Quantifying nickel in soils and plants in an ultramafic area in Philippines
Metamorphism of ultramafic rocks at Gebel Arais and Gebel Malo Grim, Eastern Desert, Egypt:...
Transcript of Metamorphism of ultramafic rocks at Gebel Arais and Gebel Malo Grim, Eastern Desert, Egypt:...
Egyptian Journal of Geology, v. 51, 2007, p. 105- 124
Metamorphism of ultramafic rocks at Gebel Arais and Gebel Malo Grim, Eastern Desert, Egypt: mineralogical and O‐H stable isotopic constraints
M. M. HAMDY* and E. M. LEBDA**
* Geology Department, Faculty of Sciences, Tanta University, 31527 Tanta, Egypt ** Biological and Geological Sciences Department, Faculty of Education, Kafr El Sheikh University,
Kafr El Sheikh, Egypt
ABSTRACT The stable oxygen and hydrogen isotopes in the serpentine minerals and the chemistry of the rock‐forming minerals in the Neoproterozoic ultramafic rocks from Gebel Arais (AR) and Gebel Malo Grim (MG) (south Eastern Desert, Egypt) were studied. The aim is to define the source of serpentinization fluids and when it took place. The studied rocks include ophiolitic peridotites (spinel harzburgite and spinel lherzolite from the AR and MG) and non‐ophiolitic (intrusive‐affiliated) olivine websterite (from the AR) occurring within the ophiolitic peridotite rocks. Source of fluids, PT conditions and the tectonic environment of serpentinization and the pathways of the later regional metamorphism of the two types of the ultramafic rocks are different. The ophiolitic peridotites were mostly serpentinized after their obduction by infiltration of metamorphic and/or hydrothermal fluids (δ18O ≤ 3.72 ‰ for AR‐rocks and δ18O ≤ 5.78 ‰ for MG‐rocks) particularly along the contact of the rocks with the lower infrastructural Pre‐Pan‐African continental rocks. This formed predominant lizardite (δD from ‐108.5 to ‐94.8 ‰). The serpentinized ultramafic rocks suffered prograde regional metamorphism causing recrystallization of the pseudomorphic textures to non‐pseudomorphic ones, alteration of aluminian chromite to Fe3+‐rich aluminian chromite, ferritchromite and Cr‐magnetite and formation of talc and anthophyllite (in MG‐rocks). This transferred the rocks from the lowest greenschist facies to the transitional greenschist‐amphibolite facies of metamorphism (T= 500–550 ºC). The non‐ophiolitic olivine websterites, on the other hand, were probably serpentinized before their incorporation into the obducted ophiolitic peridotites and during their cooling. This might took place in the continental mantle wedge over the subducted oceanic lithosphere. The water of the serpentinization (δ18O = 6.75 ‐ 6.85 ‰) was a mixture of oceanic water released from the subducted oceanic crust and the continental magma c water in the mantle wedge. Temperature of serpen niza on was between 300 and 500 ºC forming antigorite with δD from ‐74 to ‐77.2 ‰. By cooling the serpentinized rock was subjected to retrograde regional metamorphism forming tremolite, chlorite (T = 176 to 245 ºC) and lizardite on the expense of antigorite at the lowest greenschist facies of metamorphism. Keywords: Ultramafic rocks, O‐H isotopes, mineral chemistry, serpentinization, metamorphism, ED, Egypt
INTRODUCTION Ultramafic rocks of Neoproterozoic age are common in the Eastern Desert of Egypt. These rocks are commonly regarded as part of the widely distributed dismembered ophiolite sequence. However, many ultramafic intrusions are recognized, e.g. at Abu Hamamid, Genina Gharbia, G. El Motaghayerat and W. Allaqi. Dismembered ophiolite suites, island‐arc meta‐volcanosedimentary assemblages and island arc gabbro‐diorite complexes were formed during the arc stage, as early as 850 Ma ago (Stern, 2002). During the collision stage the ophiolitic ultramafic rocks, were obducted over the continental margin of a back‐arc basin (Akaad, 1996, 1997; Akaad and Abu El Ela, 2002; El Gaby, 2005). They were accreted to the East Saharan Craton (Ries et al., 1983; El Gaby et al., 1984, 1988; Abu El Ela, 1996). They were subjected to regional metamorphism during the collision stage between 650 and 620 Ma ago (Finger and Helmy, 1998). Granodiorite intrusions predominated during this stage (Moghazi, 2002). Large masses of volcanic rocks (Dokhan volcanics) and alkaline granites erupted during the extensional post‐collision stage (610–550 Ma) (Ghoneim and Lebda, 1997). The ophiolitic ultramafic rocks in the Eastern Desert are represented mainly by peridotite mantle slices, comprising predominate harzburgites and dunites together with subordinate pyroxenites and wherlites. They serpentinized to different degrees, but still enclosing few relics of unserpentinized peridotites. They suffered regional metamorphism in the greenschist‐amphibolite facies (Ghoneim and Aly, 1986; Azer and
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Khalil, 2005; Azer and Stern, 2007). The intrusive ultramafic rocks comprising spinel lherzolite and clinopyroxenite occurs as small, frequently layered intrusions and sills. Some of them are similar to the Alaskan type ultramafic‐mafic complexes (Farahat and Helmy, 2006), which are common Phanerozoic intrusions formed above the subduction zone (Tistl et al., 1994). These non‐ophiolitic ultramafic were intruded coevally with the island‐arc calc‐alkaline magmatic rocks (El Gaby, 1990). These rocks are commonly unmetamophosed, but sometimes they have clearly been metamorphosed (El Gaby, 1990). Some authors suggest that serpentinization of ultramafic rocks took place by interaction with hot water during seafloor weathering and interaction with hot seawater (Lebda, 1995; Li and Lee, 2006). Other authors believe that serpentinization took place by infiltration of metamorphic and hydrothermal waters along major tectonic fractures during or after rock exhumation (Hyndman and Peacock, 2003; Hamdy, 2004). The most proper way to decipher the source of the fluids inducing the serpentinization of ultramafic rocks is to study the O‐H isotopic composition of the serpentinites. Till to date, we lack such studies on the serpentinites from the Eastern Desert of Egypt. The main objective of the present study is to detect the source of the fluids that caused the serpentinization of the ultramafic rocks and when it took place. It also aims at estimating the PT conditions of metamorphism. Both ophiolitic and non‐ophiolitic (intrusive‐ affiliated) ultramafic rocks were selected for this study from Arais (AR) and Malo Grim (MG) areas, south Eastern Desert. To achieve the aims of study stable oxygen and hydrogen isotopes were analyzed in the serpentine minerals as well as the chemical composition of all rock‐forming minerals in the sampled ultramafic rocks were determined.
GEOLOGIC SETTING AND FIELD OBSERVATIONS The area of Gebel Arais (AR) lies between la tudes 23º 31΄ 20˝ and 23º 34΄ 10˝N and longitudes 34º 49΄ 40˝ and 34º 54΄ 45˝E, and that of Gebel Malo Grim (MG) lies between la tudes 22º 17´ 30˝ and 22º 21´N and longitudes 36º 09´30˝ and 36º 17´E. The AR‐area (Fig. 1A) is reached from 65 km from Bernice on the Bernice‐Shalatin road, and then westward along W. Marafai to Bir El‐Gahliya and then for further 33 km along W. Bitan. The area is occupied by gneisses, ophiolitic peridotites and metagabbros, island‐arc volcaniclastic metasediments, non‐ophiolitic ultramafics and syn‐collision granodiorite. The gneisses to the west represent the infrastructure upon which the supracrustal ophiolites were thrust along a major NW–SE trending thrust fault (Ghoneim et al., 2002). The ophiolitic ultramafic rocks also thrust over the island arc volcanics and volcaniclastic metasediments to the east forming an ophiolitic mélange (Ghoneim et al, 2002). The studied ophiolitic ultramafics are composed mainly of serpentinized peridotites. The serpentinized ophiolitic peridotites occur as isolated sheets or slabs (2–20 km long and 0.7–3 km width) tectonically mixed with the island arc rock assemblage. They also occur as small lenses in the metasedimentary matrix of the mélange. They are generally massive, but become sheared and foliated along their peripheries. Sometimes, these serpentinites include small pockets and veins of magnesite (Hamdy, in press) and lenses of chromitite (Ghoneim et al., 2002). The sampled non‐ophiolitic ultramafic rocks occur within the ophiolitic peridotite slabs in their west and west northern parts. Their identification and distinction from the ophiolitic peridotite rocks was not possible in the field. The contact between the two types of the rocks was also absent. Only sampling sites of these non‐ophiolitic ultramafic rocks are plotted on the geological map (Fig. 1A). Predicting origin of these non‐ophiolitic ultramafics is summarized in sec on 3. Discussing the origin of these rocks based on their mineral chemistry and whole‐rock geochemistry is the subject of further publication. The MG‐area (Fig. 1B) can be reached at 150 km from Shala n on the Shala n‐Halaib road, then going for 50 km to the west through W. Yoider. The ultramafic rocks are composed of serpentinized peridotites forming the base of the dismembered ophiolitic sequence that comprises also metagabbros, pillow lavas and pelagic sediments (Abu El Laban, 2002). They occur as sheets and lenses enclosed within the mélange matrix made of schist and metasediments. The mélange comprises beside the ophiolitic rocks, island arc metavolcanics and metagabbros; they are intruded by syn‐collision and post collision granites. Serpentinized ophiolitic perido tes form ridges about 20 km long and about 0.4–1.8 km wide, elongated in NE‐SW direction. They are mostly massive but are sometimes brecciated and fragmented along their contacts with the mélange matrix.
SAMPLES AND ANALYTICAL METHODS Complete serpentinization of some of the AR and MG ophiolitic peridotites make it rather difficult to define their composition and origin. Relics of olivine and orthopyroxene, textures of these rocks and their high normative olivine and orthopyroxene contents refer to the spinel harzburgite composition of the peridotites
Metamorphism of Ultramafic Rocks at Gebel Arais and Gebel Malo Grim, ED, Egypt 107
(Fig. 2). However, some samples have the composi on of spinel lherzolite. Some samples from AR are rich in mineral constituents of the Al‐hercynite, not similar to the ophiolitic ultramafic rocks but are at least affiliated to the Pan‐African non‐ophiolitic continental mantle‐derived ultramafic rocks. Occurrence of these non‐ophiolitic ultramafic rocks within the obducted ophiolitic peridotite indicates that these non‐ophiolitic ultramafic rocks might derive from the continental mantle wedge over the subducted oceanic lithosphere. The relics of primary minerals and the estimated normative composition of the AR‐non‐ophiolitic ultramafics classify them as olivine websterites with wide ranges of clinopyroxene and orthopyroxene modal content. They range from clinopyroxene‐rich to orthopyroxene‐rich websterites. Major and some minor elements analyses of minerals were carried out at the Institute of Geological Sciences‐Polish Academy of Sciences by JEOL‐JXA‐840A scanning electron microscope (SEM) equipped with Link Analytical AN‐1000/855 energy dispersive X‐ray spectrometer (EDS). The analy cal condi ons were 15 kV accelera ng voltage and 35 nA beam current. In order to es mate Fe2O3 from FeO in Cr‐spinels the chemical data were stoichiometrically recalculated according to Droop (1987). The O and H isotopes were analyzed in separated serpentine minerals obtained from fresh, water‐washed rock fragments (0.14–0.08 mm in size). Mineral concentrates were achieved by using heavy liquids. For final
Fig. 1. Geological maps of the areas around ultramafic rocks at A) Gebel Arais (a er Ghoneim et al., 2002) and B) Gebel Malo Grim (after Abu El‐Laban, 2002).
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Fig.2. Nomenclature of ultramafic rocks based on Ol‐Opx‐Cpx normative composition (after Streickeisen, 1976).
purification, the separated minerals were hand picked under a binocular microscope. Iron stained or composite grains and those with visible inclusions were rejected. Mineral separates were subjected to ultrasound vibration in dis lled water for 15 minutes to remove surface contaminants. The oxygen and hydrogen isotopes in the serpentine concentrates were analyzed in the Stable Isotope Laboratory of the University of Lausanne, Switzerland. The oxygen isotope composition was measured using CO2‐Laserline extraction method of Kasemann et al. (2001). Hydrogen isotopes (H, D) were analyzed using the method of carbon reduction for water as described by Sharp et al. (2001). Oxygen and hydrogen isotope composi ons are given in the standard δ‐notation, expressed relative to VSMOW in per mil (‰). Oxygen and hydrogen isotope analyses of the used standards generally have an average precision of ±0.1% and 2% respectively.
PETROGRAPHY AND MINERAL CHEMISTRY Serpentinized Ophiolitic Spinel Peridotites The studied ophiolitic peridotites suffered variable degrees of serpentinization. In completely serpentinized rocks no primary textures are detected, while original peridotite minerals and textures have been retained in partly serpentinized peridotites from AR. The serpentine minerals were properly identified using their X‐ray diffraction (XRD) spectra compared to the data of the Powder Diffrac on File (1992) of the ICDD. The XRD patterns of the serpentinites (not shown here) indicate that the dominant serpentine mineral is lizardite, whereas chrysotile is subordinate. These serpentine minerals occur as alteration products of olivines and pyroxenes, in the form of anastomizing veins, and as cross‐cutting veins. Serpentine minerals also occur as thin rims surrounding primary spinels. They appear to be accompanied by the shedding of fine‐grained magnetite, which concentrates in veins or along relict pyroxene cleavages. In addition to the serpentine minerals, all serpentinites contain chromite, magnetite and sulphides grains. The MG‐peridotites contain also talc, anthophyllite and magnesite. Lizardite forms mesh and bastite textures (pseudomorphic) and sometimes occur as interlocking and penetrating grains (non‐pseudomorphic) (Figs. 3A‐3C) confirming the harzburgite parent rocks. FeO and Cr2O3 contents in lizardite increase distinctly with increasing degree of serpentinization from the partly (e.g. samples # AR‐25, AR‐78, MG‐06, MG‐10) to the completely serpen nized rocks (e.g. samples # AR‐23, AR‐24, MG‐47, MG‐69) (from 1.80 to 3.75 wt% in AR‐rocks and from 0.83 to 6.21 wt% in MG‐rocks; from 0.12 to 0.41 wt% in AR and from 0.09 to 0.66 wt% in MG‐rocks, respectively) (Table 1). Apparently, with the increasing grade of metamorphism Fe released from olivine and orthopyroxene and Cr released from chromite are accommodated in the prograde serpentines. Chrysotile occurs as crossfiber veinlets traversing the lizardite matrix, indicating its later crystalliza on under sta c condi ons (O’Hanley and Wicks, 1995). Chrysotile in veins shows that Al and Cr are relatively immobile during recrystallization of lizardite and therefore remain in their original crystal lattice.
Metamorphism of Ultramafic Rocks at Gebel Arais and Gebel Malo Grim, ED, Egypt 109
Fig. 3. A‐D) principle textures in serpentinized mantle peridotites. A) hourglass (pseudomorphic) (sample# AR‐25); B) par al recyrstalliza on of lizardite in hourglass texture to lizardite in interpenetrating texture (non‐pseudomorphic) (sample# AR‐23); C) lizardite in interlocking texture (non‐pseudomorphic) (sample# MG‐69); D) bas te a er orthopyroxene with flakes of talc; E&F) olivine websterite with antigorite in non‐pseudomorphic textures (serrate veins (E) and interlocking (F), relics of lizardite in patchy texture (E) and growing chlorite among antigorite (F).
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Table 1. Representative microprobe analyses of mineral constituents of serpentinized ophiolitic peridotites Locality G. Arais Mineral lz lz lz lz lz lz lz lz chr chr chr chr Sample AR-25 AR-78 AR-23 AR-24 AR-24 AR-24 AR-33 AR-33 AR-23 AR-23 AR-25 AR-25 Analysis 12'-3 5'-2 6'-2 10'-2 22-5 23-6 10'-4 11'-5 11'-1 12'-2 8'-4 9'-5 SiO2 43.44 43.04 42.09 42.62 43.70 44.08 44.35 43.41 42.47 42.60 43.07 42.11 TiO2 0.04 0.15 0.00 0.01 0.01 0.02 0.00 0.02 0.00 0.01 0.00 0.00 Al2O3 0.46 0.14 0.39 1.12 0.69 0.42 0.27 0.31 0.13 0.00 0.23 0.20 Cr2O3 0.12 0.34 0.09 0.16 0.11 0.14 0.36 0.41 0.02 0.00 0.00 0.05 FeO 1.80 1.85 3.75 1.98 2.81 1.94 2.92 3.00 1.81 2.05 1.88 2.31 MnO 0.16 0.09 0.26 0.04 0.29 0.30 0.00 0.12 0.00 0.00 0.07 0.05 MgO 36.41 36.88 37.15 37.67 36.92 36.29 37.75 36.95 38.42 36.68 36.31 36.56 CaO 0.34 0.13 0.14 0.00 0.34 0.33 0.00 0.00 0.00 0.19 0.18 0.21 Na2O 0.00 0.01 0.03 0.05 0.03 0.02 0.06 0.00 0.02 0.00 0.00 0.00 K2O 0.00 0.03 0.03 0.03 0.00 0.00 0.03 0.01 0.01 0.03 0.00 0.00 Total 82.77 82.66 83.93 83.68 84.90 83.54 85.74 84.23 82.88 81.56 81.74 81.49
No of cations on the basis of 9 oxygen atoms Si 2.109 2.096 2.047 2.053 2.084 2.121 2.092 2.087 2.064 2.102 2.116 2.086 Al 0.026 0.008 0.022 0.064 0.039 0.024 0.015 0.018 0.007 0.000 0.013 0.012 Ti 0.001 0.005 0.000 0.000 0.000 0.001 0.000 0.001 0.000 0.000 0.000 0.000 Fe2+ 0.073 0.075 0.153 0.080 0.112 0.078 0.115 0.121 0.074 0.085 0.077 0.096 Cr 0.005 0.013 0.003 0.006 0.004 0.005 0.013 0.016 0.001 0.000 0.000 0.002 Mn 0.007 0.004 0.011 0.002 0.012 0.012 0.000 0.005 0.000 0.000 0.003 0.002 Mg 2.635 2.678 2.694 2.705 2.625 2.604 2.655 2.649 2.784 2.699 2.659 2.699 Ca 0.018 0.007 0.007 0.000 0.017 0.017 0.000 0.000 0.000 0.010 0.009 0.011 Na 0.000 0.001 0.003 0.005 0.003 0.002 0.005 0.000 0.002 0.000 0.000 0.000 K 0.000 0.002 0.002 0.002 0.000 0.000 0.002 0.001 0.001 0.002 0.000 0.000
Locality G. Malo Grim Mineral lz lz lz lz lz lz lz lz chr chr chr chr Sample MG-10 MG-10 MG-10 MG-47 MG-47 MG-69 MG-69 MG-69 MG-69 MG-10 MG-10 MG-47 Analysis 16-4 15-6 14-5 1 2 22-7 23-8 8'-2 13'-4 2'-5 3'-6 4'-11 SiO2 43.01 43.93 44.15 41.56 41.30 42.71 43.92 41.94 43.96 42.07 43.02 42.24 TiO2 0.13 0.05 0.00 0.01 0.00 0.00 0.00 0.03 0.00 0.00 0.00 0.00 Al2O3 0.21 0.13 0.23 0.14 0.26 0.21 0.25 0.28 0.19 0.06 0.05 0.16 Cr2O3 0.32 0.30 0.24 0.09 0.16 0.23 0.25 0.36 0.02 0.00 0.00 0.08 FeO 0.83 0.98 1.55 3.99 3.36 1.77 1.54 6.21 1.14 2.37 2.35 2.45 MnO 0.12 0.15 0.18 0.17 0.22 0.10 0.00 0.00 0.02 0.00 0.27 0.00 MgO 37.35 37.64 37.69 38.06 38.68 36.83 37.58 35.98 37.16 36.98 36.29 39.35 CaO 0.02 0.02 0.00 0.00 0.00 0.60 0.06 0.11 0.00 0.19 0.21 0.00 Na2O 0.17 0.15 0.10 0.00 0.00 0.04 0.09 0.08 0.02 0.09 0.08 0.09 K2O 0.00 0.00 0.01 0.01 0.00 0.03 0.00 0.01 0.00 0.02 0.01 0.06 Total 82.16 83.35 84.15 84.03 83.98 82.52 83.69 85.00 82.51 81.78 82.28 84.43
No of cations on the basis of 9 oxygen atoms Si 2.096 2.092 2.106 2.024 2.009 2.087 2.105 2.040 2.127 2.079 2.110 2.029 Al 0.012 0.007 0.013 0.008 0.015 0.012 0.014 0.016 0.011 0.003 0.003 0.009 Ti 0.005 0.002 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 Fe2+ 0.034 0.039 0.062 0.163 0.137 0.072 0.062 0.253 0.046 0.098 0.096 0.098 Cr 0.012 0.011 0.009 0.003 0.006 0.009 0.009 0.014 0.001 0.000 0.000 0.003 Mn 0.005 0.006 0.007 0.007 0.009 0.004 0.000 0.000 0.001 0.000 0.011 0.000 Mg 2.714 2.705 2.680 2.764 2.805 2.683 2.685 2.610 2.681 2.724 2.653 2.818 Ca 0.001 0.001 0.000 0.000 0.000 0.031 0.003 0.006 0.000 0.010 0.011 0.000 Na 0.016 0.014 0.009 0.000 0.000 0.004 0.008 0.008 0.002 0.009 0.008 0.008 K 0.000 0.000 0.001 0.001 0.000 0.002 0.000 0.001 0.000 0.001 0.001 0.004 Chromite occurs as disseminated subhedral and anhedral crystals of reddish brown colour. Detailed compositional profiles (Fig. 4) show dis nc vely three zones (core, intermediate zone and rim), some mes without apparent compositional gaps in between. The core has aluminian chromite composition. It is commonly the largest in size (~ 350 μm), except in some MG‐samples where it is the smallest in size (~ 30 μm). It usually retains the primary spinel composition. Discussing the primitive composition of the core in relation to the mantle processes is beyond the scope of this paper. The compositions of the intermediate and the rim zones are given in Table 1 and plo ed in the Al‐Cr‐Fe3+ triangle of Stevens (1944) (Fig. 5a). The variation in composition from the core to the intermediate and rim zones is the result of metamorphic altera on of the aluminian spinel (Abzalov, 1998; Barnes, 2000; Proenza et al., 2004). The altera on is accompanied by decrease in Al, Mg, and Cr contents, and consequence increase in Fe3+ and Fe2+ . Due to Al
Metamorphism of Ultramafic Rocks at Gebel Arais and Gebel Malo Grim, ED, Egypt 111
Table 1 (Continued) Locality G. Arais Mineral Al-Cr Cr-Mt Al-Cr Cr-Mt Al-Cr Cr-Mt Al-Cr Cr-Mt Al-Cr Cr-Mt Al-Cr Mt Sample AR-23 AR-23 AR-24 AR-25 AR-78 AR-33 Grain# 1 3 5 13 22 9
Analysis 2'-1 3'-1 2'-3 3'-3 2'-5 3'-5 2'-13 3'-13 2'-22 3'-22 2'-9 3'-9 Location int rim int rim int rim int rim int rim int rim SiO2 0.39 0.17 0.30 0.35 0.19 0.32 0.08 0.20 0.13 0.11 0.18 0.22 TiO2 0.05 0.00 0.06 0.00 0.00 0.00 0.10 0.00 0.23 0.01 0.08 0.00 Al2O3 12.30 0.22 12.95 0.25 13.02 0.21 12.03 1.85 14.23 0.15 12.35 0.03 FeO 18.89 90.59 19.79 88.70 20.93 87.70 27.91 79.32 26.93 87.55 31.90 92.40 Cr2O3 58.16 1.97 58.24 3.31 57.26 4.43 52.25 9.88 51.51 4.03 49.71 0.00 MnO 0.07 0.37 0.05 0.31 0.46 0.28 1.18 1.24 1.10 0.14 2.19 0.00 MgO 8.70 0.83 8.98 1.05 8.93 1.59 5.48 1.53 6.54 0.86 4.25 0.54 NiO 0.09 1.28 0.19 1.31 0.00 1.13 0.10 1.88 0.00 0.84 0.10 0.05 Total 98.65 95.43 100.56 95.28 100.79 95.66 99.03 95.90 100.70 93.69 100.76 93.24
No of cations on the basis of 32 oxygen atoms Si 0.104 0.067 0.078 0.136 0.050 0.123 0.022 0.074 0.035 0.044 0.050 0.090 Ti 0.010 0.000 0.012 0.000 0.000 0.000 0.021 0.000 0.052 0.003 0.017 0.000 Al 3.848 0.102 3.975 0.115 4.005 0.097 3.921 0.805 4.465 0.070 4.005 0.014 Fe2+ 4.519 6.981 4.460 6.878 4.421 6.647 5.462 6.209 5.154 7.174 5.724 7.652 Fe3+ 0.000 15.219 0.000 14.731 0.133 14.433 0.617 12.237 0.609 14.619 1.118 15.896 Cr 12.200 0.612 11.987 1.018 11.812 1.347 11.414 2.884 10.839 1.264 10.810 0.000 Mn 0.016 0.123 0.011 0.102 0.102 0.091 0.277 0.388 0.248 0.047 0.511 0.000 Mg 3.445 0.486 3.489 0.610 3.477 0.912 2.261 0.843 2.598 0.509 1.745 0.328 Ni 0.020 0.410 0.040 0.410 0.000 0.350 0.000 0.560 0.000 0.270 0.020 0.020 Cr# 0.77 0.04 0.75 0.06 0.74 0.08 0.72 0.18 0.68 0.08 0.68 0.00 Fe3+# 0.00 0.96 0.00 0.93 0.01 0.91 0.04 0.77 0.04 0.92 0.07 0.99 Mg# 0.43 0.07 0.44 0.08 0.44 0.12 0.29 0.12 0.34 0.07 0.23 0.04
Locality G. Malo Grim Mineral Al-Cr Cr-Mt Al-Cr Cr-Mt Al-Cr Cr-Mt Al-Cr Cr-Mt Fe-Cr Cr-Mt Fe-Cr Mt Sample MG-06 MG-06 MG-69 MG-47 MG-47 MG-10 Grain# 2' 6' 15 8' 10 12'
Analysis 2'-2 3'-2 2'-6 3'-6 2'-15 3'-15 2'-8 3'-8 2'-10 3'-10 2'-12 3'-12 Location int rim int rim int rim int rim int rim int rim SiO2 0.07 0.11 0.05 0.16 0.05 0.18 0.10 0.18 0.07 0.22 0.23 0.18 TiO2 0.36 0.00 0.23 0.07 0.15 0.00 0.00 0.00 0.21 0.02 0.09 0.00 Al2O3 8.81 0.06 9.23 0.34 5.40 0.50 15.15 1.49 3.28 0.10 2.78 0.00 FeO 34.12 90.02 34.47 90.60 34.12 85.95 25.91 80.56 36.23 91.34 37.77 90.30 Cr2O3 51.38 1.78 51.10 2.02 56.25 4.72 51.44 8.20 53.76 1.34 52.58 0.00 MnO 0.39 0.52 0.00 0.05 0.02 0.02 0.00 0.71 0.36 0.41 0.93 0.30 MgO 4.23 0.80 4.03 0.94 3.67 3.11 7.48 2.65 4.96 1.05 4.50 0.71 NiO 0.00 1.15 0.72 1.31 0.01 1.42 0.07 1.44 0.48 0.23 0.35 0.11 Total 99.36 94.44 99.83 95.49 99.67 95.90 100.15 95.23 99.35 94.71 99.23 91.60
No of cations on the basis of 32 oxygen atoms Si 0.020 0.044 0.014 0.063 0.014 0.068 0.026 0.067 0.021 0.037 0.069 0.075 Ti 0.077 0.000 0.049 0.021 0.033 0.000 0.000 0.000 0.047 0.006 0.020 0.000 Al 2.955 0.028 3.084 0.157 1.840 0.223 4.724 0.654 1.140 0.047 0.977 0.000 Fe2+ 6.110 6.979 6.125 7.024 6.412 5.809 5.038 5.873 5.617 7.171 5.680 7.417 Fe3+ 1.392 15.368 1.492 15.134 1.281 14.298 0.494 12.864 2.258 15.440 2.540 15.925 Cr 11.556 0.560 11.451 0.625 12.852 1.411 10.756 2.415 12.534 0.052 12.390 0.000 Mn 0.094 0.176 0.000 0.017 0.005 0.006 0.000 0.224 0.090 0.138 0.230 0.105 Mg 1.796 0.475 1.705 0.549 1.583 1.755 2.952 1.473 2.183 0.621 2.002 0.438 Ni 0.000 0.370 0.160 0.410 0.000 0.430 0.010 0.430 0.110 0.070 0.080 0.040 Cr# 0.73 0.04 0.72 0.04 0.81 0.09 0.67 0.15 0.79 0.03 0.78 0.00 Fe3+# 0.09 0.96 0.09 0.95 0.08 0.90 0.03 0.81 0.14 0.97 0.16 1.00 Mg# 0.23 0.06 0.22 0.07 0.20 0.23 0.37 0.20 0.28 0.08 0.26 0.06 loss relative to Cr, the intermediate zone has aluminian chromite to ferritchromite composition. The composition of the outer rim is Cr‐magnetite to magnetite, which is nearly devoid of Al and lie along the Cr–Fe3+ sideline. However, some chromite grains look homogeneous in reflected light and show cryptic elemental zoning. The Mg# [Mg/ (Mg+Fe2+)] decreases from the intermediate zone to the rim due to the Mg‐Fe2+ exchange (Table 1). The Fe3+# [Fe3+/ (Fe3++Al+Cr)] increases with decreasing Mg# from the intermediate zone to the rim due to Al‐loss and Fe3+‐enrichment (Fig. 5b). Magnetite occurs also as veinlets cutting the zoned chromite, indicating that compositional zoning of the chromite preceded the formation of the metamorphic magnetite veinlets. Magnetite as well marks the concentric layers of the serpentine in the mesh texture. Its grain size is generally very small (in the order of a few microns or below), although occasionally euhedral crystals may reach over 10 µm. This magne te is
112 M. M. HAMDY and E. M. LEBDA
Table 1 (Continued) Locality G. Malo Grim
Mineral tc tc tc ath ath ath mag mag mag mag
Sample MG-06 MG-06 MG-10 MG-47 MG-47 MG-47 MG-01 MG-10 MG-68 MG-47 Analysis 10'-3 17-4 18-5 10'-1 10'-2 10'-3 4'-1 7'-2 5'-1 6'-1
SiO2 60.92 59.98 61.09 57.21 56.78 57.36 0.04 0.08 0.09 0.09 TiO2 0.04 0.07 0.22 0.07 0.02 0.03 Al2O3 0.28 0.05 0.00 0.34 0.42 0.50 Cr2O3 0.26 0.03 0.00 0.17 0.28 0.20 0.09 0.09 0.00 0.07 FeO 3.85 2.71 3.11 5.60 6.15 5.80 13.42 9.47 13.79 10.63 MnO 0.04 0.00 0.00 0.20 0.00 0.16 0.13 0.25 0.17 0.27 MgO 26.85 27.17 27.60 34.84 34.26 34.26 36.82 38.91 36.63 38.07 CaO 0.00 0.15 0.16 0.09 0.20 0.18 0.17 0.10 0.27 0.15 CoO 0.00 0.11 0.00 0.08 NiO 0.16 0.36 0.21 0.32 Na2O 0.16 0.17 0.13 0.09 0.09 0.06 K2O 0.00 0.00 0.00 0.02 0.02 0.01 Total 92.40 90.33 92.31 98.63 98.22 98.56 50.83 49.37 51.16 49.68 O atoms 24 24 24 24 24 24 6 6 6 6 Si 8.067 8.081 8.069 7.636 7.628 7.661 0.006 0.012 0.013 0.014 Al 0.044 0.008 0.000 0.053 0.066 0.079 Ti 0.004 0.007 0.022 0.007 0.002 0.003 Fe2+ 0.426 0.305 0.344 0.625 0.691 0.648 Cr 0.027 0.003 0.000 0.018 0.030 0.021 0.001 0.002 0.003 0.003 Co 0.339 0.239 0.270 0.348 Ni 0.002 0.002 0.002 0.000 Mn 0.004 0.000 0.000 0.023 0.000 0.018 0.003 0.006 0.007 0.004 Mg 5.300 5.457 5.435 6.772 6.861 6.822 1.657 1.752 1.721 1.645 Ca 0.000 0.022 0.023 0.013 0.029 0.026 0.005 0.003 0.005 0.009 Na 0.041 0.044 0.033 0.023 0.023 0.016 0.000 0.000 0.000 0.000 K 0.000 0.000 0.000 0.003 0.003 0.002 0.000 0.010 0.010 0.010 Mg# 0.92 0.91 0.91
lz (lizardite), chr (chrysotile), Al-Cr (aluminian chromite), Cr-Mt (Cr-magnetite), Mt (magnetite), Fe-Cr (ferritchromite), tc (talc), ath (anthophyllite), mag (magnesite). produced by shedding excess iron during serpentinization reaction of somewhat iron‐rich ferromagnesian minerals. Talc is not abundant in the studied serpentinites. It is recorded only in samples # MG‐06 and MG‐10. It forms fine shreds, dense fibres and medium grained flaky crystals (0.01–0.03 mm) (Fig. 3D). Perfect cleavage, straight extinction and high interference colours are characteristic features of talc. Talc is pseudomorphous after anthophyllite. It is homogenous and commonly associated with the alteration of orthopyroxene. Representative analyses are given in Table 1. Anthophyllite is an uncommon mineral occurring only in MG‐samples. It occurs as fine to medium grained prismatic and elongated crystals (0.02–0.05 mm long). Its Mg# ranges from 0.91 to 0.92 indicating its magnesio‐anthophyllite composition (Table 1). It is a common replacement mineral of orthopyroxene, where it initially grows along cleavage planes and eventually replacing the whole grain at the end. Magnesite is encountered only in MG‐serpentinized peridotites. It occurs as granular aggregates and discrete rhombs, commonly stained with reddish iron material. It has mainly a ferrian magnesite composition (Table 1) with the formula (Mg1.645‐1.752 Fe0.239‐0.348 Ca0.003‐0.009 Mn0.003‐0.007 Ni0.000‐0.002) (CO3)2. Secondary sulphide minerals that formed during serpentinization are uncommon. They occur as fine grains (20–60 µm) among serpentine minerals and along rock cracks. The sulphides are mainly represented by pyrite and pentlandite. Serpentinized Non-Ophiolitic (Intrusive-Affiliated) Olivine Websterites The AR‐non‐ophiolitic websterites are inequigranular with grain size ranging from 0.5 to 0.9 mm across. The rocks preserve their primary textures. The primary magmatic minerals are orthopyroxene (25–55 vol.%), clinopyroxene (35–65 vol.%), and opaque minerals (Al‐hercynite, Cr‐magnetite, titanian magnetite, ilmenite and sulphides) (Fig. 3E). Trapped pore liquid components between cumulus minerals in the rocks are limited.
Metamorphism of Ultramafic Rocks at Gebel Arais and Gebel Malo Grim, ED, Egypt 113
The metamorphic minerals are mainly serpentines (mainly antigorite with subordinate lizardite and rare chrysotile), amphibole and chlorite. An gorite (20–40 vol.%) occurs as flaky crystals or interpenetrating and interlocking elongate plates forming non‐pseudomorphic textures (Figs. 3E, 3F). Some mes, they are arranged in roughly parallel aggregates. Antigorite has a wide chemical composition (Table 2), but it is common in part with that of the lizardite and chrysotile in the studied serpentinized peridotite rocks. Its composition is varied depending on the modal composition of the clino‐ and orthopyroxene. With increasing amount of clinopyroxene relative to orthopyroxene the concentration of SiO2 increases from 36.68 to 42.24 wt% whereas the concentrations of Al2O3 and FeO decrease from 6.17 to 0.13 wt% and from 7.59 to 2.39 wt%, respectively. Antigorite replaces
Fig. 4. Back sca ered electron (BSE) images and qualita ve microprobe profiles of Cr, Al and Fe along line A‐A’ in chromite from the serpentinized mantle peridotites. A) wide aluminian chromite core (dark) with thin alteration zone of Cr‐magnetite (light) (sample# AR‐25); B) zoned spinel with aluminian chromite core, Fe3+‐rich aluminian chromite intermediate zone and Cr‐magnetite wider outer zone (Sample# MG‐10); C) aluminain chromite (darker) interpenetrated with Fe3+‐richer aluminian chromite‐ferritchromite (lighter) (sample#MG‐47).
114 M. M. HAMDY and E. M. LEBDA
orthopyroxene contains also chromium. When antigorite recrystallized to chrysotile the latter shows less variation in Al and Cr content due to its low immobility during alteration and therefore remain in their original microstructural site of antigorite. Amphiboles often occur in the clinopyroxene‐rich websterite. They occur as prismatic crystals and acicular crystals along fractures in clinopyroxene. They have the composition of tremolite according to the nomenclature of Leake et al. (1997). Some mes they contain in their composi on FeO (0.79–2.08 wt%), Al2O3 (0.08–1.68 wt%) and Cr2O3 (0.0–0.32 wt%) (Table 2). Chlorite occurs as fine anhedral plates and sometimes as tabular discrete crystals (Fig. 3F). It is always bordered by abundant small iron oxide crystals forming complete or partial coronas which separate the grains from the surrounding minerals. Its composition varies widely from penninite in the Opx‐rich websterite to clinochlore in the Cpx‐rich websterite (according to the classifica on of Hey, 1954) (Fig. 6). The SiO2 and MgO contents decrease from 33.14 to 29.85 wt% and from 33.47 to 28.03 wt%, respectively. Meanwhile, Al2O3, FeO and TiO2 contents increase from 13.99 to 20.65 wt%, from 2.93 to 4.52 wt% and from 0.02 to 0.26 wt% respectively (Table 2).
Fig. 5. a) Composition of intermediate and outer zones in chromite from the serpentinized AR and MG‐mantle peridotites plotted on Al‐Cr‐Fe3+ triangle of spinel nomenclature of Stevens (1944). Spinel stability limits of Sack and Ghiorso (1991) are drawn. b) Mg# [Mg/(Mg+Fe2+)] versus Fe3+# [Fe3+/( Fe3++Cr+Al)] of intermediate zone and rim.
Fig. 6. Nomenclature of chlorite from the AR‐serpen nized websterite (a er Hey, 1954).
Metamorphism of Ultramafic Rocks at Gebel Arais and Gebel Malo Grim, ED, Egypt 115
Table 2. Representative microprobe analyses of mineral constituents of the serpentinized AR-non-ophiolitic websterite
Mineral atg atg atg atg atg atg atg chr chr Sample AR-54 AR-54 AR-54 AR-81 AR-81 AR-82 AR-82 AR-54 AR-81 Analysis 2-A38 5-A43 1-A23 2-A24 4 14 3-A39 1-A34 3-A25 SiO2 39.08 36.68 41.12 42.24 41.21 41.33 38.71 39.35 41.45 TiO2 0.00 0.11 0.10 0.00 0.13 0.03 0.01 0.00 0.09 Al2O3 1.96 6.17 0.44 0.53 0.13 0.19 1.55 1.29 0.54 Cr2O3 0.03 0.10 0.37 0.27 0.00 0.14 0.00 0.01 0.02 FeO 4.40 7.59 2.99 2.39 3.35 5.20 5.58 3.99 2.36 MnO 0.13 0.06 0.26 0.48 0.67 1.03 0.00 0.00 0.41 MgO 35.67 31.33 36.44 36.43 36.26 35.41 36.02 36.00 37.01 CaO 0.00 0.02 0.08 0.09 0.09 0.20 0.00 0.03 0.06 Na2O 0.00 0.01 0.07 0.13 0.06 0.13 0.04 0.00 0.02 K2O 0.09 0.04 0.00 0.00 0.07 0.00 0.00 0.08 0.08 Total 81.36 82.11 81.87 82.56 81.97 83.66 81.91 80.75 82.04
No of cations on the basis of 9 oxygen atoms Si 1.973 1.864 2.044 2.072 2.054 2.044 1.955 1.995 2.048 Al 0.117 0.369 0.026 0.031 0.008 0.011 0.092 0.077 0.031 Ti 0.000 0.004 0.004 0.000 0.005 0.001 0.000 0.000 0.003 Fe2+ 0.186 0.323 0.124 0.098 0.140 0.215 0.236 0.169 0.098 Cr 0.001 0.004 0.015 0.010 0.000 0.005 0.000 0.000 0.001 Mn 0.006 0.003 0.011 0.020 0.028 0.043 0.000 0.000 0.017 Mg 2.684 2.373 2.701 2.665 2.694 2.610 2.712 2.721 2.727 Ca 0.000 0.001 0.004 0.005 0.005 0.011 0.000 0.002 0.003 Na 0.000 0.001 0.007 0.012 0.006 0.012 0.004 0.000 0.002 K 0.006 0.006 0.000 0.000 0.004 0.000 0.000 0.005 0.005
Ta Mineral amph amph amph amph amph Mineral chl chl chl chl chl chl Sample AR-81 AR-81 AR-82 AR-82 AR-82 Sample AR-54 AR-55 AR-55 AR-82 AR-82 AR-82 Analysis 2 4 7 12 14 Analysis 1-A37 3-A42 4-A44 1 3 4 SiO2 57.38 57.83 54.62 53.11 57.01 SiO2 29.85 30.21 30.22 32.03 33.14 31.95 TiO2 0.00 0.15 0.00 0.00 0.04 TiO2 0.09 0.19 0.26 0.02 0.07 0.08 Al2O3 0.79 0.26 1.68 0.08 0.49 Al2O3 20.65 20.45 20.52 14.63 13.99 14.97 FeO 2.00 2.08 2.00 0.79 1.29 Cr2O3 0.13 0.17 0.04 Cr2O3 0.00 0.24 0.00 0.13 0.32 FeO 4.16 4.48 4.52 2.93 3.14 3.15 MnO 0.28 0.00 0.00 0.11 0.08 MnO 0.16 0.00 0.00 0.00 0.00 0.05 MgO 22.05 22.90 24.90 18.80 23.40 MgO 28.03 29.00 29.27 33.10 32.64 33.47 CaO 13.40 13.93 12.55 23.74 13.42 CaO 0.00 0.00 0.00 0.10 0.08 0.15 Na2O 0.32 0.22 0.43 0.03 0.16 Na2O 0.00 0.01 0.00 0.01 0.01 0.03 K2O 0.00 0.00 0.01 0.00 0.00 K2O 0.07 0.03 0.03 0.00 0.00 0.01 Total 96.22 97.61 96.19 96.79 96.21 Total 83.14 84.54 84.86 82.82 83.06 83.86 O atoms 23 23 23 23 23 O atoms 36 36 36 36 36 36 TSi 7.969 7.934 7.620 7.598 7.904 Si 5.888 5.871 5.852 6.321 6.512 6.245 TAl 0.031 0.042 0.276 0.013 0.080 AlIV 2.112 2.129 2.146 1.679 1.486 1.755 TTi 0.000 0.015 0.000 0.000 0.004 AlVI 2.685 2.551 2.531 1.721 1.749 1.691 CAl 0.098 0.000 0.000 0.000 0.000 Altot 4.797 4.680 4.677 3.400 3.235 3.446 CCr 0.000 0.026 0.000 0.015 0.035 Ti 0.013 0.028 0.038 0.003 0.010 0.012 CMg 4.565 4.684 5.000 4.010 4.837 Cr 0.020 0.026 0.006 0.000 0.000 0.000 CFe2 0.232 0.239 0.000 0.095 0.128 Mg 8.242 8.402 8.450 9.739 9.561 9.752 CMn 0.033 0.000 0.000 0.013 0.000 Fe2+ 0.686 0.728 0.732 0.484 0.516 0.514 CCa 0.071 0.052 0.000 0.868 0.000 Mn 0.027 0.000 0.000 0.000 0.000 0.008 BMg 0.000 0.000 0.178 0.000 0.000 Ca 0.000 0.000 0.000 0.021 0.017 0.032 BFe2 0.000 0.000 0.233 0.000 0.021 Na 0.000 0.004 0.000 0.004 0.003 0.011 BMn 0.000 0.000 0.000 0.000 0.009 K 0.018 0.007 0.007 0.000 0.000 0.002 BCa 1.923 1.996 1.588 2.000 1.969 Mg# 0.92 0.92 0.92 0.95 0.95 0.95 BNa 0.077 0.004 0.000 0.000 0.000 ACa 0.000 0.000 0.287 0.771 0.024 ANa 0.009 0.054 0.116 0.008 0.043 AK 0.000 0.000 0.002 0.000 0.000 atg (antigorite), chr (chrysotile), amph (amphibole), chl (chlorite)
OXYGEN AND HYDROGEN ISOTOPES IN SERPENTINES Overall, the δ18O values of the serpentines from the MG‐ophiolitic peridotites (lizardite and chrysotile) are higher than those from the serpentinized AR‐ ophiolitic peridotites (lizardite and chrysotile) and serpentines from the serpentinized AR‐non‐ophiolitic websterites (antigorite) (Table 3). The δ18O values in the serpentine minerals from the serpentinized MG‐peridotite (δ18O =7.39–7.78 ‰) are higher than those of the unaltered mantle (δ18O =5.4–5.8 ‰) (Kyser, 1987). Lizardite and chrysotile in the AR‐peridotites have δ18O values
116 M. M. HAMDY and E. M. LEBDA
varying from 5.1 to 5.72 ‰, whereas the antigorite in the AR‐websterite is enriched in 18O and has values ranging from 5.75 to 5.85 ‰. Hydrogen isotope concentration in serpentines varies widely from ‐108.5 to –74 ‰. The δD values in the serpentines from the AR and MG‐ ophiolitic peridotites have usually lower δD values (‐102.9 and ‐95.9 ‰ and ‐108.5 to ‐94.8 ‰ respectively) compared to the higher δD values (between ‐74 to ‐76.8 ‰) in the serpentines from the AR‐non‐ophiolitic websterite.
METAMORPHIC THERMOMETRY AND TEXTURAL RELATIONSHIPS Lizardite dominates in the serpentinized AR and MG‐ophiolitic peridotites indicating that the temperature of serpentinization did not exceed ~200 °C (O'Hanley, 1996; Evans, 2004). However, the recrystalliza on of lizardite to chryso le in some samples refers to higher temperatures up to 300 °C (O'Hanley, 1996). The abundance of non‐pseudomorphic textures in serpentinized MG‐peridotites compared to those from AR is an indication of higher temperature of serpentinization (O'Hanley and Wicks, 1995). The polygonal units of the hourglass texture and the penetrative fabric of the serrate veins in all serpentinized peridotites indicate that fracturing of these rocks was developed in a dynamic regime (Wicks and O'Hanley, 1988; Mével, 2003). Zoning in altered spinel grains, in the ophiolitic peridotites, from an aluminian chromite core through Fe3+‐rich aluminian chromite to ferritchromite intermediate zone followed by a Cr‐magnetite to magnetite outer zone of varied widths reflect variation in the degree of alteration. With increasing degree of alteration the size of the aluminian chromite core decreases whereas the widths of the intermediate Fe3+‐rich aluminian chromite to ferritchromite zone and the outer Cr‐magnetite to magnetite zones increase. Sack and Ghiorso (1991) showed that the Fe3+ in these alteration zones increases with increasing the temperature of their formation. In the partly serpentinized AR and MG‐ ophiolitic peridotites, narrow intermediate Fe3+‐rich aluminian chromite and outer Cr‐magnetite zones are developed. The compositions of the different zones in the chromite are plotted on the Fe3+‐Cr‐Al diagram (Fig. 5a) where they indicate temperatures of forma on of 400–450 ºC. The Fe3+‐rich aluminian chromite‐ferritchromite intermediate zone and the Cr‐magnetite zones are developed in the completely serpentinized peridotites. This corresponds to higher temperature of metamorphism up to 550 ºC. In talc‐bearing serpentinites from MG the magnetite rims become much wider surrounding lobate Fe3+‐rich aluminian chromite to ferritchromite intermediate zones. The composition of this zone corresponds to temperature of metamorphism of 500–550 ºC. The sulphide‐oxide assemblage (magnetite‐pentlandite‐pyrite) in the AR and MG serpentinized peridotites is stable at temperatures of ~200°C to 400 °C (Lorand, 1985) consistent with other temperature es mates. In the AR‐non‐ophioli c websterites chlorite formed at T between 176 and 204 ºC in Opx‐rich websterite and between 242 and 245 ºC in Cpx‐rich websterite (geothermometer of Cathelineau and Nieva, 1985 is used). Tremolite is assumed to have formed at temperatures ranging from 300 to 450 ºC (Bucher and Frey, 1994). Antigorite is suggested to have occurred at T between 300 and 500 ºC (Deer et al., 1992).
DISCUSSION The data presented above, indicate that all ultramafic rocks suffered progressive metamorphism. Two essential metamorphic events have occurred, which are represented by an earlier serpentinization followed by a later regional metamorphism. 1. Serpentinization 1.1. Source of serpentinization fluids Oxygen and hydrogen isotope data are used as tracers of fluid‐rock interaction during serpentinization of ultramafic rocks. Mantle peridotites that have not interacted with external fluids should have bulk rock oxygen isotope ratios within the range of δ18O = 5.4–5.8 ‰ (Kyser, 1987). If the rock interacted with a fluid its composition will largely depend on the isotopic composition of the interacting fluid and fluid fluxes as well as the temperature during serpen niza on. Submarine weathering and low temperature (< 225 ºC) hydrothermal alteration of the ultramafic rocks induce a general enrichment in 18O in the bulk rock, whereas higher temperature interactions deplete the rocks in 18O (Alt et al., 1986; Muehlenbachs, 1987; Früh‐Green et al., 2001). In conjunction with the δ18O values, the δD values can be used to determine the isotopic compositions of the
Metamorphism of Ultramafic Rocks at Gebel Arais and Gebel Malo Grim, ED, Egypt 117
Table 3. Oxygen and hydrogen isotopes of serpentines in AR and MG-ultramafic rocks Rock Sample δ18O ‰ δD ‰
AR-peridotite
AR-23 5.59 -97.80 AR-24 5.10 -100.90 AR-25 5.33 -102.30 AR-28 5.41 -102.50 AR-29 5.67 -98.74 AR-30 5.58 -100.73 AR-31 5.72 -100.18 AR-33 5.22 -95.90 AR-70 5.53 -99.14 AR-78 5.15 -96.10
MG-peridotite
MG-01 7.78 -99.76 MG-05 7.69 -100.24 MG-06 7.41 -97.14 MG-08 7.60 -94.80 MG-10 7.59 -102.47 MG-12 7.42 -96.74 MG-19 7.49 -98.96 MG-20 7.39 -95.10 MG-26 7.53 -100.81 MG-28 7.51 -108.50
AR-websterite
AR-53 5.81 -75.62 AR-54 5.85 -74.00 AR-80 5.77 -77.12 AR-81 5.79 -76.10 AR-82 5.75 -77.20
fluids causing the serpentinization. Figure 7 shows the oxygen and hydrogen isotope composi ons of the serpentine minerals in the studied ultramafic rocks compared with the compositions known to date of serpentinites from different tectonic environments in both marine and continental settings (Wenner and Taylor, 1973; Agrinier and Cannat, 1997; Kyser et al., 1999; Miller et al., 2001). Serpen nes from both AR and MG‐serpentinized ophiolitic peridotites have δ18O between 5.1 and 7.78 ‰ and δD between ‐108.5 and ‐94.8 ‰, i.e. lying within the field of ophiolitic lizardite and chrysotile, which overlaps considerably with the field of continental lizardite and chrysotile. In contrast, serpentine minerals in the AR‐non‐ophiolitic websterites are poorer in deuterium (δD between ‐74 and ‐77.2 ‰) and are plotted between the fields of the oceanic lizardite and chrysotile and continental lizardite and chrysotile. Thus it is concluded that the serpentinization of ophiolitic peridotites took place by fluids different from those that serpentinized the non‐ophiolitic websterites. The δ18O values of the serpentinizing fluids can be estimated using the dependency of the oxygen isotope fractionation between the serpentines and the serpentinizing fluids (δ18Oserpentine‐serpentinization fluids) on temperature (Zheng, 1993) (Fig. 8). The temperatures of serpen niza on (≤ 200 ºC for the dominant lizardite in the serpen nized ophioli c perido te and 300–500 ºC for the dominant antigorite in the serpentinized websterite) estimate δ18Oserpentine‐serpentinization fluids ≥ 2 ‰ for the lizardite in AR and MG‐serpentinized ophiolitic peridotites and between ‐1 and 1.5 ‰ for the antigorite in the AR‐serpentinized non‐ophiolitic websterite. By subtracting the δ18O values of the serpentines from the estimated δ18O serpentine‐serpentinization
fluids fractionation values, the δ18O of the serpentinization fluids was ≤ 3.72 ‰ for the serpentines in AR‐
118 M. M. HAMDY and E. M. LEBDA
Fig. 7. Oxygen and hydrogen isotope ratios of serpentine in the studied ultramafic rocks compared with the range of isotopic compositions of serpentines formed in oceanic and continental settings and those recorded in ophiolites (Wenner and Taylor, 1973; Agrinier and Cannat, 1997; Kyser et al., 1999; Miller et al., 2001). Symbols as in Fig. 2.
Fig. 8. Oxygen isotope fractionation curve for serpen ne (Zheng, 1993).
peridotites and ≤ 5.78 ‰ for the serpentines in the MG‐perido tes but lies between 6.75 and 6.85 ‰ for the serpentines in the AR‐non‐ophiolitic websterite. The estimated δ18O values of the serpentinizing fluids in both ophiolitic and non‐ophiolitic ultramafic occurrences as well as the δD values of serpentine minerals are consistent with the δ18O and δD values for the metamorphic and/or hydrothermal waters given by Sheppard (1986), Pultlitz et al. (2000) and Hoefs (2003). The metamorphic and/or hydrothermal water of serpen niza on of the AR and MG‐ophiolitic peridotites could be released from the surrounding rocks during their metamorphism or from the surrounding magmatic activity (Fig. 9). This must have acted most probably after obduction of the rocks in particular at their shear zone. Inconsistency of the isotopic composition of antigorite in the non‐ophiolitic websterite with the compositions of both oceanic and continental serpentines makes the source of serpentinization water of these non‐ophiolitic rocks is still of debate. The suggested tectonic setting of these non‐ophiolitic ultramafic rocks as the continental mantle wedge over the subducted oceanic lithosphere might explain this. The water of serpentinization could be a mixture of oceanic water released from the subducted oceanic crust due to devolatization and continental magmatic water in the mantle wedge. This water hydrated the ultramafic rocks forming antigorite with D composition in between the D compositions in oceanic and the continental serpentines. 1.2. Timing of fluid uptake As shown the ophiolitic peridotites at AR and MG were mostly subjected to serpentinization by metamorphic and/or hydrothermal fluids taking place after their obduction. The obduction of the ophiolitic ultramafic rocks was followed by the metamorphic event in the East African Orogen (Moghazi, 2002). Deformation due to the obduction helped in the infiltration of the metamorphic and/or hydrothermal fluids that caused serpentinization. This could feasibly occur along the tectonic continental crust‐oceanic mantle contact at the Pan‐African ophiolite of the Eastern Desert. The AR‐non‐ophiolitic websterite was mostly serpentinized at a different time by different water. If the sources of water PT conditions and the timing of serpentinization of the two types of the AR‐ultramafic rocks are the same, the serpentine minerals and their isotopic compositions should be similar. It is probable that the serpentinization of the non‐ophiolitic websterite took place at another time. It is worth to say that the websterite might serpentinized before its emplacement in the ophiolitic peridotite? Its serpentinization might take place probably during its up welling and cooling. The temperature of serpentinization was higher and the source of water was deeper comparing with the ophiolitic peridotites.
Metamorphism of Ultramafic Rocks at Gebel Arais and Gebel Malo Grim, ED, Egypt 119
2. Regional Metamorphism 2.1. Prograde metamorphism of the serpentinized ophiolitic peridotites The lower temperatures of serpentinization of the AR and MG‐ophiolitic peridotites led to formation of lizardite pseudomorphs after olivine and orthopyroxene. Increasing temperature with regional metamorphism caused recrystallization of lizardite in non‐pseudomorphic textures. During this regional metamorphism aluminian chromite grains are altered to Fe3+‐rich aluminian chromite, ferritchromite and Cr‐magnetite. Talc and anthophyllite formed in MG‐rocks. The alteration of aluminian chromite leads to the libera on of the Al, Mg and Ti (Abzalov, 1998; Barnes, 2000; Proenza et al., 2004). According to Barnes (2000), the Fe3+‐rich aluminian chromite‐ferritchromite zone were formed by reactions between the aluminian chromite grains and surrounding magnetite rims, where its size increases by increasing grade of metamorphism. In the AR and MG‐partly serpentinized rocks containing lizardite pseudomorphs, the thin Fe3+‐rich aluminian chromite and Cr‐magnetite zones were formed under the PT conditions up to the lower greenschist facies. As the degree of serpentinization increases and the pseudomorphic textures vanish chrysotile crystallizes and the ferritchromite intermediate zone is developed. Here, the conditions correspond to the upper greenschist facies (Fig. 10). In the serpentinized talc‐bearing MG‐peridotites, the chromite grains have wider Fe3+‐rich aluminian chromite, ferritchromite and magnetite zones separated by curved and lobate phase boundaries. In this case the PT conditions correspond to that of the upper greenschist to the transitional greenschist‐amphibolite facies. The metamorphism of the AR and MG ophiolitic peridotite rocks (up to the transitional greenschist‐amphibolite facies) are similar to those determined for many metamorphosed ultramafic rocks in the Eastern Desert (Ghoneim and Aly, 1986; Azer and Khalil, 2005). Ultramafic bodies in orogenic belts are commonly metamorphosed to the same grade as the surrounding rocks (Winter, 2001) and are described by Evans (2004) as ‘‘isofacial’’. The metamorphic facies that prevailed during the evolution of the Egyptian Neoproterozoic basement complex was up to the transitional greenschist‐amphibolite facies (Azer and Khalil, 2005). Thus most of the studied ultramafic rocks are isofacial with the Neoproterozoic basement rocks in the Eastern Desert. However, our data imply that the studied AR‐serpentinite rocks have presumably suffered only the primary hydrothermal metamorphism but escaped the later regional one. 2.2. Retrograde metamorphism of the AR-serpentinized non-ophiolitic (intrusive-affiliated) websterite The textural relationships between the metamorphic minerals in the AR‐non‐ophiolitic websterite that could be indications for the pathways of metamorphism are lack. However, the suggested origin as affiliated to the continental mantle‐derived intrusive ultramafic rocks proposes a probable retrograde metamorphic pathway, as the metamorphism could be the result of the rock cooling. The serpentinization and formation of antigorite at T < 500 ºC was the peak of metamorphism. The rock transferred from the transitional greenschist‐amphibolite facies to the lower to upper greenschist facies by forma on of tremolite at T=300‐400 ºC. Chlorite formed at temperatures between 176 and 245 ºC in the lowest greenschist to the greenschist facies (Bucher and Frey, 1994). During this stage some an gorite recrystallized to lizardite and chryso le.
CONCLUSIONS The Neoproterozoic metamorphosed ultramafic rocks at G. Arais‐AR (ophiolitic spinel peridotite and non‐ophiolitic (intrusive‐affiliated) olivine websterite) and at G. Malo Grim‐MG (ophiolitic spinel peridotite) south Eastern Desert of Egypt show distinctly variation in sources of fluids, PT conditions and pathways of metamorphism. It is concluded that after the obduction of the ophiolitic peridotites on the infrastructural Pre‐Pan‐African continental rocks, the peridotites were serpentinized by metamorphic and/or hydrothermal waters with δ18O ≤ 3.72 ‰ for AR‐rocks and δ18O ≤ 5.78 ‰ for MG‐rocks. Serpentinization of the ophiolitic peridotites formed D‐enriched lizardite (δD from ‐102.9 to 95.9 ‰ in AR and from ‐108.5 to ‐94.8 ‰ in MG). The temperature of the water infiltrating the MG‐rocks was higher than that infiltrating the AR‐rocks. The obducted ophiolitic ultramafic rocks (particularly the MG‐peridotites) underwent progressive metamorphism (regional) causing alteration of aluminian chromite to intermediate Fe3+‐rich aluminian chromite‐ferritchromite zone and Cr‐magnetite outer zone and formation of talc and anthophyllite. This transferred the peridotites from the lowest greenschist facies to the transitional greenschist‐amphibolite facies (in the MG) at T=500–550 ºC. The AR‐non‐ophiolitic olivine websterite was probably serpentinized before its emplacement in the obducted ophiolitic peridotite rocks. Serpentinization was at T< 500 ºC forming antigorite. It happened most probably by a water (δ18O = 6.75 – 6.85 ‰) mixture of an oceanic water released from the subducted oceanic
120 M. M. HAMDY and E. M. LEBDA
crust and a magmatic water from the continental mantle wedge. By progressive cooling the rock suffered retrograde metamorphism. This accompanied by formation of tremolite and chlorite and partial recrystallization of antigorite to lizardite in the lowest greenschist to the greenschist facies.
ACKNOWLEDGMENT The authors are greatly indebted to Prof. M. Ghoneim for his fruitful discussions and valuable comments. This paper has benefited from constructive reviews of Prof. S. El Gaby and Prof. Z. Abdel Kader.
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124 M. M. HAMDY and E. M. LEBDA
براهين من التركيب المعدني : مصر-تحول الصخور الفوق مافية بجبل عرايس وجبل وملوجريم ،الصحراء الشرقية ونظائر األوكسجين والهيدروجين
**المتولى محمد لبدة و *محمد محمود حمدى
،مصر ٣١٥٢٧م الجيولوجيا، كلية العلوم، جامعة طنطا، طنطا قس * قسم العلوم البيولوجية والجيولوجية، كلية التربية، جامعة كفر الشيخ، كفر الشيخ، مصر **
نظائر األكسجين والهيدروجين لمعادن السربنتين وكذا كيمياء المعادن المتحولة بالصخور الفوق مافية يتناول هذا البحث دراسة
إلى التعـرف علـى مصـدر سـوائل ةتهدف هذه الدراسو ٠بجبل عرايس وجبل ملو جريم بجنوب الصحراء الشرقية المصريةالصخور موضـوع الدراسـة مـن صـخور وتتكون ٠السربنتنة ومتى حدثت عملية السربنتنة بالنسبة لعمليات التحول التالية
بجبل عرايس و جبل ملو جريم وصخور أوليفين ويبستريت غير ) يتسبينل ليرزول –سببنل هارزبرجيت (بيريدوتيت أوفيوليتية ٠بجبل عرايس فقط) تشابه الصخور الفوق مافية المتداخلة(أوفيوليتية
يعتقد أن مصادر سوائل السربنتنة وكذا درجات الحرارة واألوضاع التكتونية ومسارات التحول اإلقليمي التالية مختلفة بين نوعى
فصخور البيريدوتيت غالبا تمت سربنتنتها بعد إنزالقها على صخور البنية األساسـية . ة التى تمت دراستهاالصخور الفوق مافيلصـخور عـرايس ‰ ٣.٧٢≤ δ18O لـه (القارية وخاصة فى مناطق التماس بينهما وذلك بماء قارى متحول أو حرمـائي
،≥δ18O له (نه األساسى ليزرديت وقد أدى هذا لتكوين سربنتين معد). لصخور ملو جرايم ‰ ٥.٧٨δD ١٠٨.٥-تتراوح من وقد عانت هذه الصخور بعد هذا من تحول إقليمي تقدمى أدى إلعادة تبلور معادن السربنتين فى أنسجة غيـر ) ‰٩٤.٨ –إلي
ي والماجنيتيت كاذبة بدال من األنسجة الكاذبة، تحلل الكروميت األلوميني إلي كروميت األلوميني غني بالحديديك وكروميت الفيريتوبهذا إنتقلت الصخور من سحنة تحول الشيست األخضر ٠الكرومي كذا تكوين التلك واألنثوفيليت وخاصة في صخور ملو جريم
أما على الجانب اآلخر فصـخور األوليفـين ٠السفلية إلي السحنة اإلنتقالية بين سحنتي الشيست األخضر واألمفيبواليت للتحولحيث تم هـذا فـى الوتـد . ربنتنتها قبل تداخلها في صخور البريدوتيت األوفيوليتية وذلك أثناء تبريدهاويبستريت فغالبا تمت س
كان خليطـا ) ‰ ٦.٨٥-٦.٧٥ δ18O = له (ويعتقد أن ماء السربنتنة ٠الوشاحي القاري الواقع فوق اللوح المحيطي المندثروسـربنتنة ٠ا وإنصهارها وماء مجماتى قارى من الوتد الوشاحىمن ماء محيطي نتج من القشرة المحيطية المندثرة بعد تحوله
مقارنة بصخور البيريدوتيت األوفيوليتية حيث تكون األنتيجويت ) م٥٠٠ºإلي ٣٠٠(هذه الصخور تمت في درجات حرارة أعلى رة الصخرة المسـربنتنة وبإنخفاض درجة حرا ٠فقيرا فى محتواه من نظير الديوتيريوم مقارنة بالليزرديت بصخور البيريدوتيت
) م ٢٤٥ºالـى ١٧٦عنـد حـرارة تراوحـت مـن (فهى غالبا تعرضت لتحول إقليميى تراجعي مكونة التريمواليت والكلوريت ٠والليزرديت على حساب األنتيجوريت وذلك فى سحنة الشيست األخضر السفلية للتحول