Determination of the Nanhua System volcanic rocks in Anyi on the western margin of the Yangtze Block

Metamorphic basic volcanic rocks composed of metamorphic basalts and some other rocks are exposed in the Dawanshan area of Anyi on the western margin of the Yangtze Block. Due to the lack of geochronologic data, previous researchers classified these rocks as belonging to the Middle Proterozoic and considered them as the main target for magnetite prospecting. The authors found that a stable sedimentary conglomerate is developed between the metamorphic basic volcanic rocks and the underlying metamorphic rocks in the Dawanshan area of Anyi. LA-ICP-MS tech was applied to measuring the U-Pb age of zircons from metamorphic basalt above most conglomerates. The ²⁰⁶Pb/²³⁸U weighted average values of 781.3±1.9Ma, and 1008±14Ma, 1142±15Ma, 2714±10Ma for ²⁰⁷Pb/²⁰⁶Pb ages of inherited magma zircon were obtained, suggesting that the metamorphic basic volcanic rocks were formed in Nanhua epoch. The authors separated them from the shallow metamorphic rock series and assigned them to Chengjiang Formation through correlation. The ages of the inherited zircons and the geological characteristics of this region suggest that the Neoarchean and Mesoproterozoic material records exist in the Anyi area on the western margin of the Yangtze Block. Based on previous research results and the age of tuff obtained from Chengjiang Formation by the authors, the temporal boundary of Chengjiang Formation in central Yunnan was defined at 820~740Ma, and the age of the bottom of Nanhua System was defined at 820Ma. Dawanshan middlelarge magnetite was produced in the metamorphic basic volcanic rocks of Chengjiang Formation on the western margin of the Yangtze Block. The main ore-forming activity occurred in the Nanhua period and may belong to the IOCG deposit.