Elemental geochemistry of the Middle Jurassic shales in the northern Qaidam Basin, northwestern China: Constraints for tectonics and paleoclimate

Abstract

The elemental geochemical characteristics of mudstones/shales are good tracers for indicating the evolution of tectonics, paleoenvironment, and paleoclimate. Based on the continuous sampling of drilling cores from the Middle Jurassic Dameigou and Shimengou Formations in the northern Qaidam Basin, the major, trace, and rare earth elements of the 31 mudstones and shales were analyzed. The information on the evolution of tectonics, provenance, and paleoclimate during Middle Jurassic was also recovered. The results show that: (1) A couple of elements consisting of Sc, Y, V, Cr, Co, Ni, Cu, Zn, Th, and U are relatively enriched, indicating that the contents of siderophile and chalcophile elements are significantly high in the Middle Jurassic samples; (2) Changes in the chemical index of alteration, Ga/Rb, and K2O/Al2O3 ratios in the mudstone/shale samples suggest that the paleoclimate was changed from warm and humid in the early stage to cold and dry in the middle stage and to hot and arid in the late stage; (3) The Middle Jurassic provenance of the northern Qaidam Basin was predicted from upper crust and felsic rocks to the mixed felsic rocks and basic rocks; (4) The Middle Jurassic tectonic background was changed from passive continental margin to active continental margin and oceanic island arc. The paleoclimatic and paleogeographic evolution of northern Qaidam Basin were closely related to the surrounding paleo-oceanic and ancient plate activities. In the early stage of the Middle Jurassic, the extensional activity in the passive continental margin and the water vapor input was caused by the Tethys Ocean, resulting in a warm and humid paleoclimate. In the late stage of the Middle Jurassic, the tectonic background of the study area tended to be an oceanic island arc caused by compressive tectonic, which blocked the monsoon input and led to a hot and arid paleoclimate. The establishment of multiple geochemical profiles can provide a scientific basis for the climate changes in greenhouse–icehouses and source–sink systems of the Middle Jurassic in northwestern China.

Investigation on Adsorption Pore and Fractal Analyses of Low-Rank Coals in the Northern Qaidam Basin

“…This indicates that the structural complexity of the western study area is higher than that of the eastern area mainly due to the different influences of regional geological context. Specially, both orogenic belts, including Altun strike-slip fault and Qilian Mountain, affect the western area, whereas only Qilian Mountain affects the eastern area, resulting in more complex coal reservoirs and strata structural complexity. − The average D 2 of Saishiteng and Yuqia coalfields in the west is 2.88, and the mean D 2 of Quanji and Delingha coalfields in the east is 2.68. Compared with D 1 and D 2 data, it can be found that the fractal dimension D 1 has a little change and a high correlation coefficient, whereas the fractal dimension D 2 is significantly different, and the D 2 of type III with a weak hysteresis loop is significantly lower than that of type I and type II with an obvious hysteresis ring (Tables and ).…”