Garnet dissolution–reprecipitation processes in leucogranitic magma and their profound geochemical consequences

Syn-magmatic garnet-bearing and garnet-free leucogranites from the Tethyan Himalaya display pronounced differences in major element, trace element, and SrNd isotope geochemistry, reflecting a complex magmatic evolution process. Compared with biotite-bearing leucogranites, the garnet-bearing leucogranites have (1) relatively higher SiO2, FeO*, and MnO contents; lower Al2O3, CaO, P2O5, and TiO2 contents; and lower A/CNK values; (2) higher Y, Yb, Zr, Hf, and Th contents and lower Ba and Sr contents; (3) notably higher heavy rare earth element contents and more pronounced negative Eu anomalies; and (4) significantly higher 87Sr/86Sr(t) ratios and εNd(t) ratios. Furthermore, the garnets in the garnet-bearing leucogranites exhibit characteristics typical of magmatic origins. On the basis of field observations and geochemical data, we propose the following model: during migration and intrusion, high-temperature granitic magma that ultimately formed the biotite-bearing leucogranites, incorporated wall rocks, thereby releasing both material and ancient garnet crystals from the wall rocks into the magma. The assimilation of the wall rocks elevated the FeO*, MnO, and HREE contents and SrNd isotopic compositions and ultimately promoted the crystallization of magmatic garnets and the formation of garnet-bearing leucogranites in the Kongbugang area. These observations provide new constraints on the mechanism responsible for the development of geochemical heterogeneities in granites and the mechanism responsible for the formation of magmatic garnets in S-type granites and highly fractionated I-type granites.