A high-quality mid-Neoproterozoic paleomagnetic pole from South China, with implications for ice ages and the breakup configuration of Rodinia

Neoproterozoic (Sinian) sediments are exceptionally well preserved in the T hree Gorges region (western Hubei Province) of the South China block. We re port new paleomagnetic results, obtained independently by two separate labo ratories, from a total of 157 samples of the 748 +/-12 Ma, basal Sinian Lia ntuo Formation at its type locality. Detailed thermal demagnetization proce dures and least-squares line analyses reveal three distinct magnetic compon ents among the suite of samples. Two overprint components can be distinguis hed from each other by their laboratory unblocking temperatures. The first to be removed ('C'), always annihilated below 600 degrees C, is common thro ughout the dataset but is amenable to least-squares line-fitting in only 37 samples. It yields a pole which in present coordinates resembles Mesozoic overprints identified from previous studies in the Three Gorges region (75. 7 degrees N, 174.3 degrees E, d(p) = 6.0 degrees, d(m) = 8.3 degrees, Q = 4 ). The higher unblocking-temperature overprint ('B'), always subsidiary to the 'A' component, is more prevalent than 'C' and was identified by line-fi tting in 67 samples. The 'B' direction is very steep and generates a paleop ole whose in situ coordinates do not resemble the Mesozoic-Cenozoic apparen t polar wander path for South China, and whose tilt-corrected coordinates ( 20.3 degrees N, 106.2 degrees E, d(p) = 7.2 degrees, d(m)= 7.3 degrees, Q = 5) bear no resemblance to any reliable Phanerozoic paleopoles from the Sou th China block. The steep 'B' direction, if an unbiased representative of a n ancient geomagnetic dipole held, was probably acquired some time in the 2 00 m.y. interval between deposition of the Liantuo Formation at similar to 750 Ma and Cambrian time. The most stable component is a two-polarity reman ence, removed at temperatures predominantly >630 degrees C, which we infer to reside in hematite. A change in polarity of this component occupies a si milar stratigraphic position (within 5 cm) among three outcrops separated b y similar to 100 m lateral distance. We calculate a mean paleomagnetic pole from each of the laboratories' datasets and combine these with a previousl y determined pole from correlative rocks in Yunnan ['N1' of Zhang and Piper , Precambrian Res. 85 (1997) 173-199], to obtain an overall weighted mean p aleomagnetic pole (04.4 degrees N, 161.1 degrees E, A(95) = 12.9 degrees, Q = 7) for the South China block at 748 +/- 12 Ma. The combined 'Z1' pole is considered to be primary based on its thermal stability, its magnetostrati graphic consistency, and a soft-sediment fold test determined by previous w ork. Results from individual sampling areas constrain the depositional pale olatitude of the Liantuo Formation and equivalent Sinian rocks to 30-40 deg rees. This result applies to one or both of the stratigraphically adjacent Chang'an and Nantuo glacial deposits; unfortunately, it is neither high nor low enough to refute any of the conceptual models for the enigmatic Neopro terozoic glaciations. The new basal Sinian paleopole, in the context of rec ent paleomagnetic and geochronological results from Australia. suggests tha t the Nantuo glaciation is pre-Marinoan. The new 'ZI' pole may also provide constraints onthe various proposed reconstructions of South China's positi on in Rodinia. In particular, a paleoposition adjacent to northwestern Aust ralia at similar to 750 Ma requires a specific relative orientation between the two blocks. Likewise, if Rodinia were still intact by 750 Ma, South Ch ina may have lain between Australia and Laurentia only in an orientation di fferent from that originally proposed in the 'missing link' hypothesis. As a final alternative. the new paleomagnetic data could be used to position South China, Australia , and Laurentia in an immediately post-Rodinian paleogeography around the n ascent Pacific Ocean. (C) 2000 Elsevier Science B.V. All rights reserved.