Large zircon age spans record multi-stage history of batholith assembly: Insights from the Late Triassic Dongcuo batholith in the eastern Tibetan Plateau

Abstract

Large and homogeneous plutons can assemble incrementally over millions of years, yielding large age spans. Determination of the age span is crucial to constrain the timing and nature of the batholith assembly. This paper reports comprehensive whole-rock geochemical data, LA-ICP-MS zircon U-Pb age, and zircon Hf isotopic data to constrain the timing and assembly process of the Late Triassic Dongcuo batholith, the largest batholith in the Yidun Terrane, eastern Tibetan Plateau. Samples from the batholith consist of metaluminous to weakly peraluminous granitoids (SiO2 = 69.91–79.85 wt%). These granitoids show large variation of zircon Hf isotopic compositions with εHf(t) of -17.15 to +0.13. The multiple zircon populations in single hand specimens revealed by kernel density estimation spread over 20 Myr (ca. 225–204 Ma), which is also confirmed by core-rim ages of single zircon grains, suggesting prolonged magmatism of the Dongcuo batholith. Our data indicate that the assembly of the batholith resulted from emplacement of discrete magma batches which derived from spatially varied sources and underwent variable differentiation. The assembly mainly involved three stages, including an early episode of magmatism (ca. 228 Ma), a high-flux magmatism between ca. 220 Ma and ca. 216 Ma, and a late-stage thermal event (ca. 211–204 Ma) possibly related to high temperature and localized remelting of the batholith. This study highlights the complexity of zircon populations in composite granitic batholith, showing that the search for single crystallization ages for a sample is fraught with difficulties and may mask rather than elucidate the crystallization history of the magmatic body.