.11

0.026 Act

7.561 0.439 0.118 0.029 0.140 1.219 0.029 3.523 1.941 0.097

0.24 16.46 12.62 0.35 0.14 0.07 97.42

52.66 0.27 3.29 11.32

Arab J Geosci

7.526 0.474 0.014 0.017 0.320 1.141 0.036 3.321 2.151 0.085 0.020 Act 7.379 0.621 0.065 0.025 0.350 1.240 0.030 3.220 2.000 0.114 0.041 Mg-Hb 7.817 0.166 0.000 0.011 0.103 1.231 0.042 3.664 1.966 0.057 0.017 Act 7.321 0.679 0.096 0.073 0.174 1.445 0.042 3.227 1.942 0.195 0.067 Mg-Hb 7.145 0.855 0.057 0.097 0.376 1.386 0.050 3.052 1.999 0.185 0.078 Mg-Hb 7.368 0.632 0.027 0.053 0.347 1.235 0.038 3.237 2.064 0.113 0.040 Mg-Hb 7.656 0.344 0.046 0.033 0.111 1.348 0.037 3.457 1.968 0.093 0.028 Act 7.735 0.265 0.000 0.012 0.159 1.147 0.037 3.625 2.021 0.071 0.013 Act 7.791 0.209 0.018 0.012 0.085 1.231 0.050 3.606 1.998 0.070 0.013 Act 6.869 0.436 0.000 0.454 0.772 0.790 0.064 2.973 2.643 0.124 0.022 Mg-Hb 7.074 0.890 0.000 0.096 0.335 1.536 0.036 2.902 2.131 0.341 0.094 Mg-Hb Tsch tschermakite, Mg-Hb magnesio-hornblende, Act actinolite

6.839 1.161 0.326 0.066 0.345 1.325 0.012 2.938 1.968 0.297 0.061 Mg-Hb 6.914 1.086 0.348 0.056 0.289 1.343 0.027 2.951 1.985 0.263 0.073 Mg-Hb

6.971 1.029 0.372 0.058 0.242 1.400 0.041 2.879 2.007 0.253 0.045 Mg-Hb

0.00 97.57 0.04 95.74

0.00 0.00 0.09 0.00 P2O5 Total 97.22 97.09 96.66 97.96 Number of cations on the basis of 23 oxygens Si 7.365 7.403 7.431 7.324 0.635 0.597 0.569 0.676 AlIV 0.245 0.186 0.158 0.216 AlVI Ti 0.047 0.023 0.022 0.031 0.105 0.180 0.225 0.198 Fe3+ 1.451 1.319 1.237 1.343 Fe2+ Mn 0.033 0.012 0.034 0.030 Mg 3.156 3.308 3.384 3.216 Ca 1.972 1.951 1.941 1.966 Na 0.161 0.152 0.121 0.165 K 0.048 0.033 0.022 0.035 Name Mg-Hb Mg-Hb Mg-Hb Mg-Hb

Table 4 (continued)

0.00 97.04

0.06 97.00

0.04 98.33

0.00 97.10

0.04 95.63

0.00 96.19

0.03 94.46

0.07 95.96

0.05 97.54

0.00 94.76

0.04 95.97

0.00 96.61

Arab J Geosci

Nasb. At the intersection between Wadi Nasb and Wadi Rahaba, the gabbro/diorite rocks intruded mafic-ultramafic rocks; the latter, interpreted to represent fault-bounded fragments of oceanic crust or fault slivers of rocks originally, were intrusive-extrusive complexes. Soliman (op. cit.) regarded all the above-mentioned rock types older than the Rutig volcanics and volcanoclastics. In addition, questionable geochemical data were given for only three samples of mafic-ultramafic rocks. They have 50.10–56.65 wt.% SiO2, 3.56–4.25 wt.% Na2O, 1.45–4.92 wt.% K2O, 542–880 ppm Ba, 1.5–5.9 ppm Co, and Y is not detected. Abdel-Karim (2013) described only a small gabbroic mass to the east of Wadi Rahaba formed of uralitized gabbro and hornblendite. It intruded the Older granitoids (calc-alkaline tonalite-granodiorite association) with sharp contacts, and in turn, intruded by Younger granites (calc-alkaline to alkaline granodiorite to alkali-feldspar granites). The uralitized gabbros are composed of cumulus augite (45 %) and hypersthene and plagioclase (40 %) with subordinate hornblende and quartz. The hornblendites are composed of cumulus hornblende (62 %) and plagioclase (30 %) with minor pyroxene, biotite, and quartz. Their parental magma is tholeiitic in nature and was mostly generated and emplaced in continental crust and tends to be formed by a transitional compressionextension regime dominated from the final arc stage to active continental margin. The above-mentioned petrographic nomenclature contradicts the IUGS classification. The petrogenetic aspects of the Rahaba intrusion as given by Abdel-Karim (2013) are difficult to accept as the transitional compressionextension regime in Sinai extending from ~740 to ~610 Ma. The present study deals with the geological, petrographical, mineralogical, and geochemical characteristics of the gabbro/ diorite intrusion at Gebel Sheikh El-Arab area (central Sinai) in order to explain its tectonic setting and petrogenesis. In addition, the new data presented here will determine if these gabbro/diorite rocks are related as previously thought to the older metagabbro/diorite complexes or to the younger gabbros. Also, the data, together with the previously published ones concerning gabbroic rocks in south Sinai, may shed light on its evolution.

Geologic setting The oldest basement rock units in Katharina province include remnants of older metavolcanics and metasediments of unknown age. A younger tectonomagmatic cal-alkaline phase in the province is characterized by vast intrusion of weakly to non-deformed calc-alkaline plutons and volcano-sedimentary succession (Rutig Formation). The calc-alkaline magmatic phase is separated from the overlying alkaline magmatic phase by Hamad Abadu unconformity and Katharina Group. The latter constitutes a stratified sequence of pyroclastics,

Arab J Geosci Table 5 Representative microprobe analyses of biotites of the studied intrusion Py-Hb gabbro

Diorite

35.84 36.33 36.86 36.57 SiO2 TiO2 3.71 3.94 2.96 2.68 Al2O3 14.09 13.70 15.53 15.32 FeO 19.52 18.99 18.30 18.25 MnO 0.20 0.22 0.14 0.24 MgO 12.96 12.44 11.49 12.14 CaO 0.04 0.13 0.06 0.11 Na2O 0.12 0.16 0.13 0.11 K2O 8.80 8.45 8.88 8.87 P2O5 0.01 0.00 0.13 0.01 Total 95.29 94.36 94.48 94.30 Number of cations on the basis of 22 oxygens Si 5.495 5.595 5.630 5.608 IV Al 2.505 2.405 2.370 2.392 AlVI 0.039 0.080 0.423 0.375 Ti 0.428 0.456 0.340 0.309 Fe2+ 2.503 2.446 2.337 2.341 Mn Mg Ca Na K Total Fe/Fe + Mg

0.026 2.962 0.007 0.036 1.721 15.722 0.46

0.029 2.856 0.021 0.048 1.660 15.596 0.46

0.018 2.616 0.010 0.038 1.730 15.512 0.47

0.031 2.776 0.018 0.033 1.735 15.618 0.46

36.49 3.03 15.12 18.41 0.19 11.57 0.05 0.19 8.91 0.03 93.99

36.76 2.68 15.32 18.45 0.14 12.25 0.07 0.11 8.70 0.02 94.50

36.27 2.67 15.31 17.91 0.21 12.21 0.03 0.16 8.91 0.01 93.69

37.13 2.10 15.72 17.94 0.17 12.24 0.01 0.12 9.13 0.00 94.56

36.70 1.96 15.77 17.83 0.18 12.29 0.04 0.16 9.01 0.04 93.98

36.37 2.24 15.82 17.78 0.20 11.83 0.00 0.13 9.31 0.00 93.68

36.32 2.73 15.05 18.05 0.18 12.36 0.03 0.20 8.84 0.00 93.75

35.51 1.20 16.19 17.71 0.20 12.59 0.04 0.09 8.58 0.05 92.17

36.51 2.13 15.09 17.70 0.20 12.51 0.04 0.14 8.45 0.02 92.79

35.26 1.10 18.26 17.13 0.16 12.77 0.09 0.08 8.87 0.05 93.77

35.24 1.04 18.75 17.04 0.17 12.84 0.04 0.12 8.98 0.00 94.22

5.621 2.379 0.364 0.351 2.372

5.618 2.382 0.375 0.308 2.358

5.595 2.405 0.377 0.310 2.311

5.662 2.338 0.485 0.241 2.288

5.631 2.369 0.481 0.226 2.288

5.613 2.387 0.488 0.260 2.295

5.602 2.398 0.336 0.317 2.328

5.554 2.446 0.536 0.141 2.317

5.662 2.338 0.416 0.248 2.295

5.400 2.600 0.693 0.127 2.194

5.370 2.630 0.735 0.119 2.172

0.025 2.657 0.008 0.057 1.751 15.585 0.47

0.018 2.791 0.011 0.033 1.696 15.59 0.46

0.027 2.808 0.005 0.048 1.754 15.64 0.45

0.022 2.782 0.002 0.035 1.776 15.631 0.45

0.023 2.811 0.007 0.048 1.764 15.648 0.45

0.026 2.722 0.000 0.039 1.833 15.663 0.46

0.024 2.842 0.005 0.060 1.740 15.652 0.45

0.026 2.936 0.007 0.027 1.712 15.702 0.44

0.028 2.892 0.007 0.042 1.672 15.60 0.44

0.021 2.916 0.015 0.024 1.733 15.723 0.43

0.022 2.917 0.007 0.035 1.746 15.753 0.43

ignimbrites together with typical rhyolitic flows of alkaline/ peralkaline affinity. The younger alkaline magmatic phase comprises riebeckite granite, monzonite, syenite, syenogranite, and perthite granite. The Sheikh El-Arab area lies at about 15 km to the southeast of Saint Katharina town. The area is characterized by moderate to high relief terrain and dissected by numerous structurally controlled wadis; namely Wadi Nasb, Wadi Wa’ara, Wadi Zera, and Wadi Rahaba. Gebel Sheikh ElArab itself is formed of granodiorite pluton surrounded by gabbro/diorite rocks exposed in an area of about 11.7 km2 and delineated by latitudes 28° 26′ 14″ to 28° 27′ 33″ N and longitudes 33° 59′ 00″ to 34° 02′ 27″ E (Fig. 2). The contact between the studied gabbro/diorite suite and its country rocks are sharp. This suite is generally undeformed and shows no signs of metamorphism. Volcanic and clastic xenoliths of Rutig Formation are included in the gabbro/diorite intrusion. The investigated gabbro/diorite intrusion crops out as five small masses separated by old alluvial fans and alluvium deposits. Amphibole granodiorite of Sheikh El-Arab pluton and biotite granodiorite of Rahaba pluton (610 Ma; Moreno et al. 2012) intruded the southern parts of the gabbro/diorite masses. The northern parts were intruded by both the Nakhila microdiorite as well as quartz monzonite and quartz syenite of

Katharina Outer Ring dyke. The westernmost gabbro/diorite mass intrudes the volcanics and clastics of the lower unit of the Rutig Formation (~630–615 Ma; Be’eri-Shlevin et al. 2011). Field study showed that the intrusion is heterogeneous comprising gabbros and dioritic rocks; the latter are exposed in small area relative to the gabbros. The contact between the gabbro and dioritic rocks is hardly discerned in the outcrop, only slight color variation can be recognized. The investigated gabbro/diorite suite is a Neoproterozoic intrusion that was emplaced prior to 610±5 Ma (Moreno et al. 2012), most probably related to the post-collisional calc-alkaline suite, even without direct geochronological data yet. The available U-Pb zircon data (Be’eri-Shlevin et al. 2011; Samuel et al. 2011) show that in Katharina province the emplacement of the lower unit of the Rutig Formation, truncated by the studied gabbro/diorite rocks, lasted from 630 to 615 Ma.

Petrography Following the IUGS recommendation (1989), the studied intrusion is lithologically classified into gabbro and diorite. The average An content of the plagioclase in the former is

Arab J Geosci Table 6 Representative microprobe analyses of Fe-Ti oxides of the studied intrusion Py-Hb gabbro

Qz-diorite

Magnetite

Ilmenite

SiO2 0.02 0.04 0.04 0.04 0.02 0.11 0.04 0.18 TiO2 0.26 0.06 0.03 0.12 0.16 0.10 0.08 48.48 Al2O3 0.05 0.10 0.11 0.13 0.14 0.11 0.12 0.02 FeO 90.17 92.55 92.12 91.57 91.83 91.59 91.89 43.91 MnO 0.07 0.00 0.02 0.05 0.00 0.06 0.02 3.68 MgO 0.00 0.00 0.05 0.01 0.00 0.05 0.00 0.04 CaO 0.01 0.00 0.00 0.00 0.01 0.02 0.01 0.56 Na2O 0.01 0.02 0.02 0.02 0.00 0.03 0.00 0.01 K2O 0.02 0.00 0.02 0.00 0.00 0.00 0.02 0.03 P2O5 0.00 0.00 0.05 0.00 0.01 0.00 0.00 0.00 Total 90.61 92.77 92.46 91.94 92.17 92.07 92.18 96.91 Number of cations on the basis of 4 oxygens for magnetite and of 3 oxygens for ilmenite Si 0.001 0.002 0.002 0.002 0.001 0.004 0.002 0.005 Al 0.002 0.005 0.005 0.006 0.006 0.005 0.006 0.001 Ti Fe3+ Fe2+ Mn Mg Ca Na K Total XUsp XIlm

0.008 1.980 1.006 0.002 0.000 0.000 0.001 0.001 3.001 2.379

0.002 1.989 1.003 0.000 0.000 0.000 0.002 0.000 3.003 2.322

0.001 1.988 0.999 0.001 0.003 0.000 0.002 0.001 3.002 2.329

0.004 1.984 1.003 0.002 0.001 0.000 0.002 0.000 3.004 2.343

0.005 1.982 1.005 0.000 0.000 0.000 0.000 0.000 2.999 2.337

0.003 1.980 1.001 0.002 0.003 0.001 0.002 0.000 3.001 2.339

0.002 1.987 1.003 0.001 0.000 0.000 0.000 0.001 3.002 2.337

Ilmenite 0.02 48.73 0.01 46.10 3.02 0.18 0.00 0.09 0.00 0.00 98.15

0.00 46.43 0.00 48.52 2.60 0.25 0.02 0.01 0.01 0.00 97.84

0.05 48.23 0.03 45.80 2.85 0.25 0.01 0.05 0.00 0.00 97.27

0.04 48.05 0.03 46.12 3.03 0.13 0.03 0.03 0.00 0.02 97.48

0.02 46.44 0.02 47.74 2.75 0.26 0.03 0.03 0.01 0.00 97.30

0.001 0.000

0.000 0.000

0.001 0.001

0.001 0.001

0.001 0.001

0.945 0.100 0.852 0.081 0.002 0.016 0.001 0.001 2.004

0.938 0.120 0.867 0.066 0.007 0.000 0.005 0.000 2.004

0.924 0.152 0.855 0.058 0.010 0.001 0.001 0.000 2.001

0.936 0.123 0.865 0.062 0.010 0.000 0.003 0.000 2.001

0.932 0.133 0.861 0.066 0.005 0.001 0.002 0.000 2.002

0.900 0.199 0.830 0.060 0.010 0.001 0.002 0.000 2.004

1.557

1.538

1.589

1.551

1.548

1.547

greater than 50 % and is represented by pyroxene-hornblende gabbro. The dioritic rocks are differentiated into diorite and quartz diorite (quartz>5 %). The most remarkable feature in all the rock types is the development of secondary amphibole

(e.g., actinolite) either as uralite after pyroxene (uralitization) or at the expense of primary amphibole (amphibolitization). Pyroxene-hornblende gabbro is fine- to medium-grained and possesses hypidiomorphic granular texture. The mineral

Fig. 3 SiO2-Al2O3 discrimination diagram of clinopyroxenes in the studied intrusion (Le Bas 1962)

Fig. 4 FeOt vs. Al2O3 biotite discriminant diagram for the analyzed biotites in rocks of the studied intrusion (Abdel-Rahman 1994)

Nb Rb Sr Zr Y V La Ce Pr Nd Sm Eu Gd Tb Dy Ho

2 38 620 37 7 214 5.45 12.3 1.60 7.6 2.00 0.71 2.29 0.40 2.19 0.46

3 33 494 21 6 236

220 66 226 42

383 67 250 38

4 40 530 29 11 226

45.88 0.22 17.96 8.00 0.19 8.86 13.51 0.77 1.08 0.08 2.83 99.38 66.7

47.70 0.50 17.20 8.58 0.18 8.11 10.96 1.40 1.63 0.33 2.89 99.48 63.1

3 32 346 20 5 240 7.1 15.7 2.17 9.6 2.45 0.74 2.45 0.41 2.35 0.44

160 96 303 45

45.35 0.35 16.07 8.65 0.19 9.38 14.00 0.84 0.98 0.15 3.74 99.70 66.3

14

4 42 555 38 7 225

337 83 188 36

47.50 0.35 18.60 7.58 0.17 7.32 11.88 1.17 1.74 0.25 3.03 99.59 63.6

15

1 43 486 39 8 220 8.3 18.9 2.59 11.6 3.09 1.03 2.97 0.49 2.86 0.62

385 74 305 42

47.75 0.45 15.03 8.58 0.19 9.06 12.46 1.01 1.64 0.25 3.78 100.2 65.7

16

7 57 1050 118 15 142 26.9 61.3 7.33 31.8 5.5 1.71 4.03 0.57 2.8 0.47

1223 112 137 28

55.54 1.28 16.57 7.61 0.14 4.51 6.33 3.82 2.13 0.34 1.10 99.37 51.8

17A

13

11

12

Diorite

Py-Hb gabbro

SiO2 48.67 TiO2 0.52 Al2O3 16.22 Fe2O3 9.29 MnO 0.16 MgO 8.08 CaO 9.95 Na2O 1.91 K2O 1.59 P2O5 0.27 L.O.I. 3.10 Total 99.76 Mg# 61.2 Trace elements (ppm) Ba 364 Ni 82 Cr 273 Co 39

Rock type

Table 7 Chemical analyses of different rock types of the studied intrusion

6 56 1097 116 16 140

1205 116 157 31

55.50 1.18 16.12 7.42 0.17 4.99 7.28 3.86 2.03 0.50 1.08 100.13 54.9

17B

5 69 1162 180 18 152 26.8 59.7 7.25 29.5 5.5 1.49 4.39 0.66 3.49 0.57

1027 83 123 24

58.48 0.95 16.82 5.97 0.10 3.66 6.02 4.05 2.26 0.36 1.00 99.67 52.6

1

Qz-diorite

6 70 967 189 19 171

992 93 152 25

59.00 1.00 15.61 6.55 0.12 4.13 5.76 3.62 2.54 0.38 0.85 99.56 53.3

2

6 69 1043 183 18 169

1016 89 129 25

58.73 1.00 15.78 6.38 0.12 3.95 5.74 3.84 2.40 0.42 0.78 99.14 52.9

3

5 70 1042 184 17 164

945 72 113 25

58.80 1.00 15.70 6.34 0.12 3.95 5.74 3.95 2.46 0.38 1.05 99.49 53.0

4A

7 68 1057 182 16 159 31.0 69.4 7.99 33.9 5.8 1.55 4.27 0.61 3.17 0.53

1013 87 117 24

57.62 1.12 15.56 7.05 0.10 4.08 5.72 3.61 2.71 0.34 1.60 99.51 51.2

4B

7 71 976 194 19 152

1008 105 113 23

59.70 0.90 16.66 6.15 0.10 3.65 5.45 3.84 2.75 0.36 0.94 100.5 51.8

4C

6 69 1026 188 18 156

905 104 121 24

58.88 0.93 16.06 6.23 0.10 3.98 5.74 3.74 2.60 0.40 0.76 99.42 53.6

4D

5 72 1011 190 19 158

1020 78 116 23

59.30 0.98 16.25 6.26 0.10 3.71 5.60 3.89 2.80 0.42 0.77 100.08 51.8

4E

6 70 1009 186 17 166

954 86 125 25

58.50 0.96 15.60 6.58 0.12 4.10 6.55 3.68 2.54 0.36 0.96 99.95 53.0

6

7 72 1035 196 18 141

878 75 122 21

58.80 0.88 16.62 5.39 0.10 3.50 5.57 3.65 2.52 0.30 1.18 99.51 54.0

7

Arab J Geosci

1.50 0.23 1.51 0.20 161.66 0.95 14.73 3.45 12.77 1.62 0.24 1.54 0.23 142.98 0.93 12.48 3.15 10.77

1

1.46 0.21 1.22 0.18 145.48 1.11 15.82 3.16 13.96 1.78 0.26 1.57 0.19 56.25 1.04 3.79 1.73 3.34

17A

1.27 0.18 1.07 0.12 46.05 0.92 4.76 1.87 4.08 Er Tm Yb Lu ∑REE (Eu/Eu*)n (La/Yb)n (La/Sm)n (Ce/Yb)n

1.42 0.21 1.46 0.19 38.28 1.01 2.68 1.76 2.34

12 11

Py-Hb gabbro Rock type

Table 7 (continued)

13

14

15

16

Diorite

17B

Qz-diorite

2

3

4A

4B

4C

4D

4E

6

7

Arab J Geosci

constituents (Table 1) are plagioclase (50–59 vol%), amphibole (17–37 vol%), pyroxene (4–13 vol%), biotite (3–7 vol%) and opaques (~3 vol%). Others are chlorite and epidote. Plagioclase occurs as large tabular crystals that may reach 4 mm in length, usually with albite and albite-Carlsbad twinning. Most of these plagioclase crystals exhibit alteration to epidote and sericite. Amphibole is commonly of green color exhibiting pleochroism variable in green; i.e., yellow green, olive green to dark green and is mainly actinolite-tremolite, replacing primary clinopyroxene. Clinopyroxene occurs as subhedral crystals uncommonly display simple twinning. It is mainly decomposed to secondary amphibole around its margin. The opaque Fe-Ti oxides are represented by homogeneous magnetite as discrete euhedral crystals; rare fine grains of ilmenite are recorded. Diorite and quartz diorite more or less have the same texture, habit and characteristics of the essential minerals. However, quartz content is higher in quartz diorite (7– 10 vol%) than in diorite (3–5 vol%). Both contain few (1– 4 vol%) subhedral to anhedral K-feldspar occur between plagioclase. Diorite and quartz diorite are holocrystalline, mainly equigranular hypidiomorphic of medium-grained size. The modal compositions of both rock types are plagioclase (64–71 vol%), amphibole (7–13 vol%), biotite (4–7 vol%), clinopyroxene (1–3 vol%), and opaques (2.5–4 vol%). Secondary phases are chlorite and epidote. The plagioclase occurs as subhedral tabular crystals displaying albite and albite-Carlsbad twinning and is partly altered to sericite. Hornblende is anhedral with bluish green to light yellow pleochroism and partly replaced by actinolite and Fe-Ti oxides. Quartz occurs as interstitial grains with sutured outlines and shows undulatory extinction. Biotite co-exists with amphibole and forms discrete anhedral to subhedral crystals and crystal aggregates. Pyroxene is a minor consistent and exhibits marginal alteration to secondary amphibole (uralitization). The opaque minerals are represented by homogeneous subhedral ilmenite grains.

Mineral chemistry Chemical composition of essential minerals was determined by electron probe microanalysis (EPMA) using CAMERA SX 100 instrument under operating conditions of 15 kV and 20 nA. Suitable synthetic and natural standards were applied for calibration. The analyses were carried out at the Institute of Mineralogy, Claustal University, Germany. The raw data were processed through Minpet Software after Richard (1995) for the calculation of the given structural formulae. Representative microprobe analyses of essential mineral phases from the investigated rocks are given in Tables 2, 3, 4, 5, and 6.

Arab J Geosci

Feldspars Feldspars were analyzed from pyroxene-hornblende gabbro and dioritic rocks (diorite and quartz diorite). The analyzed feldspars in the pyroxene-hornblende gabbro are represented mainly by bytownite; while in the diorite are andesine. In quartz diorite, both plagioclase and alkali-feldspar are present. Plagioclase is mainly oligoclase and the alkali-feldspar is almost pure orthoclase with rare anorthoclase.

magnetite and ilmenite. Primary magnetite is characteristically abundant in gabbros, whereas it is nearly absent in dioritic rocks and only occurs as fine grains at expense of decomposed ferromagnesium silicates. Magnetite analyzed from pyroxenehornblende gabbro has low TiO2 contents (0.03–0.26 %). The ulvöspinel contents in the magnetite, calculated according to Stromer (1983), are below 2.4 mol%. End-member components (Xilm and Xhem) of ilmenite are calculated according to Stromer (1983). The analyzed ilmenite is nearly pure ilmenite (0.90–0.94 % mole ilmenite) and enriched in MnO content (2.60–3.68 wt.%) relative to the magnetite.

Pyroxenes All pyroxenes of the pyroxene-hornblende gabbro and diorite are clinopyroxenes with very narrow compositional range from diopside to augite according to the classification of Morimoto (1988). The majority of the analyzed clinopyroxenes plot in the subalkaline field on the SiO2 vs. Al2O3 diagram of Le Bas (1962) (Fig. 3). Amphiboles The amphiboles of the pyroxene-hornblende gabbro and the dioritic rocks are primary and secondary amphiboles. Primary amphiboles are high in TiO2 and all contain >0.1 Ti p.f.u. (Girardeau and Mevel 1982), whereas secondary amphiboles have low TiO2, contain