Geochemical and Sr-Nd-Pb isotopic data bearing on the origin of Pan-African granitoids in the Kid area, southeast Sinai, Egypt

A. M. Moghazi, T. Andersen, G. A. Oweiss, A. M. El Bouseily

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    Abstract

    The Kid plutonic suite in southeast Sinai, Egypt is composed of: 1) a gabbro-diorite complex; 2) quartz monzodiorite; 3) granodiorite-monzogranite association; 4) syenogranite. Major- and trace element analyses and radiogenic isotope data (Rb/Sr, Sm/Nd and Pb/Pb) are presented for these rocks. Rb-Sr isochrons indicate ages of 581 ± 11 Ma, 576 ± 11 Ma and 570 ± 4 Ma for the quartz monzodiorite-granodiorite-monzogranite association and syenogranite, resepectively. The gabbro-diorite complex is a layered intrusion varying in SiO2 from 42 to 59 wt.%. It shows a calc-alkaline trend, and the trace element characteristics of arc lavas. Subsequently, this magma fractionated to give quartz monzodiorite and syenogranite magmas. The quartz monzodiorite has low initial 87Sr/86Sr ratios (0.703) and high εNd values (+4.4 ± 0.03), with trace element charcteristics of volcanic-arc granites. The granodiorite-monzogranite association has higher initial Sr ratio (0.7044), lower εNd (+1.5-+2.7) and relatively high 207Pb/204Pb and 208Pb/204Pb, significantly different from those of the older rocks. Isotopic data and REE modelling suggest that it formed by partial melting of an early Pan-African amphibolitic lower crust. The syenogranite is a typical highly fractionated post-orogenic granite, with a deep negative Eu anomaly, HFSE and REE enrichment, a 87Sr/86Sr initial ratio of 0.7033, εNd of 4.5 ± 0.5 and distinct variations in the 206Pb/204Pb ratio. The isotopic compositions of Sr, Nd and Pb indicate that the syenogranite and the quartz monzodiorite are comagmatic. Increasing fluorine concentration from the quartz monzodiorite to the syenogranite suggests that fluid fractionation involving fluorine complexing played an important role in the genesis of the syenogranite. This process led to the enrichment of HFSE and HREE, to extensive fractional crystallization, to changes in the U/Pb ratios and a distinct variation in 206Pb/204Pb.

    Original languageEnglish
    Pages (from-to)697-710
    Number of pages14
    JournalJournal of the Geological Society
    Volume155
    Issue number4
    DOIs
    Publication statusPublished - Jul 1998

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