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

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 ε_Nd 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 ε_Nd (+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, ε_Nd 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.