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

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 traceelement 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, respectively. 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 andsyenogranite magmas. The quartz monzodiorite has low initial 87 Sr/ 86 Sr ratios (0.703) and high ɛ Nd values (+4.4±0.3), with trace element characteristics of volcanic-arc granites. The granodiorite–monzograniteassociation has higher initial Sr ratio (0.7044), lower ɛ Nd (+1.5–+2.7) and relatively high 207 Pb/ 204 Pb and 208 Pb/ 204 Pb, significantly different from those of the older rocks. Isotopic data and REE modelling suggestthat it formed by partial melting of an early Pan-African amphibolitic lower crust. The syenogranite isa typical highly fractionated post-orogenic granite, with a deep negative Eu anomaly, HFSE and REEenrichment, a 87 Sr/ 86 Sr initial ratio of 0.7033, ɛ Nd of 4.5 & 0.5 and distinct variations in the 206 Pb/ 204 Pbratio. The isotopic compositions of Sr, Nd and Pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic. Increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite. This process led to the enrichment of HFSE and HREE, to extensivefractional crystallization, to changes in the U/Pb ratios and a distinct variation in 206 Pb/ 204 Pb.