Neogene weathering and supergene manganese enrichment in subtropical South China: An 40Ar/39Ar approach and paleoclimatic significance

Deep weathering profiles and associated supergene Mn-oxides deposits are widely distributed in South China. However, data on their ages are not available until most recently. 40Ar/39Ar laser incremental heating analysis of potassium-bearing supergene Mn-oxides collected from four weathering profiles in the Qinzhou-Fangcheng Mn belt, Guangxi Province, South China, provides, for the first time, numerical constraints on timing of weathering and supergene Mn enrichment. Thirty-eight of 42 cryptomelane grains yield well-defined plateau (36 grains) or pseudo plateau (2 grains) ages ranging from 17.6 ± 0.5 Ma to 4.71 ± 0.08 Ma (2σ). The results, when combined with data from Mn-oxides deposits from the adjacent Guangdong and Hunan Provinces, document a protracted history of weathering and supergene Mn enrichment, spanning from the earliest Miocene to the latest Pliocene in South China. This indicates that warm and humid climatic conditions conducive to intense weathering and secondary mineralization prevailed across South China during the whole Neogene. The climatic conditions inferred from weathering geochronology are consistent with sedimentary records and fossil floral and fauna associations from Cenozoic inland and marginal sea basins of South China. The weathering geochronology data also provide temporal constraints on the initiation and intensification of the East Asian monsoon that has been the cause for the South China's humid climate.