Origin of Halite in the Lanping–Simao Basin, Southeastern Tibetan Plateau, China: Evidence from Sr and Cl Isotopes

Abstract The Lanping–Simao Basin is located on the southeastern Tibetan Plateau, China, and contains massive evaporites. The origin of evaporites in the basin has been hotly debated because of the strong transformation by tectonic movement. Forty halite samples from borehole MK‐3 in the Mengyejing area of the basin were collected and analyzed using XRD, Cl‐Sr isotopes and chemical compositions to trace the origin of the evaporites in the basin. The Br × 10 3 /Cl ratios of the halite samples are between 0 and 0.55, most of which are synchronized with the law of seawater evaporation and at the stage of halite precipitation from seawater, indicating that the evaporites are mainly of marine origin. The 87 Sr/ 86 Sr ratios range from 0.707489 to 0.711279; after correction, the 87 Sr/ 86 Sr 145 Ma ratios range from 0.704721 to 0.707611, equivalent with the 87 Sr/ 86 Sr ratios of seawater at 145 Ma, indicating a marine origin. The decay of 87 Rb in the evaporite during deposition, change of the depositional environment and the unsealed environment at a later period resulted in the present 87 Sr/ 86 Sr ratios of some samples being high. The δ 37 Cl value compositions range from –0.38‰ to 0.83‰, which is consistent with the δ 37 Cl value composition of the world marine halite (–0.6‰ to 0.4‰), further confirming that seawater is the main origin. In addition, the high δ 37 Cl value of some samples at the boundary of the upper and lower evaporite layers might be related to the influence of δ 37 Cl‐rich brine and the incomplete dissolution of the halite.