Tetrad effect of rare earth elements caused by fractional crystallization in high-silica granites: An example from central Tibet

Determining the formation mechanism (fractional crystallization vs. melt-fluid interaction) of the REE tetrad effect in high-silica granites is critical for understanding trace elemental behaviors (e.g., enrichment of rare metals) during magmatic and hydrothermal evolution. This study reports new petrography, zircon U-Pb ages, trace elemental and Hf isotopic, and whole-rock geochemical data of high-silica granites from the Gajin Batholith in the central Lhasa Terrane, Tibet. All samples are dated at ca. 100 Ma and likely formed during Lhasa-Qiangtang collision. These samples are syenogranites and alkali-feldspar granites, belonging to high-silica I-type granites (SiO2 = 74.2–78.2 wt%), having high total-alkalis, low magnesian, marked enrichments in Pb, U, Th, and Rb, and marked depletions in Ti, Eu, P, Sr, and Ba. The petrological and geochemical data indicate moderate (0–20%) fractionation of plagioclase + K-feldspar + biotite (5:3:2). The REE data clearly define regular chondrite-normalized patterns (No-tetrad Group) and pronounced tetrad patterns (Tetrad Group). The REE tread effect in Tetrad Group can be reproduced by fractionation of monazite and allanite as indicated by a Rayleigh fractionation modeling. New modeling results for samples reported elsewhere, along with petrographic observations and decoupling between the REE tetrad effect and fractionation of twin-elements, demonstrate that the REE tetrad effect in high-silica granites is inherited from fractional crystallization rather than formed by melt-fluid interaction as suggested widely.