Compositional Variations and Tectonic Settings of Podiform Chromitites Associated with Ultramafic Rocks of the Pan-African Proterozoic Ophiolites from South Eastern Desert, Egypt

摘要

The Precambrian podiform chromitites associated with ophiolites are abundant in Pan-African belt in central Eastern Desert(CED)and south Eastern Desert(SED),Egypt and range from 690 to 890 Ma in age.The studied chromitites associated with Neoproterozoic ophiolites are distributed in southern Eastern Desert,Egypt in Baranis-Shalaten sheet and occur as lenticular bodies with variable dimensions in ultramafic component(serpentinites).We present geochemical and mineralogical data from three areas of ophiolites and associated chromitites namely Gebel Abu Dahr(D),Gebel Arais(A)and Gebel Anbat in the Wadi Hodein area(H)(Fig.1).The paper studies the compositional variations and tectonic settings of podiform chromitites associated with ultramafic rocks,in addition to the alteration process of chromite during metamorphism.The ophiolite in the present areas is composed of the ultramafic rocks(mainly serpentinites)with minor relics of fresh dunite and harzburgite.All these rocks are affected by metamorphism and subsequent retrograde during subduction and exhumation.Six samples selected from the serpentinites geochemically analyzed for major,trace and some REE elements and the geochemical results reflect that harzburgite and dunite compositions are typical of depleted mantle peridotite.Microprobe analyses and SIMS investigations were carried out for three massive podiform chromitite ore bodies and disseminated chromites in serpentinites(1215 spot probe analyses),and silicate minerals in serpentinite rocks such as serpentine and olivine(102 spots).Serpentine minerals are mainly antigorite with some chrysotile in serpentinite rocks and in chromitites,mainly filling cross-cutting veins.In this study,we consider that the alteration occurred in two stages:during the first one chromite reacted with olivine and water to form Cr-and Fe-rich,porous chromite and chlorite;during the second event magnetite filled the pores,created in the porous chromite and defused into this chromite to form homogeneous magnetite.According to this,the composition of chromite is a key factor controlling the metamorphic reaction between olivine and chromite because if the primary chromite is very poor in Al,the chlorite-forming reaction hardly takes place.In this case,during the second event,the addition of magnetite only contributes to create a magnetite corona around the former chromite grains without any diffusion at the chromite-magnetite boundary as suggested by Gerbilla et al.(2012).Barnes(2000)studied the chromite in komatiites and modification during green schist to mid amphibolite facies metamorphism.He suggested that the chromite cores continually equilibrated with magnetite rims document metamorphic grade conditions.Barnes(2000)suggested that the relative proportions of Cr3+,Al3+and Fe3+of chromite are not affected by metamorphism up to lower temperature amphibolite facies implying restricted mobility of these elements occurred under lower amphibolite facies.So,the chromite in lower temperature amphibolite facies preserves its primary igneous chemistry and can be used to estimate the metamorphic grade.Sack and Ghiorso(1991)and Barnes(2000)suggested that all chromite cores are equilibrated at temperature below^500–550℃corresponding to lowest amphibolite facies metamorphism and reflect magmatic composition not influenced by metamorphism.In this study,there is no alteration but only nearly pure magnetite deposition and development with restricted Cr-solubility at<500℃in the chromite rims on crystal boundaries and within fractures as shown in Fig 2a,b.Also magnetite alters later to hematite.The podiform chromitites are common in the Moho transition zone(MTZ)to the mantle section of ophiolites or harzburgite dominant peridotite massifs(e.g.,Arai,1997;Miura et al.,2012).They have been interpreted as a product of peridotite/melt reaction and subsequent melt mixing within the MTZ to the upper mantle;they are basically magmatic cumulates that formed at the upper mantle level(e.g.,Arai and Yurimoto,1994;Zhou et al.,1994).They are thus a good marker of peridotite/melt reaction(e.g.,Arai,1997).The Pan-African podiform chromitites mayh ave formed in the same way as the Phanerozoic,namely by melt-harzburgite reaction and subsequent melt mixing.The podiform chromatites and disseminated chromites are high-Cr chromites and have range in Cr#(Cr/Cr+Al)atomic ratio from 0.75 to 0.95 and low Ti with boninitic affinity(Fig.3a),indicating an island arc setting in supra-subduction zone setting.The present massive chromitites and disseminated chromites in serpentinites fall in the field of chromites de Bou Azer,chromites de Cordoba,Argentinia in the Cr#versus Mg#diagram(Fig.3 b,c)(Gervilla et al.,2012)The studied chromatites contain some grains of platinumgroup minerals(PGM)such as sulfides(Os-rich laurite)and Os–Ir alloy as shown in Fig.4 and as reported in South Eastern Desert by Ahmed(2007).