Origin and recharge model of the Late Cretaceous evaporites in the Khorat Plateau

Abstract

Evaporites commonly occur in the Mesozoic-Cenozoic Tethyan domain, which characterized by extensive Late Cretaceous potash deposits in the Lanping-Simao Basin (LSB) in southwestern China and in the Khorat Plateau (KP) in Thailand and Laos. The LSB and the KP are located in the eastern Tethyan tectonic belt. The origin of the Late Cretaceous evaporites in these basins is controversial; possibilities include marine, continental, or hydrothermal origins. In addition, the recharge model for the major solutes into these evaporitic basins is inadequate, whether it is from the KP to the LSB or from the Qiangtang to the LSB to the KP. In this study, 34 gypsum, anhydrite, and halite samples from two sediment cores collected from the KP were analyzed to determine their stable B-Sr-S isotopic compositions. This is the first time that δ11B values have been reported for the anhydrite in the study area. The origin and evolutionary relationships of these evaporitic basins were investigated based on the geochemical data, sedimentary features, mineral sequences, and stratigraphic ages of the evaporites in the LSB and KP. The tectonic evolution and stratigraphic comparisons during the Triassic-Cretaceous in the eastern Tethyan domain were systematically reviewed and summarized in order to determine a preferred recharge model for the evaporites in the KP. The following conclusions were reached. (1) The reconstructed δ11B values (+38.20‰ to + 51.23‰) of the parent solution, which were based on those of the anhydrite (+8.20‰ to + 21.23‰), and the isotopic fractionation levels (30.2‰ to 32.7‰), 87Sr/86Sr ratios (0.70743–0.70846), and δ34S values (+14.39‰ to +15.94‰) of the anhydrite and halite in the KP overlap with those of Late Cretaceous seawater, suggesting a marine origin. (2) The similar mineral sequences and B-Sr-S isotopic signatures, and the comparable sedimentary features and inherited ore-forming ages indicate that evaporites in the LSB and KP have similar solute sources and evolutionary relationships. (3) The tectonic evolution and stratigraphy demonstrate that during the Late Cretaceous, paleoseawater from the Shan Boundary Ocean (the eastern segment of the Meso-Tethys Ocean) most likely passed through the southwestern part (Tengchong-Baoshan block) of Sibumasu and preferably recharged marine solutes into the LSB and KP.