The rhyolitic porphyroclastic lavas from the Heishan Formation, Dongchuan Group,Eshan area,Yunnan: Zircon U-Pb age and its geological significance

The classification of the stratigraphic sequences of the Mesoproterozoic "Kunyang Group" and "Dongchuan Group" in central Yunnan is a major basic geological issue that has been controversial for a long time. In this paper,more than 100 zircon grains are selected for age dating from a piece of the sample D0023 collected from nearly 5m-thick rhyolitic porphyroclastic lavas developed in the Heishan Formation of the Dongchuan Group in the Eshan area,and give a LA-ICP-MS zircon U-Pb weighted average age of 1569.2±4.5Ma. All the selected zircons have the Th/U ratios of more than 0.5 and exhibit clear oscillatory zonal structures,suggesting that these zircons are magmatic in origin. The age determinations for the volcanic rocks in this study indicate that the volcanic rocks in the Etouchang Formation of the Dongchuan Group formed during the early Mesoproterozoic should belong to the an orogenic-associated volcanic rock series,and may represent the responses in the Yangtze continental mass to the breakup of the Colombian supercontinent. The age data of the Heishan Formation in the Dongchuan Group have provided new age data for the delineation of the strata and sequences in the Dongchuan Group,and new information for the approaches to the formation and evolution of the basement on the western margin of the Yangtze continental mass.