40Ar/39Ar geochronology of supergene K-bearing sulfate minerals: Cenozoic continental weathering and its paleoclimatic significance in the Tu–Ha Basin, northwestern China

Abstract

40Ar/39Ar incremental-heating analyses of supergene jarosite and yavapaiite were conducted for three weathering profiles at the Hongshan, Liuhuangshan, and Caihuagou deposits in the Tu–Ha Basin. The analyses provide numerical constraints on the timing and duration of weathering and supergene enrichment. Well-constrained plateau ages and best-fitting inverse isochrons yield 40Ar/39Ar ages ranging from 33.3±0.5Ma to 3.3±0.4Ma (1σ). Our 40Ar/39Ar ages, combined with the published ages, indicate that a protracted history of weathering and supergene enrichment and, by inference, arid–semiarid climate (with at least a moderate amount of precipitation (>10cm/y)) favorable to intense chemical weathering emerged at 33.3Ma, 27.7–23.3Ma, and 16.4–14.7Ma, and prevailed from 11 to 7.8Ma. Then, a progressive change from arid–semiarid toward hyperarid conditions and predominantly hyperarid conditions may have persisted since at least ca. 3.3Ma. The climatic implications inferred from the weathering geochronology are in agreement with the chemical parameters and isotopic compositions of the Cenozoic sedimentary sequence from the Lianmuqin section in the Tu–Ha Basin, attesting to the reliability of weathering geochronology by the 40Ar/39Ar method as an indicator of paleoclimate in arid areas. Our results suggest that the retreat of the Paratethys Sea, which would have reduced eastward water vapor transport by the westerlies to the Tu–Ha Basin, led to its aridification in the Oligocene and that increased rain shadow effects, resulting from uplift of the Tibetan Plateau and Tian Shan Mountains, played important roles in the aridification history of the Tu–Ha Basin since the late Miocene.