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

Abstract

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.