We present the first chemical age determination of monazite from a geochemically-specialized rare metal granite of the Central Eastern Desert (CED), Egypt. The Abu Diab composite A-type granite of the CED divides into three co-magmatic phases: two mica granite (TG), garnet-bearing muscovite granite (GMG), and muscovite granite (MG). The GMG granitic phase is a typical example of a rare metal, volatile-rich aqueous silicate magma. Its zircon has extremely high U-concentration (up to 45,000 ppm) and intense metamictization, resulting in significant radiogenic Pb-loss, as well as common Pb contamination, along fractures and other imperfections. Laser ablation (LA-ICP-MS) zircon U-Pb results yield imprecise to meaningless dates. On the other hand, the GMG exhibits two texturally and chemically distinct varieties of monazite, pristine (Mnz1) and altered (Mnz2), that yield similar electron microprobe (EMP) chemical ages of 587 ± 8 Ma (2σ) for Mnz1 and 587 ± 9 Ma (2σ) for Mnz2. These ages, combined with the inferred alteration temperature (T > 400 °C), indicate monazite alteration/replacement occurred during or soon after crystallization of the pluton, in response to expelled magmatic-hydrothermal fluids inferred to be F-rich, late-staged deuteric fluids. The alteration and elemental substitution of zircon and monazite reflect coupled dissolution-recrystallization processes during late-staged deuteric alteration when zircon and thorite were relatively vulnerable to post-magmatic hydrothermal effects because of their extremely metamict state. The ca. 587 Ma age is typical of A-type granite intrusion across the CED and the post-collisional stage of the Arabian Nubian Shield (ANS; 610 – 560 Ma), and is within analytical error of associated magmatic thorite (592 ± 8 Ma). These two ages agree with the timing of rare metal-rich intrusions (0.6−0.4 Ga) related to the Pan-African orogeny. Future research using the monazite chronometer may better constrain the timing of rare metal granitic magmatism of the ANS.

我们提出了来自埃及中东部沙漠（CED）的地球化学专业稀有金属花岗岩中独居石的首次化学年龄测定。CED的Abu Diab复合A型花岗岩分为三个共岩浆相：两个云母花岗岩（TG），带石榴石的白云母花岗岩（GMG）和白云母花岗岩（MG）。GMG花岗岩相是稀有金属，富含挥发物的含水硅酸盐岩浆的典型例子。其锆石具有极高的U浓度（高达45,000 ppm）和强烈的成矿作用，导致沿裂缝和其他缺陷的大量放射性Pb损失以及常见Pb污染。激光烧蚀（LA-ICP-MS）锆石U-Pb结果产生的结果不精确，无意义。另一方面，GMG展示了两种在质地和化学上都不同的独居石，原始（Mnz1）和改变的（Mnz2），对于Mnz1产生类似的电子微探针（EMP）化学年龄为587±8 Ma（2σ），对于Mnz2产生587±9 Ma（2σ）。这些年龄与推断的蚀变温度（T> 400°C）相结合，表明独居石蚀变/置换发生在岩体结晶期间或之后不久，这是由于推断出富含富氟，晚期的岩浆水热流体所致。氘代液体。锆石和独居石的蚀变和元素替代反应反映了后期氘代蚀变中锆石和石由于其极易成矿状态而相对易受岩浆热液作用的耦合溶解-再结晶过程。该ca。587 Ma年龄是典型的A型花岗岩侵入CED和阿拉伯努比亚盾构的碰撞后阶段（ANS； 610 – 560 Ma），并且在相关岩浆matic石的分析误差内（592±8 Ma）。这两个时代与与泛非造山运动有关的稀有金属富集侵入（0.6-0.4 Ga）的时间一致。未来使用独居石天文钟表进行的研究可能会更好地约束ANS的稀有金属花岗岩岩浆作用的时间。