Age constraints on the Manghuihe Formation in the Jiukang area, western Yunnan: evidence from zircon U-Pb age and bivalve fossils

The widely distributed volcano-sedimentary strata in the Yunxian-Jinghong area of western Yunnan not only constitutes an important part of the southern Lancangjiang tectonic zone, but also is host of sizable copper (silver) deposits.Unfortunately, the age of this strata remains a topic of heated debate, which hinders our understanding of the evolution of the Lancangjiang zone as well as the geological background of copper mineralization.In this study, a U-Pb (LA-ICP-MS) weighted average age of 237.9 ± 1.1 Ma for the zircon grains was obtained from the crystal tuff in the lower part of Jiukang section in Jinggu County.In addition, a large number of bivalve fossils of Late Triassic in age, including 7 genera and 16 species, were collected in the upper part of the study section for the first time. Detailed examination allows the establishment of the Cardium- Schafhaeutlia- Unionites? assemblage, which has been further determined to be of the middle Carnian to Rhaetian in age and comparable with the contemporaneous bivalve groups from the southwest Guizhou, southeast Yunnan and northern Guangxi.Accordingly, the volcano-sedimentary strata can be correlated with the Late Triassic Manghuihe Formation and is not of an Early Jurassic age as presumed by previously studies.This study not only is of great significance for the determination of the age, stratigraphic division and regional correlation of the highly-disputed volcano-sedimentary sequence in the southern Lancangjiang tectonic zone, but also provides new complementary data for the study of the tectonic evolution of southern Lancangjiang tectonic zone and the geological background of copper mineralization.