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

Abstract

Neoproterozoic (Sinian) sediments are exceptionally well preserved in the Three Gorges region (western Hubei Province) of the South China block. We report new paleomagnetic results, obtained independently by two separate laboratories, from a total of 157 samples of the 748±12 Ma, basal Sinian Liantuo Formation at its type locality. Detailed thermal demagnetization procedures and least-squares line analyses reveal three distinct magnetic components among the suite of samples. Two overprint components can be distinguished from each other by their laboratory unblocking temperatures. The first to be removed ('C'), always annihilated below 600°C, is common throughout 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°N, 174.3°E, d_p=6.0°, d_m=8.3°, Q=4). The higher unblocking-temperature overprint ('B'), always subsidiary to the 'A' component, is more prevalent than 'C' and was identified by line-fitting in 67 samples. The 'B' direction is very steep and generates a paleopole whose in situ coordinates do not resemble the Mesozoic–Cenozoic apparent polar wander path for South China, and whose tilt-corrected coordinates (20.3°N, 106.2°E, d_p=7.2°, d_m=7.3°, Q=5) bear no resemblance to any reliable Phanerozoic paleopoles from the South China block. The steep 'B' direction, if an unbiased representative of an ancient geomagnetic dipole field, was probably acquired some time in the 200 m.y. interval between deposition of the Liantuo Formation at ∼750 Ma and Cambrian time. The most stable component is a two-polarity remanence, removed at temperatures predominantly >630°C, which we infer to reside in hematite. A change in polarity of this component occupies a similar stratigraphic position (within 5 cm) among three outcrops separated by ∼100 m lateral distance. We calculate a mean paleomagnetic pole from each of the laboratories' datasets and combine these with a previously determined pole from correlative rocks in Yunnan ['N1' of Zhang and Piper, Precambrian Res. 85 (1997) 173–199], to obtain an overall weighted mean paleomagnetic pole (04.4°N, 161.1°E, A95=12.9°, 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 magnetostratigraphic consistency, and a soft-sediment fold test determined by previous work. Results from individual sampling areas constrain the depositional paleolatitude of the Liantuo Formation and equivalent Sinian rocks to 30–40°. 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 Neoproterozoic glaciations. The new basal Sinian paleopole, in the context of recent paleomagnetic and geochronological results from Australia, suggests that the Nantuo glaciation is pre-Marinoan. The new 'Z1' pole may also provide constraints on the various proposed reconstructions of South China's position in Rodinia. In particular, a paleoposition adjacent to northwestern Australia at ∼750 Ma requires a specific relative orientation between the two blocks. Likewise, if Rodinia were still intact by 750 Ma, South China may have lain between Australia and Laurentia only in an orientation different 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 nascent Pacific Ocean.

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