GEOCHEMISTRY , URANIUM , THORIUM AND RARE EARTH ELEMENTS OF TRACHYTE DYKES OF UMM SALATIT MOUNTAIN AREA , CENTRAL EASTERN DESERT , EGYPT

Mohamed Mahmoud Ghoneim1,2 moh.gho@mail.ru Saint Petersburg State University Saint Petersburg, Russia Gehad Mohamed Saleh2, drgehad_m@yahoo.com Maher Dawoud Dawoud3, dawoud_99@yahoo.com Mohamed Saleh Azab2, mohamedazab68@yahoo.com Mahmoud Ahmed Mohamed2, hoodageo@yahoo.com Hamdy Ahmed Awad4,5 hamdiawaad@gmail.com 1Saint Petersburg University, Saint Petersburg, Russia 2Nuclear Materials Authority, Cairo, Egypt 3Minufiya University, Faculty of Science, Geology Department, Minufiya, Egypt 4South Federal University Rostov on Don, Russia 5Al-Azhar University Assiut, Egypt


I ntroduction
Umm Salatit Mountain area is a part of the Central Eastern Desert of Egypt.It is bounded by longitudes 33°45' and 33°60' E and latitudes 25°10' and 25°20' N, covering an area of 450 km 2 .The area is accessible from Idfu-Marsa Alam road.It is traversed by the main desert road of Barramiya-Gabal El Rukham originating southward from the main road of Idfu-Marsa Alam.It is composed of ophiolitic mélange, older granitoids, biotite granites, muscovite granites and post granitic dykes and veins (Fig. 1).
A lot of researches has been done about ophiolitic rocks, ophiolitic mélange and granitoid rocks but no one study in details the volcanic trachyte dykes which cut across muscovite granites and extend through several meters.
Methodology 1 -Field work is accomplished construction of geological map with the structural features, scale 1 : 50,000 based on mosaic and field observations.It is also comprised the collection of the rock specimens from the different rock units through cross profiles covering the study area.Moreover, Spectrometric measurements of the different rock units using a portable gamma-ray spectrometer RS-230. 2 -Fifteen samples of trachyte dykes were chosen for preparation of thin sections for petrographic studies.The microscopic studies and photomicrographs were carried out using a Nikon (Optiphot-Pol) polarizing microscope equipped with a full automatic photomicrographic attachment (Microflex AFX-II). 3 -Representative seven samples trachyte dykes are selected for analyses of trace elements, REEs concentrations.These were prepared for complete chemical analysis by fused tablets with LiBO 2 and HNO 3 dissolution in the laboratory of ACME Analytical Laboratories (Vancouver) Ltd, Canada using ICP-MS.The major elements were analyzed by using an Atomic Emission Spectroscopy (AES) coupled with Inductively Plasma (IP) source.
Geological setting and petrography of trachyte dykes Trachyte dykes are the most abundant dyke rocks in the area.These dykes show variable trends to N-S and NNE-SSW trends.They are usually fine-grained, buff, pale brown to red, deep red and reddish brown in colour due to the degree of ferrugination.These dykes are altered, massive and range in thickness from 10 m to 20 m.They cut younger granitoids.Macroscopically, Trachyte dykes are fine to medium grained of buff and pale pink to red in colour.Microscopically, consist mainly of K-feldspar with relatively minor amount of plagioclase, iron oxides, quartz and biotite.Secondary minerals are represented by sericite, muscovite, chlorite, carbonates and epidote.Accessory minerals are represented by opaque minerals.
K-feldspar occurs as phenocrystals from orthoclase and perthite sometimes with twining in a matrix of anorthoclase and sanidine (Fig. 2, a, b and c).Sanidine feldspar, very commonly occurs in two generations, i. e. both as large well-shaped porphyritic crystals and smaller imperfect rods or laths forming a finely crystalline groundmass.
K-feldspars may occur as subparallel fluxional orientation forming trachytic texture or as randomly oriented crystals forming spherulitic texture (Fig. 2, d and e).Plagioclase is rare and form euhedral to subhedral crystals 0.5 × 3.5 mm in phenocryst and 0.3 × 0.8 in the groundmass.It ranges in composition from oligoclase to andesite (An 13-38 ) shows glomero-porphyritic texture (Fig. 2, f) and altered to sassurite and calcite.They occur as patches distributed through the rocks.Quartz occurs as irregular anhedral crystals with corroded plagioclase and biotite and intergrowth with K-feldspar forming graphic texture.It may be also enclosed in biotite.Amygdals are present and filled by carbonates.According to [3], the term (granophyric) implies a graphic intergrowth of quartz and either K-feldspar or Na-rich feldspar, rather than a quartz and alkali-feldspar solid solution, although [4] regarded the term as the irregular, finer-grained counter part of the graphic intergrowth.It may be due to simultaneous crystallization of quartz and alkali feldspars.Biotite occurs as subhedral to euhedral flakes with one set of cleavage and strongly pleochroic from brown to dark brown in colour.It is subjected severe alteration to chlorite, muscovite, iron oxides and epidote.Carbonates occur as anhedral crystals or as fine grained.Opaque minerals occur as anhedral crystals in the fine-grained groundmass with K-feldspar, plagioclase and quartz.
Geochemistry of trachyte dykes.Geochemical Nomenclature The geochemical nomenclature of the trachyte is studied by many workers elsewhere using the variation in the major oxides (wt.%) and trace elements (ppm).
The contents of the major oxides, trace elements in the analyzed trachyte dykes as well as CIPW norms are given in (Table 1).The average chemical composition of the studied trachyte dykes when compared with world trachyte dykes [6] and trachyte of Umm Shaghir CED, Egypt [7] 2).The relative concentrations of trace elements in the studied rocks are shown in the form of a primitive mantle-normalized diagram (Fig. 4) based on the normalized factors of [10].In general, all rocks show moderate enrichment of most large ion lithophile elements (LILE) and (HFSE) elements and depletion of P, Ti and K.The depletion of Ti and p is ascribed to fractionation of titanomagnetite and apatite.
Magma Type Several discrimination diagrams were proposed and used to distinguish and elucidate the magma types of the volcanic rocks.
The trahyte dykes can be subdivided generally into two major subgroups: alkali rock series and sub-alkalic (non alkali) rock series.Using [11], the dividing curve which is considered to give a better separation between alkalic and sub-alkalic rocks for general studies.It is evident that the trachyte samples fall entirely in the alkaline field (Fig. 5).AFM diagram originally constructed by [12] is commonly used for identifying iron depletion and iron enrichment trends characteristic of calc-alkalic and tholeiitic series, respectively [13].The analyzed samples were plotted on the AFM diagram using the dividing line suggested by [11] to separate tholeiitic and calc-alkaline composition.All the studied samples fall within the calc-alkalic field with 1971.Compressional and tensional trends (after [14] (Fig. 6, a).
Tectonic Setting An attempt is made here to identify the tectonic environment of the studied trachyte based on the presently popular discrimination diagrams using major and trace elements data [15] constructed (Al 2 O 3 /TiO 2 ) versus TiO 2 variation diagram (Fig. 6, b).The analysed trachyte samples show complete spectra of values of the completely volcanic suite that fall along the same trend.This may suggest that samples were derived from the same magma source by fractionation processes with high TiO 2 magma being the early fractionated while the lower TiO 2 magma is produced from more fractionated varieties.The relation between the SiO 2 and Ba/Nb ratio (Fig. 6, c) shows a decrease of the Ba/Nb ratio as a function of differentiation in the studied trachyte.This indicating that the high Ba/Nb in the most primitive rocks is a characteristic feature inherited from their source.

Rare Earth Elements (REEs) geochemistry
The distribution of europium (Eu) is particularly important because it can occur in two oxidation states, divalent (EuP +2P ) and trivalent (EuP +3P ) depending on oxygen fugacity.Under oxidizing conditions, Eu is trivalent and behaves as the other rare earth elements, while under reducing conditions, it occurs as divalent EuP +2P , which has relatively large ionic radius and replace mainly CaP +2P in plagioclases and rarely KP +P in K-feldspar.The average normalized REE patterns of Umm Salatit trachyte dykes display low to moderate fractionated REE pattern (Table 3), (Fig. 7) relative to chondritic values from [16] where the averages (La/Yb) n = 16.8 and have marked enrichment of the average of ∑LREE (324.6)relative to the average of ∑HREEs (26.4)where the averages (La/Sm) n = 5.6.trachyte dykes have moderate negative Eu anomaly (Eu/Eu * = 0.5), this may reflect the plagioclase and K-feldespar fractionation or due to the greater effect and higher oxygen activity of the melt, which is relates to volatile saturation (higher oxidation state) in case of the melt that formed the melt.The oxygen activity of the melt would be sufficiently high to keep Eu at the trivalent state and thus keep its incorporation into accumulating plagioclase [17].
The dose rate in the trachyte dykes ranges from 0.5 to 1.5 with an average of 1.2 (m Sv/y).The radiometric data of the radioelements for them show a wide variation in eU and eTh contents.The eU content ranges from 2 to 14 ppm with an average of 6.6 ppm and the eTh content ranges from 4 to 37 ppm with an average of 18.03 ppm.The potassium content is ranging between 1.9 % and 5.1% with an average of 3.4%.

Distribution of Radioelements in Trachyte Dykes
The relationship between eU with eTh, eTh with K%, the eU with eU/eTh contents among the trachyte dykes reflect a direct relation means the eU/eTh ratio tends to increase with uranium mobilization and post magmatic redistribution in trachyte dykes (Fig. 8).
The correlation matrix between U, Th (chemically) and some major, traces and rare earth elements For trachyte dykes both U and Th correlate similarly with other major, trace and rare earth elements, reflecting their geochemical coherence during the crystallization of the magma (Fig. 10-12).2. Geochemically, the investigated trachyte dykes were originated from an alkali magma rich in total alkalis, and the tectonic setting is continental basalt.Trachyte dykes have steep LREEs, nearly flat HREEs and a negative Eu anomaly.The negative Eu anomaly is either due to the partitioning of Eu into feldspar during fractionation, which is an important process in developing alkalinity, or the presence of residual feldspar in the source.Another alternative explanation for the negative Eu anomaly is based on the high oxygen fugacity in the melt due to volatile saturation.
3. The dose rate in the trachyte dykes ranges from 0.5 to 1.5 with an average of 1.2 (m Sv/y).The radiometric data of the radioelements for them show a wide variation in eU and eTh contents.The eU content ranges from 2 to 14 ppm with an average of 6.6 ppm and the eTh content ranges from 4 to 37 ppm with an average of 18.03 ppm.
4. Both U and Th correlate similarly with other major and trace elements, their geochemical coherence during the crystallization of the magma.
they show increasing in SiO 2 , Fe 2 O 3 , MgO, Na 2 O and CaO but decreasing in Al 2 O 3 , when compared with Average of Um Domi trachytes [8] they show high SiO 2 , Na 2 O and Al 2 O 3 contents and when compared with average of trachyte of G. El Ghorfa area, SED, Egypt [9] they show high SiO 2 and Al 2 O 3 contents but low Fe 2 O 3 and P 2 O 5 contents (Table

Conclusion 1 .
Macroscopically, Trachyte dykes are fine to medium grained of buff and pale pink to red in colour.Microscopically, consist mainly of K-feldspar with relatively few plagioclase, iron oxides, quartz and biotite.Secondary minerals are represented by sericite, muscovite, chlorite, carbonates and epidote.Accessory minerals are represented by zircon and opaque minerals.