Boron and Molybdenum Isotope Evidence for Source‐Controlled Compositional Diversity of Cenozoic Granites in the Eastern Tethyan Himalaya

Abstract

The origins of Cenozoic granites in the Himalaya are key to understanding the evolution of the Himalayan orogen. However, it is unclear whether these granites represent primary melts, and the nature of their magma source is controversial. Here, we present a systematic element and Sr–Nd–B–Mo isotope study of Cenozoic granites from the Yardoi area in the eastern Tethyan Himalaya, China. These granites can be divided into two groups: mid‐Eocene porphyritic two‐mica granites with low SiO2 contents (65.9−69.6 wt.%) and adakitic geochemical signatures, and mid‐Eocene to Miocene equigranular granites with high SiO2 contents (71.6−75.5 wt.%). The high‐SiO2 granites (HSG) have similar Sr−Nd isotope compositions to the low‐SiO2 granites (LSG), but they have distinct δ11B values of −19.4‰ to −11.4‰ and −10.6‰ to −6.89‰. This indicates that the two groups have different sources, with the LSG derived by partial melting dominantly of metamafic rocks at thickened lower crustal conditions, and the HSG generated by partial melting of the mid‐crust metasedimentary rocks with less enriched Nd isotope compositions. The δ98/95Mo of the LSG and HSG are highly variable with values of −0.68‰ to 0.12‰ and −1.13‰ to 0.46‰, respectively. δ11B values of the HSG correlate positively with δ98/95Mo and Sr/Y values and correlate negatively with K2O, Rb, Zr, and Rb/Sr, reflecting the addition of external metamorphic fluids during anatexis of the metapelites. The B–Mo isotope data robustly suggest source‐controlled compositional diversity of the Himalayan granites, which could provide clues to the physical and geochemical responses during the evolution of a large orogen.

Light Mo Isotopes of Post‐Collisional Ultrapotassic Rocks in Southern Tibet Derived From Subducted Indian Continental Crust

“…The very low B/Nb ratios of these post‐collisional ultrapotassic rocks indicate a fluid‐starved source and their low δ 11 B values cannot be derived from oceanic sediments or a stalled dehydrated oceanic slab. Combined with the light B isotope compositions of the schists and gneisses of the Tethyan Himalaya (Fan et al., 2023), Hao, Wang, Kerr, et al. (2022) proposed that the very light B isotope compositions of these post‐collisional ultrapotassic rocks in southern Tibet are most likely derived from the subducted Indian continental crust.…”

“…The early Cretaceous arc‐like basalts from the Liaodong Peninsula (Fang et al., 2022) related to continental subduction show low δ 98/95 Mo values (−0.98 to −0.16‰), low Mo/Ce ratios (Figure 4a) and enriched Nd isotopes (Figure 4d), which are similar to those of the Sailipu ultrapotassic rocks. In the case of southern Tibet, the schists and gneisses of the Tethyan Himalaya, as representative examples of Indian continental crust, have extremely low δ 98/95 Mo values (−1.03 to −0.64‰) and Mo/Ce ratios (0.0001–0.009, Fan et al., 2023) (Figure 4a). Moreover, the Sailipu post‐collisional ultrapotassic rocks have δ 98/95 Mo values and Mo/Ce ratios that are intermediate between those of MORBs and the Indian continental crust (Figure 4a).…”