Ephemeral Magma Reservoirs During the Incremental Growth of the Neoproterozoic Jiuling Composite Batholith in South China

Large‐volume magma bodies in the crust are crucial to magmatic differentiation and intracrustal mass redistribution. Their magmatic residence timescales during solidification in the upper crust reflect the intensity of magmatic activity and control the formation of economic deposits. Here we explore the use of zircon crystal size distributions (CSDs) and oxygen isotopic diffusion modeling to constrain the magmatic timescales of the four main intrusions of the Neoproterozoic Jiuling composite batholith in eastern Jiangnan Orogen, South China. We present laser‐ablation–inductively coupled plasma–mass spectrometry zircon U–Pb ages and co‐registered secondary ion mass spectrometry zircon O isotopes for the four main intrusions. The dating results show that the intrusions were emplaced at 826–810 Ma, with a few inherited zircon domains in the range of 3,800–3,835 Ma. The magmatic zircon grains/rims commonly have wide δ18O variations from 5.9‰ to 13.1‰, and magmatic centers have similar δ18O ranges (3.9‰–12.2‰) to those of inherited zircon domains (3.8‰–11.4‰). The application of zircon CSD measurements reveals short zircon crystallization timescales and thus possible magmatic residence times in the order of 103–104 yr, consistent with the maximum residence times (18–47 kyr, as an upper limit) constrained by zircon oxygen isotopic diffusion modeling. This study of such ephemeral reservoirs provides unique insights into the state of magmas and the processes that occur within magma reservoirs. Plain Language Summary Magma reservoirs in the crust are key locations for the generation of volcanic eruptions and/or intrusive rocks and have a close association with the formation of economic ore deposits. The longevity of magma reservoirs and the magmatic processes involved in their formation remain highly debated topics in geoscience communities. Here we use zircon crystal size distributions and oxygen isotopic diffusion modeling to constrain the longevity of the magmas that formed the Neoproterozoic Jiuling composite batholith in South China. Our results reveal a short magmatic lifetime (a few thousand to a few tens of thousands of years). This well‐documented example shows that melts were assembled rapidly during the Neoproterozoic era and adds to our understanding of magma reservoirs in the early Earth. Key Points Variable oxygen isotope values are obtained from magmatic zircon from the 826–810 Ma Jiuling composite batholith in South China Zircon crystal size distrib