|
Shihong Zhang, China University of Geosciences (Beijing) (China)
David A D Evans, Yale University (United States)
Huaichun Wu, China University of Geosciences (Beijing) (China)
Ganqing Jiang, University of Nevada Las Vegas (United States)
Jin Dong, China University of Geosciences (Beijing) (China)
Tim Raub, Yale University (United States)
Haiyan Li, China University of Geosciences (Beijing) (China)
Qingle Zhao, China University of Geosciences (Beijing) (China)
|
|
The Nantuo Formation is well-known as the "Marinoan" glaciogenic stratum (654-635 Ma) distributed throughout the Yangtze Platform (YP) in South China. Paleomagnetic results obtained directly from this formation are crucial to understanding the paleogeographic position of the YP among the fragmented landmasses begat by Rodinia and also assessing extreme paleoclimatic changes during the late Neoproterozoic. However, because of widespread remagnetization across the YP, the few acceptable paleomagnetic results that have been obtained from the Nantuo Formation thus far could benefit from more concrete field tests (eg., reversals) and comparison with sections possessing direct geochronological constraints. Because of the lack of reliable data directly from the Nantuo Fm., recently some researchers have even proposed that the Nantuo glaciations might have formed close to the equator, based on a shallow remanent magnetization in Doushantuo Formation (635-550 Ma) that overlies the Nantuo Fm. However, we question whether this Doushantuo component is primary in light of several new paleomagnetic data.
Our new sampling section is located between Guzhang and Jishou, western Hunan Province, where ∼40m-thick laminated red beds in the middle Nantuo formation are well-exposed along the highway from Zhangjiajie to Jishou. Ninety-four samples were collected and subjected to stepwise AF and thermal demagnetization. Remanent magnetizations were measured using a 2-G superconducting magnetometer in the paleomagnetic laboratory at Yale University. The K-T curves for selected samples and AMS for all specimens were measured using a KLY-4A kappabridge in the China University of Geosciences, Beijing. Rock-magnetic analyses have demonstrated that the main magnetic carriers are hematite and magnetite. The samples also contain pyrite, which converts to pyrrhotite upon heating over 590 degrees C. Remagnetization great circles were obtained for the majority of samples, as well as several stable endpoints. A combined analysis of the great circles and stable endpoints shows, after removal of Cenozoic and Mesozoic remagnetizations, a dual-polarity, probably primary component for the Nantuo Formation, directed eastward and moderately down and westward, moderately up. This component, yielding a medium paleolatitude, resembles that of Liantuo Formation (Evans et al., 2000) and not that of Doushantuo Formation (Macouin et al., 2004).
The significant paleomagnetic difference between Nantuo and Doushantuo formations may imply three possibilities: significant intervening age gap encompassing measurable plate motion; rapid true polar wander during this period; or shallow eastward remagnetization of Doushantuo Formation, most likely in Paleozoic time.
These new paleomagnetic results, together with those of Liantuo and selected Paleozoic data, support a South China-Australia connection with South China placed against northwest Australia.
|
