Huashan Group in Northern Margin of Yangtze Block: A Suite of Back-Arc-Basin Volcanic-Sedimentary Strata but Not Ophiolite Mélange

The Huashan Group, which was once regarded as the eastward extension part of the Paleozoic "Mianlüe ophiolite" or the Neoproterozoic subduction-accretion-complex, has been controversial so far. To identify its tectonic significance and its relationship with the Rodinia supercontinet, the petrogenesis and age of the Liufangzui Formation by clastic tracing, redox index, tectonic discrimination and zircon U-Pb dating were studied. The sedimentary basin of the Liufangzui Formation is a marginal sea with oxidation condition, in which the clastic rocks had two sources, including sand debris from the Yangtze Block and basaltic glass debris, suggesting that the strata and the basalt were associated products in the same period, but not emplacements in a melange. The basalts which have dual-characteristics between MORB and IAB, are actually BABB. 823.9±5.5 Ma and 829.9±6.9 Ma of zircon ages were obtained, as well as 1 637 Ma and 2 670 Ma of xenocrystal zircon ages from these basalts, which implies that there was probably an continental basement in the lower part of the volcanic channel, but not a mid-ocean-ridge. The Liufangzui Formation of the Huashan Group is a Neoproterozoic back-arc-basin sedimentary-volcanic strata rather than an ophiolitic mélange. In Late Neoproterozoic, a plate-subduction movement took place in the northern margin of Yangtze Block, which lagged behind the global Rodinia supercontinent breakup event. © 2021, Editorial Department of Earth Science. All right reserved.