Abstract: The Maoniushan Formation of molasses rests unconformably on pre-Devonian strata and records the closure time of the Early Paleozoic ocean. We made zircon U-Pb dating for dacite from upper volcanic rock Member of Maoniushan Formation by SHRIMP. 11 grains analyses on zircons indicate weighted mean 206Pb/238U ages of(406.1±2.9)Ma, interpreted to be the time of the formation of volcanic rocks. Therefore, it is suggested that the time of the formation of the Maoniushan Formation should be in the Early Devonian. It also supports the conclusion made from zircon U-Pb dating for rhyolite rocks from bottom volcanic rocks of Maoniushan Formation by LA-MC-ICP-MS, which the closure time of the Early Paleozoic ocean should be in the Late Silurian-Early Devonian.