Combined zircon U-Pb dating and chemical Th–U–total Pb chronology of monazite and thorite, Abu Diab A-type granite, Central Eastern Desert of Egypt: Constraints on the timing and magmatic-hydrothermal evolution of rare metal granitic magmatism in the Arabian Nubian Shield

Abstract

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.