CHEMICAL WEATHERING IN THE THREE RIVERS REGION OF EASTERN TIBET

Three large rivers-the Yangtze (Jinsha Jiang), Mekong (Lancang Jiang)and Salween (Nu Jiang)-originate in eastern Tibet and flow through suture zones generated by the collision of India and Eurasia plates. These three rivers run in close parallel over 300 km (Hallet and Molnar, 2001), but their geochemical composition is dissimilar. We sampled at 74 stations in the Three Rivers region and their widely varying compositions reflect the complex geologic makeup. The ternary diagrams indicate that the main channels of the Three Rivers have been affected by carbonate (CaCO₃) weathering. At local tributaries, however, distinctive evaporite and silicate signatures are seen. The total dissolved solids (TDS) range from 30 mg/l to 3000 mg/l. Some headwater tributaries of the upper Yangtze on the Tibetan Plateau are dominated by evaporite dissolution as evidenced by high TDS values (928 mg/l and 3036 mg/l) and Na-Cl dominant major element composition. The major element composition of the Three Rivers bears closest resemblance to the Amazon River. Four reservoirs, rain (atmosphere), silicate, carbonate and evaporite are considered an inversion model for calculating the contribution of each lithology. Carbonate weathering in the Mekong accounts for 47 %~89 % of the TZ+. Carbonate weathering also plays an important role in the Yangtze and Salween though not as great as in the Mekong. The inversion results are sensitive to end-member designation. The ⁸⁷Sr/⁸⁶Sr ratio in the Three Rivers region ranges from 0.7088 to 0.7296. Sr isotope is greater than 0.725 are found near the Linchang batholith.

Hallet, B. and P. Molnar (2001) Distorted drainage basins as markers of crustal strain east of the Himalaya. Journal of Geophysical Research 106, pp. 13697-13709.