EVIDENCE FOR EXTREME ARIDITY DURING THE EARLY AND MIDDLE TRIASSIC OF THE SICHUAN BASIN, CHINA

The Sichuan basin is located in the western part of the Yangtze plate. In the early-middle Triassic, the Sichuan basin was at about 10°N.during which time evaporates, that are part of the Jialingjiang Formation, were deposited. Fluid inclusion analysis of the halite in these evaporites indicate an extremely arid climate. The homogenization temperatures on 131 fluid inclusion range from 17.7℃ to 63.5℃with an average temperature of 33.7 ±11.0 ℃. However, average temperature of various strata ranges from a low of 22.3 ± 2.78 °C to a high of 48.5 ± 7.51°C. and indicates variable aridity or evaporation throughout the depositions history. Maximum homogenization temperature recorded in halite deposits is often interpreted to reflect maximum surface temperature of the brine (evaporated seawater). Following this paradigm maximum surface temperature appear to have oscillated from ~31°C to ~64°C during the deposition of the evaporites of the Jialingjiang Formation. LA-ICP-MS was utilized to analyze single fluid inclusion chemical composition, K range is 1.82-63.51g/l, and average 23.41 ± 39.41g/l; Mg range is 0.24-48.92 g/l, and average is 12.58± 35.13 g/l; and Ca ranges from 0.04 to 21.64 g/l, average is 4.33 ± 12.31g/l. Chemical composition suggest late stage for the evaporation of the seawater. δ D and δ¹⁸ O of the water in the fluid inclusion in halite is very negative, the average is -21.93 ±1.87 and 8.26 ±1.86 of the δ D and δ¹⁸O. δ³⁷Cl values for halite range from - 0.83 to 0.0 ‰, with an average of -0.49 ± 0.41. δ⁴⁴Ca values for anhydrite range from the 0.21 to 1.79 ‰. The negative δ³⁷Cl values and low δ⁴⁴Ca values are consistent with highly evaporated seawater. In all data are suggestive of very arid evaporative climate in the eastern Sichuan Basin during the Early to Middle Triassic.