Abstract:

 Clay mineral assemblages and chlorite composition are often analyzed to study diagenesis of sediments and burial paleotemperature in basins. A number of previous studies show that clay mineral assemblages and chemical compositions of chlorites are related to the progressive burial depth/temperature in many sedimentary basins. However, some phenomena show that this opinion could not explain features in mudstones from some particular regions on the Earth. In order to investigate the main factors that influence diagenesis in basins, this paper focuses on the characteristics of clay mineral assemblages, and composition of chlorites in the mudstone in Xinjiang Tabei Uplift, China, as well as the relationship between the characteristics and the paleosalinity of the basin. The results show the following four points. (1) The assemblages of the clay minerals in the depth range of 2777.37 m to 4604.41 m, which were generated during Jurassic Period and Eogene Period (J1-N2), are I+Chl and I+Chl+Ka; while the assemblages in the depth range of 5071.52 m to 5299.37 m, which were generated during Triassic period (T1), are R0I/S+ Chl+I and R0I/S +Chl. (2) The diagenetic chlorite in this area is brunsvlgite in TypeⅡb polymorphism; its composition features show that (a) the average value of octahedral occupancy is 11.532; (b) the octahedral Al amount is obviously greater than the tetrahedral Al amount; (c) the average value of（Fe+Mg）is 4.016; (d) the ratio of Si/Al is higher than 1. (3) According to the distribution of the mineral salts and the data of ancient salinity in the area, it is proposed that paleosalinity was the main factor that affects the clay mineral assemblages and the compositional changes of chlorites in the area. In this paper, paleosalinity is utilized to explain an abnormal phenomenon in the clay mineral assemblages. There is not a disorderly mixed layer of illite / smectite (R0I/S) in the samples from the depths less than 5000 m, while there is a disorderly mixed layer of illite/smectite(R0I/S) in the samples from the depths greater than 5000 m. Also, it is revealed that the composition of chlorites is affected by the paleosalinity in a certain depth range (2777.37~4405.27 m) and certain ancient salinity conditions (Sr/Ba ratio≥0.4 with the appearance of salt minerals). While the salinity is higher, its influence is greater. With the increase of salinity, the contents of Al, ⅣAl, Na, Na2O, Al/Si in chlorites increase, while Si/Al decreases. The temperature obtained by the chlorite composition thermometer increases with higher salinity, which indicates that the temperature calculated by the chlorite composition thermometer cannot represent the actual burial paleotemperature in the area. (4) Between the composition of the chlorites and the host rocks in the area, such as SiO2chl-SiO2rock and FeOchl -FeOrock, there are negative correlations, which are not disturbed under the influence of paleosalinity.

Key words:  TabeiUplift, Xinjiang, claymineralassemblages, chloritecomposition, paleosalinity, rockchemicalcomposition