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

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 x 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 Sr-87/Sr-86 ratios range from 0.707489 to 0.711279; after correction, the Sr-87/Sr-86 145 Ma ratios range from 0.704721 to 0.707611, equivalent with the Sr-87/Sr-86 ratios of seawater at 145 Ma, indicating a marine origin. The decay of Rb-87 in the evaporite during deposition, change of the depositional environment and the unsealed environment at a later period resulted in the present Sr-87/Sr-86 ratios of some samples being high. The delta Cl-37 value compositions range from -0.38 parts per thousand to 0.83 parts per thousand, which is consistent with the delta Cl-37 value composition of the world marine halite (-0.6 parts per thousand to 0.4 parts per thousand), further confirming that seawater is the main origin. In addition, the high delta Cl-37 value of some samples at the boundary of the upper and lower evaporite layers might be related to the influence of delta Cl-37-rich brine and the incomplete dissolution of the halite.