The major composition of a middle–late Eocene salt lake in the Yunying depression of Jianghan Basin of Middle China based on analyses of fluid inclusions in halite

• First report of pre-quaternary salt lake brine composition in China.
• Fluid inclusions in Eocene halite provide the direct brine composition in China.
• The brine composition in Eocene halite is different from seawater at same time.
• Transgression affected Eocene inland basins little according to fluid inclusion.

During the Cretaceous–Tertiary transition in eastern China, abundant halites formed in non-marine areas. Many continental salt deposits from inland salt lakes were formed in eastern China in faulted basins as a result of the northward movement and collision of the Indo-China Plate with the Eurasian Plate, including the Bohai Gulf Basin. However, a marine transgression versus a non-marine origin of these evaporites remains to be determined. Primary fluid inclusions trapped in halite deposits can directly record the composition of evaporated seawater or salt lake water, such as those in the Cretaceous halite in the Khorat Plateau (Laos and Thailand) area can resolve the origins of the evaporate deposits; recent fluid inclusions data in the Khorat Plateau coincide with the predicted secular variation of seawater and are comparable to other fluid inclusions in Cretaceous marine halite, indicating these fluid inclusions are directly related to a marine transgression. Our analyses in this study shows that the average K+, Mg2+, and SO42- contents are 8.8, 5.0, and 6.8 g/l, respectively, in the primary fluid inclusions in halite of middle–late Eocene from the Yunying depression of China. These numbers are much less than those in the contemporary Spanish primary fluid inclusions in halite precipitated from seawater (16.4, 36.3, and 12.5 g/l for K+, Mg2+, and SO42-, respectively). Furthermore, Br contents of all fluid inclusion samples in halite from the Yunying depression are lower than 2 ppm (vs. 55–58 ppm at the base of Spanish contemporary marine halite), and their δ37Cl values range from −0.11‰ to +2.94‰ (vs. −0.09‰ to −0.24‰ in sylvite of Spanish deposit), indicating that the compositions of the middle–late Eocene brines trapped in halite in the Yunying depression of China are very different from those derived from the contemporary seawater, and are considered to be resulted from evaporation of an inland saline lake water with little influence of seawater.