Sr-Nd-Hf Isotopic Disequilibrium During the Partial Melting of Metasediments: Insight From Himalayan Leucosome

Radiogenic isotopes of granitoids are widely applied to fingerprint the source of granitoids and study the magma mingling and assimilation processes, aiming to decipher the planetary differentiation. This weapon is based on the assumption that crustal melts inherit the radiogenic isotopes of protoliths. However, complicated melting processes in the crust would drive the radiogenic isotopes of melt away from the source, thus calling for a clear understanding of the behaviors of radiogenic isotopes during crustal melting. This study investigated the joint behavior of Rb-Sr, Sm-Nd, and Lu-Hf isotope systems during the melting of metasediments. Leucosome, schist, and leucogranite samples were collected from the Nyalam, South Tibet, where the leucosome was produced by muscovite dehydration melting of metapelites. Results show that the leucosome has a broad range of Sr-Nd-Hf isotopes (Sr-87/Sr-86(i): 0.763,48-0.875321, epsilon Nd-(t): 14.6-11.6, epsilon Hf-(t): 15.0-4.4) that deviate from the metasediments. We attributed it to the non-modal or disequilibrium melting of metapelites. The variation of Sr isotopes of leucosome is formed due to preferential entry into the melt of muscovite relative to plagioclase during melting. The changing Sr-87/(86)Sri and Rb-87/Sr-86 of leucosome define an errorchron at similar to 500 Ma indicating an early Paleozoic tectono-thermal event supported by the U-Pb age of zircon cores (similar to 460 Ma) in the leucosome. The Nd isotopes of leucosome are mainly controlled by the preferential dissolution of apatite relative to monazite. The inadequate dissolution of zircon caused more depleted initial Hf isotopes compositions in the leucosome than the source. However, the leucosome with a higher crystallization temperature has radiogenic isotopic composition closer to the source. It indicates that the melting temperature greatly affects the isotope equilibrium between source and melt.

Automated summary

The behavior of radiogenic isotopes during the melting of metasediments is influenced by the source and temperature of the melt. During the melting process, muscovite preferentially enters the melt relative to plagioclase, resulting in variation in Sr isotopes. In addition, the incomplete dissolution of zircon and the higher crystallization temperature result in the leucosome having Nd and Hf isotopes compositions closer to the source.