Unraveling the Genesis of Highly Fractionated Rare-Metal Granites in the Nubian Shield via the Rare-Earth Elements Tetrad Effect, Sr–Nd Isotope Systematics, and Mineral Chemistry

Abstract

The rare-metal granites of the Nubian Shield are commonly associated with Nb−Ta and Sn mineralization. The magmatic and hydrothermal evolution of these granites and their relation to the Nb−Ta and Sn mineralization need further investigations. This study reports new mineral chemistry, wholerock geochemical, and Sr−Nd isotopic data for the Abu Dabbab granites in the Central Eastern Desert of Egypt to provide new insights into the formation mechanisms of the granites and their associated rare-metal mineralization. The granites consist mainly of albite, orthoclase, quartz, and Li-phengite, with accessory minerals including cassiterite, columbite, tantalite, wodginite, and Hf-zircon. Geochemically, they are peraluminous with high SiO 2 , alkalis, Rb, Nb, Ta, Sn, F, and low contents of MgO, CaO, and P 2 O 5 . The granites are characterized by distinct rare-earth element tetrad effects (TE 1,3 = 1.8−2.5), with strongly negative Eu anomalies and are extremely depleted in Ba, Sr, P, and Ti. These features imply that they are typical muscovite peraluminous highly evolved granites. Isotopically, they display positive εNd(t) values (+1.11 to +8.32) and two-stage Nd depleted model ages (t DM2 ) between 0.68 and 1.13 Ga, suggesting a juvenile magma source of the Neoproterozoic age. The Abu Dabbab granites could be formed by dehydration melting of muscovite-rich crustal materials followed by extensive fractional crystallization with a significant role of magmatic−hydrothermal fluids during the transitional orogenic stage of the Nubian Shield.

Two Distinct Fractional Crystallization Mechanisms of A-Type Granites in the Nanling Range, South China: A Case Study of the Jiuyishan Complex Massif and Xianghualing Intrusive Stocks

“…The intensive fractionation of feldspars is also proven by a discovery that A-type granites have especially highly stable Ca isotope compositions, likely the result of the rapid or extensive crystallization of plagioclase [75]. Whole-rock Zr/Hf, Rb/Sr and K/Rb ratios commonly serve as efficient criteria to determine the influence of fractional crystallization [6,76,77]. The Shaziling, Xishan, Jinjiling and Pangxiemu granites have gradually increased Rb/Sr ratios, but decreased Zr/Hf and K/Rb ratios (Figure 13a,b).…”

“…The concept of A-type granite was first proposed by Loiselle and Wones [1], with the three basic features being "Alkali, Anhydrous, and Anorogenic". The granites that conform to the initial definition of A-type granites typically have extremely high temperatures, high alkali and low water content [2][3][4][5][6]. However, the "A-type" concept has already been broadened, and granites with only high alkali features are also called A-type.…”

Development of an analytical method for accurate and precise determination of rare earth element concentrations in geological materials using an MC-ICP-MS and group separation

“…JG2 is highly fractionated granite that exhibits the REE tetrad effect. The existence of the REE tetrad effect observed in the chondritenormalized REE plot is a special phenomenon whose formation mechanism has not been clearly identified (LeeMasuda and Kim, 1994;2013;Kawabe, 1995;Monecke et al, 2011;Sami et al, 2022). Therefore, REE data for JG2 were reported by several researchers (Kawabe, 1995;Liang et al, 2000).…”

Geochemical characteristics, hazards impact assessment and radiogenic heat production of the alkaline rocks

“…Natural radiation in alkaline rocks primarily arises from the presence of specific radioactive elements like U, Th, and K. These radioactive elements decay over time, emitting radiation 6 , 7 . The specific mineral composition of granitic and alkaline rocks, which often includes minerals like zircon, monazite, allanite, thorite, apatite and uraninite, can also influence their radioactivity due to the concentration of radioactive elements within these minerals 8 .…”

“…The radioactive elements and rare earth elements (REEs) are mainly sourced from resources such as phosphorites 2 ; ironstone 14 ; black shale 15 , rare metal granites 8 ; pegmatites 16 , carbonatites and alkaline-rich rocks 17 among others. Notable areas for REEs include China (Bayan Obo deposit) 18 , Australia (Mount Weld deposit) 19 , and Brazil (Catalão deposit) 20 .…”