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Науки о Земле _Earth sciences_
УДК 25.00.04 https://doi.org/10.21440/2307-2091-2021-2-7-15
Geological and Tectonic Setting of Andesitic Rock in Central Eastern Desert, Egypt
Hamdy Ahmed Mohamed AWAD12' Ibrahim Abu El-Leil ALI 2 Aleksey Valer'evich NASTAVKIN1 Abdellah Sadek TOLBA2 Mostafa Kamel ABDEL GHANI 2 Musab Awad Ahmed HASSAN 3 Mohamed Mahmoud Fathy GHONEIM 14 Ahmed El Sayed Abdel GAWAD 4
1Southern Federal University, Rostov-on-Don, Russia 2Al-Azhar University, Cairo, Egypt
3Peoples' Friendship University of Russia (RUDN University), Moscow, Russia "Nuclear Materials Authority, Cairo, Egypt
Abstract
Objective. The current study aims to detect the geologic features, geochemical characteristics and tectonic setting of the investigated rock using field observations and geochemical analyses.
Research methods. This work contains both field work (Collection samples and drawing of a new geological map) and laboratory work (preparation of thin sections for petrographic studies by polarizing microscope), X-ray Fluorescence analysis (XRF) in Institute of Biology, Southern Federal University and Mass-Spectrometer with Inductively Coupled Plasma (ICPMS) at the central Laboratory of Russian Geological Institute.
Result. Investigated andesitic rock belongs to Dokhan volcanic that located in the Central Eastern Desert of Egypt a long Qena-Safaga Road. It is considered as one of the most important shear zones in Eastern Desert that includes distinctive rocks and economic mineral deposits. The investigated rock belongs to late to post tectonic magmatism of the East African Orogeny (EAO). Petrographically: Dokhan volcanic is represented by andesite according to petrographical studies. It consists of plagioclase, quartz, in addition to mafic minerals. Geochemically, the investigated andesite samples plotted in calk-alkaline nature.
Conclusion. Tectonically, andesite samples fall in arc lava and continental fields. They are enriched in Ba, Sr, Rb, K, Nb and Ce with marked depletion in the most HFSEs like those of island arc calc-alkaline series.
Keywords: Tectonic setting, Andesite, Eastern Desert, Egypt.
Introduction
The investigated rock is restricted in Qena-Safaga road in the Central Eastern Desert (CED) of Egypt as shown in (fig. 1). It occurs in the Arabian Nubian Shield (ANS) and is made up of arc-inter-arc rock associations, which are represented by dismembered ophiolitic slabs thrusted over the deformed arc terrane. Generally, volcanic rocks belong to the (EAO) of Neoproterozoic rocks that related to the (ANS), are divided into three groups. First and second groups are considered as the oldest rocks that are classified into metavolcanics and younger metavolcanics, occasionally they represent in the central and southern Eastern Desert. They are characterized by mafic to intermediate with subordinate felsic rocks. Older metavolcanic rocks have low-K tholeiitic, while younger metavolcanics rocks contain low- to medium-K calc-alkaline accompanied with subduction-related processes (back-arc basins and island arcs) [1]. Otherwise, the third type includes
Hhamdiawaad@gmail.com
Dokhan volcanics and occur mainly in the northeastern desert and southern Sinai [2, 3] stated that the Hammamate sediments are younger than Dokhan volcanic, while other authors mentioned that the Hammamate sediments were associated with eruption of the Dokhan volcanic as reported by [4]. According to Rb-Sr ages; Dokhan volcanics have 610560 Ma, 600-585 Ma for the Hammamate sediments, and younger granites have 610-550 Ma [5]. Dokhan volcanics recognize varicolored and thick sequence of lava flows with their pyroclastics and represented by basalt, basaltic andesite, andesite, rhyolite, rhyodacite and dacite related to their mineral composition (Basta et al., 1980; Stern, Gottfried 1986). Numerous workers have been studied area along Qena-Safaga road [6-12]. The examined rock units are represented by Island Arc/back-arc assemblage (Oceanic crust terrain), late to post orogenic (Continental crust terrain) [13].
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Figure 1. Image shows the location map of the study area Рисунок 1. На рисунке показано местоположение изучаемой области
Materials and methods
Elements were determined using ICP-MS technique, after digestion of the fused beads with HF + HNO3. Pure solution external standers were used for calibration. Elements were determined by ELAN-DRC-6100 ICP-MS at the central Laboratory of Russian Geological Institute. Also, the major oxides and some trace elements were measured by X-ray Fluorescence analysis (XRF) in (Institute of Biology, Southern Federal University). Thin sections were investigated under microscope and the modal analyses have been calculated at Faculty of Sciences, Al-Azhar University.
Geological seating
It represents about 6% of exposed rock units and restricted in the southeastern part of the mapped area (fig. 2), it is located along the asphaltic road of Qena-Saf-aga near of Safaga city. Andesite is recognized by imperial porphyry (fig. 3, a), medium to coarse-grained massive rock, reddish in color and highly jointed in NNE-SSW directions. It has high peaks, jointed with predominant trending NNE-SSW, casing the columnar like huge plugs. It is intruded directly by granitic porphyry at the asphal-tic road found in (fig. 3, b).
Petrographical studies
According to QAP classification diagram of volcanic rocks of [14], it has been andesite in composition (fig. 4, a). It occurs as fine to medium grained rocks of gray to black color. Andesite is composed essentially of plagioclase, hornblende and biotite, with few amounts of quartz and alkali felspar. Iron oxides are the accessory
minerals. Sometimes it is characterized by porphyritic texture (fig. 4, b). Plagioclase: It occurs as andesine composition (An30-A40), covering an area up to 59% of main rock constituents. It is represented by euhedral to subhedral crystals, up to 1.6 mm in length and 0.8 mm in width, often showing albite-Carlsbad twins (figs. 4, b, c) and zonation. Sometimes, it is partly altered to sericite. Quartz: It covers an area up to 11% of rock constituents, occurs as fine-grained anhedral subordinate crystals and distributed among other mineral constituents. Hornblende: It is represented by sub-hedral to anhedral crystals, covers an area ranging from 11% to 16% of mineral compositions of rock. It is green in color pleochroic from green (=X) to pale brown (=Y) and brown (=Z) (fig. 4, c), it is partly or completely altered to chlorite. Biotite: It occurs as flaky crystals, up to 0.4 mm in length and 0.2 mm in width. Biotite covers an area ranging from 10% to 14% ofrock compositions (fig. 4, b), sometimes, it is altered to chlorite. Alkali felspar: It occurs as fine to medium grained, up to 0.04 mm in length and 0.02 mm in width. It covers an area up to 3% of rock compositions (fig. 4, b), sometimes; it is represented by microcline and orthoclase crystals associated with other minerals of rock. Iron oxides: They are represented by deep black grains scattering over the rock mineral constituents and disseminated within the different minerals (fig. 4, b). They cover an area up to 3.6 % of the rock composition.
Geochemistry of the investigated andesitic rock
The results of the complete analyses of the major oxides, trace and rare earth elements of andesite samples are given in (table. 1).
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Figure 2. Geological map of Um Taghir area formed from integrated remotely sensed data processing and field observation Рисунок 2. Геологическая карта района Ум Тагир, сформирована на основе интегрированной обработки данных дистанционного зондирования и полевых наблюдений
Figure 3. Field photographs: a - showing andesite rock (D. V) directly is intruded by granitic porphyry (Gr. d) at asphaltic road; b - imperial porphyry of Andesite
Рисунок 3. Полевые фотографии: a — андезитовая порода (D. V), непосредственно прорванная гранитным порфиром (гр. d) у асфальтированной дороги; б — "императорский порфир" андезит
Basalt
Figure 4. Photomicrographs showing: a - modal composition of the investigated andesitic rocks on QAP diagram of Streckeisen classification (1976); b - euhedral phenocryst of plagioclase surrounded by fine grains of alkali felspar (Ak), quartz (Qz) and iron oxide (In) showing porphyritic texture; c - Plagioclase phenocryst (P) associated with hornblende (Hb)
Рисунок 4. Микрофотографии, показывающие: a - модальный состав исследованных андезитовых пород на диаграмме QAP по классификации Штрекейзен (1976); б - идиоморфный вкрапленник плагиоклаза, окруженный мелкими зернами щелочного полевого шпата (Ak), кварца (Qz) и оксида железа (In), показывающий порфировую структуру; в - вкрапленник плагиоклаза (P), в ассоциации с роговой обманкой (Hb)
Geochemical classification ofthe investigated andesitic rock
A lot of parameters are used here to classify and follow up the chemical affinity of the investigated rocks. According to the classification of Na2O + K2O versus SiO2 binary diagram of [15], the plots of analyzed andesite samples fall in andesite field (fig. 5, a). Therefore, to verify this classification binary plot of [16] using a portion of immobile elements SiO2 versus Zr/TiO2 have been used, all samples plotted in the field of andesite (fig. 5, b), as well as geochemical classification of andesitic rock is coinciding with petrographic classifications as shown in (fig. 4, a).
Magma type of the investigated rock unit The magma type of the studied rock unit is discussed on the base of the following proposed diagrams. The discrimination binary diagrams of K2O-SiO2 according to [17, 18] (fig. 5, c) shows the investigated andesite samples plotted in calk-alkaline nature. The calc-alkaline nature has been confirmed by Zr versus Y binary diagram (fig. 5, d) proposed by [19].
Tectonic setting of the investigated rock unit Immobile incompatible elements were plotting in tectonic discrimination diagrams to determine the tectonic setting of andesitic lava. According to Ti versus Zr binary diagram of [20], the studied andesite has been plotted in volcanic arc setting (fig. 5, e). On the TiO2-K2O-P2O5 ternary diagram (after [21] which can be used to distinguish between oceanic and continental basalts. All the analyzed andesite samples plot in continental field (fig. 5, f).
Trace and Rare earth elements ofthe studied rock Andesite samples on the N-MORB normalized spider diagram [22] (fig. 5, g) show enrichment in Ba, Sr, Rb, K2O and Zr in addition to depletion in the most HFSEs similar to the island arc calc-alkaline affinity. The distribution of the rare earth elements in andesite show slightly depletion of REE pattern (La to Lu), and significant little or no sign of Eu-anomaly as shown in chondrite-nor-malized REE diagrams of [23] (fig. 5, h).
Table 1. Major oxides, Trace and Rare Earth Elements (REE) of andesitic rock
Таблица 1. Основные оксиды, редкие и редкоземельные элементы (РЗЭ) андезитовой породы
Andesite
Major oxides Sample number
57a 61a 63a 57b 63b 63c
SiO2 62 61.6 61.4 61 61.2 63
Al2O3 15.2 14.7 14.78 16.1 16.0 15.5
Fe2O3 8.5 6.03 7.59 9.0 8.0 7.7
FeO 1.2 1.6 1.4 1.0 1.3 1.6
CaO 3.26 4.97 4.51 3.4 3.6 2.9
MgO 2.57 2.77 2.43 2.6 2.4 2.3
P2O5 0.23 0.33 0.35 0.2 0.3 0.4
TiO2 1.06 0.93 1.3 1.0 1.1 1.2
Na2O 3.4 3.6 3.1 3.5 3.4 3.7
K22O 1.88 1.54 1.41 1.7 1.8 1.4
L.O.I. 0.4 1.22 1.09 0.5 0.9 0.16
Av 99.7
Trace Elements
Cr 34 42 61 51 46 47
Ni 96.4 107 55.8 96.0 56 54
Cu 87 90.3 44 85 87 91
Zn 92 88.3 122.2 95 121 118
Zr 145 154 132 142 137 141
Rb 19.2 18 18.45 20 19 18.4
Y 14.5 13.6 13.8 15 14 15.6
Ba 285 278 281 291 286 287
Pb 28.7 28.3 36.8 29 38 39
Sr 753 921.8 601.7 762 612 618
Ga 14 15 10 13 12 11
V 109.8 105 138.4 110 128 132
Nb 16 22 17 15 16 18
Co 45.8 54.7 69.3 47 64 61
U 0.77 0.55 0.63 0.8 0.7 0.6
Th 2.75 2.33 1.86 2.6 2.0 2.1
Rare Earth Elements (REE)
La 16.2 18 17.7 17 18 16
Ce 40 38.4 39.2 42 41 43
Pr 5 6 5.2 7 6 5
Nd 23 24.5 22.6 25 23 22
Sm 4.5 5 4.6 4.4 5.0 4.9
Eu 1.5 1.3 1.4 1.4 1.6 1.7
Gd 4.6 4.8 4.2 5.0 4.9 5.2
Tb 0.4 0.7 0 .5 0.5 0.8 0.9
Dy 3 3.2 2.9 2.5 3.1 3.2
Ho 0.8 0.4 0 .6 0.7 0.5 0.65
Er 1.5 1.7 1.4 1.5 1.6 1.8
Tm 0.3 0.4 0.2 0.3 0.4 0.35
Yb 1.6 1.8 1.4 1.5 1.7 1.54
Lu 0.1 0.4 0.2 0.3 0.2 0.18
XREE 102.5 106.6 101 109.1 108 106.4
Eu/Sm 0.33 0.26 0.3 0.31 0.32 0.34
Av(Eu/Sm) 0.31
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Figure 5. Binary diagrams showing: a - Classification of the studied andesite using total alkalis vs silica (TAS) diagram [15]; b - Chemical classification of the andesitic rock of the study area based on SiO2-Zr/TiO2 [16]; c - SiO2-KO2 diagram with fields after [17, 18] distinguishing the different melt series for andesite; d - Zr vs Y diagram proposed for the studied andesite after [19]; e - Plots the investigated andesite on Ti-Zr diagram of [20]; f - TiO2-K2O-P2O5 ternary diagram for andesitic rock after [21]; g - MORB-normalizing diagram of investigated andesitic rock [22]; h - Chondrite-normalized REE diagram [23] of investigated andesitic rock
Рисунок 5. Бинарные диаграммы: a - классификация изученного андезита по диаграмме общего содержания щелочей и кремнезема (TAS) [15]; б - химическая классификация андезитовых пород района исследований на основе SiO2-Zr/TiO2 [16]; в - SiO2-KO2 диаграмма с полями по [17,18], выделяющими разные серии расплавов для андезита; г - Zr vs Y - диаграмма, предложенная для исследуемого андезита по [19]; д - положение исследуемого андезита на диаграмме Ti-Zr [20]; е - TiO2-K2O-P2O5 тройная диаграмма для андезитовой породы по [21]; ж - диаграмма MORB-нормализации исследуемой андезитовой породы [22]; з - хондрит-нормированная диаграмма REE [23] исследуемой андезитовой породы
Conclusion
Dokhan volcanics are restricted in the Central Eastern Desert of Egypt on the Qena-Safaga road. The Qena-Safaga shear zone one of the most important structural occurrences in Central Eastern Desert of Egypt. Andesitic rock belongs to Neoproterozoic rocks that related to the Arabian Nubian Shield (ANS). According to petrographical studies Dokhan volcanics are represented by andesite, it consists of plagioclase, quartz, alkali feldspar and hornblende in addition to iron oxides.
Geochemically, the investigated andesitic rock has calc-alkaline affinity, as well as andesite has been erupted in volcanic arc setting. On the N-MORB normalized spider diagram, they show enrichment in Ba, Sr, Rb, K, Nb and Ce in addition to depletion in most HFSEs similar to the island arc calc-alkaline series. The distribution of the rare earth elements in andesite show slightly depletion of REE pattern (La to Lu), and significant little or no sign of Eu-anomaly.
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9. Fowler A.-R., Ali K. G., Omar S. M., Eliwa H. A. 2006, The significance of gneissic rocks and synmagmatic extensional ductile shear zones of the Barud area for the tectonics of the North Eastern Desert, Egypt. Journal of African Earth Sciences, vol. 46, issue 3, pp. 201-220. https://doi.org/10.1016/jjafrearsci.2006.04.011
10. Hassan S. M., Ramadan T. M. 2015, Mapping of the late Neoproterozoic Basement rocks and detection of the gold-bearing alteration zones at Abu Marawat-Semna area, Eastern Desert, Egypt using remote sensing data. Arabian Journal of Geosciences, vol. 8, no. 7, pp. 4641-4656. https://doi.org/10.1007/s12517-014-1562-0
11. El-Bialy M. Z., Omar M. M. 2015, Spatial association of Neoproterozoic continental arc I-type and post-collision A-type granitoids in the Arabian-Nubian Shield: the Wadi Al-Baroud older and younger granites, North Eastern Desert, Egypt. Journal of African Earth Sciences, vol. 103, pp. 1-29.
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The article was received on March 31, 2021
УДК 25.00.04 https://doi.org/10.21440/2307-2091-2021-2-7-15
Геологическое и тектоническое положение андезитов в Центрально-Восточной пустыне, Египет
Хамди Ахмед Мохамед АВАД1,2* Ибрагим Абу Эль-Лейл АЛИ2 Алексей Валерьевич НАСТАВКИН1 Абделлах Садек ТОЛБА2 Мостафа Камель АБДЕЛЬ ГАНИ2 Мусаб Авад Ахмед ХАССАН3 Мухамед Махмуд Фатхи ГХОНЕЙМ4 Ахмед Эль Саид Абдель ГАВАД4
1 Южный федеральный университет, Ростов-на-Дону, Россия 2Университет аль-Азхар, Каир, Египет
3Российский университет дружбы народов (РУДН), Москва, Россия "Агентство по атомной энергетике, Каир, Египет
Аннотация
Цель. Настоящее исследование направлено на выявление геологических особенностей, геохимических характеристик и тектонических условий исследуемой породы с помощью полевых наблюдений и геохимического анализа.
Методы исследования включают в себя как полевые (сбор образцов и составление новой геологической карты), так и лабораторные работы (подготовка тонких разрезов для петрографических исследований с помощью поляризационного микроскопа), атомную абсорбцию, рентгеновский флуоресцентный анализ (ХРЕ) Геологическое и тектоническое положение андезитов и масс-спектрометрия с индуктивно связанной плазмой (1СР-М8) в лаборатории Геологического института РАН.
Результат. Исследуемая андезитовая порода относится к вулкану Дохан, который расположен в Центрально-Восточной пустыне Египта, по дороге Кения-Сафага. Он считается одной из важнейших зон сдвига в Восточной пустыне, которая включает в себя отличительные породы и экономические месторождения полезных ископаемых. Исследуемая порода относится к позднему посттектоническому магматизму восточноафриканской орогении (ЕАО). Вулкан Дохан сложен андезитом по петрографическим исследованиям. Он состоит из плагиоклаза, кварца в дополнение к мафическим минералам. Геохимически исследованные образцы андезита имеют известково-щелочную природу.
Заключение. Тектонически, андезит попадает в поля островодужных и континентальных базальтов. Они обогащены Ва, 8г, М>, К2О и 7г с заметным обеднением большинства ИБ8Е, таких как известково-щелочные островодужные серии.
Ключевые слова: тектоническая обстановка, андезит, Восточная пустыня, Египет.
ЛИТЕРАТУРА
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Статья поступила в редакцию 31 марта 2021 года
