Late Mio-Pliocene chemical weathering of the Yulong porphyry Cu deposit in the eastern Tibetan Plateau constrained by goethite (U–Th)/He dating: Implication for Asian summer monsoon

Abstract

Chemical weathering has provided a potentially important feedback between tectonic forcing and climate evolution of the Asian continent, although precise constraints on the timing and history of weathering are only variably documented. Here, we use goethite (U–Th)/He and 4He/3He geochronology to constrain the timing and rates of chemical weathering at the Yulong porphyry Cu deposit on the eastern Tibetan Plateau. Goethite grains have (U–Th)/He ages ranging from 6.73±0.51 to 0.53±0.04 Ma that correlate with independent paleoclimatic proxies inferred from supergene Mn-oxides and loess deposits under variable tectonic regimes and vegetation zones over the southeastern Asia. This correlation indicates that regional climatic conditions, especially monsoonal precipitation, controlled chemical weathering and goethite precipitation in a vast area of southeastern Asia. The goethite ages suggest that the Asian summer monsoon was relatively strong from 7 to 4.6 Ma, but weakened between 4.6 and 4 Ma, and then significantly intensified from 4 to 2 Ma. The precipitation ages of goethites collected along a 100-m-thick weathering profile decrease with depth, and indicate a downward propagation of the weathering front at rates of <6.7, 53.5±10.8, and 4.8±0.6 m/Ma during the intervals of 7–4, 4–2, and 2–0.7 Ma, respectively. The rapid propagation of weathering front during 4–2 Ma was caused by abrupt lowering of the water table, which was possibly related to local surface uplift or reorganization of the river systems in southeastern Tibet during this period.