Sr isotopic characteristics and fractionation during weathering of a small granitic watershed system in the Jiuhua Mountains (eastern China)

摘要

To determine the influence of the Mesozoic Yanshanian orogeny in East Asia on contemporaneous Sr isotopic evolution of seawater, we systematically investigated the weathering profile, riverbed sediment and stream water in mono-lithological small granitic watersheds of the Jiuhua Mountains, Anhui, eastern China. Analysis based on 190 samples from 1 to 2 samplings per month during an entire hydrological year, spanning July 2014 to June 2015, shows that the infra-annual change of Sr concentration is 10-70%, but Sr-87/Sr-86 ratios only exhibit a slight change (0.709148-0.710427). This result indicates that using single sampling data to evaluate the influence of chemical weathering on the Sr-87/Sr-86 ratio evolution of seawater may cause some deviations. The Sr-87/Sr-86 ratio in the small granitic watersheds of the Jiuhua Mountains is 0.709148-0.710427 with an average of 0.710021, which is significantly higher than the lowest value (0.7068, similar to 160 Ma) of seawater in the Phanerozoic. During and after this period, the East Asian continent experienced a strong tectonic event - Yanshanian orogeny and formed widespread Jurassic-Cretaceous igneous rocks, such as the Jiuhua Mountains granite in the Yangtze Block. The Yanshanian granites in several main tectonic units in China exhibit high radiogenic Sr characteristics. Combined with the evidence of enhanced chemical weathering during Late Jurassic-Early Cretaceous, the notable increase of the seawater Sr-87/Sr-86 ratio after 160 Ma may be related to the Yanshanian orogeny in East Asia.