NANHUA PERIOD IN THE NORTHERN YANGTZE: A STANDARD CRYOGENIAN?

The Nanhua period in South China is relative to Cryogenian even both of them have no definite basal levels.

After several years detailed work, we have distinguished a suite of strata of the Nanhua period in the northern Yangtze from the metamorphic rocks involved by the Qinling-Dabie subduction belt. The strata consist of interbeded volcanic/ pyroclastic and sedimentary rocks, and begin with an angular disconformity and fast-depositing conglomerate, then follow by the basaltic pyroclastic and volcanic rocks, which indicates an explosive magmatism accompanying by the fast uplifting of the crust led by the large-scale of mantle swell.

After obvious uplifting, the strata are recorded continuously by the Macaoyuan Formation, the delta facies of the upward Shennongjia area, the Wudang Group, the fast-depositing of volcanic materials on the continental margin, and the later Yaolinghe group. Even though they did not occur in the same section, the strata of the Nanhua period continued from the top of liufangzui Formation, Macaoyuan Formation, Wudang Group and Yaolinghe Group clued by the continual volcanic depositions. The zircon U-Pb ages not only sequenced the Nanhua successionperfectly from several key layers of volcanic rocks, but also constrained the number, synchroneity and durations of glacial deposits well.

As a result, the Nanhua (Cryogenian) period should begin at 826Ma and have a basal level with a defined tectonic implication and precise age. And we suggest an about 200m-long roadcut section in the south of the Dahongshan Mountains in this area as a candidate of GSSP between Tonian and Cryogenian.