Geochemistry of Mesozoic and Cenozoic sediments in the northern Qaidam basin, northeastern Tibetan Plateau: Implications for provenance and weathering

Abstract

Whole-rock geochemical data of the Mesozoic and Cenozoic sediments in the northern Qaidam basin were used to reconstruct the provenance and chemical weathering history. Based on the fairly uniform REE patterns and trace element ratios, both the Mesozoic (LaCN/YbCN=10.37±1.75; Eu/Eu*=0.68±0.07; Th/Sc=1.11±0.38) and Cenozoic (LaCN/YbCN=9.77±0.62; Eu/Eu*=0.69±0.03; Th/Sc=0.97±0.14) mudstones were derived from a similar source area with acidic-intermediate rocks as dominant contributors, and two modeled mixtures composed of 60% granite, 35% quartzdiorite and 5% mafic rocks, and 55% granite, 40% quartzdiorite and 5% mafic rocks can be deduced as the potential source compositions for the Mesozoic and Cenozoic sediments, respectively. This conclusion is reinforced by the mudstone major element composition and sandstone petrography. The Early–Middle Jurassic mudstones have relatively high K-corrected CIA (84–93) and PIA (88–99) values, indicating intense chemical weathering conditions; while the Late Jurassic to Cenozoic sediments have variable corrected CIA (51–85) and PIA (50–92) values, implying predominantly mild to moderate weathering conditions. The chemical weathering history suggests that a warm and humid climate prevailed during the Early–Middle Jurassic, whereas a cool and semiarid to arid climate prevailed from the Late Jurassic up to now. This dramatic transition of climate may be attributed to multiple likely factors, including the restructuring of global atmospheric circulation patterns in response to the breakup of Pangea since Late Jurassic, and the orogenic rejuvenation and high elevation of the Qilian Mountains, northeastern Tibetan Plateau. Furthermore, the decreasing chemical weathering intensity over time indicates the increase of aridification and cooling of the climate culminating in the Late Cenozoic.