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Mineral chemistry and geochemical aspects of Gebel Filat granites, South Eastern Desert, Egypt

كيمياء المعادن والخصائص الجيوكيميائية لجرانيت جبل فلات ، جنوب الصحراء الشرقية، مصر

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Abstract

Gebel Filat granites form one of Egyptian younger granite intrusions in Wadi Allaqi region, South Eastern Desert of Egypt. They are perthitic monzogranites composed mainly of K-feldspars, plagioclase, and quartz with minor biotite. Plagioclase feldspars are Na-rich and have low anorthite content (An2–3). Potash feldspars are mainly perthitic microcline and have chemical formula as (Or96–96.6 Ab3.4–4 An0). Biotite is Mg-rich and seems to be derived from calc-alkaline magma. Chlorite is pycnochlorite with high Mg content, revealing its secondary derivation from biotite. The estimated formation temperatures of biotite and chlorite are (689–711°C) and (602–622°C), respectively. Gebel Filat monzogranites are metaluminous, high-K calc-alkaline, I-type granites. They are late orogenic granites related to subduction-related volcanic arc magmatism. They are enriched in LILE and depleted in HSFE indicating highly differentiation character. The REE patterns display an enrichment in LREE due to presence of zircon and allanite as accessories and depletion in HREE with slight negative Eu anomaly \( \left( {{\text{Eu}}/{\text{Eu}} * = 0.51 - 0.97} \right) \). The parent magma of Gebel Filat monzogranites were emplaced at moderate depths (20–30 km) under moderate conditions of water-vapor pressure (1–5 kbar) and crystallization temperature [700–750°C]. The source magma of these granites seems to be derived from partial melting of lower crust material rather than upper mantle. The geochemical characteristics of pegmatites revealed that they are related to post orogenic within plate magmatism and not genetically related to the parent magma of Gebel Filat monzogranites. Distribution of radioactive elements (U and Th) in the studied rocks indicates normal U–Th contents for Filat monzogranites and U–Th bearing pegmatites. The positive correlations of each of Zr and Y versus U and Th are attributed to presence of zircon and allanite as accessories which incorporate U and Th in their crystal lattice.

الملخص العربي

يعتبر جرانيت فلات أحد أشهر متداخلات الجرانيت الأحدث بمنطقة وادى العلاقى جنوب الصحراء الشرقية بمصر. وتتميز هذه المتداخلة بأنها من نوع المونزوجرانيت البرثيتى والذى يتكون أساساً من الفلسبار القلى والبلاجيوكليز والكوارتز مع قليل من البيوتيت. وقد أوضحت الدراسات المجهرية والمعدنية أن البلاجيوكلاز فلسبار غنى بالصوديوم مع نسبة ضئيلة من الأنورثيت، بينما وجد أن الفلسبار القلى يتمثل أساسا بالميكروكلين البرثيتى، فى حين بدا البيوتيت بأنه من النوع الغنى بالماغنسيوم (الناتج من مجما كلس قلية) وهذا يتشابه مع معدن الكلوريت الذى أوضحت دراسته أنه أيضا من النوع الغنى بالماغنسيوم وهذا يدلل على أنه ناتج ثانوى مشتق من البيوتيت ،وأوضحت الدراسات لكل من البيوتيت والكلوريت أن درجة حرارة كلاهما تتراوح بين (689-711 ºم) ، (602-622 ºم) على التوالى. يتميز مونزوجرانيت جبل فلات بأنه نشأ من صهارة كلس قلية ذات نسبة بوتاسيوم عالية ، من النوع النارى ذو الطبيعة ميتا ألومنيوم ، كما أن الخصائص التكتونية توضح أنه تكون فى بيئة الجزر البركانية المنزلقة فى المراحل المتأخرة من الحركات الأرضية ، وهذه المجما نشأت بالإنصهار الجزئى للمناطق العميقة من القشرة فى أعماق متوسطة (20-30 كم) وتحت ضغط متوسط (1-5 ك بار) وفى درجة حرارة متوسطة (700-750 ºم) . أوضحت الدراسات الجيوكيميائية تميز هذه الجرانيتات بثرائها بالعناصر الغير متوافقة (مثل البوتاسيوم والربيديوم والباريوم) بينما تكون فقيرة فى العناصر المتوافقة(مثل الزركونيوم والنيوبيوم والإيتريوم) وهذا يوضح أنها ذات طبيعة تمايز التبلور عالية نسبيا، وتوضح دراسة ثراء صخور الجرانيت بالعناصر الأرضية النادرة الخفيفة ويعزى هذا لوجود المعادن الثانوية مثل الزركون والألانيت ، بينما لوحظ الإفتقار بالعناصر الأرضية النادرة الثقيلة مع وجود شذوذ سلبى لعنصر الإريبيوم. وقد أوضحت خصائص توزيع العناصر الشحيحة الجيوكيميائية للبجماتيت - المصاحب لمتداخلات جرانيت جبل فلات - أنها تكونت من نشاط صهيرى فى مراحل مابعد الحركات الأرضية فى بيئة داخل الألواح القارية مما يؤكد أن هذه الصهارة ليس لها أى علاقة بمصدر الصهارة التى تكون منها مونزوجرانيت جبل فلات. وتبين دراسة توزيع العناصر المشعة (اليورانيوم والثوريوم) لكل من جرانيت فلات والبجماتيت المصاحب له أن جرانيت فلات به محتوى عادى من اليورانيوم والثوريوم ، بينما تتميز عروق البجماتيت بأنها ذات نسبة عالية من العناصر المشعة مقارنة بجرانيت فلات. وأظهرت العلاقات بين بعض العناصر الشحيحة مثل الزركونيوم والإيتريوم وكل من اليورانيوم والثوريوم تزايد تركيز اليورانيوم والثوريوم مع تزايد كل من الزركونيوم والايتريوم مما يؤكد ارتباط عميق مع الزركون والألانيت التى تكتنف هذه العناصر المشعة داخل تركيبها البلورى.

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Acknowledgement

The authors would like to express their sincere thanks to Prof. Abdalla, H.M. who facilitated the access to the electron microprobe analyses during his post doctoral fellowship at Hokkaido University, Japan. Special thanks are due to Prof. Hassaan, M.M. for his critical reviewing and insightful comments on the draft of this manuscript.

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  1. Nuclear Materials Authority, Cairo, Egypt

    N. M. Moghazy

  2. Geology Department, Aswan Faculty of Science, South Valley University, Aswan, 81528, Egypt

    A. A. Emam & E. M. Abdel Rahman

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  1. N. M. Moghazy
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Moghazy, N.M., Emam, A.A. & Abdel Rahman, E.M. Mineral chemistry and geochemical aspects of Gebel Filat granites, South Eastern Desert, Egypt. Arab J Geosci 4, 689–702 (2011). https://doi.org/10.1007/s12517-009-0073-x

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