Palaeomagnetic study of Neoproterozoic glacial rocks of the Yangzi Block: palaeolatitude and configuration of South China in the late Proterozoic Supercontinent

Abstract

Regional glaciation is recorded at two stratigraphic levels in the Neoproterozoic Sinian succession of the Yangzi (South China) Block. A palaeomagnetic study is reported here of the upper sequence (∼ 730-670 Ma) which contains red beds. An ‘A1’ component ( D I = 291 −57° , α 95 = 5.0°, Palaeopole 151.2°E, 0.2°N) is resolved at high unblocking temperatures and resides in specular haematite. A study of contemporaneous deformation structures suggests that this component was acquired at an early stage of lithification and yields a representative palaeolatitude (37°) for the glaciation. Lower blocking temperature magnetizations referred to as ‘A2’ ( D I = 297 −19° , α 95 = 7.6°, Palaeopole 171.8°E, 19.2°S) and ‘A3’ ( D I = 319 −1° , α 95 = 3.2°, Palaeopole 169.1°E, 41.8°S) are later overprints correlating with probable Late Precambrian or Cambrian palaeofields. In other regions, the ‘A’ magnetizations were found to be partially or completely overprinted by a Recent field and/or a ‘B’ remanence ( D I = 4 13° ); the latter correlates with a remagnetization widely recorded elsewhere in South China and is attributed to the Mesozoic Indosinian Orogeny. An ∼90° apparent polar wander (APW) swathe is identified from the Yangzi Block between ∼730-670 Ma and 540 Ma. It is not yet clear whether poles from older Sinian rocks record earlier magnetizations or were overprinted during this APW motion. Comparison of the Sinian-Cambrian palaeomagnetic record from South China with the contemporaneous record from Australia implies that the former block was located immediately to the north-west of Australia during these times. This reconstruction is compatible with Early Palaeozoic biogeographic data. Continuity of the ∼900 Ma Cathaysian-Yangzi suture with the ∼ 1000 Ma Albany-Fraser belt in Australia suggests an extension of this reconstruction back to the earlier part of Neoproterozoic times. A proposed location of South China adjacent to western North America (Li et al., 1995) does not conform with the palaeomagnetic data. No support is also found for the Rodinia reconstruction during the critical 750-550 Ma interval required by the stratigraphic correlation from which the reconstruction was originally derived (Jefferson, 1980). We conclude that the large-scale three-fold sedimentary cycle, which formed the basis for this reconstruction, resulted from global eustatic changes and cannot be used to infer close spatial proximity.