40Ar/39Ar dating of supergene Mn-oxides from the Zunyi Mn deposit, Guizhou Plateau, SW China: Implications for chemical weathering and paleoclimatic evolution since the late Miocene

Abstract

40Ar/39Ar laser incremental heating analyses of potassium-bearing Mn-oxides from a weathering profile at the Zunyi manganese deposit, northern Guizhou Plateau, SW China, were applied to constrain the timing and rate of chemical weathering in critical zone and to provide insights into paleoclimatic conditions under which the weathering profile and associated Mn-oxides formed. Eighteen Mn-oxide grains from nine samples at three sites in saprolite zone of a single weathering profile yield well-defined plateau or pseudo-plateau ages ranging from 13.1±0.3 to 0.1±0.4Ma (2σ). The Mn-oxide ages are progressively younger toward lower elevations (13.1±0.3 to 11.0±1.8Ma at 951m; 3.69±0.07 to 2.16±0.16Ma at 922m; <1.3±0.2Ma at 914m), yielding an average weathering front propagation rate at 3.3m/Ma for the past 13.1Ma. The calculated weathering front propagation rate corresponds to a chemical denudation rate of 4.6±0.2t/km2/yr., which is consistent with values deduced from mass balance calculation for suspended and dissolved loads of river discharge across the Guizhou Plateau. The Mn-oxide 40Ar/39Ar ages suggest that humid climate conducive to intense weathering has prevailed over the Guizhou Plateau at least since the late Miocene. The climatic conditions inferred from the weathering geochronology are consistent with the continuous sedimentary and paleontological records from many Cenozoic basins over the Guizhou Plateau, confirming that supergene Mn-oxides can serve as a paleoclimate proxy for the past climatic condition. The present 40Ar/39Ar dating results, when combined with previous geochronological data of weathering profiles in the Yunnan–Guizhou Plateau and coastal areas of South China, document progressively intensive erosion toward areas proximal to the eastern margin of the Tibet Plateau.