Geosciences Research

Number 4, 2018

9
A Note on the Chemical Composition and Origin of Peridot from the Harrat Kishb, Saudi Arabia

Authors: Adel A. Surour

Abstract

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Zabarjad (peridot in Arabic) is a common gemstone of valuable importance used in jewelry. It is the mineral olivine that has some gemological characters that classify it as peridot or gem olivine. Although it is not found in huge quantities in Saudi Arabia, there are some mining activities to extract peridot from the Harrat Kishb where the mineral is found scattered in alkali basalt or in the scree and wadi deposits. The basaltic flows at the Harrat Kishb include this gem olivine in the form of mantle xenoliths that comprise both dunite and peridotite. The size of collected peridot lies in the range of 0.4-0.8 cm and it occurs either as free olivine or in the form of cluster with components of host basalt. The collected crystals are transparent olive-green after washing and screening. Using the modern technique of analysis by the electron microprobe, the accurate chemical composition of peridot crystals was obtained. The studied peridot is a Mg-rich end-member of the foresterite (Mg) and fayalite (Fe) solid solution series where the forsterite content is Fo90.35-92.31. Such composition of olivine is similar to majority of olivine in the ultramafic mantle beneath the western Arabian Peninsula. The present work uses the concentrations of some elements such as Mn, Ni and Ca in addition to the forsterite content to characterize the Saudi peridot. It is typically terrsterial when compared to extraterrestrial (pallastic) olivine, and mostly xenocrystic and rarely magmatic. The forsterite content (Fo>90) suggests transition from a residual to cumulate origin

14
Investigating Rainfall Trend and Monitoring Meteorological Drought in a Himalayan Watershed of India

Authors: Alok Kumar Mishra, Anil Kumar

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Assessment of rainfall trend and monitoring meteorological drought and wet conditions are very important in dealing with agricultural planning and water resources management under the influence of changing climate in the Himalayan region. This study was conducted to assess rainfall trend and monitor the drought and wet pattern at four stations, namely Naula, Kedar, Chaukhutia and Mehalchauri, located in the upper Ramganga River catchment in Uttarakhand state of India in the central Himalayan region. Rainfall trend analysis at these stations was carried out using Mann-Kendall test and Sen’s slope estimator. Standardized Precipitation Index (SPI) at multiple time scales of 1-, 3-, 6-, 9-, 12- and 24-months was used to assess meteorological drought and wet pattern at Naula station. Results have indicated a significantly downward trend in December and post-monsoon season and an upward trend in September at Naula station. Significantly downward trend was also observed in July, December and monsoon season at Kedar station, and in June, July, August, monsoon and annual data series at Chaukhutia station. No significant trend was observed at Mehalchauri station. Study also revealed that there are more chances of occurrences of mild drought and wet conditions and only meager chances of moderate drought and wet conditions during monsoon season. However, the chances of severe and extreme drought and wet conditions are almost negligible. Therefore, efforts must be made to harvest the excess water during the wet periods and utilize the same during the periods of mild and moderate drought for drinking, household activities and irrigation purposes.

22
Younger Granites and Associated Pegmatites of Gabal El Fereyid – Wadi Rahaba Area, South Eastern Desert, Egypt: Geological and Geochemical Characteristics

Authors: Maher I. Dawoud, Gehad M. Saleh, Hassan A. Shahin, Farrage M. Khaleal, Bahaa M. Emad

Abstract

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Lithologically, the rock types of G. El Fereyid - W. Rahaba area are dominantly granites associated with pegmatites. The granitic rocks comprise tonalite and monzogranite with (minor acidic dykes), quartz and pegmatites occur as a vein type (granitic and perthitic pegmatites) hosted within the monzogranite. Structural analysis shows the presence of a prominent set of folds with axes striking WNW to NW and a less prominent set with pronounced ENE and NNW trends. Petrochemical studies and tectonic discrimination diagrams for the monzogranite reveal that it is classified as post orogenic granites (POG). Geochemically, monzogranite shows enrichment (peaks) of large ion lithophile elements (LILE; Pb, Rb, Ba, Sr) and high field strength elements (HFSE; Y, Zr, Nd,) and depletion (troughs) of K, P and Ti. Pegmatite shows enrichment (peaks) of large ion lithophile elements (LILE; Pb, Rb, Sr) and high field strength elements (HFSE; Y, Zr, Th, U, Nb) and depletion (troughs) of K, P and Ti. They exhibit high K-calc alkaline magma and peraluminous characteristics and considered as post orogenic granites (POG). The monzogranite reveal small to moderate negative Eu anomaly, while the pegmatites reveal moderate to large negative Eu anomaly. The pegmatite displays clear W-type tetrad effect of the REE while, the studied monzogranite do not show tetrad effect.