Ophiolite nappes, encompassing dismembered oceanic lithosphere sections, in the Eastern Desert of Egypt are mainly confined to Tonian-Cryogenian inter-terrane sutures. A cohesive geodynamic model of these ophiolites remains elusive in light of the variably obliterated field relationships and lack of ample geochronological information. Available geochemical data of the well-studied ophiolites lead to different interpretations of mid-ocean ridge, back-arc and/or fore-arc affinities. Relevant geochronological data are likewise heterogeneous in terms of methods and the dated lithologic units. Here, we attempt to constrain the timing and genesis of some Eastern Desert ophiolites by using new SIMS zircon U-Pb and oxygen isotope data of gabbros from four different ophiolites.

The new zircon U-Pb ages and oxygen isotope values integrated with available geochemical data constrain ages of 737 ±6 Ma and 720 ±6 Ma and fore-arc origin of the Wadi El-Sid and Gabal Abu Dahr-Abu Siayil ophiolites, respectively. The δ¹⁸O_(Zrn) values of these fore-arc ophiolites (+2.5 to +8.5 ‰) deviate significantly from primitive mantle values, suggestive of substantial degrees of subduction-induced metasomatism. The Gabal El-Rubshi and Wadi Ghadir back-arc ophiolites returned younger and mutually similar ages (702 ±5 Ma and 698 ±4 Ma, respectively) and less heterogeneous δ¹⁸O_(Zrn) values (+3.1 to +6.1‰). Excluding outliers, most measured δ¹⁸O_(Zrn) values are indistinguishable from the unaltered MORB values and the restricted mantle-like δ¹⁸O range.

Results of the present study suggest multiple arc-fore-arc accretion events and an extensive late Tonian back-arc basin opening event in the evolution of the Eastern Desert accretionary belt. Subduction tectonics continued to stabilize the immature arc crust ∼750-720 Ma and a tapered back-arc spreading ridge engendered at ∼700 Ma. The final terrane accretion and suturing could have been associated with subduction-related melts that circulated and locally metasomatized back-arc ophiolites. This model is in broad agreement with the recently advocated ∼720 Ma plate reorganization in the closure of the Mozambique Ocean during the course of Gondwana assembly.

蛇绿岩推覆体，包括被肢解的大洋岩石圈部分，在埃及东部沙漠中主要局限于托尼阶-低温阶地间缝合线。这些蛇绿岩的一个有凝聚力的地球动力学模型仍然难以捉摸，因为它们之间的磁场关系变化不定，而且缺乏充足的地质年代学信息。已充分研究的蛇绿岩的可用地球化学数据导致对洋中脊、弧后和/或弧前亲和力的不同解释。相关地质年代学数据在方法和年代的岩性单元方面同样具有异质性。在这里，我们试图通过使用来自四种不同蛇绿岩的辉长岩的新 SIMS 锆石 U-Pb 和氧同位素数据来限制一些东部沙漠蛇绿岩的时间和成因。

新的 锆石 U-Pb 年龄和氧同位素值与现有地球化学数据相结合，分别限制了 Wadi El-Sid 和 Gabal Abu Dahr-Abu Siayil 蛇绿岩的 737 ±6 Ma 和 720 ±6 Ma年龄和前弧成因。这些前弧蛇绿岩的 δ ¹⁸ O _(Zrn) 值（+2.5 至 +8.5 ‰）与原始地幔值显着不同，表明 俯冲引起的交代作用相当大。Gabal El-Rubshi 和 Wadi Ghadir 弧后蛇绿岩返回更年轻且相互相似的年龄（分别为 702 ±5 Ma 和 698 ±4 Ma）， δ ¹⁸ O _(Zrn) 值不均匀（+3.1 至 +6.1‰）。排除异常值，大多数测量 δ ¹⁸ O _(Zrn) 值 与 未改变的 MORB 值和受限制的类似地幔的 δ ¹⁸ O 范围无法区分。

本研究的结果表明，东部沙漠增生带演化过程中存在多次弧前增生事件和广泛的托尼阶晚期弧后盆地开放事件。俯冲构造继续稳定未成熟的弧地壳~750-720 Ma，并在~700 Ma 产生锥形弧后扩张脊。最终的地体增生和缝合可能与俯冲相关的熔体有关，这些熔体循环并局部交代了弧后蛇绿岩。该模型与最近在冈瓦纳大陆组装过程中莫桑比克洋闭合时提倡的~720 Ma 板块重组大体一致。