Abstract The Tighsi area of the Egere/Aleksod Terrane (Tuareg Shield) contains mafic eclogites interlayered within anatectic metapelites corresponding to metabasalts coeval with a shallow water marine sedimentation. In this study we present in-situ geochronological (U-Pb) and geochemical (major and trace elements, Hf and O isotopes) analyses from a 2.6 mm zircon megacryst found in a high-pressure kyanite pegmatite enclosed in the metapelites. Oscillatory zoning, HREE-enriched patterns, positive Ce anomalies and the absence of negative Eu anomalies are consistent with crystallization of the megacryst from the anatectic melt. Ti-in-zircon temperatures indicate crystallization at 811±15°C in an allanite/monazite buffered anatectic melt as evidenced by the low Th/U ratios. Oxygen isotopes yield large intra-grain variations (7.1 to 12.3‰) with a gradual lowering towards the edge of the grain in contact with secondary feldspathic veinlets. These results indicate fluid-assisted oxygen isotope disturbances, consistent with the low retentivity of O in zircon under wet conditions. Hf isotopes do not display intra-grain variations (mean eHfi =-20.7±1.0) and support production of the leucosome by melting of crustal material. U-Pb analyses of the center of the crystal provide an age of 654±5 Ma (2σ), attributed to post-peak decompression and heating. HP eclogite facies conditions in the Egere terrane are thus significantly older than HP metamorphism in the western part of the shield (610-625 Ma) in agreement with multiple subduction events along the margins of the Tuareg Shield. The rim of the megacryst, close to feldspathic veinlets, is characterized by a significant decrease of trace elements (U, Y, HFSE), but preserved identical Hf isotope ratios, which is consistent with recrystallization processes. The rim displays a Ti-in-zircon temperature of 717±28°C and a U-Pb age of 584±6 Ma (2σ) coeval with the climax of batholith intrusion in Central Hoggar. Reheating and softening of the lower/middle crust at that time may have assisted and favoured upward viscous flow of basement domes and escape tectonics along lithospheric shear zones. We propose that the final push of the Saharan metacraton in the east was responsible for the observed architecture of the Egere terrane, where anatectic elongated domes of basement gneisses alternate with HP metasedimentary synforms.

中文翻译：

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锆石巨晶原位分析（主要和微量元素、U-Pb 年代学和 Hf-O 同位素）的案例研究：对 Egéré 地体演化的影响（Central Hoggar，图阿雷格地盾，阿尔及利亚）

摘要 Egere/Aleksod Terrane（图阿雷格地盾）的 Tighsi 地区包含夹在深熔变泥质岩内的镁铁质榴辉岩，对应于与浅水海洋沉积同时期的变玄武岩。在这项研究中，我们展示了在变泥质岩中封闭的高压蓝晶石伟晶岩中发现的 2.6 毫米锆石巨晶的原位地质年代学 (U-Pb) 和地球化学（主要和微量元素、Hf 和 O 同位素）分析。振荡分带、HREE 富集模式、Ce 正异常和 Eu 负异常的缺失与深熔熔体中巨晶的结晶一致。Ti-in-zircon 温度表明在 811±15°C 的温度下结晶，在 allanite/monazite 缓冲的深熔熔体中，低 Th/U 比证明了这一点。氧同位素产生较大的晶粒内变化（7.1 到 12. 3‰) 向与次生长石细脉接触的晶粒边缘逐渐降低。这些结果表明流体辅助的氧同位素扰动，与湿条件下锆石中 O 的低保留率一致。Hf 同位素不显示颗粒内变化（平均 eHfi =-20.7±1.0）并支持通过地壳物质熔化产生白质体。晶体中心的 U-Pb 分析提供了 654±5 Ma (2σ) 的年龄，归因于峰后减压和加热。因此，与图阿雷格地盾边缘的多次俯冲事件一致，埃格尔地体中的高压榴辉岩相条件明显比地盾西部的高压变质作用（610-625 Ma）更古老。巨晶的边缘，靠近长石细脉，其特点是微量元素（U、Y、HFSE）显着减少，但保留了相同的 Hf 同位素比，这与再结晶过程一致。边缘显示锆石钛温度为 717±28°C，U-Pb 年龄为 584±6 Ma (2σ)，与中央 Hoggar 的基岩侵入高潮同时期。当时下地壳/中地壳的重新加热和软化可能有助于和有利于基底圆顶的向上粘性流动和沿岩石圈剪切带的构造逃逸。我们认为东部撒哈拉元克拉通的最后推动是观察到的埃格尔地体结构的原因，其中基底片麻岩的深熔细长圆顶与 HP 变沉积同形体交替出现。这与重结晶过程一致。边缘显示锆石钛温度为 717±28°C，U-Pb 年龄为 584±6 Ma (2σ)，与中央 Hoggar 的基岩侵入高潮同时期。当时下地壳/中地壳的重新加热和软化可能有助于和有利于基底圆顶的向上粘性流动和沿岩石圈剪切带的构造逃逸。我们认为东部撒哈拉元克拉通的最后推动是观察到的埃格尔地体结构的原因，其中基底片麻岩的深熔细长圆顶与 HP 变沉积同形体交替出现。这与重结晶过程一致。边缘显示锆石钛温度为 717±28°C，U-Pb 年龄为 584±6 Ma (2σ)，与中央 Hoggar 的基岩侵入高潮同时期。当时下地壳/中地壳的重新加热和软化可能有助于和有利于基底圆顶的向上粘性流动和沿岩石圈剪切带的构造逃逸。我们认为东部撒哈拉元克拉通的最后推动是观察到的埃格尔地体结构的原因，其中基底片麻岩的深熔细长圆顶与 HP 变沉积同形体交替出现。当时下地壳/中地壳的重新加热和软化可能有助于和有利于基底圆顶的向上粘性流动和沿岩石圈剪切带的构造逃逸。我们认为东部撒哈拉元克拉通的最后推动是观察到的埃格尔地体结构的原因，其中基底片麻岩的深熔细长圆顶与 HP 变沉积同形体交替出现。当时下地壳/中地壳的重新加热和软化可能有助于和有利于基底圆顶的向上粘性流动和沿岩石圈剪切带的构造逃逸。我们认为东部撒哈拉元克拉通的最后推动是观察到的埃格尔地体结构的原因，其中基底片麻岩的深熔细长圆顶与 HP 变沉积同形体交替出现。