Geochemistry and magmatic setting of Wadi El-Markh island-arc gabbro–diorite suite, central Eastern Desert, Egypt

Chemie der Erde 70 (2010) 257–266

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Geochemistry and magmatic setting of Wadi El-Markh island-arc gabbro–diorite suite, central Eastern Desert, Egypt Sherif Kharbish n Geology Department, Faculty of Science, Suez Canal University, Suez Branch, Suez governate, El Salam City 43518, Egypt

a r t i c l e in fo

abstract

Article history: Received 26 July 2009 Accepted 11 December 2009

Wadi El-Markh gabbro–diorite complex is composed of pyroxene hornblende gabbros, hornblende gabbros, diorites and quartz diorites. According to their bulk rock geochemistry and mineral chemistry, the gabbroic and dioritic rocks represent fractionates along a single line of descent and crystallized from a calc-alkaline mafic magma. When compared to the primitive mantle, all members of the gabbroic– dioritic rock suite are enriched in the large ion lithophile elements relative to the high field strength elements and display distinctive negative Nb and P2O5 anomalies. This signals an arc setting. Fractionation modeling involving the major elements reveals that the hornblende gabbros were generated from the parent pyroxene hornblende gabbros by 61.86% fractional crystallization. The diorites were produced from the hornblende gabbros by fractional crystallization with a 58.97% residual liquid, whereas the quartz diorites were formed from the diorites by 26.58% fractional crystallization. According to geothermobarometry based on amphibole mineral chemistry, the most primitive pyroxene hornblende gabbros crystallized at  830 1C/  5 kbar. The crystallization conditions of the quartz diorites were estimated at  570 1C/  2 kbar. In consequence the Wadi El-Markh gabbro–diorite complex represents a single magmatic suite of which fractionates crystallized in progressively shallower levels of an arc crust. & 2009 Elsevier GmbH. All rights reserved.

Keywords: Wadi El Markh Central Eastern Desert Gabbro–diorite complex Mineral chemistry Pressure–temperature Calc-alkaline Arc magmatism Geochemical modeling

1. Introduction Wadi El Markh gabbro–diorite (WMGD) complex, central Eastern Desert (ED), Egypt, is a part of the Arabian–Nubian Shield (ANS) that covers huge areas of NE Africa and the Arabian Peninsula. In Egypt, ANS occurs in the ED, Southern Sinai and limited areas of the Western Desert, where igneous and metamorphic rocks of Precambrian age cover about 100,000 km2 (Ali et al., 2009). ANS was evolved and cratonized during the Late Precambrian ‘Pan-African’ ¨ event (950–450 Ma; Kroner, 1984). It was initiated as an intraoceanic island arc that underwent obduction–accretion processes ¨ through the Neoproterozoic (Ries et al., 1983; Gass, 1981; Kroner, 1991). The Neoproterozoic ANS was characterized by high strain zones, which accommodated intensive ductile shearing. Some of these zones are reworked accretionary complexes and sutures, whereas others cut across the accreted microplates that compose the shield (Almond et al., 1989). They are mainly fold and thrust belts, characterized by the presence of allochtonous ophiolitic slices and sheets (Greiling et al., 1988). Stratigraphically, the Neoproterozoic ANS comprises four units; volcanosedimentary successions, dismembered ophiolite complexes, gabbro–diorite-tonalite complexes and

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unmetamorphosed volcanic and pyroclastic sequences that are intruded by granodiorite–granite complexes (El-Ramly, 1972). In the last decades, several important papers have been published on the geology and tectonics of Egypt (e.g. El-Nisr and El-Sayed, 2002; El-Sayed et al., 2002; Zoheir and Klemm, 2007; Ali et al., 2009). The Egyptian basement complex is characterized by abundant gabbroid intrusions of different ages and tectonomagmatic evolution. Generally, the Egyptian mafic plutonites have been classified as younger- (YG) and older-gabbros (OG) (Basta and Takla, 1974; Takla et al., 1981). The YG are mostly fresh, undeformed and unmetamorphosed (El-Sharkawy and El-Bayoumi, 1979; Takla et al., 1981; El-Gaby et al., 1990; Basta, 1998) and in many cases they are layered (Basta, 1998). The OG were described under different names; epidiorite– diorite complex (El-Ramly and Akaad, 1960), metagabbros–diorite complex (El-Ramly, 1972) and metagabbros (Akaad and Noweir, 1980). They are known either to form an integral part of obducted ophiolitic sequences (Nasseef et al., 1980; Takla et al., 1981, 1982; El-Sayed et al., 1999) or to constitute members of subductionrelated, calc-alkaline gabbro–diorite complexes (El-Gaby et al., 1988, 1990). Apart from whole-rock geochemistry, the discrimination between the ophiolitic (former oceanic crust) and island arc OG is difficult because both are regionally deformed, sheared, metamorphosed up to greenschist or lower amphibolite facies and are petrographically identical (El-Gaby et al., 1990; Basta,

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1998). However, the ophiolitic and island-arc OG originated in different tectonic regimes and should therefore have different geochemical characteristics. Only regional studies have been carried out on the area under investigation (e.g. El-Kassas, 1974; Bakhit, 1978; Greenberg, 1981; El-Kassas and Bakhit, 1992; Amer et al., 2005). In addition, detailed studies on the WMGD complex have not been published at all. Therefore, the present work aims to characterize the petrography, mineral chemistry and whole-rock major and trace element chemistry. Geochemical criteria are used to throw some light on the magmatic evolution of the studied rocks and to deduce some constraints on origin of the parent magma. A petrogenetic model, based upon combined modeling of major elements, is formulated to account for the observed geochemical variations.

2. Field observation and petrography The WMGD complex (Fig. 1) is exposed around Wadi El Markh at the central part of the exposed Precambrian basement in ED. The WMGD intrusions form elongated masses with moderate to high relief. The WMGD complex is intruded by tonalite– granodiorite complex (i.e. Egyptian Older Granites) and NNE– SSW doleritic dykes. Gabbros are dark green, medium- to coarse-grained. They can be classified into pyroxene hornblende (Px-Hb) and hornblende (Hb) gabbros. The Px-Hb gabbros are composed mainly of plagioclase (65–75 vol%), amphibole (15–20 vol%) and clinopyroxene (up to 10 vol%). Actinolite, chlorite, epidote and sericite are secondary components. Fe–Ti oxides and apatite are common accessories. Plagioclase forms discrete euhedral phenocrysts with Carlsbad twinning and represents the cumulus phase. Occasionally, it is completely masked with sericite and kaolinite. Sometimes plagioclase is partly or completely enclosed in intercumulus amphiboles giving rise to pseudo-subophitic and -ophitic textures, respectively. Primary amphiboles occur as subhedral oikocrysts and are represented by brown-green intercumulus hornblende with cumulus plagioclase and abundant inclusions of apatite, epidote and Fe–Ti oxides. Clinopyroxene transforms to actinolite and/or chlorite. The texture and essential mineral constituents of the Hb gabbros are the same as those of the PxHb gabbros. However, the clinopyroxene content is obviously lower (  5 vol%) and amphibole is higher (20–25 vol%).

Diorites are medium grained, grayish to dark green in color. They enclose gabbroic xenoliths of different shape and size. Under the petrographic microscope, diorites include two varieties (diorites and quartz-diorites), which merge imperceptibly into each other and have textural and mineralogical similarities, suggesting that they are genetically related. Diorites are composed essentially of plagioclase (70–75 vol%), amphibole (15–20 vol%), biotite (3–5 vol%) and minor quartz. Epidote and chlorite are secondary minerals. Accessory minerals include apatite and titanite with some Fe–Ti oxides. Plagioclase is commonly zoned and variably altered, resulting in sericitization and epidotization. Amphiboles are present as subhedral crystals that are commonly twinned and partly altered to chlorite. They are poikilitic, with inclusions of plagioclase, apatite and Fe–Ti oxides. Biotite, containing apatite and zircon inclusions, occurs as euhedral to subhedral flakes as well as interstitial aggregates of small tabular grains and occasionally replaces chlorite. Large, anhedral green chlorite, frequently interleaved with sericite and Fe–Ti oxides, may replace amphibole and biotite. Quartz-diorites are composed mainly of plagioclase (58–62 vol%), amphibole (18–22 vol%) and quartz (up to 10 vol%), with subordinate amounts of biotite (1–4 vol%) and K-feldspar. Chlorite and epidote are secondary phases after biotite and feldspars, respectively. Apatite, zircon, titanite and Fe–Ti oxides are common accessories. Sometimes plagioclase shows myrmekitic intergrowth at contact with the K-feldspar.

3. Analytical techniques Sixteen rock samples were analyzed for major and trace elements by X-ray fluorescence (XRF) technique using a Philips PW 2400 series spectrometer on fused and pressed powder discs, respectively, at Earth Science Institute, Vienna University, Austria. Loss on ignition (LOI) was determined after igniting 5 g of rock sample powder in porcelain crucibles at 1050 1C for 1 h. Analytical precision is better than 1% and 2–5% for major and trace elements, respectively. Mineral analyses were performed at the Institute of Mineralogy and Crystallography, University of Vienna, Austria, using a Jeol JSM-6400 Scanning Electron Microscope with an attached energy dispersive X-ray (EDX) microanalysis unit. Cobalt was used for internal gain calibration. An acceleration voltage of 20 keV was applied, the channel width was set to 20 eV, matrix absorption and fluorescence effects were corrected by the ZAF-4 algorithm (Link analytical). Silicate standards were adularia for Si, K and Al, garnet for Fe, Mg and Mn, titanite for Ca and Ti, chromite for Cr and jadeite for Na. The analytical precision was accurate to 72%. For determining the chemical compositions of the investigated minerals, a total of 350 spots [amphibole, 55 spots; pyroxene, 38 spots; plagioclase, 143 spots; biotite, 20 spots; chlorite, 17 spots; Fe-oxides, 67 spots, titanite, 10 spots] in sixteen carbon-coated polished thin sections were analyzed.

4. Mineral chemistry 4.1. Amphibole

Fig. 1. Geologic map of the Wadi El Markh gabbro–diorite complex, Eastern Desert, Egypt.

Amphibole is the main mafic mineral in the investigated rocks. The studied amphiboles have a wide range of composition with respect to their silica contents (43.38–50.77 wt%). They also show significant variations with respect to Al2O3 (6.78–10.49 wt%), FeO (0.52–17.01 wt%), MgO (10.46–21.03 wt%) and Fe2O3 (2.07– 16.16 wt%). The other major elements exhibit a restricted range (TiO2, 0.23–1.70 wt%; CaO, 9.76–12.60 wt%; Na2O, 0–3.57 wt%;

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K2O, 0–2.55 wt%; MnO, 0–0.57 wt%). Fig. 2a displays the relationship between the AlIV and AlT in the investigated amphiboles, revealing a clearly positive correlation, which reflects a systematic difference between the Al contents of the amphiboles in different rocks. On the Si and AlVI diagram (Fig. 2b) of Raase (1974), the analyzed amphiboles as plotted in the lowpressure field (less than 5 kbar). Structural formulae of the investigated amphiboles were calculated on an anhydrous basis assuming 23 O atoms per half unit cell, with the general form A0  1B2C5T8O22(OH)2 representing one formula unit. According to the IMA classification and Fe3 + calculation proposed by Leake et al. (1997) and Droop (1987), respectively, the studied amphiboles are mainly calcic [BCa41 atom per formula unit (apfu)] with subordinate amount of sodic– calcic (Na–Ca) variety. Calcic (Ca) amphiboles mostly contain high Si contents (TSi46.5 apfu) and belong to two subgroups of the IMA classification. Group 1 is defined by A(Na +K) o0.5 apfu, characteristic of magnesiohornblende. Group 2 is defined by A (Na +K) 40.5 apfu, characteristic of edenite. Regardless of the amount of Na+ K in the A site, the XMg [= Mg/(Mg+ Fe2 + )] of the investigated Ca amphiboles ranges from 0.5 to 1, making them all magnesian. The Na–Ca amphiboles are magnesiokatophrite. The composition of amphibole is a direct indicator of the magma alkalinity from which they crystallize (Giret et al., 1980). In the studied amphiboles, variation in alkalinity is concomitant

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with the composition of amphiboles. Group 1 (Ca amphibole) occurs in all rock varieties, whereas group 2 (Ca amphibole) and Na–Ca amphiboles occur only in the diorites and quartz diorites. This implies increasing alkalinity with increasing silica supersaturation.

4.2. Clinopyroxene Clinopyroxene, which belongs to the group of the chain silicates, is one of the early crystallizing mineral phases in the mafic magma. It can be described by the simple formula (M2)(M1)T2O6, and its composition can be used as an indicator of the initial composition of the coexisting melt and its tectonic environment. The SiO2 content of the studied clinopyroxene varies, where the highest SiO2 content (53.2 wt%–1.98 apfu) corresponds to the Px-Hb gabbro and the lowest SiO2 content (46.8 wt%–1.74 apfu) to the Hb gabbro. The investigated clinopyroxene contains low contents of TiO2 (0.3–0.9 wt%), Na2O (0–1.7 wt%), K2O (0–0.4 t%) and MnO (0–0.5 wt%). The FeO and Fe2O3 contents are strongly varying. The maximum FeO content amounts to 16.3 wt% (0.49 apfu) and the minimum content to 0.31 wt% (0.01 apfu). The highest Fe2O3 content is 9.74 wt% (0.27 apfu) and the lowest is zero. The Al2O3, MgO and CaO contents are relatively high. Maximum values reach up to

Fig. 2. (a) AlIV and AlT and (b) Si vs.AlVI (Raase 1974) binary diagrams for amphiboles from Wadi El Markh gabbro–diorite complex, (c) MgO vs. Al2O3 binary diagram for the investigated biotites (Abdel-Rahman, 1994) and (d) Ternary plot of cation proportions of Fe2 + , Fe3 + and Ti4 + showing the composition of the studied magnetites and ilmenites after Robinson et al. (2001).

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6.48 wt% (0.28 apfu) for Al2O3, 21.93 wt% (1.21 apfu) for MgO and 13.51 wt% (0.54 apfu) for CaO. The lowest Al2O3, MgO and CaO contents amount to 1.64 wt% (0.07 apfu), 16.13 wt% (0.89 apfu) and 1.64 wt% (0.07 apfu), respectively. The structural formulae of pyroxenes were calculated on the basis of 6 O as recommended by Morimoto (1988). In the Wo–En–Fs diagram (Morimoto, 1988; not shown), the investigated clinopyroxenes are mainly classified as diopside and sometimes traverse the boundary into the augite field (Wo21.8–25.6En63.9–58.2Fs14.3–16.3 for Px-Hb gabbro; Wo21.3–27.8En57.3–46.1Fs21.5–26.0 for Hb gabbro). According to the (Ca+Na) vs. Ti diagram (Leterrier et al., 1982; not shown) the studied clinopyroxenes have calc-alkaline character. In the Ca–Ti discrimination diagram all analyzed pyroxenes plot in the orogenic field (Leterrier et al., 1982; not shown), consistent with formation in an arc-related tectonic setting.

correlate negatively, suggesting that their sodic nature is due to magma fractionation. 4.4. Biotite The studied biotites are magmatic, calc alkaline orogenic and Mg-rich (Fig. 2c, Abdel-Rahman, 1994). The high Mg contents (10.5–15.2 wt%, 2.44–3.37 apfu) in biotites suggest a higher temperature of crystallization (Klob, 1970). In the tetrahedral site, there is no coupled substitution between AlIV and Ti, but on the other hand there is more substitution of Si by and AlIV, which also reflects a higher crystallization temperature of biotites (Deer et al., 1966). The low Mn contents (max. 0.63 wt%, 0.08 apfu) in biotite may be due to the presence of pyroxene or amphibole, indicating that the studied rocks are rich in mafic minerals and are derived from mafic magma.

4.3. Plagioclase

4.5. Chlorite and titanite

Plagioclase has a wide range of An-content, where it ranges from An62 to An45 for Px-Hb gabbros, from An49 to An39 for Hb gabbros, from An42 to An 33 for diorites and from An35 to An25 for quartz diorites. The Na and Ca contents of the studied plagioclase

Chlorite is ripidolite and Pynochlorite according to the Si vs. Fe + 2 +Fe + 3 classification diagram of Hey (1954). It is characterized by a relatively high Fe/(Fe+Mg) ratio (0.28–0.36 apfu) and low Si (5.57–5.43 apfu).

Table 1 Element concentrations in plutonic rocks of the wadi El Markh. Px-Hb gabbros G1

G2

Major oxides (wt%) 51.40 52.32 SiO2 TiO2 0.63 0.62 Al2O3 18.75 18.16 FeOtot 6.88 6.97 MnO 0.13 0.14 MgO 8.03 7.37 CaO 9.91 9.28 Na2O 2.97 3.14 K2O 0.60 0.63 P2O5 0.15 0.26 LOI 0.26 1.07 Total 99.71 99.96 FeO 5.45 5.53 Fe2O3 0.82 0.83 Mg# 53.86 51.39 Trace elements (ppm) As 0.7 1.5 Ba 131 144 Ce 6 12 Co 32 37 Cr 428 414 Cu 11 14 Ga 17 17 Hf 0.5 0.7 La 3 7 Mo 0.4 0.2 Nb 0.4 1.7 Nd 13 14 Ni 49 73 Pb 7 6 Rb 26 21 Sc 38 45 Sr 521 481 Th 1.0 1.4 U 1.1 0.9 V 119 123 W 1.3 1.2 Y 14 18 Zn 64 46 Zr 35 55

Hb gabbros G12

52.33 0.62 18.49 6.75 0.13 7.78 10.04 2.42 0.80 0.11 0.31 99.77 5.35 0.80 53.53

0.6 82 15 33 503 9 14 0.8 7 0.8 2.4 15 66 11 29 39 482 0.8 1.2 131 1.5 13 61 25

G14

53.25 0.61 18.23 6.62 0.13 7.52 10.17 2.99 0.79 0.07 0.35 100.73 5.25 0.79 53.18

2.3 115 12 36 448 24 13 1.0 10 0.5 1.3 12 91 11 36 44 359 0.8 1.3 140 1.9 17 76 43

G6

54.27 0.71 17.26 6.71 0.12 6.12 8.21 3.41 0.86 0.34 1.12 99.13 5.32 0.80 47.70

2.7 245 35 33 375 9 20 2.1 22 0.5 5.9 24 77 10 32 29 782 2.1 2.7 208 3.9 16 107 86

G7

55.27 0.78 17.61 6.49 0.12 6.62 8.48 3.38 1.02 0.20 0.43 100.40 5.15 0.77 50.50

1.7 226 30 31 246 67 18 1.7 10 0.2 1.8 19 54 4 30 31 609 2.3 3.2 155 3.2 21 71 64

Diorites G77

55.50 0.75 17.44 6.56 0.12 6.12 8.31 3.01 0.88 0.19 0.69 99.57 5.20 0.78 48.28

0.8 297 38 27 312 44 19 2.4 15 0.4 4.3 19 45 7 39 36 511 1.1 4.3 163 2.8 28 74 109

G3

D4

56.97 0.75 17.44 6.48 0.11 5.63 8.24 3.26 1.19 0.04 0.53 100.64 5.14 0.77 46.49

58.91 0.85 16.46 6.21 0.11 4.54 6.88 3.93 1.43 0.05 1.01 100.38 4.92 0.74 42.23

0.3 174 21 26 216 48 16 1.8 10 0.6 2.2 21 35 9 35 32 390 0.4 0.9 115 2.2 29 48 78

0.5 258 35 24 102 34 14 2.7 14 0.7 2.0 20 40 10 40 19 440 1.1 0.8 136 2.1 22 55 123

Quartz diorites D19

59.22 0.81 16.00 5.77 0.10 5.23 7.14 3.27 1.08 0.23 0.71 99.56 4.57 0.69 47.55

1.2 306 44 20 120 23 15 3.2 19 0.6 1.5 19 46 11 50 24 373 1.2 1.5 145 1.4 21 43 100

D9

60.15 0.89 16.26 5.97 0.10 4.08 6.38 3.72 1.46 0.06 0.87 99.92 4.73 0.71 40.57

0.9 384 36 26 178 57 16 2.5 11 0.5 1.2 19 56 7 52 25 512 1.7 2.4 128 1.6 30 61 122

D10

61.38 0.92 16.06 5.73 0.09 3.61 5.87 4.03 1.49 0.07 0.72 99.97 4.54 0.68 38.65

5.4 312 30 21 125 17 16 3.7 18 0.5 1.1 22 34 7 48 16 554 2.2 3.2 138 0.8 21 36 147

T11

63.42 0.99 15.09 5.32 0.08 2.68 4.95 3.98 1.73 0.22 0.93 99.39 4.22 0.63 33.50

3.7 388 23 16 81 54 17 3.7 15 0.9 4.4 21 47 24 62 12 518 1.9 3.0 140 0.1 16 65 184

T13

64.05 0.91 14.96 5.49 0.08 2.79 4.51 4.08 2.03 0.21 0.65 99.76 4.35 0.65 33.70

6.0 454 41 17 94 55 17 4.9 18 1.0 5.5 29 51 17 57 14 460 1.8 1.3 129 0.6 21 72 171

T5

64.93 0.90 14.72 5.11 0.07 2.68 4.56 3.86 1.89 0.20 0.74 99.66 4.05 0.61 34.40

3.0 522 46 14 94 16 16 4.7 22 0.9 5.2 22 50 8 60 14 467 1.9 1.6 123 1.1 22 54 193

T12

66.11 1.01 13.34 4.85 0.08 2.23 4.32 4.25 2.03 0.24 1.04 99.50 3.85 0.58 31.50

4.2 486 46 11 77 62 17 4.7 15 1.1 1.2 25 41 6 66 6 503 1.6 1.9 151 0.7 25 47 205

S. Kharbish / Chemie der Erde 70 (2010) 257–266

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Accessory titanite has SiO2 (27.5–33.1 wt%), TiO2 (32.8– 38.9 wt%) and CaO (24.2–32.9 wt%). The concentration of Al2O3, FeO, MgO and MnO are (0.8–2.2 wt%), (0.1–6.4 wt%), (0.0– 1.04 wt%) and (0.0–0.76 wt%), respectively.

allow applying several conventional geothermobarometers to infer the pressure–temperature conditions.

4.6. Fe–Ti oxides

The pressure conditions of crystallization have been observed to be correlated linearly with the Al-content in amphibole and many calibrations of the Al in amphibole barometer have been published (Hammarstrom and Zen, 1986; Hollister et al., 1987; Johnson and Rutherford, 1988; Rutter, 1989; Schmidt, 1992; Anderson and Smith, 1995; Ernst and Liu, 1998). The calibration of Schmidt (1992) for calculating amphibole crystallization pressures has been used for the investigated rocks. The calculated bulk pressures of crystallization for the investigated rocks lie between 4.77 and 2.40 kbar but usually 3.0–2.5 kbar. The maximum total pressure is recorded from Px-Hb gabbros, whereas the minimum is from quartz diorites.

The most common opaque phases are magnetite and ilmenite. Magnetite is characterized by low contents of TiO2 (0.2–2.6 wt%), MgO (0.2–2 wt%), MnO (0.2–0.7 wt%) and Al2O3 (0.1–1.7 wt%) and of rather homogeneous composition. On the Fe2 + –Ti4 + –Fe3 + diagram (Fig. 2d) of Robinson et al. (2001), the analyses of magnetites show them to have nearly end member composition. Ilmenites show low TiO2 content (33.6–45.7 wt%), indicating extensive hematite exsolution (Fig. 2d). Although hematite lamellae are abundant, probably none were coarse enough to obtain pure titanohematite analyses. MnO content is relatively high (up to 4 wt%) indicating the possibility of a solid solution between FeTiO3 and MnTiO3. The low MgO content (0.2–0.5 wt%) reflects that the ilmenite crystallized late in the mafic melt (Reynolds, 1983).

5. Pressure–temperature conditions Knowledge of pressure and temperature at which the various minerals crystallized facilitate the understanding of the evolution of an igneous pluton. The mineral assemblages in the WMGD

5.1. Al in amphibole barometer

5.2. Amphibole plagioclase geothermometer Blundy and Holland (1990) proposed a geothermometer based on coexisting amphibole plagioclase pairs. This calibration has a reported uncertainty of 775 C. The analyses of coexisting amphibole-plagioclase pairs from many samples representing each rock type have been used to calculate the temperature. In most cases, amphibole and plagioclase sharing a common grain boundary have been selected. The calculated bulk temperatures of

Fig. 3. (a) SiO2–K2O graph (Peccerillo and Taylor, 1976), (b) Log Cr vs. SiO2 diagram after Miyashiro and Shido (1975) and (c) AFM diagram of Irivine and Baragar (1971) for the Wadi El Markh gabbro–diorite complex. Symbols as for Fig. 2.

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crystallization for the studied rocks lie between 826 and 577 1C but usually 4650 1C. The highest temperatures are recorded from Px-Hb gabbros, while the lowest are from quartz diorites.

6. Major and trace elements The studied rocks have a wide range of composition with respect to their silica contents (51.40–66.11 wt%) (Table 1). They also show significant variations with respect to Al2O3, FeO, MgO, CaO, Na2O and K2O, whereas the other major elements exhibit a restricted range (Table 1). The concentrations of Co, Ni, Cr and V range between 11–37, 34–91, 77–503 and 115–208 ppm, respectively. The large ion lithophile elements (LILE) Ba, Sr and Rb define a considerable chemical variation between 82–522, 359–782 and 21–67 ppm, respectively (Table 1). In the K2O vs. SiO2 graph (Fig. 3a, Peccerillo and Taylor, 1976), the relatively medium K content designates the investigated suite as being largely calc-alkaline. The SiO2 vs. Cr variation diagram (Fig. 3b) of Miyashiro and Shido (1975) and the AFM diagram (Fig. 3c, Irvine and Baragar, 1971) corroborate the typical calcalkaline character of the studied rocks. Some of the major and trace elements of the studied rocks are plotted against SiO2 on Harker variation diagrams as a differentiation index (Fig. 4). Except for elements linked to feldspar (Na2O, K2O, Ba and Rb) and Zr, major and trace elements compatible with pyroxene, amphibole and biotite (i.e. Al2O3,

FeOtot, MgO, CaO, Co and Cr) exhibit generally a good negative correlation with increasing silica [the correlation coefficient (r) is typically 40.88)]. No obvious inflections, indicative of changes in fractionating mineral assemblages, are evident within these compositional trends. Such compositional variations are typical for an evolving calc-alkaline magmatic suite, originated from one magma source.

7. Tectonic setting High field strength elements (HFSE) are used to deduce the tectonic setting of the studied rocks due to their immobility up to the medium grade of metamorphism (Pearce and Cann, 1973; Pearce and Norry, 1979). On the Th–Zr–Nb diagram (Fig. 5a, Wood et al., 1979), the investigated samples straddle the destructive plate margin field (i.e. island and volcanic arcs) and are absent from constructive plate margin [mid-oceanic ridge (MOR)] and within-plate fields. The Ti/Cr vs. Ni plot (Fig. 5b, Beccaluva et al., 1979) and the MgO–FeOtot–Al2O3 ternary diagrams (Fig. 5c, Pearce et al., 1977) clearly confirm that the investigated suites were formed in an island arc environment. The tectonic discrimination diagram (Fig. 5d) of Floyd (1991) indicates that the investigated rocks have a geochemical attribute similar to island arc basalts. The incompatible trace elements of the investigated rocks are normalized to the primitive mantle abundances given by Sun and

Fig. 4. Harker variation diagram of some major oxide and trace element contents for the Wadi El Markh gabbro–diorite complex. Symbols as for Fig. 2.

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McDonough (1989) and plotted on spider diagrams (Fig. 6). The WMGD suite shows close similarity with most of the incompatible elements, when compared to the average calcalkaline island arc basalts of Sun (1980). In addition, the relative depletion in Nb and P2O5 constitutes negative Nb–P2O5 anomalies typical of arc related rocks (Fig. 6).

8. Magma generation and geochemical modeling In the investigated suite, the calc-alkaline nature, the strongly LILE enrichment, the negative Nb–P2O5 anomalies and the spiky patterns of the normalized elements relative to MORB smooth pattern (Fig. 6) are considered to be the ‘‘hallmark’’ of most arc magmatism (Pearce, 1982; Best and Christiansen, 2001). Therefore, the WMGD magma generation was derived from the asthenospheric mantle wedge material that was affected by aqueous fluids from a subducted oceanic slab. The Mg-number [Mg #= Mg/(Mg+ Fe2 + )  100] is relatively insensitive to the degree of partial melting but is strongly influenced by fractional crystallization and should be between 68 and 75 in the primitive magmas (Roeder and Emslie, 1970; Best and Christiansen, 2001). In the investigated rocks, the low Mg # ( = 53.86) for the most mafic sample [G1, (SiO2 = 51.40 wt%, Table 1)] indicates that the

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primary magma of the examined rocks is not a direct partial melt of asthenospheric mantle wedge material and has been subjected to some degree of fractional crystallization. Furthermore, the systematic variation of major and trace elements (Fig. 4) provides additional evidence that these rocks were derived from one magma series by subsequent fractional crystallization processes. Therefore, fractional crystallization processes were investigated using major element modeling. The major element modeling was based on the general leastsquares mixing equation of Stormer and Nicholls (1978) and was performed using the PetroGraph package (Petrelli et al., 2005). Those oxides that had very low concentrations in all the studied minerals and rocks (MnO and P2O5) were omitted; total iron content (FeOtot) was used instead of the separate FeO and Fe2O3. Typical mineral analyses for each rock were used; the choice of whole-rock endmembers was based on Harker plots. The quality of the model was assessed by the sum of squares of the residuals (R2), with R2 =0 for the ideal fit; R2 o1 was considered to be acceptable. This approach was repeated for several parent–daughter combinations to check the robustness of the modeling; the ranges of obtained solutions are given below and representative examples are given in Table 2. Major element modeling suggests that the Hb gabbros (e.g. G1) can be produced by 61.86% fractional crystallization of 53.88% plagioclase, 25.60% pyroxene 11.11% amphibole and 9.40% biotite from the PxHb gabbros (e.g. G77). Diorites can be produced from the parent Hb

Fig. 5. (a) Th–Zr/117-Nb/16 discrimination diagram (Wood et al., 1979), (b) Ti/Cr–Ni diagram after Beccaluva et al. (1979), (c) MgO–FeOtot–Al2O3 plot after Pearce et al. (1977) and (d) Ba against Zr plots (Floyd, 1991) for the Wadi El Markh gabbro–diorite complex. Symbols as for Fig. 2.

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Fig. 6. Spidergrams of trace element concentrations for El Markh gabbro–diorite complex normalized to primitive mantle after Sun and McDonough (1989), compared with average chemical composition of island arc basalts and MORB after Sun (1980) and Sun and McDonough (1989), respectively.

Table 2 Examples of major-element based modeling of fractional crystallization using the least-squares model of Stormer and Nicholls (1978). Observed parent Px-Hb gabbro (100%) G1

SiO2 TiO2 Al2O3 FeOtot MgO CaO Na2O K2O Total

51.83 0.64 18.91 6.94 8.10 9.99 2.99 0.61

Observed daughter

Solid removed (38.14%) 53.88% 11.11% 25.60% 9.40% a RR2 =0.54

Plagioclase Amphibole Pyroxene Biotite

Observed parent Hb gabbro (100%) G7

SiO2 TiO2 Al2O3 FeOtot MgO CaO Na2O K2O Total

55.46 0.78 17.67 6.51 6.64 8.51 3.39 1.02 100.00

Solid removed (41.03%) 47.55% 46.49% 5.96% RR2 = 0.87

Plagioclase Amphibole Pyroxene

Observed parent Diorite (100%) D10

SiO2 TiO2 Al2O3

61.94 0.93 16.21

Solid removed (73.42%)

Fractionating phases recalculated to 100% Hb gabbro (61.86%) Plagioclase Amphibole G7 G1-110 G1-124

Pyroxene G1-97

Biotite G1-44

55.46 0.78 17.67 6.51 6.64 8.51 3.39 1.02 100.00

51.16 0.00 29.19 1.11 0.00 13.62 4.62 0.30 100.00

52.56 0.57 3.97 9.61 22.07 10.49 0.72 0.00 100.00

35.93 2.93 16.21 20.86 13.71 0.00 0.00 10.37 100.00

Observed daughter Diorite (58.97%) D10

Fractionating phases recalculated to 100% Plagioclase Amphibole Pyroxene G7-121 G7-203 G7-64

61.94 0.93 16.21 5.78 3.64 5.92 4.07 1.50 100.00

54.26 0.00 28.18 0.33 0.00 9.55 7.44 0.24 100.00

Observed daughter Quartz diorite (26.58%) T5

Fractionating phases recalculated to 100% Plagioclase Amphibole Biotite G7-110 G7-210 G7-20

Ilmenite G7-73

65.82

55.15 0.00 27.85

3.58 42.30 1.45

0.91 14.92

45.92 0.77 7.92 16.95 15.00 11.20 1.56 0.68 100.00

46.27 1.20 7.56 15.25 15.99 11.75 1.39 0.59 100.00

46.07 0.47 7.24

33.47 3.90 16.11 23.41 13.04 0.00 0.00 10.07 100.00

37.42 2.16 15.08

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Table 2. (continued ) FeOtot MgO CaO Na2O K2O Total a

5.78 3.64 5.92 4.07 1.50 100.00

58.93% 38.89% 2.10% 0.08% RR2 = 0.16

Plagioclase Amphibole Biotite Ilmenite

5.18 2.72 4.62 3.91 1.92 100.00

0.29 0.00 8.22 8.23 0.27 100.00

19.08 13.44 11.61 1.39 0.70 100.00

19.48 15.11 0.32 0.59 9.83 100.00

49.39 1.39 0.48 1.29 0.13 100.00

SR2: Sum of squares of the residuals.

gabbros by fractional crystallization of 46.49% plagioclase, 47.55% amphibole and 5.96% biotite (Table 2), with a residual liquid of about 58.97%. Quartz diorites were generated from the parent diorites by 26.58% fractional crystallization of 58.93% plagioclase, 38.89% amphibole, 2.10% biotite and 0.08% ilmenite.

9. Summary and conclusion The investigated Wadi El Markh intrusive complex represents a unimodal plutonic suite, which mainly consists of Px-Hb gabbros, Hb gabbros, diorites and quartz diorites. The examined rocks form a continuum in composition with a wide range in SiO2, Al2O3, MgO, CaO, Ba, Cr and Sr. The lack of a compositional gap is consistent with their identification as a unimodal suite. Coexisting mineral phases and their compositions from the WMGD complex were used to estimate the physicochemical parameters of their crystallizing parent magmas. The studied rocks were crystallized, under conditions of  5–2 kbar pressure and  830–570 1C temperature, passing from the least evolved gabbro to the most evolved quartz diorites. The decrease in P–T conditions can be related to the concomitant ascent, cooling and geochemical differentiation in the magma chamber. The investigated rocks are calc-alkaline, enriched in LILE and have distinctive negative Nb–P2O5 anomalies, which are characteristic features of arc-related magmas. The gabbroic magma of the WMGD suite was generated in an island arc regime, where the mantle wedge and a subducted oceanic slab played a vital role in their genesis. The most mafic sample in the investigated samples cannot be considered to be derived from a primitive magma source because of its low Mg #. The less fractionated Hb gabbros (daughter) were formed from Px-Hb gabbros (parent) by fractional crystallization with about 61.86% residual liquid. The examined diorites (daughter) were produced from the Hb gabbros (parent) with a 58.97% residual liquid. The investigated tonalites (daughter) were produced from the diorites (parent) with a 26.58% residual liquid.

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