Geochronology of the Baye Mn oxide deposit, southern Yunnan Plateau: Implications for the late Miocene to Pleistocene paleoclimatic conditions and topographic evolution

Abstract

40Ar/39Ar laser incremental heating analyses of supergene K-Mn oxides from weathering profiles at the Baye Mn deposit, southern Yunnan Plateau, SW China, were carried out to place constraints on the timing of weathering and derive insights into local paleoclimatic and landscape evolution. Weathering profiles in the Baye Mn mine are dominated by 20–30m thick saprolites, which are locally covered by unconsolidated alluvial deposits. We analyzed 70 grains from 35 hand samples collected from four sites located at distinct elevations. In most cases, different grains from the same hand sample and different samples from the same site have reproducible 40Ar/39Ar plateau ages. The plateau ages of individual grains are also consistent with the respective isochron, integrated, and ideogram ages. This age consistency attests to the reliability of the geochronological results as numerical constraints on the formation and evolution of these weathering profiles.Sixty-four grains from 32 hand samples collected from weathering profiles at four sites (A to D) yield well-defined plateau or pseudoplateau ages ranging from 2.98±0.07 to <0.16Ma (2σ). The age results show that Mn oxides become progressively younger towards lower elevations: samples collected from the highest elevation site (∼1855m a.s.l.) have the oldest ages at 2.98±0.07 to 2.83±0.13Ma; samples from intermediate elevations (1821 to 1815m a.s.l.) yield ages ranging from 2.91±0.10 to 2.08±0.11Ma; and those from the lowest sites (1768 to 1753m a.s.l.) are younger than 1.25±0.08Ma. The age versus elevation relationship indicates fast downward propagation of weathering front since the late Pliocene, and permits estimating the weathering rates at 3.3±3.8, 6.6±1.2, and 11.1±1.9mMyr−1 during the time periods of 2.98–2.83, 2.91–2.08, and 1.25 to <0.16Ma, respectively. The average river incision rates estimated from the surface exposed ages range from 235±10 to 416±19mMyr−1 during the past 3 million years. The incision rate calculated from minimum exposed ages at sites B–D is 33±2mMyr−1 for the time period of 2.91–1.25Ma, which is consistent with the slow incision of streams on the ridges over the Yunnan Plateau since the early Pleistocene.Three Mn oxide pebbles contained in the alluvial deposits overlying the saprolite-dominated weathering profiles at the two highest sites provided additional information on the timing of weathering. Four grains from two pebbles yield plateau ages of 6.32±0.19 to 5.27±0.10Ma, whereas the other two grains from the third pebble indicate the minimum formation ages of 8.2±0.4 and 9.3±0.3Ma. These ages confirm the existence of older weathering profiles, now dismantled, in the region. Manganese oxide 40Ar/39Ar ages of the Baye deposit, when combined with results from other localities, indicate that lateritic weathering and supergene Mn enrichment and, by inference, warm and humid climates conducive to intense weathering have prevailed over the Plateau since the middle to late Miocene. The climatic conditions inferred from the weathering geochronology are consistent with multiple independent marine and terrestrial sedimentary and paleontological records, confirming that supergene Mn oxides can be used as a useful proxy for past climate. Age clusters of Mn oxides at 2.9–2.4, 1.2–0.8, and 0.6–0.4Ma are broadly coincident with and thus likely reflect intensification events of the Indian Summer Monsoon that brings moisture and abundant precipitation to the Yunnan Plateau. These clusters also coincide with the periods of significant surface uplift in the Yunnan Plateau, demonstrating a causal link between topographic evolution, plateau uplift, and intensification of the monsoonal climate.