Graphs and TIMA images from the paper.
Although low Zr/Hf and Nb/Ta ratios (a) are usually taken to be indicators of extreme crystal fractionation of fluid-rock interaction, in the Himalayan leucogranites they instead indicate mixing between muscovite-rich (b) and (biotite+chlorite)-rich (c) end members, represented by the TESCAN Integrated Mineral Analyzer (TIMA) maps (d-e). These mineralogical and geochemical signatures are shown to be inherited from mixing of different sedimentary sources during melting and do not require fractionation or mantle input. Credit: Huang et al. [2025], Figure 7
Editors’ Highlights are summaries of recent papers by AGU’s journal editors.
Source: Geochemistry, Geophysics, Geosystems

Geologists are responding to increasing demand for a variety of rare metals by focusing attention on the origins of high silica leucogranites that often host high concentrations of valuable metals such as niobium (Nb), tantalum (Ta), zirconium (Zr), hafnium (Hf), tin (Sn), and lanthanides. Many of these rocks have anomalous trace-element signatures (distinctively low ratios of Zr/Hf, Nb/Ta, and europium (Eu)/(gadolinium (Gd) + samarium (Sm)) that have long been thought to indicate extensive fractional crystallization or interaction with large volumes of fluid. They may also have unradiogenic Hf isotope ratios suggestive of input from depleted mantle sources despite their presence in thick crustal orogenic belts.

Huang et al. [2025] contribute measurements of the stable isotope ratio of boron (B) and the radiogenic neodymium (Nd) system from a belt of Paleozoic leucogranites in the Qilian orogenic belt in central China. The results show decoupling of Nd and Hf isotope signatures, not consistent with simple crust/mantle mixing, but correlation of Hf and B isotope signatures with trace element ratios that fingerprint mixing of various sedimentary rocks in the sources of the granites. The authors conclude that these are pure S-type (sediment-derived) magmas, whose budget of valuable metals was scavenged from the Paleozoic crust rather than concentrated by extreme fractionation of mantle-derived magma, overturning the common interpretation based on Hf isotope data alone. 

Citation: Huang, H., Niu, Y., Romer, R. L., Zhang, Y., He, M., & Li, W. (2025). High silica leucogranites result from sedimentary rock melting—Evidence from trace elements and Nd-Hf-B isotopes. Geochemistry, Geophysics, Geosystems, 26, e2024GC012024. https://doi.org/10.1029/2024GC012024

—Paul Asimow, Editor, G-Cubed

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