Sulfur isotopic composition of sulfate and its geological significance of the Yunlong formation in the Lanping Basin,Yunnan Province

The Lanping Basin is situated in the south part of the Sanjiang Tethys.The Yunlong Formation in the Lanping Basin contains gypsum-rock salts,which is significant to origin of potash salts and Jinding Zn-Pb deposit.Recent advances in the Simao Basin inferred that evaporates in the equivalent Mengyejing Formation originated from marine incursion of the Neo-Tethys Ocean.Did evaporates of the Yunlong Formation originate from the Neo-Tethys Ocean.A combined analysis of petrology and sulfur isotopic geochemistry was performed.The results show that sulfur isotope values of primary gypsum samples from Jinding,Xiaoyanjing,Lajing,and Hexi are between 12.6‰~17.6‰,with most values in the range of 13‰~15‰.These values,which have no effect of bacteria sulfate reduction,are similar to that of contemporaneous marine anhydrites in the Vientiane and Khorat basins and anhydrite in the Mengyejing Formation.Most values are in the range of δ³⁴ SV-CDT of the Cretaceous sea water indicating marine origin.Together with previous palaeogeographic and palaeomagnetic data,we speculated that a vast evaporatic basinsystem was formed by marine incursion during the Late Cretaceous.